Frictional melting of clayey gouge during seismic fault slip: Experimental observation and implications

NASA Astrophysics Data System (ADS)

Han, Raehee; Hirose, Takehiro; Jeong, Gi Young; Ando, Jun-ichi; Mukoyoshi, Hideki

2014-08-01

Clayey gouges are common in fault slip zones at shallow depths. Thus, the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths. We conducted rotary shear tests on clayey gouge (~41 wt % clay minerals) at a seismic slip rate of 1.3 m/s. Here we report that the gouge was melted at 5 MPa of normal stress and room humidity conditions. The initial local melting was followed by melt layer formation. Clay minerals (e.g., smectite and illite) and plagioclase were melted and quenched to glass with numerous vesicles. Both flash heating and bulk temperature increases appear to be responsible for the melting. This observation of clayey gouge melting is comparable to that of natural faults (e.g., Chelungpu fault, Taiwan). Due to heterogeneous fault zone properties (e.g., permeability), frictional melting may be one of the important processes in clayey slip zones at shallow depths.

Evolution of the Median Tectonic Line fault zone, SW Japan, during exhumation

NASA Astrophysics Data System (ADS)

Shigematsu, Norio; Kametaka, Masao; Inada, Noriyuki; Miyawaki, Masahiro; Miyakawa, Ayumu; Kameda, Jun; Togo, Tetsuhiro; Fujimoto, Koichiro

2017-01-01

Like many crustal-scale fault zones, the Median Tectonic Line (MTL) fault zone in Japan preserves fault rocks that formed across a broad range of physical conditions. We examined the architecture of the MTL at a large new outcrop in order to understand fault behaviours under different crustal levels. The MTL here strikes almost E-W, dips to the north, and juxtaposes the Sanbagawa metamorphic rocks to the south against the Izumi Group sediments to the north. The fault core consists mainly of Sanbagawa-derived fault gouges. The fault zone can be divided into several structural units, including two slip zones (upper and lower slip zones), where the lower slip zone is more conspicuous. Crosscutting relationships among structures and kinematics indicate that the fault zone records four stages of deformation. Microstructures and powder X-ray diffraction (XRD) analyses indicate that the four stages of deformation occurred under different temperature conditions. The oldest deformation (stage 1) was widely distributed, and had a top-to-the-east (dextral) sense of slip at deep levels of the seismogenic zone. Deformation with the same sense of slip, then became localised in the lower slip zone (stage 2). Subsequently, the slip direction in the lower slip zone changed to top-to-the-west (sinistral-normal) (stage 3). The final stage of deformation (stage 4) involved top-to-the-north normal faulting along the two slip zones within the shallow crust (near the surface). The widely distributed stage 1 damage zone characterises the deeper part of the seismogenic zone, while the sets of localised principal slip zones and branching faults of stage 4 characterise shallow depths. The fault zone architecture described in this paper leads us to suggest that fault zones display different behaviours at different crustal levels.

Aftershocks to Philippine quake found within nearby megathrust fault

NASA Astrophysics Data System (ADS)

Schultz, Colin

2013-02-01

On 31 August 2012 a magnitude 7.6 earthquake ruptured deep beneath the sea floor of the Philippine Trench, a powerful intraplate earthquake centered seaward of the plate boundary. In the wake of the main shock, sensors detected a flurry of aftershocks, counting 110 in total. Drawing on seismic wave observations and rupture mechanisms calculated for the aftershocks, Ye et al. found that many were located near the epicenter of the main intraplate quake but at shallower depth; all involved normal faulting. Some shallow thrusting aftershocks were located farther to the west, centered within the potentially dangerous megathrust fault formed by the subduction of the Philippine Sea plate beneath the Philippine microplate, the piece of crust housing the Philippine Islands.

Seismic reflection images of shallow faulting, northernmost Mississippi embayment, north of the New Madrid seismic zone

USGS Publications Warehouse

McBride, J.H.; Nelson, W.J.

2001-01-01

High-resolution seismic reflection surveys document tectonic faults that displace Pleistocene and older strata just beyond the northeast termination of the New Madrid seismic zone, at the northernmost extent of the Mississippi embayment. These faults, which are part of the Fluorspar Area fault complex in southeastern Illinois, are directly in line with the northeast-trending seismic zone. The reflection data were acquired using an elastic weight-drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10-ft (??3.0m) station interval. Recognizable reflections were recorded to about 200 msec (100-150 m). The effects of multiple reflections, numerous diffractions, low apparent velocity (i.e., steeply dipping) noise, and the relatively low-frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets. Data processing steps that were critical to the detection of faults included residual statics, post-stack migration, deconvolution, and noise-reduction filtering. Seismic migration was crucial for detecting and mitigating complex fault-related diffraction patterns, which produced an apparent 'folding' of reflectors on unmigrated sections. Detected individual offsets of shallow reflectors range from 5 to 10 m for the top of Paleozoic bedrock and younger strata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grabens. Tilting or folding of stratal reflectors associated with faulting is also locally observed. At one site, the observed faulting is superimposed over a prominent antiformal structure, which may itself be a product of the Quaternary deformation that produced the steep normal and reverse faults. Our results suggest that faulting of the Paleozoic bedrock and younger sediments of the northern Mississippi embayment is more pervasive and less localized than previously thought.

Dynamic Rupture Simulations of 11 March 2011 Tohoku Earthquake

NASA Astrophysics Data System (ADS)

Kozdon, J. E.; Dunham, E. M.

2012-12-01

There is strong observational evidence that the 11 March 2011 Tohoku earthquake rupture reached the seafloor. This was unexpected because the shallow portion of the plate interface is believed to be frictionally stable and thus not capable of sustaining coseismic rupture. In order to explore this seeming inconsistency we have developed a two-dimensional dynamic rupture model of the Tohoku earthquake. The model uses a complex fault, seafloor, and material interface structure as derived from seismic surveys. We use a rate-and-state friction model with steady state shear strength depending logarithmically on slip velocity, i.e., there is no dynamic weakening in the model. The frictional parameters are depth dependent with the shallowest portions of the fault beneath the accretionary prism being velocity strengthening. The total normal stress on the fault is taken to be lithostatic and the pore pressure is hydrostatic until a maximum effective normal stress is reached (40 MPa in our preferred model) after which point the pore pressure follows the lithostatic gradient. We also account for poroelastic buffering of effective normal stress changes on the fault. The off-fault response is linear elastic. Using this model we find that large stress changes are dynamically transmitted to the shallowest portions of the fault by waves released by deep slip that are reflected off the seafloor. These stress changes are significant enough to drive the rupture through a velocity strengthening region that is tens of kilometers long. Rupture to the trench is therefore consistent with standard assumptions about depth-dependence of subduction zone properties, and does not require extreme dynamic weakening, shallow high stress drop asperities, or other exceptional processes. We also make direct comparisons with measured seafloor deformation and onshore 1-Hz GPS data from the Tohoku earthquake. Through these comparisons we are able to determine the sensitivity of these data to several dynamic source parameters (prestress, seismogenic depth, and the extent and frictional properties of the shallow plate interface). We find that there is a trade-off between the near-trench frictional properties and effective normal stress, particularly for onshore measurements. That is, the data can be equally well fit by either a velocity strengthening or velocity weakening near-trench fault segment, provided that compensating adjustments are also made to the maximum effective normal stress on the fault. On the other hand, the seismogenic depth is fairly well constrained from the static displacement field, independent of effective normal stress and near-trench properties. Finally, we show that a water layer (modeled as an isotropic linear acoustic material) has a negligible effect on the rupture process. That said, the inclusion of a water layer allows us to make important predictions concerning hydroacoustic signals that were observed by ocean bottom pressure sensors.

Strike-Slip Fault Deformation and Its Control in Hydrocarbon Trapping in Ketaling Area, Jambi Subbasin, Indonesia

NASA Astrophysics Data System (ADS)

Ramadhan, Aldis; Badai Samudra, Alexis; Jaenudin; Puji Lestari, Enik; Saputro, Julian; Sugiono; Hirosiadi, Yosi; Amrullah, Indi

2018-03-01

Geologically, Ketaling area consists of a local high considered as flexure margin of Tempino-Kenali Asam Deep in west part and graben in east part also known as East Ketaling Deep. Numerous proven plays were established in Ketaling area with reservoir in early Miocene carbonate and middle Miocene sand. This area underwent several major deformations. Faults are developed widely, yet their geometrical features and mechanisms of formation remained so far indistinct, which limited exploration activities. With new three-dimensional seismic data acquired in 2014, this area evidently interpreted as having strike-slip mechanism. The objective of this study is to examine characteristic of strike slip fault and its affect to hydrocarbon trapping in Ketaling Area. Structural pattern and characteristic of strike slip fault deformation was examined with integration of normal seismic with variance seismic attribute analysis and the mapping of Syn-rift to Post-rift horizon. Seismic flattening on 2D seismic cross section with NW-SE direction is done to see the structural pattern related to horst (paleohigh) and graben. Typical flower structure, branching strike-slip fault system and normal fault in synrift sediment clearly showed in section. An echelon pattern identified from map view as the result of strike slip mechanism. Detail structural geology analysis show the normal fault development which has main border fault in the southern of Ketaling area dipping to the Southeast-East with NE-SW lineament. These faults related to rift system in Ketaling area. NW-SE folds with reactive NE-SW fault which act as hydrocarbon trapping in the shallow zone. This polyphase tectonic formed local graben, horst and inverted structure developed a good kitchen area (graben) and traps (horst, inverted structure). Subsequently, hydrocarbon accumulation potentials such as basement fractures, inverted syn-rift deposit and shallow zone are very interesting to explore in this area.

Normal fault earthquakes or graviquakes

PubMed Central

Doglioni, C.; Carminati, E.; Petricca, P.; Riguzzi, F.

2015-01-01

Earthquakes are dissipation of energy throughout elastic waves. Canonically is the elastic energy accumulated during the interseismic period. However, in crustal extensional settings, gravity is the main energy source for hangingwall fault collapsing. Gravitational potential is about 100 times larger than the observed magnitude, far more than enough to explain the earthquake. Therefore, normal faults have a different mechanism of energy accumulation and dissipation (graviquakes) with respect to other tectonic settings (strike-slip and contractional), where elastic energy allows motion even against gravity. The bigger the involved volume, the larger is their magnitude. The steeper the normal fault, the larger is the vertical displacement and the larger is the seismic energy released. Normal faults activate preferentially at about 60° but they can be shallower in low friction rocks. In low static friction rocks, the fault may partly creep dissipating gravitational energy without releasing great amount of seismic energy. The maximum volume involved by graviquakes is smaller than the other tectonic settings, being the activated fault at most about three times the hypocentre depth, explaining their higher b-value and the lower magnitude of the largest recorded events. Having different phenomenology, graviquakes show peculiar precursors. PMID:26169163

Evolution of oceanic core complex domes and corrugations

NASA Astrophysics Data System (ADS)

Cann, J.; Escartin, J.; Smith, D.; Schouten, H.

2007-12-01

In regions of the oceans where detachment faulting is developed widely, individual core complex domes (elevated massifs capped by corrugated detachment surfaces) show a consistent morphology. At their outward sides, most core complex domes are attached to a planar slope, interpreted (Smith et al., 2006) as an originally steep inward-facing normal fault that has been rotated to shallower angles. We suggest that the break in slope where the originally steep normal fault meets the domal corrugated surface marks the trace of the brittle-ductile transition at the base of the original normal fault. The steep faults originate within a short distance of the spreading axis. This means that the arcuate shape of the intersection of the steep fault with the dome must indicate the shape of the brittle-ductile transition very close to the spreading axis. The transition must be very shallow close to the summit of the dome and deeper on each flank. Evidence from drilling of some core complexes (McCaig et al, 2007) shows that while the domal detachment faults are active they may channel hydrothermal flow at black smoker temperatures and may be simultaneously injected by magma from below. This indicates a close link between igneous activity, hydrothermal flow and deformation while a core complex is forming. Once the shape of the core complex dome is established, it persists as the ductile footwall mantle rising from below is shaped by the overlying brittle hanging wall that has been cooled by the hydrothermal circulation. The corrugations in the footwall must be moulded into it by irregularities in the brittle hanging wall, as suggested by Spencer (1999). The along-axis arched shape of the hanging wall helps to stabilise the domal shape of the footwall as it rises and cools.

Structure of a normal seismogenic fault zone in carbonates: The Vado di Corno Fault, Campo Imperatore, Central Apennines (Italy)

NASA Astrophysics Data System (ADS)

Demurtas, Matteo; Fondriest, Michele; Balsamo, Fabrizio; Clemenzi, Luca; Storti, Fabrizio; Bistacchi, Andrea; Di Toro, Giulio

2016-09-01

The Vado di Corno Fault Zone (VCFZ) is an active extensional fault cutting through carbonates in the Italian Central Apennines. The fault zone was exhumed from ∼2 km depth and accommodated a normal throw of ∼2 km since Early-Pleistocene. In the studied area, the master fault of the VCFZ dips N210/54° and juxtaposes Quaternary colluvial deposits in the hangingwall with cataclastic dolostones in the footwall. Detailed mapping of the fault zone rocks within the ∼300 m thick footwall-block evidenced the presence of five main structural units (Low Strain Damage Zone, High Strain Damage Zone, Breccia Unit, Cataclastic Unit 1 and Cataclastic Unit 2). The Breccia Unit results from the Pleistocene extensional reactivation of a pre-existing Pliocene thrust. The Cataclastic Unit 1 forms a ∼40 m thick band lining the master fault and recording in-situ shattering due to the propagation of multiple seismic ruptures. Seismic faulting is suggested also by the occurrence of mirror-like slip surfaces, highly localized sheared calcite-bearing veins and fluidized cataclasites. The VCFZ architecture compares well with seismological studies of the L'Aquila 2009 seismic sequence (mainshock MW 6.1), which imaged the reactivation of shallow-seated low-angle normal faults (Breccia Unit) cut by major high-angle normal faults (Cataclastic Units).

Postseismic deformation associated with the 2008 Mw 7.9 Wenchuan earthquake, China: Constraining fault geometry and investigating a detailed spatial distribution of afterslip

NASA Astrophysics Data System (ADS)

Jiang, Zhongshan; Yuan, Linguo; Huang, Dingfa; Yang, Zhongrong; Chen, Weifeng

2017-12-01

We reconstruct two types of fault models associated with the 2008 Mw 7.9 Wenchuan earthquake, one is a listric fault connecting a shallowing sub-horizontal detachment below ∼20 km depth (fault model one, FM1) and the other is a group of more steeply dipping planes further extended to the Moho at ∼60 km depth (fault model two, FM2). Through comparative analysis of the coseismic inversion results, we confirm that the coseismic models are insensitive to the above two type fault geometries. We therefore turn our attention to the postseismic deformation obtained from GPS observations, which can not only impose effective constraints on the fault geometry but also, more importantly, provide valuable insights into the postseismic afterslip. Consequently, FM1 performs outstandingly in the near-, mid-, and far-field, whether considering the viscoelastic influence or not. FM2 performs more poorly, especially in the data-model consistency in the near field, which mainly results from the trade-off of the sharp contrast of the postseismic deformation on both sides of the Longmen Shan fault zone. Accordingly, we propose a listric fault connecting a shallowing sub-horizontal detachment as the optimal fault geometry for the Wenchuan earthquake. Based on the inferred optimal fault geometry, we analyse two characterized postseismic deformation phenomena that differ from the coseismic patterns: (1) the postseismic opposite deformation between the Beichuan fault (BCF) and Pengguan fault (PGF) and (2) the slightly left-lateral strike-slip motions in the southwestern Longmen Shan range. The former is attributed to the local left-lateral strike-slip and normal dip-slip components on the shallow BCF. The latter places constraints on the afterslip on the southwestern BCF and reproduces three afterslip concentration areas with slightly left-lateral strike-slip motions. The decreased Coulomb Failure Stress (CFS) change ∼0.322 KPa, derived from the afterslip with viscoelastic influence removed at the hypocentre of the Lushan earthquake, indicates that the postseismic left-lateral strike-slip and normal dip-slip motions may have a mitigative effect on the fault loading in the southwestern Longmen Shan range. Nevertheless, it is much smaller than the total increased CFS changes (∼8.368 KPa) derived from the coseismic and viscoelastic deformations.

Preferred-rupture propagation to the hangingwall of the shallow part of the out-of-sequence thrust: Ishido Fault in Boso Peninsula, central Japan

NASA Astrophysics Data System (ADS)

Yamamoto, Y.; Fukuyama, M.; Ujiie, K.; Hirose, T.; Hamada, Y.; Kitamura, M.; Kamiya, N.

2016-12-01

Although earthquake ruptures in shallow portion of plate boundary have recently been identified (e.g. Tohoku, Nankai, etc.), their mechanisms why the shallow portion of plate boundary composed mainly of clay minerals can accumulate strain and make seismic slip are under controversial. An ancient out-of-sequence thrust which divided the early and late Miocene accretionary complexes in the Boso Peninsula, central Japan records rupture propagation to the shallow portion of accretionary prism (< 2 km). The fault core is composed of black-colored thin (<1 mm) pseudotachylite and fluidized fault gouge. The former is characterized by homogeneous glassy matrix including fragments of quartz/feldspar, submicron-sized Fe-rich spherules, and vesicles. Based on the mineralogy of the host rock and EDS analyses of matrices, origin of the pseudotachylite was apparently frictional melting of smectite containing Fe. Fe-rich spherules formed by rapid cooling of pseudotachylite. On the other hand, overturned fault-related drag fold developed in the footwall, within about 30 m. Although some Riedel sheared normal faults developed in the overturned footwall, no other brittle deformations were identified. These occurrences imply coexistence of low- and high-speed slips along the same thrust fault. The whole-rock major and trace elemental analyses using XRF and ICP-MS show that mudstone in the hangingwall has similar chemical composition to those of pseudotachylite and fluidized fault gouge with REE enriched patterns, whereas the footwall has different chemical characteristics with relatively flat REE pattern and low LOI. Therefore, the protolith of pseudotachylite and fluidized fault gouge is mudstone in the hangingwall. These data imply that rupture propagation preferably occurred in the hangingwall along the fault zone. The footwall was also deformed apparently during slow-slip deformation leading to formation of the overturn, whereas only the hangingwall, just side of the fault zone, slipped under high-speed shear.

Spatial arrangement and size distribution of normal faults, Buckskin detachment upper plate, Western Arizona

NASA Astrophysics Data System (ADS)

Laubach, S. E.; Hundley, T. H.; Hooker, J. N.; Marrett, R. A.

2018-03-01

Fault arrays typically include a wide range of fault sizes and those faults may be randomly located, clustered together, or regularly or periodically located in a rock volume. Here, we investigate size distribution and spatial arrangement of normal faults using rigorous size-scaling methods and normalized correlation count (NCC). Outcrop data from Miocene sedimentary rocks in the immediate upper plate of the regional Buckskin detachment-low angle normal-fault, have differing patterns of spatial arrangement as a function of displacement (offset). Using lower size-thresholds of 1, 0.1, 0.01, and 0.001 m, displacements range over 5 orders of magnitude and have power-law frequency distributions spanning ∼ four orders of magnitude from less than 0.001 m to more than 100 m, with exponents of -0.6 and -0.9. The largest faults with >1 m displacement have a shallower size-distribution slope and regular spacing of about 20 m. In contrast, smaller faults have steep size-distribution slopes and irregular spacing, with NCC plateau patterns indicating imposed clustering. Cluster widths are 15 m for the 0.1-m threshold, 14 m for 0.01-m, and 1 m for 0.001-m displacement threshold faults. Results demonstrate normalized correlation count effectively characterizes the spatial arrangement patterns of these faults. Our example from a high-strain fault pattern above a detachment is compatible with size and spatial organization that was influenced primarily by boundary conditions such as fault shape, mechanical unit thickness and internal stratigraphy on a range of scales rather than purely by interaction among faults during their propagation.

Cenozoic structural inversion from transtension to transpression in Yingxiong Range, western Qaidam Basin: New insights into strike-slip superimposition controlled by Altyn Tagh and Eastern Kunlun Faults

NASA Astrophysics Data System (ADS)

Cheng, Xiang; Zhang, Daowei; Jolivet, Marc; Yu, Xiangjiang; Du, Wei; Liu, Runchao; Guo, Zhaojie

2018-01-01

A Cenozoic structural inversion event from transtension to transpression involving salt tectonics has been uncovered in the Yingxiong Range, the western Qaidam Basin. Seismic reflection data show that there are two common structural styles in the Yingxiong Range: (1) the positive flower structure; (2) the thrust-controlled fold at shallow depth and the positive inverted flower structure at deep levels, which are separated by a salt layer in the upper Xiaganchaigou Formation. The Yingxiong Range experienced a first stage of transtension in the Eocene, induced by the Altyn Tagh Fault, and a second stage of transpression from the early Miocene to present, jointly controlled by the Altyn Tagh and Eastern Kunlun Faults. The Eocene transtension produced numerous NW-striking right-stepping en-échelon transtensional normal faults or fractures in the Yingxiong Range. At the same time, evaporites and mudstone were deposited in the vicinity of these faults. In the early Miocene, the Eocene transtensional normal faults were reactivated in a reverse sense, and the thrust-controlled folds at shallow depth started to form simultaneously. With transpression enhanced in the late Cenozoic, positive flower structures directly formed in places without evaporites. The Cenozoic transtension to transpression inversion of the Yingxiong Range is the result of strike-slip superimposition controlled by the Altyn Tagh and Eastern Kunlun Faults in time and space.

Rupture Process During the Mw 8.1 2017 Chiapas Mexico Earthquake: Shallow Intraplate Normal Faulting by Slab Bending

NASA Astrophysics Data System (ADS)

Okuwaki, R.; Yagi, Y.

2017-12-01

A seismic source model for the Mw 8.1 2017 Chiapas, Mexico, earthquake was constructed by kinematic waveform inversion using globally observed teleseismic waveforms, suggesting that the earthquake was a normal-faulting event on a steeply dipping plane, with the major slip concentrated around a relatively shallow depth of 28 km. The modeled rupture evolution showed unilateral, downdip propagation northwestward from the hypocenter, and the downdip width of the main rupture was restricted to less than 30 km below the slab interface, suggesting that the downdip extensional stresses due to the slab bending were the primary cause of the earthquake. The rupture front abruptly decelerated at the northwestern end of the main rupture where it intersected the subducting Tehuantepec Fracture Zone, suggesting that the fracture zone may have inhibited further rupture propagation.

Teleseismic body waves from dynamically rupturing shallow thrust faults: Are they opaque for surface-reflected phases?

USGS Publications Warehouse

Smith, D.E.; Aagaard, Brad T.; Heaton, T.H.

2005-01-01

We investigate whether a shallow-dipping thrust fault is prone to waveslip interactions via surface-reflected waves affecting the dynamic slip. If so, can these interactions create faults that are opaque to radiated energy? Furthermore, in this case of a shallow-dipping thrust fault, can incorrectly assuming a transparent fault while using dislocation theory lead to underestimates of seismic moment? Slip time histories are generated in three-dimensional dynamic rupture simulations while allowing for varying degrees of wave-slip interaction controlled by fault-friction models. Based on the slip time histories, P and SH seismograms are calculated for stations at teleseismic distances. The overburdening pressure caused by gravity eliminates mode I opening except at the tip of the fault near the surface; hence, mode I opening has no effect on the teleseismic signal. Normalizing by a Haskell-like traditional kinematic rupture, we find teleseismic peak-to-peak displacement amplitudes are approximately 1.0 for both P and SH waves, except for the unrealistic case of zero sliding friction. Zero sliding friction has peak-to-peak amplitudes of 1.6 for P and 2.0 for SH waves; the fault slip oscillates about its equilibrium value, resulting in a large nonzero (0.08 Hz) spectral peak not seen in other ruptures. These results indicate wave-slip interactions associated with surface-reflected phases in real earthquakes should have little to no effect on teleseismic motions. Thus, Haskell-like kinematic dislocation theory (transparent fault conditions) can be safety used to simulate teleseismic waveforms in the Earth.

The great 1933 Sanriku-oki earthquake: reappraisal of the main shock and its aftershocks and implications for its tsunami using regional tsunami and seismic data

NASA Astrophysics Data System (ADS)

Uchida, Naoki; Kirby, Stephen H.; Umino, Norihito; Hino, Ryota; Kazakami, Tomoe

2016-09-01

The aftershock distribution of the 1933 Sanriku-oki outer trench earthquake is estimated by using modern relocation methods and a newly developed velocity structure to examine the spatial extent of the source-fault and the possibility of a triggered interplate seismicity. In this study, we first examined the regional data quality of the 1933 earthquake based on smoked-paper records and then relocated the earthquakes by using the 3-D velocity structure and double-difference method. The improvements of hypocentre locations using these methods were confirmed by the examination of recent earthquakes that are accurately located based on ocean bottom seismometer data. The results show that the 1933 aftershocks occurred under both the outer- and inner-trench-slope regions. In the outer-trench-slope region, aftershocks are distributed in a ˜280-km-long area and their depths are shallower than 50 km. Although we could not constrain the fault geometry from the hypocentre distribution, the depth distribution suggests the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks under the inner trench slope was also confirmed by an investigation of waveform frequency difference between outer and inner trench earthquakes as recorded at Mizusawa. The earthquakes under the inner trench slope were shallow (depth ≦30 km) and the waveforms show a low-frequency character similar to the waveforms of recent, precisely located earthquakes in the same area. They are also located where recent activity of interplate thrust earthquakes is high. These suggest that the 1933 outer-trench-slope main shock triggered interplate earthquakes, which is an unusual case in the order of occurrence in contrast with the more common pairing of a large initial interplate shock with subsequent outer-slope earthquakes. The off-trench earthquakes are distributed about 80 km width in the trench perpendicular direction. This wide width cannot be explained from a single high-angle fault confined at a shallow depth (depth ≦50 km). The upward motion of the 1933 tsunami waveform records observed at Sanriku coast also cannot be explained from a single high-angle west-dipping normal fault. If we consider additional fault, involvement of high-angle, east-dipping normal faults can better explain the tsunami first motion and triggering of the aftershock in a wide area under the outer trench slope. Therefore multiple off-trench normal faults may have activated during the 1933 earthquake. We also relocated recent (2001-2012) seismicity by the same method. The results show that the present seismicity in the outer-trench-slope region can be divided into several groups along the trench. Comparison of the 1933 rupture dimensions based on our aftershock relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited to the region where fault scarps are largely trench parallel and cross cut the seafloor spreading fabric. These findings imply that bending geometry and structural segmentation of the incoming plate largely controls the spatial extent of the 1933 seismogenic faulting. In this shallow rupture model for this largest outer trench earthquake, triggered seismicity in the forearc and structural control of faulting represent an important deformation styles for off-trench and shallow megathrust zones.

Does permanent extensional deformation in lower forearc slopes indicate shallow plate-boundary rupture?

NASA Astrophysics Data System (ADS)

Geersen, J.; Ranero, C. R.; Kopp, H.; Behrmann, J. H.; Lange, D.; Klaucke, I.; Barrientos, S.; Diaz-Naveas, J.; Barckhausen, U.; Reichert, C.

2018-05-01

Seismic rupture of the shallow plate-boundary can result in large tsunamis with tragic socio-economic consequences, as exemplified by the 2011 Tohoku-Oki earthquake. To better understand the processes involved in shallow earthquake rupture in seismic gaps (where megathrust earthquakes are expected), and investigate the tsunami hazard, it is important to assess whether the region experienced shallow earthquake rupture in the past. However, there are currently no established methods to elucidate whether a margin segment has repeatedly experienced shallow earthquake rupture, with the exception of mechanical studies on subducted fault-rocks. Here we combine new swath bathymetric data, unpublished seismic reflection images, and inter-seismic seismicity to evaluate if the pattern of permanent deformation in the marine forearc of the Northern Chile seismic gap allows inferences on past earthquake behavior. While the tectonic configuration of the middle and upper slope remains similar over hundreds of kilometers along the North Chilean margin, we document permanent extensional deformation of the lower slope localized to the region 20.8°S-22°S. Critical taper analyses, the comparison of permanent deformation to inter-seismic seismicity and plate-coupling models, as well as recent observations from other subduction-zones, including the area that ruptured during the 2011 Tohoku-Oki earthquake, suggest that the normal faults at the lower slope may have resulted from shallow, possibly near-trench breaking earthquake ruptures in the past. In the adjacent margin segments, the 1995 Antofagasta, 2007 Tocopilla, and 2014 Iquique earthquakes were limited to the middle and upper-slope and the terrestrial forearc, and so are upper-plate normal faults. Our findings suggest a seismo-tectonic segmentation of the North Chilean margin that seems to be stable over multiple earthquake cycles. If our interpretations are correct, they indicate a high tsunami hazard posed by the yet un-ruptured southern segment of the seismic gap.

Geological perspectives of shallow slow earthquakes deduced from deformation in subduction mélanges

NASA Astrophysics Data System (ADS)

Ujiie, K.; Saishu, H.; Kinoshita, T.; Nishiyama, N.; Otsubo, M.; Ohta, K.; Yamashita, Y.; Ito, Y.

2017-12-01

Shallow (< 15 km depth) slow earthquakes are important to understand, as they occur along the subduction thrust where devastating tsunamis are generated. Geophysical studies have revealed that shallow slow earthquakes are not restricted to specific temperature conditions and depths but occur in regions of high fluid pressure. In the Nankai subduction zone, the shallow slow slip appears to trigger tremor and very-low-frequency-earthquake. However, the geologic perspectives for shallow slow earthquakes remain enigmatic. The Makimine mélange in the Late Cretaceous Shimanto accretionary complex of southwest Japan was formed during the subduction of young oceanic plate. Within the mélange, the quartz-filled veins and viscous shear zones are concentrated in the zones of 10 to 60 m-thick. The veins consist of shear veins showing low-angle thrust or normal faulting mechanisms and extension veins parallel or at high angle to mélange foliation. The geometrical relationship between shear and extension veins indicates that shear slip and tensile fracturing occur by small differential stress under elevated fluid pressure. The shear and extension veins typically show crack-seal textures defined by the solid inclusions bands. The time scale of each crack-seal event, which is determined from the quartz kinetics considering inclusion band spacing and vein length, is a few years. The shear slip increments estimated from the spacing of inclusions bands at dilational jogs are 0.1 mm. The viscous shear is accommodated by pressure solution creep and consistently shows low-angle thrust shear sense. These geologic features are suggested to explain seismogenic environment for shallow slow earthquakes. The shear veins and viscous shear zones showing low-angle thrust faulting mechanism could represent episodic tremor and slip, while the shear veins showing low-angle normal faulting mechanism may represent the tremor that occurred after the passage of slow slip front.

Shallow subsurface imaging of the Piano di Pezza active normal fault (central Italy) by high-resolution refraction and electrical resistivity tomography coupled with time domain electromagnetic data

NASA Astrophysics Data System (ADS)

Villani, Fabio; Tulliani, Valerio; Fierro, Elisa; Sapia, Vincenzo; Civico, Riccardo

2015-04-01

The Piano di Pezza fault is the north-westernmost segment of the >20 km long Ovindoli-Pezza active normal fault-system (central Italy). Although existing paleoseismic data document high vertical Holocene slip rates (~1 mm/yr) and a remarkable seismogenic potential of this fault, its subsurface setting and Pleistocene cumulative displacement are still poorly known. We investigated for the first time by means of high-resolution seismic and electrical resistivity tomography coupled with time domain electromagnetic (TDEM) measurements the shallow subsurface of a key section of the Piano di Pezza fault. Our surveys cross a ~5 m-high fault scarp that was generated by repeated surface-rupturing earthquakes displacing some Late Holocene alluvial fans. We provide 2-D Vp and resistivity images which clearly show significant details of the fault structure and the geometry of the shallow basin infill material down to 50 m depth. We can estimate the dip (~50°) and the Holocene vertical displacement of the master fault (~10 m). We also recognize in the hangingwall some low-velocity/low-resistivity regions that we relate to packages of colluvial wedges derived from scarp degradation, which may represent the record of several paleo-earthquakes older than the Late Holocene events previously recognized by paleoseismic trenching. Conversely, due to the limited investigation depth of seismic and electrical tomography, the estimation of the cumulative amount of Pleistocene throw is hampered. Therefore, to increase the depth of investigation, we performed 7 TDEM measurements along the electrical profile using a 50 m loop size both in central and offset configuration. The recovered 1-D resistivity models show a good match with 2-D resistivity images in the near surface. Moreover, TDEM inversion results indicate that in the hangingwall, ~200 m away from the surface fault trace, the carbonate pre-Quaternary basement may be found at ~90-100 m depth. The combined approach of electrical and seismic data coupled with TDEM measurements provides a robust constraint to the Piano di Pezza fault cumulative offset. Our data are useful for better reconstructing the deep structural setting of the Piano di Pezza basin and assessing the role played by extensional tectonics in its Quaternary evolution.

Continentward-Dipping Normal Faults, Boudinage and Ductile Shear at Rifted Passive Margins

NASA Astrophysics Data System (ADS)

Clerc, C. N.; Ringenbach, J. C.; Jolivet, L.; Ballard, J. F.

2017-12-01

Deep structures resulting from the rifting of the continental crust are now well imaged by seismic profiles. We present a series of recent industrial profiles that allow the identification of various rift-related geological processes such as crustal boudinage, ductile shear of the base of the crust and low-angle detachment faulting. Along both magma-rich and magma-poor rifted margins, we observe clear indications of ductile deformation of the deep continental crust. Large-scale shallow dipping shear zones are identified with a top-to-the-continent sense of shear. This sense of shear is consistent with the activity of the Continentward-Dipping Normal Faults (CDNF) that accommodate the extension in the upper crust. This pattern is responsible for an oceanward migration of the deformation and of the associated syn-tectonic deposits (sediments and/or volcanics). We discuss the origin of the Continentward-Dipping Normal Faults (CDNF) and investigate their implications and the effect of sediment thermal blanketing on crustal rheology. In some cases, low-angle shear zones define an anastomosed pattern that delineates boudin-like structures that seem to control the position and dip of upper crustal normal faults. We present some of the most striking examples from several locations (Uruguay, West Africa, South China Sea…), and discuss their rifting histories that differ from the classical models of oceanward-dipping normal faults.

A case study on pseudo 3-D Chirp sub-bottom profiler (SBP) survey for the detection of a fault trace in shallow sedimentary layers at gas hydrate site in the Ulleung Basin, East Sea

NASA Astrophysics Data System (ADS)

Kim, Young-Jun; Koo, Nam-Hyung; Cheong, Snons; Kim, Jung-Ki; Chun, Jong-Hwa; Shin, Sung-Ryul; Riedel, Michael; Lee, Ho-Young

2016-10-01

A pseudo 3-D Chirp sub-bottom profiler (SBP) survey was conducted to define the extension of a fault that was previously identified on low-resolution 2-D seismic data with an emphasis on the shallow sedimentary layers and to determine if the fault extends to the seafloor. The geophysical survey was conducted as part of an environmental impact assessment for a proposed gas hydrate production test in the Ulleung Basin, East Sea. The Chirp SBP raw data were acquired over an area of 1 km × 1 km with an average line spacing of 20 m. To produce a 3-D Chirp SBP volume, we developed an optimal processing sequence that was divided into two steps. The first phase of 2-D data processing included a sweep signature estimation, correlation, deconvolution, swell effect correction, and migration. The second phase of 3-D data processing was composed of a bin design, bin gathering of the final processed 2-D data set, amplitude normalization, and residual statics correction. The 3-D Chirp SBP volume provides enhanced imaging especially due to the residual static processing using a moving average method and shows better continuity of the sedimentary layers and consistency of the reflection events than the individual 2-D lines. Deformation of the seafloor as a result of the fault was detected, and the fault offset increases in the deeper sedimentary layers. We also determined that the fault strikes northwest-southeast. However, the shallow sub-seafloor sediments have high porosities and therefore do not exhibit brittle fault-behavior but rather deform continuously due to fault movement.

Integrated study on the topographic and shallow subsurface expression of the Grote Brogel Fault at the boundary of the Roer Valley Graben, Belgium

NASA Astrophysics Data System (ADS)

Deckers, Jef; Van Noten, Koen; Schiltz, Marco; Lecocq, Thomas; Vanneste, Kris

2018-01-01

The Grote Brogel Fault (GBF) is a major WNW-ESE striking normal fault in Belgium that diverges westward from the NW-SE striking western border fault system of the Roer Valley Graben. The GBF delimits the topographically higher Campine Block from the subsiding Roer Valley Graben, and is expressed in the Digital Terrain Model (DTM) by relief gradients or scarps. By integrating DTM, Electrical Resistivity Tomography (ERT), Cone Penetration Test (CPT) and borehole data, we studied the Quaternary activity of the GBF and its effects on local hydrogeology. In the shallow subsurface (< 50 m) underneath these scarps, fault splays of the GBF were interpreted on newly acquired ERT profiles at two investigation sites: one on the eastern section and the other on the western section, near the limit of the visible surface trace of the fault. Borehole and CPT data enabled stratigraphic interpretations of the ERT profiles and thereby allowed measuring vertical fault offsets at the base of Pleistocene fluvial deposits of up to 12 m. Groundwater measurements in the boreholes and CPTs indicate that the GBF acts as a hydrologic boundary that prevents groundwater flow from the elevated footwall towards the hangingwall, resulting in hydraulic head differences of up to 12.7 m. For the two investigation sites, the hydraulic head changes correlate with the relief gradient, which in turn correlates with the Quaternary vertical offset of the GBF. ERT profiles at the eastern site also revealed a local soft-linked stepover in the shallow subsurface, which affects groundwater levels in the different fault blocks, and illustrates the complex small-scale geometry of the GBF.

Strength evolution of simulated carbonate-bearing faults: The role of normal stress and slip velocity

NASA Astrophysics Data System (ADS)

Mercuri, Marco; Scuderi, Marco Maria; Tesei, Telemaco; Carminati, Eugenio; Collettini, Cristiano

2018-04-01

A great number of earthquakes occur within thick carbonate sequences in the shallow crust. At the same time, carbonate fault rocks exhumed from a depth < 6 km (i.e., from seismogenic depths) exhibit the coexistence of structures related to brittle (i.e., cataclasis) and ductile deformation processes (i.e., pressure-solution and granular plasticity). We performed friction experiments on water-saturated simulated carbonate-bearing faults for a wide range of normal stresses (from 5 to 120 MPa) and slip velocities (from 0.3 to 100 μm/s). At high normal stresses (σn > 20 MPa) fault gouges undergo strain-weakening, that is more pronounced at slow slip velocities, and causes a significant reduction of frictional strength, from μ = 0.7 to μ = 0.47. Microstructural analysis show that fault gouge weakening is driven by deformation accommodated by cataclasis and pressure-insensitive deformation processes (pressure solution and granular plasticity) that become more efficient at slow slip velocity. The reduction in frictional strength caused by strain weakening behaviour promoted by the activation of pressure-insensitive deformation might play a significant role in carbonate-bearing faults mechanics.

Simulations of tremor-related creep reveal a weak crustal root of the San Andreas Fault

USGS Publications Warehouse

Shelly, David R.; Bradley, Andrew M.; Johnson, Kaj M.

2013-01-01

Deep aseismic roots of faults play a critical role in transferring tectonic loads to shallower, brittle crustal faults that rupture in large earthquakes. Yet, until the recent discovery of deep tremor and creep, direct inference of the physical properties of lower-crustal fault roots has remained elusive. Observations of tremor near Parkfield, CA provide the first evidence for present-day localized slip on the deep extension of the San Andreas Fault and triggered transient creep events. We develop numerical simulations of fault slip to show that the spatiotemporal evolution of triggered tremor near Parkfield is consistent with triggered fault creep governed by laboratory-derived friction laws between depths of 20–35 km on the fault. Simulated creep and observed tremor northwest of Parkfield nearly ceased for 20–30 days in response to small coseismic stress changes of order 104 Pa from the 2003 M6.5 San Simeon Earthquake. Simulated afterslip and observed tremor following the 2004 M6.0 Parkfield earthquake show a coseismically induced pulse of rapid creep and tremor lasting for 1 day followed by a longer 30 day period of sustained accelerated rates due to propagation of shallow afterslip into the lower crust. These creep responses require very low effective normal stress of ~1 MPa on the deep San Andreas Fault and near-neutral-stability frictional properties expected for gabbroic lower-crustal rock.

Shallow high-resolution geophysical investigation along the western segment of the Victoria Lines Fault (island of Malta)

NASA Astrophysics Data System (ADS)

Villani, Fabio; D'Amico, Sebastiano; Panzera, Francesco; Vassallo, Maurizio; Bozionelos, George; Farrugia, Daniela; Galea, Pauline

2018-01-01

The Victoria Lines Fault (island of Malta) is a >15 km-long and N260°-striking segmented normal fault-system, which is probably inactive since the late Pliocene. In the westernmost part, the Fomm Ir-Rih segment displays comparable geologic throw and escarpment height ( 150-170 m), moreover its hangingwall hosts thin patches of Middle Pleistocene clastic continental deposits (red beds), which are poorly preserved elsewhere. We acquired two seismic transects, by collecting ambient vibration recordings, processed by using horizontal-to-vertical spectral ratios, complemented by one high-resolution 2-D refraction tomography survey crossing this fault where it is locally covered by red beds and recent colluvial deposits. We found a resonance peak at 1.0 Hz in the hangingwall block, whereas clear peaks in the range 5.0-10.0 Hz appear when approaching the subsurface fault, and we relate them to the fractured bedrock within the fault zone. The best-fit tomographic model shows a relatively high-Vp shallow body (Vp 2200-2400 m/s) that we relate to the weathered top of the Miocene Upper Coralline Limestone Fm., bounded on both sides by low-Vp regions (<1400 m/s). The latter are the smeared images of steep fault zones. Tomography further reveals a thick ( 15-20 m) low-Vp (<1000 m/s) zone, which could be a syn-tectonic wedge of colluvial deposits developed in the downthrown block. Surface waves analysis indicates lateral changes of the average shallow shear wave velocity, with Vs 130 m/s within the inferred fault zone, and Vs >230 m/s above the weathered top-bedrock. Our results depict a clear seismic signature of the Victoria Lines Fault, characterized by low seismic velocity and high amplification of ground motion. We hypothesize that, during the Middle Pleistocene, faulting may have affected the basal part of the red beds, so that this part of the investigated complex fault-system may be considered inactive since 0.6 Myr ago.

Frictional properties of the Nankai frontal thrust explain recurring shallow slow slip events

NASA Astrophysics Data System (ADS)

Saffer, D. M.; Ikari, M.; Kopf, A.; Roesner, A.

2017-12-01

Recent observations provide evidence for shallow slip reaching to the trench on subduction megathrusts, both in earthquakes and slow slip events (SSE). This is at odds with existing friction studies, which report primarily velocity-strengthening behavior (friction increases with slip velocity) for subduction fault material and synthetic analogs, which leads only to stable sliding. We report on direct shearing experiments on fault rocks from IODP Site C0007, which sampled the frontal thrust of the Nankai accretionary prism. This fault has been implicated in both coseimic slip and recurring SSE. We focus on material from 437.2 meters below seafloor, immediately above a localized shear zone near the base of the fault. In our experiments, a 25 mm diameter cylindrical specimen is loaded in an assembly of two steel plates. After application of normal stress (3, 10, or 17 MPa) and subsequent equilibration, the lower plate is driven at a constant velocity while the upper plate remains stationary; this configuration forces shear to localize between the two plates. After reaching a steady state residual friction coefficient (µss), we conducted velocity-stepping tests to measure the friction rate parameter (a-b), defined as the change in friction for a change in velocity: (a-b) = Δuss/ln(V/Vo), over a range of velocities from 0.1-100 µm s-1. We find that µss ranges from 0.26 to 0.32 and exhibits a slight decrease with normal stress. We observe velocity-weakening behavior at low normal stresses (3-10 MPa) and for low sliding velocities (< 3-10 µm s-1). Values of (a-b)_increase systematically from -0.007 to -0.005 at velocities of 0.3-1 µm s-1, to 0.001-0.045 at velocities >30 µm s-1. At higher normal stress (17 MPa), we observe dominantly velocity-strengthening, consistent with previously reported measurements for 25 MPa normal stress. Our observation of rate weakening at slip rates matching those of SSE in the outer Nankai forearc provide a potential explanation for periodic strain accumulation and subsequent release during SSE near the trench. The observation of rate weakening behavior only at low normal stresses also suggests that nucleation of these SSE should be restricted to shallow depths (< 2-5 km) or zones of elevated pore fluid pressure.

Frictional properties of the Nankai frontal thrust explain recurring shallow slow slip events

NASA Astrophysics Data System (ADS)

Scholz, J. R.; Davy, C.; Barruol, G.; Fontaine, F. R.; Cordier, E.

2016-12-01

Recent observations provide evidence for shallow slip reaching to the trench on subduction megathrusts, both in earthquakes and slow slip events (SSE). This is at odds with existing friction studies, which report primarily velocity-strengthening behavior (friction increases with slip velocity) for subduction fault material and synthetic analogs, which leads only to stable sliding. We report on direct shearing experiments on fault rocks from IODP Site C0007, which sampled the frontal thrust of the Nankai accretionary prism. This fault has been implicated in both coseimic slip and recurring SSE. We focus on material from 437.2 meters below seafloor, immediately above a localized shear zone near the base of the fault. In our experiments, a 25 mm diameter cylindrical specimen is loaded in an assembly of two steel plates. After application of normal stress (3, 10, or 17 MPa) and subsequent equilibration, the lower plate is driven at a constant velocity while the upper plate remains stationary; this configuration forces shear to localize between the two plates. After reaching a steady state residual friction coefficient (µss), we conducted velocity-stepping tests to measure the friction rate parameter (a-b), defined as the change in friction for a change in velocity: (a-b) = Δuss/ln(V/Vo), over a range of velocities from 0.1-100 µm s-1. We find that µss ranges from 0.26 to 0.32 and exhibits a slight decrease with normal stress. We observe velocity-weakening behavior at low normal stresses (3-10 MPa) and for low sliding velocities (< 3-10 µm s-1). Values of (a-b)_increase systematically from -0.007 to -0.005 at velocities of 0.3-1 µm s-1, to 0.001-0.045 at velocities >30 µm s-1. At higher normal stress (17 MPa), we observe dominantly velocity-strengthening, consistent with previously reported measurements for 25 MPa normal stress. Our observation of rate weakening at slip rates matching those of SSE in the outer Nankai forearc provide a potential explanation for periodic strain accumulation and subsequent release during SSE near the trench. The observation of rate weakening behavior only at low normal stresses also suggests that nucleation of these SSE should be restricted to shallow depths (< 2-5 km) or zones of elevated pore fluid pressure.

Geometry and slip rates of active blind thrusts in a reactivated back-arc rift using shallow seismic imaging: Toyama basin, central Japan

NASA Astrophysics Data System (ADS)

Ishiyama, Tatsuya; Kato, Naoko; Sato, Hiroshi; Koshiya, Shin; Toda, Shigeru; Kobayashi, Kenta

2017-10-01

Active blind thrust faults, which can be a major seismic hazard in urbanized areas, are commonly difficult to image with seismic reflection surveys. To address these challenges in coastal plains, we collected about 8 km-long onshore high-resolution two-dimensional (2D) seismic reflection data using a dense array of 800 geophones across compressionally reactivated normal faults within a failed rift system located along the southwestern extension of the Toyama trough in the Sea of Japan. The processing of the seismic reflection data illuminated their detailed subsurface structures to depths of about 3 km. The interpreted depth-converted section, correlated with nearby Neogene stratigraphy, indicated the presence of and along-strike variation of previously unrecognized complex thrust-related structures composed of active fault-bend folds coupled with pairs of flexural slip faults within the forelimb and newly identified frontal active blind thrusts beneath the alluvial plain. In addition, growth strata and fold scarps that deform lower to upper Pleistocene units record the recent history of their structural growth and fault activity. This case shows that shallow seismic reflection imaging with densely spaced seismic recorders is a useful tool in defining locations, recent fault activity, and complex geometry of otherwise inaccessible active blind thrust faults.

Source model for the Copahue volcano magma plumbing system constrained by InSAR surface deformation observations

NASA Astrophysics Data System (ADS)

Lundgren, Paul; Nikkhoo, Mehdi; Samsonov, Sergey V.; Milillo, Pietro; Gil-Cruz, Fernando; Lazo, Jonathan

2017-07-01

Copahue volcano straddling the edge of the Agrio-Caviahue caldera along the Chile-Argentina border in the southern Andes has been in unrest since inflation began in late 2011. We constrain Copahue's source models with satellite and airborne interferometric synthetic aperture radar (InSAR) deformation observations. InSAR time series from descending track RADARSAT-2 and COSMO-SkyMed data span the entire inflation period from 2011 to 2016, with their initially high rates of 12 and 15 cm/yr, respectively, slowing only slightly despite ongoing small eruptions through 2016. InSAR ascending and descending track time series for the 2013-2016 time period constrain a two-source compound dislocation model, with a rate of volume increase of 13 × 106 m3/yr. They consist of a shallow, near-vertical, elongated source centered at 2.5 km beneath the summit and a deeper, shallowly plunging source centered at 7 km depth connecting the shallow source to the deeper caldera. The deeper source is located directly beneath the volcano tectonic seismicity with the lower bounds of the seismicity parallel to the plunge of the deep source. InSAR time series also show normal fault offsets on the NE flank Copahue faults. Coulomb stress change calculations for right-lateral strike slip (RLSS), thrust, and normal receiver faults show positive values in the north caldera for both RLSS and normal faults, suggesting that northward trending seismicity and Copahue fault motion within the caldera are caused by the modeled sources. Together, the InSAR-constrained source model and the seismicity suggest a deep conduit or transfer zone where magma moves from the central caldera to Copahue's upper edifice.

Southeastern extension of the Lake Basin fault zone in south- central Montana: implications for coal and hydrocarbon exploration ( USA).

USGS Publications Warehouse

Robinson, L.N.; Barnum, B.E.

1986-01-01

The Lake Basin fault zone consists mainly of en echelon NE-striking normal faults that have been interpreted to be surface expressions of left-lateral movement along a basement wrench fault. Information gathered from recent field mapping of coal beds and from shallow, closely-spaced drill holes resulted in detailed coal bed correlations, which revealed another linear zone of en echelon faulting directly on the extended trend of the Lake Basin fault zone. This faulted area, referred to as the Sarpy Creek area, is located 48 km E of Hardin, Montana. It is about 16 km long, 13 km wide, and contains 21 en echelon normal faults that have an average strike of N 63oE. We therefore extend the Lake Basin fault zone 32 km farther SE than previously mapped to include the Sarpy Creek area. The Ash Creek oil field, Wyoming, 97 km due S of the Sarpy Creek area, produces from faulted anticlinal structues that have been interpreted to be genetically related to the primary wrench-fault system known as the Nye-Bowler fault zone. The structural similarities between the Sarpy Creek area and the Ash Creek area indicate that the Sarpy Creek area is a possible site for hydrocarbon accumulation.-from Authors

Near-surface, marine seismic-reflection data defines potential hydrogeologic confinement bypass in a tertiary carbonate aquifer, southeastern Florida

USGS Publications Warehouse

Cunningham, Kevin J.; Walker, Cameron; Westcott, Richard L.

2012-01-01

Approximately 210 km of near-surface, high-frequency, marine seismic-reflection data were acquired on the southeastern part of the Florida Platform between 2007 and 2011. Many high-resolution, seismic-reflection profiles, interpretable to a depth of about 730 m, were collected on the shallow-marine shelf of southeastern Florida in water as shallow as 1 m. Landward of the present-day shelf-margin slope, these data image middle Eocene to Pleistocene strata and Paleocene to Pleistocene strata on the Miami Terrace. This high-resolution data set provides an opportunity to evaluate geologic structures that cut across confining units of the Paleocene to Oligocene-age carbonate rocks that form the Floridan aquifer system.Seismic profiles image two structural systems, tectonic faults and karst collapse structures, which breach confining beds in the Floridan aquifer system. Both structural systems may serve as pathways for vertical groundwater flow across relatively low-permeability carbonate strata that separate zones of regionally extensive high-permeability rocks in the Floridan aquifer system. The tectonic faults occur as normal and reverse faults, and collapse-related faults have normal throw. The most common fault occurrence delineated on the reflection profiles is associated with karst collapse structures. These high-frequency seismic data are providing high quality structural analogs to unprecedented depths on the southeastern Florida Platform. The analogs can be used for assessment of confinement of other carbonate aquifers and the sealing potential of deeper carbonate rocks associated with reservoirs around the world.

Fault creep rates of the Chaman fault (Afghanistan and Pakistan) inferred from InSAR

NASA Astrophysics Data System (ADS)

Barnhart, William D.

2017-01-01

The Chaman fault is the major strike-slip structural boundary between the India and Eurasia plates. Despite sinistral slip rates similar to the North America-Pacific plate boundary, no major (>M7) earthquakes have been documented along the Chaman fault, indicating that the fault either creeps aseismically or is at a late stage in its seismic cycle. Recent work with remotely sensed interferometric synthetic aperture radar (InSAR) time series documented a heterogeneous distribution of fault creep and interseismic coupling along the entire length of the Chaman fault, including an 125 km long creeping segment and an 95 km long locked segment within the region documented in this study. Here I present additional InSAR time series results from the Envisat and ALOS radar missions spanning the southern and central Chaman fault in an effort to constrain the locking depth, dip, and slip direction of the Chaman fault. I find that the fault deviates little from a vertical geometry and accommodates little to no fault-normal displacements. Peak-documented creep rates on the fault are 9-12 mm/yr, accounting for 25-33% of the total motion between India and Eurasia, and locking depths in creeping segments are commonly shallower than 500 m. The magnitude of the 1892 Chaman earthquake is well predicted by the total area of the 95 km long coupled segment. To a first order, the heterogeneous distribution of aseismic creep combined with consistently shallow locking depths suggests that the southern and central Chaman fault may only produce small to moderate earthquakes (

The January 2014 Northern Cuba Earthquake Sequence - Unusual Location and Unexpected Source Mechanism Variability

NASA Astrophysics Data System (ADS)

Braunmiller, J.; Thompson, G.; McNutt, S. R.

2017-12-01

On 9 January 2014, a magnitude Mw=5.1 earthquake occurred along the Bahamas-Cuba suture at the northern coast of Cuba revealing a surprising seismic hazard source for both Cuba and southern Florida where it was widely felt. Due to its location, the event and its aftershocks (M>3.5) were recorded only at far distances (300+ km) resulting in high-detection thresholds, low location accuracy, and limited source parameter resolution. We use three-component regional seismic data to study the sequence. High-pass filtered seismograms at the closest site in southern Florida are similar in character suggesting a relatively tight event cluster and revealing additional, smaller aftershocks not included in the ANSS or ISC catalogs. Aligning on the P arrival and low-pass filtering (T>10 s) uncovers a surprise polarity flip of the large amplitude surface waves on vertical seismograms for some aftershocks relative to the main shock. We performed regional moment tensor inversions of the main shock and its largest aftershocks using complete three-component seismograms from stations distributed throughout the region to confirm the mechanism changes. Consistent with the GCMT solution, we find an E-W trending normal faulting mechanism for the main event and for one immediate aftershock. Two aftershocks indicate E-W trending reverse faulting with essentially flipped P- and T-axes relative to the normal faulting events (and the same B-axes). Within uncertainties, depths of the two event families are indistinguishable and indicate shallow faulting (<10 km). One intriguing possible interpretation is that both families ruptured the same fault with reverse mechanisms compensating for overshooting. However, activity could also be spatially separated either vertically (with reverse mechanisms possibly below extension) or laterally. The shallow source depth and the 200-km long uplifted chain of islands indicate that larger, shallow and thus potentially tsunamigenic earthquakes could occur just offshore of northern Cuba posing a potential hazard to Florida and the Bahamas.

STRUCTURAL CONTROLS OF THE EMERSON PASS GEOTHERMAL SYSTEM, NORTHWESTERN NEVADA: CHARACTERIZATION OF A "BLIND" SYSTEM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Anderson, Ryan B; Faulds, James E

The Pyramid Lake area is favorable for geothermal development due to the tectonic setting of the region. The Walker Lane belt, a dextral shear zone that accommodates ~20% relative motion between the Pacific and North American plates, terminates northwestward in northeast California. NW-directed dextral shear is transferred to WNW extension accommodated by N-to -NNE striking normal faults of the Basin and Range. As a consequence, enhanced dilation occurs on favorably oriented faults generating high geothermal potential in the northwestern Great Basin. The NW-striking right-lateral Pyramid Lake fault, a major structure of the northern Walker Lane, terminates at the southern endmore » of Pyramid Lake and transfers strain to the NNE-striking down to the west Lake Range fault, resulting in high geothermal potential. Known geothermal systems in the area have not been developed due to cultural considerations of the Pyramid Lake Paiute Tribe. Therefore, exploration has been focused on discovering blind geothermal systems elsewhere on the reservation by identifying structurally favorable settings and indicators of past geothermal activity. One promising area is the northeast end of Pyramid Lake, where a broad left step between the west-dipping range-bounding faults of the Lake and Fox Ranges has led to the formation of a broad, faulted relay ramp. Furthermore, tufa mounds, mineralized veins, and altered Miocene rocks occur proximal to a thermal anomaly discovered by a 2-m shallow temperature survey at the north end of the step-over in Emerson Pass. Detailed geologic mapping has revealed a system of mainly NNE-striking down to the west normal faults. However, there are three notable exceptions to this generality, including 1) a prominent NW-striking apparent right-lateral fault, 2) a NW-striking down to the south fault which juxtaposes the base of the mid-Miocene Pyramid sequence against younger late Tertiary sedimentary rocks, and 3) a NNE-striking down to the east normal fault, which accommodates motion such that the Mesozoic Nightingale sequence is juxtaposed with late Tertiary sedimentary rocks. The NW dextral fault, the NNE-down to east fault, and several NNE-down to the west faults intersect roughly at the thermal anomaly in Emerson Pass. This suggests that fault intersections locally control upwelling of geothermal fluids within the step-over. Based on this assumption, it is proposed that the area near Buckbrush Springs be investigated further for geothermal potential. At this location, a NNE-down to the west normal fault, with >1 km of offset, intersects a NW-striking down to the south fault at a small left step in the NNE fault. Further studies will include collection of available kinematic indicators near the shallow thermal anomaly in Emerson Pass, geothermometry on Buckbrush Spring, and possibly drilling of temperature gradient wells in Emerson Pass and at Buckbrush Spring.« less

Continuous forearc extension following the 2010 Maule megathrust earthquake: InSAR and seismic observations and modelling

NASA Astrophysics Data System (ADS)

Bie, L.; Rietbrock, A.; Agurto-Detzel, H.

2017-12-01

The forearc region in subduction zones deforms in response to relative movement on the plate interface throughout the earthquake cycle. Megathrust earthquakes may alter the stress field in the forearc areas from compression to extension, resulting in normal faulting earthquakes. Recent cases include the 2011 Iwaki sequence following the Tohoku-Oki earthquake in Japan, and 2010 Pichilemu sequence after the Maule earthquake in central Chile. Given the closeness of these normal fault events to residential areas, and their shallow depth, they may pose equivalent, if not higher, seismic risk in comparison to earthquakes on the megathrust. Here, we focus on the 2010 Pichilemu sequence following the Mw 8.8 Maule earthquake in central Chile, where the Nazca Plate subducts beneath the South American Plate. Previous studies have clearly delineated the Pichilemu normal fault structure. However, it is not clear whether the Pichilemu events fully released the extensional stress exerted by the Maule mainshock, or the forearc area is still controlled by extensional stress. A 3 months displacement time-series, constructed by radar satellite images, clearly shows continuous aseismic deformation along the Pichilemu fault. Kinematic inversion reveals peak afterslip of 25 cm at shallow depth, equivalent to a Mw 5.4 earthquake. We identified a Mw 5.3 earthquake 2 months after the Pichilemu sequence from both geodetic and seismic observations. Nonlinear inversion from geodetic data suggests that this event ruptured a normal fault conjugate to the Pichilemu fault, at a depth of 4.5 km, consistent with the result obtained from independent moment tensor inversion. We relocated aftershocks in the Pichilemu area using relative arrivals time and a 3D velocity model. The spatial correlation between geodetic deformation and aftershocks reveals three additional areas which may have experienced aseismic slip at depth. Both geodetic displacement and aftershock distribution show a conjugated L-shape feature. This pattern coincides with weak zones depicted by high vp/vs and low vs in the upper crust of this region, suggesting fluid control of seismic and aseismic activities in the Pichilemu area.

Magnetic and clast fabrics as measurements of grain-scale processes within the Death Valley shallow crustal detachment faults

NASA Astrophysics Data System (ADS)

Hayman, Nicholas W.; Housen, B. A.; Cladouhos, T. T.; Livi, K.

2004-05-01

The rock product of shallow-crustal faulting includes fine-grained breccia and clay-rich gouge. Many gouges and breccias have a fabric produced by distributed deformation. The orientation of fabric elements provides constraints on the kinematics of fault slip and is the structural record of intrafault strain not accommodated by planar and penetrative surfaces. However, it can be difficult to quantify the deformational fabric of fault rocks, especially the preferred orientations of fine-grained minerals, or to uniquely determine the relationship between fabric geometry and finite strain. Here, we present the results of a fabric study of gouge and breccia sampled from low-angle normal (detachment) faults in the Black Mountains, Death Valley, CA. We measured a preferred orientation of the long axes of the clasts inherited from the crystalline footwall of the fault and compared the shape preferred orientation to the anisotropy of magnetic susceptibility of the fault rocks. The two measurements of fabric exhibit systematic similarities and differences in orientation and anisotropy that are compatible with the large-scale kinematics of fault slip. The dominant carriers of the magnetic susceptibility are micron- and sub-micron scale iron oxides and clay minerals. Therefore even the finest grains in the fault rock were sensitive to the distributed deformation and the micro-mechanics of particle interaction must have departed from those assumed by the passive-marker kinematic model that best explains the fabric.

Fault kinematics and active tectonics of the Sabah margin: Insights from the 2015, Mw 6.0, Mt. Kinabalu earthquake

NASA Astrophysics Data System (ADS)

Wang, Y.; Wei, S.; Tapponnier, P.; WANG, X.; Lindsey, E.; Sieh, K.

2016-12-01

A gravity-driven "Mega-Landslide" model has been evoked to explain the shortening seen offshore Sabah and Brunei in oil-company seismic data. Although this model is considered to account simultaneously for recent folding at the edge of the submarine NW Sabah trough and normal faulting on the Sabah shelf, such a gravity-driven model is not consistent with geodetic data or critical examination of extant structural restorations. The rupture that produced the 2015 Mw6.0 Mt. Kinabalu earthquake is also inconsistent with the gravity-driven model. Our teleseismic analysis shows that the centroid depth of that earthquake's mainshock was 13 to 14 km, and its favored fault-plane solution is a 60° NW-dipping normal fault. Our finite-rupture model exhibits major fault slip between 5 and 15 km depth, in keeping with our InSAR analysis, which shows no appreciable surface deformation. Both the hypocentral depth and the depth of principal slip are far too deep to be explained by gravity-driven failure, as such a model would predict a listric normal fault connecting at a much shallower depth with a very gentle detachment. Our regional mapping of tectonic landforms also suggests the recent rupture is part of a 200-km long system of narrowly distributed active extension in northern Sabah. Taken together, the nature of the 2015 rupture, the belt of active normal faults, and structural consideration indicate that active tectonic shortening plays the leading role in controlling the overall deformation of northern Sabah and that deep-seated, onland normal faulting likely results from an abrupt change in the dip-angle of the collision interface beneath the Sabah accretionary prism.

Initial results from the Nankai Trough shallow splay and frontal thrust (IODP Expedition 316): Implications for fluid flow

NASA Astrophysics Data System (ADS)

Screaton, E.; Kimura, G.; Curewitz, D.; Scientists, E.

2008-12-01

Integrated Ocean Drilling Program (IODP) Expedition 316 examined the frontal thrust and the shallow portion of the megasplay fault offshore of the Kii peninsula, and was the third drilling expedition of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE). NanTroSEIZE will integrate seafloor observations, drilling, and observatories to investigate the processes controlling slip along subduction zone plate boundary fault systems. Site C0004 examined a shallow portion of the splay fault system where it overrides slope basin sediments. Site C0008, located in the slope basin 1 km seaward of Site C0004, provided a reference site for the footwall sediments. Results of drilling indicate that the footwall sediments have dewatered significantly, suggesting permeable routes for fluid escape. These high-permeability pathways might be provided by coarse-grained layers within the slope sediments. In situ dewatering and multiple fluid escape paths will tend to obscure any geochemical signature of flow from depth. Sites C0006 and C0007 examined the frontal thrust system. Although poorly recovered, coarse-grained trench sediments were sampled within the footwall. These permeable sediments would be expected to allow rapid escape of any fluid pressures due to loading. At both sites, low porosities are observed at shallow depths, suggesting removal of overlying material. This observation is consistent with interpretations that the prism is unstable and currently in a period of collapse. Anomalously low temperatures were measured within boreholes at these sites. One possible explanation for the low temperatures is circulation of seawater along normal faults in the unstable prism.

Focal Mechanisms From Moment Tensor Solutions and First Motion Polarities of Shallow to Deep Local Earthquakes in Eastern Nepal and Southern Tibet

NASA Astrophysics Data System (ADS)

de La Torre, T. L.; Sheehan, A. F.; Monsalve, G.; Wu, F.

2004-12-01

We determined focal mechanisms using waveforms and first motion polarities from local earthquakes recorded during the Himalayan Nepal Tibet Seismic Experiment (HIMNT). The HIMNT experiment included the deployment of 28 broad band seismometers in eastern Nepal and southern Tibet from September 2001 to April 2003. Using a regional moment tensor method (Ammon and Randall, 2001) and first motion polarities for displaying double-couple focal mechanisms (Snokes, 2003), we analyzed the fault plane solutions at three distinct zones of seismicity. Characteristic focal mechanisms in seismically concentrated areas may indicate the presence of fault ramps or a decollement in the Himalayan collision zone. Previous studies of focal mechanisms on the Tibetan Plateau predominantly indicate east-west extension and shallow thrusting at the Himalayan collision zone for shallow to intermediate earthquakes (Ni and Barazangi, 1984; Molnar and Lyon-Caen, 1989; Randall et al., 1995) and east-west extension for intermediate to deep earthquakes (Zhu and Helmberger, 1996; Chen and Yang, 2004). The first zone in southeast Nepal between the Main Boundary and Main Frontal faults consist of earthquakes < Mw 4.0 at depths 40 - 60 km near the epicenter of the 1988 Udaypur earthquake, Mb 6.1, depth 57 km. The second zone north of the Main Central Thrust outcrop in eastern Nepal consists of 14 earthquakes 3.0 - 5.0 Mw at depths < 30 km that indicate north-south strike normal faulting and east-west strike thrust faulting. The third zone is an arc parallel to the Himalayas in southern Tibet and a cluster in northeast Nepal. This zone consists of 45 earthquakes < 4.0 Mw at depths > 50 km. Four earthquakes indicate northwest-southeast compression resulting in northeast strike strike-slip faulting while one earthquake in the northeast cluster indicates east-west compression at a source depth below the crust-mantle boundary. Focal mechanisms from full waveform moment tensor inversions are cross checked with first motion solutions for selected events. Source depths as determined from normalized error of the sum of the squared differences between the data and synthetic seismogram coincide with the source depths determined from the travel time residual inversion.

The 2011 Hawthorne, Nevada, Earthquake Sequence; Shallow Normal Faulting

NASA Astrophysics Data System (ADS)

Smith, K. D.; Johnson, C.; Davies, J. A.; Agbaje, T.; Knezevic Antonijevic, S.; Kent, G.

2011-12-01

An energetic sequence of shallow earthquakes that began in early March 2011 in western Nevada, near the community of Hawthorne, has slowly decreased in intensity through mid-2011. To date about 1300 reviewed earthquake locations have been compiled; we have computed moment tensors for the larger earthquakes and have developed a set of high-precision locations for all reviewed events. The sequence to date has included over 50 earthquakes ML 3 and larger with the largest at Mw 4.6. Three 6-channel portable stations configured with broadband sensors and accelerometers were installed by April 20. Data from the portable instruments is telemetered through NSL's microwave backbone to Reno where it is integrated with regional network data for real-time notifications, ShakeMaps, and routine event analysis. The data is provided in real-time to NEIC, CISN and the IRIS DMC. The sequence is located in a remote area about 15-20 km southwest of Hawthorne in the footwall block of the Wassuk Range fault system. An initial concern was that the sequence might be associated with volcanic processes due to the proximity of late Quaternary volcanic flows; there have been no volcanic signatures observed in near source seismograms. An additional concern, as the sequence has proceeded, was a clear progression eastward toward the Wassuk Range front fault. The east dipping range bounding fault is capable of M 7+ events, and poses a significant hazard to the community of Hawthorne and local military facilities. The Hawthorne Army Depot is an ordinance storage facility and the nation's storage site for surplus mercury. The sequence is within what has been termed the 'Mina Deflection' of the Central Walker Lane Belt. Faulting along the Whiskey Flat section of the Wassuk front fault would be primarily down-to-the-east, with an E-W extension direction; moment tensors for the 2011 earthquake show a range of extension directions from E-W to NW-SE, suggesting a possible dextral component to the Wassuk Range front fault at this latitude. At least two faults have been imaged within the sequence; these structures are at shallow depth (3-6 km), strike NE, and dip ~NW. Prior to temporary station installation event depths were poorly constrained, with the nearest network station 25 km from the source area. Early sequence moment tensor solutions show depths are on the order of 2-6 km and locations using the near source stations also confirm the shallow depths of the Hawthorne sequence. S-P times of 0.5 sec and less have been observed on a near-source station, illustrating extremely shallow source depths for some events. Along with the 2011 Hawthorne activity, very shallow depths in Nevada have been observed from near source stations in the 2008 west Reno earthquake sequence (primarily strike-slip faulting; main shock Mw 5.0) and the 1993 Rock Valley sequence in southern NNSS (strike-slip faulting; main shock Mw 4.0). These shallow sequences tend to include high rates of low magnitude earthquakes continuing over several months duration.

Integrated geophysical and hydrothermal models of flank degassing and fluid flow at Masaya Volcano, Nicaragua

USGS Publications Warehouse

Sanford, Ward E.; Pearson, S.C.P.; Kiyosugi, K.; Lehto, H.L.; Saballos, J.A.; Connor, C.B.

2012-01-01

We investigate geologic controls on circulation in the shallow hydrothermal system of Masaya volcano, Nicaragua, and their relationship to surface diffuse degassing. On a local scale (~250 m), relatively impermeable normal faults dipping at ~60° control the flowpath of water vapor and other gases in the vadose zone. These shallow normal faults are identified by modeling of a NE-SW trending magnetic anomaly of up to 2300 nT that corresponds to a topographic offset. Elevated SP and CO2 to the NW of the faults and an absence of CO2 to the SE suggest that these faults are barriers to flow. TOUGH2 numerical models of fluid circulation show enhanced flow through the footwalls of the faults, and corresponding increased mass flow and temperature at the surface (diffuse degassing zones). On a larger scale, TOUGH2 modeling suggests that groundwater convection may be occurring in a 3-4 km radial fracture zone transecting the entire flank of the volcano. Hot water rising uniformly into the base of the model at 1 x 10-5 kg/m2s results in convection that focuses heat and fluid and can explain the three distinct diffuse degassing zones distributed along the fracture. Our data and models suggest that the unusually active surface degassing zones at Masaya volcano can result purely from uniform heat and fluid flux at depth that is complicated by groundwater convection and permeability variations in the upper few km. Therefore isolating the effects of subsurface geology is vital when trying to interpret diffuse degassing in light of volcanic activity.

Preliminary atlas of active shallow tectonic deformation in the Puget Lowland, Washington

USGS Publications Warehouse

Barnett, Elizabeth A.; Haugerud, Ralph A.; Sherrod, Brian L.; Weaver, Craig S.; Pratt, Thomas L.; Blakely, Richard J.

2010-01-01

This atlas presents an up-to-date map compilation of the geological and geophysical observations that underpin interpretations of active, surface-deforming faults in the Puget Lowland, Washington. Shallow lowland faults are mapped where observations of deformation from paleoseismic, seismic-reflection, and potential-field investigations converge. Together, results from these studies strengthen the identification and characterization of regional faults and show that as many as a dozen shallow faults have been active during the Holocene. The suite of maps presented in our atlas identifies sites that have evidence of deformation attributed to these shallow faults. For example, the paleoseismic-investigations map shows where coseismic surface rupture and deformation produced geomorphic scarps and deformed shorelines. Other maps compile results of seismic-reflection and potential-field studies that demonstrate evidence of deformation along suspected fault structures in the subsurface. Summary maps show the fault traces derived from, and draped over, the datasets presented in the preceding maps. Overall, the atlas provides map users with a visual overview of the observations and interpretations that support the existence of active, shallow faults beneath the densely populated Puget Lowland.

Shallow fluid pressure transients caused by seismogenic normal faults

NASA Astrophysics Data System (ADS)

Fleischmann, Karl Henry

1993-10-01

Clastic dikes, induced by paleo-seismic slip along the Jonesboro Fault, can be used to estimate the magnitude of shallow fluid pressure transients. Fractures show evidence of two phases of seismically induced dilation by escaping fluids. Initial dilation and propagation through brittle rocks was caused by expulsion of trapped reducing fluids from beneath a clay cap. Second phase fluids were thixotropic clays which flowed vertically from clay beds upwards into the main fracture. Using the differential dilation and fracture trace lengths, the fluid pressure pulse is estimated to have ranged from 0.312-0.49 MPa, which is approximately equal to the vertical load during deformation. Field observations in adjacent rocks record evidence of large-magnitude seismic events, which are consistent with the large nature of the fluid pressure fluctuation.

The Padul normal fault activity constrained by GPS data: Brittle extension orthogonal to folding in the central Betic Cordillera

NASA Astrophysics Data System (ADS)

Gil, Antonio J.; Galindo-Zaldívar, Jesús; Sanz de Galdeano, Carlos; Borque, Maria Jesús; Sánchez-Alzola, Alberto; Martinez-Martos, Manuel; Alfaro, Pedro

2017-08-01

The Padul Fault is located in the Central Betic Cordillera, formed in the framework of the NW-SE Eurasian-African plate convergence. In the Internal Zone, large E-W to NE-SW folds of western Sierra Nevada accommodated the greatest NW-SE shortening and uplift of the cordillera. However, GPS networks reveal a present-day dominant E-W to NE-SW extensional setting at surface. The Padul Fault is the most relevant and best exposed active normal fault that accommodates most of the NE-SW extension of the Central Betics. This WSW-wards dipping fault, formed by several segments of up to 7 km maximum length, favored the uplift of the Sierra Nevada footwall away from the Padul graben hanging wall. A non-permanent GPS network installed in 1999 constrains an average horizontal extensional rate of 0.5 mm/yr in N66°E direction. The fault length suggests that a (maximum) 6 magnitude earthquake may be expected, but the absence of instrumental or historical seismic events would indicate that fault activity occurs at least partially by creep. Striae on fault surfaces evidence normal-sinistral kinematics, suggesting that the Padul Fault may have been a main transfer fault of the westernmost end of the Sierra Nevada antiform. Nevertheless, GPS results evidence: (1) shortening in the Sierra Nevada antiform is in its latest stages, and (2) the present-day fault shows normal with minor oblique dextral displacements. The recent change in Padul fault kinematics will be related to the present-day dominance of the ENE-WSW regional extension versus NNW-SSE shortening that produced the uplift and northwestwards displacement of Sierra Nevada antiform. This region illustrates the importance of heterogeneous brittle extensional tectonics in the latest uplift stages of compressional orogens, as well as the interaction of folding during the development of faults at shallow crustal levels.

3D resistivity survey for shallow subsurface fault investigations

NASA Astrophysics Data System (ADS)

Petrit, Kraipat; Klamthim, Poonnapa; Duerrast, Helmut

2018-03-01

The shallow subsurface is subject to various human activities, and the place of occurrence of geohazards, e.g. shallow active faults. The identification of the location and orientation of such faults can be vital for infrastructure development. The aim of this study was to develop a low-cost 3D resistivity survey system, with reasonable survey time for shallow fault investigations. The study area in Songkhla Province, Thailand is located in an old quarry where faults could be identified in outcrops. The study area was designed to cover the expected fault with 100 electrodes arranged in a 10×10 square grid with an electrode spacing of 3 meters in x and y axis. Each electrode in turn was used as a current and potential electrode using a dipole-dipole array. Field data have been processed and interpreted using 3DResINV. Results, presented in horizontal depth slices and vertical xz- and yz-cross sections, revealed through differences in resistivity down to 8 m depths a complex structural setting with two shallow faults and dipping sedimentary rock layers. In conclusion, this study has shown that a 3D resistivity survey can imagine complex tectonic structures, thus providing a far more insight into the shallow subsurface.

Rupture Dynamics along Thrust Dipping Fault: Inertia Effects due to Free Surface Wave Interactions

NASA Astrophysics Data System (ADS)

Vilotte, J. P.; Scala, A.; Festa, G.

2017-12-01

We numerically investigate the dynamic interaction between free surface and up-dip, in-plane rupture propagation along thrust faults, under linear slip-weakening friction. With reference to shallow along-dip rupture propagation during large subduction earthquakes, we consider here low dip-angle fault configurations with fixed strength excess and depth-increasing initial stress. In this configuration, the rupture undergoes a break of symmetry with slip-induced normal stress perturbations triggered by the interaction with reflected waves from the free surface. We found that both body-waves - behind the crack front - and surface waves - at the crack front - can trigger inertial effects. When waves interact with the rupture before this latter reaches its asymptotic speed, the rupture can accelerate toward the asymptotic speed faster than in the unbounded symmetric case, as a result of these inertial effects. Moreover, wave interaction at the crack front also affects the slip rate generating large ground motion on the hanging wall. Imposing the same initial normal stress, frictional strength and stress drop while varying the static friction coefficient we found that the break of symmetry makes the rupture dynamics dependent on the absolute value of friction. The higher the friction the stronger the inertial effect both in terms of rupture acceleration and slip amount. When the contact condition allows the fault interface to open close to the free surface, the length of the opening zone is shown to depend on the propagation length, the initial normal stress and the static friction coefficient. These new results are shown to agree with analytical results of rupture propagation in bounded media, and open new perspectives for understanding the shallow rupture of large subduction earthquakes and tsunami sources.

Late Oligocene to present contractional structure in and around the Susitna basin, Alaska—Geophysical evidence and geological implications

USGS Publications Warehouse

Saltus, Richard W.; Stanley, Richard G.; Haeussler, Peter J.; Jones, James V.; Potter, Christopher J.; Lewis, Kristen A.

2016-01-01

The Cenozoic Susitna basin lies within an enigmatic lowland surrounded by the Central Alaska Range, Western Alaska Range (including the Tordrillo Mountains), and Talkeetna Mountains in south-central Alaska. Some previous interpretations show normal faults as the defining structures of the basin (e.g., Kirschner, 1994). However, analysis of new and existing geophysical data shows predominantly (Late Oligocene to present) thrust and reverse fault geometries in the region, as previously proposed by Hackett (1978). A key example is the Beluga Mountain fault where a 50-mGal gravity gradient, caused by the density transition from the igneous bedrock of Beluga Mountain to the >4-km-thick Cenozoic sedimentary section of Susitna basin, spans a horizontal distance of ∼40 km and straddles the topographic front. The location and shape of the gravity gradient preclude a normal fault geometry; instead, it is best explained by a southwest-dipping thrust fault, with its leading edge located several kilometers to the northeast of the mountain front, concealed beneath the shallow glacial and fluvial cover deposits. Similar contractional fault relationships are observed for other basin-bounding and regional faults as well. Contractional structures are consistent with a regional shortening strain field inferred from differential offsets on the Denali and Castle Mountain right-lateral strike-slip fault systems.

Coastal land loss and gain as potential earthquake trigger mechanism in SCRs

NASA Astrophysics Data System (ADS)

Klose, C. D.

2007-12-01

In stable continental regions (SCRs), historic data show earthquakes can be triggered by natural tectonic sources in the interior of the crust and also by sources stemming from the Earth's sub/surface. Building off of this framework, the following abstract will discuss both as potential sources that might have triggered the 2007 ML4.2 Folkestone earthquake in Kent, England. Folkestone, located along the Southeast coast of Kent in England, is a mature aseismic region. However, a shallow earthquake with a local magnitude of ML = 4.2 occurred on April 28 2007 at 07:18 UTC about 1 km East of Folkestone (51.008° N, 1.206° E) between Dover and New Romney. The epicentral error is about ±5 km. While coastal land loss has major effects towards the Southwest and the Northeast of Folkestone, research observations suggest that erosion and landsliding do not exist in the immediate Folkestone city area (<1km). Furthermore, erosion removes rock material from the surface. This mass reduction decreases the gravitational stress component and would bring a fault away from failure, given a tectonic normal and strike-slip fault regime. In contrast, land gain by geoengineering (e.g., shingle accumulation) in the harbor of Folkestone dates back to 1806. The accumulated mass of sand and gravel accounted for a 2.8·109 kg (2.8 Mt) in 2007. This concentrated mass change less than 1 km away from the epicenter of the mainshock was able to change the tectonic stress in the strike-slip/normal stress regime. Since 1806, shear and normal stresses increased at most on oblique faults dipping 60±10°. The stresses reached values ranging between 1.0 KPa and 30.0 KPa in up to 2 km depth, which are critical for triggering earthquakes. Furthermore, the ratio between holding and driving forces continuously decreased for 200 years. In conclusion, coastal engineering at the surface most likely dominates as potential trigger mechanism for the 2007 ML4.2 Folkestone earthquake. It can be anticipated that the mainshock nucleated at shallower depth (<500 m) near the Paleozoic surface a) where differential stresses are generally maximum and b) because earthquakes in aseismic regions are generally overestimated by 88% due to sparse instrumental coverage. The latter was suggested by recent research on shallow seismicitiy (<10 km) in SCRs in northeastern USA and eastern Canada. Data of the focal mechanism provided by the British Geological Survey (BGS) confirm fault zone orientations of 326°/74° (strike-slip fault component) and 71°/48° (normal fault component).

Magma-tectonic Interaction at Laguna del Maule, Chile

NASA Astrophysics Data System (ADS)

Keranen, K. M.; Peterson, D. E.; Miller, C. A.; Garibaldi, N.; Tikoff, B.; Williams-Jones, G.

2016-12-01

The Laguna del Maule Volcanic Field (LdM), Chile, the largest concentration of rhyolite <20 kyr globally, exhibits crustal deformation at rates higher than any non-erupting volcano. The interaction of large magmatic systems with faulting is poorly understood, however, the Chaitén rhyolitic system demonstrated that faults can serve as magma pathways during an eruption. We present a complex fault system at LdM in close proximity to the magma reservoir. In March 2016, 18 CHIRP seismic reflection lines were acquired at LdM to identify faults and analyze potential spatial and temporal impacts of the fault system on volcanic activity. We mapped three key horizons on each line, bounding sediment packages between Holocene onset, 870 ybp, and the present date. Faults were mapped on each line and offset was calculated across key horizons. Our results indicate a system of normal-component faults in the northern lake sector, striking subparallel to the mapped Troncoso Fault SW of the lake. These faults correlate to prominent magnetic lineations mapped by boat magnetic data acquired February 2016 which are interpreted as dykes intruding along faults. We also imaged a vertical fault, interpreted as a strike-slip fault, and a series of normal faults in the SW lake sector near the center of magmatic inflation. Isochron and fault offset maps illuminate areas of growth strata and indicate migration and increase of fault activity from south to north through time. We identify a domal structure in the SW lake sector, coincident with an area of low magnetization, in the region of maximum deformation from InSAR results. The dome experienced 10 ms TWT ( 10 meters) of uplift throughout the past 16 kybp, which we interpret as magmatic inflation in a shallow magma reservoir. This inflation is isolated to a 1.5 km diameter region in the hanging wall of the primary normal fault system, indicating possible fault-facilitated inflation.

The Kumamoto Mw7.1 mainshock: deep initiation triggered by the shallow foreshocks

NASA Astrophysics Data System (ADS)

Shi, Q.; Wei, S.

2017-12-01

The Kumamoto Mw7.1 earthquake and its Mw6.2 foreshock struck the central Kyushu region in mid-April, 2016. The surface ruptures are characterized with multiple fault segments and a mix of strike-slip and normal motion extended from the intersection area of Hinagu and Futagawa faults to the southwest of Mt. Aso. Despite complex surface ruptures, most of the finite fault inversions use two fault segments to approximate the fault geometry. To study the rupture process and the complex fault geometry of this earthquake, we performed a multiple point source inversion for the mainshock using the data on 93 K-net and Kik-net stations. With path calibration from the Mw6.0 foreshock, we selected the frequency ranges for the Pnl waves (0.02 0.26 Hz) and surface waves (0.02 0.12 Hz), as well as the components that can be well modeled with the 1D velocity model. Our four-point-source results reveal a unilateral rupture towards Mt. Aso and varying fault geometries. The first sub-event is a high angle ( 79°) right-lateral strike-slip event at the depth of 16 km on the north end of the Hinagu fault. Notably the two M>6 foreshocks is located by our previous studies near the north end of the Hinagu fault at the depth of 5 9 km, which may give rise to the stress concentration at depth. The following three sub-events are distributed along the surface rupture of the Futagawa fault, with focal depths within 4 10 km. Their focal mechanisms present similar right-lateral fault slips with relatively small dip angles (62 67°) and apparent normal-fault component. Thus, the mainshock rupture initiated from the relatively deep part of the Hinagu fault and propagated through the fault-bend toward NE along the relatively shallow part of the Futagawa fault until it was terminated near Mt. Aso. Based on the four-point-source solution, we conducted a finite-fault inversion and obtained a kinematic rupture model of the mainshock. We then performed the Coulomb Stress analyses on the two foreshocks and the mainshock. The results support that the stress alternation after the foreshocks may have triggered the failure on the fault plane of the Mw7.1 earthquake. Therefore, the 2016 Kumamoto earthquake sequence is dominated by a series of large triggering events whose initiation is associated with the geometric barrier in the intersection of the Futagawa and Hinagu faults.

The Corinth Rift Laboratory, Greece (CRL) : A Multidisciplinary Near Fault Observatory (NFO) on a Fast Rifting System

NASA Astrophysics Data System (ADS)

Bernard, P.; Lyon-Caen, H.; Deschamps, A.; Briole, P.; Lambotte, S.; Ford, M.; Scotti, O.; Beck, C.; Hubert-Ferrari, A.; Boiselet, A.; Godano, M.; Matrullo, E.; Meyer, N.; Albini, P.; Elias, P.; Nercessian, A.; Katsonopoulou, D.; Papadimitriou, P.; Voulgaris, N.; Kapetanidis, V.; Sokos, E.; Serpetsidaki, A.; el Arem, S.; Dublanchet, P.; Duverger, C.; Makropoulos, K.; Tselentis, A.

2014-12-01

The western rift of Corinth (Greece) is one of the most active tectonic structures of the euro-mediterranean area. Its NS opening rate is 1.5 cm/yr ( strain rate of 10-6/yr) results into a high microseismicity level and a few destructive, M>6 earthquakes per century, activating a system of mostly north dipping normal faults. Since 2001, monitoring arrays of the European Corinth Rift Laboratory (CRL, www.crlab.eu) allowed to better track the mechanical processes at work, with short period and broad band seismometers, cGPS, borehole strainmeters, EM stations, …). The recent (300 kyr) tectonic history has been revealed by onland (uplifted fan deltas and terraces) and offshore geological studies (mapping, shallow seismic, coring), showing a fast evolution of the normal fault system. The microseismicity, dominated by swarms lasting from days to months, mostly clusters in a layer 1 to 3 km thick, between 6 and 9 km in depth, dipping towards north, on which most faults are rooting. The diffusion of the microseismicity suggests its triggering by pore pressure transients, with no or barely detected strain. Despite a large proportion of multiplets, true repeaters seem seldom, suggesting a minor contribution of creep in their triggering, although transient or steady creep is clearly detected on the shallow part of some majors faults. The microseismic layer may thus be an immature, downward growing detachment, and the dominant rifting mechanism might be a mode I, anelastic strain beneath the rift axis , for which a mechanical model is under development. Paleoseismological (trenching, paleoshorelines, turbidites), archeological and historical studies completed the catalogues of instrumental seismicity, motivating attempts of time dependent hazard assessment. The Near Fault Observatory of CRL is thus a multidisciplinary research infrastructure aiming at a better understanding and modeling of multiscale, coupled seismic/aseismic processes on fault systems.

Influence of slip-surface geometry on earth-flow deformation, Montaguto earth flow, southern Italy

USGS Publications Warehouse

Guerriero, L.; Coe, Jeffrey A.; Revellio, P.; Grelle, G.; Pinto, F.; Guadagno, F.

2016-01-01

We investigated relations between slip-surface geometry and deformational structures and hydrologic features at the Montaguto earth flow in southern Italy between 1954 and 2010. We used 25 boreholes, 15 static cone-penetration tests, and 22 shallow-seismic profiles to define the geometry of basal- and lateral-slip surfaces; and 9 multitemporal maps to quantify the spatial and temporal distribution of normal faults, thrust faults, back-tilted surfaces, strike-slip faults, flank ridges, folds, ponds, and springs. We infer that the slip surface is a repeating series of steeply sloping surfaces (risers) and gently sloping surfaces (treads). Stretching of earth-flow material created normal faults at risers, and shortening of earth-flow material created thrust faults, back-tilted surfaces, and ponds at treads. Individual pairs of risers and treads formed quasi-discrete kinematic zones within the earth flow that operated in unison to transmit pulses of sediment along the length of the flow. The locations of strike-slip faults, flank ridges, and folds were not controlled by basal-slip surface topography but were instead dependent on earth-flow volume and lateral changes in the direction of the earth-flow travel path. The earth-flow travel path was strongly influenced by inactive earth-flow deposits and pre-earth-flow drainages whose positions were determined by tectonic structures. The implications of our results that may be applicable to other earth flows are that structures with strikes normal to the direction of earth-flow motion (e.g., normal faults and thrust faults) can be used as a guide to the geometry of basal-slip surfaces, but that depths to the slip surface (i.e., the thickness of an earth flow) will vary as sediment pulses are transmitted through a flow.

Shallow Seismic Reflection Study of Recently Active Fault Scarps, Mina Deflection, Western Nevada

NASA Astrophysics Data System (ADS)

Black, R. A.; Christie, M.; Tsoflias, G. P.; Stockli, D. F.

2006-12-01

During the spring and summer of 2006 University of Kansas geophysics students and faculty acquired shallow, high resolution seismic reflection data over actively deforming alluvial fans developing across the Emmigrant Peak (in Fish Lake Valley) and Queen Valley Faults in western Nevada. These normal faults represent a portion of the transition from the right-lateral deformation associated with the Walker Lane/Eastern California Shear Zone to the normal and left-lateral faulting of the Mina Deflection. Data were gathered over areas of recent high resolution geological mapping and limited trenching by KU students. An extensive GPR data grid was also acquired. The GPR results are reported in Christie, et al., 2006. The seismic data gathered in the spring included both walkaway tests and a short CMP test line. These data indicated that a very near-surface P-wave to S-wave conversion was taking place and that very high quality S-wave reflections were probably dominating shot records to over one second in time. CMP lines acquired during the summer utilized a 144 channel networked Geode system, single 28 hz geophones, and a 30.06 downhole rifle source. Receiver spacing was 0.5 m, source spacing 1.0m and CMP bin spacings were 0.25m for all lines. Surveying was performed using an RTK system which was also used to develop a concurrent high resolution DEM. A dip line of over 400m and a strike line over 100m in length were shot across the active fan scarp in Fish Lake Valley. Data processing is still underway. However, preliminary interpretation of common-offset gathers and brute stacks indicates very complex faulting and detailed stratigraphic information to depths of over 125m. Depth of information was actually limited by the 1024ms recording time. Several west-dipping normal faults downstep towards the basin. East-dipping antithetic normal faulting is extensive. Several distinctive stratigraphic packages are bound by the faults and apparent unconformitites. A CMP dip line was also run across a large active scarp in Queen Valley near Boundary Peak. Due to slope steepness and extensive boulder armoring shot and receiver locations had to be skipped within several meters of the actual scarp location. Initial structural and stratigraphic interpretations are similar to those in the Fish Lake Valley location. Overall the data prove that the actively deforming fans can be imaged in detail sufficient to perform structural and possibly seismic stratigraphic analysis within the upper one hundred meters of the fans, if not deeper.

Structural controls on fractured coal reservoirs in the southern Appalachian Black Warrior foreland basin

USGS Publications Warehouse

Groshong, R.H.; Pashin, J.C.; McIntyre, M.R.

2009-01-01

Coal is a nearly impermeable rock type for which the production of fluids requires the presence of open fractures. Basin-wide controls on the fractured coal reservoirs of the Black Warrior foreland basin are demonstrated by the variability of maximum production rates from coalbed methane wells. Reservoir behavior depends on distance from the thrust front. Far from the thrust front, normal faults are barriers to fluid migration and compartmentalize the reservoirs. Close to the thrust front, rates are enhanced along some normal faults, and a new trend is developed. The two trends have the geometry of conjugate strike-slip faults with the same ??1 direction as the Appalachian fold-thrust belt and are inferred to be the result of late pure-shear deformation of the foreland. Face cleat causes significant permeability anisotropy in some shallow coal seams but does not produce a map-scale production trend. ?? 2008 Elsevier Ltd. All rights reserved.

Kinematics of shallow backthrusts in the Seattle fault zone, Washington State

USGS Publications Warehouse

Pratt, Thomas L.; Troost, K.G.; Odum, Jackson K.; Stephenson, William J.

2015-01-01

Near-surface thrust fault splays and antithetic backthrusts at the tips of major thrust fault systems can distribute slip across multiple shallow fault strands, complicating earthquake hazard analyses based on studies of surface faulting. The shallow expression of the fault strands forming the Seattle fault zone of Washington State shows the structural relationships and interactions between such fault strands. Paleoseismic studies document an ∼7000 yr history of earthquakes on multiple faults within the Seattle fault zone, with some backthrusts inferred to rupture in small (M ∼5.5–6.0) earthquakes at times other than during earthquakes on the main thrust faults. We interpret seismic-reflection profiles to show three main thrust faults, one of which is a blind thrust fault directly beneath downtown Seattle, and four small backthrusts within the Seattle fault zone. We then model fault slip, constrained by shallow deformation, to show that the Seattle fault forms a fault propagation fold rather than the alternatively proposed roof thrust system. Fault slip modeling shows that back-thrust ruptures driven by moderate (M ∼6.5–6.7) earthquakes on the main thrust faults are consistent with the paleoseismic data. The results indicate that paleoseismic data from the back-thrust ruptures reveal the times of moderate earthquakes on the main fault system, rather than indicating smaller (M ∼5.5–6.0) earthquakes involving only the backthrusts. Estimates of cumulative shortening during known Seattle fault zone earthquakes support the inference that the Seattle fault has been the major seismic hazard in the northern Cascadia forearc in the late Holocene.

Gravity evidence for shaping of the crustal structure of the Ameca graben (Jalisco block northern limit). Western Mexico

NASA Astrophysics Data System (ADS)

Alatorre-Zamora, Miguel Angel; Campos-Enríquez, José Oscar; Fregoso-Becerra, Emilia; Quintanar-Robles, Luis; Toscano-Fletes, Roberto; Rosas-Elguera, José

2018-03-01

The Ameca tectonic depression (ATD) is located at the NE of the Jalisco Block along the southwestern fringe of the NW-SE trending Tepic-Zacoalco Rift, in the west-central part of the Trans-Mexican Volcanic Belt, western Mexico. To characterize its shallow crustal structure, we conducted a gravity survey based on nine N-S gravity profiles across the western half of the Ameca Valley. The Bouguer residual anomalies are featured by a central low between two zones of positive gravity values with marked gravity gradients. These anomalies have a general NW-SE trend similar to the Tepic-Zacoalco Rift general trend. Basement topography along these profiles was obtained by means of: 1) a Tsuboi's type inverse modeling, and 2) forward modeling. Approximately northward dipping 10° slopes are modeled in the southern half, with south tilted down faulted blocks of the Cretaceous granitic basement and its volcano-sedimentary cover along sub-vertical and intermediate normal faults, whereas southward dipping slopes of almost 15° are observed at the northern half. According to features of the obtained models, this depression corresponds to a slight asymmetric graben. The Ameca Fault is part of the master fault system along its northern limit. The quantitative interpretation shows an approximately 500 to 1100 m thick volcano-sedimentary infill capped by alluvial products. This study has several implications concerning the limit between the Jalisco Block and the Tepic-Zacoalco Rift. The established shallow crustal structure points to the existence of a major listric fault with its detachment surface beneath the Tepic-Zacoalco Rift. The Ameca Fault is interpreted as a secondary listric fault. The models indicate the presence of granitic bodies of the Jalisco Block beneath the TMVB volcanic products of the Tepic-Zacoalco rift. This implies that the limit between these two regional structures is not simple but involves a complex transition zone. A generic model suggests that the extension related normal faulting has been operating as a mechanism in the evolution of this rift. Analysis of seismicity affecting the study area and neighborhood indicates the inferred faults are active.

High resolution shallow imaging of the mega-splay fault in the central Nankai Trough off Kumano

NASA Astrophysics Data System (ADS)

Ashi, J.

2012-12-01

Steep slopes are continuously developed at water depths between 2200 to 2800 m at the Nankai accretionary prism off Kumano. These slopes are interpreted to be surface expressions caused by the megasplay fault on seismic reflection profiles. The fault plane has been drilled at multiple depths below seafloor by IODP NanTroSEIZE project. Mud breccias only recognized at the hanging wall of the fault (Site C0004) by Xray CT scanner are interpreted be formed by strong ground shaking and the age of the shallowest event of mud breccia layers suggests deformation in 1944 Tonankai earthquake (Sakaguchi et al., 2011). Detailed structures around the fault have been examined by seismic reflection profiles including 3D experiments. Although the fault plane deeper than 100 m is well imaged, the structure shallower than 100 m is characterized by obscure sediment veneer suggesting no recent fault activity. Investigation of shallow deformation structures is significant for understanding of recent tectonic activity. Therefore, we carried out deep towed subbottom profile survey by ROV NSS (Navigable Sampling System) during Hakuho-maru KH-11-9 cruise. We introduced a chirp subbottom profiling system of EdgeTech DW-106 for high resolution mapping of shallow structures. ROV NSS also has capability to take a long core with a pinpoint accuracy. The subbottom profiler crossing the megasplay fault near Site C0004 exhibits a landward dipping reflector suggesting the fault plane. The shallowest depth of the reflector is about 10 m below seafloor and the strata above it shows reflectors parallel to the seafloor without any topographic undulation. The fault must have displaced the shallow formation because intense deformation indicated by mud breccia was restricted to near fault zone. Slumping or sliding probably modified the shallow formation after the faulting. The shallow deformations near the megasplay fault were well imaged at the fault scarp 20 km southwest of Site C0004. Although the fault plane itself is not recognized, displacements of sedimentary layers are observed along the fault up to 30 meter below the seafloor. Landward dip of the fault is estimated to be 30 degrees. Displacements of strata are about 3 m near the surface and about 5 m at 7 m below the seafloor suggesting accumulation of fault displacement. The structure more than 30 m below the seafloor is obscure due to decrease of acoustic signal. Active cold seep is expected in this site by high heat flow (Yamano et al., 2012) and many trails of Calyptogena detected by seafloor observations. These results are consistent with the shallow structures reveled by our subbottom profiling survey. References Sakaguchi, A. et al., Geology 39, 919-922, 2011. Yamano, M. et al., JpGU Meeting abstract, SSS38-P23, 2012

High Resolution Vp and Vp/Vs Local Earthquake Tomography of the Val d'Agri Region (Southern Apennines, Italy).

NASA Astrophysics Data System (ADS)

Improta, L.; Bagh, S.; De Gori, P.; Pastori, M.; Piccinini, D.; Valoroso, L.; Anselmi, M.; Buttinelli, M.; Chiarabba, C.

2015-12-01

The Val d'Agri (VA) Quaternary basin in the southern Apennines extensional belt hosts the largest oilfield in onshore Europe and normal-fault systems with high (up to M7) seismogenic potential. Frequent small-magnitude swarms related to both active crustal extension and anthropogenic activity have occurred in the region. Causal factors for induced seismicity are a water impoundment with severe seasonal oscillations and a high-rate wastewater injection well. We analyzed around 1200 earthquakes (ML<3.3) occurred in the VA and surrounding regions between 2001-2014. We integrated waveforms recorded at 46 seismic stations belonging to 3 different networks: a dense temporary network installed by INGV in 2005-2006, the permanent national network of INGV, and the trigger-mode monitoring network managed by the local operator ENI petroleum company. We used local earthquake tomography to investigate static and transient features of the crustal velocity structure and to accurately locate earthquakes. Vp and Vp/Vs models are parameterized by a 3x3x2 km spacing and well resolved down to about 12 km depth. The complex Vp model illuminates broad antiformal structures corresponding to wide ramp-anticlines involving Mesozoic carbonates of the Apulia hydrocarbon reservoir, and NW-SE trending low Vp regions related to thrust-sheet-top clastic basins. The VA basin corresponds to shallow low-Vp region. Focal mechanisms show normal faulting kinematics with minor strike slip solutions in agreement with the local extensional regime. Earthquake locations and focal solutions depict shallow (< 5 km depth) E-dipping extensional structures beneath the artificial lake located in the southern sector of the basin, and along the western margin of the VA. A few swarms define relatively deep transfer structures accommodating the differential extension between main normal faults. The spatio-temporal distribution of around 220 events correlates with wastewater disposal activity, illuminating a NE-dipping fault between 2-5 km depth in the carbonate reservoir. The fault measures 5 km along dip and corresponds to a pre-existing thrust fault favorably oriented with respect to the local extensional field.

Strain partitioning and deformation mode analysis of the normal faults at Red Mountain, Birmingham, Alabama

NASA Astrophysics Data System (ADS)

Wu, Schuman

1989-12-01

In a low-temperature environment, the thin-section scale rock-deformation mode is primarily a function of confining pressure and total strain at geological strain rates. A deformation mode diagram is constructed from published experimental data by plotting the deformation mode on a graph of total strain versus the confining pressure. Four deformation modes are shown on the diagram: extensional fracturing, mesoscopic faulting, incipient faulting, and uniform flow. By determining the total strain and the deformation mode of a naturally deformed sample, the confining pressure and hence the depth at which the rock was deformed can be evaluated. The method is applied to normal faults exposed on the gently dipping southeast limb of the Birmingham anticlinorium in the Red Mountain expressway cut in Birmingham, Alabama. Samples of the Ordovician Chickamauga Limestone within and adjacent to the faults contain brittle structures, including mesoscopic faults and veins, and ductile deformation features including calcite twins, intergranular and transgranular pressure solution, and deformed burrows. During compaction, a vertical shortening of about 45 to 80% in shale is indicated by deformed burrows and relative compaction of shale to burrows, about 6% in limestone by stylolites. The normal faults formed after the Ordovician rocks were consolidated because the faults and associated veins truncate the deformed burrows and stylolites, which truncate the calcite cement. A total strain of 2.0% was caused by mesoscopic faults during normal faulting. A later homogenous deformation, indicated by the calcite twins in veins, cement and fossil fragments, has its major principal shortening strain in the dip direction at a low angle (about 22°) to bedding. The strain magnitude is about 2.6%. By locating the observed data on the deformation mode diagram, it is found that the normal faulting characterized by brittle deformation occurred under low confining pressure (< 18 MPa) at shallow depth (< 800 m), and the homogenous horizontal compression characterized by uniform flow occurred under higher confining pressure (at least 60 MPa) at greater depth (> 2.5 km).

Tectono-stratigraphic evolution of normal fault zones: Thal Fault Zone, Suez Rift, Egypt

NASA Astrophysics Data System (ADS)

Leppard, Christopher William

The evolution of linkage of normal fault populations to form continuous, basin bounding normal fault zones is recognised as an important control on the stratigraphic evolution of rift-basins. This project aims to investigate the temporal and spatial evolution of normal fault populations and associated syn-rift deposits from the initiation of early-formed, isolated normal faults (rift-initiation) to the development of a through-going fault zone (rift-climax) by documenting the tectono-stratigraphic evolution of the Sarbut EI Gamal segment of the exceptionally well-exposed Thai fault zone, Suez Rift, Egypt. A number of dated stratal surfaces mapped around the syn-rift depocentre of the Sarbut El Gamal segment allow constraints to be placed on the timing and style of deformation, and the spatial variability of facies along this segment of the fault zone. Data collected indicates that during the first 3.5 My of rifting the structural style was characterised by numerous, closely spaced, short (< 3 km), low displacement (< 200 m) synthetic and antithetic normal faults within 1 - 2 km of the present-day fault segment trace, accommodating surface deformation associated with the development of a fault propagation monocline above the buried, pre-cursor strands of the Sarbut El Gamal fault segment. The progressive localisation of displacement onto the fault segment during rift-climax resulted in the development of a major, surface-breaking fault 3.5 - 5 My after the onset of rifting and is recorded by the death of early-formed synthetic and antithetic faults up-section, and thickening of syn-rift strata towards the fault segment. The influence of intrabasinal highs at the tips of the Sarbut EI Gamal fault segment on the pre-rift sub-crop level, combined with observations from the early-formed structures and coeval deposits suggest that the overall length of the fault segment was fixed from an early stage. The fault segment is interpreted to have grown through rapid lateral propagation and early linkage of the precursor fault strands at depth before the fault segment broke surface, followed by the accumulation of displacement on the linked fault segment with minimal lateral propagation. This style of fault growth contrasts conventional fault growth models by which growth occurs through incremental increases in both displacement and length through time. The evolution of normal fault populations and fault zones exerts a first- order control on basin physiography and sediment supply, and therefore, the architecture and distribution of coeval syn-rift stratigraphy. The early syn-rift continental, Abu Zenima Formation, to shallow marine, Nukhul Formation show a pronounced westward increase in thickness controlled by the series of synthetic and antithetic faults up to 3 km west of present day Thai fault. The orientation of these faults controlled the location of fluvial conglomerates, sandstones and mudstones that shifted to the topographic lows created. The progressive localisation of displacement onto the Sarbut El Gamal fault segment during rift-climax resulted in an overall change in basin geometry. Accelerated subsidence rates led to sedimentation rates being outpaced by subsidence resulting in the development of a marine, sediment-starved, underfilled hangingwall depocentre characterised by slope-to-basinal depositional environments, with a laterally continuous slope apron in the immediate hangingwall, and point-sourced submarine fans. Controls on the spatial distribution, three dimensional architecture, and facies stacking patterns of coeval syn-rift deposits are identified as: I) structural style of the evolution and linkage of normal fault populations, ii) basin physiography, iii) evolution of drainage catchments, iv) bedrock lithology, and v) variations in sea/lake level.

3D Model of the Tuscarora Geothermal Area

DOE Data Explorer

Faulds, James E.

2013-12-31

The Tuscarora geothermal system sits within a ~15 km wide left-step in a major west-dipping range-bounding normal fault system. The step over is defined by the Independence Mountains fault zone and the Bull Runs Mountains fault zone which overlap along strike. Strain is transferred between these major fault segments via and array of northerly striking normal faults with offsets of 10s to 100s of meters and strike lengths of less than 5 km. These faults within the step over are one to two orders of magnitude smaller than the range-bounding fault zones between which they reside. Faults within the broad step define an anticlinal accommodation zone wherein east-dipping faults mainly occupy western half of the accommodation zone and west-dipping faults lie in the eastern half of the accommodation zone. The 3D model of Tuscarora encompasses 70 small-offset normal faults that define the accommodation zone and a portion of the Independence Mountains fault zone, which dips beneath the geothermal field. The geothermal system resides in the axial part of the accommodation, straddling the two fault dip domains. The Tuscarora 3D geologic model consists of 10 stratigraphic units. Unconsolidated Quaternary alluvium has eroded down into bedrock units, the youngest and stratigraphically highest bedrock units are middle Miocene rhyolite and dacite flows regionally correlated with the Jarbidge Rhyolite and modeled with uniform cumulative thickness of ~350 m. Underlying these lava flows are Eocene volcanic rocks of the Big Cottonwood Canyon caldera. These units are modeled as intracaldera deposits, including domes, flows, and thick ash deposits that change in thickness and locally pinch out. The Paleozoic basement of consists metasedimenary and metavolcanic rocks, dominated by argillite, siltstone, limestone, quartzite, and metabasalt of the Schoonover and Snow Canyon Formations. Paleozoic formations are lumped in a single basement unit in the model. Fault blocks in the eastern portion of the model are tilted 5-30 degrees toward the Independence Mountains fault zone. Fault blocks in the western portion of the model are tilted toward steeply east-dipping normal faults. These opposing fault block dips define a shallow extensional anticline. Geothermal production is from 4 closely-spaced wells, that exploit a west-dipping, NNE-striking fault zone near the axial part of the accommodation zone.

Anomalous Seismic Radiation in the Shallow Subduction Zone Explained by Extensive Poroplastic Deformation in the Overriding Wedge

NASA Astrophysics Data System (ADS)

Hirakawa, E. T.; Ma, S.

2012-12-01

The deficiency of high-frequency seismic radiation from shallow subduction zone earthquakes was first recognized in tsunami earthquakes (Kanamori, 1972), which produce larger tsunamis than expected from short-period (20 s) surface wave excitation. Shallow subduction zone earthquakes were also observed to have unusually low energy-to-moment ratios compared to regular subduction zone earthquakes (e.g., Newman and Okal, 1998; Venkataraman and Kanamori, 2004; Lay et al., 2012). What causes this anomalous radiation and how it relates to large tsunami generation has remained unclear. Here we show that these anomalous observations can be due to extensive poroplastic deformation in the overriding wedge, which provides a unifying interpretation. Ma (2012) showed that the pore pressure increase in the wedge due to up-dip rupture propagation significantly weakens the wedge, leading to widespread Coulomb failure in the wedge. Widespread failure gives rise to slow rupture velocity and large seafloor uplift (landward from the trench) in the case of a shallow fault dip. Here we extend this work and demonstrate that the large seafloor uplift due to the poroplastic deformation significantly dilates the fault behind the rupture front, which reduces the normal stress on the fault and increases the stress drop, slip, and rupture duration. The spectral amplitudes of the moment-rate time function is significantly less at high frequencies than those from elastic simulations. Large tsunami generation and deficiency of high-frequency radiation are thus two consistent manifestations of the same mechanism (poroplastic deformation). Although extensive poroplastic deformation in the wedge represents a significant portion of total seismic moment release, the plastic deformation is shown to act as a large energy sink, leaving less energy to be radiated and leading to low energy-to-moment ratios as observed for shallow subduction zone earthquakes.

Exploring geothermal structures in the Ilan Plain, Taiwan

NASA Astrophysics Data System (ADS)

Wang, Chien-Ying; Shih, Ruey-Chan; Chung, Chen-Tung; Huang, Ming-Zi; Kuo, Hsuan-Yu

2017-04-01

The Ilan Plain in northeast Taiwan is located at the southwestern tip of the Okinawa Trough, which extends westward into the Taiwan orogeny. The Ilan Plain covered by thick sediments is clipped by the Hsuehshan Range in the northern side and the Central Range in the southern side. High geothermal gradients with plenteous hot springs of this area may result from igneous intrusion associated with the back-arc spreading of the Okinawa Trough. In this study, we use reflection seismic survey to explore underground structures in the whole Ilan Plain, especially in SanShin, Wujie, and Lize area. We aim to find the relationship between underground structures and geothermal forming mechanism. The research uses reflection seismic survey to investigate the high geothermal gradient area with two mini-vibrators and 240-channel system. The total length of seismic lines is more than 30 kilometers. The results show that alluvial sediments covering the area about 400 600 meters thick and then thin out to the west in SanShin area. In SanShin , the Taiyaqiao anticline in Hsuehshan Range has entered the plain area and is bounded by the Zhuoshui fault (south) and the Zailian fault (north). In Wujie and Lize , Zhuoshui fault cut through a strong reflector which is the top of the gravel layer near the bottom of the alluvial layer, while the SanShin fault seems to cut near very shallow strata. These two faults are a strike-slip fault with a bit of normal fault component distributing over a range of 600 meters. In Ilan Plain, the geothermal forming mechanism is controlled by anticlines and faults. The hydrothermal solution which migrates upward along these anticline or fault zones to the shallow part causing high geothermal gradients in these areas.

Slumping and shallow faulting related to the presence of salt on the Continental Slope and rise off North Carolina

USGS Publications Warehouse

Cashman, K.V.; Popenoe, P.

1985-01-01

Seismic reflection profiles and long- and medium-range sidescan sonar were used to investigate a salt diapir complex and area of slope instability near the base of the Continental Slope off North Carolina. Within the area of investigation three diapirs are bounded on their upslope side by a scarp 60 m high and 50 km long. The slope above the scarp is characterized by a series of shallow rotational normal faults. The bottom below the scarp is furrowed by slide tracks, which were probably carved by large blocks that broke off the scarp face and slid downslope leaving rubble and scree lobes. Extensive slumping in this area appears to be a result of uplift and faulting associated with salt intrusion, which has fractured and oversteepened the slope leading to instability and failure. Sharply defined slide tracks suggest that slope failure above the breached diapir complex is a continuing process, in contrast to much of the surrounding slope area where few instability features were observed. 

Rupture dynamics along dipping thrust faults: free surface interaction and the case of Tohoku earthquake

NASA Astrophysics Data System (ADS)

Festa, Gaetano; Scala, Antonio; Vilotte, Jean-Pierre

2017-04-01

To address the influence of the free surface interaction on rupture propagating along subduction zones, we numerically investigate dynamic interactions, involving coupling between normal and shear tractions, between in-plane rupture propagating along dipping thrust faults and a free surface for different structural and geometrical conditions. When the rupture occurs along reverse fault with a dip angle different from 90° the symmetry is broken as an effect of slip-induced normal stress perturbations and a larger ground motion is evidenced on the hanging wall. The ground motion is amplified by multiple reflections of waves trapped between the fault and the free surface. This effect is shown to occur when the rupture tip lies on the vertical below the intersection between the S-wave front and the surface that is when waves along the surface start to interact with the rupture front. This interaction is associated with a finite region where the rupture advances in a massive regime preventing the shrinking of the process zone and the emission of high-frequency radiation. The smaller the dip angle the larger co-seismic slip in the shallow part as an effect of the significant break of symmetry. Radiation from shallow part is still depleted in high frequencies due to the massive propagating regime and the interaction length dominating the rupture dynamics. Instantaneous shear response to normal traction perturbations may lead to unstable solutions as in the case of bimaterial rupture. A parametric study has been performed to analyse the effects of a regularised shear traction response to normal traction variations. Finally the case of Tohoku earthquake is considered and we present 2D along-dip numerical results. At first order the larger slip close to the trench can be ascribed to the break of symmetry and the interaction with free surface. When shear/normal coupling is properly regularised the signal from the trench is depleted in high frequencies whereas during deep propagation high-frequency radiations emerge associated to geometrical and structural complexities or to frictional strength asperities.

Fault zone architecture of a major oblique-slip fault in the Rawil depression, Western Helvetic nappes, Switzerland

NASA Astrophysics Data System (ADS)

Gasser, D.; Mancktelow, N. S.

2009-04-01

The Helvetic nappes in the Swiss Alps form a classic fold-and-thrust belt related to overall NNW-directed transport. In western Switzerland, the plunge of nappe fold axes and the regional distribution of units define a broad depression, the Rawil depression, between the culminations of Aiguilles Rouge massif to the SW and Aar massif to the NE. A compilation of data from the literature establishes that, in addition to thrusts related to nappe stacking, the Rawil depression is cross-cut by four sets of brittle faults: (1) SW-NE striking normal faults that strike parallel to the regional fold axis trend, (2) NW-SE striking normal faults and joints that strike perpendicular to the regional fold axis trend, and (3) WNW-ESE striking normal plus dextral oblique-slip faults as well as (4) WSW-ENE striking normal plus dextral oblique-slip faults that both strike oblique to the regional fold axis trend. We studied in detail a beautifully exposed fault from set 3, the Rezli fault zone (RFZ) in the central Wildhorn nappe. The RFZ is a shallow to moderately-dipping (ca. 30-60˚) fault zone with an oblique-slip displacement vector, combining both dextral and normal components. It must have formed in approximately this orientation, because the local orientation of fold axes corresponds to the regional one, as does the generally vertical orientation of extensional joints and veins associated with the regional fault set 2. The fault zone crosscuts four different lithologies: limestone, intercalated marl and limestone, marl and sandstone, and it has a maximum horizontal dextral offset component of ~300 m and a maximum vertical normal offset component of ~200 m. Its internal architecture strongly depends on the lithology in which it developed. In the limestone, it consists of veins, stylolites, cataclasites and cemented gouge, in the intercalated marls and limestones of anastomosing shear zones, brittle fractures, veins and folds, in the marls of anastomosing shear zones, pressure solution seams and veins and in the sandstones of coarse breccia and veins. Later, straight, sharp fault planes cross-cut all these features. In all lithologies, common veins and calcite-cemented fault rocks indicate the strong involvement of fluids during faulting. Today, the southern Rawil depression and the Rhone Valley belong to one of the seismically most active regions in Switzerland. Seismogenic faults interpreted from earthquake focal mechanisms strike ENE-WSW to WNW-ESE, with dominant dextral strike-slip and minor normal components and epicentres at depths of < 15 km. All three Neogene fault sets (2-4) could have been active under the current stress field inferred from the current seismicity. This implies that the same mechanisms that formed these fault zones in the past may still persist at depth. The Rezli fault zone allows the detailed study of a fossil fault zone that can act as a model for processes still occurring at deeper levels in this seismically active region.

Maximum spectral demands in the near-fault region

USGS Publications Warehouse

Huang, Yin-Nan; Whittaker, Andrew S.; Luco, Nicolas

2008-01-01

The Next Generation Attenuation (NGA) relationships for shallow crustal earthquakes in the western United States predict a rotated geometric mean of horizontal spectral demand, termed GMRotI50, and not maximum spectral demand. Differences between strike-normal, strike-parallel, geometric-mean, and maximum spectral demands in the near-fault region are investigated using 147 pairs of records selected from the NGA strong motion database. The selected records are for earthquakes with moment magnitude greater than 6.5 and for closest site-to-fault distance less than 15 km. Ratios of maximum spectral demand to NGA-predicted GMRotI50 for each pair of ground motions are presented. The ratio shows a clear dependence on period and the Somerville directivity parameters. Maximum demands can substantially exceed NGA-predicted GMRotI50 demands in the near-fault region, which has significant implications for seismic design, seismic performance assessment, and the next-generation seismic design maps. Strike-normal spectral demands are a significantly unconservative surrogate for maximum spectral demands for closest distance greater than 3 to 5 km. Scale factors that transform NGA-predicted GMRotI50 to a maximum spectral demand in the near-fault region are proposed.

Maximum spectral demands in the near-fault region

USGS Publications Warehouse

Huang, Y.-N.; Whittaker, A.S.; Luco, N.

2008-01-01

The Next Generation Attenuation (NGA) relationships for shallow crustal earthquakes in the western United States predict a rotated geometric mean of horizontal spectral demand, termed GMRotI50, and not maximum spectral demand. Differences between strike-normal, strike-parallel, geometric-mean, and maximum spectral demands in the near-fault region are investigated using 147 pairs of records selected from the NGA strong motion database. The selected records are for earthquakes with moment magnitude greater than 6.5 and for closest site-to-fault distance less than 15 km. Ratios of maximum spectral demand to NGA-predicted GMRotI50 for each pair of ground motions are presented. The ratio shows a clear dependence on period and the Somerville directivity parameters. Maximum demands can substantially exceed NGA-predicted GMRotI50 demands in the near-fault region, which has significant implications for seismic design, seismic performance assessment, and the next-generation seismic design maps. Strike-normal spectral demands are a significantly unconservative surrogate for maximum spectral demands for closest distance greater than 3 to 5 km. Scale factors that transform NGA-predicted GMRotI50 to a maximum spectral demand in the near-fault region are proposed. ?? 2008, Earthquake Engineering Research Institute.

Depth-varying seismogenesis on an oceanic detachment fault at 13°20‧N on the Mid-Atlantic Ridge

NASA Astrophysics Data System (ADS)

Craig, Timothy J.; Parnell-Turner, Ross

2017-12-01

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived faults called oceanic detachments. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths. The commonly-observed result is a domed, sub-horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains subject to debate. Here we present a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13°20‧N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger teleseismically-observed earthquakes. The unique coincidence of these two datasets provides a comprehensive definition of rupture on the fault, from the uppermost mantle to the seabed. Our results demonstrate that although slip on the deep, steeply-dipping portion of detachment faults is accommodated by failure in numerous microearthquakes, the shallow, gently-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. This result brings into question the current paradigm that the shallow sections of oceanic detachment faults are dominated by low-friction mineralogies and therefore slip aseismically, but is consistent with observations from continental detachment faults. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths.

Three Dimensional Seismic Tomography of the Shallow Subsurface Structure Under the Meihua Lake in Ilan, Northeastern Taiwan

NASA Astrophysics Data System (ADS)

Shih, R.

2008-12-01

The island of Taiwan is located at an ongoing collision boundary between two plates. The Philippine Sea plate and the Eurasian plate collided at the Longitudinal Valley of eastern Taiwan, and the Philippine Sea plate subducted northward beneath the Eurasian plate along the Ryukyu trench in eastern Taiwan at the Hualien area. Further northward in the island, the opening Okinawa trough ended at the Ilan area in northeastern Taiwan. The Ilan area is over populated and potentially able to produce large earthquake; however, since that are is densely covered with forests, due to lack of geologic and geomorphologic evidences, known active faults are still unclear. Recently, a series of topographic offsets of several meters distributed in a zone were found by using the LiDAR DTM data, indicating active normal faulting was activated in the past. Besides, several small sag ponds were mapped to support the active normal faulting activities. Later on, core borings in one of the small ponds (the Meihua Lake, diameter of about 700m) were conducted and the records showed obvious difference of depths in the adjacent boreholes at a very short distance. In order to realize the variation of the distribution of sediments under the Meihua Lake, we conducted a 3d seismic tomography survey at the lake, hopefully to help to verify the faults. In this paper, we will show results of using a 120-channel shallow seismic recording system for mapping the shallow subsurface structure of sediments under the Meihua Lake. During the experiment, we deployed the geophone groups of three geophones at every 6m along the bank of the lake and fired the shots at every 80m around the lake. An impactor of energy 2200 joule per shot was used as a seismic source. We stacked the energy at each shot point around 60 times for receiving clear signals. Since the total extension of recording system is 720m, about one third of the perimeter around the lake, 2,200m, we moved the geophone deployments 3 times to circulate the entire lake. Clear signals were received in the field. The data were then analyzed by using a 3d tomographic method. The inverted images of the shallow subsurface structure under the lake will be incorporated with the records from boreholes, and hopefully for the first time to provide the evidence of the faults in this area.

Shallow observatory installations unravel earthquake processes in the Nankai accretionary complex (IODP Expedition 365)

NASA Astrophysics Data System (ADS)

Kopf, A.; Saffer, D. M.; Toczko, S.

2016-12-01

NanTroSEIZE is a multi-expedition IODP project to investigate fault mechanics and seismogenesis along the Nankai Trough subduction zone through direct sampling, in situ measurements, and long-term monitoring. Recent Expedition 365 had three primary objectives at a major splay thrust fault (termed the "megasplay") in the forearc: (1) retrieval of a temporary observatory (termed a GeniusPlug) that has been monitoring temperature and pore pressure within the fault zone at 400 meters below seafloor for since 2010; (2) deployment of a complex long-term borehole monitoring system (LTBMS) across the same fault; and (3) coring of key sections of the hanging wall, deformation zone and footwall of the shallow megasplay. Expedition 365 achieved its primary monitoring objectives, including recovery of the GeniusPlug with a >5-year record of pressure and temperature conditions, geochemical samples, and its in situ microbial colonization experiment; and installation of the LTBMS. The pressure records from the GeniusPlug include high-quality records of formation and seafloor responses to multiple fault slip events, including the 2011 M9 Tohoku and the 1 April Mie-ken Nanto-oki M6 earthquakes. The geochemical sampling coils yielded in situ pore fluids from the fault zone, and microbes were successfully cultivated from the colonization unit. The LTBMS incorporates multi-level pore pressure sensing, a volumetric strainmeter, tiltmeter, geophone, broadband seismometer, accelerometer, and thermistor string. This multi-level hole completion was meanwhile connected to the DONET seafloor cabled network for tsunami early warning and earthquake monitoring. Coring the shallow megasplay site in the Nankai forearc recovered ca. 100m of material across the fault zone, which contained indurated silty clay with occasional ash layers and sedimentary breccias in the hangingwall and siltstones in the footwall of the megasplay. The mudstones show different degrees of deformation spanning from occasional fractures to intensely fractured scaly claystones of up to >10 cm thickness. Sparse faulting with low displacement (usually <2cm) is seen with both normal and reverse sense of slip. Post-cruise rock deformation experiments will relate physical properties to the earthquake response monitored by the observatory array.

Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

USGS Publications Warehouse

Choy, G.L.; Kirby, S.H.

2004-01-01

The behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (?? a = ??Es/Mo, where E s is radiated energy and Mo is seismic moment) of all globally distributed shallow (depth, ?? 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower ??a (< 1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average ?? a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average ??a with focal mechanism and tectonic regime suggests that the level of ?? a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high ??a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal earthquake occurred remarkably close in space and time. The intraslab-normal member of each pair radiated anomalously high amounts of energy compared to its thrust-fault counterpart. These intraslab earthquakes probably ruptured intact slab mantle and are dramatic examples in which Mc (an energy magnitude) is shown to be a far better estimate of the potential for earthquake damage than Mw. This discovery may help explain why loss of life as a result of intraslab earthquakes was greater in the 20th century in Latin America than the fatalities associated with interplate-thrust events that represented much higher total moment release. ?? 2004 RAS.

Geochemical and microstructural evidence for interseismic changes in fault zone permeability and strength, Alpine Fault, New Zealand

NASA Astrophysics Data System (ADS)

Boulton, Carolyn; Menzies, Catriona D.; Toy, Virginia G.; Townend, John; Sutherland, Rupert

2017-01-01

Oblique dextral motion on the central Alpine Fault in the last circa 5 Ma has exhumed garnet-oligoclase facies mylonitic fault rocks from ˜35 km depth. During exhumation, deformation, accompanied by fluid infiltration, has generated complex lithological variations in fault-related rocks retrieved during Deep Fault Drilling Project (DFDP-1) drilling at Gaunt Creek, South Island, New Zealand. Lithological, geochemical, and mineralogical results reveal that the fault comprises a core of highly comminuted cataclasites and fault gouges bounded by a damage zone containing cataclasites, protocataclasites, and fractured mylonites. The fault core-alteration zone extends ˜20-30 m from the principal slip zone (PSZ) and is characterized by alteration of primary phases to phyllosilicate minerals. Alteration associated with distinct mineral phases occurred proximal the brittle-to-plastic transition (T ≤ 300-400°C, 6-10 km depth) and at shallow depths (T = 20-150°C, 0-3 km depth). Within the fault core-alteration zone, fractures have been sealed by precipitation of calcite and phyllosilicates. This sealing has decreased fault normal permeability and increased rock mass competency, potentially promoting interseismic strain buildup.

Frictional properties of fault rocks along the shallow part of the Japan Trench décollement: insights from samples recovered during the Integrated Ocean Drilling Project Expedition 343 (the JFAST project)

NASA Astrophysics Data System (ADS)

Remitti, Francesca; Smith, Steven; Gualtieri, Alessandro; Di Toro, Giulio; Nielsen, Stefan

2014-05-01

The Japan Trench Fast Drilling Project (JFAST), Integrated Ocean Drilling Program (IODP) Expedition 343, successfully located and sampled the shallow slip zone of the Mw =9.0 Tohoku-Oki earthquake where the largest coseismic slip occurred (c. 50 m). Logging-while-drilling, core-sample observations and the analysis of temperature data recovered from a third borehole show that a thin (<5 m), smectite rich plate-boundary fault accommodated the large slip of the Tohoku-Oki Earthquake rupture, as well as most of the interplate motion at the drill site. Effective normal stress along the shallow plate-boundary fault is estimated to be c. 7 MPa. Single-velocity and velocity-stepping rotary-shear friction experiments on fault material were performed with the Slow to HIgh Velocity Apparatus (SHIVA) installed at INGV in Rome. Quantitative phase analysis using the combined Rietveld and R.I.R. method indicates that the starting material is mainly composed of smectite (56 wt%) and illite/mica (21 wt%) and minor quartz, kaolinite, plagioclase and K-feldspar. The amount of amorphous fraction has also been calculated and it is close to the detection limit. Each experiment used 3.5 g of loosely disaggregated gouge, following sieving to a particle size fraction <1 mm. Experiments were performed either 1) "room-dry" (40-60% humidity) at 8.5 MPa normal stress (one test at 12.5 MPa), or 2) "water-dampened" (0.5 ml distilled water added to the gouge layers) at 3.5 MPa normal stress. Slip velocities ranged over nearly seven orders of magnitude (10-5 - 3 m s-1). Total displacement is always less than 1 m. The peak and steady-state frictional strengths of the gouges are significantly lower under water-dampened conditions, with mean steady-state friction coefficients (μ, shear stress/normal stress) at all investigated velocities of 0.04<μ<0.1. This is consistent with the small measured frictional heat anomaly along the plate boundary fault ~1.5 years after the Tohoku-Oki earthquake. Under room-dry conditions the gouge material is velocity-strengthening at intermediate velocities (0.001 - 0.1 m s-1), but strongly velocity-weakening at > 0.1 m s-1. Instead, under water-dampened conditions, the gouge is velocity-neutral to velocity-weakening at all investigated velocities. In other words, the intermediate-velocity strengthening, which would probably act as a "barrier" to rupture propagation in the dry gouges, disappears in water-dampened gouges. This result is compatible with propagation of the Tohoku rupture to the trench, and also with large coseismic slip at shallow depths. Quantitative phase analysis using the combined Rietveld and R.I.R. method has been performed also on six post-experiment gouges for the determination of both the crystalline and amorphous fractions. Preliminary results show that the mineralogical assemblage is basically the same after the experiments, with both smectite and illite phases preserved, this suggests that the weakening mechanism operating in this material is active at low temperature.

Seismic rupture and ground accelerations induced by CO 2 injection in the shallow crust

DOE PAGES

Cappa, Frédéric; Rutqvist, Jonny

2012-09-01

We present that because of the critically stressed nature of the upper crust, the injection of large volumes of carbon dioxide (CO 2) into shallow geological reservoirs can trigger seismicity and induce ground deformations when the injection increases the fluid pressure in the vicinity of potentially seismic faults. The increased fluid pressure reduces the strength against fault slip, allowing the stored elastic energy to be released in seismic events that can produce felt ground accelerations. Here, we seek to explore the likelihood ground motions induced by a CO 2 injection using hydromechanical modelling with multiphase fluid flow and dynamic rupture,more » including fault-frictional weakening. We extend the previous work of Cappa and Rutqvist, in which activation of a normal fault at critical stress may be possible for fast rupture nucleating by localized increase in fluid pressure and large decrease in fault friction. In this paper, we include seismic wave propagation generated by the rupture. For our assumed system and injection rate, simulations show that after a few days of injection, a dynamic fault rupture of few centimetres nucleates at the base of the CO 2 reservoir and grows bilaterally, both toward the top of the reservoir and outside. The rupture is asymmetric and affects a larger zone below the reservoir where the rupture is self-propagating (without any further pressure increase) as a result of fault-strength weakening. The acceleration and deceleration of the rupture generate waves and result in ground accelerations (~0.1–0.6 g) consistent with observed ground motion records. Finally, the maximum ground acceleration is obtained near the fault, and horizontal accelerations are generally markedly higher than vertical accelerations.« less

Large earthquakes and creeping faults

USGS Publications Warehouse

Harris, Ruth A.

2017-01-01

Faults are ubiquitous throughout the Earth's crust. The majority are silent for decades to centuries, until they suddenly rupture and produce earthquakes. With a focus on shallow continental active-tectonic regions, this paper reviews a subset of faults that have a different behavior. These unusual faults slowly creep for long periods of time and produce many small earthquakes. The presence of fault creep and the related microseismicity helps illuminate faults that might not otherwise be located in fine detail, but there is also the question of how creeping faults contribute to seismic hazard. It appears that well-recorded creeping fault earthquakes of up to magnitude 6.6 that have occurred in shallow continental regions produce similar fault-surface rupture areas and similar peak ground shaking as their locked fault counterparts of the same earthquake magnitude. The behavior of much larger earthquakes on shallow creeping continental faults is less well known, because there is a dearth of comprehensive observations. Computational simulations provide an opportunity to fill the gaps in our understanding, particularly of the dynamic processes that occur during large earthquake rupture and arrest.

In-situ stress and fracture permeability in a fault-hosted geothermal reservoir at Dixie Valley, Nevada

USGS Publications Warehouse

Hickman, Stephen; Barton, Colleen; Zoback, Mark; Morin, Roger; Sass, John; Benoit, Richard; ,

1997-01-01

As part of a study relating fractured rock hydrology to in-situ stress and recent deformation within the Dixie Valley Geothermal Field, borehole televiewer logging and hydraulic fracturing stress measurements were conducted in a 2.7-km-deep geothermal production well (73B-7) drilled into the Stillwater fault zone. Borehole televiewer logs from well 73B-7 show numerous drilling-induced tensile fractures, indicating that the direction of the minimum horizontal principal stress, Shmin, is S57 ??E. As the Stillwater fault at this location dips S50 ??E at approximately 3??, it is nearly at the optimal orientation for normal faulting in the current stress field. Analysis of the hydraulic fracturing data shows that the magnitude of Shmin is 24.1 and 25.9 MPa at 1.7 and 2.5 km, respectively. In addition, analysis of a hydraulic fracturing test from a shallow well 1.5 km northeast of 73B-7 indicates that the magnitude of Shmin is 5.6 MPa at 0.4 km depth. Coulomb failure analysis shows that the magnitude of Shmin in these wells is close to that predicted for incipient normal faulting on the Stillwater and subparallel faults, using coefficients of friction of 0.6-1.0 and estimates of the in-situ fluid pressure and overburden stress. Spinner flowmeter and temperature logs were also acquired in well 73B-7 and were used to identify hydraulically conductive fractures. Comparison of these stress and hydrologic data with fracture orientations from the televiewer log indicates that hydraulically conductive fractures within and adjacent to the Stillwater fault zone are critically stressed, potentially active normal faults in the current west-northwest extensional stress regime at Dixie Valley.

Overview of the Mechanics of the Active Mai'iu Low Angle Normal Fault (Dayman Dome), Southeastern Papua New Guinea

NASA Astrophysics Data System (ADS)

Little, T. A.; Boulton, C. J.; Webber, S. M.; Mizera, M.; Oesterle, J.; Ellis, S. M.; Norton, K. P.; Wallace, L.; Biemiller, J.; Seward, D.; Boles, A.

2016-12-01

The Mai'iu Fault is a corrugated low-angle normal fault (LANF) that has slipped >24 km. It emerges near sea level at 21° N dip, and flattens southward over the dome crest at 3000 m. This reactivated Paleogene suture is slipping at up to 1 cm/year based on previous GPS data and preliminary 10Be cosmogenic nuclide exposure scarp dating. An alignment of microseismicity (Eilon et al. 2015) suggests a dip of 30° N at 15-25 km depth. Pseudotachylites are abundant in lower, mylonitic parts of the footwall. One vein yielded 40Ar/39Ar ages of 1.9-2.2 Ma, implying seismicity at 8-10 km depth at the above slip rate. Widespread, antithetic normal faults in the footwall are attributed to rolling-hinge controlled yielding during exhumation. A single rider block is downfolded into synformal megamullion. Unconformities within this block, and ductile folding and conjugate strike-slip faulting of mylonitic footwall fabrics record prolonged EW shortening and constriction. Many normal and strike-slip faults cut the metabasaltic footwall recording Andersonian stresses and flipping between σ1 and σ2. To exhume the steep faults, the LANF must have remained active despite differential stress being locally high enough to initiate well-oriented faults—relationships that bracket the frictional strength of the LANF. Quantitative XRD on mafic and serpentinitic gouges reveal the Mai'iu fault core is enriched in weak clays corrensite and saponite. Hydrothermal friction experiments were done at effective normal stresses of 30-210 MPa, and temperatures of 50-450oC. At shallow depths (T≤200 oC), clay-rich fault gouges are frictionally weak (μ=0.13-0.15 and 0.20-0.28) and velocity-strengthening. At intermediate depths (T>200 oC), the footwall is frictionally strong (μ=0.71-0.78 and 0.50-0.64) and velocity-weakening. Velocity-strengthening is observed at T≥400 oC. The experiments provide evidence for deep unstable slip, consistent with footwall pseudotachylites and microseismicity at depth

Investigating The Relationship Between Structural Geology and Wetland Loss Near Golden Meadow, Louisiana By Utilizing 3D Seismic Reflection and Well Log Data

NASA Astrophysics Data System (ADS)

Johnston, A. S.; Zhang, R.; Gottardi, R.; Dawers, N. H.

2017-12-01

Wetland loss is one of the greatest environmental and economic threats in the deltaic plain of the Gulf Coast. This loss is controlled by subsidence, sea level rise, decreased sediment supply rates, movement along normal faults, salt tectonics, fluid extraction related to oil, gas and water exploration, and compaction. However, the interplay and feedback between these different processes are still poorly understood. In this study, we investigate the role of active faulting and salt tectonics on wetland loss in an area located between Golden Meadow and Leeville, Louisiana. Using industry 3D seismic and well log data, we investigate key segments of the Golden Meadow fault zone and map shallow faults that overlie the Leeville salt dome, to compare those fault planes with areas of wetland loss and subsidence. Faults were mapped to a depth of 1200 m, and well logs were tied to the upper 180 m of the seismic data to make accurate projections of the faults to the surface. Preliminary results highlight a graben structure south of a segment of the Golden Meadow fault. Well log and published data from shallow borings reveal a thicker Holocene accumulation at the center of the graben, up to 45 m than on the flanks of the graben. The location of this graben spatially correlates with Catfish Lake, and part of it overlies salt adjacent to the main fault surface. Bayou Lafourche, the main distributary channel of the Lafourche lobe of the Mississippi River delta complex, appears to have its path controlled by faults. Bayou Lafourche changes orientation and flows parallel to, and on the downthrown side of, two radial faults associated with the Leeville salt dome. These preliminary results indicate that there is a relationship between surface geomorphology and subsurface structures that, at least in part, exert a control on wetland loss in southern Louisiana.

Thermal effects on shearing resistance of fractures in Tak granite

NASA Astrophysics Data System (ADS)

Khamrat, S.; Thongprapha, T.; Fuenkajorn, K.

2018-06-01

Triaxial shear tests have been performed on tension-induced fractures and smooth saw-cut surfaces in Tak granite under temperatures up to 773 K. The objective is to gain an understanding of the movement of shallow faults that cause seismic activities in the Tak batholith in the north of Thailand. The results indicate that the peak and residual shear strengths and fracture dilations notably decrease as the temperatures increase. The thermal effect is enhanced under higher confining pressures. The areas of the sheared-off asperities increase with temperature and confining pressure. A power equation can describe the increase of shear strengths with normal stress where the normal stress exponent is a linear function of the temperature. The strain energy principle is applied to incorporate the principal stresses and strains into a strength criterion. A linear relation between the distortional strain energy (Wd) and the mean strain energy (Wm) of the fractures is obtained. The Wd-Wm slope depends on the fracture roughness and strength of the asperities, which can be defined as a function of shear and mean strains and dilation of the fractures. This may allow predicting the peak strength of the shallow faults in the Tak batholith.

Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009.

PubMed

Beavan, J; Wang, X; Holden, C; Wilson, K; Power, W; Prasetya, G; Bevis, M; Kautoke, R

2010-08-19

The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.

Comparative study of two tsunamigenic earthquakes in the Solomon Islands: 2015 Mw 7.0 normal-fault and 2013 Santa Cruz Mw 8.0 megathrust earthquakes

NASA Astrophysics Data System (ADS)

Heidarzadeh, Mohammad; Harada, Tomoya; Satake, Kenji; Ishibe, Takeo; Gusman, Aditya Riadi

2016-05-01

The July 2015 Mw 7.0 Solomon Islands tsunamigenic earthquake occurred ~40 km north of the February 2013 Mw 8.0 Santa Cruz earthquake. The proximity of the two epicenters provided unique opportunities for a comparative study of their source mechanisms and tsunami generation. The 2013 earthquake was an interplate event having a thrust focal mechanism at a depth of 30 km while the 2015 event was a normal-fault earthquake occurring at a shallow depth of 10 km in the overriding Pacific Plate. A combined use of tsunami and teleseismic data from the 2015 event revealed the north dipping fault plane and a rupture velocity of 3.6 km/s. Stress transfer analysis revealed that the 2015 earthquake occurred in a region with increased Coulomb stress following the 2013 earthquake. Spectral deconvolution, assuming the 2015 tsunami as empirical Green's function, indicated the source periods of the 2013 Santa Cruz tsunami as 10 and 22 min.

Interpretation of the Seattle uplift, Washington, as a passive-roof duplex

USGS Publications Warehouse

Brocher, T.M.; Blakely, R.J.; Wells, R.E.

2004-01-01

We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive-roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short-wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene. Ages from offset soils in trenches across the fault scarps and from abruptly raised shorelines indicate that the splay, roof, and floor thrusts of the Seattle and Tacoma faults ruptured about 1100 years ago.

Tectono-seismic characteristics of faults in the shallow portion of an accretionary prism

NASA Astrophysics Data System (ADS)

Hirono, Tetsuro; Ishikawa, Tsuyoshi

2018-01-01

To understand the tectono-seismic evolution of faults in the shallow part of a subduction-accretion system, we examined major faults in a fossil accretionary prism, the Emi Group (Hota Group), Boso Peninsula, Japan, by performing multiple structural, geochemical, and mineralogical analyses. Because the strata are relatively shallow (burial depth, 1-4 km), early stage deformation related to subduction, accretion, and uplifting processes is well preserved in three dominant fault zones. On the basis of both previous findings and our geochemical and mineralogical results, we inferred that early stage faulting in a near-trench setting under high pore fluid pressure and second stage faulting at relatively deep along subduction corresponded to aseismic deformations, as shown by velocity strengthening characteristics; and during late stage faulting, probably in association with accretion and uplift processes, a high-temperature fluid, revealed by a geochemical temperature proxy, triggered fault weakening by a thermal pressurization mechanism, and potentially led to the generation of a tsunami.

Late 20th Century Deep-seated Vertical Motions in New Orleans and implications for Gulf Coast Subsidence

NASA Astrophysics Data System (ADS)

Dokka, R. K.

2010-12-01

Subsidence of the Mississippi River delta and adjoining coastal areas is widely thought to be dominated by compaction of Holocene sediments. Current public policies regarding hurricane protection and ecosystems restoration are founded on this interpretation. To test this hypothesis, monuments that penetrate the entire Holocene section were measured using geodetic leveling and water gauges attached to bridge foundations. Results show that the entire sampling area subsided between 1955 and 1995 in amounts unanticipated by previous models. Subsidence due to processes originating below the Holocene section locally exceeded 0.9 m between 1955 and 1995. The maxima of deep subsidence occurred in the urbanized and industrialized sections of eastern New Orleans. Subsidence decreased away from urbanized areas and north of the belt of active basin margin normal faults; this decrease in subsidence continued to the north and east along the Mississippi coast. These independent measurements provide insights into the complexity and causes of modern landscape change in the region. Modern subsidence is clearly not dominated solely by shallow processes such as natural compaction, Deep subsidence occurring east and north of the basin margin faults can be explained by regional tectonic loading of the lithosphere by the modern Mississippi River delta and local groundwater withdrawal. Sharp, local changes in subsidence coincide with strands of the basin margin normal fault system. Deep subsidence of the New Orleans area can be explained by a combination of groundwater withdrawal from shallow upper Pleistocene aquifers, the aforementioned lithospheric loading, and non-groundwater-related faulting. Subsidence due to groundwater extraction from aquifers ~160 to 200 m deep dominated the urbanized areas from ~1960 to the early 1990s and is likely responsible for lowering flood protection structures and bridges in the area by as much as ~0.8 m.

Neogene Development of the Terror Rift, western Ross Sea, Antarctica

NASA Astrophysics Data System (ADS)

Sauli, C.; Sorlien, C. C.; Busetti, M.; De Santis, L.; Wardell, N.; Henrys, S. A.; Geletti, R.; Wilson, T. J.; Luyendyk, B. P.

2015-12-01

Terror Rift is a >300 km-long, 50-70 km-wide, 14 km-deep sedimentary basin at the edge of the West Antarctic Rift System, adjacent to the Transantarctic Mountains. It is cut into the broader Victoria Land Basin (VLB). The VLB experienced 100 km of mid-Cenozoic extension associated with larger sea floor spreading farther north. The post-spreading (Neogene) development of Terror Rift is not well understood, in part because of past use of different stratigraphic age models. We use the new Rossmap seismic stratigraphy correlated to Cape Roberts and Andrill cores in the west and to DSDP cores in the distant East. This stratigraphy, and new fault interpretations, was developed using different resolutions of seismic reflection data included those available from the Seismic Data Library System. Depth conversion used a new 3D velocity model. A 29 Ma horizon is as deep as 8 km in the south, and a 19 Ma horizon is >5 km deep there and 4 km-deep 100 km farther north. There is a shallower northern part of Terror Rift misaligned with the southern basin across a 50 km right double bend. It is bounded by steep N-S faults down-dropping towards the basin axis. Between Cape Roberts and Ross Island, the Oligocene section is also progressively-tilted. This Oligocene section is not imaged within northern Terror Rift, but the simplest hypothesis is that some of the Terror Rift-bounding faults were active at least during Oligocene through Quaternary time. Many faults are normal separation, but some are locally vertical or even reverse-separation in the upper couple of km. However, much of the vertical relief of the strata is due to progressive tilting (horizontal axis rotation) and not by shallow faulting. Along the trend of the basin, the relief alternates between tilting and faulting, with a tilting margin facing a faulted margin across the Rift, forming asymmetric basins. Connecting faults across the basin form an accommodation zone similar to other oblique rifts. The Neogene basin is shallow to non-existent 300 km north of Ross Island, near -74° 30' latitude. We propose that the Neogene history of Terror Rift has been highly-oblique right-lateral, terminating northward into right lateral faults of northern Victoria Land.

Magnetotelluric and Audio-magnetotelluric measurements in Alasehir Graben for geothermal exploration purposes

NASA Astrophysics Data System (ADS)

Tekesin-Cankurtaranlar, Ozge; Tuysuz, Okan; Riza Kilic, Ali

2017-04-01

In this study, we present the results of Magnetotelluric (MT) and Audio-magnetotelluric (AMT) soundings over a potential geothermal field. Study area is located in the northeasternmost part of the Alasehir (or Gediz) Graben, Western Anatolia, which is delimited by NW-SE trending fault systems and is filled by Miocene to Recent sediments. Study area is also very close to the Kula Quaternary volcanic region, a possible geothermal heat source for the region, last eruption of which was 12.000 years ago. Relatively thin crust, high heat flow values and intense tectonic activity of the Western Anatolia possibly refers to the high geothermal potential. In fact, along the southern and central part of the graben there are many productive areas reaching up to 300 degrees Celsius. By this motivation, to determine the geothermal potential of the study area MT and AMT measurements had been carried out on a total of 45 stations covering about 8 km2 area. All profiles shows higher resistivity values (>140 ohm.m) at greater depths, possibly indicating a metamorphic basement covered by Miocene to Recent sediments. This metamorphic basement gets shallower towards the North where the geothermally weathered schists and marbles crop out. Furthermore, a normal fault interface between metamorphic basement and Neogene sediments shows high resistivity contrast. Results indicate that the metamorphic basement is a less conductive block located at a depth of 1500 - 2000 m at the south and gets shallower towards the north as normal fault blocks.

The structures, stratigraphy and evolution of the Gulf of Corinth rift, Greece

NASA Astrophysics Data System (ADS)

Taylor, Brian; Weiss, Jonathan R.; Goodliffe, Andrew M.; Sachpazi, Maria; Laigle, Mireille; Hirn, Alfred

2011-06-01

A multichannel seismic and bathymetry survey of the central and eastern Gulf of Corinth (GoC), Greece, reveals the offshore fault geometry, seismic stratigraphy and basin evolution of one of Earths most active continental rift systems. Active, right-stepping, en-echelon, north-dipping border faults trend ESE along the southern Gulf margin, significantly overlapping along strike. The basement offsets of three (Akrata-Derveni, Sithas and Xylocastro) are linked. The faults are biplanar to listric: typically intermediate angle (˜35° in the centre and 45-48° in the east) near the surface but decreasing in dip and/or intersecting a low- or shallow-angle (15-20° in the centre and 19-30° in the east) curvi-planar reflector in the basement. Major S-dipping border faults were active along the northern margin of the central Gulf early in the rift history, and remain active in the western Gulf and in the subsidiary Gulf of Lechaio, but unlike the southern border faults, are without major footwall uplift. Much of the eastern rift has a classic half-graben architecture whereas the central rift has a more symmetric w- or u-shape. The narrower and shallower western Gulf that transects the >40-km-thick crust of the Hellenides is associated with a wider distribution of overlapping high-angle normal faults that were formerly active on the Peloponnesus Peninsula. The easternmost sector includes the subsidiary Gulfs of Lechaio and Alkyonides, with major faults and basement structures trending NE, E-W and NW. The basement faults that control the rift architecture formed early in the rift history, with little evidence (other than the Vrachonisida fault along the northern margin) in the marine data for plan view evolution by subsequent fault linkage. Several have maximum offsets near one end. Crestal collapse graben formed where the hanging wall has pulled off the steeper onto the shallower downdip segment of the Derveni Fault. The dominant strikes of the Corinth rift faults gradually rotate from 090-120° in the basement and early rift to 090-100° in the latest rift, reflecting a ˜10° rotation of the opening direction to the 005° presently measured by GPS. The sediments include a (locally >1.5-km-) thick, early-rift section, and a late-rift section (also locally >1.5-km-thick) that we subdivide into three sequences and correlate with seven 100-ka glacio-eustatic cycles. The Gulf depocentre has deepened through time (currently >700 mbsl) as subsidence has outpaced sedimentation. We measure the minimum total horizontal extension across the central and eastern Gulf as varying along strike between 4 and 10 km, and estimate full values of 6-11 km. The rift evolution is strongly influenced by the inherited basement fabric. The regional NNW structural fabric of the Hellenic nappes changes orientation to ESE in the Parnassos terrane, facilitating the focused north-south extension observed offshore there. The basement-penetrating faults lose seismic reflectivity above the 4-14-km-deep seismogenic zone. Multiple generations and dips of normal faults, some cross-cutting, accommodate extension beneath the GoC, including low-angle (15-20°) interfaces in the basement nappes. The thermally cool forearc setting and cross-orogen structures unaccompanied by magmatism make this rift a poor analogue and unlikely precursor for metamorphic core complex formation.

Dynamic Simulations for the Seismic Behavior on the Shallow Part of the Fault Plane in the Subduction Zone during Mega-Thrust Earthquakes

NASA Astrophysics Data System (ADS)

Tsuda, K.; Dorjapalam, S.; Dan, K.; Ogawa, S.; Watanabe, T.; Uratani, H.; Iwase, S.

2012-12-01

The 2011 Tohoku-Oki earthquake (M9.0) produced some distinct features such as huge slips on the order of several ten meters around the shallow part of the fault and different areas with radiating seismic waves for different periods (e.g., Lay et al., 2012). These features, also reported during the past mega-thrust earthquakes in the subduction zone such as the 2004 Sumatra earthquake (M9.2) and the 2010 Chile earthquake (M8.8), get attentions as the distinct features if the rupture of the mega-thrust earthquakes reaches to the shallow part of the fault plane. Although various kinds of observations for the seismic behavior (rupture process and ground motion characteristics etc.) on the shallow part of the fault plane during the mega-trust earthquakes have been reported, the number of analytical or numerical studies based on dynamic simulation is still limited. Wendt et al. (2009), for example, revealed that the different distribution of initial stress produces huge differences in terms of the seismic behavior and vertical displacements on the surface. In this study, we carried out the dynamic simulations in order to get a better understanding about the seismic behavior on the shallow part of the fault plane during mega-thrust earthquakes. We used the spectral element method (Ampuero, 2009) that is able to incorporate the complex fault geometry into simulation as well as to save computational resources. The simulation utilizes the slip-weakening law (Ida, 1972). In order to get a better understanding about the seismic behavior on the shallow part of the fault plane, some parameters controlling seismic behavior for dynamic faulting such as critical slip distance (Dc), initial stress conditions and friction coefficients were changed and we also put the asperity on the fault plane. These understandings are useful for the ground motion prediction for future mega-thrust earthquakes such as the earthquakes along the Nankai Trough.

Predicted variation of stress orientation with depth near an active fault: application to the Cajon Pass Scientific Drillhole, southern California

USGS Publications Warehouse

Wesson, R.L.

1988-01-01

Preliminary measurements of the stress orientation at a depth of 2 km interpreted to indicate that the regional orientation of the maximum compression is normal to the fault, and taken as evidence for a very weak fault. The orientation expected from plate tectonic arguments is about 66?? NE from the strike of the fault. Geodetic data indicate that the orientation of maximum compressive strain rate is about 43?? NE from the strike of the fault, and show nearly pure right-lateral shear acting parallel to the fault. These apparent conflicts in the inferred orientation of the axis of maximum compression may be explained in part by a model in which the fault zone is locked over a depth interval in the range of 2-5 to 15 km, but is very weak above and below that interval. This solution does require, however, a few mm/yr of creep at the surface on the San Andreas or nearby sub-parallel faults (such as the San Jacinto), which has not yet been observed, or a shallow zone near the faults of distributed deformation. -from Author

Coseismic fault zone deformation caused by the 2014 Mw=6.2 Nagano-ken-hokubu, Japan, earthquake on the Itoigawa-Shizuoka Tectonic Line revealed with differential LiDAR

NASA Astrophysics Data System (ADS)

Toda, S.; Ishimura, D.; Homma, S.; Mukoyama, S.; Niwa, Y.

2015-12-01

The Mw = 6.2 Nagano-ken-hokubu earthquake struck northern Nagano, central Japan, on November 22, 2014, and accompanied a 9-km-long surface rupture mostly along the previously mapped N-NW trending Kamishiro fault, one of the segments of the 150-km-long Itoigawa-Shizuoka Tectonic Line active fault system. While we mapped the rupture and measured vertical displacement of up to 80 cm at the field, interferometric synthetic aperture radar (InSAR) shows densely spaced fringes on the hanging wall side, suggesting westward or uplift movement associated with thrust faulting. The mainshock focal mechanism and aftershock hypocenters indicate the source fault dips to the east but the InSAR images cannot exactly differentiate between horizontal and vertical movements and also lose coherence within and near the fault zone itself. To reveal near-field deformation and shallow fault slip, here we demonstrate a differential LiDAR analysis using a pair of 1 m-resolution pre-event and post-event bare Earth digital terrain models (DTMs) obtained from commercial LiDAR provider. We applied particle image velocity (PIV) method incorporating elevation change to obtain 3-D vectors of coseismic displacements (Mukoyama, 2011, J. Mt. Sci). Despite sporadic noises mostly due to local landslides, we detected up to 1.5 m net movement at the tip of the hanging wall, more than the field measurement of 80 cm. Our result implies that a 9-km-long rupture zone is not a single continuous fault but composed of two bow-shaped fault strands, suggesting a combination of shallow fault dip and modest amount (< 1.5 m) of slip. Eastward movement without notable subsidence on the footwall also supports the low angle fault dip near the surface, and significant fault normal contraction, observed as buckled cultural features across the fault zone. Secondary features, such as subsidiary back-thrust faults confirmed at the field, are also visible as a significant contrast of vector directions and slip amounts.

Effective stress, friction and deep crustal faulting

USGS Publications Warehouse

Beeler, N.M.; Hirth, Greg; Thomas, Amanda M.; Burgmann, Roland

2016-01-01

Studies of crustal faulting and rock friction invariably assume the effective normal stress that determines fault shear resistance during frictional sliding is the applied normal stress minus the pore pressure. Here we propose an expression for the effective stress coefficient αf at temperatures and stresses near the brittle-ductile transition (BDT) that depends on the percentage of solid-solid contact area across the fault. αf varies with depth and is only near 1 when the yield strength of asperity contacts greatly exceeds the applied normal stress. For a vertical strike-slip quartz fault zone at hydrostatic pore pressure and assuming 1 mm and 1 km shear zone widths for friction and ductile shear, respectively, the BDT is at ~13 km. αf near 1 is restricted to depths where the shear zone is narrow. Below the BDT αf = 0 is due to a dramatically decreased strain rate. Under these circumstances friction cannot be reactivated below the BDT by increasing the pore pressure alone and requires localization. If pore pressure increases and the fault localizes back to 1 mm, then brittle behavior can occur to a depth of around 35 km. The interdependencies among effective stress, contact-scale strain rate, and pore pressure allow estimates of the conditions necessary for deep low-frequency seismicity seen on the San Andreas near Parkfield and in some subduction zones. Among the implications are that shear in the region separating shallow earthquakes and deep low-frequency seismicity is distributed and that the deeper zone involves both elevated pore fluid pressure and localization.

Frictional behaviour and evolution of rough faults in limestone

NASA Astrophysics Data System (ADS)

Harbord, C. W. A.; Nielsen, S. B.; De Paola, N.; Holdsworth, R.

2017-12-01

Fault roughness is an important parameter which influences the frictional behaviour of seismically active faults, in particular the nucleation stage of earthquakes. Here we investigate frictional sliding and stability of roughened micritic limestone surfaces from the seismogenic layer in Northern-Central Apennines of Italy. Samples are roughened using #60, #220 and #400 grit and deformed in a direct shear configuration at conditions typical of the shallow upper crust (15-60 MPa normal stress). We perform velocity steps between 0.01-1 μm s-1 to obtain rate-and-state friction parameters a, b and L. At low normal stress conditions (30 MPa) and at displacements of <3-4mm there is a clear 2 state evolution of friction with two state parameters, b1 and b2, and accompanying critical slip distances L1 and L2 for all roughnesses. In some cases, on smooth faults (#400 grit), the short term evolution leads to silent slow instability which is modulated by the second state evolution. With increasing slip displacement (>2-4 mm) friction can be modelled with a single state parameter, b, as the short frictional evolution disappears. The longer term state evolution, b2, gives negative values of b, reminiscent of plastic creep experiments at high temperature, reaching a steady state at 3-4 mm displacement. Microstructural observations reveal shiny surfaces decorated by nanometric gouge particles with variable porosity. When normal stress is increased, rough faults (#60 grit) revert to a single state evolution with positive values of b, whilst smoother faults (#220 & #400 grit) retain a two state evolution with negative b2 values. These observations suggest that on carbonate hosted faults sliding may be controlled by plastic processes which can lead to slow stick-slip instability, which may be supressed by frictional wear and accompanying gouge build-up.

Tectonic context of moderate to large historical earthquakes in the Lesser Antilles and mechanical coupling with volcanoes

NASA Astrophysics Data System (ADS)

Feuillet, Nathalie; Beauducel, FrançOis; Tapponnier, Paul

2011-10-01

The oblique convergence between North American and Caribbean plates is accommodated in a bookshelf faulting manner by active, oblique-normal faults in the northern part of the Lesser Antilles arc. In the last 20 years, two M > 6 earthquakes occurred along a large, arc parallel, en echelon fault system, the 16 March 1985 in Redonda and 21 November 2004 in Les Saintes. A better understanding of active faulting in this region permit us to review the location and magnitude of historical earthquakes by using a regional seismic attenuation law. Several others moderate earthquakes may have occurred along the en echelon fault system implying a strong seismic hazard along the arc. These faults control the effusion of volcanic products and some earthquakes seem to be correlated in time with volcanic unrest. Shallow earthquakes on intraplate faults induced normal stress and pressure changes around neighboring volcano and may have triggered volcanic activity. The Redonda earthquake could have initiated the 1995 eruption of Montserrat's Soufrière Hills by compressing its plumbing system. Conversely, pressure changes under the volcano increased Coulomb stress changes and brought some faults closer to failure, promoting seismicity. We also discuss the magnitude of the largest 11 January 1839 and 8 February 1843 megathrust interplate earthquakes. We calculate that they have increased the stress on some overriding intraplate faults and the extensional strain beneath several volcanoes. This may explain an increase of volcanic and seismic activity in the second half of the 19th century culminating with the devastating, 1902 Mount Pelée eruption.

An L-band interferometric synthetic aperture radar study on the Ganos section of the north Anatolian fault zone between 2007 and 2011: Evidence for along strike segmentation and creep in a shallow fault patch.

PubMed

de Michele, Marcello; Ergintav, Semih; Aochi, Hideo; Raucoules, Daniel

2017-01-01

We utilize L-band interferometric synthetic aperture radar (InSAR) data in this study to retrieve a ground velocity map for the near field of the Ganos section of the north Anatolian fault (NAF) zone. The segmentation and creep distribution of this section, which last ruptured in 1912 to generate a moment magnitude (Mw)7.3 earthquake, remains incompletely understood. Because InSAR processing removes the mean orbital plane, we do not investigate large scale displacements due to regional tectonics in this study as these can be determined using global positioning system (GPS) data, instead concentrating on the close-to-the-fault displacement field. Our aim is to determine whether, or not, it is possible to retrieve robust near field velocity maps from stacking L-band interferograms, combining both single and dual polarization SAR data. In addition, we discuss whether a crustal velocity map can be used to complement GPS observations in an attempt to discriminate the present-day surface displacement of the Ganos fault (GF) across multiple segments. Finally, we characterize the spatial distribution of creep on shallow patches along multiple along-strike segments at shallow depths. Our results suggest the presence of fault segmentation along strike as well as creep on the shallow part of the fault (i.e. the existence of a shallow creeping patch) or the presence of a smoother section on the fault plane. Data imply a heterogeneous fault plane with more complex mechanics than previously thought. Because this study improves our knowledge of the mechanisms underlying the GF, our results have implications for local seismic hazard assessment.

An L-band interferometric synthetic aperture radar study on the Ganos section of the north Anatolian fault zone between 2007 and 2011: Evidence for along strike segmentation and creep in a shallow fault patch

PubMed Central

Ergintav, Semih; Aochi, Hideo; Raucoules, Daniel

2017-01-01

We utilize L-band interferometric synthetic aperture radar (InSAR) data in this study to retrieve a ground velocity map for the near field of the Ganos section of the north Anatolian fault (NAF) zone. The segmentation and creep distribution of this section, which last ruptured in 1912 to generate a moment magnitude (Mw)7.3 earthquake, remains incompletely understood. Because InSAR processing removes the mean orbital plane, we do not investigate large scale displacements due to regional tectonics in this study as these can be determined using global positioning system (GPS) data, instead concentrating on the close-to-the-fault displacement field. Our aim is to determine whether, or not, it is possible to retrieve robust near field velocity maps from stacking L-band interferograms, combining both single and dual polarization SAR data. In addition, we discuss whether a crustal velocity map can be used to complement GPS observations in an attempt to discriminate the present-day surface displacement of the Ganos fault (GF) across multiple segments. Finally, we characterize the spatial distribution of creep on shallow patches along multiple along-strike segments at shallow depths. Our results suggest the presence of fault segmentation along strike as well as creep on the shallow part of the fault (i.e. the existence of a shallow creeping patch) or the presence of a smoother section on the fault plane. Data imply a heterogeneous fault plane with more complex mechanics than previously thought. Because this study improves our knowledge of the mechanisms underlying the GF, our results have implications for local seismic hazard assessment. PMID:28961264

Characterising the range of seismogenic behaviour on detachment faults - the case of 13o20'N, Mid Atlantic Ridge.

NASA Astrophysics Data System (ADS)

Craig, T. J.; Parnell-Turner, R.

2017-12-01

Extension at slow- and intermediate-spreading mid-ocean ridges is commonly accommodated through slip on long-lived detachment faults. These curved, convex-upward faults consist of a steeply-dipping section thought to be rooted in the lower crust or upper mantle which rotates to progressively shallower dip-angles at shallower depths, resulting in a domed, sub-horizontal oceanic core complex at the seabed. Although it is accepted that detachment faults can accumulate kilometre-scale offsets over millions of years, the mechanism of slip, and their capacity to sustain the shear stresses necessary to produce large earthquakes, remains debated. In this presentation we will show a comprehensive seismological study of an active oceanic detachment fault system on the Mid-Atlantic Ridge near 13o20'N, combining the results from a local ocean-bottom seismograph deployment with waveform inversion of a series of larger, teleseismically-observed earthquakes. The coincidence of these two datasets provides a more complete characterisation of rupture on the fault, from its initial beginnings within the uppermost mantle to its exposure at the surface. Our results demonstrate that although slip on the steeply-dipping portion of detachment fault is accommodated by failure in numerous microearthquakes, the shallower-dipping section of the fault within the upper few kilometres is relatively strong, and is capable of producing large-magnitude earthquakes. Slip on the shallow portion of active detachment faults at relatively low angles may therefore account for many more large-magnitude earthquakes at mid-ocean ridges than previously thought, and suggests that the lithospheric strength at slow-spreading mid-ocean ridges may be concentrated at shallow depths.

InSAR Evidence for an active shallow thrust fault beneath the city of Spokane Washington, USA

USGS Publications Warehouse

Wicks, Charles W.; Weaver, Craig S.; Bodin, Paul; Sherrod, Brian

2013-01-01

In 2001, a nearly five month long sequence of shallow, mostly small magnitude earthquakes occurred beneath the city of Spokane, a city with a population of about 200,000, in the state of Washington. During most of the sequence, the earthquakes were not well located because seismic instrumentation was sparse. Despite poor-quality locations, the earthquake hypocenters were likely very shallow, because residents near the city center both heard and felt many of the earthquakes. The combination of poor earthquake locations and a lack of known surface faults with recent movement make assessing the seismic hazards related to the earthquake swarm difficult. However, the potential for destruction from a shallow moderate-sized earthquake is high, for example Christchurch New Zealand in 2011, so assessing the hazard potential of a seismic structure involved in the Spokane earthquake sequence is important. Using interferometric synthetic aperture radar (InSAR) data from the European Space Agency ERS2 and ENVISAT satellites and the Canadian Space Agency RADARSAT-1, satellite we are able to show that slip on a shallow previously unknown thrust fault, which we name the Spokane Fault, is the source of the earthquake sequence. The part of the Spokane Fault that slipped during the 2001 earthquake sequence underlies the north part of the city, and slip on the fault was concentrated between ~0.3 and 2 km depth. Projecting the buried fault plane to the surface gives a possible surface trace for the Spokane Fault that strikes northeast from the city center into north Spokane.

Structural changes and shallow geological structure of the isolated basins in the forearc slope of the Japan Trench

NASA Astrophysics Data System (ADS)

Misawa, A.; Arai, K.; Fujiwara, T.; Sato, M.; Shin'ichiro, Y.; Hirata, K.; Kanamatsu, T.

2017-12-01

On the forearc slope of the Japan Trench is a typical subsidence region associated with the subduction erosion in the Japan Trench. Arai et al. (2014) reported the existence of the isolated basins with widths of up to several tens of kilometers using the seismic profiles that acquired before the 2011 Tohoku earthquake (Mw 9.0) in the forearc slope. The isolated basin probably formed due to subsidence accompanying the regional activity of normal fault systems in the forearc slope. Arai et al. (2014) suggested that the geological structures of the forearc slope along the Japan Trench are typical of those resulting from subduction erosion and proposed that the episodic subsidence accompanied by normal faulting is the most recent deformation. During the 2011 large earthquake, seafloor on the landward slope of the Japan Trench moved 50 m east-southeast toward trench (Fujiwara et al., 2011). In addition, aftershock activity after the 2011 large earthquake have predominated in the activity of the normal fault system. Therefore, there have a possibility that new isolated basin is formed after the 2011 large earthquake in the forearc slope of the Japan Trench. In order to capture the structural change in the isolated basins, we compared the seismic profiles acquired before (Multi-Channel Seismic (MCS) data acquired with KR07-05 cruise) and after (Single-Channel Seismic (SCS) data acquired with NT15-07 cruise) the 2011 large earthquake. However, the large-scale structural changes are not identified around the isolated basin. In order to capture the small-scale structural change in the shallow part of the isolated basins using high-resolution data, we make an attempt at the marine geological and geophysical survey in the offshore Tohoku region using R/V Shinsei-Maru of JAMSTEC (KS-17-8 cruise) in August 2017. In this cruise, we plan to carry out the following surveys; (1) swath bathymetric survey, (2) high-resolution parametric subbottom profiler (SBP) survey, (3) geomagnetic survey. In this presentation, we will show the latest results about the shallow structure of the isolated basin in the forearc slope.

Impact of tectonic and volcanism on the Neogene evolution of isolated carbonate platforms (SW Indian Ocean)

NASA Astrophysics Data System (ADS)

Courgeon, S.; Jorry, S. J.; Jouet, G.; Camoin, G.; BouDagher-Fadel, M. K.; Bachèlery, P.; Caline, B.; Boichard, R.; Révillon, S.; Thomas, Y.; Thereau, E.; Guérin, C.

2017-06-01

Understanding the impact of tectonic activity and volcanism on long-term (i.e. millions years) evolution of shallow-water carbonate platforms represents a major issue for both industrial and academic perspectives. The southern central Mozambique Channel is characterized by a 100 km-long volcanic ridge hosting two guyots (the Hall and Jaguar banks) and a modern atoll (Bassas da India) fringed by a large terrace. Dredge sampling, geophysical acquisitions and submarines videos carried out during recent oceanographic cruises revealed that submarine flat-top seamounts correspond to karstified and drowned shallow-water carbonate platforms largely covered by volcanic material and structured by a dense network of normal faults. Microfacies and well-constrained stratigraphic data indicate that these carbonate platforms developed in shallow-water tropical environments during Miocene times and were characterized by biological assemblages dominated by corals, larger benthic foraminifera, red and green algae. The drowning of these isolated carbonate platforms is revealed by the deposition of outer shelf sediments during the Early Pliocene and seems closely linked to (1) volcanic activity typified by the establishment of wide lava flow complexes, and (2) to extensional tectonic deformation associated with high-offset normal faults dividing the flat-top seamounts into distinctive structural blocks. Explosive volcanic activity also affected platform carbonates and was responsible for the formation of crater(s) and the deposition of tuff layers including carbonate fragments. Shallow-water carbonate sedimentation resumed during Late Neogene time with the colonization of topographic highs inherited from tectonic deformation and volcanic accretion. Latest carbonate developments ultimately led to the formation of the Bassas da India modern atoll. The geological history of isolated carbonate platforms from the southern Mozambique Channel represents a new case illustrating the major impact of tectonic and volcanic activity on the long-term evolution of shallow-water carbonate platforms.

Magnetic Fabric Associated with Faulting of Poorly Consolidated Basin Sediments of the Rio Grande Rift, New Mexico, USA

NASA Astrophysics Data System (ADS)

Hudson, M. R.; Minor, S. A.; Caine, J. S.

2015-12-01

Permanent strain in sediments associated with shallow fault zones can be difficult to characterize. Anisotropy of magnetic susceptibility (AMS) data were obtained from 120 samples at 6 sites to assess the nature of fault-related AMS fabrics for 4 faults cutting Miocene-Pliocene basin fill sediments of the Rio Grande rift of north-central New Mexico. The San Ysidro (3 sites), Sand Hill, and West Paradise faults within the northern Albuquerque basin have normal offset whereas an unnamed fault near Buckman in the western Española basin has oblique strike-slip offset. Previous studies have shown that detrital magnetite controls magnetic susceptibility in rift sandstones, and in a 50-m-long hanging wall traverse of the San Ysidro fault, non-gouge samples have typical sedimentary AMS fabrics with Kmax and Kint axes (defining magnetic foliation) scattered within bedding. For the 5 normal-fault sites, samples from fault cores or adjacent mixed zones that lie within 1 m of the principal slip surface developed common deformation fabrics with (1) magnetic foliation inclined in the same azimuth but more shallowly dipping than the fault plane, and (2) magnetic lineation plunging down foliation dip with nearly the same trend as the fault striae, although nearer for sand versus clay gouge samples. These relations suggest that the sampled fault materials deformed by particulate flow with alignment of magnetite grains in the plane of maximum shortening. For a 2-m-long traverse at the Buckman site, horizontal sedimentary AMS foliation persists to < 15 cm to the fault slip surface, wherein foliation in sand and clay gouge rotates toward the steeply dipping fault plane in a sense consistent with sinistral offset. Collectively these data suggest permanent deformation fabrics were localized within < 1 m of fault surfaces and that AMS fabrics from gouge samples can provide kinematic information for faults in unconsolidated sediments which may lack associated slickenlines.

The Olmsted fault zone, southernmost Illinois: A key to understanding seismic hazard in the northern new Madrid seismic zone

USGS Publications Warehouse

Bexfield, C.E.; McBride, J.H.; Pugin, Andre J.M.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.

2005-01-01

Geological deformation in the northern New Madrid seismic zone, near Olmsted, Illinois (USA), is analyzed using integrated compressional-wave (P) and horizontally polarized-wave (SH) seismic reflection and regional and dedicated borehole information. Seismic hazards are of special concern because of strategic facilities (e.g., lock and dam sites and chemical plants on the Ohio River near its confluence with the Mississippi River) and because of alluvial soils subject to high amplification of earthquake shock. We use an integrated approach starting with lower resolution, but deeper penetration, P-wave reflection profiles to identify displacement of Paleozoic bedrock. Higher resolution, but shallower penetration, SH-wave images show deformation that has propagated upward from bedrock faults into Pleistocene loess. We have mapped an intricate zone more than 8 km wide of high-angle faults in Mississippi embayment sediments localized over Paleozoic bedrock faults that trend north to northeast, parallel to the Ohio River. These faults align with the pattern of epicenters in the New Madrid seismic zone. Normal and reverse offsets along with positive flower structures imply a component of strike-slip; the current stress regime favors right-lateral slip on northeast-trending faults. The largest fault, the Olmsted fault, underwent principal displacement near the end of the Cretaceous Period 65 to 70 million years ago. Strata of this age (dated via fossil pollen) thicken greatly on the downthrown side of the Olmsted fault into a locally subsiding basin. Small offsets of Tertiary and Quaternary strata are evident on high-resolution SH-wave seismic profiles. Our results imply recent reactivation and possible future seismic activity in a critical area of the New Madrid seismic zone. This integrated approach provides a strategy for evaluating shallow seismic hazard-related targets for engineering concerns. ?? 2005 Elsevier B.V. All rights reserved.

Splay fault slip in a subduction margin, a new model of evolution

NASA Astrophysics Data System (ADS)

Conin, Marianne; Henry, Pierre; Godard, Vincent; Bourlange, Sylvain

2012-08-01

In subduction zones, major thrusts called splay faults are thought to slip coseismically during large earthquakes affecting the main plate interface. We propose an analytical condition for the activation of a splay fault based on force balance calculations and suggest thrusting along the splay fault is generally conditioned by the growth of the accretionary wedge, or by the erosion of the hanging wall. In theory, normal slip on the splay fault may occur when the décollement has a very low friction coefficient seaward. Such a low friction also implies an unstable extensional state within the outer wedge. Finite element elasto-plastic calculations with a geometry based on the Nankai Kumano section were performed and confirm that this analytical condition is a valid approximation. Furthermore, localized extension at a shallow level in the splay hanging wall is observed in models for a wide range of friction coefficients (from ∼0 to the value of internal friction coefficient of the rock, here equals to 0.4). The timing of slip established for the splay fault branch drilled on Nankai Kumano transect suggests a phase of concurrent splay and accretionary wedge growth ≈2 Ma to ≈1.5 Ma, followed by a locking of the splay ≈1.3 Ma. Active extension is observed in the hanging wall. This evolution can be explained by the activation of a deeper and weaker décollement, followed by an interruption of accretion. Activation of a splay as a normal fault, as hypothesized in the case of the Tohoku 2011 earthquake, can be achieved only if the friction coefficient on the décollement drops to near zero. We conclude that the tectonic stress state largely determines long-term variations of tightly related splay fault and outer décollement activity and thus influences where and how coseismic rupture ends, but that occurrence of normal slip on a splay fault requires coseismic friction reduction.

The Bear River Fault Zone, Wyoming and Utah: Complex Ruptures on a Young Normal Fault

NASA Astrophysics Data System (ADS)

Schwartz, D. P.; Hecker, S.; Haproff, P.; Beukelman, G.; Erickson, B.

2012-12-01

The Bear River fault zone (BRFZ), a set of normal fault scarps located in the Rocky Mountains at the eastern margin of Basin and Range extension, is a rare example of a nascent surface-rupturing fault. Paleoseismic investigations (West, 1994; this study) indicate that the entire neotectonic history of the BRFZ may consist of two large surface-faulting events in the late Holocene. We have estimated a maximum per-event vertical displacement of 6-6.5 m at the south end of the fault where it abuts the north flank of the east-west-trending Uinta Mountains. However, large hanging-wall depressions resulting from back rotation, which front scarps that locally exceed 15 m in height, are prevalent along the main trace, obscuring the net displacement and its along-strike distribution. The modest length (~35 km) of the BRFZ indicates ruptures with a large displacement-to-length ratio, which implies earthquakes with a high static stress drop. The BRFZ is one of several immature (low cumulative displacement) normal faults in the Rocky Mountain region that appear to produce high-stress drop earthquakes. West (1992) interpreted the BRFZ as an extensionally reactivated ramp of the late Cretaceous-early Tertiary Hogsback thrust. LiDAR data on the southern section of the fault and Google Earth imagery show that these young ruptures are more extensive than currently mapped, with newly identified large (>10m) antithetic scarps and footwall graben. The scarps of the BRFZ extend across a 2.5-5.0 km-wide zone, making this the widest and most complex Holocene surface rupture in the Intermountain West. The broad distribution of Late Holocene scarps is consistent with reactivation of shallow bedrock structures but the overall geometry of the BRFZ at depth and its extent into the seismogenic zone are uncertain.

Radon concentration distributions in shallow and deep groundwater around the Tachikawa fault zone.

PubMed

Tsunomori, Fumiaki; Shimodate, Tomoya; Ide, Tomoki; Tanaka, Hidemi

2017-06-01

Groundwater radon concentrations around the Tachikawa fault zone were surveyed. The radon concentrations in shallow groundwater samples around the Tachikawa fault segment are comparable to previous studies. The characteristics of the radon concentrations on both sides of the segment are considered to have changed in response to the decrease in groundwater recharge caused by urbanization on the eastern side of the segment. The radon concentrations in deep groundwater samples collected around the Naguri and the Tachikawa fault segments are the same as those of shallow groundwater samples. However, the radon concentrations in deep groundwater samples collected from the bedrock beside the Naguri and Tachikawa fault segments are markedly higher than the radon concentrations expected from the geology on the Kanto plane. This disparity can be explained by the development of fracture zones spreading on both sides of the two segments. The radon concentration distribution for deep groundwater samples from the Naguri and the Tachikawa fault segments suggests that a fault exists even at the southern part of the Tachikawa fault line. Copyright © 2017 Elsevier Ltd. All rights reserved.

Extensional tectonics, graben development and fault terminations in the eastern Rif (Bokoya-Ras Afraou area)

NASA Astrophysics Data System (ADS)

Galindo-Zaldívar, Jesús; Azzouz, Omar; Chalouan, Ahmed; Pedrera, Antonio; Ruano, Patricia; Ruiz-Constán, Ana; Sanz de Galdeano, Carlos; Marín-Lechado, Carlos; López-Garrido, Angel Carlos; Anahnah, Farida; Benmakhlouf, Mohamed

2015-11-01

Westward motion of the Alboran Domain between the Eurasian and African plate boundaries determined crustal thickening along the southern border of the Gibraltar Arc, forming the Rif Cordillera. This process developed major sinistral NE-SW to ENE-WSW faults (such as the Nekor Fault), inactive since the Late Miocene. However, the Neogene-Quaternary Boudinar and Nekor basins underwent very intense recent tectonic and seismic activity related to N-S faults. Kinematics of this fault set changes with depth. While at ~ 10 km faults have a sinistral strike-slip kinematics, they become normal to normal-oblique at surface (Sfeha, Trougout and Boudinar faults). Their different kinematics could be explained by the existence of a crustal detachment separating two differently pre-structured domains. Shallow transtensive N-S faults trend orthogonal to the coastline, decreasing their slip southwards until disappearing. Paleostress analysis shows a progressive change from E-W extension near the coastline up to radial extension in southern areas of major fault terminations. The behavior of each fault-bounded block is conditioned by its inherited rheological features. The sequence of horsts (Bokoya, Ras Tarf, Ras Afraou) corresponds mainly to resistant rocks (volcanics or limestones), whereas the grabens (Nekor and Boudinar basins) are generally floored by weak metapelites and flysch. The presence of liquefaction structures, interpreted as seismites, underlines the continued recent seismic activity of the region. The recent structures deforming the two Alboran Sea margins come to support the continuity, at present, of orogenic processes undergone by the eastern internal regions of the Gibraltar Arc, involving regional E-W extension in the framework of NW-SE to N-S Eurasian-African convergence.

Estimating Stresses, Fault Friction and Fluid Pressure from Topography and Coseismic Slip Models

NASA Astrophysics Data System (ADS)

Styron, R. H.; Hetland, E. A.

2014-12-01

Stress is a first-order control on the deformation state of the earth. However, stress is notoriously hard to measure, and researchers typically only estimate the directions and relative magnitudes of principal stresses, with little quantification of the uncertainties or absolute magnitude. To improve upon this, we have developed methods to constrain the full stress tensor field in a region surrounding a fault, including tectonic, topographic, and lithostatic components, as well as static friction and pore fluid pressure on the fault. Our methods are based on elastic halfspace techniques for estimating topographic stresses from a DEM, and we use a Bayesian approach to estimate accumulated tectonic stress, fluid pressure, and friction from fault geometry and slip rake, assuming Mohr-Coulomb fault mechanics. The nature of the tectonic stress inversion is such that either the stress maximum or minimum is better constrained, depending on the topography and fault deformation style. Our results from the 2008 Wenchuan event yield shear stresses from topography up to 20 MPa (normal-sinistral shear sense) and topographic normal stresses up to 80 MPa on the faults; tectonic stress had to be large enough to overcome topography to produce the observed reverse-dextral slip. Maximum tectonic stress is constrained to be >0.3 * lithostatic stress (depth-increasing), with a most likely value around 0.8, trending 90-110°E. Minimum tectonic stress is about half of maximum. Static fault friction is constrained at 0.1-0.4, and fluid pressure at 0-0.6 * total pressure on the fault. Additionally, the patterns of topographic stress and slip suggest that topographic normal stress may limit fault slip once failure has occurred. Preliminary results from the 2013 Balochistan earthquake are similar, but yield stronger constraints on the upper limits of maximum tectonic stress, as well as tight constraints on the magnitude of minimum tectonic stress and stress orientation. Work in progress on the Wasatch fault suggests that maximum tectonic stress may also be able to be constrained, and that some of the shallow rupture segmentation may be due in part to localized topographic loading. Future directions of this work include regions where high relief influences fault kinematics (such as Tibet).

High resolution shallow co-seismic and post-seismic slip from the 2016 central Italy earthquake sequence captured using terrestrial laser scanning, structure from motion and low-cost near-field GNSS

NASA Astrophysics Data System (ADS)

Wedmore, L. N. J.; Gregory, L. C.; McCaffrey, K. J. W.; Wilkinson, M.; Walters, R. J.

2017-12-01

Coseismic fault slip in the shallow crust is poorly constrained by many of the conventional tools used to record deformation during earthquakes. GNSS stations are often distributed too far from faults and radar images tend to decorrelate across earthquake surface ruptures. As a result, our understanding of near-field fault slip, shallow slip deficits, and off-fault deformation is limited. We present evidence from the 2016 central Italy earthquake sequence, during which we captured shallow coseismic and post-seismic slip using a combination of terrestrial laser scanning (TLS), structure-from-motion (SfM), and near-field low-cost GNSS recording at 1Hz. Three Mw>6 earthquakes on the 24th August, 26th and 30th October all involved slip on the Mt Vettore-Mt Bove fault system. We collected TLS and SfM point clouds across three separate segments of this system. Each segment experienced a different record of slip during the earthquake sequence; all three ruptured in the largest event (Mw 6.6. on October 30th) but two segments also ruptured during either the 24th August or the 26th October earthquakes. Following the Mw 6.6 earthquake, the faults were repeatedly surveyed using TLS, with the first scan collected c. 5 hours following the earthquake. This represents the first known instance where shallow co-seismic slip has been recorded by pre- and post-event terrestrial laser scanning. Displacement continuously measured across GNSS pairs at 1 Hz demonstrates that permanent near field displacement developed across the fault in the immediate seconds following the initiation of the rupture. However, a discrepancy between on-fault field measurements of surface displacement and the GNSS recorded displacement over 1km long baselines hints at a more complex rupture processes and the possibility of high slip gradients in the shallow subsurface. Displacement measured by differential TLS confirms the presence of these shallow slip deficits but suggests that shallow slip gradient may be controlled by the pattern and timing of slip in the preceding earthquakes. Postseismic afterslip captured by repeated TLS surveys hints at more complicated temporal evolution of nearfield afterslip than is currently predicted by logarithmic models for this process.

Field and experimental evidence for coseismic ruptures along shallow creeping faults in forearc sediments of the Crotone Basin, South Italy

NASA Astrophysics Data System (ADS)

Balsamo, Fabrizio; Aldega, Luca; De Paola, Nicola; Faoro, Igor; Storti, Fabrizio

2014-05-01

Large seismic slip occurring along shallow creeping faults in tectonically active areas represents an unsolved paradox, which is largely due to our poor understanding of the mechanics governing creeping faults, and to the lack of documented geological evidence showing how coseismic rupturing overprints creep in near-surface conditions. In this contribution we integrate field, petrophysical, mineralogical and friction data to characterize the signature of coseismic ruptures propagating along shallow creeping faults affecting unconsolidated forearc sediments of the seismically active Crotone Basin, in South Italy. Field observations of fault zones show widespread foliated cataclasites in fault cores, locally overprinted by sharp slip surfaces decorated by thin (0.5-1.5 cm) black gouge layers. Compared to foliated cataclasites, black gouges have much lower grain size, porosity and permeability, which may have facilitated slip weakening by thermal fluid pressurization. Moreover, black gouges are characterized by distinct mineralogical assemblages compatible with high temperatures (180-200°C) due to frictional heating during seismic slip. Foliated cataclasites and black gouges were also produced by laboratory friction experiments performed on host sediments at sub-seismic (≤ 0.1 m/s) and seismic (1 m/s) slip rates, respectively. Black gouges display low friction coefficients (0.3) and velocity-weakening behaviours, as opposed to high friction coefficients (0.65) and velocity-strengthening behaviours shown by the foliated cataclasites. Our results show that narrow black gouges developed within foliated cataclasites represent a potential diagnostic marker for episodic seismic activity in shallow creeping faults. These findings can help understanding the time-space partitioning between aseismic and seismic slip of faults at shallow crustal levels, impacting on seismic hazard evaluation of subduction zones and forearc regions affected by destructive earthquakes and tsunamis.

A shallow fault-zone structure illuminated by trapped waves in the Karadere-Duzce branch of the North Anatolian Fault, western Turkey

USGS Publications Warehouse

Ben-Zion, Y.; Peng, Z.; Okaya, D.; Seeber, L.; Armbruster, J.G.; Ozer, N.; Michael, A.J.; Baris, S.; Aktar, M.

2003-01-01

We discuss the subsurface structure of the Karadere-Duzce branch of the North Anatolian Fault based on analysis of a large seismic data set recorded by a local PASSCAL network in the 6 months following the Mw = 7.4 1999 Izmit earthquake. Seismograms observed at stations located in the immediate vicinity of the rupture zone show motion amplification and long-period oscillations in both P- and S-wave trains that do not exist in nearby off-fault stations. Examination of thousands of waveforms reveals that these characteristics are commonly generated by events that are well outside the fault zone. The anomalous features in fault-zone seismograms produced by events not necessarily in the fault may be referred to generally as fault-zone-related site effects. The oscillatory shear wave trains after the direct S arrival in these seismograms are analysed as trapped waves propagating in a low-velocity fault-zone layer. The time difference between the S arrival and trapped waves group does not grow systematically with increasing source-receiver separation along the fault. These observations imply that the trapping of seismic energy in the Karadere-Duzce rupture zone is generated by a shallow fault-zone layer. Traveltime analysis and synthetic waveform modelling indicate that the depth of the trapping structure is approximately 3-4 km. The synthetic waveform modelling indicates further that the shallow trapping structure has effective waveguide properties consisting of thickness of the order of 100 m, a velocity decrease relative to the surrounding rock of approximately 50 per cent and an S-wave quality factor of 10-15. The results are supported by large 2-D and 3-D parameter space studies and are compatible with recent analyses of trapped waves in a number of other faults and rupture zones. The inferred shallow trapping structure is likely to be a common structural element of fault zones and may correspond to the top part of a flower-type structure. The motion amplification associated with fault-zone-related site effects increases the seismic shaking hazard near fault-zone structures. The effect may be significant since the volume of sources capable of generating motion amplification in shallow trapping structures is large.

High-angle faults control the geometry and morphology of the Corinth Rift

NASA Astrophysics Data System (ADS)

Bell, R. E.; Duclaux, G.; Nixon, C.; Gawthorpe, R.; McNeill, L. C.

2016-12-01

Slip along low-angle normal faults is mechanically difficult, and the existence of low angle detachment faults presents one of most important paradoxes in structural geology. Only a few examples of young continental rifts where low-angle faults may be a mechanism for accommodating strain have been described in the literature, and an important example is the Gulf of Corinth, central Greece. Here, microseismicity, the geometry of onshore faults and deep seismic reflection images have been used to argue for the presence of <30o dipping faults. However, new and reinterpreted data calls into question whether low-angle faults have been influential in controlling rift geometry. We seek to definitively test whether slip on a mature low-angle normal fault can reproduce the long-term geometry and morphology of the Corinth Rift, which involves i) significant uplift of the southern margin, ii) long-term uplift to subsidence ratios across south coast faults of 1 -2, and iii) a northern margin that does not undergo significant long-term uplift. We use PyLith, an open-source finite-element code for quasi-static viscoelastic simulations of crustal deformation and model the uplift and subsidence fields associated with the following fault geometries: i) planar faults with dips of 45-60° that sole onto a 10° detachment at a depth of 6 to 8 km, ii) 45-60° faults, which change to a dip angle of 25-45° at a depth of 3 km and continue to a brittle-ductile transition at 10 km and iii) planar faults which dip 45-60° to the brittle-ductile transition at a depth of 10 km. We show that models involving low-angle detachments, shallower than 8 km produce very minor coseismic uplift of the southern margin and post-seismic relaxation results in the southern margin experiencing net subsidence over many seismic cycles, incompatible with geological observations. Models involving planar faults produce long-term displacement fields involving uplifted southern margin with uplift to subsidence ratios of c. 1:2 and subsidence of the northern margin, compatible with geological observations. We propose that low-angle detachment faults cannot have controlled the long-term geometry of the Corinth rift, and that the rift should no longer be used as an example of low-angle normal faulting.

Stress shadow prohibits low interseismic coupling on shallow megathrusts, even where they are frictionally unlocked

NASA Astrophysics Data System (ADS)

Lindsey, E. O.; Almeida, R. V.; Bradley, K. E.; Hubbard, J.; Sathiakumar, S.; Mallick, R.; Barbot, S.; Hill, E.

2017-12-01

The possibility of large coseismic slip on the shallow part of megathrusts represents a significant hazard, due to both tsunami risk (for subduction zones) and strong seismic shaking hazard (for subaerial megathrusts). Laboratory experiments have long suggested that the shallow part of these faults may be frictionally unlocked. However, evidence for the predicted shallow creep remains scarce, and in the case of more accessible subaerial megathrusts such as the Himalaya, appears to be virtually absent. We propose that the lack of shallow creep can be due to the presence of a stress shadow from the down-dip locked portion of the fault and therefore that interseismic geodetic observations (even at the seafloor) are typically insufficient to constrain the frictional locking on the shallow portion of the fault. We use a boundary element model with rate-strengthening friction up-dip of a locked fault segment and find that a coupling value of zero at the trench is physically unrealistic even if only a small portion of the downdip fault zone is locked. For a typical subduction zone fault geometry which is frictionally unlocked from the surface to half the maximum seismogenic depth (considered an extreme case) we find that a coupling ratio of less than 0.7 at the trench is prohibited under most conditions. For a narrower zone of up-dip unlocking, the coupling ratio should be even higher. This result highlights the difference between frictional locking (a mechanical parameter) and geodetic coupling (a kinematic parameter), and indicates that published models of coupling that assume or predict low coupling at the trench need to be reevaluated. To assess how the up-dip slip deficit is ultimately released in the case of a frictionally unlocked (but geodetically coupled) zone, we conduct a series of earthquake cycle models based on rate- and state-dependent friction in two and three dimensions. The results show that strain in the shallow part of the fault is typically released as coseismic and postseismic slip, except under conditions favoring nucleation of shallow earthquakes or SSEs. During the interseismic period, our results suggest that seafloor geodetic instruments with an uncertainty of cm/yr may not have the resolution to distinguish shallow frictional locking (with zero slip) from unlocking (with slip <30% of plate rate).

The 2006-2007 Kuril Islands great earthquake sequence

USGS Publications Warehouse

Lay, T.; Kanamori, H.; Ammon, C.J.; Hutko, Alexander R.; Furlong, K.; Rivera, L.

2009-01-01

The southwestern half of a ???500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an MS 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an MW = 8.3 thrust event on 15 November 2006. This event ruptured ???250 km along the seismic gap, just northeast of the great 1963 Kuril Island (Mw = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal-faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an MW = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal-faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short-period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (Mw = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention. Copyright 2009 by the American Geophysical Union.

Earthquake scaling laws for rupture geometry and slip heterogeneity

NASA Astrophysics Data System (ADS)

Thingbaijam, Kiran K. S.; Mai, P. Martin; Goda, Katsuichiro

2016-04-01

We analyze an extensive compilation of finite-fault rupture models to investigate earthquake scaling of source geometry and slip heterogeneity to derive new relationships for seismic and tsunami hazard assessment. Our dataset comprises 158 earthquakes with a total of 316 rupture models selected from the SRCMOD database (http://equake-rc.info/srcmod). We find that fault-length does not saturate with earthquake magnitude, while fault-width reveals inhibited growth due to the finite seismogenic thickness. For strike-slip earthquakes, fault-length grows more rapidly with increasing magnitude compared to events of other faulting types. Interestingly, our derived relationship falls between the L-model and W-model end-members. In contrast, both reverse and normal dip-slip events are more consistent with self-similar scaling of fault-length. However, fault-width scaling relationships for large strike-slip and normal dip-slip events, occurring on steeply dipping faults (δ~90° for strike-slip faults, and δ~60° for normal faults), deviate from self-similarity. Although reverse dip-slip events in general show self-similar scaling, the restricted growth of down-dip fault extent (with upper limit of ~200 km) can be seen for mega-thrust subduction events (M~9.0). Despite this fact, for a given earthquake magnitude, subduction reverse dip-slip events occupy relatively larger rupture area, compared to shallow crustal events. In addition, we characterize slip heterogeneity in terms of its probability distribution and spatial correlation structure to develop a complete stochastic random-field characterization of earthquake slip. We find that truncated exponential law best describes the probability distribution of slip, with observable scale parameters determined by the average and maximum slip. Applying Box-Cox transformation to slip distributions (to create quasi-normal distributed data) supports cube-root transformation, which also implies distinctive non-Gaussian slip distributions. To further characterize the spatial correlations of slip heterogeneity, we analyze the power spectral decay of slip applying the 2-D von Karman auto-correlation function (parameterized by the Hurst exponent, H, and correlation lengths along strike and down-slip). The Hurst exponent is scale invariant, H = 0.83 (± 0.12), while the correlation lengths scale with source dimensions (seismic moment), thus implying characteristic physical scales of earthquake ruptures. Our self-consistent scaling relationships allow constraining the generation of slip-heterogeneity scenarios for physics-based ground-motion and tsunami simulations.

Geophysical characterization of transtensional fault systems in the Eastern California Shear Zone-Walker Lane Belt

NASA Astrophysics Data System (ADS)

McGuire, M.; Keranen, K. M.; Stockli, D. F.; Feldman, J. D.; Keller, G. R.

2011-12-01

The Eastern California Shear Zone (ECSZ) and Walker Lane belt (WL) accommodate ~25% of plate motion between the North American and Pacific plates. Faults within the Mina deflection link the ECSZ and the WL, transferring strain from the Owens Valley and Death Valley-Fish Lake Valley fault systems to the transcurrent faults of the central Walker Lane. During the mid to late Miocene the majority of strain between these systems was transferred through the Silver Peak-Lone Mountain (SPLM) extensional complex via a shallowly dipping detachment. Strain transfer has since primarily migrated north to the Mina Deflection; however, high-angle faults bounding sedimentary basins and discrepancies between geodetic and geologic models indicate that the SPLM complex may still actively transfer a portion of the strain from the ECSZ to the WL on a younger set of faults. Establishing the pattern and amount of active strain transfer within the SPLM region is required for a full accounting of strain accommodation, and provides insight into strain partitioning at the basin scale within a broader transtensional zone. To map the active structures in and near Clayton Valley, within the SPLM region, we collected seismic reflection and refraction profiles and a dense grid of gravity readings that were merged with existing gravity data. The primary goals were to determine the geometry of the high-angle fault system, the amount and sense of offset along each fault set, connectivity of the faults, and the relationship of these faults to the Miocene detachment. Seismic reflection profiles imaged the high-angle basin-bounding normal faults and the detachment in both the footwall and hanging wall. The extensional basin is ~1 km deep, with a steep southeastern boundary, a gentle slope to the northwest, and a sharp boundary on the northwest side, suggestive of another fault system. Two subparallel dip-slip faults bound the southeast (deeper) basin margin with a large lateral velocity change (from ~2.0 km/sec in the basin fill to 4.5-5.5 km/sec in the footwall) across the basin-bounding normal fault system. Very fast (approaching 6.0 km/sec) basement underlies the basin fill. The residual gravity anomaly indicates that Clayton Valley is divided into a shallower northern basin, imaged by the seismic lines, and a deeper, more asymmetric southern basin. Faults within Clayton Valley are curvilinear in nature, similar to faults observed in other step-over systems (e.g., the Mina Deflection). Gravity profiles support the seismic reflection interpretation and indicate a high angle fault (>60 degrees) bounding the northern sub-basin on its southeast margin, with a shallower fault bounding it to the northwest. A basement high trends west-northwest and separates the northern and southern basins, and is likely bounded on its southern edge by a predominantly strike-slip fault crossing the valley. Much of the strain accommodated within the southern sub-basin appears to be transferred into southern Big Smoky Valley, northwest of Clayton Valley, via these dextral strike-slip faults that obliquely cross Clayton Valley.

Regional pore-fluid pressures in the active western Taiwan thrust belt: A test of the classic Hubbert-Rubey fault-weakening hypothesis

NASA Astrophysics Data System (ADS)

Yue, Li-Fan; Suppe, John

2014-12-01

We document regional pore-fluid pressures in the active Taiwan thrust belt using 55 deep boreholes to test the classic Hubbert-Rubey hypothesis that high static fluid pressures (depth normalized as λ = Pf/ρrgz) account for the extreme weakness of thrust faults, since effective friction μf∗ =μf(1 - λ) . Taiwan fluid pressures are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth zFRD ≈ 3 km, below which sediments are increasingly undercompacted and overpressured. The Hubbert-Rubey fault weakening coefficient is a simple function of depth (1 - λ) ≈ 0.6zFRD/z. We map present-day and pre-erosion fluid pressures and weakening (1 - λ) regionally and show that active thrusts are too shallow relative to zFRD for the classic Hubbert-Rubey mechanism to be important, which requires z ≥ ˜4zFRD ≈ 12 km to have the required order-of-magnitude Hubbert-Rubey fault weakening of (1 - λ) ≤ ˜0.15. The best-characterized thrust is the Chelungpu fault that slipped in the 1999 (Mw = 7.6) Chi-Chi earthquake, which has a low effective friction μf∗ ≈ 0.08- 0.12 , yet lies near the base of the hydrostatic zone at depths of 1-5 km with a modest Hubbert-Rubey weakening of (1 - λ) ≈ 0.4-0.6. Overpressured Miocene and Oligocene detachments at 5-7 km depth have (1 - λ) ≈ 0.3. Therefore, other mechanisms of fault weakening are required, such as the dynamical mechanisms documented for the Chi-Chi earthquake.

The South Sandwich "Forgotten" Subduction Zone and Tsunami Hazard in the South Atlantic

NASA Astrophysics Data System (ADS)

Okal, E. A.; Hartnady, C. J. H.; Synolakis, C. E.

2009-04-01

While no large interplate thrust earthquakes are know at the "forgotten" South Sandwich subduction zone, historical catalogues include a number of events with reported magnitudes 7 or more. A detailed seismological study of the largest event (27 June 1929; M (G&R) = 8.3) is presented. The earthquake relocates 80 km North of the Northwestern corner of the arc and its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg. respectively). The seismic moment of 1.7*10**28 dyn*cm supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. This event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations were performed using the MOST method [Titov and Synolakis, 1997]. Computed deep-water tsunami amplitudes of 30cm and 20cm were found off the coast of Brazil and along the Gulf of Guinea (Ivory Coast, Ghana) respectively. The 1929 moment was assigned to the geometries of other know earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. Tsunami hydrodynamic simulation of these scenarios revealed strong focusing of tsunami wave energy by the SAR, the SWIOR and the Agulhas Rise, in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

South Sandwich: The Forgotten Subduction Zone and Tsunami Hazard in the South Atlantic

NASA Astrophysics Data System (ADS)

Okal, E. A.; Hartnady, C. J.

2008-12-01

While no large interplate thrust earthquakes are known at the South Sandwich subduction zone, historical catalogues include a number of earthquakes with reported magnitudes of 7 or more. We present a detailed seismological study of the largest one (27 June 1929; M (G&R) = 8.3). The earthquake relocates 80 km North of the Northwestern corner of the arc. Its mechanism, inverted using the PDFM method, features normal faulting on a steeply dipping fault plane (phi, delta, lambda = 71, 70, 272 deg.). The seismic moment, 1.7 10**28 dyn*cm, supports Gutenberg and Richter's estimate, and is 28 times the largest shallow CMT in the region. The 1929 event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed "STEP faults" by Gover and Wortel [2005]. Hydrodynamic simulations using the MOST method [Titov and Synolakis, 1997] suggest deep-water tsunami amplitudes reaching 30 cm off the coast of Brazil, where it should have had observable run-up, and 20 cm along the Gulf of Guinea (Ivory Coast, Ghana). We also simulate a number of potential sources obtained by assigning the 1929 moment to the geometries of other known earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the Southern corner, where the youngest lithosphere is subducted. A common feature of these models is the strong focusing of tsunami waves by the SAR, the SWIOR, and the Agulhas Rise, resulting in amplitudes always enhanced in Ghana, Southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

Strong interseismic coupling, fault afterslip, and viscoelastic flow before and after the Oct. 9, 1995 Colima-Jalisco earthquake: continuous GPS measurements from Colima, Mexico

USGS Publications Warehouse

Azua, B.M.; DeMets, C.; Masterlark, Timothy

2002-01-01

Continuous GPS measurements from Colima, Mexico during 4/93-6/01, bracketing the Oct. 9, 1995 M = 8.0 Colima-Jalisco earthquake, provide new constraints on Rivera plate subduction mechanics. Modeling of margin-normal strain accumulation before the earthquake suggests the Rivera-North America subduction interface was fully locked. Transient postseismic motion from 10/ 95-6/97 is well fit by a model that includes logarithmically-decaying fault afterslip, elastic strain from shallow fault relocking, and possibly a minor viscoelastic response, but is fit poorly by models that assume a dominant Maxwell viscoelastic response of the lower crust and upper mantle, independent of the assumed viscosities. Landward, margin-normal motion since mid-1997 is parallel to but ??? 75% slower than the pre-seismic velocity. Afterslip alone fails to account for this slowdown. The viscoelastic response predicted by a FEM correctly resolves the remaining velocity difference within the uncertainties. Both processes thus offset elastic strain accumulating from the relocked subduction interface.

Disparate Tectonic Settings of Devastating Earthquakes in Mexico, September 2017

NASA Astrophysics Data System (ADS)

Li, J.; Chen, W. P.; Ning, J.

2017-12-01

Large earthquakes associated with thrust faulting along the plate interface typically pose the highest seismic risk along subduction zones. However, both damaging earthquakes in Mexico of September 2017 are notable exceptions. The Tehuantepec event on the 8th (Mw 8.1) occurred just landward of the trench but is associated with normal faulting, akin to the large (Ms 8) historical event of 1931 that occurred about 200 km to the northwest along this subduction zone. The Puebla earthquake (on the 19th, Mw 7.1) occurred almost 300 km away from the trench where seismic imaging had indicated that the flat-lying slab steepens abruptly and plunges aseismically into the deep mantle. Here we show that both types of tectonic settings are in fact common along a large portion of the Mexican subduction zone, thus identifying source zones of potentially damaging earthquakes away from the plate interface. Additionally, modeling of broadband waveforms made clear that another significant event (Mw 6.1) on the 23rd, is associated with shallow normal faulting in the upper crust, not directly related to the two damaging earthquakes.

Implications of seismic reflection and potential field geophysical data on the structural framework of the Yucca Mountain-Crater Flat region, Nevada

USGS Publications Warehouse

Brocher, T.M.; Hunter, W.C.; Langenheim, V.E.

1998-01-01

Seismic reflection and gravity profiles collected across Yucca Mountain, Nevada, together with geologic data, provide evidence against proposed active detachment faults at shallow depth along the pre-Tertiary-Tertiary contact beneath this potential repository for high-level nuclear waste. The new geophysical data show that the inferred pre-Tertiary-Tertiary contact is offset by moderate- to high-angle faults beneath Crater Flat and Yucca Mountain, and thus this shallow surface cannot represent an active detachment surface. Deeper, low-angle detachment surface(s) within Proterozoic-Paleozoic bedrock cannot be ruled out by our geophysical data, but are inconsistent with other geologic and geophysical observations in this vicinity. Beneath Crater Flat, the base of the seismogenic crust at 12 km depth is close to the top of the reflective (ductile) lower crust at 14 to 15 km depth, where brittle fault motions in the upper crust may be converted to pure shear in the ductile lower crust. Thus, our preferred interpretation of these geophysical data is that moderate- to high-angle faults extend to 12-15-km depth beneath Yucca Mountain and Crater Flat, with only modest changes in dip. The reflection lines reveal that the Amargosa Desert rift zone is an asymmetric half-graben having a maximum depth of about 4 km and a width of about 25 km. The east-dipping Bare Mountain fault that bounds this graben to the west can be traced by seismic reflection data to a depth of at least 3.5 km and possibly as deep as 6 km, with a constant dip of 64????5??. Within Crater Flat, east-dipping high-angle normal faults offset the pre-Tertiary-Tertiary contact as well as a reflector within the Miocene tuff sequence, tilting both to the west. The diffuse eastern boundary of the Amargosa Desert rift zone is formed by a broad series of high-angle down-to-the-west normal faults extending eastward across Yucca Mountain. Along our profile the transition from east- to west-dipping faults occurs at or just west of the Solitario Canyon fault, which bounds the western side of Yucca Mountain. The interaction at depth of these east- and west-dipping faults, having up to hundreds of meters offset, is not imaged by the seismic reflection profile. Understanding potential seismic hazards at Yucca Mountain requires knowledge of the subsurface geometry of the faults near Yucca Mountain, since earthquakes generally nucleate and release the greatest amount of their seismic energy at depth. The geophysical data indicate that many fault planes near the potential nuclear waste facility dip toward Yucca Mountain, including the Bare Mountain range-front fault and several west-dipping faults east of Yucca Mountain. Thus, earthquake ruptures along these faults would lie closer to Yucca Mountain than is often estimated from their surface locations and could therefore be more damaging.

Transfer zones and fault reactivation in inverted rift basins: Insights from physical modelling

NASA Astrophysics Data System (ADS)

Konstantinovskaya, Elena A.; Harris, Lyal B.; Poulin, Jimmy; Ivanov, Gennady M.

2007-08-01

Lateral transfer zones of deformation and fault reactivation were investigated in multilayered silicone-sand models during extension and subsequent co-axial shortening. Model materials were selected to meet similarity criteria and to be distinguished on CT scans; this approach permitted non-destructive visualisation of the progressive evolution of structures. Transfer zones were initiated by an orthogonal offset in the geometry of a basal mobile aluminium sheet and/or by variations of layer thickness or material rheology in basal layers. Transfer zones affected rift propagation and fault kinematics in models. Propagation and overlapping rift culminations occurred in transfer zones during extension. During shortening, deviation in the orientation of frontal thrusts and fold axes occurred within transfer zones in brittle and ductile layers, respectively. CT scans showed that steep (58-67°) rift-margin normal faults were reactivated as reverse faults. The reactivated faults rotated to shallower dips (19-38°) with continuing shortening after 100% inversion. Rotation of rift phase faults appears to be due to deep level folding and uplift during the inversion phase. New thrust faults with shallow dips (20-34°) formed outside the inverted graben at late stages of shortening. Frontal ramps propagated laterally past the transfer structure during shortening. During inversion, the layers filling the rift structures underwent lateral compression at the depth, the graben fill was pushed up and outwards creating local extension near the surface. Sand marker layers in inverted graben have showed fold-like structures or rotation and tilting in the rifts and on the rift margins. The results of our experiments conform well to natural examples of inverted graben. Inverted rift basins are structurally complex and often difficult to interpret in seismic data. The models may help to unravel the structure and evolution of these systems, leading to improved hydrocarbon exploration assessments. Model results may also be used to help predict the location of basement discontinuities which may have focused hydrothermal fluids during basin formation and inversion.

Possible strain partitioning structure between the Kumano fore-arc basin and the slope of the Nankai Trough accretionary prism

NASA Astrophysics Data System (ADS)

Martin, Kylara M.; Gulick, Sean P. S.; Bangs, Nathan L. B.; Moore, Gregory F.; Ashi, Juichiro; Park, Jin-Oh; Kuramoto, Shin'ichi; Taira, Asahiko

2010-05-01

A 12 km wide, 56 km long, three-dimensional (3-D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan, images the accretionary prism, fore-arc basin, and subducting Philippine Sea Plate. We have analyzed an unusual, trench-parallel depression (a "notch") along the seaward edge of the fore-arc Kumano Basin, just landward of the megasplay fault system. This bathymetric feature varies along strike, from a single, steep-walled, ˜3.5 km wide notch in the northeast to a broader, ˜5 km wide zone with several shallower linear depressions in the southwest. Below the notch we found both vertical faults and faults which dip toward the central axis of the depression. Dipping faults appear to have normal offset, consistent with the extension required to form a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is difficult to determine, but the along-strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. By considering only the along-strike variability of the megasplay fault, we could not explain a transform feature at the scale of the notch. Strike-slip faulting at the seaward edge of fore-arc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollement strength variations which control the location of the fore-arc basins may therefore play a role in the position where an along-strike component of strain is localized. While the obliquity of convergence in the Nankai Trough is comparatively small (˜15°), we believe it generated the Kumano Basin Edge Fault Zone, which has implications for interpreting local measured stress orientations and suggests potential locations for strain-partitioning-related deformation in other subduction zones.

Dynamic Evolution Of Off-Fault Medium During An Earthquake: A Micromechanics Based Model

NASA Astrophysics Data System (ADS)

Thomas, Marion Y.; Bhat, Harsha S.

2018-05-01

Geophysical observations show a dramatic drop of seismic wave speeds in the shallow off-fault medium following earthquake ruptures. Seismic ruptures generate, or reactivate, damage around faults that alter the constitutive response of the surrounding medium, which in turn modifies the earthquake itself, the seismic radiation, and the near-fault ground motion. We present a micromechanics based constitutive model that accounts for dynamic evolution of elastic moduli at high-strain rates. We consider 2D in-plane models, with a 1D right lateral fault featuring slip-weakening friction law. The two scenarios studied here assume uniform initial off-fault damage and an observationally motivated exponential decay of initial damage with fault normal distance. Both scenarios produce dynamic damage that is consistent with geological observations. A small difference in initial damage actively impacts the final damage pattern. The second numerical experiment, in particular, highlights the complex feedback that exists between the evolving medium and the seismic event. We show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture that could be potentially seen as a geological signature of this transition. These scenarios presented here underline the importance of incorporating the complex structure of fault zone systems in dynamic models of earthquakes.

Dynamic Evolution Of Off-Fault Medium During An Earthquake: A Micromechanics Based Model

NASA Astrophysics Data System (ADS)

Thomas, M. Y.; Bhat, H. S.

2017-12-01

Geophysical observations show a dramatic drop of seismic wave speeds in the shallow off-fault medium following earthquake ruptures. Seismic ruptures generate, or reactivate, damage around faults that alter the constitutive response of the surrounding medium, which in turn modifies the earthquake itself, the seismic radiation, and the near-fault ground motion. We present a micromechanics based constitutive model that accounts for dynamic evolution of elastic moduli at high-strain rates. We consider 2D in-plane models, with a 1D right lateral fault featuring slip-weakening friction law. The two scenarios studied here assume uniform initial off-fault damage and an observationally motivated exponential decay of initial damage with fault normal distance. Both scenarios produce dynamic damage that is consistent with geological observations. A small difference in initial damage actively impacts the final damage pattern. The second numerical experiment, in particular, highlights the complex feedback that exists between the evolving medium and the seismic event. We show that there is a unique off-fault damage pattern associated with supershear transition of an earthquake rupture that could be potentially seen as a geological signature of this transition. These scenarios presented here underline the importance of incorporating the complex structure of fault zone systems in dynamic models of earthquakes.

High resolution seismic imaging of faults beneath Limón Bay, northern Panama Canal, Republic of Panama

USGS Publications Warehouse

Pratt, Thomas L.; Holmes, Mark; Schweig, Eugene S.; Gomberg, Joan S.; Cowan, Hugh A.

2003-01-01

High-resolution seismic reflection profiles from Limo??n Bay, Republic of Panama, were acquired as part of a seismic hazard investigation of the northern Panama Canal region. The seismic profiles image gently west and northwest dipping strata of upper Miocene Gatu??n Formation, unconformably overlain by a thin (<20 m) sequence of Holocene muds. Numerous faults, which have northeast trends where they can be correlated between seismic profiles, break the upper Miocene strata. Some of the faults have normal displacement, but on many faults, the amount and type of displacement cannot be determined. The age of displacement is constrained to be Late Miocene or younger, and regional geologic considerations suggest Pliocene movement. The faults may be part of a more extensive set of north- to northeast-trending faults and fractures in the canal region of central Panama. Low topography and the faults in the canal area may be the result of the modern regional stress field, bending of the Isthmus of Panama, shearing in eastern Panama, or minor deformation of the Panama Block above the Caribbean subduction zone. For seismic hazard analysis of the northern canal area, these faults led us to include a source zone of shallow faults proximal to northern canal facilities. ?? 2003 Elsevier B.V. All rights reserved.

Physicochemical Processes and the Evolution of Strength in Calcite Fault Gouge at Room Temperature

NASA Astrophysics Data System (ADS)

Carpenter, B. M.; Viti, C.; Collettini, C.

2015-12-01

The presence of calcite in and near faults, as the dominant material, cement, or vein fill, indicates that the mechanical behavior of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. Furthermore, a variety of physical and chemical processes control the evolution of strength and style of slip along seismogenic faults and thus play a critical role in the seismic cycle. Determining the role and contributions of these types of mechanisms is essential to furthering our understanding of the processes and timescales that lead to the strengthening of faults during interseismic periods and their behavior during the earthquake nucleation process. To further our understanding of these processes, we performed laboratory-shearing experiments on calcite gouge at normal stresses from 1 to 100 MPa, under conditions of saturation and at room temperature. We performed velocity stepping (0.1-1000μm/s) and slide-hold-slide (1-3000s) tests, to measure the velocity dependence of friction and the amount of frictional strengthening respectively, under saturated conditions with pore fluid that was in equilibrium with CaCO3. At 5 MPa normal stress, we also varied the environmental conditions by performing experiments under conditions of 5% RH and 50 % RH, and saturation with: silicone oil, demineralized water, and the equilibrated solution combined with 0.5M NaCl. Finally, we collected post experimental samples for microscopic analysis. Our combined analyses of rate-dependence, strengthening behavior, and microstructures show that calcite fault gouge transitions from brittle to semi-brittle behavior at high normal stress and low sliding velocities. Furthermore, our results also highlight how changes in pore water chemistry can have significant influence on the mechanical behavior of calcite gouge in both the laboratory and in natural faults. Our observations have important implications for earthquake nucleation and propagation on faults in carbonate-dominated lithologies.

Refining interseismic fault slip and shallow creep on the Hayward and Calaveras Faults, California, using UAVSAR, satellite InSAR and GPS data

NASA Astrophysics Data System (ADS)

Farge, G.; Delbridge, B. G.; Materna, K.; Johnson, C. W.; Chaussard, E.; Jones, C. E.; Burgmann, R.

2016-12-01

Understanding the role of the Hayward/Calaveras fault junction in major earthquake ruptures in the East San Francisco Bay Area is a major challenge in trying to assess the regional seismic hazard. We use updated GPS velocities, and surface geodetic measurements from both traditional space-based InSAR and the NASA JPL's Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) system to quantify the deep long-term interseismic deformation and shallow temporally variable fault creep. Here, we present a large data set of interseismic deformation over the Hayward/Calaveras fault system, combining far-field deformation from 1992-2011 ERS and Envisat InSAR data, near-field deformation from 2009-2016 UAVSAR data and 1997-2016 regional GPS measurements from the Bay Area Velocity Unification model (BAVU4) in both near-field and far field. We perform a joint inversion of the data to obtain the long-term slip on deep through-going dislocations and the distribution of shallow creep on a 3D model of the Hayward and Calaveras faults. Spatially adaptative weights are given to each data set in order to account for its importance in constraining slip at different depths. The coherence and resolution of the UAVSAR data allow us to accurately resolve the near-field fault deformation, thus providing stronger constraints on the location of active strands of the southern Hayward and Calaveras faults and their shallow interseismic creep distribution.

Source model for the Copahue volcano magmaplumbing system constrained by InSARsurface deformation observations

NASA Astrophysics Data System (ADS)

Lundgren, P.; Nikkhoo, M.; Samsonov, S. V.; Milillo, P.; Gil-Cruz, F., Sr.; Lazo, J.

2017-12-01

Copahue volcano straddling the edge of the Agrio-Caviahue caldera along the Chile-Argentinaborder in the southern Andes has been in unrest since inflation began in late 2011. We constrain Copahue'ssource models with satellite and airborne interferometric synthetic aperture radar (InSAR) deformationobservations. InSAR time series from descending track RADARSAT-2 and COSMO-SkyMed data span theentire inflation period from 2011 to 2016, with their initially high rates of 12 and 15 cm/yr, respectively,slowing only slightly despite ongoing small eruptions through 2016. InSAR ascending and descending tracktime series for the 2013-2016 time period constrain a two-source compound dislocation model, with a rate ofvolume increase of 13 × 106 m3/yr. They consist of a shallow, near-vertical, elongated source centered at2.5 km beneath the summit and a deeper, shallowly plunging source centered at 7 km depth connecting theshallow source to the deeper caldera. The deeper source is located directly beneath the volcano tectonicseismicity with the lower bounds of the seismicity parallel to the plunge of the deep source. InSAR time seriesalso show normal fault offsets on the NE flank Copahue faults. Coulomb stress change calculations forright-lateral strike slip (RLSS), thrust, and normal receiver faults show positive values in the north caldera forboth RLSS and normal faults, suggesting that northward trending seismicity and Copahue fault motion withinthe caldera are caused by the modeled sources. Together, the InSAR-constrained source model and theseismicity suggest a deep conduit or transfer zone where magma moves from the central caldera toCopahue's upper edifice.

The regional structural setting of the 2008 Wells earthquake and Town Creek Flat Basin: implications for the Wells earthquake fault and adjacent structures

USGS Publications Warehouse

Henry, Christopher S.; Colgan, Joseph P.

2011-01-01

The 2008 Wells earthquake occurred on a northeast-striking, southeast-dipping fault that is clearly delineated by the aftershock swarm to a depth of 10-12 km below sea level. However, Cenozoic rocks and structures around Wells primarily record east-west extension along north- to north-northeast-striking, west-dipping normal faults that formed during the middle Miocene. These faults are responsible for the strong eastward tilt of most basins and ranges in the area, including the Town Creek Flat basin (the location of the earthquake) and the adjacent Snake Mountains and western Windermere Hills. These older west-dipping faults are locally overprinted by a younger generation of east-dipping, high-angle normal faults that formed as early as the late Miocene and have remained active into the Quaternary. The most prominent of these east-dipping faults is the set of en-échelon, north-striking faults that bounds the east sides of the Ruby Mountains, East Humboldt Range, and Clover Hill (about 5 km southwest of Wells). The northeastern-most of these faults, the Clover Hill fault, projects northward along strike toward the Snake Mountains and the approximately located surface projection of the Wells earthquake fault as defined by aftershock locations. The Clover Hill fault also projects toward a previously unrecognized, east-facing Quaternary fault scarp and line of springs that appear to mark a significant east-dipping normal fault along the western edge of Town Creek Flat. Both western and eastern projections may be northern continuations of the Clover Hill fault. The Wells earthquake occurred along this east-dipping fault system. Two possible alternatives to rupture of a northern continuation of the Clover Hill fault are that the earthquake fault (1) is antithetic to an active west-dipping fault or (2) reactivated a Mesozoic thrust fault that dips east as a result of tilting by the west-dipping faults along the west side of the Snake Mountains. Both alternatives are precluded by the depths of the earthquake and aftershocks, about 8 km and as deep as 12 km, respectively. These depths are below where an antithetic fault would intersect any main fault, and a tilted, formerly shallow and sub-horizontal thrust fault would not extend to depths of more than about 5–6 km. The east-dipping, high-angle, earthquake fault cuts older west-dipping faults rather than reactivating them, highlighting a change in the structural style of Basin and Range extension in this region from closely-spaced, west-dipping faults that rotated significantly during slip and accommodated large-magnitude extension, to widely-spaced, high-angle faults that accommodate much less total strain over a long time span.

Is low-angle normal fault slip aided by local stress rotations?: Assessment of paleostress inversion methods

NASA Astrophysics Data System (ADS)

Luther, A. L.; Axen, G. J.; Selverstone, J.; Khalsa, N.

2009-12-01

Classical fault mechanic theory does not adequately explain slip on “weak” faults oriented at high angles to the regional maximum stress direction, such as the San Andreas Fault and low-angle normal faults. One hypothesis is that stress rotation due to fault-weakening mechanisms allows slip, which may be testable using detailed paleostress analyses of minor faults and tensile fractures. Preliminary data from the footwalls of the Whipple detachment (WD) and the West Salton detachment (WSD) suggest lateral and/or vertical stress rotations. Three inversion programs that use different fault-slip datasets are compared. 1) FaultKin (Marrett and Allmendinger ‘90; Cladouhos and Allmendinger ‘93) determines the principal strain directions using only faults with striae and known slip senses; principal stress orientations are determined assuming coaxiality. To date, FaultKin results appear to be the most reproducible, but it is difficult to find enough faults with striae and slip sense in the small outcrop areas of our study. 2) Slick.bas (Ramsey and Lisle ‘00) uses a grid search to find the best-fit stress tensor from fault and striae orientations, but does not accept slip sense. This program can yield erroneous stress fields that predict slip senses opposite those known for some faults (particularly faults at a high angle to sigma 1). 3) T-TECTO 2.0 (Zalohar and Vrabec ‘07) applies a Gaussian approach, using orientations of faults and striae, the slip senses of any faults for which it is known, plus tensile fractures. We expect that this flexibility of input data types will be best, but testing is preliminary. Paleostress analyses assume that minor faults slipped in response to constant, homogeneous stress fields. We use shear and tensile fractures and cross-cutting relationships from the upper ~25 m of both footwalls to test for spatial and temporal changes to the paleostress field. Paleostress analysis of fractures ~0.3 - 2 m below the WSD on the N limb of an antiform suggests that sigma 3 plunges moderately (~45 degrees) W, sigma 1 plunges gently S, and sigma 2 is steep, consistent with wrench-related folding about E-W trends during WSD slip. However, tensile fractures in the immediately overlying ultracataclasite yield sigma 3 with a shallow W plunge (~4 degrees). In a synformal trough, Reidel shears in the upper 1-2 m of the WSD footwall suggest a moderately (~50 degrees) E plunging sigma 1. Deeper (2-10 m) in the footwall, shear fractures have different but consistent orientations, suggesting a change in the stress field. Preliminary results from several sets of shear fractures in the WD footwall suggest that sigma 1 is steep (~75-90 degrees) in the chlorite breccia zone (implying low shear traction) but is shallower (~45 degrees) in the deeper damage zone. Prior work (Axen & Selverstone ‘94) found that sigma 1 becomes steep again at greater depths. Continued testing of paleostress analysis methods and several other datasets are in progress to confirm our results.

First Results from a Forward, 3-Dimensional Regional Model of a Transpressional San Andreas Fault System

NASA Astrophysics Data System (ADS)

Fitzenz, D. D.; Miller, S. A.

2001-12-01

We present preliminary results from a 3-dimensional fault interaction model, with the fault system specified by the geometry and tectonics of the San Andreas Fault (SAF) system. We use the forward model for earthquake generation on interacting faults of Fitzenz and Miller [2001] that incorporates the analytical solutions of Okada [85,92], GPS-constrained tectonic loading, creep compaction and frictional dilatancy [Sleep and Blanpied, 1994, Sleep, 1995], and undrained poro-elasticity. The model fault system is centered at the Big Bend, and includes three large strike-slip faults (each discretized into multiple subfaults); 1) a 300km, right-lateral segment of the SAF to the North, 2) a 200km-long left-lateral segment of the Garlock fault to the East, and 3) a 100km-long right-lateral segment of the SAF to the South. In the initial configuration, three shallow-dipping faults are also included that correspond to the thrust belt sub-parallel to the SAF. Tectonic loading is decomposed into basal shear drag parallel to the plate boundary with a 35mm yr-1 plate velocity, and East-West compression approximated by a vertical dislocation surface applied at the far-field boundary resulting in fault-normal compression rates in the model space about 4mm yr-1. Our aim is to study the long-term seismicity characteristics, tectonic evolution, and fault interaction of this system. We find that overpressured faults through creep compaction are a necessary consequence of the tectonic loading, specifically where high normal stress acts on long straight fault segments. The optimal orientation of thrust faults is a function of the strike-slip behavior, and therefore results in a complex stress state in the elastic body. This stress state is then used to generate new fault surfaces, and preliminary results of dynamically generated faults will also be presented. Our long-term aim is to target measurable properties in or around fault zones, (e.g. pore pressures, hydrofractures, seismicity catalogs, stress orientation, surface strain, triggering, etc.), which may allow inferences on the stress state of fault systems.

Plate boundary and major fault system in the overriding plate within the Shumagin gap at the Alaska-Aleutian subduction zone

NASA Astrophysics Data System (ADS)

Becel, A.; Shillington, D. J.; Nedimovic, M. R.; Keranen, K. M.; Li, J.; Webb, S. C.; Kuehn, H.

2013-12-01

Structure in the overriding plate is one of the parameters that may increase the tsunamigenic potential of a subduction zone but also influence the seismogenic behavior and segmentation of great earthquake rupture. The Alaska-Aleutian margin is characterized by along-strike changes in plate interface coupling over relatively small distances. Here, we present trench normal multichannel seismic (MCS) profiles acquired across the Shumagin gap that has not broken in many decades and appears to be weakly coupled. The high fold, deep penetration (636 channel, 8-km long streamer, 6600 cu.in airgun source) MCS data were acquired as part of the ALEUT project. This dataset gives us critical new constraints on the interplate boundary that can be traced over ~100 km distance beneath the forearc with high variation in its reflection response with depth. These profiles also reveal the detailed upper plate fault structure and forearc morphology. Clear reflections in the overriding plate appear to delineate one or more large faults that cross the shelf and the upper slope. These faults are observed 75 km back from the trench and seem to branch at depth and connect to the plate interface within this gap at ~11 s twtt. We compare the reflective structure of these faults to that of the plate boundary and examine where it intersects the megathrust with respect of the expected downdip limit of coupling. We also compare this major structure with the seismicity recorded in this sector. The imaged fault system is associated with a large deep basin (~6s twt) that is an inherited structure formed during the pre-Aleutian period. Basins faults appear to have accommodated primarily normal motion, although folding of sediments near the fault and complicated fault geometries in the shallow section may indicate that this fault has accommodated other types of motion during its history that may reflect the stress-state at the megathrust over time. The deformation within the youngest sediment also suggests also that this fault system might be still active. The coincident wide-angle seismic data coincident with one MCS profile allow the addition of more information about the deep P-wave velocity structure whereas the streamer tomography (Michaelson-Rotermund et al., this session) around the fault system add more detailed view into the complex structure in the shallow portions (upper 2km) of these structures showing a low velocity zone along one large fault suggesting that this fault is still active. These large-scale structures imaged in the overriding plate within the Shumagin gap are probably sufficiently profound to play a major role in the behavior of the megathrust in this area, segmentation of great earthquake rupture area, tsunami generation and may influence the frictional properties of the seismogenic zone at depth.

The shallow structure of Solfatara Volcano, Italy, revealed by dense, wide-aperture seismic profiling.

PubMed

Bruno, Pier Paolo G; Maraio, Stefano; Festa, Gaetano

2017-12-12

Two active-source, high-resolution seismic profiles were acquired in the Solfatara tuff cone in May and November 2014, with dense, wide-aperture arrays. Common Receiver Surface processing was crucial in improving signal-to-noise ratio and reflector continuity. These surveys provide, for the first time, high-resolution seismic images of the Solfatara crater, depicting a ~400 m deep asymmetrical crater filled by volcanoclastic sediments and rocks and carved within an overall non-reflective pre-eruptive basement showing features consistent with the emplacement of shallow intrusive bodies. Seismic reflection data were interpreted using the trace complex attributes and clearly display several steep and segmented collapse faults, generally having normal kinematics and dipping toward the crater centre. Fault/fracture planes are imaged as sudden amplitude drops that generate narrow low-similarity and high-dip attributes. Uprising fluids degassed by a magmatic source are the most probable cause of the small-scale amplitude reduction. Seismic data also support the interpretation of the shallow structure of the Solfatara crater as a maar. Our results provides a solid framework to constrain the near-surface geological interpretation of such a complex area, which improves our understanding of the temporal changes of the structure in relation with other geophysical and geochemical measurements.

Size matters: The effects of displacement magnitude on the fluid flow properties of faults in poorly lithified sediments

NASA Astrophysics Data System (ADS)

Loveless, S. E.; Bense, V.; Turner, J.

2011-12-01

Many aquifers worldwide occur in poorly lithified sediments, often in regions that experience active tectonic deformation. Faulting of these sediments introduces heterogeneities that may affect aquifer porosity and permeability, and consequently subsurface fluid flow and groundwater storage. The specific hydrogeological effects of faults depend upon the fault architecture and deformation mechanisms. These are controlled by factors such as rheology, stratigraphy and burial depth. Here, we analyse fault permeability in poorly lithified sediments as a function of fault displacement. We have carried out detailed outcrop studies of minor normal faults at five study sites within the rapidly extending Corinth rift, Central Greece. Gravel conglomerates of giant Gilbert delta facies form productive but localised shallow aquifers within the region. Exposures reveal dense (average 20 faults per 100 m) networks of minor (0.1 to 50 m displacement) normal faults within the uplifted sequences, proximal to many of the crustal-scale normal faults. Analysis of 42 faults shows that fault zones are primarily composed of smeared beds that can either retain their definition or mix with surrounding sediment. Lenses or blocks of sediment are common in fault zones that cut beds with contrasting rheology, and a few faults have a clay core and/or damage zone. Fault thickness increases at a rate of about 0.4 m per 10 m increase in displacement. Comparison of sediment micro-structures from the field, hand samples and thin sections show grain-scale sediment mixing, fracturing of clasts, and in some cases cementation, within fault zones. In faults with displacements >12 m we also find a number of roughly parallel, highly indurated shear planes, up to 20 mm in thickness, composed of highly fragmented clasts and a fine grained matrix. Image analysis of thin sections from hand samples collected in the field was used to quantify the porosity of fault zones and adjacent undeformed sediment. These data show a reduction in average porosity from 21% (± 4) in undisturbed sediments to 14% (± 8) within fault zones. We find that fault zone porosity decreases by approximately 5% per 1 m displacement (up to 2 m displacement), as sediments undergo greater micro-scale deformation. Porosity within the shear planes of larger displacement faults (> 12 m) is significantly less than 5%. In summary, with an increase in fault displacement there is an increase in fault thickness and decrease in fault zone porosity, in addition to the occurrence of extremely low porosity shear planes. Consequently, the impact of faults in poorly lithified sediment on fluid flow is, to a large degree, dependent upon the magnitude of fault displacement.

Numerical experiments of volcanic dominated rifts and passive margins

NASA Astrophysics Data System (ADS)

Korchinski, Megan; Teyssier, Christian; Rey, Patrice; Whitney, Donna; Mondy, Luke

2017-04-01

Continental rifting is driven by plate tectonic forces (passive rifting), thermal thinning of the lithosphere over a hotspot (active rifting), or a combination of the two. Successful rifts develop into passive margins where pre-drift stretching is accompanied by normal faulting, clastic sedimentation, and various degrees of magmatism. The structure of volcanic passive margins (VPM) differs substantially from margins that are dominated by sedimentation. VPMs are typically narrow, with a lower continental crust that is intruded by magma and can flow as a low-viscosity layer. To investigate the role of the deep crust in the early development of VPMs, we have developed a suite of 2D thermomechanical numerical experiments (Underworld code) in which the density and viscosity of the deep crust and the density of the rift basin fill are systematically varied. Our experiments show that, for a given rifting velocity, the viscosity of the deep crust and the density of the rift basin fill exert primary controls on early VPM development. The viscosity of the deep crust controls the degree to which the shallow crust undergoes localised faulting or distributed thinning. A weak deep crust localises rifting and is efficiently exhumed to the near-surface, whereas a strong deep crust distributes shallow crust extension and remains buried. A high density rift basin fill results in gravitational loading and increased subsidence rate in cases in which the viscosity of the deep crust is sufficiently low to allow that layer to be displaced by the sinking basin fill. At the limit, a low viscosity deep crust overlain by a thick basalt-dominated fill generates a gravitational instability, with a drip of cool basalt that sinks and ponds at the Moho. Experiment results indicate that the deep crust plays a critical role in the dynamic development of volcanic dominated rifts and passive margins. During rifting, the deep continental crust is heated and readily responds to solicitations of the shallow crust (rooting of normal faults, exhumation of the deep crust in normal fault footwalls). Gravitational instabilities caused by high density rift infill similar to those observed in our numerical experiments may be present in the Mesoproterozoic ( 1100 Ma) North American Midcontinent Rift System (MRS). The MRS is a failed rift that is filled with >20 km of dominantly basaltic volcanic deposits, and therefore represents an end member VPM (high density basin fill) where the initial structure of a pre-drift VPM is preserved. Magmatism occurred in two pulses over <15 Ma involving deep mantle melting first (>150 km), then shallow melting (40-70 km). Post-rift subsidence accumulated up to 10 km of clastic sediments in the center of the basin. Evidence of cool, dense rocks sinking into a low-viscosity deep crust as predicted in our numerical experiments may be present in the western arm of the MRS, where crustal density analyses suggest the presence of dense bodies (eclogite) at the base of the crust.

Active and long-lived permanent forearc deformation driven by the subduction seismic cycle

NASA Astrophysics Data System (ADS)

Aron Melo, Felipe Alejandro

I have used geological, geophysical and engineering methods to explore mechanisms of upper plate, brittle deformation at active forearc regions. My dissertation particularly addresses the permanent deformation style experienced by the forearc following great subduction ruptures, such as the 2010 M w8.8 Maule, Chile and 2011 Mw9.0 Tohoku, Japan earthquakes. These events triggered large, shallow seismicity on upper plate normal faults above the rupture reaching Mw7.0. First I present new structural data from the Chilean Coastal Cordillera over the rupture zone of the Maule earthquake. The study area contains the Pichilemu normal fault, which produced the large crustal aftershocks of the megathrust event. Normal faults are the major neotectonic structural elements but reverse faults also exist. Crustal seismicity and GPS surface displacements show that the forearc experiences pulses of rapid coseismic extension, parallel to the heave of the megathrust, and slow interseismic, convergence-parallel shortening. These cycles, over geologic time, build the forearc structural grain, reactivating structures properly-oriented respect to the deformation field of each stage of the interplate cycle. Great subduction events may play a fundamental role in constructing the crustal architecture of extensional forearc regions. Static mechanical models of coseismic and interseismic upper plate deformation are used to explore for distinct features that could result from brittle fracturing over the two stages of the interplate cycle. I show that the semi-elliptical outline of the first-order normal faults along the Coastal Cordillera may define the location of a characteristic, long-lived megathrust segment. Finally, using data from the Global CMT catalog I analyzed the seismic behavior through time of forearc regions that have experienced great subduction ruptures >Mw7.7 worldwide. Between 61% and 83% of the cases where upper plate earthquakes exhibited periods of increased seismicity above background levels occurred contemporaneous to megathrust ruptures. That correlation is stronger for normal fault events than reverse or strike-slip crustal earthquakes. More importantly, for any given megathrust the summation of the Mw accounted by the forearc normal fault aftershocks appears to have a positive linear correlation with the Mw of the subduction earthquake -- the larger the megathrust the larger the energy released by forearc events.

Magnetic character of a large continental transform: an aeromagnetic survey of the Dead Sea Fault

USGS Publications Warehouse

ten Brink, Uri S.; Rybakov, Michael; Al-Zoubi, Abdallah S.; Rotstein, Yair

2007-01-01

New high-resolution airborne magnetic (HRAM) data along a 120-km-long section of the Dead Sea Transform in southern Jordan and Israel shed light on the shallow structure of the fault zone and on the kinematics of the plate boundary. Despite infrequent seismic activity and only intermittent surface exposure, the fault is delineated clearly on a map of the first vertical derivative of the magnetic intensity, indicating that the source of the magnetic anomaly is shallow. The fault is manifested by a 10–20 nT negative anomaly in areas where the fault cuts through magnetic basement and by a

The shallow boreholes at The AltotiBerina near fault Observatory (TABOO; northern Apennines of Italy)

NASA Astrophysics Data System (ADS)

Chiaraluce, L.; Collettini, C.; Cattaneo, M.; Monachesi, G.

2014-04-01

As part of an interdisciplinary research project, funded by the European Research Council and addressing the mechanics of weak faults, we drilled three 200-250 m-deep boreholes and installed an array of seismometers. The array augments TABOO (The AltotiBerina near fault ObservatOry), a scientific infrastructure managed by the Italian National Institute of Geophysics and Volcanology. The observatory, which consists of a geophysical network equipped with multi-sensor stations, is located in the northern Apennines (Italy) and monitors a large and active low-angle normal fault. The drilling operations started at the end of 2011 and were completed by July 2012. We instrumented the boreholes with three-component short-period (2 Hz) passive instruments at different depths. The seismometers are now fully operational and collecting waveforms characterised by a very high signal to noise ratio that is ideal for studying microearthquakes. The resulting increase in the detection capability of the seismic network will allow for a broader range of transients to be identified.

Lithospheric "corner flow" via extensional faulting and tectonic rotation at non-volcanic, slow-spreading ridges

NASA Astrophysics Data System (ADS)

Schroeder, T.; Cheadle, M. J.; Dick, H. J.; Faul, U.

2005-12-01

Large degrees (up to 90°) of tectonic rotation may be the norm at slow-spreading, non-volcanic ridges. Vertically upwelling mantle beneath all mid-ocean ridges must undergo corner flow to move horizontally with the spreading plate. Because little or no volcanic crust is produced at some slow-spreading ridges, the uppermost lithospheric mantle must undergo this rotation in the regime of localized, rather than distributed deformation. Anomalous paleomagnetic inclinations in peridotite and gabbro cores drilled near the 15-20 Fracture Zone (Mid-Atlantic Ridge, ODP Leg 209) support such large rotations, with sub-Curie-temperature rotations up to 90° (Garces et al., 2004). Here, we present two end-member tectonic mechanisms, with supporting data from Leg 209 cores and bathymetry, to show how rotation is accomplished via extensional faults and shear zones: 1) long-lived detachment faults, and 2) multiple generations of high-angle normal faults. Detachment faults accommodate rotation by having a moderate to steep dip at depth, and rotating to horizontal through a rolling hinge as the footwall is tectonically denuded. Multiple generations of high-angle normal faults accommodate large rotations in a domino fashion; early faults become inactive when rotated to inopportune slip angles, and are cut by younger high-angle faults. Thus, each generation of high-angle faults accommodates part of the total rotation. There is likely a gradation between the domino and detachment mechanisms; transition from domino to detachment faulting occurs when a single domino fault remains active at inopportune slip angles and evolves into a detachment that accommodates all corner flow for that region. In both cases, the original attitude of layering within mantle-emplaced gabbro bodies must be significantly different than present day observed attitudes; sub-horizontal bodies may have been formed sub-vertically and vice-versa. Leg 209 cores record an average major brittle fault spacing of approximately 100 m, suggesting that the width of individual rotating fault blocks may be on the order of 100-200 m. Numerous fault bounded domino slices could therefore be formed within a 10km wide axial valley, with large rotations (and commensurate extension) leading to the exposure of 1km wide shallow-dipping fault surfaces, as are seen in the 15-20 FZ region bathymetry. The region's bathymetry is dominated by irregular, low-relief ridges that were likely formed by domino faulting of lithosphere with a small elastic thickness. The region contains relatively few corrugated detachment fault domes, suggesting that domino faulting may be the normal mode of lithospheric corner flow at non-volcanic ridges.

The 29 July 2014 (Mw 6.4) Southern Veracruz, Mexico Earthquake: Scenary Previous to Its Occurrence.

NASA Astrophysics Data System (ADS)

Yamamoto, J.

2014-12-01

On 29 July 2014 (10:46 UTC) a magnitude 6.4 (Mw) earthquake occurred at the southern Veracruz, Mexico region. The epicenter was preliminary located at 17.70° N and 95.63° W. It was a normal fault event with the slip on a fault that trend NNW and a focus approximately 117 km below the surface of the Gulf of Mexico costal plane. The earthquake was widely felt through centro and southern Mexico. In Oaxaca City 133 km to the south a person die of a hearth attack. No damages were reported. Most prominent moderate-sized earthquakes occurring in the southern Veracruz region since 1959 has been concentrated along two well defined seismic belts. One belt runs off the coast following nearly its contour. Here the earthquakes are shallow depth and mostly show a reverse fault mechanism. This belt of seismicity begins at the Los Tuxtlas volcanic field. Another seismic belt is located inland 70 km to the west. Here most earthquakes are of intermediate-depth (108-154 km) focus and normal faulting mechanism. The July 2014 earthquake is located near to this second seismic belt. In the present paper we discuss, within the regional geotectonic framework, the location and some aspects of the rupture process of the July 2014 earthquake.

Dissolved noble gases and stable isotopes as tracers of preferential fluid flow along faults in the Lower Rhine Embayment, Germany

NASA Astrophysics Data System (ADS)

Gumm, L. P.; Bense, V. F.; Dennis, P. F.; Hiscock, K. M.; Cremer, N.; Simon, S.

2016-02-01

Groundwater in shallow unconsolidated sedimentary aquifers close to the Bornheim fault in the Lower Rhine Embayment (LRE), Germany, has relatively low δ2H and δ18O values in comparison to regional modern groundwater recharge, and 4He concentrations up to 1.7 × 10-4 cm3 (STP) g-1 ± 2.2 % which is approximately four orders of magnitude higher than expected due to solubility equilibrium with the atmosphere. Groundwater age dating based on estimated in situ production and terrigenic flux of helium provides a groundwater residence time of ˜107 years. Although fluid exchange between the deep basal aquifer system and the upper aquifer layers is generally impeded by confining clay layers and lignite, this study's geochemical data suggest, for the first time, that deep circulating fluids penetrate shallow aquifers in the locality of fault zones, implying that sub-vertical fluid flow occurs along faults in the LRE. However, large hydraulic-head gradients observed across many faults suggest that they act as barriers to lateral groundwater flow. Therefore, the geochemical data reported here also substantiate a conduit-barrier model of fault-zone hydrogeology in unconsolidated sedimentary deposits, as well as corroborating the concept that faults in unconsolidated aquifer systems can act as loci for hydraulic connectivity between deep and shallow aquifers. The implications of fluid flow along faults in sedimentary basins worldwide are far reaching and of particular concern for carbon capture and storage (CCS) programmes, impacts of deep shale gas recovery for shallow groundwater aquifers, and nuclear waste storage sites where fault zones could act as potential leakage pathways for hazardous fluids.

Stress Fields Along Okinawa Trough and Ryukyu Arc Inferred From Regional Broadband Moment Tensors

NASA Astrophysics Data System (ADS)

Kubo, A.; Fukuyama, E.

2001-12-01

Most shallow earthquakes along Okinawa trough and Ryukyu arc are relatively small (M<5.5). Focal mechanism estimations for such events were difficult due to insufficient dataset. However, this situation is improved by regional broadband network (FREESIA). Lower limit of magnitude of the earthquakes determined becomes 1.5 smaller in M{}w than that of Harvard moment tensors. As a result, we could examine the stress field in more detail than Fournier et al.(2001, JGR, 106, 13751-) did based on surface geology and teleseismic moment tensors. In the NE Okinawa trough, extension axes are oblique to the trough strike, while in SW Okinawa trough, they are perpendicular to the trough. Fault type in SW is normal fault and gradually changes to mixture of normal and strike slip toward NE. In the Ryukyu arc, extension axes are parallel to the arc. Although this feature is not clear in the NW Ryukyu arc, arc parallel extension may be a major property of entire arc. Dominant fault type is normal fault and several strike slips with the same extensional component are included. The volcanic train is located at the edge of arc parallel extension field faced A simple explanation of the arc parallel extension is the response to the opening motion of the Okinawa trough. Another possible mechanism is forearc movement due to oblique subduction which is enhanced in SW. We consider that the Okinawa trough and the Ryukyu arc are independent stress provinces.

A plastic flow model for the Acquara - Vadoncello landslide in Senerchia, Southern Italy

USGS Publications Warehouse

Savage, W.; Wasowski, J.

2006-01-01

A previously developed model for stress and velocity fields in two-dimensional Coulomb plastic materials under self-weight and pore pressure predicts that long, shallow landslides develop slip surfaces that manifest themselves as normal faults and normal fault scarps at the surface in areas of extending flow and as thrust faults and thrust fault scarps at the surface in areas of compressive flow. We have applied this model to describe the geometry of slip surfaces and ground stresses developed during the 1995 reactivation of the Acquara - Vadoncello landslide in Senerchia, southern Italy. This landslide is a long and shallow slide in which regions of compressive and extending flow are clearly identified. Slip surfaces in the main scarp region of the landslide have been reconstructed using surface surveys and subsurface borehole logging and inclinometer observations made during retrogression of the main scarp. Two of the four inferred main scarp slip surfaces are best constrained by field data. Slip surfaces in the toe region are reconstructed in the same way and three of the five inferred slip surfaces are similarly constrained. The location of the basal shear surface of the landslide is inferred from borehole logging and borehole inclinometry. Extensive data on material properties, landslide geometries, and pore pressures collected for the Acquara - Vadoncello landslide give values for cohesion, friction angle, and unit weight, plus average basal shear-surface slopes, and pore-pressures required for modelling slip surfaces and stress fields. Results obtained from the landslide-flow model and the field data show that predicted slip surface shapes are consistent with inferred slip surface shapes in both the extending flow main scarp region and in the compressive flow toe region of the Acquara - Vadoncello landslide. Also predicted stress distributions are found to explain deformation features seen in the toe and main scarp regions of the landslide. ?? 2005 Elsevier B.V. All rights reserved.

Imaging b-value depth variations within the Cocos and Rivera plates at the Mexican subduction zone

NASA Astrophysics Data System (ADS)

Rodríguez-Pérez, Quetzalcoatl; Zuñiga, F. Ramón

2018-06-01

By a systematic mapping of the b-value along profiles perpendicular to the Mexican Wadati-Benioff zone, we obtained important characteristics pertaining the stress state and faulting style related to the subduction process. To this purpose, we used data from the earthquake catalog reported by the Servicio Sismologico Nacional (1988-2016). We investigate depth variations of the b-value for the Cocos and Rivera under North American plates interface, by a detailed analysis of 15 cross-sections. The obtained b-value profiles vary from 0.50 to 2.50, which nevertheless appear related to the faulting style and stress state. By comparing the locations and focal mechanism of the largest events with the b-values of the surrounding regions, our analysis corroborates the dependence of the b-value on the faulting style. Thrust events occur in regions of low and high b-value at depths <50 km. Normal-faulting events occur mainly in high b-value regions at all shallow (Z < 30 km) and intermediate depths (Z > 30 km), in agreement with global studies. These results support the hypothesis that differential stress processes may be behind the occurrence of the different faulting style. On the contrary, by analyzing the mean b-values for both types of faulting mechanism at each of the cross-sections, we found a significantly lower mean b-value related to normal faulting for those regions where the 8 (Mw 8.2) and 19 (Mw 7.1) September 2017 earthquakes occur. These results lead us to conclude that those regions experienced an increased stress state prone to the occurrence of normal-intraplate events. We also compare the b-value distribution with Vp and Q tomography studies obtaining a good correlation between them. We found evidence to relate b-value variations with subduction processes such as stress state due to tectonic and flexural conditions, and to a lesser extent to material heterogeneity and fluid dehydration.

Anatomy of a Plate Boundary at Shallow Crustal Levels: a Composite Section from the Alpine Fault, New Zealand

NASA Astrophysics Data System (ADS)

Barth, N. C.; Toy, V. G.; Boulton, C. J.; Carpenter, B. M.

2010-12-01

New Zealand's Alpine Fault is mostly a moderately SE-dipping dextral reverse plate boundary structure, but at its southern end, strike-slip-normal motion is indicated by offset of recent surfaces, juxtaposition of sediments, and both brittle and ductile shear sense indicators. At the location of uplift polarity reversal fault rocks exhumed from both the hangingwall Pacific and footwall Australian Plates are juxtaposed, offering a remarkably complete cross section of the plate boundary at shallow crustal levels. We describe Alpine Fault damage zone and fault core structures overprinted on Pacific and Australian plate mylonites of a variety of compositions, in a fault-strike perpendicular composite section spanning the reversal in dip-slip polarity. The damage zone is asymmetric; on the Australian Plate 160m of quartzose paragneiss-derived mylonites are overprinted by brittle faults and fractures that increase in density towards the principal slip surface (PSS). This damage zone fabric consists of 1-10m-spaced, moderately to steeply-dipping, 1-20cm-thick gouge-filled faults, overprinted on and sub-parallel to a mylonitic foliation sub-parallel to the PSS. On the Pacific Plate, only 40m of the 330m section of volcaniclastic-derived mylonites have brittle damage in the form of unhealed fractures and faults, as well as a pervasive greenschist facies hydrothermal alteration absent in the footwall. These damage-related structures comprise a network of small-offset faults and fractures with increasing density and intensity towards the PSS. The active Pacific Plate fault core is composed of ~1m of cataclasite grading into folded protocataclasite that is less folded and fractured with increasing distance from the PSS. The active Australian Plate fault core is <1.5m wide and consists of 3 distinct foliated clay gouges, as well as a 4cm thick brittle ultracataclasite immediately adjacent to the active PSS. The Australian Plate foliated clay gouge contains stringers of quartz that become less continuous and more sigmoidal toward the PSS, indicating a strain gradient across the gouge zone. Gouge textures are consistent with deformation by pressure solution. Intact wafers from one of the gouges, experimentally -sheared in a biaxial configuration under true-triaxial loading at σn’= 31MPa and Pf = 10MPa, yielded a friction coefficient, μss = 0.32 and displayed velocity strengthening behavior. No significant re-strengthening was observed during hold periods of slide-hold tests. Well-cemented glacial till (~8000 years old), which caps many outcrops, is a marker that shows that the damage zone is not active in the near-surface, but most of the fault core is. The active near-surface damage zone here is <40m wide and the active fault core is <2.5m wide. Both overprint a much wider, inactive damage zone. The combination of rheologically-weak Australian Plate fault rocks with surface rupture traces indicates distinctly different coseismic and interseismic behaviors along the southern strike-slip-normal segment of the Alpine Fault.

Linking megathrust earthquakes to brittle deformation in a fossil accretionary complex

PubMed Central

Dielforder, Armin; Vollstaedt, Hauke; Vennemann, Torsten; Berger, Alfons; Herwegh, Marco

2015-01-01

Seismological data from recent subduction earthquakes suggest that megathrust earthquakes induce transient stress changes in the upper plate that shift accretionary wedges into an unstable state. These stress changes have, however, never been linked to geological structures preserved in fossil accretionary complexes. The importance of coseismically induced wedge failure has therefore remained largely elusive. Here we show that brittle faulting and vein formation in the palaeo-accretionary complex of the European Alps record stress changes generated by subduction-related earthquakes. Early veins formed at shallow levels by bedding-parallel shear during coseismic compression of the outer wedge. In contrast, subsequent vein formation occurred by normal faulting and extensional fracturing at deeper levels in response to coseismic extension of the inner wedge. Our study demonstrates how mineral veins can be used to reveal the dynamics of outer and inner wedges, which respond in opposite ways to megathrust earthquakes by compressional and extensional faulting, respectively. PMID:26105966

Tidal triggering of low frequency earthquakes near Parkfield, California: Implications for fault mechanics within the brittle-ductile transition

USGS Publications Warehouse

Thomas, A.M.; Burgmann, R.; Shelly, David R.; Beeler, Nicholas M.; Rudolph, M.L.

2012-01-01

Studies of nonvolcanic tremor (NVT) have established the significant impact of small stress perturbations on NVT generation. Here we analyze the influence of the solid earth and ocean tides on a catalog of ∼550,000 low frequency earthquakes (LFEs) distributed along a 150 km section of the San Andreas Fault centered at Parkfield. LFE families are identified in the NVT data on the basis of waveform similarity and are thought to represent small, effectively co-located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km. We calculate the sensitivity of each of these 88 LFE families to the tidally induced right-lateral shear stress (RLSS), fault-normal stress (FNS), and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity. LFE occurrence is most strongly modulated by fluctuations in shear stress, with the majority of families demonstrating a correlation with RLSS at the 99% confidence level or above. Producing the observed LFE rate modulation in response to shear stress perturbations requires low effective stress in the LFE source region. There are substantial lateral and vertical variations in tidal shear stress sensitivity, which we interpret to reflect spatial variation in source region properties, such as friction and pore fluid pressure. Additionally, we find that highly episodic, shallow LFE families are generally less correlated with tidal stresses than their deeper, continuously active counterparts. The majority of families have weaker or insignificant correlation with positive (tensile) FNS. Two groups of families demonstrate a stronger correlation with fault-normal tension to the north and with compression to the south of Parkfield. The families that correlate with fault-normal clamping coincide with a releasing right bend in the surface fault trace and the LFE locations, suggesting that the San Andreas remains localized and contiguous down to near the base of the crust. The deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth. Synthesizing our observations with those of other LFE-hosting localities will help to develop a comprehensive understanding of transient fault slip below the “seismogenic zone” by providing constraints on parameters in physical models of slow slip and LFEs.

Imaging the Shallow Crust in the Epicentral Area of the 1857 M7 Agri Valley Earthquake (Southern Italy) by Combined Traveltime and Full-Waveform Tomography

NASA Astrophysics Data System (ADS)

Improta, L.; Operto, S.; Piromallo, C.; Valoroso, L.

2008-12-01

The Agri Valley is a Quaternary extensional basin located in the Southern Apennines range. This basin was struck by a M7 earthquake in 1857. In spite of extensive morphotectonic surveys and hydrocarbon exploration, major unsolved questions remain about the upper crustal structure, the recent tectonic evolution and seismotectonics of the area. Most authors consider a SW-dipping normal-fault system bordering the basin to the East as the major seismogenic source. Alternatively, some authors ascribe the high seismogenic potential of the region to NE-dipping normal faults identified by morphotectonic surveys along the ridge bounding the basin to the West. These uncertainties mainly derive from the poor performance of commercial reflection profiling that suffers from an extreme structural complexity and unfavorable near-surface conditions. To overcome these drawbacks, ENI and Shell Italia carried out a non-conventional wide-aperture survey with densely spaced sources (60 m) and receivers (90 m). The 18-km-long wide-aperture profile crosses the basin, yielding a unique opportunity to get new insights into the crustal structure by using advanced imaging techniques. Here, we apply a two-step imaging procedure. We start determining multi- scale Vp images down to 2.5 km depth by using a non-linear traveltime tomographic technique able to cope with strongly heterogeneous media. Assessment of an accurate reference Vp model is indeed crucial for the subsequent application of a frequency-domain full-waveform inversion aimed at improving spatial resolution of the velocity images. Frequency components of the data are then iteratively inverted from low to high frequency values in order to progressively incorporate smaller wavelength components into the model. Inversion results accurately image the shallow crust, yielding valuable constraints for a better understanding of the recent basin evolution and of the surrounding normal-fault systems.

Faulting apparently related to the 1994 Northridge, California, earthquake and possible co-seismic origin of surface cracks in Potrero Canyon, Los Angeles County, California

USGS Publications Warehouse

Catchings, R.D.; Goldman, M.R.; Lee, W.H.K.; Rymer, M.J.; Ponti, D.J.

1998-01-01

Apparent southward-dipping, reverse-fault zones are imaged to depths of about 1.5 km beneath Potrero Canyon, Los Angeles County, California. Based on their orientation and projection to the surface, we suggest that the imaged fault zones are extensions of the Oak Ridge fault. Geologic mapping by others and correlations with seismicity studies suggest that the Oak Ridge fault is the causative fault of the 17 January 1994 Northridge earthquake (Northridge fault). Our seismically imaged faults may be among several faults that collectively comprise the Northridge thrust fault system. Unusually strong shaking in Potrero Canyon during the Northridge earthquake may have resulted from focusing of seismic energy or co-seismic movement along existing, related shallow-depth faults. The strong shaking produced ground-surface cracks and sand blows distributed along the length of the canyon. Seismic reflection and refraction images show that shallow-depth faults may underlie some of the observed surface cracks. The relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have developed from nontectonic alluvial movement, but others may be fault related. Immediately beneath the surface cracks, P-wave velocities are unusually low (<400 m/sec), and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m. On the basis of velocity data, we suggest that unconsolidated soils (<800 m/sec) extend to depths of about 15 to 20 m beneath our datum (<25 m below ground surface). The underlying rocks range in velocity from about 1000 to 5000 m/sec in the upper 100 m. This study illustrates the utility of high-resolution seismic imaging in assessing local and regional seismic hazards.

The shallow sedimentary and structural deformation in the southern Longmen Shan: constraints on the seismotectonics of the 2013 Lushan Mw6.7 Earthquake

NASA Astrophysics Data System (ADS)

Lu, R.; Xu, X.; He, D.; Suppe, J.

2017-12-01

On April 20, 2013, an unexpected Mw 6.7 earthquake occurred in Lushan County at the southern Longmen Shan, the eastern margin of the Tibetan Plateau. After this Lushan earthquake, whether the seismogenic fault is a high-angle or low-angle fault? The structural characteristics, attribution, and the seismotectonic model of this earthquake have many debates and problems. In this study, a high-resolution seismic reflection profile was combined with near-surface geological data, earthquake relocation and geodetic measurements, and a recent deep artificial seismic reflection profile to identify the active fault and seismotectonics of this earthquake. Three-dimensional imaging of the aftershocks was used to identify two planar faults that together form a y-shape (f1 and f2). Seismic interpretations suggest that the seismogenic fault f1 is a typical basement blind fault that did not penetrate into the overlying Mesozoic and Cenozoic units, and it is not a Shuangshi-Dachuan fault (F4) or the frontal Dayi buried fault (F6). Geodetic measurements suggest that the coseismic deformation is consistent with the geometry and kinematics of shear fault-bend folding (FBF). The history of tectonic evolution since the Paleozoic in Longmen Shan area also referred. There are three major detachments control the structural deformation of the upper crust in the Longmen Shan and Western Sichuan Basin, resulting in multiple superimposed deformation events. Deep seismic data indicate the syndepositional nature of fault f1 a preexisting normal fault older than the Triassic, which underwent positive inversion tectonics during the Late Cenozoic. A thrust fault f3 converges with f1 at a depth of approximately12 km with an accumulated slip 3.6 km. This 2013 Lushan earthquake triggered by blind faults is a hidden earthquake. Since the Late Cenozoic, with the strong and on-going compression of the Qinghai-Tibet Plateau to the Sichuan Basin, the early-period normal faults were activated after inversion and triggered Lushan earthquakes. Blind and reactivated faults increase the potential risk and uncertainty related to earthquakes in the eastern margin of the Tibetan Plateau.

CO2/Brine transport into shallow aquifers along fault zones.

PubMed

Keating, Elizabeth H; Newell, Dennis L; Viswanathan, Hari; Carey, J W; Zyvoloski, G; Pawar, Rajesh

2013-01-02

Unintended release of CO(2) from carbon sequestration reservoirs poses a well-recognized risk to groundwater quality. Research has largely focused on in situ CO(2)-induced pH depression and subsequent trace metal mobilization. In this paper we focus on a second mechanism: upward intrusion of displaced brine or brackish-water into a shallow aquifer as a result of CO(2) injection. Studies of two natural analog sites provide insights into physical and chemical mechanisms controlling both brackish water and CO(2) intrusion into shallow aquifers along fault zones. At the Chimayó, New Mexico site, shallow groundwater near the fault is enriched in CO(2) and, in some places, salinity is significantly elevated. In contrast, at the Springerville, Arizona site CO(2) is leaking upward through brine aquifers but does not appear to be increasing salinity in the shallow aquifer. Using multiphase transport simulations we show conditions under which significant CO(2) can be transported through deep brine aquifers into shallow layers. Only a subset of these conditions favor entrainment of salinity into the shallow aquifer: high aspect-ratio leakage pathways and viscous coupling between the fluid phases. Recognition of the conditions under which salinity is favored to be cotransported with CO(2) into shallow aquifers will be important in environmental risk assessments.

A preliminary study on surface ground deformation near shallow foundation induced by strike-slip faulting

NASA Astrophysics Data System (ADS)

Wong, Pei-Syuan; Lin, Ming-Lang

2016-04-01

According to investigation of recent earthquakes, ground deformation and surface rupture are used to map the influenced range of the active fault. The zones of horizontal and vertical surface displacements and different features of surface rupture are investigated in the field, for example, the Greendale Fault 2010, MW 7.1 Canterbury earthquake. The buildings near the fault rotated and displaced vertically and horizontally due to the ground deformation. Besides, the propagation of fault trace detoured them because of the higher rigidity. Consequently, it's necessary to explore the ground deformation and mechanism of the foundation induced by strike-slip faulting for the safety issue. Based on previous study from scaled analogue model of strike-slip faulting, the ground deformation is controlled by material properties, depth of soil, and boundary condition. On the condition controlled, the model shows the features of ground deformation in the field. This study presents results from shear box experiment on small-scale soft clay models subjected to strike-slip faulting and placed shallow foundations on it in a 1-g environment. The quantifiable data including sequence of surface rupture, topography and the position of foundation are recorded with increasing faulting. From the result of the experiment, first en echelon R shears appeared. The R shears rotated to a more parallel angle to the trace and cracks pulled apart along them with increasing displacements. Then the P shears crossed the basement fault in the opposite direction appears and linked R shears. Lastly the central shear was Y shears. On the other hand, the development of wider zones of rupture, higher rising surface and larger the crack area on surface developed, with deeper depth of soil. With the depth of 1 cm and half-box displacement 1.2 cm, en echelon R shears appeared and the surface above the fault trace elevated to 1.15 mm (Dv), causing a 1.16 cm-wide zone of ground-surface rupture and deformation (W). Compared to the investigation in field, rupture of the Greendale Fault, produced a 30-km-long, 300-m-wide zone of ground-surface rupture and deformation (W), involving 5.29 m maximum horizontal , 1.45 m maximum vertical (Dv, max) and 2.59 m average net displacement. Meanwhile, en echelon R shears and cracks were recorded in some region. Besides, the 400-m depth of deep sedimentation (Ds) in the Christchurch City area. Greendale Fault showed close ratio Dv/Ds and W/Ds compared to the experimental case (in the same order), which indicated the wide zone of ground-surface rupture and deformation may be normalized with the vertical displacement (Dv). The foundation located above the basement-fault trace had obvious horizontal displacements and counter-clockwise rotation with increasing displacement. Horizontal displacements and rotation decreased with deeper depth of soil. The deeper embedded foundation caused more rotation. Besides, the soil near the foundation is confined and pressed when it rotates. Key words: strike-slip fault, shallow foundation, ground deformation

A comparison of long-baseline strain data and fault creep records obtained near Hollister, California

USGS Publications Warehouse

Slater, L.E.; Burford, R.O.

1979-01-01

A comparison of creepmeter records from nine sites along a 12-km segment of the Calaveras fault near Hollister, California and long-baseline strain changes for nine lines in the Hollister multiwavelength distance-measuring (MWDM) array has established that episodes of large-scale deformation both preceded and accompanied periods of creep activity monitored along the fault trace during 1976. A concept of episodic, deep-seated aseismic slip that contributes to loading and subsequent aseismic failure of shallow parts of the fault plane seems attractive, implying that the character of aseismic slip sensed along the surface trace may be restricted to a relatively shallow (~ 1-km) region on the fault plane. Preliminary results from simple dislocation models designed to test the concept demonstrate that extending the time-histories and amplitudes of creep events sensed along the fault trace to depths of up to 10 km on the fault plane cannot simulate adequately the character and amplitudes of large-scale episodic movements observed at points more than 1 km from the fault. Properties of a 2-3-km-thick layer of unconsolidated sediments present in Hollister Valley, combined with an essentially rigid-block behavior in buried basement blocks, might be employed in the formulation of more appropriate models that could predict patterns of shallow fault creep and large-scale displacements much more like those actually observed. ?? 1979.

Influence of lithostatic stress on earthquake stress drops in North America

USGS Publications Warehouse

Boyd, Oliver; McNamara, Daniel E.; Hartzell, Stephen; Choy, George

2017-01-01

We estimate stress drops for earthquakes in and near the continental United States using the method of spectral ratios. The ratio of acceleration spectra between collocated earthquakes recorded at a given station removes the effects of path and recording site and yields source parameters including corner frequency for, and the ratio of seismic moment between, the two earthquakes. We determine stress drop from these parameters for 1121 earthquakes greater than M∼3 in 60 earthquake clusters. We find that the average Brune stress drop for the few eastern United States (EUS) tectonic mainshocks studied (2.6–36 MPa) is about three times greater than that of tectonic mainshocks in the western United States (WUS, 1.0–7.9 MPa) and five times greater than mainshocks potentially induced by wastewater injection in the central United States (CUS, 0.6–5.6 MPa). EUS events tend to be deeper thrusting events, whereas WUS events tend to be shallower but have a wide range of focal mechanisms. CUS events tend to be shallow with strike‐slip to normal‐faulting mechanisms. With the possible exception of CUS aftershocks, we find that differences in stress drop among all events can be taken into account, within one standard deviation of significance, by differences in the shear failure stress as outlined by Mohr–Coulomb theory. The shear failure stress is a function of vertical stress (or depth), the fault style (normal, strike slip, or reverse), and coefficient of friction (estimated here to be, on average, 0.64). After accounting for faulting style and depth dependence, we find that the average Brune stress drop is about 3% of the failure stress. These results suggest that high‐frequency shaking hazard (>∼1  Hz) from shallow induced events and aftershocks is reduced to some extent by lower stress drop. However, the shallow hypocenters will increase hazard within several kilometers of the source.

From Extension to Transcurrence: Regime Transition as a new key to Interpret Seismogenesis in the Southern Apennines (Italy)

NASA Astrophysics Data System (ADS)

Fracassi, U.; Vannoli, P.; Burrato, P.; Basili, R.; Tiberti, M. M.; di Bucci, D.; Valensise, G.

2006-12-01

The backbone of the Southern Apennines is perhaps the largest seismic moment release area in Italy. The region is dominated by an extensional regime dating back to the Middle Pleistocene, with maximum extension striking SW-NE (i.e. orthogonal to the mountain belt). The full length (~ 200 km) of the mountain range has been the locus of several destructive earthquakes occurring in the uppermost 10-12 km of the crust. This seismicity is due to a well documented normal faulting mechanism. Instrumental earthquakes (e.g. 5 May 1990, 31 Oct 2002, 1 Nov 2002; all M 5.8) that have occurred in the foreland, east of the Southern Apennines, have posed new questions concerning seismogenic processes in southern Italy. Although of moderate magnitude, these events unveiled the presence of E-W striking, deeper (13-25 km) strike-slip faults. Recent studies suggest that these less known faults belong to inherited shear zones with a multi-phase tectonic history, the most recent phase being a right-lateral reactivation. The direction of the maximum horizontal extension of these faults (in a transcurrent regime) coincides with the maximum horizontal extension in the core of the Southern Apennines (in an extensional regime) and both are compatible with the general framework provided by the Africa-Europe convergence. However, the regional extent along strike of the E-W shear zones poses the issue of their continuity from the foreland towards the thrust-belt. The 1456 (M 6.9) and 1930 (M 6.7) earthquakes, that occurred just east of the main extensional axis, were caused by faults having a strike intermediate between the E-W, deeper strike-slip faults in the foreland and the NW-SE-trending, shallower normal faults in the extensional belt. Hence, the location and geometry of these seismogenic sources suggests that there could be a transition zone between the crustal volumes affected by the extensional and transcurrent regimes. To image such transition, we built a 3D model that incorporates data available from surface and subsurface geology (published and unpublished), seismogenic faults, seismicity, focal mechanisms, and gravity anomalies. We explored the mechanisms of fault interaction in the Southern Apennines between the extensional upper portion and the transcurrent deeper portion of the seismogenic layer. In particular, we studied (a) how the reactivation of regional shear zones interacts with an adjacent, although structurally independent, extensional belt; (b) at what depth range the interaction occurs; and (c1) whether oblique slip in earthquakes like the 1930 event is merely due to the geometry of the causative fault, or (c2) such geometry and kinematics are the result of oblique slip due to fault interaction. We propose that (a) the 1456 and 1930 earthquakes are the expression of the transition between the two tectonic regimes, and that (b) these events can be seen as templates of the seismogenic oblique-slip faulting that occurs at intermediate depths between the shallower extensional faults and the deeper strike-slip faults. These findings suggest that a transtensional faulting mechanism governs the release of major earthquakes in the transition zone between extensional and transcurrent domains.

Hydrochemical-isotopic and hydrogeological conceptual model of the Las Tres Vírgenes geothermal field, Baja California Sur, México

NASA Astrophysics Data System (ADS)

Portugal, E.; Birkle, P.; Barragán R, R. M.; Arellano G, V. M.; Tello, E.; Tello, M.

2000-09-01

Based on geological, structural, hydrochemical and isotopic data, a hydrogeological conceptual model for the geothermal reservoir, shallow wells and springs at the Las Tres Vírgenes geothermal field and its surroundings is proposed. The model explains the genesis of different types of thermal and cold groundwater in the NW (El Azufre Valley, Las Tres Vírgenes and Aguajito complex), NE (Reforma complex) and S (Sierra Mezquital) areas. Shallow groundwater of sulfate type in the NW zone is explained by the rise of CO2-H2S vapor from a shallow magma chamber and the subsequent heating up of a shallow aquifer. Vertical communication between the reservoir and the surface is facilitated by a series of extensional, NW-SE-trending normal faults, forming the graben structures of the Santa Rosalía Basin. Low-permeability characteristics of the geological formations of the study area support the hypothesis of a fracture and fault-dominated, subterranean-flow circulation system. The Na- (Cl-HCO3) composition of springs in the NE and SE zones indicates influence of ascending geothermal fluids, facilitated by radial fault systems of the Reforma caldera and probably the existence of a shallow magma chamber. Close to the surface, the rising geothermal fluids are mixed up with meteoric water from a shallow aquifer. The Las Tres Vírgenes and the Reforma complex are separated by younger, N-S-trending lateral shearing faults, such as the Cimarrón fault; such disposition explains the genesis of different hydrogeological flow regimes on both sides. HCO3-type surface water from the southern zone between San Ignacio and Mezquital is of typical meteoric origin, with no influence of geothermal fluids. Due to arid climatic conditions in the study zone, recent recharge in the geothermal area seems improbable; thus, recent interaction between the surface and the geothermal reservoir can be excluded. Furthermore, isotopic and hydrochemical data exclude the presence of marine water from the Gulf of California in the deep reservoir. Both conditions indicate recharge of the reservoir by meteoric water during glacial periods in Holocene or Pleistocene time, or a magmatic origin of the reservoir fluids. The slightly positive slope of the δ18O-δD line of geothermal fluids and its intermediate isotopic composition-between the surface samples and magmatic (;andesitic;) water indicate that magmatic (;andesitic;) water contributes approximately 30% to the geothermal fluid composition, whereas ;fossil; meteoric water represents the major component (70%). The geothermal reservoir is considered to represent a hydrostatic, stagnant flow system. Based on the observed linear correlation between the isotopic composition and the altitude of the surface manifestations, the isotopic composition and altitude of the former recharge were determined as δ18O=-9.7‰ and δD=-67.3‰, and 350 m.a.s.l., respectively. This altitude is interpreted as mean (average) recharge elevation. Scarcity of permanent rivers, low density of springs and domestic wells, as well as low precipitation rates, reflect restricted distribution of shallow groundwater systems in the study zone. These systems are related to isolated, local aquifers composed of valley fillings.

How Might Draining Lake Campotosto Affect Stress and Seismicity on the Monte Gorzano Normal Fault, Central Italy?

NASA Astrophysics Data System (ADS)

Verdecchia, A.; Deng, K.; Harrington, R. M.; Liu, Y.

2017-12-01

It is broadly accepted that large variations of water level in reservoirs may affect the stress state on nearby faults. While most studies consider the relationship between lake impoundment and the occurrence of large earthquakes or seismicity rate increases in the surrounding region, very few examples focus on the effects of lake drainage. The second largest reservoir in Europe, Lake Campotosto, is located on the hanging wall of the Monte Gorzano fault, an active normal fault responsible for at least two M ≥ 6 earthquakes in historical times. The northern part of this fault ruptured during the August 24, 2016, Mw 6.0 Amatrice earthquake, increasing the probability for a future large event on the southern section where an aftershock sequence is still ongoing. The proximity of the Campotosto reservoir to the active fault aroused general concern with respect to the stability of the three dams bounding the reservoir if the southern part of the Monte Gorzano fault produces a moderate earthquake. Local officials have proposed draining the reservoir as hazard mitigation strategy to avoid possible future catastrophes. In efforts to assess how draining the reservoir might affect earthquake nucleation on the fault, we use a finite-element poroelastic model to calculate the evolution of stress and pore pressure in terms of Coulomb stress changes that would be induced on the Monte Gorzano fault by emptying the Lake Campotosto reservoir. Preliminary results show that an instantaneous drainage of the lake will produce positive Coulomb stress changes, mostly on the shallower part of the fault (0 to 2 km), while a stress drop of the order of 0.2 bar is expected on the Monte Gorzano fault between 0 and 8 km depth. Earthquake hypocenters on the southern portion of the fault currently nucleate between 5 and 13 km depth, with activity distributed nearby the reservoir. Upcoming work will model the effects of varying fault geometry and elastic parameters, including geological layering. In addition, we will consider more realistic unloading strategies to test the time-dependent stress and pore pressure changes on the fault.

Shallow Lunar Seismic Activity and the Current Stress State of the Moon

NASA Technical Reports Server (NTRS)

Watters, Thomas R.; Weber, Renee C.; Collins, Geoffrey C.; Johnson, Catherine L.

2017-01-01

A vast, global network of more than 3200 lobate thrust fault scarps has been revealed in high resolution Lunar Reconnaissance Orbiter Camera (LROC) images. The fault scarps are very young, less than 50 Ma, based on their small scale and crisp appearance, crosscutting relations with small-diameter impact craters, and rates of infilling of associated small, shallow graben and may be actively forming today. The population of young thrust fault scarps provides a window into the recent stress state of the Moon and offers insight into the origin of global lunar stresses. The distribution of orientations of the fault scarps is non-random, inconsistent with isotropic stresses from late-stage global contraction as the sole source of stress. Modeling shows that tidal stresses contribute significantly to the current stress state of the lunar crust. Tidal stresses (orbital recession and diurnal tides) superimposed on stresses from global contraction result in non-isotropic compressional stress and may produce thrust faults consistent with lobate scarp orientations. At any particular point on the lunar surface, peak compressive stress will be reached at a certain time in the diurnal cycle. Coseismic slip events on currently active thrust faults are expected to be triggered when peak stresses are reached. Analysis of the timing of the 28 the shallow moonquakes recorded by the Apollo seismic network shows that 19 indeed occur when the Moon is closer to apogee, while only 9 shallow events occur when the Moon is closer to perigee. Here we report efforts to refine the model for the current stress state of the Moon by investigating the contribution of polar wander. Progress on relocating the epicentral locations of the shallow moonquakes using an algorithm designed for sparse networks is also reported.

Criteria for Seismic Splay Fault Activation During Subduction Earthquakes

NASA Astrophysics Data System (ADS)

Dedontney, N.; Templeton, E.; Bhat, H.; Dmowska, R.; Rice, J. R.

2008-12-01

As sediment is added to the accretionary prism or removed from the forearc, the material overlying the plate interface must deform to maintain a wedge structure. One of the ways this internal deformation is achieved is by slip on splay faults branching from the main detachment, which are possibly activated as part of a major seismic event. As a rupture propagates updip along the plate interface, it will reach a series of junctions between the shallowly dipping detachment and more steeply dipping splay faults. The amount and distribution of slip on these splay faults and the detachment determines the seafloor deformation and the tsunami waveform. Numerical studies by Kame et al. [JGR, 2003] of fault branching during dynamic slip-weakening rupture in 2D plane strain showed that branch activation depends on the initial stress state, rupture velocity at the branching junction, and branch angle. They found that for a constant initial stress state, with the maximum principal stress at shallow angles to the main fault, branch activation is favored on the compressional side of the fault for a range of branch angles. By extending the part of their work on modeling the branching behavior in the context of subduction zones, where critical taper wedge concepts suggest the angle that the principal stress makes with the main fault is shallow, but not horizontal, we hope to better understand the conditions for splay fault activation and the criteria for significant moment release on the splay. Our aim is to determine the range of initial stresses and relative frictional strengths of the detachment and splay fault that would result in seismic splay fault activation. In aid of that, we conduct similar dynamic rupture analyses to those of Kame et al., but use explicit finite element methods, and take fuller account of overall structure of the zone (rather than focusing just on the branching junction). Critical taper theory requires that the basal fault be weaker than the overlying material, so we build on previous work by incorporating the effect of strength contrasts between the basal and splay faults. The relative weakness of the basal fault is often attributed to high pore pressures, which lowers the effective normal stress and brings the basal fault closer to failure. We vary the initial stress state, while maintaining a constant principal stress orientation, to see how the closeness to failure affects the branching behavior for a variety of branch step-up angles.

Frictional power dissipation on plate boundary faults: Implications for coseismic slip propagation at near-surface depths

NASA Astrophysics Data System (ADS)

Ikari, M.; Kopf, A.; Saffer, D. M.; Marone, C.; Carpenter, B. M.

2013-12-01

The general lack of earthquake slip at shallow (< ~4 km) depths on plate-boundary faults suggests that they creep stably, a behavior associated with laboratory observations that disaggregated fault gouges commonly strengthen with increasing sliding velocity (i.e. velocity-strengthening friction), which precludes strain energy release via stress drops. However, the 2011 Tohoku earthquake demonstrated that coseismic rupture and slip can sometimes propagate to the surface in subduction zones. Surface rupture is also known to occur on other plate boundary faults, such as the Alpine Fault in New Zealand. It is uncertain how the extent of coseismic slip propagation from depth is controlled by the frictional properties of the near-surface portion of major faults. In these situations, it is common for slip to localize within gouge having a significant component of clay minerals, which laboratory experiments have shown are generally weak and velocity strengthening. However, low overall fault strength should facilitate coseismic slip, while velocity-strengthening behavior would resist it. In order to investigate how frictional properties may control the extent of coseismic slip propagation at shallow depths, we compare frictional strength and velocity-dependence measurements using samples from three subduction zones known for hosting large magnitude earthquakes. We focus on samples recovered during scientific drilling projects from the Nankai Trough, Japan, the Japan Trench in the region of the Tohoku earthquake, and the Middle America Trench, offshore Costa Rica; however we also include comparisons with other major fault zones sampled by drilling. In order to incorporate the combined effects of overall frictional strength and friction velocity-dependence, we estimate shear strength as a function of slip velocity (at constant effective normal stress), and integrate this function to obtain the areal power density, or frictional power dissipation capability of the fault zone. We also explore the role of absolute shear stress level before arrival of a propagating rupture. Preliminary results show that weak, velocity-strengthening fault zones have a low net power density, but are unlikely to contribute to instability via dynamic stress drops unless they are initially very close to failure. By contrast, strong and velocity-weakening faults will tend to resist coseismic slip by consuming energy if stresses are initially low; however their velocity-weakening nature means that they can support a stress drop even if relatively far below their failure strength.

Possible Strain Partitioning Between the Kumano Forearc Basin and the Slope of the Nankai Trough Accretionary Prism

NASA Astrophysics Data System (ADS)

Martin, K. M.; Gulick, S. P.; Bangs, N. L.; Ashi, J.; Moore, G. F.; Nakamura, Y.; Tobin, H. J.

2008-12-01

A 12 km wide, 56 km long, three-dimensional (3-D) seismic volume acquired over the Nankai Trough offshore the Kii Peninsula, Japan images the Nankai accretionary prism, forearc basin and the subducting Philippine Sea Plate. We have analyzed an unusual, trench-parallel ~1200 m deep depression (a "notch") along the seaward edge of the Kumano forearc basin, just landward of the shallowest branch of the previously- mapped splay-fault system. The shape of this feature varies along strike, from a single, steep-walled, ~3.5 km wide notch in the northeast, to a broader, ~6 km wide zone with several shallower linear bathymetric lows in the southwest. We have mapped the area below the notch and found both vertical faults and faults which dip toward the central axis of the depression. Some dipping faults appear to have normal offset, consistent with the formation of a bathymetric low. Some of these dipping faults may join the central vertical fault(s) at depth, creating apparent flower structures. Offset on the vertical faults is more difficult to determine, but the dip and along-strike geometry of these faults makes predominantly normal or thrust motion unlikely. We conclude, therefore, that the notch feature is the bathymetric expression of a transtensional fault system. Possible causes for such a system in the forearc include variations in splay fault geometry and strain partitioning. By considering only the along-strike variability of the mapped splay fault, we were unable to explain a transform feature at the scale of the notch. Strike-slip faulting at the seaward edge of forearc basins is also observed in Sumatra and is there attributed to strain partitioning due to oblique convergence. The wedge and décollment strength variations which control the location of the forearc basins may therefore play a role in the position where the along-strike component of deformation is localized. While the obliquity of convergence in the Nankai trough is comparatively small (13-30 degrees), we believe it is still significant enough to account for the formation of the observed notch.

The July 12, 1993, Hokkaido-Nansei-Oki, Japan, earthquake: Coseismic slip pattern from strong-motion and teleseismic recordings

USGS Publications Warehouse

Mendoza, C.; Fukuyama, E.

1996-01-01

We employ a finite fault inversion scheme to infer the distribution of coseismic slip for the July 12, 1993, Hokkaido-Nansei-Oki earthquake using strong ground motions recorded by the Japan Meteorological Agency within 400 km of the epicenter and vertical P waveforms recorded by the Global Digital Seismograph Network at teleseismic distances. The assumed fault geometry is based on the location of the aftershock zone and comprises two fault segments with different orientations: a northern segment striking at N20??E with a 30?? dip to the west and a southern segment with a N20??W strike. For the southern segment we use both westerly and easterly dip directions to test thrust orientations previously proposed for this portion of the fault. The variance reduction is greater using a shallow west dipping segment, suggesting that the direction of dip did not change as the rupture propagated south from the hypocenter. This indicates that the earthquake resulted from the shallow underthrusting of Hokkaido beneath the Sea of Japan. Static vertical movements predicted by the corresponding distribution of fault slip are consistent with the general pattern of surface deformation observed following the earthquake. Fault rupture in the northern segment accounts for about 60% of the total P wave seismic moment of 3.4 ?? 1020 N m and includes a large circular slip zone (4-m peak) near the earthquake hypocenter at depths between 10 and 25 km. Slip in the southern segment is also predominantly shallower than 25 km, but the maximum coseismic displacements (2.0-2.5 m) are observed at a depth of about 5 km. This significant shallow slip in the southern portion of the rupture zone may have been responsible for the large tsunami that devastated the small offshore island of Okushiri. Localized shallow faulting near the island, however, may require a steep westerly dip to reconcile the measured values of ground subsidence.

Cenozoic stratigraphy and structure of the Chesapeake Bay region

USGS Publications Warehouse

Powars, David S.; Edwards, Lucy E.; Kidwell, Susan M.; Schindler, J. Stephen

2015-01-01

The Salisbury embayment is a broad tectonic downwarp that is filled by generally seaward-thickening, wedge-shaped deposits of the central Atlantic Coastal Plain. Our two-day field trip will take us to the western side of this embayment from the Fall Zone in Washington, D.C., to some of the bluffs along Aquia Creek and the Potomac River in Virginia, and then to the Calvert Cliffs on the western shore of the Chesapeake Bay. We will see fluvial-deltaic Cretaceous deposits of the Potomac Formation. We will then focus on Cenozoic marine deposits. Transgressive and highstand deposits are stacked upon each other with unconformities separating them; rarely are regressive or lowstand deposits preserved. The Paleocene and Eocene shallow shelf deposits consist of glauconitic, silty sands that contain varying amounts of marine shells. The Miocene shallow shelf deposits consist of diatomaceous silts and silty and shelly sands. The lithology, thickness, dip, preservation, and distribution of the succession of coastal plain sediments that were deposited in our field-trip area are, to a great extent, structurally controlled. Surficial and subsurface mapping using numerous continuous cores, auger holes, water-well data, and seismic surveys has documented some folds and numerous high-angle reverse and normal faults that offset Cretaceous and Cenozoic deposits. Many of these structures are rooted in early Mesozoic and/or Paleozoic NE-trending regional tectonic fault systems that underlie the Atlantic Coastal Plain. On Day 1, we will focus on two fault systems (stops 1–2; Stafford fault system and the Skinkers Neck–Brandywine fault system and their constituent fault zones and faults). We will then see (stops 3–5) a few of the remaining exposures of largely unlithified marine Paleocene and Eocene strata along the Virginia side of the Potomac River including the Paleocene-Eocene Thermal Maximum boundary clay. These exposures are capped by fluvial-estuarine Pleistocene terrace deposits. On Day 2, we will see (stops 6–9) the classic Miocene section along the ~25 miles (~40 km) of Calvert Cliffs in Maryland, including a possible fault and structural warping. Cores from nearby test holes will also be shown to supplement outcrops.

Crustal architecture of an inverted back arc rift basin, Niigata, central Japan

NASA Astrophysics Data System (ADS)

Sato, H.; Abe, S.; Kawai, N.; Saito, H.; Kato, N.; Ishiyama, T.; Iwasaki, T.; Kurashimo, E.; Inaba, M.; Van Horne, A.

2012-04-01

A back arc rift basin, formed during the Miocene opening of the Japan Sea, now uplifted and exposed in Niigata, central Japan, provides an exceptional opportunity to study a back arc rift formed on a short time scale and in a still active setting for the present day shortening deformation. Due to stress build up before the 2011 Tohoku earthquake (M9), two damaging earthquakes (M6.8) occurred in 2004 and 2007 in this inverted rift basin. Deep seismic profiling was performed along four seismic lines between 2008 and 2011. We used onshore-offshore deep seismic reflection profiling to examine the crustal architecture of the back arc basin, in particular the geometry of the source faults. We further applied refraction tomography analysis to distinguish between previously undifferentiated syn-rift volcanics and pre-rift Mesozoic rock based on P-wave velocity. Our findings indicate that the Miocene rift structure created during the extensional phase regulates the style of deformation and the geometry of the source faults in the current compressional regime. Syn-rift volcanics with a maximum thickness of 6 km filled the fault controlled basins as rifting proceeded. The volcanism was bimodal, comprising a reflective unit of mafic rocks around the rift axis and a non-reflective unit of felsic rocks near the margins of the basins. Once rifting ended, thermal subsidence, and subsequently, mechanical subsidence related to the onset of the compressional regime, allowed deposition of up to 5 km of post-rift, deep marine to fluvial sedimentation, including the Teradomari Formation, an over-pressured mudstone in the middle of the section that later became an important shallow detachment layer. Continued compression has caused fault-related fold and wedge thrusting in the post-rift sedimentary strata which are highly deformed by thin-skin style deformation. Since the Pliocene, normal faults created during the rift phase have been reactivated as reverse faults, including a shallow detachment in the Teradomari Formation which forms a complicated shortened deformation structure. Quaternary geomorphology suggests ongoing shortening. Transform faults inherited from the rift stage control the extent of present day reverse source faults and more importantly, earthquake magnitude.

Geophysical Characterization of Groundwater-Fault Dynamics at San Andreas Oasis

NASA Astrophysics Data System (ADS)

Faherty, D.; Polet, J.; Osborn, S. G.

2017-12-01

The San Andreas Oasis has historically provided a reliable source of fresh water near the northeast margin of the Salton Sea, although since the recent completion of the Coachella Canal Lining Project and persistent drought in California, surface water at the site has begun to disappear. This may be an effect of the canal lining, however, the controls on groundwater are complicated by the presence of the Hidden Springs Fault (HSF), a northeast dipping normal fault that trends near the San Andreas Oasis. Its surface expression is apparent as a lineation against which all plant growth terminates, suggesting that it may form a partial barrier to subsurface groundwater flow. Numerous environmental studies have detailed the chemical evolution of waters resources at San Andreas Spring, although there remains a knowledge gap on the HSF and its relation to groundwater at the site. To better constrain flow paths and characterize groundwater-fault interactions, we have employed resistivity surveys near the surface trace of the HSF to generate profiles of lateral and depth-dependent variations in resistivity. The survey design is comprised of lines installed in Wenner Arrays, using an IRIS Syscal Kid, with 24 electrodes, at a maximum electrode spacing of 5 meters. In addition, we have gathered constraints on the geometry of the HSF using a combination of ground-based magnetic and gravity profiles, conducted with a GEM walking Proton Precession magnetometer and a Lacoste & Romberg gravimeter. Seventeen gravity measurements were acquired across the surface trace of the fault. Preliminary resistivity results depict a shallow conductor localized at the oasis and discontinuous across the HSF. Magnetic data reveal a large contrast in subsurface magnetic susceptibility that appears coincident with the surface trace and trend of the HSF, while gravity data suggests a shallow, relatively high density anomaly centered near the oasis. These data also hint at a second, previously undocumented fault bounding the opposite margin of the oasis and trending subparallel to the HSF. We thus speculate that the Hidden Springs Fault and this possible secondary fault act as partial barriers to lateral subsurface flow and form a structural wedge, localizing groundwater beneath the oasis.

Pore Pressure Evolution in Shallow Subduction Earthquake Sequences and Effects on Aseismic Slip Transients -- Numerical Modeling With Rate and State Friction

NASA Astrophysics Data System (ADS)

Liu, Y.; Rice, J. R.

2005-12-01

In 3D modeling of long tectonic loading and earthquake sequences on a shallow subduction fault [Liu and Rice, 2005], with depth-variable rate and state friction properties, we found that aseismic transient slip episodes emerge spontaneously with only a simplified representation of effects of metamorphic fluid release. That involved assumption of a constant in time but uniformly low effective normal stress in the downdip region. As suggested by observations in several major subduction zones [Obara, 2002; Rogers and Dragert, 2003; Kodaira et al, 2004], the presence of fluids, possibly released from dehydration reactions beneath the seismogenic zone, and their pressurization within the fault zone may play an important role in causing aseismic transients and associated non-volcanic tremors. To investigate the effects of fluids in the subduction zone, particularly on the generation of aseismic transients and their various features, we develop a more complete physical description of the pore pressure evolution (specifically, pore pressure increase due to supply from dehydration reactions and shear heating, decrease due to transport and dilatancy during slip), and incorporate that into the rate and state based 3D modeling. We first incorporated two important factors, dilatancy and shear heating, following Segall and Rice [1995, 2004] and Taylor [1998]. In the 2D simulations (slip varies with depth only), a dilatancy-stabilizing effect is seen which slows down the seismic rupture front and can prevent rapid slip from extending all the way to the trench, similarly to Taylor [1998]. Shear heating increases the pore pressure, and results in faster coseismic rupture propagation and larger final slips. In the 3D simulations, dilatancy also stabilizes the along-strike rupture propagation of both seismic and aseismic slips. That is, aseismic slip transients migrate along the strike faster with a shorter Tp (the characteristic time for pore pressure in the fault core to re-equilibrate with that of its surroundings). This is consistent with our previous simulations, which show that the aseismic transients migrate along the strike at a higher speed under a lower, constant in time, effective normal stress. As a combination of the two factors, we show the pore pressure evolution with drops (due to dilatancy during slip) and then rises (due to shear heating) on the fault over multiple time scales. We next plan to formulate, and merge with the slip-rupture analysis, fuller fluid release models based on phase equilibria and models of transport in which the average fault-parallel permeability is a decreasing function of the effective normal stress. The thrust fault zone, at seismogenic depths and slightly downdip, is represented in a conceptually similar manner to the well-studied major continental faults, assuming the fault core materials have a lower permeability than the neighboring damaged zone. Heat diffusion in the fault core and damaged zone will also be considered in the modeling. The simulation results may help to improve our understanding of the processes of the aseismic transients observed within a transform plate boundary along the SAF near Cholame, California [Nadeau and Dolenc, 2005].

Normal-faulting stress state associated with low differential stress in an overriding plate in northeast Japan prior to the 2011 Mw 9.0 Tohoku earthquake

NASA Astrophysics Data System (ADS)

Otsubo, Makoto; Miyakawa, Ayumu; Imanishi, Kazutoshi

2018-03-01

Spatial and temporal variations in inland crustal stress prior to the 2011 Mw 9.0 Tohoku earthquake are investigated using focal mechanism solutions for shallow seismicity in Iwaki City, Japan. The multiple inverse method of stress tensor inversion detected two normal-faulting stress states that dominate in different regions. The stress field around Iwaki City changed from a NNW-SSE-trending triaxial extensional stress (stress regime A) to a NW-SE-trending axial tension (stress regime B) between 2005 and 2008. These stress changes may be the result of accumulated extensional stress associated with co- and post-seismic deformation due to the M7 class earthquakes. In this study we suggest that the stress state around Iwaki City prior to the 2011 Tohoku earthquake may have been extensional with a low differential stress. High pore pressure is required to cause earthquakes under such small differential stresses.

High-resolution seismic reflection imaging of growth folding and shallow faults beneath the Southern Puget Lowland, Washington State

USGS Publications Warehouse

Clement, C.R.; Pratt, T.L.; Holmes, M.L.; Sherrod, B.L.

2010-01-01

Marine seismic reflection data from southern Puget Sound, Washington, were collected to investigate the nature of shallow structures associated with the Tacoma fault zone and the Olympia structure. Growth folding and probable Holocene surface deformation were imaged within the Tacoma fault zone beneath Case and Carr Inlets. Shallow faults near potential field anomalies associated with the Olympia structure were imaged beneath Budd and Eld Inlets. Beneath Case Inlet, the Tacoma fault zone includes an ???350-m wide section of south-dipping strata forming the upper part of a fold (kink band) coincident with the southern edge of an uplifted shoreline terrace. An ???2 m change in the depth of the water bottom, onlapping postglacial sediments, and increasing stratal dips with increasing depth are consistent with late Pleistocene to Holocene postglacial growth folding above a blind fault. Geologic data across a topographic lineament on nearby land indicate recent uplift of late Holocene age. Profiles acquired in Carr Inlet 10 km to the east of Case Inlet showed late Pleistocene or Holocene faulting at one location with ???3 to 4 m of vertical displacement, south side up. North of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad Tacoma fault zone. Seismic reflection profiles across part of the Olympia structure beneath southern Puget Sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding. Directly beneath one of these faults, a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw, suggesting strike-slip motion. Deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset, consistent with strike-slip faulting. These faults and folds indicate that the Tacoma fault and Olympia structure include active structures with probable postglacial motion.

High-resolution seismic reflection imaging of growth folding and shallow faults beneath the Southern Puget Lowland, Washington State

USGS Publications Warehouse

Odum, Jackson K.; Stephenson, William J.; Pratt, Thomas L.; Blakely, Richard J.

2016-01-01

Marine seismic reflection data from southern Puget Sound, Washington, were collected to investigate the nature of shallow structures associated with the Tacoma fault zone and the Olympia structure. Growth folding and probable Holocene surface deformation were imaged within the Tacoma fault zone beneath Case and Carr Inlets. Shallow faults near potential field anomalies associated with the Olympia structure were imaged beneath Budd and Eld Inlets. Beneath Case Inlet, the Tacoma fault zone includes an ∼350-m wide section of south-dipping strata forming the upper part of a fold (kink band) coincident with the southern edge of an uplifted shoreline terrace. An ∼2 m change in the depth of the water bottom, onlapping postglacial sediments, and increasing stratal dips with increasing depth are consistent with late Pleistocene to Holocene postglacial growth folding above a blind fault. Geologic data across a topographic lineament on nearby land indicate recent uplift of late Holocene age. Profiles acquired in Carr Inlet 10 km to the east of Case Inlet showed late Pleistocene or Holocene faulting at one location with ∼3 to 4 m of vertical displacement, south side up. North of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad Tacoma fault zone. Seismic reflection profiles across part of the Olympia structure beneath southern Puget Sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding. Directly beneath one of these faults, a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw, suggesting strike-slip motion. Deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset, consistent with strike-slip faulting. These faults and folds indicate that the Tacoma fault and Olympia structure include active structures with probable postglacial motion.

Insights on the seismotectonics of the western part of northern Calabria (southern Italy) by integrated geological and geophysical data: Coexistence of shallow extensional and deep strike-slip kinematics

NASA Astrophysics Data System (ADS)

Ferranti, L.; Milano, G.; Pierro, M.

2017-11-01

We assess the seismotectonics of the western part of the border area between the Southern Apennines and Calabrian Arc, centered on the Mercure extensional basin, by integrating recent seismicity with a reconstruction of the structural frame from surface to deep crust. The analysis of low-magnitude (ML ≤ 3.5) events occurred in the area during 2013-2017, when evaluated in the context of the structural model, has revealed an unexpected complexity of seismotectonics processes. Hypocentral distribution and kinematics allow separating these events into three groups. Focal mechanisms of the shallower (< 9 km) set of events show extensional kinematics. These results are consistent with the last kinematic event recorded on outcropping faults, and with the typical depth and kinematics of normal faulting earthquakes in the axial part of southern Italy. By contrast, intermediate ( 9-17 km) and deep ( 17-23 km) events have fault plane solutions characterized by strike- to reverse-oblique slip, but they differ from each other in the orientation of the principal axes. The intermediate events have P axes with a NE-SW trend, which is at odds with the NW-SE trend recorded by strike-slip earthquakes affecting the Apulia foreland plate in the eastern part of southern Italy. The intermediate events are interpreted to reflect reactivation of faults in the Apulia unit involved in thrust uplift, and appears aligned along an WNW-ESE trending deep crustal, possibly lithospheric boundary. Instead, deep events beneath the basin, which have P-axis with a NW-SE trend, hint to the activity of a deep overthrust of the Tyrrhenian back-arc basin crust over the continental crust of the Apulia margin, or alternatively, to a tear fault in the underthrust Apulia plate. Results of this work suggest that extensional faulting, as believed so far, does not solely characterizes the seismotectonics of the axial part of the Southern Apennines.

Focal mechanisms and tidal modulation for tectonic tremors in Taiwan

NASA Astrophysics Data System (ADS)

Ide, S.; Yabe, S.; Tai, H. J.; Chen, K. H.

2015-12-01

Tectonic tremors in Taiwan have been discovered beneath the southern Central Range, but their hosting structure has been unknown. Here we constrain the focal mechanism of underground deformation related to tremors, using moment tensor inversion in the very low frequency band and tidal stress analysis. Three types of seismic data are used for two analysis steps: detection of tremors and the moment tensor inversion. Short-period seismograms from CWBSN are used for tremor detection. Broadband seismograms from BATS and the TAIGER project are used for both steps. About 1000 tremors were detected using an envelope correlation method in the high frequency band (2-8 Hz). Broadband seismograms are stacked relative to the tremor timing, and inverted for a moment tensor in the low frequency band (0.02-0.05 Hz). The best solution was obtained at 32 km depth, as a double-couple consistent with a low-angle thrust fault dipping to the east-southeast, or a high-angle thrust with a south-southwest strike. Almost all tremors occur when tidal shear stress is positive and normal stress is negative (clamping). Since the clamping stress is high for a high-angle thrust fault, the low-angle thrust fault is more likely to be the fault plane. Tremor rate increases non-linearly with increasing shear stress, suggesting a velocity strengthening friction law. The high tidal sensitivity is inconsistent with horizontal slip motion suggested by previous studies, and normal faults that dominates regional shallow earthquakes. Our results favor thrust slip on a low-angle fault dipping to the east-southeast, consistent with the subduction of the Eurasian plate. The tremor region is characterized by a deep thermal anomaly with decrease normal stress. This region has also experienced enough subduction to produce metamorphic fluids. A large amount of fluid and low vertical stress may explain the high tidal sensitivity.

Syn-Extensional Constrictional Folding of the Gwoira Rider Block, a Large Fault-Bounded Slice Atop the Mai'iu Low-Angle Normal Fault, Woodlark Rift.

NASA Astrophysics Data System (ADS)

Little, T. A.; Webber, S. M.; Norton, K. P.; Mizera, M.; Oesterle, J.; Ellis, S. M.

2016-12-01

The Mai'iu Fault is an active and corrugated low-angle normal fault (LANF) in Woodlark Rift, Eastern Papua New Guinea, which dips 21° NNE, accommodating rapid N-S extension. The Gwoira rider block is a large fault-bounded sedimentary slice comprising the Gwoira Conglomerate, located within a large synformal megamullion in the Mai'iu Fault surface. The Gwoira Conglomerate was originally deposited on the Mai'iu Fault hanging wall concurrent with extension, and has since been buried to a maximum depth of 1600-2100 m (evidenced by vitrinite reflectance data), back-tilted, and synformally folded. Both the Gwoira Conglomerate (former hanging wall) and mylonitic foliation (footwall) of the Mai'iu Fault have been shortened E-W, perpendicular to the extension direction. We show that E-W synformal folding of the Gwoira Conglomerate was concurrent with ongoing sedimentation and extension on the Mai'iu Fault. Structurally shallower Gwoira Conglomerate strata are folded less than deeper strata, indicating that folding was progressively accrued concurrent with N-S extension. We also show that abandonment of the inactive strand of the Mai'iu Fault in favor of the Gwoira Fault, which resulted in formation of the Gwoira rider block, occurred in response to progressive megamullion amplification and resultant misorientation of the inactive strand of the Mai'iu Fault. We attribute E-W folding to extension-perpendicular constriction. This is consistent with observations of outcrop-scale conjugate strike-slip faults that deform the footwall and hanging wall of the Mai'iu Fault, and accommodate E-W shortening. Constrictional folding remains active in the near-surface as evidenced by synformal tilting of inferred Late Quaternary fluvial terraces atop the Gwoira rider block. This sequence of progressive constrictional folding is dated using 26Al/10Be terrestrial cosmogenic nuclide burial dating of the Gwoira Conglomerate. Finally, because rider block formation records abandonment of the uppermost part of a LANF, Coulomb fault mechanical analysis (after Choi and Buck, 2012) can be applied to field observations to provide an upper limit on LANF frictional strength (µf). Modelling constrains the µf for the Mai'iu Fault to ≤0.25, which suggests that the Mai'iu Fault is frictionally very weak.

Capturing Postseismic Processes of the 2016 Mw 7.1 Kumamoto Earthquake, Japan, Using Dense, Continuous GPS and Short-repeat Time ALOS-2 InSAR Data: Implications for the Shallow Slip Deficit Problem

NASA Astrophysics Data System (ADS)

Milliner, C. W. D.; Burgmann, R.; Wang, T.; Inbal, A.; Bekaert, D. P.; Liang, C.; Fielding, E. J.

2017-12-01

Separating the contribution of shallow coseismic slip from rapidly decaying, postseismic afterslip in surface rupturing events has been difficult to resolve due to the typically sparse configuration of GPS networks and long-repeat time of InSAR acquisitions. Whether shallow fault motion along surface ruptures is a result of coseismic slip, or largely a product of rapid afterslip occurring within the first minutes to days, has significant implications for our understanding of the mechanics and frictional behavior of faulting in the shallow crust. To test this behavior in the case of a major surface rupturing event, we attempt to quantify the co- and postseismic slip of the 2016 Mw 7.1 Kumamoto earthquake sequence using a dense and continuous GPS network ( 10 km spacing), with short-repeat time, ALOS-2 InSAR data. Using the Network Inversion Filter method, we jointly invert the GPS and InSAR data to obtain a time history of afterslip in the first minutes to months following the mainshock. From our initial results, we find no clear evidence of significant shallow afterslip (i.e., no observable slip > 30 cm at depths of < 3 km, a minimum resolvable value), that could account for the 1 m of coseismic deficit of shallow slip inferred from our static finite-fault inversion. Our results show, aside from significant volumetric changes related to poroelastic processes, the majority of shallow fault slip was largely complete after rupture cessation. We also attempt to improve our coseismic slip model by implementing a method that inverts changes in seismicity rates for coseismic slip, helping constrain parts of the model space at depth where geodetic data loses resolving power. The use of geodetic data with the ability to resolve near-field, coseismic deformation and rapidly decaying postseismic processes will aid in our understanding of the frictional properties of shallow faulting, giving more reliable predictions for ground motion simulations and seismic hazard assessments.

Preliminary Geophysical Characterization of a CO2-Driven Geyser in the Rio Grande Rift, New Mexico

NASA Astrophysics Data System (ADS)

Feucht, D. W.; Jensen, K. J.; Kelly, C.; Ryan, J. C.; Ferriz, H.; Kanjorski, N.; Ferguson, J. F.; McPhee, D. K.; Pellerin, L.

2009-12-01

As part of the Summer of Applied Geophysical Experience (SAGE) a preliminary geophysical investigation was conducted in the vicinity of a cold CO2-driven geyser located at Chimayó, NM, along the eastern margin of the Rio Grand Rift. This geyser is of interest as a possible analog for CO2 leakage from deep saline-aquifer carbon sequestration projects. Observed water chemistry variations can be explained by mixing of a CO2-rich, high salinity brine rising into, and mixing with a shallow freshwater aquifer. Several large, basin bounding faults and numerous smaller normal faults cut the area of the well and may constitute the necessary conduit for the deep water. Geophysical methods were used to characterize the subsurface properties at the Chimayó geyser as well as regional structures that may influence groundwater flow in the area. Shallow transient electromagnetic (TEM) data and capactively-coupled resistivity (CCR) data were acquired in close proximity to the geyser. The CCR shows a near-surface resistive feature, possibly hematite-cemented Tesuque formation sediment, in close proximity to the geyser. A shallow, highly conductive layer delineated through modeling of the TEM data is postulated to be a fluid consistent with high levels of Total Dissolved Solid (TDS) content. The well is located almost directly on the Roberts fault, which is antithetic to the basin bounding Chimayó fault 1.5 km to the east. Previously published hydrogeochemical studies associate this fault with high CO2 and TDS water along its strike. Deeper sounding TEM and audiomagnetotelluric (AMT) data were acquired along the Alamo Arroyo, 3 km to the southwest of the well. The Kelley Federal #1 Well located in this arroyo provides deep stratigraphic control to Pennsylvanian carbonate basement at 740 m. Tesuque formation conglomeritic alluvial fan deposits occur between 230 and 708 m and are overlain by finer grained basin floor deposits. The deep, coarse grained unit is thought to be a good, freshwater aquifer. A 2-D model of the AMT data indicates a resistive layer at about 200 m depth, which may correspond to the aquifer. High conductivity observed in the TEM and AMT data likely reflects saline fluids. Gravity data were collected at approximately 500 m station spacing along transects in Arroyo Alamo and Arroyo Seco 2 km to the south, and throughout the Chimayó Valley. The new data, combined with previously collected data, were used to create regional complete Bouguer anomaly and depth to basement maps. Two-dimensional forward models were constructed to provide a geohydrologic framework consistent with migration of CO2 rich brine up the Chimayó fault to mix with freshwater in the conglomeritic aquifer. The shallow, mixed groundwater emerges at the surface along the Roberts fault and drives the geyser at the Roberts well.

Clay-clast aggregates: A new textural evidence for seismic fault sliding?

NASA Astrophysics Data System (ADS)

Boutareaud, Sébastien; Calugaru, Dan-Gabriel; Han, Raehee; Fabbri, Olivier; Mizoguchi, Kazuo; Tsutsumi, Akito; Shimamoto, Toshihiko

2008-03-01

To determine the processes responsible for slip-weakening in clayey gouge zones, rotary-shear experiments were conducted at seismic slip rates (equivalent to 0.9 and 1.3 m/s) at 0.6 MPa normal stress on a natural clayey gouge for saturated and non-saturated initial conditions. The mechanical behavior of the simulated faults shows a reproducible slip-weakening behavior, whatever initial moisture conditions. Examination of gouge obtained at the residual friction stage in saturated and non-saturated initial conditions allows the definition of two types of microstructures: a foliated type reflecting strain localization, and a non-foliated type composed of spherical aggregates. Friction experiments demonstrate that liquid-vapor transition of water within gouge due to frictional heating has a high capacity to explain the formation of spherical aggregates in the first meters of displacement. This result suggests that the occurrence of spherical aggregates in natural clayey fault gouges can constitute a new textural evidence for shallow depth pore water phase transition at seismic slip velocity and consequently for past seismic fault sliding.

Loading Rate Variations Along a Midcrustal Shear Zone Preceding the Mw6.0 Earthquake of 24 August 2016 in Central Italy

NASA Astrophysics Data System (ADS)

Vuan, A.; Sugan, M.; Chiaraluce, L.; Di Stefano, R.

2017-12-01

To identify greater detail in the seismicity pattern preceding the 24 August 2016 Mw6.0 earthquake in Central Italy, we apply waveform matching using 1,028 events as templates. In the 8 months before the mainshock, we find 2,000 additional earthquakes mostly located along a subhorizontal shear zone (SZ) bounding at depth the extensional fault system. Asynchrony is observed in the occurrence of events nucleating along the SZ compared to the ones on fault portions embedded in the shallower upper crust, with the former anticipating the latter. Within the SZ, we also observe along-strike seismic migration episodes with earthquakes pointing toward the Mw6.0 mainshock nucleation zone. These episodes are followed by an apparent quiescence within the main fault area. We suggest that the variations in the seismic activity along the SZ represent the brittle signature of the tectonic loading process enabling portions of the overlaying normal faults to become unlocked.

Interactions between Polygonal Normal Faults and Larger Normal Faults, Offshore Nova Scotia, Canada

NASA Astrophysics Data System (ADS)

Pham, T. Q. H.; Withjack, M. O.; Hanafi, B. R.

2017-12-01

Polygonal faults, small normal faults with polygonal arrangements that form in fine-grained sedimentary rocks, can influence ground-water flow and hydrocarbon migration. Using well and 3D seismic-reflection data, we have examined the interactions between polygonal faults and larger normal faults on the passive margin of offshore Nova Scotia, Canada. The larger normal faults strike approximately E-W to NE-SW. Growth strata indicate that the larger normal faults were active in the Late Cretaceous (i.e., during the deposition of the Wyandot Formation) and during the Cenozoic. The polygonal faults were also active during the Cenozoic because they affect the top of the Wyandot Formation, a fine-grained carbonate sedimentary rock, and the overlying Cenozoic strata. Thus, the larger normal faults and the polygonal faults were both active during the Cenozoic. The polygonal faults far from the larger normal faults have a wide range of orientations. Near the larger normal faults, however, most polygonal faults have preferred orientations, either striking parallel or perpendicular to the larger normal faults. Some polygonal faults nucleated at the tip of a larger normal fault, propagated outward, and linked with a second larger normal fault. The strike of these polygonal faults changed as they propagated outward, ranging from parallel to the strike of the original larger normal fault to orthogonal to the strike of the second larger normal fault. These polygonal faults hard-linked the larger normal faults at and above the level of the Wyandot Formation but not below it. We argue that the larger normal faults created stress-enhancement and stress-reorientation zones for the polygonal faults. Numerous small, polygonal faults formed in the stress-enhancement zones near the tips of larger normal faults. Stress-reorientation zones surrounded the larger normal faults far from their tips. Fewer polygonal faults are present in these zones, and, more importantly, most polygonal faults in these zones were either parallel or perpendicular to the larger faults.

Volcanic facies architecture of an intra-arc strike-slip basin, Santa Rita Mountains, Southern Arizona

NASA Astrophysics Data System (ADS)

Busby, Cathy J.; Bassett, Kari N.

2007-09-01

The three-dimensional arrangement of volcanic deposits in strike-slip basins is not only the product of volcanic processes, but also of tectonic processes. We use a strike-slip basin within the Jurassic arc of southern Arizona (Santa Rita Glance Conglomerate) to construct a facies model for a strike-slip basin dominated by volcanism. This model is applicable to releasing-bend strike-slip basins, bounded on one side by a curved and dipping strike-slip fault, and on the other by curved normal faults. Numerous, very deep unconformities are formed during localized uplift in the basin as it passes through smaller restraining bends along the strike-slip fault. In our facies model, the basin fill thins and volcanism decreases markedly away from the master strike-slip fault (“deep” end), where subsidence is greatest, toward the basin-bounding normal faults (“shallow” end). Talus cone-alluvial fan deposits are largely restricted to the master fault-proximal (deep) end of the basin. Volcanic centers are sited along the master fault and along splays of it within the master fault-proximal (deep) end of the basin. To a lesser degree, volcanic centers also form along the curved faults that form structural highs between sub-basins and those that bound the distal ends of the basin. Abundant volcanism along the master fault and its splays kept the deep (master fault-proximal) end of the basin overfilled, so that it could not provide accommodation for reworked tuffs and extrabasinally-sourced ignimbrites that dominate the shallow (underfilled) end of the basin. This pattern of basin fill contrasts markedly with that of nonvolcanic strike-slip basins on transform margins, where clastic sedimentation commonly cannot keep pace with subsidence in the master fault-proximal end. Volcanic and subvolcanic rocks in the strike-slip basin largely record polygenetic (explosive and effusive) small-volume eruptions from many vents in the complexly faulted basin, referred to here as multi-vent complexes. Multi-vent complexes like these reflect proximity to a continuously active fault zone, where numerous strands of the fault frequently plumb small batches of magma to the surface. Releasing-bend extension promotes small, multivent styles of volcanism in preference to caldera collapse, which is more likely to form at releasing step-overs along a strike-slip fault.

Coseismic deformation of the 2001 El Salvador and 2002 Denali fault earthquakes from GPS geodetic measurements

NASA Astrophysics Data System (ADS)

Hreinsdottir, Sigrun

2005-07-01

GPS geodetic measurements are used to study two major earthquakes, the 2001 MW 7.7 El Salvador and 2002 MW 7.9 Denali Fault earthquakes. The 2001 MW 7.7 earthquake was a normal fault event in the subducting Cocos plate offshore El Salvador. Coseismic displacements of up to 15 mm were measured at permanent GPS stations in Central America. The GPS data were used to constrain the location of and slip on the normal fault. One month later a MW 6.6 strike-slip earthquake occurred in the overriding Caribbean plate. Coulomb stress changes estimated from the M W 7.7 earthquake suggest that it triggered the MW 6.6 earthquake. Coseismic displacement from the MW 6.6 earthquake, about 40 mm at a GPS station in El Salvador, indicates that the earthquake triggered additional slip on a fault close to the GPS station. The MW 6.6 earthquake further changed the stress field in the overriding Caribbean plate, with triggered seismic activity occurring west and possibly also to the east of the rupture in the days to months following the earthquake. The MW 7.9 Denali Fault earthquake ruptured three faults in the interior of Alaska. It initiated with a thrust motion on the Susitna Glacier fault but then ruptured the Denali and Totschunda faults with predominantly right-lateral strike-slip motion unilaterally from west to east. GPS data measured in the two weeks following the earthquake suggest a complex coseismic rupture along the faults with two main regions of moment release along the Denali fault. A large amount of additional data were collected in the year following the earthquake which greatly improved the resolution on the fault, revealing more details of the slip distribution. We estimate a total moment release of 6.81 x 1020 Nm in the earthquake with a M W 7.2 thrust subevent on Susitna Glacier fault. The slip on the Denali fault is highly variable, with 4 main pulses of moment release. The largest moment pulse corresponds to a MW 7.5 subevent, about 40 km west of the Denali-Totschunda fault junction. We estimate relatively low and shallow slip on the Totschunda fault.

Shallow seismic structure of Kunlun fault zone in northern Tibetan Plateau, China: Implications for the 2001 M s8.1 Kunlun earthquake

USGS Publications Warehouse

Wang, Chun-Yong; Mooney, W.D.; Ding, Z.; Yang, J.; Yao, Z.; Lou, H.

2009-01-01

The shallow seismic velocity structure of the Kunlun fault zone (KLFZ) was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions. The data were collected after the 2001 Kunlun M s8.1 earthquake in the northern Tibetan Plateau. Seismic phases for the in-line record sections (26 records up to a distance of 15 km) along the fault zone were analysed, and 1-D P- and S-wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method. Sixteen seismic stations were deployed along the off-line profile perpendicular to the fault zone. Fault-zone trapped waves appear clearly on the record sections, which were simulated with a 3-D finite difference algorithm. Quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the Kunlun fault-zone width is 300 m, and S-wave quality factor Q within the fault zone is 15. Significantly, S-wave velocities within the fault zone are reduced by 30-45 per cent from surrounding rocks to a depth of at least 1-2 km, while P-wave velocities are reduced by 7-20 per cent. A fault-zone with such P- and S-low velocities is an indication of high fluid pressure because Vs is affected more than Vp. The low-velocity and low-Q zone in the KLFZ model is the effect of multiple ruptures along the fault trace of the 2001 M s8.1 Kunlun earthquake. ?? 2009 The Authors Journal compilation ?? 2009 RAS.

Fault-scale controls on rift geometry: the Bilila-Mtakataka Fault, Malawi

NASA Astrophysics Data System (ADS)

Hodge, M.; Fagereng, A.; Biggs, J.; Mdala, H. S.

2017-12-01

Border faults that develop during initial stages of rifting determine the geometry of rifts and passive margins. At outcrop and regional scales, it has been suggested that border fault orientation may be controlled by reactivation of pre-existing weaknesses. Here, we perform a multi-scale investigation on the influence of anisotropic fabrics along a major developing border fault in the southern East African Rift, Malawi. The 130 km long Bilila-Mtakataka fault has been proposed to have slipped in a single MW 8 earthquake with 10 m of normal displacement. The fault is marked by an 11±7 m high scarp with an average trend that is oblique to the current plate motion. Variations in scarp height are greatest at lithological boundaries and where the scarp switches between following and cross-cutting high-grade metamorphic foliation. Based on the scarp's geometry and morphology, we define 6 geometrically distinct segments. We suggest that the segments link to at least one deeper structure that strikes parallel to the average scarp trend, an orientation consistent with the kinematics of an early phase of rift initiation. The slip required on a deep fault(s) to match the height of the current scarp suggests multiple earthquakes along the fault. We test this hypothesis by studying the scarp morphology using high-resolution satellite data. Our results suggest that during the earthquake(s) that formed the current scarp, the propagation of the fault toward the surface locally followed moderately-dipping foliation well oriented for reactivation. In conclusion, although well oriented pre-existing weaknesses locally influence shallow fault geometry, large-scale border fault geometry appears primarily controlled by the stress field at the time of fault initiation.

Style of extensional tectonism during rifting, Red Sea and Gulf of Aden

USGS Publications Warehouse

Bohannon, R.G.

1989-01-01

Geologic and geophysical studies from the Arabian continental margin in the southern Red Sea and LANDSAT analysis of the northern Somalia margin in the Gulf of Aden suggest that the early continental rifts were long narrow features that formed by extension on closely spaced normal faults above moderate- to shallow-dipping detachments with break-away zones defining one rift flank and root zones under the opposing rift flank. The rift flanks presently form the opposing continental margins across each ocean basin. The detachment on the Arabian margin dips gently to the west, with a breakaway zone now eroded above the deeply dissected terrain of the Arabian escarpment. A model is proposed in which upper crustal breakup occurs on large detachment faults that have a distinct polarity. -from Author

Shallow seismic imaging of folds above the Puente Hills blind-thrust fault, Los Angeles, California

USGS Publications Warehouse

Pratt, T.L.; Shaw, J.H.; Dolan, J.F.; Christofferson, S.A.; Williams, R.A.; Odum, J.K.; Plesch, A.

2002-01-01

High-resolution seismic reflection profiles image discrete folds in the shallow subsurface (<600 m) above two segments of the Puente Hills blind-thrust fault system, Los Angeles basin, California. The profiles demonstrate late Quaternary activity at the fault tip, precisely locate the axial surfaces of folds within the upper 100 m, and constrain the geometry and kinematics of recent folding. The Santa Fe Springs segment of the Puente Hills fault zone shows an upward-narrowing kink band with an active anticlinal axial surface, consistent with fault-bend folding above an active thrust ramp. The Coyote Hills segment shows an active synclinal axial surface that coincides with the base of a 9-m-high scarp, consistent with tip-line folding or the presence of a backthrust. The seismic profiles pinpoint targets for future geologic work to constrain slip rates and ages of past events on this important fault system.

Postmylonitic deformation in the Raft River metamorphic core complex, northwestern Utah: Evidence of a rolling hinge

NASA Astrophysics Data System (ADS)

Manning, Andrew H.; Bartley, John M.

1994-06-01

Much of the recent debate over low-angle normal faults exposed in metamorphic core complexes has centered on the rolling hinge model. The model predicts tilting of seismogenic high-angle normal faults to lower dips by footwall deformation in response to isostatic forces caused by footwall exhumation. This shallow brittle deformation should visibly overprint the mylonitic fabric in the footwall of a metamorphic core complex. The predicted style and magnitude of rolling hinge strain depends upon the macroscopic mechanism by which the footwall deforms. Two end-members have been proposed: subvertical simple shear and flexural failure. Each mechanism should generate a distinctive pattern of structures that strike perpendicular to the regional extension direction. Subvertical simple shear (SVSS) should generate subvertical faults and kink bands with a shear sense antithetic to the detachment. For an SVSS hinge, the hinge-related strain magnitude should depend only on initial fault dip; rolling hinge structures should shorten the mylonitic foliation by >13% for an initial fault dip of >30°. In flexural failure the footwall behaves as a flexed elastic beam that partially fails in response to bending stresses. Resulting structures include conjugate faults and kink bands that both extend and contract the mylonitic foliation. Extensional sets could predominate as a result of superposition of far-field and flexural stresses. Strain magnitudes do not depend on fault dip but depend on the thickness and radius of curvature of the flexed footwall beam and vary with location within that beam. Postmylonitic structures were examined in the footwall of the Raft River metamorphic core complex in northwestern Utah to test these predictions. Observed structures strike perpendicular to the regional extension direction and include joints, normal faults, tension-gash arrays, and both extensional and contractional kink bands. Aside from the subvertical joints, the extensional structures dip moderately to steeply and are mainly either synthetic to the detachment or form conjugate sets. Range-wide, the extensional structures accomplish about 4% elongation of the mylonitic foliation. Contractional structures dip steeply, mainly record shear antithetic to the detachment, and accomplish <1% contraction of the foliation. These observations are consistent with the presence of a rolling hinge in the Raft River Mountains, but a rolling hinge that reoriented a high-angle normal fault by SVSS is excluded. The pattern and magnitudes of strain favor hinge-related deformation mainly by flexural failure with a subordinate component of SVSS.

A 2006 earthquakes series at the Colima rift and its relationship to the Rivera-Cocos plate boundary

NASA Astrophysics Data System (ADS)

Yamamoto, J.; Jimenez, Z.

2013-12-01

From July 31 through 13 August 2006 a series of fourteen earthquakes (M 3.9 to 6.1) occurred in the western end of the Central Mexican Volcanic Belt (CMVB) in twenty five days period. The most prominent earthquake (Mw 6.1) occurred on 11 August 2006 at 14:30 UTC (9:30 local time) approximately at 18.37° N, 101.25° W and 81 km depth. The epicenter was less than 40 km from Huetamo, Michoacan a 41,250-inhabitant city and 60 km from the El Infiernillo dam embayment the third largest hydroelectric plant in Mexico. This earthquake was widely felt through out the region with minor to moderate reported damage. In Mexico City 250 km away from the epicenter the earthquake, produced alarm among the population and several buildings evacuated. The earthquake series developed into two activity clusters one centered in the coast and separated about 300 km from a second inland cluster. The initial coastal cluster consisted of a nearly linear activity distribution which includes two shallow-depth earthquakes and reverse faulting mechanism with a slight left lateral strike-slip component and a possible fault planes trending roughly east-west. Two normal faulting earthquakes located at the extremes of the graben system, and fault planes oriented in a nearly north-south direction followed. The earthquakes are located approximately between the trench and the coast along the El Gordo-Colima graben system, which has been proposed as the continuation of the diffuse boundary between the Rivera and Cocos plates. The reverse faulting earthquakes are congruent either, with the expected subduction of the Rivera or Cocos plate under the North America plate and the normal faulting earthquake that can be associated to motions in the graben.

Is there a "blind" strike-slip fault at the southern end of the San Jacinto Fault system?

NASA Astrophysics Data System (ADS)

Tymofyeyeva, E.; Fialko, Y. A.

2015-12-01

We have studied the interseismic deformation at the southern end of the San Jacinto fault system using Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data. To complement the continuous GPS measurements from the PBO network, we have conducted campaign-style GPS surveys of 19 benchmarks along Highway 78 in the years 2012, 2013, and 2014. We processed the campaign GPS data using GAMIT to obtain horizontal velocities. The data show high velocity gradients East of the surface trace of the Coyote Creek Fault. We also processed InSAR data from the ascending and descending tracks of the ENVISAT mission between the years 2003 and 2010. The InSAR data were corrected for atmospheric artifacts using an iterative common point stacking method. We combined average velocities from different look angles to isolate the fault-parallel velocity field, and used fault-parallel velocities to compute strain rate. We filtered the data over a range of wavelengths prior to numerical differentiation, to reduce the effects of noise and to investigate both shallow and deep sources of deformation. At spatial wavelengths less than 2km the strain rate data show prominent anomalies along the San Andreas and Superstition Hills faults, where shallow creep has been documented by previous studies. Similar anomalies are also observed along parts of the Coyote Creek Fault, San Felipe Fault, and an unmapped southern continuation of the Clark strand of the San Jacinto Fault. At wavelengths on the order of 20km, we observe elevated strain rates concentrated east of the Coyote Creek Fault. The long-wavelength strain anomaly east of the Coyote Creek Fault, and the localized shallow creep observed in the short-wavelength strain rate data over the same area suggest that there may be a "blind" segment of the Clark Fault that accommodates a significant portion of the deformation on the southern end of the San Jacinto Fault.

SAR-revealed slip partitioning on a bending fault plane for the 2014 Northern Nagano earthquake at the northern Itoigawa-Shizuoka tectonic line

NASA Astrophysics Data System (ADS)

Kobayashi, Tomokazu; Morishita, Yu; Yarai, Hiroshi

2018-05-01

By applying conventional cross-track synthetic aperture radar interferometry (InSAR) and multiple aperture InSAR techniques to ALOS-2 data acquired before and after the 2014 Northern Nagano, central Japan, earthquake, a three-dimensional ground displacement field has been successfully mapped. Crustal deformation is concentrated in and around the northern part of the Kamishiro Fault, which is the northernmost section of the Itoigawa-Shizuoka tectonic line. The full picture of the displacement field shows contraction in the northwest-southeast direction, but northeastward movement along the fault strike direction is prevalent in the northeast portion of the fault, which suggests that a strike-slip component is a significant part of the activity of this fault, in addition to a reverse faulting. Clear displacement discontinuities are recognized in the southern part of the source region, which falls just on the previously known Kamishiro Fault trace. We inverted the SAR and GNSS data to construct a slip distribution model; the preferred model of distributed slip on a two-plane fault surface shows a combination of reverse and left-lateral fault motions on a bending east-dipping fault surface with a dip of 30° in the shallow part and 50° in the deeper part. The hypocenter falls just on the estimated deeper fault plane where a left-lateral slip is inferred, whereas in the shallow part, a reverse slip is predominant, which causes surface ruptures on the ground. The slip partitioning may be accounted for by shear stress resulting from a reverse fault slip with left-lateral component at depth, for which a left-lateral slip is suppressed in the shallow part where the reverse slip is inferred. The slip distribution model with a bending fault surface, instead of a single fault plane, produces moment tensor solution with a non-double couple component, which is consistent with the seismically estimated mechanism.

Characterizing flow pathways in a sandstone aquifer at multiple depths

NASA Astrophysics Data System (ADS)

Medici, Giacomo; West, Jared; Mountney, Nigel

2017-04-01

Sandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavily fractured where rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. This presentation reports well-test results and outcrop-scale studies that reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤ 150 m), similar to limestone and crystalline aquifers. The Triassic St Bees Sandstone Formation of the UK East Irish Sea Basin represents an optimum succession for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This sedimentary succession of fluvial origin accumulated in rapidly subsiding basins, which typically favour preservation of complete depositional cycles, including fine-grained mudstone and silty sandstone layers of floodplain origin interbedded with sandstone-dominated fluvial channel deposits. Vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding-parallel discontinuities. Additionally, normal faults are present through the succession and record development of open-fractures in their damage zones. Here, the shallow aquifer (depth ≤150 m BGL) was characterized in outcrop and well tests. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicate that zones of fault-related open fractures typically represent ˜ 50% of well transmissivity. The remaining flow component is dominated by bed-parallel fractures. However, such sub-horizontal fractures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas. Optical televiewer logs show development of karst-like conduits in correspondence of bedding fractures and faults up to 150 m below the ground surface, where recharge water containing dissolved carbonic acid enlarges fractures; these features may be responsible for the relatively high field-scale permeability (K˜0.1-1 m/day) of the phreatic zone at these depths. Below this 'karstifed' zone, field-scale permeability progressively decreases from K˜10-2 to 10-4 m/day from 150 m to 1100 m depth. Notably, differences between plug and field-scale permeability, and frequency of well in-flows seen in temperature and conductivity logs, also decrease between intermediate (150 to 450 m) and elevated (450 to 1100 m) depths. This confirms how fracture closure leads to a progressively more important matrix contribution to flow with increasing lithostatic stress, leading to intergranular flow dominance at ˜ 1 km depth.

Linkages Between the Megathrust and Upper-plate Deformation: Lessons From the Deformational Dichotomy of the 2016 Kaikoura New Zealand Earthquake

NASA Astrophysics Data System (ADS)

Furlong, K. P.; Herman, M. W.

2017-12-01

Following the 2016 Mw 7.8 Kaikoura earthquake, the nature of the coseismic rupture was unclear. Seismological and tsunami evidence pointed to significant involvement of the subduction megathrust, while geodetic and field observations pointed to a shallow set of intra-crustal faults as the main participants during the earthquake. It now appears that the Kaikoura earthquake produced synchronous faulting on the plate boundary subduction interface - the megathrust - and on a suite of crustal faults above the rupture zone in the overlying plate. This Kaikoura-style earthquake, involving synchronous ruptures on multiple components of the plate boundary, may be an important mode of plate boundary deformation affecting seismic hazard along subduction zones. Here we propose a model to explain how these upper-plate faults are loaded during the periods between megathrust earthquakes and subsequently can rupture synchronously with the megathrust. Between megathrust earthquakes, horizontal compression, driven by plate convergence, locks the upper-plate faults, particularly those at higher angles to the convergence direction and the oblique plate motion of the subducting Pacific plate deforms the upper-plate in bulk shear. During the time interval of megathrust rupture, two things happen which directly affect the stress conditions acting on these upper-plate faults: (1) slip on the megathrust and the associated `rebound' of the upper plate reduces the compressive or normal stress acting on the upper plate faults, and (2) the base of the upper plate faults (and the upper plate itself) is decoupled from the slab in the region above rupture area. The reduction in normal stress acting on these faults increases their Coulomb Stress state to strongly favor strike-slip fault slip, and the basal decoupling of the upper plate allows it to undergo nearly complete stress recovery in that region; enabling the occurrence of very large offsets on these faults - offsets that exceed the slip on the plate interface. With these results it is clear that the 2016 Kaikoura NZ earthquake represents a mode of subduction zone rupture that must be considered in other regions.

Fault geometry inversion and slip distribution of the 2010 Mw 7.2 El Mayor-Cucapah earthquake from geodetic data

NASA Astrophysics Data System (ADS)

Huang, Mong-Han; Fielding, Eric J.; Dickinson, Haylee; Sun, Jianbao; Gonzalez-Ortega, J. Alejandro; Freed, Andrew M.; Bürgmann, Roland

2017-01-01

The 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake in Baja, California, and Sonora, Mexico, had primarily right-lateral strike-slip motion and a minor normal-slip component. The surface rupture extended about 120 km in a NW-SE direction, west of the Cerro Prieto fault. Here we use geodetic measurements including near- to far-field GPS, interferometric synthetic aperture radar (InSAR), and subpixel offset measurements of radar and optical images to characterize the fault slip during the EMC event. We use dislocation inversion methods and determine an optimal nine-segment fault geometry, as well as a subfault slip distribution from the geodetic measurements. With systematic perturbation of the fault dip angles, randomly removing one geodetic data constraint, or different data combinations, we are able to explore the robustness of the inferred slip distribution along fault strike and depth. The model fitting residuals imply contributions of early postseismic deformation to the InSAR measurements as well as lateral heterogeneity in the crustal elastic structure between the Peninsular Ranges and the Salton Trough. We also find that with incorporation of near-field geodetic data and finer fault patch size, the shallow slip deficit is reduced in the EMC event by reductions in the level of smoothing. These results show that the outcomes of coseismic inversions can vary greatly depending on model parameterization and methodology.

Shallow near-fault material self organizes so it is just nonlinear in typical strong shaking

NASA Astrophysics Data System (ADS)

Sleep, N. H.

2011-12-01

Cracking within shallow compliant fault zones self-organizes so that strong dynamic stresses marginally exceed the elastic limit. To the first order, the compliant material experiences strain boundary conditions imposed by underlying stiffer rock. A major strike-slip fault yields simple dimensional relations. The near-field velocity pulse is essentially a Love wave. The dynamic strain is the ratio of the measured particle velocity over the deep S-wave velocity. The shallow dynamic stress is this quantity times the local shear modulus. I obtain the equilibrium shear modulus by starting a sequence of earthquakes with intact stiff rock surrounding the shallow fault zone. The imposed dynamic strain in stiff rock causes Coulomb failure and leaves cracks in it wake. Cracked rock is more compliant than the original intact rock. Each subsequent event causes more cracking until the rock becomes compliant enough that it just reaches its elastic limit. Further events maintain the material at the shear modulus where it just fails. Analogously, shallow damaged regolith forms with its shear modulus and S-wave velocity increasing with depth so it just reaches failure during typical strong shaking. The general conclusion is that shallow rocks in seismically active areas just become nonlinear during typical shaking. This process causes transient changes in S-wave velocity, but not strong nonlinear attenuation of seismic waves. Wave amplitudes significantly larger than typical ones would strongly attenuate and strongly damage the rock. The equilibrium shear modulus and S-wave velocity depend only modestly on the effective coefficient of internal friction.

First-order and subsidiary faults controlling the time-space evolution of the Central Italy 2016 seismic sequence - a multi-source data detailed 3D reconstruction

NASA Astrophysics Data System (ADS)

Lavecchia, Giusy; de nardis, Rita; Ferrarini, Federica; Cirillo, Daniele; Brozzetti, Francesco

2017-04-01

The Central Italy 2016 seismic sequence, with its three major events (24 August, Mw 6.0/6.2; 26 October Mw5.9/6.0; 30 October Mw6.5/6.6), activated a well-known active west-dipping extensional fault alignment of central Italy (Vettore-Gorzano faults, VEGO). Soon after the first event, based on geological, interferometric and at that moment available seismological data, a preliminary 3D fault model of VEGO was built. Such a model is here updated and improved at the light of a large amount of relocated earthquake data (time interval 24 August to 30 November 2016, 0.1≤ML ≤6.5, Chiaraluce at al., submitted to SRL) plus additional geological information. The 3D modeling was done using the software package MOVE from the Midland Valley. All the available data were taken into consideration (surface traces, fault-slip data, primary co-seismic surface fractures, geological maps and cross-sections, hypocentral locations and focal mechanisms of both background seismicity and seismic sequences). The VEGO geometric configuration did not substantially changed with respect to the previous model, but some additional structures involved in the sequence were reconstructed. In particular, four additional faults are well evident: a NE-dipping normal fault (dip-angle 50˚ ) antithetic to Vettore Fault, located at depths between 1 and 5 km; a WNW dipping plane (dip-angle 30˚ ) located at depth between 1 and 4 km within the Vettore footwall volume; this structure represents a splay of the late Miocene Sibillini thrust, which is evidently cross-cut and dislocated by the Vettore normal fault; a SW-dipping normal fault representing an unknown northward prosecution of the VEGO alignment, where since 26 October a relevant seismic activity was released; an unknown east-dipping low-angle detachment, where VEGO detaches at a depth of about 10-11 km. An uninterrupted microseismic activity has illuminated such a detachment not only during the overall sequence, but also in the previous months. At the light of the reconstructed geometric pattern integrated with the evidences of primary co-seismic fractures, it results evident that the Central Italy seismic sequence represents a "classic", although complex, intra-Apennine normal-faulting event, reactivating a long-term quiescent seismogenic alignment (e.g. VEGO). The reactivated and inverted compressional structures are confined at shallow depth within the Vettore footwall, and in no way control the major events of the sequence. Conversely, an important regional role is played by the east-dipping detachment. It represents the missing geometric link between the Altotiberina LANF of northern Umbria and the recently discovered LANF of Latium-Abruzzi.

Seismological analyses of the 2010 March 11, Pichilemu, Chile Mw 7.0 and Mw 6.9 coastal intraplate earthquakes

USGS Publications Warehouse

Ruiz, Javier A.; Hayes, Gavin P.; Carrizo, Daniel; Kanamori, Hiroo; Socquet, Anne; Comte, Diana

2014-01-01

On 2010 March 11, a sequence of large, shallow continental crust earthquakes shook central Chile. Two normal faulting events with magnitudes around Mw 7.0 and Mw 6.9 occurred just 15 min apart, located near the town of Pichilemu. These kinds of large intraplate, inland crustal earthquakes are rare above the Chilean subduction zone, and it is important to better understand their relationship with the 2010 February 27, Mw 8.8, Maule earthquake, which ruptured the adjacent megathrust plate boundary. We present a broad seismological analysis of these earthquakes by using both teleseismic and regional data. We compute seismic moment tensors for both events via a W-phase inversion, and test sensitivities to various inversion parameters in order to assess the stability of the solutions. The first event, at 14 hr 39 min GMT, is well constrained, displaying a fault plane with strike of N145°E, and a preferred dip angle of 55°SW, consistent with the trend of aftershock locations and other published results. Teleseismic finite-fault inversions for this event show a large slip zone along the southern part of the fault, correlating well with the reported spatial density of aftershocks. The second earthquake (14 hr 55 min GMT) appears to have ruptured a fault branching southward from the previous ruptured fault, within the hanging wall of the first event. Modelling seismograms at regional to teleseismic distances (Δ > 10°) is quite challenging because the observed seismic wave fields of both events overlap, increasing apparent complexity for the second earthquake. We perform both point- and extended-source inversions at regional and teleseismic distances, assessing model sensitivities resulting from variations in fault orientation, dimension, and hypocentre location. Results show that the focal mechanism for the second event features a steeper dip angle and a strike rotated slightly clockwise with respect to the previous event. This kind of geological fault configuration, with secondary rupture in the hanging wall of a large normal fault, is commonly observed in extensional geological regimes. We propose that both earthquakes form part of a typical normal fault diverging splay, where the secondary fault connects to the main fault at depth. To ascertain more information on the spatial and temporal details of slip for both events, we gathered near-fault seismological and geodetic data. Through forward modelling of near-fault synthetic seismograms we build a kinematic k−2 earthquake source model with spatially distributed slip on the fault that, to first-order, explains both coseismic static displacement GPS vectors and short-period seismometer observations at the closest sites. As expected, the results for the first event agree with the focal mechanism derived from teleseismic modelling, with a magnitude Mw 6.97. Similarly, near-fault modelling for the second event suggests rupture along a normal fault, Mw 6.90, characterized by a steeper dip angle (dip = 74°) and a strike clockwise rotated (strike = 155°) with respect to the previous event.

The effect of heterogeneous crust on the earthquake -- The case study of the 2004 Chuetsu, Japan, earthquake

NASA Astrophysics Data System (ADS)

Miyatake, T.; Kato, N.; Yin, J.; Kato, A.

2010-12-01

The 2004, Chuetsu, Japan, earthquake of Mw 6.6 occurred as shallow thrust event and the detailed kinematic source model was obtained by Hikima and Koketsu (2005). Just after the event, a dense temporal seismic network was deployed, and the detailed structure was elucidated (A. Kato et al. 2006). The seismic velocities in the hanging wall above the main shock fault are lower than those in the footwall, with the velocity contrast extending to a depth of approximately 10 km (A. Kato et al. 2006). Their results also show the high velocity on the asperity. We investigate that effect of the structure heterogeneity on fault rupture. First, we model the structure of the source region of 100km x 100km x 40km as simple as possible, and then solve the static elastic equation of motion with gravity effect by using finite difference method and GeoFEM. Our structure model consists of two layers, in which the boundary is a dipping surface from ground surface to 10km depth and bend to horizontal plane. The slope of the boundary corresponds to the earthquake fault and a bump located on the asperity between the depths of 4km and 10km. Finite difference grid size is 0.25km horizontally and 0.4km vertically. Ratio of the horizontal to vertical grids corresponds to the dip angle of the main shock. We simply assume the rigidity of 30GPa for lower sediment part and 40GPa for hard rock part. The boundary conditions imposed are, 1) stress free on the ground surface, 2) depth dependent or uniform normal stress are added on the sides that cause horizontal maximum stress, 3) Lithostatic vertical stress on the bottom. The calculated stress field on the main shock fault has the following features, 1) The high shear stress peaks appear around the depth of hypocenter and the top edge of the asperity, corresponding to the depths of the velocity contrast. These high stress zones are caused by stress concentration of the low rigidity wedge shaped sediment. 2) Expected stress drop distribution is around the top edge of the asperity. 3) Strength excess increases with depth. Combining with 2), the rupture expect to propagate toward shallower asperity than deeper part. 4) Uniform normal stress boundary condition seems to be unreasonable because of high stress drop in shallower part. These are important clues to investigate the physical process of the earthquake.

Physical and Transport Property Variations Within Carbonate-Bearing Fault Zones: Insights From the Monte Maggio Fault (Central Italy)

NASA Astrophysics Data System (ADS)

Trippetta, F.; Carpenter, B. M.; Mollo, S.; Scuderi, M. M.; Scarlato, P.; Collettini, C.

2017-11-01

The physical characterization of carbonate-bearing normal faults is fundamental for resource development and seismic hazard. Here we report laboratory measurements of density, porosity, Vp, Vs, elastic moduli, and permeability for a range of effective confining pressures (0.1-100 MPa), conducted on samples representing different structural domains of a carbonate-bearing fault. We find a reduction in porosity from the fault breccia (11.7% total and 6.2% connected) to the main fault plane (9% total and 3.5% connected), with both domains showing higher porosity compared to the protolith (6.8% total and 1.1% connected). With increasing confining pressure, P wave velocity evolves from 4.5 to 5.9 km/s in the fault breccia, is constant at 5.9 km/s approaching the fault plane and is low (4.9 km/s) in clay-rich fault domains. We find that while the fault breccia shows pressure sensitive behavior (a reduction in permeability from 2 × 10-16 to 2 × 10-17 m2), the cemented cataclasite close to the fault plane is characterized by pressure-independent behavior (permeability 4 × 10-17 m2). Our results indicate that the deformation processes occurring within the different fault structural domains influence the physical and transport properties of the fault zone. In situ Vp profiles match well the laboratory measurements demonstrating that laboratory data are valuable for implications at larger scale. Combining the experimental values of elastic moduli and frictional properties it results that at shallow crustal levels, M ≤ 1 earthquakes are less favored, in agreement with earthquake-depth distribution during the L'Aquila 2009 seismic sequence that occurred on carbonates.

Numerical Simulation of Strong Ground Motion at Mexico City:A Hybrid Approach for Efficient Evaluation of Site Amplification and Path Effects for Different Types of Earthquakes

NASA Astrophysics Data System (ADS)

Cruz, H.; Furumura, T.; Chavez-Garcia, F. J.

2002-12-01

The estimation of scenarios of the strong ground motions caused by future great earthquakes is an important problem in strong motion seismology. This was pointed out by the great 1985 Michoacan earthquake, which caused a great damage in Mexico City, 300 km away from the epicenter. Since the seismic wavefield is characterized by the source, path and site effects, the pattern of strong motion damage from different types of earthquakes should differ significantly. In this study, the scenarios for intermediate-depth normal-faulting, shallow-interplate thrust faulting, and crustal earthquakes have been estimated using a hybrid simulation technique. The character of the seismic wavefield propagating from the source to Mexico City for each earthquake was first calculated using the pseudospectral method for 2D SH waves. The site amplifications in the shallow structure of Mexico City are then calculated using the multiple SH wave reverberation theory. The scenarios of maximum ground motion for both inslab and interplate earthquakes obtained by the simulation show a good agreement with the observations. This indicates the effectiveness of the hybrid simulation approach to investigate the strong motion damage for future earthquakes.

Evidence of shallow fault zone strengthening after the 1992 M7.5 Landers, California, earthquake

USGS Publications Warehouse

Li, Y.-G.; Vidale, J.E.; Aki, K.; Xu, Fei; Burdette, T.

1998-01-01

Repeated seismic surveys of the Landers, California, fault zone that ruptured in the magnitude (M) 7.5 earthquake of 1992 reveal an increase in seismic velocity with time. P, S, and fault zone trapped waves were excited by near-surface explosions in two locations in 1994 and 1996, and were recorded on two linear, three-component seismic arrays deployed across the Johnson Valley fault trace. The travel times of P and S waves for identical shot-receiver pairs decreased by 0.5 to 1.5 percent from 1994 to 1996, with the larger changes at stations located within the fault zone. These observations indicate that the shallow Johnson Valley fault is strengthening after the main shock, most likely because of closure of cracks that were opened by the 1992 earthquake. The increase in velocity is consistent with the prevalence of dry over wet cracks and with a reduction in the apparent crack density near the fault zone by approximately 1.0 percent from 1994 to 1996.

Fluid-driven normal faulting earthquake sequences in the Taiwan orogen

NASA Astrophysics Data System (ADS)

Wang, Ling-hua; Rau, Ruey-Juin; Lee, En-Jui

2017-04-01

Seismicity in the Central Range of Taiwan shows normal faulting mechanisms with T-axes directing NE, subparallel to the strike of the mountain belt. We analyze earthquake sequences occurred within 2012-2015 in the Nanshan area of northern Taiwan which indicating swarm behavior and migration characteristics. We select events larger than 2.0 from Central Weather Bureau catalog and use the double-difference relocation program hypoDD with waveform cross-correlation in the Nanshan area. We obtained a final count of 1406 (95%) relocated earthquakes. Moreover, we compute focal mechanisms using USGS program HASH by P-wave first motion and S/P ratio picking and 114 fault plane solutions with M 3.0-5.87 were determined. To test for fluid diffusion, we model seismicity using the equation of Shapiro et al. (1997) by fitting earthquake diffusing rate D during the migration period. According to the relocation result, seismicity in the Taiwan orogenic belt present mostly N25E orientation parallel to the mountain belt with the same direction of the tension axis. In addition, another seismic fracture depicted by seismicity rotated 35 degree counterclockwise to the NW direction. Nearly all focal mechanisms are normal fault type. In the Nanshan area, events show N10W distribution with a focal depth range from 5-12 km and illustrate fault plane dipping about 45-60 degree to SW. Three months before the M 5.87 mainshock which occurred in March, 2013, there were some foreshock events occurred in the shallow part of the fault plane of the mainshock. Half a year following the mainshock, earthquakes migrated to the north and south, respectively with processes matched the diffusion model at a rate of 0.2-0.6 m2/s. This migration pattern and diffusion rate offer an evidence of 'fluid-driven' process in the fault zone. We also find the upward migration of earthquakes in the mainshock source region. These phenomena are likely caused by the opening of the permeable conduit due to the M 5.87 earthquake and the rise of the high pressure fluid.

Is Downtown Seattle on the Hanging Wall of the Seattle Fault?

NASA Astrophysics Data System (ADS)

Pratt, T. L.

2008-12-01

The Seattle fault is an ~80-km-long thrust or reverse fault that trends east-west beneath the Puget Lowland of western Washington State, and is interpreted to extend beneath the Seattle urban area just south of the downtown area. The fault ruptured about A.D. 930 in a large earthquake that uplifted parts of the Puget Sound shoreline as much as 7 m, caused a tsunami in Puget Sound and extensive landslides throughout the area. Seismic reflection profiles indicate that the fault has 3 or more fault splays that together form the Seattle fault zone. Models for the Seattle fault zone vary considerably, but most models place the northern edge of the Seattle fault zone south of the downtown area. These interpretations require that the fault zone shifts about 2 km to the south in the Seattle area relative to its location to the east (Bellevue) and west (Bainbridge Island). Potential field anomalies, particularly prominent magnetic highs associated with dipping, shallow conglomerate layers, are not continuous in the downtown Seattle area as observed to the east and west. Compilation and re-interpretation of all the existing seismic profiles in the area indicate that the northern strand of the Seattle fault, specifically a fold associated with the northernmost, blind fault strand, lies beneath the northern part of downtown Seattle, about 1.5 to 2 km farther north than has previously been interpreted. This study focuses on one previously unpublished seismic profile in central Puget Sound that shows a remarkable image of the Seattle fault, with shallow subhorizontal layers disrupted or folded by at least two thrust faults and several shallow backthrusts. These apparently Holocene layers are arched gently upwards, with the peak of the anticline in line with Alki and Restoration Points on the east and west sides of Puget Sound, respectively. The profile shows that the shallow part of the northern fault strand dips to the south at about 35 degrees, consistent with the 35 to 40 degree dip previously interpreted from tomography data. A second fault strand about 2 km south of the northern strand causes gentle folding of the Holocene strata. Two prominent backthrusts occur on the south side of the anticline, with the southern backthrust on strike with a prominent scarp on the eastern shoreline. A large erosional paleochannel beneath west Seattle and the Duwamish waterway extends beneath Elliot Bay and obscures potential field anomalies and seismic reflection evidence for the fault strands. However, hints of fault-related features on the profiles in Elliot Bay, and clear images in Lake Washington, indicate that the fault strands extend beneath the city of Seattle in the downtown area. If indeed the northern strand of the Seattle fault lies beneath the northern part of downtown Seattle, the downtown area may experience ground deformation during a major Seattle fault earthquake and that focusing of energy in the fault zone may occur farther north than previously estimated.

Shallow Lunar Seismic Activity and the Current Stress State of the Moon

NASA Technical Reports Server (NTRS)

Watters, T. R.; Weber, R. C.; Collins, G. C.; Johnson, C. L.

2017-01-01

A vast, global network of more than 3200 lobate thrust fault scarps has been revealed in high resolution Lunar Reconnaissance Orbiter Camera (LROC) images. The fault scarps very young, less than 50 Ma, based on their small scale and crisp appearance, crosscutting relations with small-diameter impact craters, and rates of infilling of associated small, shallow graben and may be actively forming today. The population of young thrust fault scarps provides a window into the recent stress state of the Moon and offers insight into the origin of global lunar stresses. The distribution of orientations of the fault scarps is non-random, inconsistent with isotropic stresses from late-stage global contraction as the sole source of stress Modeling shows that tidal stresses contribute significantly to the current stress state of the lunar crust. Tidal stresses (orbital recession and diurnal tides) superimposed on stresses from global contraction result in non-isotropic compressional stress and thrust faults consistent with lobate scarp orientations. Stresses due to orbital recession do not change with orbital position, thus it is with the addition of diurnal stresses that peak stresses are reached. At apogee, diurnal and recession stresses are most compressive near the tidal axis, while at perigee they are most compressive 90 degrees away from the tidal axis. Coseismic slip events on currently active thrust faults are expected to be triggered when peak stresses are reached. Analysis of the timing of the 28 the shallow moonquakes recorded by the Apollo seismic network shows that 19 indeed occur when the Moon is closer to apogee, while only 9 shallow events occur when the Moon is closer to perigee. Here we show the results of relocating the shallow moonquake using an algorithm designed for sparse networks to better constrain their epicentral locations in order to compare them with stress models. The model for the current stress state of the Moon is refined by investigating the contribution of polar wander.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nugraha, Andri Dian, E-mail: andridn104@gmail.com; Widiyantoro, Sri; Shiddiqi, Hasbi Ash

Indonesian archipelago region is located in active tectonic setting and high seismicity zone. During the last decade, Indonesian was experienced with destructive major earthquakes causing damage and victims. The information of precise earthquake location parameters are very important in partular for earthquake early warning to the society and for advance seismic studies. In this study, we attempted to improve hypocenter location compiled by BMKG for time periods of April, 2009 up to June, 2014 for about 22,000 earthquake events around Indonesian region. For the firts time, we applied teleseismic double-difference relocation algorithm (teletomoDD) to improve hypocenter region in Indonesia regionmore » combining regional and teleseismic stations. Hypocenter relocation was performed utilizing local, regional, and teleseismic P-wave arrival time data. Our relocation result show that travel-time RMS errors were greatly reduced compared to the BMKG catalog. Seismicity at shallower depth (less than 50 km) shows significantly improvement especially in depth, and refined shallow geological structures, e.g. trench and major strike slip faults. Clustered seismicity is also detected beneath volcanic region, and probably related volcano activities and also major faults nearby. In the Sunda arc region, seismicity at shallower depth centered at two major distributions parallel to the trench strike direction, i.e. around fore-arc and in mainland that related to major fault, e.g. the Sumatran fault, and volcanic fronts. Below Central Java region, relocated hypocenter result showed double seismic zone pattern. A seismic gap is detected around the Sunda-Banda transition zone where transition between oceanic subduction to continental crust collision of Australian plate occurs. In Eastern Indonesia region, shallow earthquakes are observed related to major strike slip faults, e.g. Sorong and Palu-Koro fault, volcanism, and shallow part of subduction and collision zones. We also compare our result in the Sunda Arc region with slab1.0 model and our relocated seismicity shows good agreement with the previous slab geometry. Horizontal position shift of relocated events are mostly perpendicular to the trench directions.« less

Refining the shallow slip deficit

NASA Astrophysics Data System (ADS)

Xu, Xiaohua; Tong, Xiaopeng; Sandwell, David T.; Milliner, Christopher W. D.; Dolan, James F.; Hollingsworth, James; Leprince, Sebastien; Ayoub, Francois

2016-03-01

Geodetic slip inversions for three major (Mw > 7) strike-slip earthquakes (1992 Landers, 1999 Hector Mine and 2010 El Mayor-Cucapah) show a 15-60 per cent reduction in slip near the surface (depth < 2 km) relative to the slip at deeper depths (4-6 km). This significant difference between surface coseismic slip and slip at depth has been termed the shallow slip deficit (SSD). The large magnitude of this deficit has been an enigma since it cannot be explained by shallow creep during the interseismic period or by triggered slip from nearby earthquakes. One potential explanation for the SSD is that the previous geodetic inversions lack data coverage close to surface rupture such that the shallow portions of the slip models are poorly resolved and generally underestimated. In this study, we improve the static coseismic slip inversion for these three earthquakes, especially at shallow depths, by: (1) including data capturing the near-fault deformation from optical imagery and SAR azimuth offsets; (2) refining the interferometric synthetic aperture radar processing with non-boxcar phase filtering, model-dependent range corrections, more complete phase unwrapping by SNAPHU (Statistical Non-linear Approach for Phase Unwrapping) assuming a maximum discontinuity and an on-fault correlation mask; (3) using more detailed, geologically constrained fault geometries and (4) incorporating additional campaign global positioning system (GPS) data. The refined slip models result in much smaller SSDs of 3-19 per cent. We suspect that the remaining minor SSD for these earthquakes likely reflects a combination of our elastic model's inability to fully account for near-surface deformation, which will render our estimates of shallow slip minima, and potentially small amounts of interseismic fault creep or triggered slip, which could `make up' a small percentages of the coseismic SSD during the interseismic period. Our results indicate that it is imperative that slip inversions include accurate measurements of near-fault surface deformation to reliably constrain spatial patterns of slip during major strike-slip earthquakes.

Effect of Sediments on Rupture Dynamics of Shallow Subduction Zone Earthquakes and Tsunami Generation

NASA Astrophysics Data System (ADS)

Ma, S.

2011-12-01

Low-velocity fault zones have long been recognized for crustal earthquakes by using fault-zone trapped waves and geodetic observations on land. However, the most pronounced low-velocity fault zones are probably in the subduction zones where sediments on the seafloor are being continuously subducted. In this study I focus on shallow subduction zone earthquakes; these earthquakes pose a serious threat to human society in their ability in generating large tsunamis. Numerous observations indicate that these earthquakes have unusually long rupture durations, low rupture velocities, and/or small stress drops near the trench. However, the underlying physics is unclear. I will use dynamic rupture simulations with a finite-element method to investigate the dynamic stress evolution on faults induced by both sediments and free surface, and its relations with rupture velocity and slip. I will also explore the effect of off-fault yielding of sediments on the rupture characteristics and seafloor deformation. As shown in Ma and Beroza (2008), the more compliant hanging wall combined with free surface greatly increases the strength drop and slip near the trench. Sediments in the subduction zone likely have a significant role in the rupture dynamics of shallow subduction zone earthquakes and tsunami generation.

Seismic constraints and coulomb stress changes of a blind thrust fault system, 2: Northridge, California

USGS Publications Warehouse

Stein, Ross S.; Lin, Jian

2006-01-01

We review seismicity, surface faulting, and Coulomb stress changes associated with the 1994 Northridge, California, earthquake. All of the observed surface faulting is shallow, extending meters to tens of meters below the surface. Relocated aftershocks reveal no seismicity shallower than 2 km depth. Although many of the aftershocks lie along the thrust fault and its up-dip extension, there are also a significant number of aftershocks in the core of the gentle anticline above the thrust, and elsewhere on the up-thrown block. These aftershocks may be associated with secondary ramp thrusts or flexural slip faults at a depth of 2-4 km. The geological structures typically associated with a blind thrust fault, such as anticlinal uplift and an associated syncline, are obscured and complicated by surface thrust faults associated with the San Fernando fault that overly the Northridge structures. Thus the relationship of the geological structure and topography to the underlying thrust fault is much more complex for Northridge than it is for the 1983 Coalinga, California, earthquake. We show from a Coulomb stress analysis that secondary surface faulting, diffuse aftershocks, and triggered sequences of moderate-sized mainshocks, are expected features of moderate-sized blind thrust earthquakes.

Is the Central America forearc sliver part of the North America plate?

NASA Astrophysics Data System (ADS)

Guzman-Speziale, M.

2012-04-01

The Central America Forearc sliver is located between the Central America volcanic arc and the Middle America trench. Several authors have suggested that the forearc is being displaced to the northwest with respect to the Caribbean plate; they point to right-lateral, normal-faulting earthquakes along the Central America volcanic arc as prime evidence of this displacement. Apparently, the forearc continues to the northwest into southeastern Mexico, although this portion of the forearc is not being displaced. I present evidence that suggests that the forearc indeed continues into southeastern Mexico and that it belongs to the North America plate. Physiographically, there is a continuity of the forearc into the Coastal plains of southeastern (Chiapas) Mexico, across the Motagua and Polochic faults. Offshore, cross-sections of the Middle America trench are similar along the mexican (Chiapas) segment, and the Central American segment. Furthermore, at the northwestern end of the coastal plain there are no compressive structures, which suggests that the coastal plain is not being displaced to the northwest. As a matter of fact, fault-plane solutions for shallow earthquakes show extension rather than compression. Shallow, interplate earthquakes along the trench show similar parameters along both segments. P-axes and earthquake slip vectors have consistent azimuths, which relate better with Cocos-North America convergence than with Cocos-Caribbean. Azimuth of T-axes for normal-faulting earthquakes also agree well with Cocos-North America convergence. Similarity in several parameters is thus found across both segments, the Chiapas coastal plain and the Central America forearc sliver proper. This suggests that both segments are continuous and probably one and the same, and belonging to the North America plate. Perhaps more properly, the forearc sliver extends into southeastern Mexico and is part of the zone of deformation associated to the Cocos-North America-Caribbean plates triple junction. Right-lateral, strike-slip faulting along the volcanic arc, and GPS results for the forearc sliver indicate that the the forearc sliver is moving with respect to the Caribbean plate. In the model presented here, I propose that it is the Chortis block (the northwestern corner of the Caribbean plate) is the one moving with respect to the forearc. In fact, I have presented evidence elsewhere attesting to this.

Geodesy cannot presently detect the up-dip limit of frictional locking on megathrusts

NASA Astrophysics Data System (ADS)

Almeida, R. V.; Lindsey, E. O.; Bradley, K.; Hubbard, J.; Sathiakumar, S.; Malick, R.; Hill, E.

2017-12-01

Most discussions of interseismic behavior on megathrust faults focus on whether they are frictionally locked or creeping. Unfortunately, many geodetic studies of subduction zone megathrusts equate fault coupling with frictional locking. This comparison is not appropriate, as one reflects the physical properties of the fault, and the other reflects the kinematics of the fault. Much of the uncertainty about slip behavior is because in subduction zones, the shallow part of the fault is far from land, and therefore creep is not detectable by land-based GPS. Published coupling maps of subduction zone megathrusts often assume a low coupling ratio near the trench, updip from fully coupled regions. Yet, if the megathrust attains a coupling ratio of 1 anywhere on the fault (i.e., the hanging wall is moving with the same velocity as the footwall), a lower value of coupling updip of this location requires interseismic extension at a rate proportional to the decrease (Wang and Dixon, 2004). We argue that the shallow region of megathrusts lie in updip stress shadows, and do not (except under rare circumstances) experience appropriate driving forces to cause significant creep during the interseismic period. Therefore it may not be possible to determine whether these regions are frictionally locked by examining interseismic geodetic records. We demonstrate this effect using a boundary element model with rate-strengthening friction and a simplified subduction zone geometry. We show that a coupling value of zero at the trench is physically unrealistic even if only a small portion of the downdip fault zone is locked. The maximum creep at the trench depends on the width of the transition of the frictionally locked zone, but should be small (<30% of plate rate) under most circumstances. During the interseismic period, even if the shallow parts of megathrusts are frictionally unlocked, creep is likely smaller than the resolution of current seafloor geodetic techniques (which is currently in the range of cms/yr). These results have important implications for various aspects of subduction studies, including physical limits on geodetic coupling inversions, the hazard posed by slip on shallow decollements (tsunamigenic or otherwise), the seismotectonic interpretation of shallow seismicity, and the utility of seafloor geodetic measurements.

Audio-frequency magnetotelluric imaging of the Hijima fault, Yamasaki fault system, southwest Japan

NASA Astrophysics Data System (ADS)

Yamaguchi, S.; Ogawa, Y.; Fuji-Ta, K.; Ujihara, N.; Inokuchi, H.; Oshiman, N.

2010-04-01

An audio-frequency magnetotelluric (AMT) survey was undertaken at ten sites along a transect across the Hijima fault, a major segment of the Yamasaki fault system, Japan. The data were subjected to dimensionality analysis, following which two-dimensional inversions for the TE and TM modes were carried out. This model is characterized by (1) a clear resistivity boundary that coincides with the downward projection of the surface trace of the Hijima fault, (2) a resistive zone (>500 Ω m) that corresponds to Mesozoic sediment, and (3) shallow and deep two highly conductive zones (30-40 Ω m) along the fault. The shallow conductive zone is a common feature of the Yamasaki fault system, whereas the deep conductor is a newly discovered feature at depths of 800-1,800 m to the southwest of the fault. The conductor is truncated by the Hijima fault to the northeast, and its upper boundary is the resistive zone. Both conductors are interpreted to represent a combination of clay minerals and a fluid network within a fault-related fracture zone. In terms of the development of the fluid networks, the fault core of the Hijima fault and the highly resistive zone may play important roles as barriers to fluid flow on the northeast and upper sides of the conductive zones, respectively.

High-resolution shallow reflection seismic image and surface evidence of the Upper Tiber Basin active faults (Northern Apennines, Italy)

USGS Publications Warehouse

Donne, D.D.; Plccardi, L.; Odum, J.K.; Stephenson, W.J.; Williams, R.A.

2007-01-01

Shallow seismic reflection prospecting has been carried out in order to investigate the faults that bound to the southwest and northeast the Quaternary Upper Tiber Basin (Northern Apennines, Italy). On the northeastern margin of the basin a ??? 1 km long reflection seismic profile images a fault segment and the associated up to 100 meters thick sediment wedge. Across the southwestern margin a 0.5 km-long seismic profile images a 50-55??-dipping extensional fault, that projects to the scarp at the base of the range-front, and against which a 100 m thick syn-tectonic sediment wedge has formed. The integration of surface and sub-surface data allows to estimate at least 190 meters of vertical displacement along the fault and a slip rate around 0.25 m/kyr. Southwestern fault might also be interpreted as the main splay structure of regional Alto Tiberina extensional fault. At last, the 1917 Monterchi earthquake (Imax=X, Boschi et alii, 2000) is correlable with an activation of the southwestern fault, and thus suggesting the seismogenic character of this latter.

Preliminary geophysical interpretations of regional subsurface geology near the Questa Mine Tailing Facility and Guadalupe Mountain, Taos County, New Mexico

USGS Publications Warehouse

Grauch, V.J.S.; Drenth, Benjamin J.; Thompson, Ren A.; Bauer, Paul W.

2015-08-01

This report presents geophysical interpretations of regional subsurface geology in the vicinity of the Tailing Facility of the Questa Mine near Guadalupe Mountain, Taos County, New Mexico, in cooperation with the New Mexico Environment Department. The interpretations were developed from aeromagnetic data, regional gravity data, data from four ground magnetic traverses, geologic mapping, a digital elevation model, and information from a few shallow wells. The resolution of the geophysical data is only appropriate for a broad assessment of the regional setting. Aeromagnetic data provided the most comprehensive information for interpretation. Qualitative and semiquantitative interpretations indicate the nature and extent of volcanic rocks, their relative depths, and inferred contacts between them, as well as conjectured locations of faults. In particular, the aeromagnetic data indicate places where volcanic rocks extend at shallow depths under sedimentary cover. Trachydacites of Guadalupe Mountain are magnetic, but their associated aeromagnetic anomalies are opposite in sign over the northern versus the southern parts of the mountain. The difference indicates that lavas erupted during different magnetic-polarity events in the north (reverse polarity) versus the south (normal polarity) and therefore have different ages. We postulate a buried volcano with reverse-polarity magnetization lies under the northeast side of Guadalupe Mountain, which likely predated the exposed trachydacites. Faults interpreted for the study area generally align with known fault zones. We interpret a northern extension to one of these faults that crosses northwesterly underneath the Tailing Facility. Gravity data indicate that Guadalupe Mountain straddles the western margin of a subbasin of the Rio Grande rift and that significant (>400 meters) thicknesses of both volcanic and sedimentary rocks underlie the mountain.

New Insight into the Role of Tectonics versus Gravitational Deformation in Development of Surface Ruptures along the Ragged Mountain Fault, Katalla, Alaska USA: Applications of High-Resolution Three-Dimensional Terrain Models

NASA Astrophysics Data System (ADS)

Heinlein, S. N.; Pavlis, T. L.; Bruhn, R. L.; McCalpin, J. P.

2017-12-01

This study evaluates a surface structure using 3D visualization of LiDAR and aerial photography then analyzes these datasets using structure mapping techniques. Results provide new insight into the role of tectonics versus gravitational deformation. The study area is located in southern Alaska in the western edge of the St. Elias Orogen where the Yakutat microplate is colliding into Alaska. Computer applications were used to produce 3D terrain models to create a kinematic assessment of the Ragged Mountain fault which trends along the length of the east flank of Ragged Mountain. The area contains geomorphic and structural features which are utilize to determine the type of displacement on the fault. Previous studies described the Ragged Mountain fault as a very shallow (8°), west-dipping thrust fault that reactivated in the Late Holocene by westward-directed gravity sliding and inferred at least 180 m of normal slip, in a direction opposite to the (relative) eastward thrust transport of the structure inferred from stratigraphic juxtaposition. More recently this gravity sliding hypothesis has been questioned and this study evaluates one of these alternative hypotheses; that uphill facing normal fault-scarps along the Ragged Mountain fault trace represent extension above a buried ramp in a thrust and is evaluated with a fault-parallel flow model of hanging-wall folding and extension. Profiles across the scarp trace were used to illustrate the curvature of the topographic surfaces adjacent to the scarps system and evaluate their origin. This simple kinematic model tests the hypothesis that extensional fault scarps at the surface are produced by flexure above a deeper ramp in a largely blind thrust system. The data in the context of this model implies that the extensional scarp structures previously examined represent a combination of erosionally modified features overprinted by flexural extension above a thrust system. Analyses of scarp heights along the structure are combined with the model to suggest a decrease in Holocene slip from south to north along the Ragged Mountain fault from 11.3 m to 0.2 m, respectively.

Time-series analysis of surface deformation at Brady Hot Springs geothermal field (Nevada) using interferometric synthetic aperture radar

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ali, S. T.; Akerley, J.; Baluyut, E. C.

We analyze interferometric synthetic aperture radar (InSAR) data acquired between 2004 and 2014, by the ERS-2, Envisat, ALOS and TerraSAR-X/TanDEM-X satellite missions to measure and characterize time-dependent deformation at the Brady Hot Springs geothermal field in western Nevada due to extraction of fluids. The long axis of the ~4 km by ~1.5 km elliptical subsiding area coincides with the strike of the dominant normal fault system at Brady. Within this bowl of subsidence, the interference pattern shows several smaller features with length scales of the order of ~1 km. This signature occurs consistently in all of the well-correlated interferometric pairsmore » spanning several months. Results from inverse modeling suggest that the deformation is a result of volumetric contraction in shallow units, no deeper than 600 m, likely associated with damaged regions where fault segments mechanically interact. Such damaged zones are expected to extend downward along steeply dipping fault planes, providing a high permeability conduit to the production wells. Using time series analysis, we test the hypothesis that geothermal production drives the observed deformation. We find a good correlation between the observed deformation rate and the rate of production in the shallow wells. We also explore mechanisms that could potentially cause the observed deformation, including thermal contraction of rock, decline in pore pressure and dissolution of minerals over time.« less

An ocean bottom seismometer study of shallow seismicity near the Mid- America Trench offshore Guatemala ( Pacific).

USGS Publications Warehouse

Ambos, E.L.; Hussong, D.M.; Holman, C.E.

1985-01-01

Five ocean bottom seismometers recorded seismicity near the Mid-America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench. Approximately 170 events were recorded by 3 or more seismometers, and almost half were located with statistical hypocentral errors of <10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone NW of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate-plate boundary. Most magnitudes ranged between 3.0 and 4.0 mb, and the threshold magnitude of locatable events was about 2.8 mb. Two distinct composite focal mechanisms were determined. One appears to indicate high- angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone NW of the array, seems to show normal faulting along possible fault planes oriented quasi-perpendicular to the trench axis. Projection of our seismicity sample and of well-located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati-Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling.-from Authors

Antecedent rivers and early rifting: a case study from the Plio-Pleistocene Corinth rift, Greece

NASA Astrophysics Data System (ADS)

Hemelsdaël, Romain; Ford, Mary; Malartre, Fabrice

2016-04-01

Models of early rifting present syn-rift sedimentation as the direct response to the development of normal fault systems where footwall-derived drainage supplies alluvial to lacustrine sediments into hangingwall depocentres. These models often include antecedent rivers, diverted into active depocentres and with little impact on facies distributions. However, antecedent rivers can supply a high volume of sediment from the onset of rifting. What are the interactions between major antecedent rivers and a growing normal fault system? What are the implications for alluvial stratigraphy and facies distributions in early rifts? These questions are investigated by studying a Plio-Pleistocene fluvial succession on the southern margin of the Corinth rift (Greece). In the northern Peloponnese, early syn-rift deposits are preserved in a series of uplifted E-W normal fault blocks (10-15 km long, 3-7 km wide). Detailed sedimentary logging and high resolution mapping of the syn-rift succession (400 to 1300 m thick) define the architecture of the early rift alluvial system. Magnetostratigraphy and biostratigraphic markers are used to date and correlate the fluvial succession within and between fault blocks. The age of the succession is between 4.0 and 1.8 Ma. We present a new tectonostratigraphic model for early rift basins based on our reconstructions. The early rift depositional system was established across a series of narrow normal fault blocks. Palaeocurrent data show that the alluvial basin was supplied by one major sediment entry point. A low sinuosity braided river system flowed over 15 to 30 km to the NE. Facies evolved downstream from coarse conglomerates to fined-grained fluvial deposits. Other minor sediment entry points supply linked and isolated depocentres. The main river system terminated eastward where it built stacked small deltas into a shallow lake (5 to 15 m deep) that occupied the central Corinth rift. The main fluvial axis remained constant and controlled facies distribution throughout the early rift evolution. We show that the length scale of fluvial facies transitions is greater than and therefore not related to fault spacing. First order facies variations instead occur at the scale of the full antecedent fluvial system. Strike-parallel subsidence variations in individual fault blocks represent a second order controlling factor on stratigraphic architecture. As depocentres enlarged through time, sediments progressively filled palaeorelief, and formed a continuous alluvial plain above active faults. There was limited creation of footwall relief and thus no significant consequent drainage system developed. Here, instead of being diverted toward subsiding zones, the drainage system overfilled the whole rift from the onset of faulting. Moreover, the zones of maximum subsidence on individual faults are aligned across strike parallel to the persistent fluvial axis. This implies that long-term sediment loading influenced the growth of normal faults. We conclude that a major antecedent drainage system inherited from the Hellenide mountain belt supplied high volumes of coarse sediment from the onset of faulting in the western Corinth rift (around 4 Ma). These observations demonstrate that antecedent drainage systems can be important in the tectono-sedimentary evolution of rift basins.

The application of active-source seismic imaging techniques to transtensional problems the Walker Lane and Salton Trough

NASA Astrophysics Data System (ADS)

Kell, Anna Marie

The plate margin in the western United States is an active tectonic region that contains the integrated deformation between the North American and Pacific plates. Nearly focused plate motion between the North American and Pacific plates within the northern Gulf of California gives way north of the Salton Trough to more diffuse deformation. In particular a large fraction of the slip along the southernmost San Andreas fault ultimately bleeds eastward, including about 20% of the total plate motion budget that finds its way through the transtensional Walker Lane Deformation Belt just east of the Sierra Nevada mountain range. Fault-bounded ranges combined with intervening low-lying basins characterize this region; the down-dropped features are often filled with water, which present opportunities for seismic imaging at unprecedented scales. Here I present active-source seismic imaging from the Salton Sea and Walker Lane Deformation Belt, including both marine applications in lakes and shallow seas, and more conventional land-based techniques along the Carson range front. The complex fault network beneath the Salton Trough in eastern California is the on-land continuation of the Gulf of California rift system, where North American-Pacific plate motion is accommodated by a series of long transform faults, separated by small pull-apart, transtensional basins; the right-lateral San Andreas fault bounds this system to the north where it carries, on average, about 50% of total plate motion. The Salton Sea resides within the most youthful and northerly "spreading center" in this several thousand-kilometer-long rift system. The Sea provides an ideal environment for the use of high-data-density marine seismic techniques. Two active-source seismic campaigns in 2010 and 2011 show progression of the development of the Salton pull-apart sub-basin and the northerly propagation of the Imperial-San Andreas system through time at varying resolutions. High fidelity seismic imagery documents the timing of strain transfer from the Imperial fault onto the San Andreas fault through the application of sequence stratigraphy. Evidence shows that the formation of the Salton and Mesquite sub-basins and the associated change of strain partitioning occurred within the last 20-40 k.y., essentially modifying a broader zone of transtension bounding the Imperial and San Andreas faults into two smaller zones of focused extension. The north-central Walker Lane contains a diffuse network of both strike-slip and normal faults, with some degree of strain partitioning characterized by normal faulting to the west along the eastern edge of the Sierra Nevada mountain range, and strike-slip faults to the east that define a diffuse boundary against the Basin and Range proper. A seismic study across the Mount Rose fault zone, bounding the Carson Range near Reno, Nevada, was carried out to investigate slip across a potential low-angle normal fault. A hammer seismic reflection and refraction profile combined with airborne LiDAR (light detection and ranging) imagery highlights fault scarp modification through minor slumping/landslides, providing a better understanding of the nature of slip on this fault. The northeastern margin of the Walker Lane is a region where both "Basin and Range" style normal faults and dextral strike-slip faults contribute to the northward propagation of the Walker Lane (essentially parallel to an equivalent northward propagation of the Mendocino triple junction). Near this intersection lies Pyramid Lake, bounded to the southwest by the dextral Pyramid Lake fault and to the northeast by the normal Lake Range fault. A high-resolution (sub-meter) seismic CHIRP survey collected in 2010 shows intriguing relationships into fault architecture beneath Pyramid Lake. Over 500 line-km of seismic data reveal a polarity flip in basin structure as down-to-the-east motion at the northern end of the Pyramid Lake fault rapidly gives way to down-to-the-west normal motion along the Lake Range fault. Alternating patterns of asymmetric and symmetric stratal patterns west of the Lake Range fault provides some evidence for segmentation of total slip along this large normal fault. Using dated sediment cores, slip rate for the Lake Range fault was found to be approximately 1 mm/yr during the Holocene. A complex zone of transtenstion was also observed in seismic CHIRP data in the northwest quadrant of the lake, where short, discontinuous faults hint at the development of a nascent shear zone trending to the northwest. (Abstract shortened by UMI.)

Changes in In Situ Stress Across the Nankai and Cascadia Convergent Margins From Borehole Breakout Measurements During Ocean Drilling

NASA Astrophysics Data System (ADS)

McNeill, L.; Moore, J. C.; Yamada, Y.; Chang, C.; Tobin, H.; Kinoshita, M.; Gulick, S.; Moore, G.; Iodp Exp. 314/315/316 Science Party, &

2008-12-01

Borehole breakouts are commonly observed in borehole images shortly after drilling of continental margin sites. This study aims to compile and compare these results to determine what in situ shallow stress measurements can tell us about the larger scale tectonic regime. Recent Logging While Drilling resistivity images across the Kumano transect of the Nankai subduction zone, during Expedition 314, Stage 1 of the IODP NanTroSEIZE project, add to this dataset. Expedition 314 site data within the prism (C0001, C0004, C0006, including the megasplay fault system which may overlie the seismogenic updip limit) suggest maximum compressive stress (SHmax) is perpendicular to the margin (not parallel to the convergence vector) but is rotated through 90° at the forearc basin site (C0002). These results may point to changes in stress state of the shallow forearc from east to west: compression in the aseismic active prism (with evidence of strain partitioning of oblique convergence); and extension above the updip seismogenic zone suggesting focus of plate coupling at the plate boundary and not in the shallow forearc. Further south, ODP Leg 196 drilled the prism toe (808) with breakouts indicating SHmax parallel to the convergence vector, in contrast to Exp. 314 results. The stress state in the shallow prism at Site 808 may be affected by nearby seamount subduction or may represent differences in strain partitioning. On the Cascadia margin, two drilling legs have collected LWD borehole images (Leg 204 and Exp. 311). Leg 204 drilled 3 sites at hydrate ridge in the C Cascadia outer prism with breakout orientations variable between closely spaced sites. Prism fold axes are parallel to the margin so we might expect SHmax perpendicular to the margin as in Exp. 314. Deviations from this orientation may reflect local and surface effects (Goldberg and Janik, 2006). Exp. 311, N Cascadia, drilled 5 sites across the prism with breakouts in LWD images. Subduction is not oblique here, in contrast to the other sites discussed, and most sites indicate SHmax almost parallel to convergence and normal to major fold axes. In one case, the in situ stress orientation is also compatible with shallow normal faulting from seismic data. Site 1325, in a slope basin, deviates from this orientation and may reflect local processes. Borehole breakouts within the shallow forearc of convergent margins are often in agreement with other indications of regional tectonic stress and may be indicative of processes at depth. Deviations may represent local stresses due to gravitational processes.

Topography of closed depressions, scarps, and grabens in the North Tharsis region of Mars: implications for shallow crustal discontinuities and graben formation

USGS Publications Warehouse

Davis, Philip A.; Tanaka, Kenneth L.; Golombek, Matthew P.

1995-01-01

Using Viking Orbiter images, detailed photoclinometric profiles were obtained across 10 irregular depressions, 32 fretted fractures, 49 troughs and pits, 124 solitary scarps, and 370 simple grabens in the north Tharsis region of Mars. These data allow inferences to be made on the shallow crustal structure of this region. The frequency modes of measured scarp heights correspond with previous general thickness estimates of the heavily cratered and ridged plains units. The depths of the flat-floored irregular depressions (55-175 m), fretted fractures (85-890 m), and troughs and pits (60-1620 m) are also similar to scarp heights (thicknesses) of the geologic units in which these depressions occur, which suggests that the depths of these flat-floored features were controlled by erosional base levels created by lithologic contacts. Although the features have a similar age, both their depths and their observed local structural control increase in the order listed above, which suggests that the more advanced stages of associated fracturing facilitated the development of these depressions by increasing permeability. If a ground-ice zone is a factor in development of these features, as has been suggested, our observation that the depths of these features decrease with increasing latitude suggests that either the thickness of the ground-ice zone does not increase poleward or the depths of the depressions were controlled by the top of the ground-ice zone whose depth may decrease with latitude. Deeper discontinuities are inferred from fault-intersection depths of 370 simple grabens (assuming 60° dipping faults that initiate at a mechanical discontinuity) in Tempe Terra and Alba Patera and from the depths of the large, flat-floored troughs in Tempe Terra. The frequency distributions of these fault-intersection and large trough depths show a concentration at 1.0-1.6 km depth, similar to data obtained for Syria, Sinai, and Lunae Plana. The consistency of these depth data over such a large region of western Mars suggests that a discontinuity or a process that transcends local and regional geology is responsible for the formation of these features. If this discontinuity is represented by the base of the cryosphere, its uniform depth over 55° of latitude suggests that the cryosphere did not thicken poleward. Alternatively, the concentration of depths at 1.0-1.6 km may represent the upper level of noneruptive dike ascent (lateral dike propagation) of Mars, which is controlled by gravity and atmospheric pressure and magma and country-rock characteristics, and was probably controlled, in part, by ground ice. Fault-intersection depths in the north Tharsis region locally extend down to a depth of 5-7 km. The depth data between 2 and 3 km are attributed to the discontinuity at the interface of megaregolith and basement or to the upper limit of noneruptive dike ascent of magma with a high volatile content. Intersection depths greater than 3 km, which were found at Alba Patera, may be due to the megaregolith-basement discontinuity, which was buried and depressed by volcanic loading, or to the upper level of noneruptive dike ascent of magma with a low volatile content. The near absence of narrow simple grabens with fault-initiation depths less than 0.6-1.0 km in this study area, as well as in most of western Mars, suggests that this depth represents the minimum depth that normal faults can initiate; at shallower depths tension cracks or joints would form instead. This hypothesis is supported by the application of the Griffith failure criterion to this minimum depth of normal fault initiation, which suggests that shallow crustal materials have a tensile strength of 2-4 MPa throughout most of western Mars, in close agreement with previous estimates of tensile strength of martian basaltic rock.

3D Dynamic Rupture Simulations along Dipping Faults, with a focus on the Wasatch Fault Zone, Utah

NASA Astrophysics Data System (ADS)

Withers, K.; Moschetti, M. P.

2017-12-01

We study dynamic rupture and ground motion from dip-slip faults in regions that have high-seismic hazard, such as the Wasatch fault zone, Utah. Previous numerical simulations have modeled deterministic ground motion along segments of this fault in the heavily populated regions near Salt Lake City but were restricted to low frequencies ( 1 Hz). We seek to better understand the rupture process and assess broadband ground motions and variability from the Wasatch Fault Zone by extending deterministic ground motion prediction to higher frequencies (up to 5 Hz). We perform simulations along a dipping normal fault (40 x 20 km along strike and width, respectively) with characteristics derived from geologic observations to generate a suite of ruptures > Mw 6.5. This approach utilizes dynamic simulations (fully physics-based models, where the initial stress drop and friction law are imposed) using a summation by parts (SBP) method. The simulations include rough-fault topography following a self-similar fractal distribution (over length scales from 100 m to the size of the fault) in addition to off-fault plasticity. Energy losses from heat and other mechanisms, modeled as anelastic attenuation, are also included, as well as free-surface topography, which can significantly affect ground motion patterns. We compare the effect of material structure and both rate and state and slip-weakening friction laws have on rupture propagation. The simulations show reduced slip and moment release in the near surface with the inclusion of plasticity, better agreeing with observations of shallow slip deficit. Long-wavelength fault geometry imparts a non-uniform stress distribution along both dip and strike, influencing the preferred rupture direction and hypocenter location, potentially important for seismic hazard estimation.

Strain Variation in Accretionary Prisms Across Space and Time: Insights from the Makran Subduction Zone

NASA Astrophysics Data System (ADS)

Penney, C.; Tavakoli, F.; Saadat, A.; Nankali, H. R.; Sedighi, M.; Khorrami, F.; Sobouti, F.; Rafi, Z.; Copley, A.; Jackson, J. A.; Priestley, K. F.

2017-12-01

The Makran is one of the world's least-studied subduction zones. In particular, little is known about the accumulation and accommodation of strain in the onshore part of the subduction zone, which parallels the coasts of southern Iran and Pakistan. The deformation of the Makran accretionary prism results from both its subduction zone setting and N-S right-lateral shear between central Iran and Afghanistan. North of the Makran, this shear is accommodated by a series of right-lateral faults which offset the rocks of the Sistan Suture Zone, an abandoned accretionary prism. However, these right-lateral faults are not observed south of 27°N, and no major N-S faults cut the E-W trending structures of the Makran. How this right-lateral motion is accommodated at the southern end of the Sistan Suture Zone is a long-standing tectonic question. By combining results from geomorphology, GPS, seismology and modelling we conclude that right-lateral motion is transferred across the depression north of the accretionary prism to the region of right-lateral shear at the western end of the accretionary prism. This requires the Jaz Murian depression to be bounded by normal faults, consistent with the basin geomorphology. However, GPS data show compression across the margins of the basin, and no shallow normal-faulting earthquakes have been observed in the region. We therefore suggest that the behaviour of these faults may be time-dependent and controlled by the megathrust seismic cycle, as has been suggested elsewhere (e.g. Chile). Recent strike-slip earthquakes, including the 2013 Balochistan earthquake, have clustered at the prism's lateral edges, showing the importance of spatial, as well as temporal, variations in strain. These earthquakes have reactivated thrust faults in the Makran accretionary prism, showing that the style of strain within accretionary prisms can vary on multiple timescales and allowing us to calculate the coefficient of friction on the underlying megathrust.

Shallow deformation of the San Andreas fault 5 years following the 2004 Parkfield earthquake (Mw6) combining ERS2 and Envisat InSAR.

PubMed

Bacques, Guillaume; de Michele, Marcello; Raucoules, Daniel; Aochi, Hideo; Rolandone, Frédérique

2018-04-16

This study focuses on the shallow deformation that occurred during the 5 years following the Parkfield earthquake (28/09/2004, Mw 6, San Andreas Fault, California). We use Synthetic Aperture Radar interferometry (InSAR) to provide precise measurements of transient deformations after the Parkfield earthquake between 2005 and 2010. We propose a method to combine both ERS2 and ENVISAT interferograms to increase the temporal data sampling. Firstly, we combine 5 years of available Synthetic Aperture Radar (SAR) acquisitions including both ERS-2 and Envisat. Secondly, we stack selected interferograms (both from ERS2 and Envisat) for measuring the temporal evolution of the ground velocities at given time intervals. Thanks to its high spatial resolution, InSAR could provide new insights on the surface fault motion behavior over the 5 years following the Parkfield earthquake. As a complement to previous studies in this area, our results suggest that shallow transient deformations affected the Creeping-Parkfield-Cholame sections of the San Andreas Fault after the 2004 Mw6 Parkfield earthquake.

Seismic potential of weak, near-surface faults revealed at plate tectonic slip rates

PubMed Central

Ikari, Matt J.; Kopf, Achim J.

2017-01-01

The near-surface areas of major faults commonly contain weak, phyllosilicate minerals, which, based on laboratory friction measurements, are assumed to creep stably. However, it is now known that shallow faults can experience tens of meters of earthquake slip and also host slow and transient slip events. Laboratory experiments are generally performed at least two orders of magnitude faster than plate tectonic speeds, which are the natural driving conditions for major faults; the absence of experimental data for natural driving rates represents a critical knowledge gap. We use laboratory friction experiments on natural fault zone samples at driving rates of centimeters per year to demonstrate that there is abundant evidence of unstable slip behavior that was not previously predicted. Specifically, weak clay-rich fault samples generate slow slip events (SSEs) and have frictional properties favorable for earthquake rupture. Our work explains growing field observations of shallow SSE and surface-breaking earthquake slip, and predicts that such phenomena should be more widely expected. PMID:29202027

Active faulting on the island of Crete (Greece)

NASA Astrophysics Data System (ADS)

Caputo, Riccardo; Catalano, Stefano; Monaco, Carmelo; Romagnoli, Gino; Tortorici, Giuseppe; Tortorici, Luigi

2010-10-01

ABSTRACT In order to characterize and quantify the Middle-Late Quaternary and ongoing deformation within the Southern Aegean forearc, we analyse the major tectonic structures affecting the island of Crete and its offshore. The normal faults typically consist of 4-30-km-long dip-slip segments locally organised in more complex fault zones. They separate carbonate and/or metamorphic massifs, in the footwall block, from loose to poorly consolidated alluvial and colluvial materials within the hangingwall. All these faults show clear evidences of recent re-activations and trend parallel to two principal directions: WNW-ESE and NNE-SSW. Based on all available data for both onland and offshore structures (morphological and structural mapping, satellite imagery and airphotographs remote sensing as well as the analysis of seismic profiles and the investigation of marine terraces and Holocene raised notches along the island coasts), for each fault we estimate and constrain some of the principal seismotectonic parameters and particularly the fault kinematics, the cumulative amount of slip and the slip-rate. Following simple assumptions and empirical relationships, maximum expected magnitudes and mean recurrence periods are also suggested. Summing up the contribution to crustal extension provided by the two major fault sets we calculate both arc-normal and arc-parallel long-term strain rates. The occurrence of slightly deeper and more external low-angle thrust planes associated with the incipient continental collision occurring in western Crete is also analysed. Although these contractional structures can generate stronger seismic events (M ~ 7.5.) they are probably much rarer and thus providing a minor contribution to the overall morphotectonic evolution of the island and the forearc. A comparison of our geologically-based results with those obtained from GPS measurements show a good agreement, therefore suggesting that the present-day crustal deformation is probably active since Middle Quaternary and mainly related to the seismic activity of upper crustal normal faults characterized by frequent shallow (<20 km) moderate-to-strong seismic events seldom alternating with stronger earthquakes occurring along blind low-angle thrust planes probably ramping from a deeper aseismic detachment (ca. 25 km). This apparently contradicting co-existence of juxtaposed upper tensional and lower compressional tectonic regimes is in agreement with the geodynamics of the region characterised by continental collision with Nubia and the Aegean mantle wedging.

Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region

USGS Publications Warehouse

McGuire, Jeffrey J.; Lohman, Rowena B.; Catchings, Rufus D.; Rymer, Michael J.; Goldman, Mark R.

2015-01-01

The Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high-temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3-D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high-temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first-order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high-temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M > 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high-temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

Sudden aseismic fault slip on the south flank of Kilauea volcano.

PubMed

Cervelli, Peter; Segall, Paul; Johnson, Kaj; Lisowski, Michael; Miklius, Asta

2002-02-28

One of the greatest hazards associated with oceanic volcanoes is not volcanic in nature, but lies with the potential for catastrophic flank failure. Such flank failure can result in devastating tsunamis and threaten not only the immediate vicinity, but coastal cities along the entire rim of an ocean basin. Kilauea volcano on the island of Hawaii, USA, is a potential source of such flank failures and has therefore been monitored by a network of continuously recording geodetic instruments, including global positioning system (GPS) receivers, tilt meters and strain meters. Here we report that, in early November 2000, this network recorded transient southeastward displacements, which we interpret as an episode of aseismic fault slip. The duration of the event was about 36 hours, it had an equivalent moment magnitude of 5.7 and a maximum slip velocity of about 6[?]cm per day. Inversion of the GPS data reveals a shallow-dipping thrust fault at a depth of 4.5[?]km that we interpret as the down-dip extension of the Hilina Pali--Holei Pali normal fault system. This demonstrates that continuously recording geodetic networks can detect accelerating slip, potentially leading to warnings of volcanic flank collapse.

Neotectonics in the foothills of the southernmost central Andes (37°-38°S): Evidence of strike-slip displacement along the Antiñir-Copahue fault zone

NASA Astrophysics Data System (ADS)

Folguera, AndréS.; Ramos, VíCtor A.; Hermanns, Reginald L.; Naranjo, José

2004-10-01

The Antiñir-Copahue fault zone (ACFZ) is the eastern orogenic front of the Andes between 38° and 37°S. It is formed by an east vergent fan of high-angle dextral transpressive and transtensive faults, which invert a Paleogene intra-arc rift system in an out of sequence order with respect to the Cretaceous to Miocene fold and thrust belt. 3.1-1.7 Ma volcanic rocks are folded and fractured through this belt, and recent indicators of fault activity in unconsolidated deposits suggest an ongoing deformation. In spite of the absence of substantial shallow seismicity associated with the orogenic front, neotectonic studies show the existence of active faults in the present mountain front. The low shallow seismicity could be linked to the high volumes of retroarc-derived volcanic rocks erupted through this fault system during Pliocene and Quaternary times. This thermally weakened basement accommodates the strain of the Antiñir-Copahue fault zone, absorbing the present convergence between the South America and Nazca plates.

Stress Transfer Processes during Great Plate Boundary Thrusting Events: A Study from the Andaman and Nicobar Segments

NASA Astrophysics Data System (ADS)

Andrade, V.; Rajendran, K.

2010-12-01

The response of subduction zones to large earthquakes varies along their strike, both during the interseismic and post-seismic periods. The December 26, 2004 earthquake nucleated at 3° N latitude and its rupture propagated northward, along the Andaman-Sumatra subduction zone, terminating at 15°N. Rupture speed was estimated at about 2.0 km per second in the northern part under the Andaman region and 2.5 - 2.7 km per second under southern Nicobar and North Sumatra. We have examined the pre and post-2004 seismicity to understand the stress transfer processes within the subducting plate, in the Andaman (10° - 15° N ) and Nicobar (5° - 10° N) segments. The seismicity pattern in these segments shows distinctive characteristics associated with the outer rise, accretionary prism and the spreading ridge, all of which are relatively better developed in the Andaman segment. The Ninety East ridge and the Sumatra Fault System are significant tectonic features in the Nicobar segment. The pre-2004 seismicity in both these segments conform to the steady-state conditions wherein large earthquakes are fewer and compressive stresses dominate along the plate interface. Among the pre-2004 great earthquakes are the 1881 Nicobar and 1941 Andaman events. The former is considered to be a shallow thrust event that generated a small tsunami. Studies in other subduction zones suggest that large outer-rise tensional events follow great plate boundary breaking earthquakes due to the the up-dip transfer of stresses within the subducting plate. The seismicity of the Andaman segment (1977-2004) concurs with the steady-state stress conditions where earthquakes occur dominantly by thrust faulting. The post-2004 seismicity shows up-dip migration along the plate interface, with dominance of shallow normal faulting, including a few outer rise events and some deeper (> 100 km) strike-slip faulting events within the subducting plate. The September 13, 2002, Mw 6.5 thrust faulting earthquake at Diglipur (depth: 21 km) and the August 10, 2009, Mw 7.5 normal faulting earthquake near Coco Island (depth: 22 km), within the northern terminus of the 2004 rupture are cited as examples of the alternating pre and post earthquake stress conditions. The major pre and post 2004 clusters were associated with the Andaman Spreading Ridge (ASR). In the Nicobar segment, the most recent earthquake on June 12, 2010, Mw 7.5 (focal depth: 35 km) occurred very close to the plate boundary, through left lateral strike-slip faulting. A segment that does not feature any active volcanoes unlike its northern and southern counterparts, this part of the plate boundary has generated several right lateral strike-slip earthquakes, mostly on the Sumatra Fault System. The left-lateral strike-slip faulting associated with the June 12 event on a nearly N-S oriented fault plane consistent with the trend of the Ninety East ridge and the occasional left-lateral earthquakes prior to the 2004 mega-thrust event suggest the involvement of the Ninety East ridge in the subduction process.

Frictional properties of DFDP-1 Alpine Fault rocks under hydrothermal conditions and high shear strain

NASA Astrophysics Data System (ADS)

Niemeijer, André R.; Boulton, Carolyn; Toy, Virginia; Townend, John; Sutherland, Rupert

2015-04-01

The Alpine Fault, New Zealand, is a major plate-bounding fault that accommodates 65-75% of the total relative motion between the Australian and Pacific plates. Paleoseismic evidence of large-displacement surface-rupturing events, as well as an absence of measurable contemporary surface deformation, indicates that the fault slips mostly in quasi-periodic large-magnitude earthquakes (< Mw 8.0). To understand the mechanics of earthquakes, it is important to study the evolution of frictional properties of the fault rocks under conditions representative of the potential hypocentral depth. Here, we present data obtained on drill core samples of rocks that surround the principal slip zone(s) (PSZ) of the Alpine Fault and the PSZ itself. The drill core samples were obtained during phase 1 of the Deep Fault Drilling Project (DFDP-1) in 2011 at relatively shallow depths (down to ~150 m). Simulated fault gouges were sheared under elevated pressure and temperature conditions in a hydrothermal ring shear apparatus. We performed experiments at temperatures of 25, 150, 300, 450 ° C, and 600 oC. Using the shallow geothermal gradient of 63 ° C/km determined in DFDP-1, our highest temperature corresponds to a depth of ~7 km (Sutherland et al. 2012); it would correspond to 10 km depth using a more moderate geotherm of 45 oC/km (Toy et al. 2010). All samples show a transition from velocity-strengthening behavior, i.e. a positive value of (a-b), to velocity-weakening behavior, i.e. a negative value of (a-b) at a temperature of 150 ° C. The transition depends on the absolute value of sliding velocity, with velocity-weakening dominating at lower sliding velocities. At 600 oC, velocity-strengthening dominates at low sliding velocity, whereas the high-velocity steps are all velocity-weakening. Moreover, shear stress depends linearly on effective normal stress at 600 oC, indicating that shearing is essentially frictional and that no transition to ductile (normal stress independent) flow has occurred. Thus, depending on the background (nucleation) strain rate, our data indicate that the Alpine Fault should be able to generate earthquakes at all temperatures above room temperature. However, at the highest temperature investigated (600 oC), the transition to velocity-weakening is postponed to slip rates above 10 mm/s (strain rate ~10-2 s-1). This observation, combined with the absence of strength recovery after long holds, suggests that seismic slip may propagate into regions of the fault unlikely to nucleate earthquakes. We propose that in our porous gouges, thermally activated processes operate simultaneously with granular flow, postponing ductile flow to higher temperatures or lower strain rates. Sutherland, R., V.G. Toy, J. Townend, S.C. Cox, J.D. Eccles, D.R. Faulkner, D.J. Prior, R.J.Norris, E. Mariani, C. Boulton, B.M. Carpenter, C.D. Menzies, T.A. Little, M. Hasting, G.De Pascale, R.M. Langridge, H.R. Scott, Z. Reid-Lindroos, B. Fleming (2012), Drilling reveals fluid control on architecture and rupture of the Alpine Fault, New Zealand, Geology,40, 1143-1146, doi:10.1130/G33614.1. Toy, V.G., Craw, D., Cooper, A.F., and R.J. Norris (2010), Thermal regime in the central Alpine Fault zone, New Zealand: Constraints from microstructures, biotite chemistry and fluid inclusion data, Tectonophysics, doi:10.1016/j.tecto.2009.12.013

Geophysical data reveal the crustal structure of the Alaska Range orogen within the aftershock zone of the Mw 7.9 Denali fault earthquake

USGS Publications Warehouse

Fisher, M.A.; Ratchkovski, N.A.; Nokleberg, W.J.; Pellerin, L.; Glen, J.M.G.

2004-01-01

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 Mw 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25?? north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (>10 ??-m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the after-shock zone.

Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case

NASA Astrophysics Data System (ADS)

Tibaldi, A.; Bonali, F. L.; Corazzato, C.

2017-03-01

Assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo-hazards. We analyse the distribution of 1518 monogenic and polygenic volcanoes of Miocene-Quaternary age of the Central Volcanic Zone of the Andes (Chile-Bolivia-Argentina), and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones. Then we compare these data with outcropping dykes, tectonic structures and state of stress. Most magma paths trend N-S, NW-SE, and NE-SW, in decreasing order of frequency. The N-S and NW-SE paths coexist in the northern and southern part of the study area, whereas N-S paths dominate east of the Salar de Atacama. Outcropping dykes show the same trends. The regional Holocene stress state is given by an E-W greatest horizontal principal stress. N-S and NNE-SSW reverse faults and folds affect deposits of 4.8, 3.2 and 1.3 Ma BP, especially in the central and southern study areas. A few NW-SE left-lateral strike-slip faults are present in the interior of the volcanic arc, part of which belong to the Calama-Olacapato-El Toro fault. The volcanic chain is also affected by several N-S- and NW-SE-striking normal faults that offset Pliocene and Quaternary deposits. The results indicate different scenarios of magma-tectonic interaction, given by N-S normal and reverse faults and N-S fold hinges that guide volcano emplacement and magma paths. Magma paths are also guided by strike-slip and normal NW-SE faults, especially in the northern part of the study area. Zones with verticalized strata, with bedding striking NE-SW, also acted as preferential magma paths. These data suggest that at convergence zones with continental crust, shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress.

Dating faults by quantifying shear heating

NASA Astrophysics Data System (ADS)

Maino, Matteo; Casini, Leonardo; Langone, Antonio; Oggiano, Giacomo; Seno, Silvio; Stuart, Finlay

2017-04-01

Dating brittle and brittle-ductile faults is crucial for developing seismic models and for understanding the geological evolution of a region. Improvement the geochronological approaches for absolute fault dating and its accuracy is, therefore, a key objective for the geological community. Direct dating of ancient faults may be attained by exploiting the thermal effects associated with deformation. Heat generated during faulting - i.e. the shear heating - is perhaps the best signal that provides a link between time and activity of a fault. However, other mechanisms not instantaneously related to fault motion can generate heating (advection, upwelling of hot fluids), resulting in a difficulty to determine if the thermal signal corresponds to the timing of fault movement. Recognizing the contribution of shear heating is a fundamental pre-requisite for dating the fault motion through thermochronometric techniques; therefore, a comprehensive thermal characterization of the fault zone is needed. Several methods have been proposed to assess radiometric ages of faulting from either newly grown crystals on fault gouges or surfaces (e.g. Ar/Ar dating), or thermochronometric reset of existing minerals (e.g. zircon and apatite fission tracks). In this contribution we show two cases of brittle and brittle-ductile faulting, one shallow thrust from the SW Alps and one HT, pseudotachylite-bearing fault zone in Sardinia. We applied, in both examples, a multidisciplinary approach that integrates field and micro-structural observations, petrographical characterization, geochemical and mineralogical analyses, fluid inclusion microthermometry and numerical modeling with thermochronometric dating of the two fault zones. We used the zircon (U-Th)/He thermochronometry to estimate the temperatures experienced by the shallow Alpine thrust. The ZHe thermochronometer has a closure temperature (Tc) of 180°C. Consequently, it is ideally suited to dating large heat-producing faults that were active at shallow depths (<6-7 km) where wall-rock temperature does not exceed Tc. On the other hand, the retrogressed pseudotachylites from the Variscan basement of Sardina developed in deeper crustal levels and produced considerably higher temperatures (>800 °C). They have been dated using laser ablation ICP-MS on monazites and zircons. This large dataset provides the necessary constraints to explore the potential causes of heating, its timing and how it is eventually related to fault motion.

Structural inheritance versus magmatic weakening: What controls the style of deformation at rift segment boundaries in the Gulf of California, Mexico?

NASA Astrophysics Data System (ADS)

Seiler, Christian; Gleadow, Andrew; Kohn, Barry

2013-04-01

Rifts are commonly segmented into several hundred kilometre long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the Gulf of California, a youthful oceanic basin that is currently undergoing the rift-drift transition. However, detailed field studies have so far failed to identify suitable structures that could accommodate the obvious deformation gradients between different rift segments, and the nature of strain transfer at segment boundaries remains enigmatic. The Bocana transfer zone (BTZ) in central Baja California is a linear, WNW striking structural discontinuity separating two rift segments with different magnitudes and styles of extensional deformation. North of the BTZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway fault that separates the Gulf of California rift to the east from the relatively stable western portion of the Baja peninsula. The N-striking Libertad escarpment developed during the Late Miocene (~10-8Ma) and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. Finite displacement decreases from ~1000m in the central fault segment to ~500m further south, where the fault bends SE and merges with the BTZ. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margins by two moderate-displacement E-dipping normal faults. South of the BTZ, extension was much less than further north, which explains the comparatively subdued relief and generally shallower tilt of pre-rift strata in this area. The BTZ itself is characterized by two en echelon WNW-ESE striking dextral-oblique transfer faults with a significant down-to-the-NNE extensional component. Strain is transferred from the Libertad breakaway fault onto the transfer faults over a distance of >20km through a network of interacting normal, oblique and strike-slip faults. The shape, location and orientation of the main faults were strongly influenced by pre-existing rheological heterogeneities. Major normal faults are parallel to either the Mesozoic metamorphic foliation or Cretaceous intrusive contacts, and developed where the foliation was at a high angle to the extension direction. In contrast, the oblique-slip faults of the BTZ formed parallel to the metamorphic foliation where formlines are at a small angle to the regional extension direction. Compared to the BTZ, deformation in other known accommodation zones of the Gulf of California rift occurred distributed across a much wider zone, and appropriate transfer faults are either lacking or minor. In these cases, however, the accommodation zones coincide with the locations of significant pre- and synrift volcanism, suggesting that thermal weakening associated with magmatic activity may have promoted the distribution of strain across a wider region instead of localising it into discrete transfer faults.

High-Resolution Aeromagnetic Survey To Image Shallow Faults, Poncha Springs and Vicinity, Chaffee County, Colorado

USGS Publications Warehouse

Grauch, V.J.S.; Drenth, Benjamin J.

2009-01-01

High-resolution aeromagnetic data were acquired over the town of Poncha Springs and areas to the northwest to image faults, especially where they are concealed. Because this area has known hot springs, faults or fault intersections at depth can provide pathways for upward migration of geothermal fluids or concentrate fracturing that enhances permeability. Thus, mapping concealed faults provides a focus for follow-up geothermal studies. Fault interpretation was accomplished by synthesizing interpretative maps derived from several different analytical methods, along with preliminary depth estimates. Faults were interpreted along linear aeromagnetic anomalies and breaks in anomaly patterns. Many linear features correspond to topographic features, such as drainages. A few of these are inferred to be fault-related. The interpreted faults show an overall pattern of criss-crossing fault zones, some of which appear to step over where they cross. Faults mapped by geologists suggest similar crossing patterns in exposed rocks along the mountain front. In low-lying areas, interpreted faults show zones of west-northwest-, north-, and northwest-striking faults that cross ~3 km (~2 mi) west-northwest of the town of Poncha Springs. More easterly striking faults extend east from this juncture. The associated aeromagnetic anomalies are likely caused by magnetic contrasts associated with faulted sediments that are concealed less than 200 m (656 ft) below the valley floor. The faults may involve basement rocks at greater depth as well. A relatively shallow (<300 m or <984 ft), faulted basement block is indicated under basin-fill sediments just north of the hot springs and south of the town of Poncha Springs.

Active faults and deformation of the Catania margin (Eastern Sicily): preliminary results from the CRACK marine geophysical survey (Aug./Sep. 2016 R/V Tethys2)

NASA Astrophysics Data System (ADS)

Gutscher, M. A.; Dellong, D.; Graindorge, D.; Le Roy, P., Sr.; Dominguez, S.; Barreca, G.; Cunarro, D.; Petersen, F.; Urlaub, M.; Krastel, S.; Gross, F.; Kopp, H.

2016-12-01

The marine geophysical survey entitled CRACK (Catania margin, Relief, ACtive faults and historical earthquaKes) aims to investigate active faults offshore eastern Sicily. Several faults have been mapped onshore on the SE flank of Mt. Etna and recently a major strike-slip fault system was mapped in the deeper offshore area. The purpose of this study is to perform shallow water bathymetric mapping and a high-resolution sparker seismic survey in the shelf zone between the deep offshore and the onshore areas, a zone less well studied. Aside from the two fault systems mentioned above, there is also the Malta escarpment, the onshore (but buried) blind-thrust of the Gela Nappe and the lateral ramp thrust of the Calabrian accretionary wedge. Somehow all these structures connect offshore Catania, though exactly how is still unknown. The study will take place between 18 Aug. and 4 Sept. 2016 using the 25m long coastal research vessel Tethys2 and will consist of three 5-day legs. The first leg (zone 2) will be purely sparker seismics and legs 2 and 3 will be combined seismics and bathymetry along the shallow submarine SE flank of Mt. Etna (zone 1) and shallow continental shelf SE of Catania (zone 3). Some time during the first leg will also be devoted to submarine geodesy. Five submarine geodetic stations were deployed along the dextral strike-slip "North Alfeo - Etna" fault by the German GEOMAR Helmholtz Centre for Ocean Research Kiel in April 2016 (R/V Poseidon). The long-term monitoring campaign should help indicate in the future if this fault is slowly creeping or not currently moving. The first five months of data will be downloaded during the CRACK cruise.

Global Sampling for Integrating Physics-Specific Subsystems and Quantifying Uncertainties of CO 2 Geological Sequestration

DOE PAGES

Sun, Y.; Tong, C.; Trainor-Guitten, W. J.; ...

2012-12-20

The risk of CO 2 leakage from a deep storage reservoir into a shallow aquifer through a fault is assessed and studied using physics-specific computer models. The hypothetical CO 2 geological sequestration system is composed of three subsystems: a deep storage reservoir, a fault in caprock, and a shallow aquifer, which are modeled respectively by considering sub-domain-specific physics. Supercritical CO 2 is injected into the reservoir subsystem with uncertain permeabilities of reservoir, caprock, and aquifer, uncertain fault location, and injection rate (as a decision variable). The simulated pressure and CO 2/brine saturation are connected to the fault-leakage model as amore » boundary condition. CO 2 and brine fluxes from the fault-leakage model at the fault outlet are then imposed in the aquifer model as a source term. Moreover, uncertainties are propagated from the deep reservoir model, to the fault-leakage model, and eventually to the geochemical model in the shallow aquifer, thus contributing to risk profiles. To quantify the uncertainties and assess leakage-relevant risk, we propose a global sampling-based method to allocate sub-dimensions of uncertain parameters to sub-models. The risk profiles are defined and related to CO 2 plume development for pH value and total dissolved solids (TDS) below the EPA's Maximum Contaminant Levels (MCL) for drinking water quality. A global sensitivity analysis is conducted to select the most sensitive parameters to the risk profiles. The resulting uncertainty of pH- and TDS-defined aquifer volume, which is impacted by CO 2 and brine leakage, mainly results from the uncertainty of fault permeability. Subsequently, high-resolution, reduced-order models of risk profiles are developed as functions of all the decision variables and uncertain parameters in all three subsystems.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, Y.; Tong, C.; Trainor-Guitten, W. J.

The risk of CO 2 leakage from a deep storage reservoir into a shallow aquifer through a fault is assessed and studied using physics-specific computer models. The hypothetical CO 2 geological sequestration system is composed of three subsystems: a deep storage reservoir, a fault in caprock, and a shallow aquifer, which are modeled respectively by considering sub-domain-specific physics. Supercritical CO 2 is injected into the reservoir subsystem with uncertain permeabilities of reservoir, caprock, and aquifer, uncertain fault location, and injection rate (as a decision variable). The simulated pressure and CO 2/brine saturation are connected to the fault-leakage model as amore » boundary condition. CO 2 and brine fluxes from the fault-leakage model at the fault outlet are then imposed in the aquifer model as a source term. Moreover, uncertainties are propagated from the deep reservoir model, to the fault-leakage model, and eventually to the geochemical model in the shallow aquifer, thus contributing to risk profiles. To quantify the uncertainties and assess leakage-relevant risk, we propose a global sampling-based method to allocate sub-dimensions of uncertain parameters to sub-models. The risk profiles are defined and related to CO 2 plume development for pH value and total dissolved solids (TDS) below the EPA's Maximum Contaminant Levels (MCL) for drinking water quality. A global sensitivity analysis is conducted to select the most sensitive parameters to the risk profiles. The resulting uncertainty of pH- and TDS-defined aquifer volume, which is impacted by CO 2 and brine leakage, mainly results from the uncertainty of fault permeability. Subsequently, high-resolution, reduced-order models of risk profiles are developed as functions of all the decision variables and uncertain parameters in all three subsystems.« less

Post 4 Ma initiation of normal faulting in southern Tibet. Constraints from the Kung Co half graben

NASA Astrophysics Data System (ADS)

Mahéo, G.; Leloup, P. H.; Valli, F.; Lacassin, R.; Arnaud, N.; Paquette, J.-L.; Fernandez, A.; Haibing, L.; Farley, K. A.; Tapponnier, P.

2007-04-01

The timing of E-W extension of the Tibetan plateau provides a test of mechanical models of the geodynamic evolution of the India-Asia convergence zone. In this work we focus on the Kung Co half graben (Southern Tibet, China), bounded by an active N-S normal fault with a minimum vertical offset of 1600 m. To estimate the onset of normal faulting we combined high and medium temperature (U-Pb, Ar/Ar) and low temperature ((U-Th)/He) thermochronometry of the Kung Co pluton, a two-mica granite of the northern Himalayan granitic belt that outcrop in the footwall of the fault. Biotite and muscovite Ar/Ar ages , are close from each other [˜ 16 Ma ± 0.2 (Ms) and ˜ 15 ± 0.4 Ma (Bt)], which is typical of fast cooling. The zircon and apatite (U-Th)/He ages range from 11.3 to 9.6 Ma and 9.9 to 3.7 Ma respectively. These He ages are indicative of (1) fast initial cooling, from 11.3 to ˜ 9 Ma, gradually decreasing with time and (2) a high geothermal gradient (˜ 400 °C/km), close to the surface at ˜ 10 Ma. The Kung Co pluton was emplaced at about 22 Ma (U-Pb on zircon) at less than 10 km depth and 520-545 °C. Subsequent to its shallow emplacement, the pluton underwent fast thermal re-equilibration ending around 7.5 Ma, followed by a period of slow cooling caused either by the end of the thermal re-equilibration or by very slow exhumation (0.02-0.03 mm/yr) from ˜ 7.5 Ma to at least 4 Ma. In either case the data suggest that the exhumation rate increased after 4 Ma. We infer this increase to be related to the initiation of the Kung Co normal fault. A critical examination of previously published data show that most ˜ N-S Tibetan normal faults may have formed less than 5 Ma ago rather than in the Miocene as assumed by several authors. Such a young age implies that E-W extension is not related to the Neogene South Tibetan magmatism (25 to 8 Ma). Consequently, models relating E-W extension to magmatism, such as convective removal of the lower lithosphere, may be inappropriate. We rather think that this extension is related with local accommodation of boundary forces and displacements.

Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake.

PubMed

Chester, Frederick M; Rowe, Christie; Ujiie, Kohtaro; Kirkpatrick, James; Regalla, Christine; Remitti, Francesca; Moore, J Casey; Toy, Virginia; Wolfson-Schwehr, Monica; Bose, Santanu; Kameda, Jun; Mori, James J; Brodsky, Emily E; Eguchi, Nobuhisa; Toczko, Sean

2013-12-06

The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

Studying the Effects of Transparent vs. Opaque Shallow Thrust Faults Using Synthetic P and SH Seismograms

NASA Astrophysics Data System (ADS)

Smith, D. E.; Aagaard, B. T.; Heaton, T. H.

2001-12-01

It has been hypothesized (Brune, 1996) that teleseismic inversions may underestimate the moment of shallow thrust fault earthquakes if energy becomes trapped in the hanging wall of the fault, i.e. if the fault boundary becomes opaque. We address this by creating and analyzing synthetic P and SH seismograms for a variety of friction models. There are a total of five models: (1) crack model (slip weakening) with instantaneous healing (2) crack model without healing (3) crack model with zero sliding friction (4) pulse model (slip and rate weakening) (5) prescribed model (Haskell-like rupture with the same final slip and peak slip-rate as model 4). Models 1-4 are all dynamic models where fault friction laws determine the rupture history. This allows feedback between the ongoing rupture and waves from the beginning of the rupture that hit the surface and reflect downwards. Hence, models 1-4 can exhibit opaque fault characteristics. Model 5, a prescribed rupture, allows for no interaction between the rupture and reflected waves, therefore, it is a transparent fault. We first produce source time functions for the different friction models by rupturing shallow thrust faults in 3-D dynamic finite-element simulations. The source time functions are used as point dislocations in a teleseismic body-wave code. We examine the P and SH waves for different azimuths and epicentral distances. The peak P and S first arrival displacement amplitudes for the crack, crack with healing and pulse models are all very similar. These dynamic models with opaque faults produce smaller peak P and S first arrivals than the prescribed, transparent fault. For example, a fault with strike = 90 degrees, azimuth = 45 degrees has P arrivals smaller by about 30% and S arrivals smaller by about 15%. The only dynamic model that doesn't fit this pattern is the crack model with zero sliding friction. It oscillates around its equilibrium position; therefore, it overshoots and yields an excessively large peak first arrival. In general, it appears that the dynamic, opaque faults have smaller peak teleseismic displacements that would lead to lower moment estimates by a modest amount.

Tectonic activity and stratigraphic history over the last 130-540 ka on the Southern Shelf of the Sea of Marmara, western North Anatolian Fault, Turkey

NASA Astrophysics Data System (ADS)

Smith, W. H.; Grall, C.; Sorlien, C. C.; Steckler, M. S.; Okay, S.; Cormier, M. H.; Seeber, L.; Cifci, G.; Dondurur, D.

2016-12-01

The submerged section of the North Anatolian Fault in the Sea of Marmara, which corresponds to the dextral plate boundary between Eurasia and Anatolia, poses strong hazard for earthquakes and subsequent submarine landslides and tsunamis in the vicinity of the highly populated region of Istanbul. Most of the right-lateral slip is accommodated by the Northern Branch of the North Anatolian Fault (NAF-N), which crosses the central part of the Sea of Marmara and is capable of an earthquake with a magnitude greater than 7. However, both the geology and the geodesy suggest that the NAF-N accommodates only 3/4 of the total slip between the plates. The deformation mechanisms for the rest of the strain (slip distributed on secondary faults, strain partitioning, and diffuse deformation) remains unexplained. Other fault systems, primarily south of the NAF-N, are shown to be important regarding the tectonic evolution of the Sea of Marmara. However, the activity of these peripheral fault systems as well as their relationships with the NAF-N need to be further constrained. For this purpose, a dense dataset of 2D geophysical images (high-resolution seismic reflection data, sparker reflection, CHIRP sub-bottom profiling), as well as multibeam bathymetry, have been acquired in 2008, 2010, 2013 and 2014 during TAMAM and SOMAR cruises, primarily in the southern shelf of the Sea of Marmara. The 15-20 km-wide southern shelf ledge is relatively flat and mostly shallower than 90 m. In this shallow marine region, we have been able to image the detailed stratigraphic record associated with the 125 ka and younger glacio-eustatic cycles and, notably, to identify paleo-shorelines at water depths shallower than 100 m. Several erosional unconformities, laterally correlative to low-stand deltas have been regionally linked to the stratigraphic boundaries previously defined for the last 130-540 ka. While the present-day shelf is relatively flat, a shallow ridge separates the inner and outer parts of the shelf. This ridge exhibits erosional unconformities, and a set of transtensive faults are mapped along its length. We show that parts of these faults were active during the last 540 ka. By estimating fault slip and folding rates along these structures, we estimate the deformation that they accommodated over this time-frame.

An overview of results from the CO2SINK 3D baseline seismic survey at Ketzin, Germany

NASA Astrophysics Data System (ADS)

Juhlin, C.; Giese, R.; Cosma, C.; Kazemeini, H.; Juhojuntti, N.; Lüth, S.; Norden, B.; Förster, A.; Yordkayhun, S.

2009-04-01

A 3D seismic survey was acquired at the CO2SINK project site over the Ketzin anticline in the fall of 2005. Main objectives of the survey were (1) to verify earlier geological interpretations of the structure based on vintage 2D seismic and borehole data, (2) to provide, if possible, an understanding of the structural geometry for flow pathways within the reservoir, (3) a baseline for later evaluation of the time evolution of rock properties as CO2 is injected into the reservoir, and (4) detailed sub-surface images near the injection borehole for planning of the drilling operations. Overlapping templates with 5 receiver lines containing 48 active channels in each template were used for the acquisition. In each template, 200 nominal source points were activated using an accelerated weight drop, giving a nominal fold of 25. Due to logistics, the number of actual source points in each template varied. In spite of the relatively low fold and the simple source used, data quality is generally good with the uppermost 1000 m being well imaged. Data processing results clearly show a fault system across the top of the Ketzin anticline that is termed the Central Graben Fault Zone (CGFZ). The fault zone consists of west-southwest-east-northeast- to east-west-trending normal faults bounding a 600-800 m wide graben. Within the Jurassic section, discrete faults are well developed, and the main graben-bounding faults have throws of up to 30 m. At shallower levels, the fault system appears to disappear in the Tertiary Rupelian clay. The main bounding faults of the CGFZ can be traced downwards to the top of the Weser Formation and possibly to the Stuttgart level, the target formation for CO2 injection. No faults were imaged near the injection site on the southern limb of the anticline. Remnant gas, cushion and residual gas from a previous natural gas storage facility at the site, is present near the top of the anticline in the depth interval of about 250-400 m and has a clear seismic signature. In addition to the standard processing and interpretation applied, attribute analysis, detailed shallow reflection seismic processing, tomographic inversion of first arrival times, and initial seismic modeling of the CO2 response have been performed. Attribute analysis of the target horizon using the continuous wavelet transform indicates that the injection site penetrates the target reservoir near the edge of a north-northwest-south-southeast striking channel.

Recent deformation along the offshore Malibu Coast, Dume, and related faults west of Point Dume, southern California

USGS Publications Warehouse

Fisher, M.A.; Langenheim, V.E.; Sorlien, C.C.; Dartnell, P.; Sliter, R.W.; Cochrane, G.R.; Wong, F.L.

2005-01-01

Offshore faults west of Point Dume, southern California, are part of an important regional fault system that extends for about 206 km, from near the city of Los Angeles westward along the south flank of the Santa Monica Mountains and through the northern Channel Islands. This boundary fault system separates the western Transverse Ranges, on the north, from the California Continental Borderland, on the south. Previous research showed that the fault system includes many active fault strands; consequently, the entire system is considered a serious potential earthquake hazard to nearby Los Angeles. We present an integrated analysis of multichannel seismic- and high-resolution seismic-reflection data and multibeam-bathymetric information to focus on the central part of the fault system that lies west of Point Dume. We show that some of the main offshore faults have cumulative displacements of 3-5 km, and many faults are currently active because they deform the seafloor or very shallow sediment layers. The main offshore fault is the Dume fault, a large north-dipping reverse fault. In the eastern part of the study area, this fault offsets the seafloor, showing Holocene displacement. Onshore, the Malibu Coast fault dips steeply north, is active, and shows left-oblique slip. The probable offshore extension of this fault is a large fault that dips steeply in its upper part but flattens at depth. High-resolution seismic data show that this fault deforms shallow sediment making up the Hueneme fan complex, indicating Holocene activity. A structure near Sycamore knoll strikes transversely to the main faults and could be important to the analysis of the regional earthquake hazard because the structure might form a boundary between earthquake-rupture segments.

Large‐displacement, hydrothermal frictional properties of DFDP‐1 fault rocks, Alpine Fault, New Zealand: Implications for deep rupture propagation

PubMed Central

Boulton, C.; Toy, V. G.; Townend, J.; Sutherland, R.

2016-01-01

Abstract The Alpine Fault, New Zealand, is a major plate‐bounding fault that accommodates 65–75% of the total relative motion between the Australian and Pacific plates. Here we present data on the hydrothermal frictional properties of Alpine Fault rocks that surround the principal slip zones (PSZ) of the Alpine Fault and those comprising the PSZ itself. The samples were retrieved from relatively shallow depths during phase 1 of the Deep Fault Drilling Project (DFDP‐1) at Gaunt Creek. Simulated fault gouges were sheared at temperatures of 25, 150, 300, 450, and 600°C in order to determine the friction coefficient as well as the velocity dependence of friction. Friction remains more or less constant with changes in temperature, but a transition from velocity‐strengthening behavior to velocity‐weakening behavior occurs at a temperature of T = 150°C. The transition depends on the absolute value of sliding velocity as well as temperature, with the velocity‐weakening region restricted to higher velocity for higher temperatures. Friction was substantially lower for low‐velocity shearing (V < 0.3 µm/s) at 600°C, but no transition to normal stress independence was observed. In the framework of rate‐and‐state friction, earthquake nucleation is most likely at an intermediate temperature of T = 300°C. The velocity‐strengthening nature of the Alpine Fault rocks at higher temperatures may pose a barrier for rupture propagation to deeper levels, limiting the possible depth extent of large earthquakes. Our results highlight the importance of strain rate in controlling frictional behavior under conditions spanning the classical brittle‐plastic transition for quartzofeldspathic compositions. PMID:27610290

Discrete element simulations of gravitational volcanic deformation. 1; Deformation structures and geometries

NASA Technical Reports Server (NTRS)

Morgan, Julia K.; McGovern, Patrick J.

2005-01-01

We have carried out two-dimensional particle dynamics simulations of granular piles subject to frictional Coulomb failure criteria to gain a first-order understanding of different modes of gravitational deformation within volcanoes. Under uniform basal and internal strength conditions, granular piles grow self-similarly, developing distinctive stratigraphies, morphologies, and structures. Piles constructed upon cohesive substrates exhibit particle avalanching, forming outward dipping strata and angle of repose slopes. Systematic decreases in basal strength lead to progressively deeper and steeper internal detachment faults and slip along a basal decollement; landslide forms grade from shallow slumps to deep-seated landslide and, finally, to axial subsidence and outward flank displacements, or volcanic spreading. Surface slopes decrease and develop concave up morphologies with decreasing decollement strength; depositional layers tilt progressively inward. Spatial variations in basal strength cause lateral transitions in pile structure, stratigraphy, and morphology. This approximation of volcanoes as Coulomb granular piles reproduces the richness of deformational structures and surface morphologies in many volcanic settings. The gentle slopes of Hawaiian volcanoes and Olympus Mons on Mars suggest weak basal decollements that enable volcanic spreading. High-angle normal faults, favored above weak decollements, are interpreted in both settings and may explain catastrophic sector collapse in Hawaii and broad aureole deposits surrounding Olympus Mons. In contrast, steeper slopes and shallow detachment faults predominate in the Canary Islands, thought to lack a weak decollement, favoring smaller, more frequent slope failures than predicted for Hawaii. The numerical results provide a useful predictive tool for interpreting dynamic behavior and associated geologic hazards of active volcanoes.

Initiation of a Low-Angle Normal Fault Active Across the Upper Brittle-Plastic Transition, Chemehuevi Mountains, CA

NASA Astrophysics Data System (ADS)

LaForge, J.; John, B. E.; Grimes, C. B.; Stunitz, H.; Heilbronner, R.

2016-12-01

The Chemehuevi detachment fault system, part of the regionally developed Colorado River extensional corridor, hosts exceptional exposures of a denuded fault system related to Miocene extension. Here, we characterize the early history of extension associated with a small slip (1-2 km) low-angle normal fault, the Mohave Wash fault (MWF), initially active across the brittle-plastic transition. Strain localized in three principal ways across the 23-km down-dip exposure (T <150° to >400°C): a brittle fault zone, localized, disseminated quartz mylonites, and syntectonic dikes hosting mylonitic fabrics. Brittle deformation in these crystalline rocks was concentrated into a 10-62-m thick brittle fault zone hosting localized, unmineralized to chlorite-epidote-quartz mineralized zones of cataclasite series fault rocks ≤3 m thick and rare pseudotachylite. Mylonitic deformation played an increased role in deformation down dip (NE), with mylonites increasing in quantity and average thickness. At shallow structural levels, footwall mylonites are absent; at 9-18 km down dip, cm-scale quartz mylonites are common; ≥18 km down dip, meter-scale syntectonic intermediate-felsic dikes are mylonitic, are attenuated into parallelism with the MWF, and host well-developed L-S fabric; 23 km down dip, the footwall hosts meter-thick zones of disseminated mylonitic quartz of varying intensities. These mylonites host microstructures that record progressively higher deformation temperature down dip, with dislocation-creep in quartz indicative of T of 280-400°C to ≥500°C, and diffusion creep with grain boundary sliding in dikes suggestive of even higher T deformation. Dike emplacement in the system is syntectonic with MWF slip; mafic-intermediate composition dikes intruded damage zone fractures and cataclasites, and were in turn fractured; Pb/U zircon ages of intermediate-felsic dikes range from ca. 1.5 ± 1 Ma to 3.8 ± 1 Ma after the onset of regional extension, but predate rapid slip. Cross cutting relations and absolute dating suggest the early history of the MWF evolved in two distinct phases: 1) seismogenic rupture with contemporaneous localized footwall mylonitization, followed by 2) additional cataclasis, episodic localized and magmatism, mylonitization and fluid-flow.

Epigenetic lead, zinc, silver, antimony, tin, and gold veins in Boulder Basin, Blaine and Custer counties, Idaho; potential for economic tin mineralization

USGS Publications Warehouse

Ratchford, Michael E.

2002-01-01

Boulder Basin is in a northwest-trending belt of allochthonous Paleozoic rocks in the Boulder Mountains of central Idaho. Regional Tertiary extension resulted in widespread normal faulting and coeval emplacement of shallow-level intrusions and extrusive rocks of the Challis Volcanic Group. Epigenetic lead-zinc-silver-antimony-tin-gold vein deposits formed during Tertiary extension and are hosted within Paleozoic strata. The major orebodies are in the lower plate of the Boulder Basin thrust fault, in massive quartzite of the Middle Pennsylvanian to Lower Permian Wood River Formation. Anomalous concentrations of tin are present in the base-metal mineral assemblage of the Boulder Basin ore deposits. The tin-bearing veins in Boulder Basin are strikingly similar to Bolivian tin deposits. The deposit model for Bolivian tin deposits identifies buried tin porphyry below the tin-bearing vein system.

Ambient tremors in a collisional orogenic belt

USGS Publications Warehouse

Chuang, Lindsay Yuling; Chen, Kate Huihsuan; Wech, Aaron G.; Byrne, Timothy; Peng, Wei

2014-01-01

Deep-seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth, which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt. In this study we explore the general features of collisional tremors in Taiwan and discuss the possible generation mechanism. In the 4 year data, we find 231 ambient tremor episodes with durations ranging from 5 to 30 min. In addition to a coseismic slip-induced stress change from nearby major earthquake, increased tremor rate is also highly correlated with the active, normal faulting earthquake swarms at the shallower depth. Both the tremor and earthquake swarm activities are confined in a small, area where the high attenuation, high thermal anomaly, the boundary between high and low resistivity, and localized veins on the surfaces distributed, suggesting the involvement of fluids from metamorphic dehydration within the orogen.

Distributed deformation and block rotation in 3D

NASA Technical Reports Server (NTRS)

Scotti, Oona; Nur, Amos; Estevez, Raul

1990-01-01

The authors address how block rotation and complex distributed deformation in the Earth's shallow crust may be explained within a stationary regional stress field. Distributed deformation is characterized by domains of sub-parallel fault-bounded blocks. In response to the contemporaneous activity of neighboring domains some domains rotate, as suggested by both structural and paleomagnetic evidence. Rotations within domains are achieved through the contemporaneous slip and rotation of the faults and of the blocks they bound. Thus, in regions of distributed deformation, faults must remain active in spite of their poor orientation in the stress field. The authors developed a model that tracks the orientation of blocks and their bounding faults during rotation in a 3D stress field. In the model, the effective stress magnitudes of the principal stresses (sigma sub 1, sigma sub 2, and sigma sub 3) are controlled by the orientation of fault sets in each domain. Therefore, adjacent fault sets with differing orientations may be active and may display differing faulting styles, and a given set of faults may change its style of motion as it rotates within a stationary stress regime. The style of faulting predicted by the model depends on a dimensionless parameter phi = (sigma sub 2 - sigma sub 3)/(sigma sub 1 - sigma sub 3). Thus, the authors present a model for complex distributed deformation and complex offset history requiring neither geographical nor temporal changes in the stress regime. They apply the model to the Western Transverse Range domain of southern California. There, it is mechanically feasible for blocks and faults to have experienced up to 75 degrees of clockwise rotation in a phi = 0.1 strike-slip stress regime. The results of the model suggest that this domain may first have accommodated deformation along preexisting NNE-SSW faults, reactivated as normal faults. After rotation, these same faults became strike-slip in nature.

Microgravity changes at the Laguna del Maule volcanic field: Magma-induced stress changes facilitate mass addition

NASA Astrophysics Data System (ADS)

Miller, C. A.; Le Mével, H.; Currenti, G.; Williams-Jones, G.; Tikoff, B.

2017-04-01

Time-dependent, or 4-D, microgravity changes observed at the Laguna del Maule volcanic field, Chile, since 2013, indicate significant (1.5 × 1011 kg) ongoing mass injection. Mass injection is focused along the Troncoso fault, and subparallel structures beneath the lake at 1.5-2 km depth, and is best modeled by a vertical rectangular prism source. The low-density change (156 to 307 kg/m3) and limited depth extent suggest a mechanism of hydrothermal fluid intrusion into existing voids, or voids created by the substantial uplift, rather than deeper-sourced dike intrusion of rhyolite or basalt magma. Although the gravity changes are broadly spatially coincident with ongoing surface deformation, existing models that explain the deformation are deeper sourced and cannot explain the gravity changes. To account for this discrepancy and the correspondence in time of the deformation and gravity changes, we explore a coupled magmatectonic interaction mechanism that allows for shallow mass addition, facilitated by deeper magma injection. Computing the strain, and mean, normal, and Coulomb stress changes on northeast trending faults, caused by the opening of a sill at 5 km depth, shows an increase in strain and mean and normal stresses along these faults, coincident with the areas of mass addition. Seismic swarms in mid-2012 to the west and southwest of the mass intrusion area may be responsible for dynamically increasing permeability on the Troncoso fault, promoting influx of hydrothermal fluids, which in turn causes larger gravity changes in the 2013 to 2014 interval, compared to the subsequent intervals.

Trapping and migration of methane associated with the gas hydrate stability zone at the Blake Ridge Diapir: New insights from seismic data

USGS Publications Warehouse

Taylor, M.H.; Dillon, William P.; Pecher, I.A.

2000-01-01

The Blake Ridge Diapir is the southernmost of a line of salt diapirs along the Carolina trough. Diapirs cause faulting of the superjacent sediments, creating pathways for migration of fluids and gas to the seafloor. We analyzed reflection seismic data from the Blake Ridge Diapir, which is located in a region with known abundant gas hydrate occurrence. A striking feature in these data is a significant shallowing of the base of gas hydrate stability (BGHS) over the center of the diapir. The seafloor is warped up by about 100 m above the diapir, from about 2300 m to about 2200 m. The BGHS, as indicated by a bottom simulating reflection (BSR), is about 4.5 s off the flanks of the diapir, rising to about 4.15 s at the center. Above the diapir, a fault system appears to rise vertically from the BGHS to about 0.05 s below the seafloor (40-50 m); it then diverges into several steeply dipping faults that breach the seafloor and cover an area ~700 m in diameter. Other secondary faults diverge from the main fault or emerge directly from the BGHS near the crest of the diapir. Gas and other fluids may migrate upward through the faults. We performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections. A low-frequency anomaly over the center of the diapir indicates high seismic attenuation. This is interpreted to be caused by migration of fluids (probably methane) along fault pathways. The migration of gas (i.e. probably mainly methane) through the gas hydrate stability zone is not yet understood. We speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate, even at depths where gas hydrate is stable away from the diapir. Alternatively, gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates. The shallowing of the BSR may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids. High pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in BSR shallowing. We, therefore, investigated the possible effect of pore water salinity on shallowing of the BSR. We found that BSR shallowing may theoretically be entirely caused by increased salinity over the diapir, although geologically this would not be reasonable. This observation demonstrates the potential importance of pore water salinity for lateral variations of BSR depths, in particular, above salt structures: (C) 2000 Elsevier Science B.V.

New perspectives on the evolution of narrow, modest extension continental rifts: Embryonic core complexes and localized, rapid Quaternary extension in the Rio Grande rift, central New Mexico

NASA Astrophysics Data System (ADS)

Ricketts, J.; Karlstrom, K. E.; Kelley, S.

2013-12-01

Updated models for continental rift zones need to address the role and development of low-angle normal fault networks, episodicity of extension, and interaction of 'active and passive' driving mechanisms. In the Rio Grande rift, USA, low-angle normal faults are found throughout the entire length of the rift, but make up a small percentage of the total fault population. The low-angle Jeter and Knife Edge faults, for example, crop out along the SW and NE margins of the Albuquerque basin, respectively. Apatite fission track (AFT) age-elevation data and apatite (U-Th)/He (AHe) ages from these rift flank uplifts record cooling between ~21 - 16 Ma in the NE rift flank and ~20 - 10 Ma in the SW, which coincides with times of rapid extension and voluminous syntectonic sedimentation. The timing of exhumation is also similar to rift flanks farther north in active margins based on AFT data alone. In addition, synthetic faults in the hanging wall of each low-angle fault become progressively steeper and younger basinward, and footwall blocks are the highest elevation along the rift flanks. These observations are consistent with a model where initially high-angle faults are shallowed in regions of maximum extension. As they rotate, new intrabasinal faults emerge which also can be rotated if extension continues. These relationships are similarly described in mature core complexes, and if these processes continued in the Rio Grande rift, it could eventually result in mid-crustal ductily deformed rocks in the footwall placed against surficial deposits in the hanging wall across faults that have been isostatically rotated to shallow dips. Although existing data are consistent with highest strain rates during a pulse of extension along the entire length of the rift 20-10 Ma., GPS-constrained measurements suggest that the rift is still actively-extending at 1.23-1.39 nstr/yr (Berglund et al., 2012). Additional evidence for Quaternary extension comes from travertine deposits that are cut by multiple tensile vein sets along the western margin of the Albuquerque basin in the Lucero uplift. At this location, U-series ages on travertine deposits are used to calculate strain rates at this location. These strain rates (15-105 nstr/yr) are higher than both the modern strain rates as well as the average long-term strain rates (3-14 nstr/yr) obtained from restored cross-sections across different basins in the rift. To explain these observations, we propose a model involving high fluid pressures, which promote the formation of tensile veins that are oriented with respect to the modern day stress field in the rift. These regions of anomalously-high strain need not be widespread, and are only active on timescales of the hydraulic system, but they are nevertheless an underappreciated mechanism of progressive extension in the rift. Berglund, H.T., Sheehan, A.F., Murray, M.H., Roy, M., Lowry, A.R., Nerem, R.S., and Blume, F., 2012, Distributed deformation across the Rio Grande Rift, Great Plains, and Colorado Plateau: Geology, v. 40, p. 23-26.

Lithospheric Structure and Active Deformation in the Salton Trough from Coseismic and Postseismic Models of the 2010 Mw 7.2 El Mayor-Cucapah Earthquake

NASA Astrophysics Data System (ADS)

Fielding, E. J.; Huang, M. H.; Dickinson, H.; Freed, A. M.; Burgmann, R.; Gonzalez-Ortega, J. A.; Andronicos, C.

2016-12-01

The 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) Earthquake ruptured about 120 km along several NW-striking faults to the west of the Cerro Prieto Fault in the Salton Trough of Baja California, Mexico. We analyzed interferometric synthetic aperture radar (SAR), SAR and optical pixel offsets, and continuous and campaign GPS data to optimize an EMC coseismic rupture model with 9 fault segments, which fits the complex structure of the faults. Coseismic slip inversion with a layered elastic model shows that largely right-lateral slip is confined to upper 10 km with strong variations along strike. Near-field GPS measures slip on a north-striking normal fault that ruptured at the beginning of the earthquake, previously inferred from seismic waveforms. EMC Earthquake postseismic deformation shows the Earth's response to the large coseismic stress changes. InSAR shows rapid shallow afterslip at the north and south ends of the main ruptures. Continuous GPS from the Plate Boundary Observatory operated by UNAVCO measures the first six years of postseismic deformation, extremely rapid near the rupture. Afterslip on faults beneath the coseismic rupture cannot explain far-field displacements that are best explained by viscoelastic relaxation of the lower crust and upper mantle. We built a viscoelastic 3D finite element model of the lithosphere and asthenosphere based on available data for the region with the EMC coseismic faults embedded inside. Coseismic slip was imposed on the model, allowed to relax for 5 years, and then compared to the observed surface deformation. Systematic exploration of the viscoelastic parameters shows that horizontal and vertical heterogeneity is required to fit the postseismic deformation. Our preferred viscoelastic model has weaker viscosity layers beneath the Salton Trough than adjacent blocks that are consistent with the inferred differences in the geotherms. Defining mechanical lithosphere as rocks that have viscosities greater than 10^19 Pa s (able to sustain stresses for more than 100 years), we infer the thickness of lithosphere beneath the Salton Trough to be 32 km and 65 km beneath the Peninsula Ranges to the west. These mechanical lithosphere-asthenosphere boundaries (LABs) are shallower than the observed seismic LABs, but probably better represent the strength of the blocks in this area.

Relating Mechanical Behavior and Microstructural Observations in Calcite Fault Gouge

NASA Astrophysics Data System (ADS)

Carpenter, B. M.; Di Stefano, G.; Viti, C.; Collettini, C.

2013-12-01

Many important earthquakes, magnitude 5-7, nucleate and/or propagate through carbonate-dominated lithologies. Additionally, the presence of precipitated calcite in (cement) and near (vein fill) faults indicates that the mechanical behavior of carbonate-dominated material likely plays an important role in shallow- and mid-crustal faulting. We report on laboratory experiments designed to explore the mechanical behavior of calcite and relate that behavior to post experiment microstructural observations. We sheared powdered gouge of Carrara Marble, >98% CaCO3, at constant normal stresses between 1 and 50 MPa under saturated conditions at room temperature. We performed velocity-stepping tests, 0.1-1000 μm/s, to evaluate frictional stability, and slide-hold-slide tests, 1-10,000 seconds, to measure the amount of frictional healing. Small subsets of experiments were performed under different environmental conditions and shearing velocities to better elucidate physicochemical processes and their role in the mechanical behavior of calcite fault gouge. All experimental samples were collected for SEM analysis. We find that the frictional healing rate is 7X higher under saturated conditions than under nominally dry conditions. We also observe a divergence between the rates of creep relaxation (increasing) and frictional healing (decreasing) as shear velocity is increased from 1 to 3000 μm/s. Our highest healing rates are observed at our lowest normal stresses. We observe a frictional strength of μ = 0.64, consistent with previous data under similar conditions. Furthermore, although we observe velocity-weakening frictional behavior in both the saturated and dry cases, rate- and-state friction parameters are distinctly different for each case. Our combined observations of rapid healing and of velocity-weakening frictional behavior indicate that faults where calcite-dominated gouge is present are likely to be seismic and have the ability to regain their strength quickly. Furthermore, our mechanical results highlight the important role of fluids in the evolution of frictional strength and thus fault behavior.

Fault zone hydrogeology

NASA Astrophysics Data System (ADS)

Bense, V. F.; Gleeson, T.; Loveless, S. E.; Bour, O.; Scibek, J.

2013-12-01

Deformation along faults in the shallow crust (< 1 km) introduces permeability heterogeneity and anisotropy, which has an important impact on processes such as regional groundwater flow, hydrocarbon migration, and hydrothermal fluid circulation. Fault zones have the capacity to be hydraulic conduits connecting shallow and deep geological environments, but simultaneously the fault cores of many faults often form effective barriers to flow. The direct evaluation of the impact of faults to fluid flow patterns remains a challenge and requires a multidisciplinary research effort of structural geologists and hydrogeologists. However, we find that these disciplines often use different methods with little interaction between them. In this review, we document the current multi-disciplinary understanding of fault zone hydrogeology. We discuss surface- and subsurface observations from diverse rock types from unlithified and lithified clastic sediments through to carbonate, crystalline, and volcanic rocks. For each rock type, we evaluate geological deformation mechanisms, hydrogeologic observations and conceptual models of fault zone hydrogeology. Outcrop observations indicate that fault zones commonly have a permeability structure suggesting they should act as complex conduit-barrier systems in which along-fault flow is encouraged and across-fault flow is impeded. Hydrogeological observations of fault zones reported in the literature show a broad qualitative agreement with outcrop-based conceptual models of fault zone hydrogeology. Nevertheless, the specific impact of a particular fault permeability structure on fault zone hydrogeology can only be assessed when the hydrogeological context of the fault zone is considered and not from outcrop observations alone. To gain a more integrated, comprehensive understanding of fault zone hydrogeology, we foresee numerous synergistic opportunities and challenges for the discipline of structural geology and hydrogeology to co-evolve and address remaining challenges by co-locating study areas, sharing approaches and fusing data, developing conceptual models from hydrogeologic data, numerical modeling, and training interdisciplinary scientists.

InSAR observations of aseismic slip associated with an earthquake swarm in the Columbia River flood basalts

USGS Publications Warehouse

Wicks, Charles; Thelen, W.; Weaver, C.; Gomberg, J.; Rohay, A.; Bodin, P.

2011-01-01

In 2009 a swarm of small shallow earthquakes occurred within the basalt flows of the Columbia River Basalt Group (CRBG). The swarm occurred within a dense seismic network in the U.S. Department of Energys Hanford Site. Data from the seismic network along with interferometric synthetic aperture radar (InSAR) data from the European Space Agencys (ESA) ENVISAT satellite provide insight into the nature of the swarm. By modeling the InSAR deformation data we constructed a model that consists of a shallow thrust fault and a near horizontal fault. We suggest that the near horizontal lying fault is a bedding-plane fault located between basalt flows. The geodetic moment of the modeled fault system is about eight times the cumulative seismic moment of the swarm. Precise location estimates of the swarm earthquakes indicate that the area of highest slip on the thrust fault, ???70mm of slip less than ???0.5km depth, was not located within the swarm cluster. Most of the slip on the faults appears to have progressed aseismically and we suggest that interbed sediments play a central role in the slip process. Copyright 2011 by the American Geophysical Union.

Impact of sea-level rise on earthquake and landslide triggering offshore the Alentejo margin (SW Iberia)

NASA Astrophysics Data System (ADS)

Neves, M. C.; Roque, C.; Luttrell, K. M.; Vázquez, J. T.; Alonso, B.

2016-12-01

Earthquakes and submarine landslides are recurrent and widespread manifestations of fault activity offshore SW Iberia. The present work tests the effects of sea-level rise on offshore fault systems using Coulomb stress change calculations across the Alentejo margin. Large-scale faults capable of generating large earthquakes and tsunamis in the region, especially NE-SW trending thrusts and WNW-ESE trending dextral strike-slip faults imaged at basement depths, are either blocked or unaffected by flexural effects related to sea-level changes. Large-magnitude earthquakes occurring along these structures may, therefore, be less frequent during periods of sea-level rise. In contrast, sea-level rise promotes shallow fault ruptures within the sedimentary sequence along the continental slope and upper rise within distances of <100 km from the coast. The results suggest that the occurrence of continental slope failures may either increase (if triggered by shallow fault ruptures) or decrease (if triggered by deep fault ruptures) as a result of sea-level rise. Moreover, observations of slope failures affecting the area of the Sines contourite drift highlight the role of sediment properties as preconditioning factors in this region.

Earthquake rupture dynamics in poorly lithified sediments

NASA Astrophysics Data System (ADS)

De Paola, N.; Bullock, R. J.; Holdsworth, R.; Marco, S.; Nielsen, S. B.

2017-12-01

Several recent large earthquakes have generated anomalously large slip patches when propagating through fluid-saturated, clay-rich sediments near the surface. Friction experiments at seismic slip rates show that such sediments are extremely weak and deform with very little energy dissipation, which facilitates rupture propagation. Although dynamic weakening may explain the ease of rupture propagation through such sediments, it cannot account for the peculiar slow rupture velocity and low radiation efficiency exhibited by some large, shallow ruptures. Here, we integrate field and experimental datasets to describe on- and off-fault deformation in natural syn-depositional seismogenic faults (< 35 ka) in shallow, clay-rich, poorly lithified sediments from the Dead Sea Fault system, Israel. The data are then used to estimate the energy dissipated by on- and off-fault damage during earthquake rupture through shallow, clay-rich sediments. Our mechanical and field data show localised principal slip zones (PSZs) that deform by particulate flow, with little energy dissipated by brittle fracturing with cataclasis. Conversely, we show that coseismic brittle and ductile deformation in the damage zones outwith the PSZ, which cannot be replicated in small-scale laboratory experiments, is a significant energy sink, contributing to an energy dissipation that is one order of magnitude greater than that estimated from laboratory experiments alone. In particular, a greater proportion of dissipated energy would result in lower radiation efficiency, due to a reduced proportion of radiated energy, plus slower rupture velocity and more energy radiation in the low frequency range than might be anticipated from laboratory experiments alone. This result is in better agreement with seismological estimates of fracture energy, implying that off-fault damage can account for the geophysical characteristics of earthquake ruptures as they pass through clay-rich sediments in the shallow crust.

Earthquake activity along the Himalayan orogenic belt

NASA Astrophysics Data System (ADS)

Bai, L.; Mori, J. J.

2017-12-01

The collision between the Indian and Eurasian plates formed the Himalayas, the largest orogenic belt on the Earth. The entire region accommodates shallow earthquakes, while intermediate-depth earthquakes are concentrated at the eastern and western Himalayan syntaxis. Here we investigate the focal depths, fault plane solutions, and source rupture process for three earthquake sequences, which are located at the western, central and eastern regions of the Himalayan orogenic belt. The Pamir-Hindu Kush region is located at the western Himalayan syntaxis and is characterized by extreme shortening of the upper crust and strong interaction of various layers of the lithosphere. Many shallow earthquakes occur on the Main Pamir Thrust at focal depths shallower than 20 km, while intermediate-deep earthquakes are mostly located below 75 km. Large intermediate-depth earthquakes occur frequently at the western Himalayan syntaxis about every 10 years on average. The 2015 Nepal earthquake is located in the central Himalayas. It is a typical megathrust earthquake that occurred on the shallow portion of the Main Himalayan Thrust (MHT). Many of the aftershocks are located above the MHT and illuminate faulting structures in the hanging wall with dip angles that are steeper than the MHT. These observations provide new constraints on the collision and uplift processes for the Himalaya orogenic belt. The Indo-Burma region is located south of the eastern Himalayan syntaxis, where the strike of the plate boundary suddenly changes from nearly east-west at the Himalayas to nearly north-south at the Burma Arc. The Burma arc subduction zone is a typical oblique plate convergence zone. The eastern boundary is the north-south striking dextral Sagaing fault, which hosts many shallow earthquakes with focal depth less than 25 km. In contrast, intermediate-depth earthquakes along the subduction zone reflect east-west trending reverse faulting.

GIS based 3D visualization of subsurface and surface lineaments / faults and their geological significance, northern tamil nadu, India

NASA Astrophysics Data System (ADS)

Saravanavel, J.; Ramasamy, S. M.

2014-11-01

The study area falls in the southern part of the Indian Peninsular comprising hard crystalline rocks of Archaeozoic and Proterozoic Era. In the present study, the GIS based 3D visualizations of gravity, magnetic, resistivity and topographic datasets were made and therefrom the basement lineaments, shallow subsurface lineaments and surface lineaments/faults were interpreted. These lineaments were classified as category-1 i.e. exclusively surface lineaments, category-2 i.e. surface lineaments having connectivity with shallow subsurface lineaments and category-3 i.e. surface lineaments having connectivity with shallow subsurface lineaments and basement lineaments. These three classified lineaments were analyzed in conjunction with known mineral occurrences and historical seismicity of the study area in GIS environment. The study revealed that the category-3 NNE-SSW to NE-SW lineaments have greater control over the mineral occurrences and the N-S, NNE-SSW and NE-SW, faults/lineaments control the seismicities in the study area.

Shallow very-low-frequency earthquakes accompany slow slip events in the Nankai subduction zone.

PubMed

Nakano, Masaru; Hori, Takane; Araki, Eiichiro; Kodaira, Shuichi; Ide, Satoshi

2018-03-14

Recent studies of slow earthquakes along plate boundaries have shown that tectonic tremor, low-frequency earthquakes, very-low-frequency events (VLFEs), and slow-slip events (SSEs) often accompany each other and appear to share common source faults. However, the source processes of slow events occurring in the shallow part of plate boundaries are not well known because seismic observations have been limited to land-based stations, which offer poor resolution beneath offshore plate boundaries. Here we use data obtained from seafloor observation networks in the Nankai trough, southwest of Japan, to investigate shallow VLFEs in detail. Coincident with the VLFE activity, signals indicative of shallow SSEs were detected by geodetic observations at seafloor borehole observatories in the same region. We find that the shallow VLFEs and SSEs share common source regions and almost identical time histories of moment release. We conclude that these slow events arise from the same fault slip and that VLFEs represent relatively high-frequency fluctuations of slip during SSEs.

Strain Localisation at Rift Segment Boundaries: An Example from the Bocana Transfer Zone in Central Baja California, Mexico

NASA Astrophysics Data System (ADS)

Seiler, C.; Gleadow, A. J.; Kohn, B. P.

2012-12-01

Rifts are commonly segmented into several hundred kilometre long zones of opposing upper-plate transport direction with boundaries defined by accommodation and transfer zones. A number of such rift segments have been recognized in the northern Gulf of California, a youthful oceanic basin that is currently undergoing the rift-drift transition. However, detailed field studies have so far failed to identify suitable structures that could accommodate the obvious deformation gradients between different rift segments, and the nature of strain transfer at segment boundaries remains enigmatic. The situation is even less clear in central and southern Baja California, where a number of rift segments have been hypothesized but it is unknown whether the intervening segment boundaries facilitate true reversals in the upper-plate transport direction, or whether they simply accommodate differences in the timing, style or magnitude of deformation. The Bocana transfer zone (BTZ) in central Baja California is a linear, WNW-ESE striking structural discontinuity separating two rift segments with different magnitudes and styles of extensional deformation. North of the BTZ, the Libertad fault is part of the Main Gulf Escarpment, which represents the breakaway fault that separates the Gulf of California rift to the east from the relatively stable western portion of the Baja peninsula. The N-striking Libertad escarpment developed during the Late Miocene (~10-8Ma) and exhibits a topographic relief of ca. 1,000m along a strike-length of ca. 50km. Finite displacement decreases from ~1000m in the central fault segment to ~500m further south, where the fault bends SE and merges with the BTZ. In the hanging wall of the Libertad fault, a series of W-tilted horsts are bound along their eastern margins by two moderate-displacement E-dipping normal faults. South of the BTZ, extension was much less than further north, which explains the comparatively subdued relief and generally shallower tilt of pre-rift strata in this area. The BTZ itself is characterized by two en echelon WNW-ESE striking dextral-oblique transfer faults with a significant down-to-the-NNE extensional component. Strain is transferred from the Libertad breakaway fault onto the transfer faults over a distance of >20km through a network of interacting normal, oblique and strike-slip faults. The shape, location and orientation of the main faults were strongly influenced by pre-existing rheological heterogeneities. Major normal faults are parallel to either the Mesozoic metamorphic foliation or Cretaceous intrusive contacts, and developed where the foliation was at a high angle to the extension direction. In contrast, the oblique-slip faults of the BTZ formed parallel to the metamorphic foliation where formlines are at a small angle to the regional extension direction. Compared to other, less well-understood accommodation zones in the Gulf of California rift, the BTZ shows a distinct lack of volcanic activity, which may help explain the different exposure and structural expression of the various segment boundaries.

Shallow seismic reflection profiles and geological structure in the Benton Hills, southeast Missouri

USGS Publications Warehouse

Palmer, J.R.; Hoffman, D.; Stephenson, W.J.; Odum, J.K.; Williams, R.A.

1997-01-01

During late May and early June of 1993, we conducted two shallow, high-resolution seismic reflection surveys (Mini-Sosie method) across the southern escarpment of the Benton Hills segment of Crowleys Ridge. The reflection profiles imaged numerous post-late Cretaceous faults and folds. We believe these faults may represent a significant earthquake source zone. The stratigraphy of the Benton Hills consists of a thin, less than about 130 m, sequence of mostly unconsolidated Cretaceous, Tertiary and Quaternary sediments which unconformably overlie a much thicker section of Paleozoic carbonate rocks. The survey did not resolve reflectors within the upper 75-100 ms of two-way travel time (about 60-100 m), which would include all of the Tertiary and Quaternary and most of the Cretaceous. However, the Paleozoic-Cretaceous unconformity (Pz) produced an excellent reflection, and, locally a shallower reflector within the Cretaceous (K) was resolved. No coherent reflections below about 200 ms of two-way travel time were identified. Numerous faults and folds, which clearly offset the Paleozoic-Cretaceous unconformity reflector, were imaged on both seismic reflection profiles. Many structures imaged by the reflection data are coincident with the surface mapped locations of faults within the Cretaceous and Tertiary succession. Two locations show important structures that are clearly complex fault zones. The English Hill fault zone, striking N30??-35??E, is present along Line 1 and is important because earlier workers indicated it has Pleistocene Loess faulted against Eocene sands. The Commerce fault zone striking N50??E, overlies a major regional basement geophysical lineament, and is present on both seismic lines at the southern margin of the escarpment. The fault zones imaged by these surveys are 30 km from the area of intense microseismicity in the New Madrid seismic zone (NMSZ). If these are northeast and north-northeast oriented fault zones like those at Thebes Gap they are favorably oriented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones outside of the NMSZ. We anticipate that future studies will provide evidence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represented by faults outside the NMSZ may be significant.

Shallow properties of faults in carbonate rocks - The Jandaíra Formation, Potiguar Basin, Brazil

NASA Astrophysics Data System (ADS)

Bezerra, F. H.; Bertotti, G.; Rabelo, J.; Silva, A. T.; Carneiro, M. A.; Cazarin, C. L.; Silva, C. C.; Vieira, M. M.; Bisdom, K.; Moraes, A.

2014-12-01

We studied the development of shallow faults in the Jandaíra Formation, a Turonian-Campanian carbonate platform in the Potiguar Basin, northeastern Brazil. Our main goal was to characterize fault geometry and properties such as porosity and permeability, and associate these results with fluid flow in shallow conditions. We used an integrated multidisciplinary approach, which combined Quickbird satellite and an unmanned aerial vehicle (UAV, drone) imagery, structural and sedimentary-facies mapping, and petrographic and petrophysical analyses. The Jandaíra Formation presents a variety of carbonate facies, which include mudstones to bioclastic, peloidal, intraclastic, and oolitic grainstones. We modeled our remote sensing and structural data using a finite element analysis system for 2D deformation modeling. We applied the magnitudes and directions of the present-day stress field to simulate depths as deep as 500 m. These stress data were derived from borehole breakout data and drilling-induced tensile fractures observed in resistivity image logs. Our results indicate the occurrence of dilation processes along three sets of joints that were reactivated as faults in the upper crust: N-S, NE-, and E-W-striking faults. These faults provided preferential leaching pathways to fresh water percolation, contributing to localized dissolution and increased secondary porosity and permeability. The results also indicate that the tectonic stresses are concentrated in preferred structural zones such as fault intersection and termination, which are sites of increased fracturing and dissolution. Dissolution by fluids increased permeability in carbonate rocks from primary values of 0.0-0.94 mD to as much as 1370.11 mD. This process is mostly Cenozoic.

Contrasting fault fluids along high-angle faults: a case study from Southern Apennines (Italy)

NASA Astrophysics Data System (ADS)

Sinisi, Rosa; Petrullo, Angela Vita; Agosta, Fabrizio; Paternoster, Michele; Belviso, Claudia; Grassa, Fausto

2016-10-01

This work focuses on two fault-controlled deposits, the Atella and Rapolla travertines, which are associated with high-angle extensional faults of the Bradano Trough, southern Apennines (Italy). The Atella travertine is along a NW-SE striking, deep-seated extensional fault, already described in literature, which crosscuts both Apulian carbonates and the overlying foredeep basin infill. The Rapolla travertine is on top of a NE-SW striking, shallow-seated fault, here described for the first time, which is interpreted as a tear fault associated with a shallow thrust displacing only the foredeep basin infill. The results of structural, sedimentological, mineralogical, and C and O isotope analyses are here reported and discussed to assess the provenance of mineralizing fluids, and to evaluate the control exerted by the aforementioned extensional faults on deep, mantle-derived and shallow, meteoric fluids. Sedimentological analysis is consistent with five lithofacies in the studied travertines, which likely formed in a typical lacustrine depositional environment. Mineralogical analysis show that travertines mainly consist of calcite, and minor quartz, feldspar and clay minerals, indicative of a terrigenous supply during travertine precipitation. The isotope signature of the two studied travertines shows different provenance for the mineralizing fluids. At the Atella site, the δ13CPDB values range between + 5.2 and + 5.7‰ and the δ18OPDB values between - 9.0 and - 7.3‰, which are consistent with a mantle-derived CO2 component in the fluid. In contrast, at the Rapolla site the δ13CPDB values vary from - 2.7 to + 1.5‰ and the δ18OPDB values from - 6.8 to - 5.4‰, suggesting a mixed CO2 source with both biogenic-derived and mantle-derived fluids. The results of structural analyses conducted along the footwall damage zone of the fault exposed at the Rapolla site, show that the whole damage zone, in which fractures and joints likely channeled the mixed fluids, acted as a distributed conduit for both fault-parallel and cross-fault fluid migration.

A shallow crustal earthquake doublet from the Trans-Mexican volcanic belt (Central Mexico)

NASA Astrophysics Data System (ADS)

Quintanar, L.; Rodríguez-González, M.; Campos-Enríquez, O.

2003-04-01

The trans-Mexican volcanic belt is an active volcanic arc related to subduction along the Middle America trench and characterized by shallow seismicity and synvolcanic to postvolcanic extensional arc-parallel faulting. The Mezquital graben is a major intra-arc basin of the central trans-Mexican volcanic belt. A doublet of moderate shallow shocks occurred in March and October 1976 in the region of this graben. These earthquakes were recorded by the Mexican National Seismological network, in particular by the Bosch-Omori seismograph (T_0 = 18 s) at the Tacubaya Observatory in Mexico City. We have carefully relocated the two main shocks and their major aftershocks by reading the original records and using a modified crustal velocity model for this region. A difference of ˜50 km is observed between the locations reported by the Mexican Seismological Service and those obtained in this study, which are additionally supported by the damage distribution of these earthquakes. A first motion analysis, based on regional and teleseismic records, defines for the March and October shocks normal fault mechanisms, characterized by E-W striking fault planes, which coincides with the orientation of the master faults of the Mezquital graben. After calculating the instrumental response, the source parameters were obtained from the Bosch-Omori seismograph records by body-wave modeling. For the March earthquake, we estimate a seismic moment of 4.5×1023 dyne-cm (equivalent to M_w=5.0) and a stress drop of 0.7 MPa assuming a circular rupture model (radius = 3 km). Given the poor quality of the Bosch-Omori record for the October earthquake, we used the comparison, between both events, of long-period (T=20 sec) teleseismic records at 2 stations to obtain its corresponding source parameters. By assuming a similar stress drop as for the March event, we obtain a M_0 of 5.6×1023 dyne-cm and M_w = 5.1 with a rupture length of 6.5 km. According to gravity data, the regional E-W faults are longer than reported. In particular, our detailed measurements indicate that the master of the Mezquital graben continue westward in the subsurface. In a more regional context, additional magnetic available information points that El Mezquital graben and Aljibes half-graben might be connected, as suggested by other authors, at their northern limits, by part of the same fault system. This is the first earthquake doublet reported from Central Mexico and the biggest seismic event in that zone during the last 50 years. The presence of moderate seismicity in this zone should therefore be taken into account when assessing its seismic hazard.

High-frequency Born synthetic seismograms based on coupled normal modes

USGS Publications Warehouse

Pollitz, Fred F.

2011-01-01

High-frequency and full waveform synthetic seismograms on a 3-D laterally heterogeneous earth model are simulated using the theory of coupled normal modes. The set of coupled integral equations that describe the 3-D response are simplified into a set of uncoupled integral equations by using the Born approximation to calculate scattered wavefields and the pure-path approximation to modulate the phase of incident and scattered wavefields. This depends upon a decomposition of the aspherical structure into smooth and rough components. The uncoupled integral equations are discretized and solved in the frequency domain, and time domain results are obtained by inverse Fourier transform. Examples show the utility of the normal mode approach to synthesize the seismic wavefields resulting from interaction with a combination of rough and smooth structural heterogeneities. This approach is applied to an ∼4 Hz shallow crustal wave propagation around the site of the San Andreas Fault Observatory at Depth (SAFOD).

10Be erosion rates controlled by normal fault activity through incision and landslide occurrence

NASA Astrophysics Data System (ADS)

Roda-Boluda, Duna; D'Arcy, Mitch; Whittaker, Alex; Gheorghiu, Delia; Rodes, Angel

2017-04-01

Quantifying erosion rates, and how they compare to rock uplift rates, is fundamental for understanding the evolution of relief and the associated sediment fluxes. The competing effects of rock uplift and erosion are clearly captured by river incision and landsliding, but linking these four important landscape processes remains a major challenge. We address these questions using field data from southern Italy, and quantify the geomorphic response to tectonic forcing. We present 15 new 10Be catchment-averaged erosion rates, collected from catchments along five active normal faults with excellent slip rate constraints. We find that erosion rates are strongly controlled by fault slip rates and that this relationship is mediated by the degree of catchment incision and landslide activity. We find that 10Be samples from low-relief, unincised areas above knickpoints yield consistent erosion rates of ˜ 0.12 mm/yr, while samples collected below knickpoints have erosion rates of ˜ 0.2 - 1.0 mm/yr. This comparison allows us to quantify the impact that transient incisional response has on erosion rates. We demonstrate that in this area incision is associated with frequent, shallow landsliding, and we show that the volumes of landslides stored in the catchments are highly correlated with 10Be-derived sediment flux estimates, suggesting that landslides are likely to be a major contributor to erosional fluxes. Despite widespread landsliding, CRN samples from the studied catchments do provide reliable estimates of catchment-averaged erosion rates, as these are consistent with fault throw patterns and rates. We suggest that this is because landslides are frequent, small and shallow, and are stored on the hillslopes for up to ˜ 103 yrs, representing the integrated record of landsliding over several seismic cycles; and test this hypothesis using a numerical model of landsliding and CRN dynamics. Our results show that adequate CRN mixing can occur through runoff as landslides are stored on the hillslopes, as long as landslide recurrence intervals are short, which is supported by the erosion rate magnitudes and previous landslide studies in the area. This study contributes to our understanding of erosion and sediment supply in tectonically-active areas, and offers novel insights into the use of CRN to infer erosion rates in areas of intense landslide activity.

Marine geology and earthquake hazards of the San Pedro Shelf region, southern California

USGS Publications Warehouse

Fisher, Michael A.; Normark, William R.; Langenheim, V.E.; Calvert, Andrew J.; Sliter, Ray

2004-01-01

High-resolution seismic-reflection data have been com- bined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro Shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington Graben, the Palos Verdes Fault Zone, various faults below the western part of the shelf and slope, and the deep-water San Pedro Basin. The structure of the Palos Verdes Fault Zone changes mark- edly southeastward across the San Pedro Shelf and slope. Under the northern part of the shelf, this fault zone includes several strands, but the main strand dips west and is probably an oblique-slip fault. Under the slope, this fault zone con- sists of several fault strands having normal separation, most of which dip moderately east. To the southeast near Lasuen Knoll, the Palos Verdes Fault Zone locally is a low-angle fault that dips east, but elsewhere near this knoll the fault appears to dip steeply. Fresh sea-floor scarps near Lasuen Knoll indi- cate recent fault movement. The observed regional structural variation along the Palos Verdes Fault Zone is explained as the result of changes in strike and fault geometry along a master strike-slip fault at depth. The shallow summit and possible wavecut terraces on Lasuen knoll indicate subaerial exposure during the last sea-level lowstand. Modeling of aeromagnetic data indicates the presence of a large magnetic body under the western part of the San Pedro Shelf and upper slope. This is interpreted to be a thick body of basalt of Miocene(?) age. Reflective sedimentary rocks overlying the basalt are tightly folded, whereas folds in sedimentary rocks east of the basalt have longer wavelengths. This difference might mean that the basalt was more competent during folding than the encasing sedimentary rocks. West of the Palos Verdes Fault Zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because age dates on deformed or offset sediment are lacking.

Strain indicators and magnetic fabric in intraplate fault zones: Case study of Daroca thrust, Iberian Chain, Spain

NASA Astrophysics Data System (ADS)

Casas-Sainz, A. M.; Gil-Imaz, A.; Simón, J. L.; Izquierdo-Llavall, E.; Aldega, L.; Román-Berdiel, T.; Osácar, M. C.; Pueyo-Anchuela, Ó.; Ansón, M.; García-Lasanta, C.; Corrado, S.; Invernizzi, C.; Caricchi, C.

2018-04-01

Anisotropy of magnetic susceptibility (AMS) has been applied to the study of shallow fault zones, although interpretation of the results requires establishing clear relationships between petrofabric and magnetic features, magnetic behaviour of fault rocks, and an extensive knowledge of P-T conditions. In this work, we demonstrate that magnetic methods can be applied to the study of heterogeneous fault zones, provided that a series of requisites are met. A major fault zone within the Iberian plate (Daroca thrust), showing transpressional movements during Cenozoic time was chosen for this purpose, because of the exceptional outcrops of fault gouge and microbreccia and its relevance within the context of the northeastern Iberian Plate. Magnetic fabrics were analysed and the results were compared with foliation and S-C structures measured within the fault zone. Clay mineral assemblages suggest maximum burial depths shallower than 2 km (<60-70 °C) for fault rocks in the footwall of the Daroca thrust. The orientation of the AMS axes is consistent with mesostructural strain indicators: kmin parallels the mean pole to S, or it is intermediate between S and C poles; kmax is oriented at a high angle (nearly orthogonal in overall) to the transport direction, which can be explained from both deformational and mineralogical controls. Both magnetic fabrics and kinematic indicators are consistent with a reverse movement for most of the fault zone.

Geology and geophysics of the southern Raft River Valley geothermal area, Idaho, USA

USGS Publications Warehouse

Williams, Paul L.; Mabey, Don R.; Zohdy, Adel A.R.; Ackermann, Hans D.; Hoover, Donald B.; Pierce, Kenneth L.; Oriel, Steven S.

1976-01-01

The Raft River valley, near the boundary of the Snake River plain with the Basin and Range province, is a north-trending late Cenozoic downwarp bounded by faults on the west, south, and east. Pleistocene alluvium and Miocene-Pliocene tuffaceous sediments, conglomerate, and felsic volcanic rocks aggregate 2 km in thickness. Large gravity, magnetic, and total field resistivity highs probably indicate a buried igneous mass that is too old to serve as a heat source. Differing seismic velocities relate to known or inferred structures and to a suspected shallow zone of warm water. Resistivity anomalies reflect differences of both composition and degree of alteration of Cenozoic rocks. Resistivity soundings show a 2 to 5 ohm·m unit with a thickness of 1 km beneath a large part of the valley, and the unit may indicate partly hot water and partly clayey sediments. Observed self-potential anomalies are believed to indicate zones where warm water rises toward the surface. Boiling wells at Bridge, Idaho are near the intersection of north-northeast normal faults which have moved as recently as the late (?) Pleistocene, and an east-northeast structure, probably a right-lateral fault. Deep circulation of ground water in this region of relatively high heat flow and upwelling along faults is the probable cause of the thermal anomaly.

Stress Orientations in the Fort Worth Basin, Texas, Determined from Earthquake Focal Mechanisms

NASA Astrophysics Data System (ADS)

Quinones, L. A.; DeShon, H. R.

2017-12-01

Since October 2008 the Fort Worth Basin (FWB), an active shale gas production field in northeastern Texas, has experienced over 30 M3.0+ earthquakes, including one M4.0. These events have primarily occurred on faults in the Precambrian basement and within the overlying Ellenburger Limestone formation, which acts as the primary wastewater disposal unit in the FWB. We generate focal mechanism catalogs for the 2013-2015 Azle-Reno, 2014-present Irving-Dallas, and 2015 Venus earthquake sequences using P-wave first motion and S-to-P wave amplitude ratio data collected from the local seismic networks operating in the region. The mechanisms show little variability when compared to natural intraplate sequences, and are most consistent with failure on NE-SW striking normal faults. Stress inversions indicate maximum regional horizontal stress in the basement strikes 20-30° N of E, consistent with shallower borehole breakout data for the basin, and within this stress regime that all seismogenic faults in the FWB are optimally oriented for failure. We show via Mohr circle diagrams that small stress perturbations on these preexisting basement faults, of magnitudes similar to those observed or modeled to be associated with wastewater disposal, are capable of inducing the earthquakes that occurred in the Azle-Reno, Irving-Dallas, and Venus earthquake sequences.

High-Resolution Seismic Reflection Profiling Across the Black Hills Fault, Clark County, Nevada: Preliminary Results

NASA Astrophysics Data System (ADS)

Zaragoza, S. A.; Snelson, C. M.; Jernsletten, J. A.; Saldana, S. C.; Hirsch, A.; McEwan, D.

2005-12-01

The Black Hills fault (BHF) is located in the central Basin and Range Province of western North America, a region that has undergone significant Cenozoic extension. The BHF is an east-dipping normal fault that forms the northwestern structural boundary of the Eldorado basin and lies ~20 km southeast of Las Vegas, Nevada. A recent trench study indicated that the fault offsets Holocene strata, and is capable of producing Mw 6.4-6.8 earthquakes. These estimates indicate a subsurface rupture length at least 10 km greater than the length of the scarp. This poses a significant hazard to structures such as the nearby Hoover Dam Bypass Bridge, which is being built to withstand a Mw 6.2-7.0 earthquake on local faults. If the BHF does continue in the subsurface, this structure, as well as nearby communities (Las Vegas, Boulder City, and Henderson), may not be as safe as previously expected. Previous attempts to image the fault with shallow seismics (hammer source) were inconclusive. However, gravity studies imply that the fault continues south of the scarp. Therefore, a new experiment utilizing high-resolution seismic reflection was performed to image subsurface geologic structures south of the scarp. At each shot point, a stack of four 30-160 Hz vibroseis sweeps of 15 s duration was recorded on a 60-channel system with 40 Hz geophones. This produced two 300 m reflection profiles, with a maximum depth of 500-600 m. A preliminary look at these data indicates the existence of two faults, potentially confirming that the BHF continues in the subsurface south of the scarp.

Refining the Magnitude of the Shallow Slip Deficit

NASA Astrophysics Data System (ADS)

Xu, X.; Tong, X.; Sandwell, D. T.; Milliner, C. W. D.

2014-12-01

Geodetic inversions for slip versus depth for several major (Mw > 7) strike-slip earthquakes (e.g. 1992 Landers, 1999 Hector Mine, 2010 El_Mayor-Cucapah) show a 10% to 40% reduction in slip near surface (depth < 2 km) compared to the slip at deeper depths (5 to 8 km). This has been called the shallow slip deficit (SSD). The large magnitude of this deficit has been an enigma since it cannot be explained by shallow creep during the interseismic period or by triggered slip from nearby earthquakes. One potential explanation for the SSD is that the previous geodetic inversions used incomplete data that do not go close to fault so the shallow portions of the slip models were poorly resolved and generally underestimated. In this study we improve the geodetic inversion, especially at shallow depth by: 1) refining the InSAR processing with non-boxcar phase filtering, model-dependent range corrections, more complete phase unwrapping by SNAPHU using a correlation mask and allowing a phase discontinuity along the rupture; 2) including near-fault offset data from optical imagery and SAR azimuth offsets; 3) using more detailed fault geometry; 4) and using additional campaign GPS data. With these improved observations, the slip inversion has significantly increased resolution at shallow depth. For the Landers rupture the SSD is reduced from 45% to 16%. Similarly for the Hector Mine rupture the SSD is reduced from 15% to 5%. We are assembling all the relevant co-seismic data for the El Major-Cucapah earthquake and will report the inversion result with its SSD at the meeting.

Aseismic deformation of a fold-and-thrust belt imaged by SAR interferometry near Shahdad, southeast Iran

NASA Technical Reports Server (NTRS)

Fielding, Eric J.; Wright, Tim J.; Muller, Jordan; Parsons, Barry E.; Walker, Richard

2004-01-01

At depth, many fold-and-thrust belts are composed of a gently dipping, basal thrust fault and steeply dipping, shallower splay faults that terminate beneath folds at the surface. Movement on these buried faults is difficult to observe, but synthetic aperture radar (SAR) interferometry has imaged slip on at least 600 square kilometers of the Shahdad basal-thrust and splay-fault network in southeast Iran.

Deep crustal faults and the origin and long-term flank stability of Mt. Etna : First results from the CIRCEE cruise (Oct. 2013)

NASA Astrophysics Data System (ADS)

Gutscher, Marc-Andre; Dominguez, Stephane; Mercier de Lepinay, Bernard; Pinheiro, Luis; Babonneau, Nathalie; Cattaneo, Antonio; LeFaou, Yann; Barreca, Giovanni; Micallef, Aaron; Rovere, Marzia

2014-05-01

The relation between deep crustal faults and the origin of Mount Etna, the largest and most active volcano in Europe has long been suspected due to its unusual geodynamic location. Results from a new marine geophysical survey offshore Eastern Sicily reveal the detailed geometry (location, length, dip and orientation) of a two-branched 200-km long, lithospheric scale fault system, long sought for as being the cause of Mount Etna. Using high-resolution bathymetry and seismic profiling, we image a 60-km long, previously unidentified, NW trending fault with evidence of recent displacement at the seafloor, offsetting Holocene sediments. This newly identified fault connects NE of Catania, to a known 40-km long, offshore-onshore fault system dissecting the southeastern flank of Mount Etna, generally interpreted as purely gravitational collapse structures. Geological and morphological field studies together with earthquake focal mechanisms indicate active dextral strike-slip motion along the onshore and shallow offshore portion of this 40 + 60 km long segment. The southern 100 km branch of the fault is associated with a sub-vertical lithospheric scale tear fault showing pure down to the East normal faulting and a 500+m thick elongate basin marked by syn-tectonic Plio-quaternary sediment fill. Together they represent two kinematically distinct strands of the long sought "STEP" (Subduction Tear Edge Propagator) fault, whose expression at depth controls the position of Mount Etna. Both 100-km long branches of the fault system are mechanically capable of generating magnitude 7 earthquakes (e.g. - like the 1693 Catania earthquake, the strongest in Italian history, causing 40,000 deaths). We conclude this deep-rooted lithospheric weakness guides gradual down slope creep of Mount Etna and may lead to long-term catastrophic flank collapse with associated tsunami by large-scale mass wasting.

Experimental Modeling of Dynamic Shallow Dip-Slip Faulting

NASA Astrophysics Data System (ADS)

Uenishi, K.

2010-12-01

In our earlier study (AGU 2005, SSJ 2005, JPGU 2006), using a finite difference technique, we have conducted some numerical simulations related to the source dynamics of shallow dip-slip earthquakes, and suggested the possibility of the existence of corner waves, i.e., shear waves that carry concentrated kinematic energy and generate extremely strong particle motions on the hanging wall of a nonvertical fault. In the numerical models, a dip-slip fault is located in a two-dimensional, monolithic linear elastic half space, and the fault plane dips either vertically or 45 degrees. We have investigated the seismic wave field radiated by crack-like rupture of this straight fault. If the fault rupture, initiated at depth, arrests just below or reaches the free surface, four Rayleigh-type pulses are generated: two propagating along the free surface into the opposite directions to the far field, the other two moving back along the ruptured fault surface (interface) downwards into depth. These downward interface pulses may largely control the stopping phase of the dynamic rupture, and in the case the fault plane is inclined, on the hanging wall the interface pulse and the outward-moving Rayleigh surface pulse interact with each other and the corner wave is induced. On the footwall, the ground motion is dominated simply by the weaker Rayleigh pulse propagating along the free surface because of much smaller interaction between this Rayleigh and the interface pulse. The generation of the downward interface pulses and corner wave may play a crucial role in understanding the effects of the geometrical asymmetry on the strong motion induced by shallow dip-slip faulting, but it has not been well recognized so far, partly because those waves are not expected for a fault that is located and ruptures only at depth. However, the seismological recordings of the 1999 Chi-Chi, Taiwan, the 2004 Niigata-ken Chuetsu, Japan, earthquakes as well as a more recent one in Iwate-Miyagi Inland, Japan in 2008, for example, seem to support the need for careful mechanical consideration. In this contribution, utilizing two-dimensional dynamic photoelasticity in conjunction with high speed digital cinematography, we try to perform "fully controlled" laboratory experiments of dip-slip faulting and observe the propagation of interface pulses and corner waves mentioned above. A birefringent material containing a (model) dip-slip fault plane is prepared, and rupture is initiated in that material using an Nd:YAG laser system, and the evolution of time-dependent isochromatic fringe patterns (contours of maximum in-plane shear stress) associated with the dynamic process of shallow dip-slip faulting is recorded. Use of Nd:YAG laser pulses, instead of ignition of explosives, for rupture initiation may enhance the safety of laboratory fracture experiments and enable us to evaluate the energy entering the material (and hence the energy balance in the system) more precisely, possibly in a more controlled way.

New insight on the Great Sumatra Fault, offshore NW Sumatra, from recent marine data

NASA Astrophysics Data System (ADS)

Ghosal, D.; Singh, S. C.; Chauhan, A.; Hananto, N. D.

2009-12-01

The Sumatra subduction system is a classic example of an oblique subduction where the slip is portioned into pure thrust along the Sumatra-Andaman megathrust and strike-slip along the Great Sumatra Fault (GSF). Only in the last five years there have been three great pure thrust earthquakes along the Sumatran subduction zone. However, the 1900 km long GSF has been rather silent and is likely to produce a large earthquake in the near future, and hence it is important study the GSF in order to mitigate seismic risks. Over the last 20 years, GSF has been studied on land, but we have no information about its offshore extension NW of Sumatra. The problem is further complicated by its vicinity with the volcanic arc, which switches back and forth centering the GSF. Here we present analyses of recently acquired high-resolution bathymetry and shallow and deep reflection seismic data. We find that GSF bifurcates into two branches north of Banda Aceh, both producing 15-20 km wide deep adjacent basins. Southern basin is 1-2 km deep and has a flower structure with a push-up ridge, suggesting the presence of an active strike-slip fault. The presence of strike-slip earthquakes beneath this basin further suggests that GSF passes through this basin. The northern basin is up to 4 km deep, bounded by normal faults. The absence of recent sediments on the basin floor suggests that the basin is very young. The presence of a chain of volcanoes in the centre of the basin suggests that the volcanic arc passes through this basin. The fact that the basin is 4 km deep in the presence of volcanoes, which tend to fill in a basin and hence make them shallower, suggests that this might be the site of an onset of back-arc spreading centre. We shall examine all the new observations in the light of plate motion, local deformation and possible seismic risk.

Incorporation of experimentally derived friction laws in numerical simulations of earthquake generated tsunamis

NASA Astrophysics Data System (ADS)

Murphy, Shane; Spagnuolo, Elena; Lorito, Stefano; Di Toro, Giulio; Scala, Antonio; Festa, Gaetano; Nielsen, Stefan; Piatanesi, Alessio; Romano, Fabrizio; Aretusini, Stefano

2016-04-01

Seismological, tsunami and geodetic observations have shown that subduction zones are complex systems where the properties of earthquake rupture vary with depth. For example nucleation and high frequency radiation generally occur at depth but low frequency radiation and large tsunami-genic slip appear to occur in the shallow crustal depth. Numerical simulations used to describe these features predominantly use standardised theoretical equations or experimental observations often assuming that their validity extends to all slip-rates, lithologies and tectonic environments. However recent rotary-shear experiments performed on a range of diverse materials and experimental conditions highlighted the large variability of the evolution of friction during slipping pointing to a more complex relationship between material type, slip rate and normal stress. Simulating dynamic rupture using a 2D spectral element methodology on a Tohoku like fault, we apply experimentally derived friction laws (i.e. thermal slip distance friction law, Di Toro et al. 2011) Choice of parameters for the friction law are based on expected material type (e.g. cohesive and non-cohesive clay rich material representative of an accretionary wedge), the normal stress which is controlled by the interaction between the regional stress field and the fault geometry. The shear stress distribution on the fault plane is fractal with the yield stress dependent on the static coefficient of friction and the normal stress, parameters that are dependent on the material type and geometry. We use metrics such as the slip distribution, ground motion and fracture energy to explore the effect of frictional behaviour, fault geometry and stress perturbations and its potential role in tsunami generation. Preliminary results will be presented. This research is funded by the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 603839 (Project ASTARTE - Assessment, Strategy and Risk Reduction for Tsunamis in Europe) and by the ERC CoG NOFEAR project 614705

The upper crust laid on its side: tectonic implications of steeply tilted crustal slabs for extension in the basin and range

USGS Publications Warehouse

Howard, Keith A.

2005-01-01

Tilted slabs expose as much as the top 8–15 km of the upper crust in many parts of the Basin and Range province. Exposures of now-recumbent crustal sections in these slabs allow analysis of pre-tilt depth variations in dike swarms, plutons, and thermal history. Before tilting the slabs were panels between moderately dipping, active Tertiary normal faults. The slabs and their bounding normal faults were tilted to piggyback positions on deeper footwalls that warped up isostatically beneath them during tectonic unloading. Stratal dips within the slabs are commonly tilted to vertical or even slightly overturned, especially in the southern Basin and Range where the thin stratified cover overlies similarly tilted basement granite and gneiss. Some homoclinal recumbent slabs of basement rock display faults that splay upward into forced folds in overlying cover sequences, which thereby exhibit shallower dips. The 15-km maximum exposed paleodepth for the slabs represents the base of the brittle upper crust, as it coincides with the depth of the modern base of the seismogenic zone and the maximum focal depths of large normal-fault earthquakes in the Basin and Range. Many upended slabs accompany metamorphic core complexes, but not all core complexes have corresponding thick recumbent hanging-wall slabs. The Ruby Mountains core complex, for example, preserves only scraps of upper-plate rocks as domed-up extensional klippen, and most of the thick crustal section that originally overlay the uplifted metamorphic core now must reside below little-tilted hanging-wall blocks in the Elko-Carlin area to the west. The Whipple and Catalina Mountains core complexes in contrast are footwall to large recumbent hanging-wall slabs of basement rock exposing 8-15 km paleodepths that originally roofed the metamorphic cores; the exposed paleodepths require that a footwall rolled up beneath the slabs.

Elastic Properties of Subduction Zone Materials in the Large Shallow Slip Environment for the Tohoku 2011 Earthquake: Laboratory data from JFAST Core Samples

NASA Astrophysics Data System (ADS)

Jeppson, T.; Tobin, H. J.

2014-12-01

The 11 March 2011 Tohoku-Oki earthquake (Mw=9.0) produced large displacements of ~50 meters near the Japan Trench. In order to understand earthquake propagation and slip stabilization in this environment, quantitative values of the real elastic properties of fault zones and their surrounding wall rock material is crucial. Because elastic and mechanical properties of faults and wallrocks are controlling factors in fault strength, earthquake generation and propagation, and slip stabilization, an understanding of these properties and their depth dependence is essential to understanding and accurately modeling earthquake rupture. In particular, quantitatively measured S-wave speeds, needed for estimation of elastic properties, are scarce in the literature. We report laboratory ultrasonic velocity measurements performed at elevated pressures, as well as the calculated dynamic elastic moduli, for samples of the rock surrounding the Tohoku earthquake principal fault zone recovered by drilling during IODP Expedition 343, Japan Trench Fast Drilling Project (JFAST). We performed measurements on five samples of gray mudstone from the hanging wall and one sample of underthrust brown mudstone from the footwall. We find P- and S-wave velocities of 2.0 to 2.4 km/s and 0.7 to 1.0 km/s, respectively, at 5 MPa effective pressure. At the same effective pressure, the hanging wall samples have shear moduli ranging from 1.4 to 2.2 GPa and the footwall sample has a shear modulus of 1.0 GPa. While these values are perhaps not surprising for shallow, clay-rich subduction zone sediments, they are substantially lower than the 30 GPa commonly assumed for rigidity in earthquake rupture and propagation models [e.g., Ide et al., 1993; Liu and Rice, 2005; Loveless and Meade, 2011]. In order to better understand the elastic properties of shallow subduction zone sediments, our measurements from the Japan Trench are compared to similar shallow drill core samples from the Nankai Trough, Costa Rica, Cascadia, and Barbados ridge subduction zones. We find that shallow subduction zone sediments in general have similarly low rigidity. These data provide important ground-truth values that can be used to parameterize fault slip models addressing the problem of shallow, tsunamigenic propagation of megathrust earthquakes.

Character of High Temperature Mylonitic Shear Zones Associated with Oceanic Detachment Faults at the Ultra-Slow Mid-Cayman Rise

NASA Astrophysics Data System (ADS)

Marr, C.; John, B. E.; Cheadle, M. J.; German, C. R.

2014-12-01

Two well-preserved core complexes at the Mid-Cayman Rise (MCR), Mt Dent and Mt Hudson, provide an opportunity to examine the deformation history and rheology of detachment faults at an ultra-slow spreading ridge. Samples from the CAYTROUGH (1976-77) project and the Nautilus NA034 cruise (2013) were selected for detailed petrographic and microstructural study. Surface samples from Mt. Dent (near the center of the MCR) provide insight into lateral variation in footwall rock type and deformation history across a core complex in both the across and down dip directions. In contrast, sampling of Mt. Hudson (SE corner of the MCR) focuses on a high-angle, crosscutting normal fault scarp, which provides a cross section of the detachment fault system. Sampling across Mt Dent reveals that the footwall is composed of heterogeneously-distributed gabbro (47%) and peridotite (20%) with basaltic cover (33%) dominating the top of the core complex. Sampling of Mt Hudson is restricted to the normal fault scarp cutting the core complex and suggests the interior is dominated by gabbro (85% gabbro, 11% peridotite, 4% basalt). At Mt. Dent, peridotite is exposed within ~4km of the breakaway indicating that the Mt. Dent detachment does not cut Penrose-style oceanic crust. The sample set provides evidence of a full down-temperature sequence of detachment related-fault rocks, from possible granulite and clear amphibolite mylonitizatization to prehnite-pumpellyite brittle deformation. Both detachments show low-temperature brittle deformation overprinting higher temperature plastic fabrics. Fe-Ti oxide gabbro mylonites dominate the sample set, and plastic deformation of plagioclase is recorded in samples collected as near as ~4km from the inferred breakaway along the southern flank of Mt. Dent, suggesting the brittle-plastic transition was initially at ~3km depth. Recovered samples suggest strain associated with both detachment systems is localized into discrete mylonitic shear zones (~1-10cm thick), implying that the plastic portion of the fault consists of a broad zone of thin, anastomosing shear zones. Concentrations of Ti-rich magmatic hornblende and interstitial Fe-Ti oxides in the high strain horizons are consistent with the lowermost part of the fault(s) localizing in the margins of the mush zone of a shallow magma chamber.

Influence of fault trend, fault bends, and fault convergence on shallow structure, geomorphology, and hazards, Hosgri strike-slip fault, offshore central California

NASA Astrophysics Data System (ADS)

Johnson, S. Y.; Watt, J. T.; Hartwell, S. R.

2012-12-01

We mapped a ~94-km-long portion of the right-lateral Hosgri Fault Zone from Point Sal to Piedras Blancas in offshore central California using high-resolution seismic reflection profiles, marine magnetic data, and multibeam bathymetry. The database includes 121 seismic profiles across the fault zone and is perhaps the most comprehensive reported survey of the shallow structure of an active strike-slip fault. These data document the location, length, and near-surface continuity of multiple fault strands, highlight fault-zone heterogeneity, and demonstrate the importance of fault trend, fault bends, and fault convergences in the development of shallow structure and tectonic geomorphology. The Hosgri Fault Zone is continuous through the study area passing through a broad arc in which fault trend changes from about 338° to 328° from south to north. The southern ~40 km of the fault zone in this area is more extensional, resulting in accommodation space that is filled by deltaic sediments of the Santa Maria River. The central ~24 km of the fault zone is characterized by oblique convergence of the Hosgri Fault Zone with the more northwest-trending Los Osos and Shoreline Faults. Convergence between these faults has resulted in the formation of local restraining and releasing fault bends, transpressive uplifts, and transtensional basins of varying size and morphology. We present a hypothesis that links development of a paired fault bend to indenting and bulging of the Hosgri Fault by a strong crustal block translated to the northwest along the Shoreline Fault. Two diverging Hosgri Fault strands bounding a central uplifted block characterize the northern ~30 km of the Hosgri Fault in this area. The eastern Hosgri strand passes through releasing and restraining bends; the releasing bend is the primary control on development of an elongate, asymmetric, "Lazy Z" sedimentary basin. The western strand of the Hosgri Fault Zone passes through a significant restraining bend and dies out northward where we propose that its slip transfers to active structures in the Piedras Blancas fold belt. Given the continuity of the Hosgri Fault Zone through our study area, earthquake hazard assessments should incorporate a minimum rupture length of 110 km. Our data do not constrain lateral slip rates on the Hosgri, which probably vary along the fault (both to the north and south) as different structures converge and diverge but are likely in the geodetically estimated range of 2 to 4 mm/yr. More focused mapping of lowstand geomorphic features (e.g., channels, paleoshorelines) has the potential to provide better constraints. The post-Last-Glacial Maximum unconformity is an important surface for constraining vertical deformation, yielding local fault offset rates that may be as high as 1.4 mm/yr and off-fault deformation rates as high as 0.5 mm/yr. These vertical rates are short-term and not sustainable over longer geologic time, emphasizing the complex evolution and dynamics of strike-slip zones.

Detachment Fault Behavior Revealed by Micro-Seismicity at 13°N, Mid-Atlantic Ridge

NASA Astrophysics Data System (ADS)

Parnell-Turner, R. E.; Sohn, R. A.; MacLeod, C. J.; Peirce, C.; Reston, T. J.; Searle, R. C.

2016-12-01

Under certain tectono-magmatic conditions, crustal accretion and extension at slow-spreading mid-ocean ridges is accommodated by low-angle detachment faults. While it is now generally accepted that oceanic detachments initiate on steeply dipping faults that rotate to low-angles at shallow depths, many details of their kinematics remain unknown. Debate has continued between a "continuous" model, where a single, undulating detachment surface underlies an entire ridge segment, and a "discrete" (or discontinuous) model, where detachments are spatially restricted and ephemeral. Here we present results from a passive microearthquake study of detachment faulting at the 13°N region of the Mid-Atlantic Ridge. This study is one component of a joint US-UK seismic study to constrain the sub-surface structure and 3-dimensional geometry of oceanic detachment faults. We detected over 300,000 microearthquakes during a 6-month deployment of 25 ocean bottom seismographs. Events are concentrated in two 1-2 km wide ridge-parallel bands, located between the prominent corrugated detachment fault surface at 13°20'N and the present-day spreading axis, separated by a 1-km wide patch of reduced seismicity. These two bands are 7-8 km in length parallel to the ridge and are clearly limited in spatial extent to the north and south. Events closest to the axis are generally at depths of 6-8 km, while those nearest to the oceanic detachment fault are shallower, at 4-6 km. There is an overall trend of deepening seismicity northwards, with events occurring progressively deeper by 4 km over an along-axis length of 8 km. Events are typically very small, and range in local magnitude from ML -1 to 3. Focal mechanisms indicate two modes of deformation, with extension nearest to the axis and compression at shallower depths near to the detachment fault termination.

Magma-Tectonic Interactions in the Main Ethiopian Rift; Insights into Rifting Processes

NASA Astrophysics Data System (ADS)

Greenfield, T.; Keir, D.; Tessema, T.; Lloyd, R.; Biggs, J.; Ayele, A.; Kendall, J. M.

2017-12-01

We report observations made around the Bora-Tulu Moye volcanic field, in the Main Ethiopian Rift (MER). A network of seismometers deployed around the volcano for one and a half years reveals the recent state of the volcano. Accurate earthquake locations and focal mechanisms are combined with surface deformation and mapping of faults, fissures and geothermally active areas to reveal the interaction between magmatism and intra-rift faulting. More than 1000 earthquakes are detected and located, making the Bora-Tulu Moye volcanic field one of the most seismically active regions of the MER. Earthquakes are located at depths of less than 5 km below the surface and range between magnitudes of 1.5 - 3.5. Surface deformation of Bora-Tulu Moye is observed using satellite based radar interferometry (InSAR) recorded before and during the seismic deployment. Since 2004, deformation has oscillated between uplift and subsidence centered at the same spatial location but different depths. We constrain the source of the uplift to be at 7 km depth while the source of the subsidence is shallower. Micro-earthquake locations reveal that earthquakes are located around the edge of the observed deformation and record the activation of normal faults orientated at 025°. The spatial link between surface deformation and brittle failure suggest that significant hydrothermal circulation driven by an inflating shallow heat source is inducing brittle failure. Elsewhere, seismicity is focused in areas of significant surface alteration from hydrothermal processes. We use shear wave splitting using local earthquakes to image the stress state of the volcano. A combination of rift parallel and rift-oblique fast directions are observed, indicating the volcano has a significant influence on the crustal stresses. Volcanic activity around Bora-Tulu Moye has migrated eastwards over time, closer to the intra-rift fault system, the Wonji Fault Belt. How and why this occurs relates to changes in the melt supply to the upper crust from depth and has implications for the early stages of rift evolution and for volcanic and tectonic hazard in Ethiopia and rifts generally.

Map of normal faults and extensional folds in the Tendoy Mountains and Beaverhead Range, Southwest Montana and eastern Idaho

USGS Publications Warehouse

Janecke, S.U.; Blankenau, J.J.; VanDenburg, C.J.; VanGosen, B.S.

2001-01-01

Compilation of a 1:100,000-scale map of normal faults and extensional folds in southwest Montana and adjacent Idaho reveals a complex history of normal faulting that spanned at least the last 50 m.y. and involved six or more generations of normal faults. The map is based on both published and unpublished mapping and shows normal faults and extensional folds between the valley of the Red Rock River of southwest Montana and the Lemhi and Birch Creek valleys of eastern Idaho between latitudes 45°05' N. and 44°15' N. in the Tendoy and Beaverhead Mountains. Some of the unpublished mapping has been compiled in Lonn and others (2000). Many traces of the normal faults parallel the generally northwest to north-northwest structural grain of the preexisting Sevier fold and thrust belt and dip west-southwest, but northeastand east-striking normal faults are also prominent. Northeaststriking normal faults are subparallel to the traces of southeast-directed thrusts that shortened the foreland during the Laramide orogeny. It is unlikely that the northeast-striking normal faults reactivated fabrics in the underlying Precambrian basement, as has been documented elsewhere in southwestern Montana (Schmidt and others, 1984), because exposures of basement rocks in the map area exhibit north-northwest- to northwest-striking deformational fabrics (Lowell, 1965; M’Gonigle, 1993, 1994; M’Gonigle and Hait, 1997; M’Gonigle and others, 1991). The largest normal faults in the area are southwest-dipping normal faults that locally reactivate thrust faults (fig. 1). Normal faulting began before middle Eocene Challis volcanism and continues today. The extension direction flipped by about 90° four times.

Fault zone characteristics and basin complexity in the southern Salton Trough, California

USGS Publications Warehouse

Persaud, Patricia; Ma, Yiran; Stock, Joann M.; Hole, John A.; Fuis, Gary S.; Han, Liang

2016-01-01

Ongoing oblique slip at the Pacific–North America plate boundary in the Salton Trough produced the Imperial Valley (California, USA), a seismically active area with deformation distributed across a complex network of exposed and buried faults. To better understand the shallow crustal structure in this region and the connectivity of faults and seismicity lineaments, we used data primarily from the Salton Seismic Imaging Project to construct a three-dimensional P-wave velocity model down to 8 km depth and a velocity profile to 15 km depth, both at 1 km grid spacing. A VP = 5.65–5.85 km/s layer of possibly metamorphosed sediments within, and crystalline basement outside, the valley is locally as thick as 5 km, but is thickest and deepest in fault zones and near seismicity lineaments, suggesting a causative relationship between the low velocities and faulting. Both seismicity lineaments and surface faults control the structural architecture of the western part of the larger wedge-shaped basin, where two deep subbasins are located. We estimate basement depths, and show that high velocities at shallow depths and possible basement highs characterize the geothermal areas.

Geophysical investigation of the Denali fault and Alaska Range orogen within the aftershock zone of the October-November 2002, M = 7.9 Denali fault earthquake

USGS Publications Warehouse

Fisher, M.A.; Nokleberg, W.J.; Ratchkovski, N.A.; Pellerin, L.; Glen, J.M.; Brocher, T.M.; Booker, J.

2004-01-01

The aftershock zone of the 3 November 2002, M = 7.9 earthquake that ruptured along the right-slip Denali fault in south-central Alaska has been investigated by using gravity and magnetic, magnetotelluric, and deep-crustal, seismic reflection data as well as outcrop geology and earthquake seismology. Strong seismic reflections from within the Alaska Range orogen north of the Denali fault dip as steeply as 25°N and extend to depths as great as 20 km. These reflections outline a relict crustal architecture that in the past 20 yr has produced little seismicity. The Denali fault is nonreflective, probably because this fault dips steeply to vertical. The most intriguing finding from geophysical data is that earthquake aftershocks occurred above a rock body, with low electrical resistivity (>10 Ω·m), that is at depths below ∼10 km. Aftershocks of the Denali fault earthquake have mainly occurred shallower than 10 km. A high geothermal gradient may cause the shallow seismicity. Another possibility is that the low resistivity results from fluids, which could have played a role in locating the aftershock zone by reducing rock friction within the middle and lower crust.

Effects of fault dip and slip rake angles on near-source ground motions: Why rupture directivity was minimal in the 1999 Chi-Chi, Taiwan, earthquake

USGS Publications Warehouse

Aagaard, Brad T.; Hall, J.F.; Heaton, T.H.

2004-01-01

We study how the fault dip and slip rake angles affect near-source ground velocities and displacements as faulting transitions from strike-slip motion on a vertical fault to thrust motion on a shallow-dipping fault. Ground motions are computed for five fault geometries with different combinations of fault dip and rake angles and common values for the fault area and the average slip. The nature of the shear-wave directivity is the key factor in determining the size and distribution of the peak velocities and displacements. Strong shear-wave directivity requires that (1) the observer is located in the direction of rupture propagation and (2) the rupture propagates parallel to the direction of the fault slip vector. We show that predominantly along-strike rupture of a thrust fault (geometry similar in the Chi-Chi earthquake) minimizes the area subjected to large-amplitude velocity pulses associated with rupture directivity, because the rupture propagates perpendicular to the slip vector; that is, the rupture propagates in the direction of a node in the shear-wave radiation pattern. In our simulations with a shallow hypocenter, the maximum peak-to-peak horizontal velocities exceed 1.5 m/sec over an area of only 200 km2 for the 30??-dipping fault (geometry similar to the Chi-Chi earthquake), whereas for the 60??- and 75??-dipping faults this velocity is exceeded over an area of 2700 km2 . These simulations indicate that the area subjected to large-amplitude long-period ground motions would be larger for events of the same size as Chi-Chi that have different styles of faulting or a deeper hypocenter.

Stress rotations due to the M6.5 foreshock and M7.3 main shock in the 2016 Kumamoto, SW Japan, earthquake sequence

NASA Astrophysics Data System (ADS)

Yoshida, Keisuke; Hasegawa, Akira; Saito, Tatsuhiko; Asano, Youichi; Tanaka, Sachiko; Sawazaki, Kaoru; Urata, Yumi; Fukuyama, Eiichi

2016-10-01

A shallow M7.3 event with a M6.5 foreshock occurred along the Futagawa-Hinagu fault zone in Kyushu, SW Japan. We investigated the spatiotemporal variation of the stress orientations in and around the source area of this 2016 Kumamoto earthquake sequence by inverting 1218 focal mechanisms. The results show that the σ3 axis in the vicinity of the fault plane significantly rotated counterclockwise after the M6.5 foreshock and rotated clockwise after the M7.3 main shock in the Hinagu fault segment. This observation indicates that a significant portion of the shear stress was released both by the M6.5 foreshock and M7.3 main shock. It is estimated that the stress release by the M6.5 foreshock occurred in the shallower part of the Hinagu fault segment, which brought the stress concentration in its deeper part. This might have caused the M7.3 main shock rupture mainly along the deeper part of the Hinagu fault segment after 28 h.

Complex rupture during the 12 January 2010 Haiti earthquake

USGS Publications Warehouse

Hayes, G.P.; Briggs, R.W.; Sladen, A.; Fielding, E.J.; Prentice, C.; Hudnut, K.; Mann, P.; Taylor, F.W.; Crone, A.J.; Gold, R.; Ito, T.; Simons, M.

2010-01-01

Initially, the devastating Mw 7.0, 12 January 2010 Haiti earthquake seemed to involve straightforward accommodation of oblique relative motion between the Caribbean and North American plates along the Enriquillog-Plantain Garden fault zone. Here, we combine seismological observations, geologic field data and space geodetic measurements to show that, instead, the rupture process may have involved slip on multiple faults. Primary surface deformation was driven by rupture on blind thrust faults with only minor, deep, lateral slip along or near the main Enriquillog-Plantain Garden fault zone; thus the event only partially relieved centuries of accumulated left-lateral strain on a small part of the plate-boundary system. Together with the predominance of shallow off-fault thrusting, the lack of surface deformation implies that remaining shallow shear strain will be released in future surface-rupturing earthquakes on the Enriquillog-Plantain Garden fault zone, as occurred in inferred Holocene and probable historic events. We suggest that the geological signature of this earthquakeg-broad warping and coastal deformation rather than surface rupture along the main fault zoneg-will not be easily recognized by standard palaeoseismic studies. We conclude that similarly complex earthquakes in tectonic environments that accommodate both translation and convergenceg-such as the San Andreas fault through the Transverse Ranges of Californiag-may be missing from the prehistoric earthquake record. ?? 2010 Macmillan Publishers Limited. All rights reserved.

Seismicity in the source areas of the 1896 and 1933 Sanriku earthquakes and implications for large near-trench earthquake faults

NASA Astrophysics Data System (ADS)

Obana, Koichiro; Nakamura, Yasuyuki; Fujie, Gou; Kodaira, Shuichi; Kaiho, Yuka; Yamamoto, Yojiro; Miura, Seiichi

2018-03-01

In the northern part of the Japan Trench, the 1933 Showa-Sanriku earthquake (Mw 8.4), an outer-trench, normal-faulting earthquake, occurred 37 yr after the 1896 Meiji-Sanriku tsunami earthquake (Mw 8.0), a shallow, near-trench, plate-interface rupture. Tsunamis generated by both earthquakes caused severe damage along the Sanriku coast. Precise locations of earthquakes in the source areas of the 1896 and 1933 earthquakes have not previously been obtained because they occurred at considerable distances from the coast in deep water beyond the maximum operational depth of conventional ocean bottom seismographs (OBSs). In 2015, we incorporated OBSs designed for operation in deep water (ultradeep OBSs) in an OBS array during two months of seismic observations in the source areas of the 1896 and 1933 Sanriku earthquakes to investigate the relationship of seismicity there to outer-rise normal-faulting earthquakes and near-trench tsunami earthquakes. Our analysis showed that seismicity during our observation period occurred along three roughly linear trench-parallel trends in the outer-trench region. Seismic activity along these trends likely corresponds to aftershocks of the 1933 Showa-Sanriku earthquake and the Mw 7.4 normal-faulting earthquake that occurred 40 min after the 2011 Tohoku-Oki earthquake. Furthermore, changes of the clarity of reflections from the oceanic Moho on seismic reflection profiles and low-velocity anomalies within the oceanic mantle were observed near the linear trends of the seismicity. The focal mechanisms we determined indicate that an extensional stress regime extends to about 40 km depth, below which the stress regime is compressional. These observations suggest that rupture during the 1933 Showa-Sanriku earthquake did not extend to the base of the oceanic lithosphere and that compound rupture of multiple or segmented faults is a more plausible explanation for that earthquake. The source area of the 1896 Meiji-Sanriku tsunami earthquake is characterized by an aseismic region landward of the trench axis. Spatial heterogeneity of seismicity and crustal structure might indicate the near-trench faults that could lead to future hazardous events such as the 1896 and 1933 Sanriku earthquakes, and should be taken into account in assessment of tsunami hazards related to large near-trench earthquakes.

Bayesian inference and interpretation of centroid moment tensors of the 2016 Kumamoto earthquake sequence, Kyushu, Japan

NASA Astrophysics Data System (ADS)

Hallo, Miroslav; Asano, Kimiyuki; Gallovič, František

2017-09-01

On April 16, 2016, Kumamoto prefecture in Kyushu region, Japan, was devastated by a shallow M JMA7.3 earthquake. The series of foreshocks started by M JMA6.5 foreshock 28 h before the mainshock. They have originated in Hinagu fault zone intersecting the mainshock Futagawa fault zone; hence, the tectonic background for this earthquake sequence is rather complex. Here we infer centroid moment tensors (CMTs) for 11 events with M JMA between 4.8 and 6.5, using strong motion records of the K-NET, KiK-net and F-net networks. We use upgraded Bayesian full-waveform inversion code ISOLA-ObsPy, which takes into account uncertainty of the velocity model. Such an approach allows us to reliably assess uncertainty of the CMT parameters including the centroid position. The solutions show significant systematic spatial and temporal variations throughout the sequence. Foreshocks are right-lateral steeply dipping strike-slip events connected to the NE-SW shear zone. Those located close to the intersection of the Hinagu and Futagawa fault zones are dipping slightly to ESE, while those in the southern area are dipping to WNW. Contrarily, aftershocks are mostly normal dip-slip events, being related to the N-S extensional tectonic regime. Most of the deviatoric moment tensors contain only minor CLVD component, which can be attributed to the velocity model uncertainty. Nevertheless, two of the CMTs involve a significant CLVD component, which may reflect complex rupture process. Decomposition of those moment tensors into two pure shear moment tensors suggests combined right-lateral strike-slip and normal dip-slip mechanisms, consistent with the tectonic settings of the intersection of the Hinagu and Futagawa fault zones.[Figure not available: see fulltext.

Structure of the western Rif (Morocco): Possible hydrocarbon plays

DOE Office of Scientific and Technical Information (OSTI.GOV)

Flinch, J.

1995-08-01

Seismic data offshore and onshore northwestern Morocco (i.e. Atlantic margin, Rharb Basin, Rif foothills) provided a detailed picture of the Western Rif Cordillera. The most external units of the folded-belt consist of allochthonous Cretaceous and Neogene strongly deformed sediments that constitute a westward-directed accretionary wedge. The structure of the accretionary wedge consist of a complex set of thrust and normal faults. The inner part of the study area consist of NW-SE trending thrust faults, partially exposed in the foothills of the Western Rif. Proceeding towards the foreland, thrust faults are offset by low-angle extensional detachments characterized by anastomosing extensional horses.more » Widespread extension overlying the accretionary wedge defines a Late Neogene episode of extensional collapse. Extension is not characterized by localized conventional half-grabens but consists of a complex extensional system with variable orientation. Locally shale ridges and toe-thrusts characterized by rear extension and frontal compression define a set of mixed extensional-compressional satellite basins that significantly differ from conventional thrust-related piggy-back basins. Satellite basins are filled with Upper Tortonian to Pliocene sediments. Shallow fields of biogenic gas are present in this Upper Neogene succession of the satellite basins. The frontalmost part of the wedge consist of WNW-ESE trending thrust imbricates. A flexural basin (foredeep) developed as a result of the accretionary prism loading. The foredeep basin discordantly overlies thinn Cretaceous and Lower-Middle Miocene shallow-water sediments that indistinctly cover Plaeozoic basement rocks and Triassic half-grabens. Pre-foredeep units are related to rifting and passive margin development of the Atlantic Ocean. East from the Rharb Basin the Rif Cordillera is essentially unexplored. Few scattered seismic sections display subsurface ramp anticlines similar to those exposed in the mountain belt.« less

Application of ERTS and EREP images to geologic investigations of the basin and range: Colorado plateau boundary in northwestern and north-central Arizona

NASA Technical Reports Server (NTRS)

Goetz, A. F. H. (Principal Investigator); Billingsley, F. C.; Elston, D. P.; Lucchita, I.; Shoemaker, E. M.

1974-01-01

The author has identified the following significant results. In the course of the ERTS investigation in the Cataract Creek Basin of the Coconino Plateau it was recognized that shallow perched ground water associated with the Kaibab Limestone could be discovered by means of drilling guided by geologic mapping aided by the use of ERTS imagery. At the Globe Ranch, the perched water table is only 5 meters beneath the surface at the site of the original, hand dug well. Recharge occurs from local runoff and from direct precipitation on the outcrop belt of the sandstone. This well provides water for the ranch at the rate of about 1,000 gallons a week. In order to explore the possibility of further developing this aquifer, unit 5 was mapped over an area of about 50 square miles in the vicinity of the hand-dug well, with negative results. A new location was then picked for drilling based on the occurrence of unit 5 in a favorable structural setting. This location was along a normal fault, and it was anticipated that water might be structurally trapped within the down-dropped block of the fault. Four shallow testholes were drilled and all encountered water. These four water-bearing holes are currently being monitored and will be tested to determine potential production of water from the local sandstone aquifer.

Earthquakes in the Orozco transform zone: seismicity, source mechanisms, and tectonics

USGS Publications Warehouse

Tréhu, Anne M.; Solomon, Sean C.

1983-01-01

As part of the Rivera Ocean Seismic Experiment, a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the Orozco transform fault in the central eastern Pacific. We present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network. All epicenters are within the transform region of the Orozco Fracture Zone and clearly delineate the active plate boundary. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone (region 1). Most focal depths for these events are very shallow, within 4 km of the seafloor; several well-determined focal depths, however, are as great as 7 km. No shallowing of seismic activity is observed as the rise-transform intersection is approached; to the contrary, the deepest events are within 10 km of the intersection. First motion polarities for most of the earthquakes in region 1 are compatible with right-lateral strike slip faulting along a nearly vertical plane, striking parallel to the spreading direction. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity in this region is more scattered than in the first, and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. Pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions. The effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three-dimensional velocity grid. While findings for events in region 1 are not significantly affected, in region 2, epicentral mislocations of up to 10 km and azimuthal deflections of up to 45° may result from assuming a laterally homogeneous velocity structure. When corrected for the effects of lateral heterogeneity, the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending NW–SE; the focal depths, however, are poorly constrained. These results suggest an en echelon spreading center or leaky transform regime in the central transform region.

Mapping b-value for 2009 Harrat Lunayyir earthquake swarm, western Saudi Arabia and Coulomb stress for its mainshock

NASA Astrophysics Data System (ADS)

Abdelfattah, Ali K.; Mogren, Saad; Mukhopadhyay, Manoj

2017-01-01

The Harrat Lunayyir (HL) earthquake swarm of 2009 originated in the HL volcanic field and attracted global attention mainly due to three factors: (i) its relatively short life span that ushered a large frequency of the swarm population (30,000 events in < 2 years), (ii) the swarm epicenter zone was contained within a small crustal volume under the HL and (iii) the migratory nature of the swarm following the tectonic trend of a normal fault zone beneath HL. The HL belongs to the Large Igneous Province of Saudi Arabia (LIP-SA) where it correlates to the Great Dikes locally. Our aim in this study is to describe the spatial distribution of the hypocenters, b-value character, and Coulomb stress failure (CSF) in an attempt to analyze the underlying geodynamic process that caused the swarm. We utilize the relocated hypocenters monitored by local networks to examine the b-value characteristics for the swarm. This is best represented in a cross section showing two domains of higher b-value anomalies: two patches occurring at shallow depth and at the deeper crust to the SE from the mainshock originated at the shallower depth northwestward. Consistently positive ΔCFF pattern with a large percentage of aftershocks imply how the mainshock rupture controlled the aftershocks activity. This implies that the failure along the NNW fault trend is due to the prevailing ambient stress field imparted to the swarm. We model this by CSF associated with the mainshock for three time dependent situations: (a) foreshock and aftershock epicenters, (b) foreshock hypocenters, and (c) aftershock hypocenters. In actuality, multiple factors might have controlled the aftershock activity as we speculate that positive Coulomb stress was associated in an area where the higher b-value prevails. The CSF produced by the mainshock illustrates how the stress dissipated along the NNW normal fault zone that interrupts the Great Dykes along the Red Sea coast. These results further suggest that the crustal heterogeneity under HL act as an asperity in the epicentral area, whose origin may relate to magma intrusion into upper crust. However, seismic survey is required for detailing this geologic inference.

Typhoon-driven landsliding induces earthquakes: example of the 2009 Morakot typhoon

NASA Astrophysics Data System (ADS)

Steer, Philippe; Jeandet, Louise; Cubas, Nadaya; Marc, Odin; Meunier, Patrick; Hovius, Niels; Simoes, Martine; Cattin, Rodolphe; Shyu, J. Bruce H.; Liang, Wen-Tzong; Theunissen, Thomas; Chiang, Shou-Hao

2017-04-01

Extreme rainfall events can trigger numerous landslides in mountainous areas and a prolonged increase of river sediment load. The resulting mass transfer at the Earth surface in turn induces stress changes at depth, which could be sufficient to trigger shallow earthquakes. The 2009 Morakot typhoon represents a good case study as it delivered 3 m of precipitation in 3 days and caused some of the most intense erosion ever recorded. Analysis of seismicity time-series before and after the Morakot typhoon reveals a systematic increase of shallow (i.e. 0-15 km of depth) earthquake frequency in the vicinity of the areas displaying a high spatial density of landslides. This step-like increase in frequency lasts for at least 2-3 years and does not follow an Omori-type aftershock sequence. Rather, it is associated to a step change of the Gutenberg-Richter b-value of the earthquake catalog. Both changes occurred in mountainous areas of southwest Taiwan, where typhoon Morakot caused extensive landsliding. These spatial and temporal correlations strongly suggest a causal relationship between the Morakot-triggered landslides and the increase of earthquake frequency and their associated b-value. We propose that the progressive removal of landslide materials from the steep mountain landscape by river sediment transport acts as an approximately constant increase of the stress rate with respect to pre-typhoon conditions, and that this in turn causes a step-wise increase in earthquake frequency. To test this hypothesis, we investigate the response of a rate-and-state fault to stress changes using a 2-D continuum elasto-dynamic model. Consistent with the results of Ader et al. (2013), our preliminary results show a step-like increase of earthquake frequency in response to a step-like decrease of the fault normal stress. We also investigate the sensitivity of the amplitude and time-scale of the earthquake frequency increase to the amplitude of the normal stress change and to rheological parameters. Our study offers new insights on the potential influence of extreme erosional events on the short-time scale dynamics of faults and earthquakes.

Proposed Drill Sites

DOE Data Explorer

Lane, Michael

2013-06-28

Proposed drill sites for intermediate depth temperature gradient holes and/or deep resource confirmation wells. Temperature gradient contours based on shallow TG program and faults interpreted from seismic reflection survey are shown, as are two faults interpreted by seismic contractor Optim but not by Oski Energy, LLC.

Characterizing the potential for fault reactivation related to CO2 injection through subsurface structural mapping and stress field analysis, Wellington Field, Sumner County, KS

NASA Astrophysics Data System (ADS)

Schwab, D.; Bidgoli, T.; Taylor, M. H.

2015-12-01

South-central Kansas has experienced an unprecedented increase in seismic activity since 2013. The spatial and temporal relationship of the seismicity with brine disposal operations has renewed interest in the role of fluids in fault reactivation. This study focuses on determining the suitability of CO2 injection into a Cambro-Ordovician reservoir for long-term storage and a Mississippian reservoir for enhanced oil recovery in Wellington Field, Sumner County, Kansas. Our approach for determining the potential for induced seismicity has been to (1) map subsurface faults and estimate in-situ stresses, (2) perform slip and dilation tendency analysis to identify optimally-oriented faults relative to the estimated stress field, and (3) monitor surface deformation through cGPS data and InSAR imaging. Through the use of 3D seismic reflection data, 60 near vertical, NNE-striking faults have been identified. The faults range in length from 140-410 m and have vertical separations of 3-32m. A number of faults appear to be restricted to shallow intervals, while others clearly cut the top basement reflector. Drilling-induced tensile fractures (N=78) identified from image logs and inversion of earthquake focal mechanism solutions (N=54) are consistent with the maximum horizontal stress (SHmax) oriented ~E-W. Both strike-slip and normal-slip fault plane solutions for earthquakes near the study area suggest that SHmax and Sv may be similar in magnitude. Estimates of stress magnitudes using step rate tests (Shmin = 2666 psi), density logs (Sv = 5308 psi), and calculations from wells with drilling induced tensile fractures (SHmax = 4547-6655 psi) are determined at the gauge depth of 4869ft. Preliminary slip and dilation tendency analysis indicates that faults striking 0°-20° are stable, whereas faults striking 26°-44° may have a moderate risk for reactivation with increasing pore-fluid pressure.

Controls on the Seafloor Exposure of Detachment Fault Surfaces

NASA Astrophysics Data System (ADS)

Olive, J. A. L.; Parnell-Turner, R. E.; Escartin, J.; Smith, D. K.; Petersen, S.

2017-12-01

Morphological and seismological evidence suggests that asymmetric accretion involving oceanic detachment faulting takes place along 40% of the Northern Mid-Atlantic Ridge. However, seafloor exposures of corrugated slip surfaces -a telltale sign of this kind of faulting- remain scarce and spatially limited according to multibeam bathymetric surveys. This raises the question of whether geomorphic processes can hinder the exposure of pristine fault surfaces during detachment growth. We address this problem by analyzing ≤2-m resolution bathymetry data from four areas where corrugated surfaces emerge from the seafloor (13º20'N, 16º25'N, 16º36'N, and TAG). We identify two key processes capable of degrading or masking a corrugated large-offset fault surface. The first is gravitational mass wasting of steep (>25º) slopes, which is widespread in the breakaway region of most normal faults. The second is blanketing of the shallow-dipping termination area by a thin apron of hanging wall-derived debris. We model this process using critical taper theory, and infer low effective friction coefficients ( 0.15) on the emerging portion of detachment faults. A corollary to this result is that faults emerging from the seafloor with an angle <10º are more likely to blanket themselves under an apron of hanging wall debris. Optimal exposure of detachment surfaces therefore occurs when the fault emerges at slopes between 10° and 25º. We generalize these findings into a simple model for the progressive exhumation and flexural rotation of detachment footwalls, which accounts for the continued action of seafloor geomorphic processes. Our model suggests that many moderate-offset `blanketed' detachments may exist along slow mid-ocean ridges, but their corrugated surfaces are unlikely to be detected in shipboard multibeam bathymetry (e.g., TAG). Furthermore, many `irregular massifs' may correspond to the degraded footwalls of detachment faults.

Shallow plumbing systems inferred from spatial analysis of pockmark arrays

NASA Astrophysics Data System (ADS)

Maia, A.; Cartwright, J. A.; Andersen, E.

2016-12-01

This study describes and analyses an extraordinary array of pockmarks at the modern seabed of the Lower Congo Basin (offshore Angola), in order to understand the fluid migration routes and shallow plumbing system of the area. The 3D seismic visualization of feeding conduits (pipes) allowed the identification of the source interval for the fluids expelled during pockmark formation. Spatial statistics are used to show the relationship between the underlying (polarised) polygonal fault (PPFs) patterns and seabed pockmarks distributions. Our results show PPFs control the linear arrangement of pockmarks and feeder pipes along fault strike, but faults do not act as conduits. Spatial statistics also revealed pockmark occurrence is not considered to be random, especially at short distances to nearest neighbours (<200m) where anti-clustering distributions suggest the presence of an exclusion zone around each pockmark in which no other pockmark will form. The results of this study are relevant for the understanding of shallow fluid plumbing systems in offshore settings, with implications on our current knowledge of overall fluid flow systems in hydrocarbon-rich continental margins.

Quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 Landers, California, earthquake: Evidence for a shallow trapping structure

USGS Publications Warehouse

Peng, Z.; Ben-Zion, Y.; Michael, A.J.; Zhu, L.

2003-01-01

We analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 Landers earthquake. A grid-search method with station delay corrections is used to locate events that do not have catalogue locations. The quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct S waves at stations close to and off the fault zone. Approximately 70 per cent of the events with S-P times of less than 2 s, including many clearly off the fault, produce considerable trapped wave energy. This distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the Landers rupture zone. The time difference between the S arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth. The dispersion measured from the trapped waves is weak. These results imply that the seismic trapping structure at the Landers rupture zone is shallow and does not extend continuously along-strike by more than a few kilometres. Synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2-4 km, a width of approximately 200 m, an S-wave velocity reduction relative to the host rock of approximately 30-40 per cent and an S-wave attenuation coefficient of approximately 20-30. The fault zone waveguide north of the array appears to be shallower and weaker than that south of the array. The waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break.

Imaging Complex Fault Slip of the 2016 MeiNong and Kumamoto Earthquakes with Sentinel-1 InSAR and Other Geodetic and Seismic Data

NASA Astrophysics Data System (ADS)

Fielding, E. J.; Huang, M. H.; Liang, C.; Yue, H.; Agram, P. S.; Simons, M.; Fattahi, H.; Tung, H.; Hu, J. C.; Huang, C.

2016-12-01

We map complex fault ruptures of the February 2016 MeiNong earthquake in Taiwan and the April 2016 Kumamoto earthquake sequence in Japan by analysis of Synthetic Aperture Radar (SAR) data from the Copernicus Sentinel-1A (S1A) satellite operated by the European Space Agency and the Advanced Land Observation Satellite-2 (ALOS-2) satellite operated by the Japanese Aerospace Exploration Agency (JAXA). Our analysis shows that the MeiNong main rupture at lower crustal depth triggered slip on another fault at upper crustal depth and shallow slip on several faults in the upper few km. The Kumamoto earthquake sequence ruptured two major fault systems over two days and triggered shallow slip on a large number of shallow faults. We combine less precise analysis of large scale displacements from the SAR images of the two satellites by pixel offset tracking or sub-pixel correlation, including the along-track component of surface motion, with the more precise SAR interferometry (InSAR) measurements in the radar line-of-sight direction to estimate all three components of the surface displacement for the events. Data was processed with customized workflows based on modules in the InSAR Scientific Computing Environment (ISCE). Joint inversion of S1A and ALOS-2 InSAR, GPS, and strong motion seismograms for the Mw6.4 MeiNong earthquake shows that the main thrust rupture with N61°W strike and 15° dip at 15-20 km depth explains nearly all of the seismic waveforms but leaves a substantial uplift residual in the InSAR and GPS offsets estimated 4 hours after the earthquake. We model this residual with slip on a N8°E-trending thrust fault dipping 30° at depths between 5-10 km. This fault strike is parallel to surface faults and we interpret it as fault slip within a mid-crustal duplex that was triggered by the main rupture within 4 hours of the mainshock. In addition, InSAR shows sharp discontinuities at many locations that are likely due to shallow triggered slip, but the timing of these is uncertain. The Kumamoto earthquake sequence in Japan started with Mw 6.2 and 6.0 earthquakes on 14 April (UTC) followed on 15 April by the Mw 7.0 mainshock. JAXA acquired one ALOS-2 scene between the foreshocks and mainshock that enables some separation of the surface deformation. InSAR shows M6 foreshocks were deeper, while M7 mainshock ruptured surface in many places.

Actively dewatering fluid-rich zones along the Costa Rica plate boundary fault

NASA Astrophysics Data System (ADS)

Bangs, N. L.; McIntosh, K. D.; Silver, E. A.; Kluesner, J. W.; Ranero, C. R.; von Huene, R.

2012-12-01

New 3D seismic reflection data reveal distinct evidence for active dewatering above a 12 km wide segment of the plate boundary fault within the Costa Rica subduction zone NW of the Osa Peninsula. In the spring of 2011 we acquired a 11 x 55 km 3D seismic reflection data set on the R/V Langseth using four 6,000 m streamers and two 3,300 in3 airgun arrays to examine the structure of the Costa Rica margin from the trench into the seismogenic zone. We can trace the plate-boundary interface from the trench across our entire survey to where the plate-boundary thrust lies > 10 km beneath the margin shelf. Approximately 20 km landward of the trench beneath the mid slope and at the updip edge of the seismogenic zone, a 12 km wide zone of the plate-boundary interface has a distinctly higher-amplitude seismic reflection than deeper or shallower segments of the fault. Directly above and potentially directly connected with this zone are high-amplitude, reversed-polarity fault-plane reflections that extend through the margin wedge and into overlying slope sediment cover. Within the slope cover, high-amplitude reversed-polarity reflections are common within the network of closely-spaced nearly vertical normal faults and several broadly spaced, more gently dipping thrust faults. These faults appear to be directing fluids vertically toward the seafloor, where numerous seafloor fluid flow indicators, such as pockmarks, mounds and ridges, and slope failure features, are distinct in multibeam and backscatter images. There are distinctly fewer seafloor and subsurface fluid flow indicators both updip and downdip of this zone. We believe these fluids come from a 12 km wide fluid-rich segment of the plate-boundary interface that is likely overpressured and has relatively low shear stress.

Intra-arc Seismicity: Geometry and Kinematic Constraints of Active Faulting along Northern Liquiñe-Ofqui and Andean Transverse Fault Systems [38º and 40ºS, Southern Andes

NASA Astrophysics Data System (ADS)

Sielfeld, G.; Lange, D.; Cembrano, J. M.

2017-12-01

Intra-arc crustal seismicity documents the schizosphere tectonic state along active magmatic arcs. At oblique-convergent margins, a significant portion of bulk transpressional deformation is accommodated in intra-arc regions, as a consequence of stress and strain partitioning. Simultaneously, crustal fluid migration mechanisms may be controlled by the geometry and kinematics of crustal high strain domains. In such domains shallow earthquakes have been associated with either margin-parallel strike-slip faults or to volcano-tectonic activity. However, very little is known on the nature and kinematics of Southern Andes intra-arc crustal seismicity and its relation with crustal faults. Here we present results of a passive seismicity study based on 16 months of data collected from 33 seismometers deployed along the intra-arc region of Southern Andes between 38˚S and 40˚S. This region is characterized by a long-lived interplay among margin-parallel strike-slip faults (Liquiñe-Ofqui Fault System, LOFS), second order Andean-transverse-faults (ATF), volcanism and hydrothermal activity. Seismic signals recorded by our network document small magnitude (0.2P and 2,796 S phase arrival times have been located with NonLinLoc. First arrival polarities and amplitude ratios of well-constrained events, were used for focal mechanism inversion. Local seismicity occurs at shallow levels down to depth of ca. 16 km, associated either with stratovolcanoes or to master, N10˚E, and subsidiary, NE to ENE, striking branches of the LOFS. Strike-slip focal mechanisms are consistent with the long-term kinematics documented by field structural-geology studies. Unexpected, well-defined NW-SE elongated clusters are also reported. In particular, a 72-hour-long, N60˚W-oriented seismicity swarm took place at Caburgua Lake area, describing a ca. 36x12x1km3 faulting crustal volume. Results imply a unique snapshot on shallow crustal tectonics, contributing to the understanding of faulting processes in volcanic arcs and ultimately, providing useful knowledge to improve the quality of hazard assessment communities emplaced in Southern Andes volcanic arc.

Active Tectonics Around Pisagua, Northern Chile Gap: Seismological and Neotectonic Approaches

NASA Astrophysics Data System (ADS)

Comte, D.; Carrizo, D.; Peyrat, S.

2013-12-01

Northern Chile is a recognized mature seismic gap that is reaching the end of its megathrust cycle. Deformation associated with the convergence between the Nazca and the South American Plates is mainly absorbed along the interplate contact, but also partially accommodated along the upper plate. Even though distribution of the active deformation along this plate has been documented mainly in the backarc region, Late Cenozoic structures have been recognized along the forearc suggesting that some part of this deformation is also accommodated along the coastal region. Recent paleoseismological studies suggest that some of these structures are tectonically active and some could be potentially active, capable to generate shallow intraplate earthquakes (Mw˜7). However, seismological and geodetical evidences of the fault activation mechanisms are poorly documented, and the activation process remain not elucidate. Currently, Northern Chile seismic gap is monitored by regional seismic networks and partially studied by temporary local seismological experiments. Results of these studies suggest the presence of shallow seismicity along the forearc, but the relationships between upper plate faults and the seismicity has not been yet explored. We perform a detailed seismotectonic analysis of the subduction-forearc system in the central part of the Northern Chile seismic gap to establish relationships between the plate contact deformation and the upper plate faults. We present preliminary results of data recorded by a dense seismic network (three components continuous recording) deployed around Pisagua, between the coastline and the Central Depression, during several months. Pisagua region was chosen because the forearc faults exhibit an extraordinary well-preserved morphotectonic expression, and the upper part of the seismogenic interplate contact shows abundant continental intraplate seismicity that could be associated with the faults systems. The data recorded in this area allow us to better constrain the 3D geometry of faults related to plate contact using morphotectonis fault signature, well-located shallow seismicity and passive tomography. By this way, the architecture of the major forearc faults in the study area is determined for the first time using geological and geophysical approaches. Through this work, we contribute to better understand the physical relations between dynamics of the plate contact and the coastal fault activation.

Multi-Scale Structure and Earthquake Properties in the San Jacinto Fault Zone Area

NASA Astrophysics Data System (ADS)

Ben-Zion, Y.

2014-12-01

I review multi-scale multi-signal seismological results on structure and earthquake properties within and around the San Jacinto Fault Zone (SJFZ) in southern California. The results are based on data of the southern California and ANZA networks covering scales from a few km to over 100 km, additional near-fault seismometers and linear arrays with instrument spacing 25-50 m that cross the SJFZ at several locations, and a dense rectangular array with >1100 vertical-component nodes separated by 10-30 m centered on the fault. The structural studies utilize earthquake data to image the seismogenic sections and ambient noise to image the shallower structures. The earthquake studies use waveform inversions and additional time domain and spectral methods. We observe pronounced damage regions with low seismic velocities and anomalous Vp/Vs ratios around the fault, and clear velocity contrasts across various sections. The damage zones and velocity contrasts produce fault zone trapped and head waves at various locations, along with time delays, anisotropy and other signals. The damage zones follow a flower-shape with depth; in places with velocity contrast they are offset to the stiffer side at depth as expected for bimaterial ruptures with persistent propagation direction. Analysis of PGV and PGA indicates clear persistent directivity at given fault sections and overall motion amplification within several km around the fault. Clear temporal changes of velocities, probably involving primarily the shallow material, are observed in response to seasonal, earthquake and other loadings. Full source tensor properties of M>4 earthquakes in the complex trifurcation area include statistically-robust small isotropic component, likely reflecting dynamic generation of rock damage in the source volumes. The dense fault zone instruments record seismic "noise" at frequencies >200 Hz that can be used for imaging and monitoring the shallow material with high space and time details, and numerous minute local earthquakes that contribute to the high frequency "noise". Updated results will be presented in the meeting. *The studies have been done in collaboration with Frank Vernon, Amir Allam, Dimitri Zigone, Zach Ross, Gregor Hillers, Ittai Kurzon, Michel Campillo, Philippe Roux, Lupei Zhu, Dan Hollis, Mitchell Barklage and others.

The 2017/09/08 Mw 8.2 Tehuantepec, Mexico Earthquake: A Large but Compact Dip-Slip Faulting Event Severing the Slab

NASA Astrophysics Data System (ADS)

Hjorleifsdottir, V.; Iglesias, A.; Suarez, G.; Santoyo, M. A.; Villafuerte, C. D.; Ji, C.; Franco-Sánchez, S. I.; Singh, S. K.; Cruz-Atienza, V. M.; Ando, R.

2017-12-01

The Mw 8.2 September 8 earthquake occurred in the middle of the "Tehuantepec Gap", a segment of the Mexican subduction zone that has no historical mentions of a large earthquake. It was, however, not the expected subduction megathrust earthquake, but rather an intraplate, normal faulting event, in the subducting oceanic Cocos plate. The earthquake rupture initiated at a depth of 50 km and propagated NW on a near-vertical plane, breaking towards the surface. Most of the slip was concentrated in the distance range 30-100 km from the hypocenter and at depth between 15 and 50 km, with maximum slip of 15m. The earthquake seems to have broken the entire lithosphere, estimated to be 35 km thick. The strike of the fault is about 20 degrees oblique to the trench but aligned with the existing fabric on the incoming oceanic plate, suggesting a structural control by preexisting intraslab fractures and activation by the extensional stress due to the slab bending and pulling. Aftershocks occurred along the fault plane during the first day after the event, with activation of other parallel structures within the subducting plate, towards the east, as well as in upper plate, in the following days. Coulomb stress modeling suggests that the stress on the plate interface above the rupture was significantly increased where shallow thrust aftershoks took place, and reduced updip of the earthquake. There are several other examples of large intraslab normal faulting earthquakes, near the downdip edge (1931 Mw 7.8 and 1999 Mw 7.5, Oaxaca) or directly below (1997 Mw 7.1, Michoacan) the coupled plate interface, along the Mexican subduction zone. The possibility of events of similar magnitude to the 2017 earthquake occurring close to the coastline, all along this part of the subduction zone, cannot be ruled out.

Early Jurassic extensional inheritance in the Lurestan region of the Zagros fold-and-thrust belt, Iran.

NASA Astrophysics Data System (ADS)

Tavani, Stefano; Parente, Mariano; Vitale, Stefano; Puzone, Francesco; Erba, Elisabetta; Bottini, Cinzia; Morsalnejad, Davoud; Mazzoli, Stefano

2017-04-01

It has long been recognized that the tectonic architecture of the Zagros mountain belt was strongly controlled by inherited structures previously formed within the Arabian plate. These preexisting features span in age from the pre-Cambrian to the Mesozoic, showing different trends and deformation styles. Yet, these structures are currently not fully understood. This uncertainty is partly related with the paucity of exposures, which rarely allows a direct observation of these important deformation features. The Lurestan Province of Iran provides a remarkable exception, since it is one of the few places of the Zagros mountain belt where exposures of Triassic and Jurassic rocks are widespread. In this area we carried out structural observations on Mesozoic extensional structures developed at the southern margin of the Neo-Tethyan basin. Syn-sedimentary extensional faults are hosted within the Triassic-Cretaceous succession, being particularly abundant in the Jurassic portion of the stratigraphy. Early to Middle Jurassic syn-sedimentary faults are observed in different paleogeographic domains of the area, and their occurrence is coherent with the subsequent transition from shallow-water to deep-sea basin environments, observed in a wide portion of the area. Most of the thrusts exposed in the area may indeed be interpreted as reactivated Jurassic extensional faults, or as reverse faults whose nucleation was controlled by the location of preexisting normal faults, as a result of positive inversion during crustal shortening and mountain building.

Fluid pathways from mantle wedge up to forearc seafloor in the coseismic slip area of the 2011 Tohoku earthquake

NASA Astrophysics Data System (ADS)

Park, J. O.; Tsuru, T.; Fujie, G.; Kagoshima, T.; Sano, Y.

2017-12-01

A lot of fluids at subduction zones are exchanged between the solid Earth and ocean, affecting the earthquake and tsunami generation. New multi-channel seismic reflection and sub-bottom profiling data reveal normal and reverse faults as the fluid pathways in the coseismic slip area of the 2011 Tohoku earthquake (M9.0). Based on seismic reflection characteristics and helium isotope anomalies, we recognize variations in fluid pathways (i.e., faults) from the mantle wedge up to forearc seafloor in the Japan Trench margin. Some fluids are migrated from the mantle wedge along plate interface and then normal or reverse faults cutting through the overriding plate. Others from the mantle wedge are migrated directly up to seafloor along normal faults, without passing through the plate interface. Locations of the normal faults are roughly consistent with aftershocks of the 2011 Tohoku earthquake, which show focal mechanism of normal faulting. It is noticeable that landward-dipping normal faults developing down into Unit C (Cretaceous basement) from seafloor are dominant in the middle slope region where basal erosion is inferred to be most active. A high-amplitude, reverse-polarity reflection of the normal faults within Unit C suggests that the fluids are locally trapped along the faults in high pore pressures. The 2011 Tohoku mainshock and subsequent aftershocks could lead the pre-existing normal faults to be reactive and more porous so that the trapped fluids are easily transported up to seafloor through the faults. Elevated fluid pressures can decrease the effective normal stress for the fault plane, allowing easier slip of the landward-dipping normal fault and also enhancing its tsunamigenic potential.

Constraints on Shallow Crustal Structure across the San Andreas Fault Zone, Coachella Valley, Southern California: Results from the Salton Seismic Imaging Project (SSIP)

NASA Astrophysics Data System (ADS)

Hernandez, A.; Persaud, P.; Bauer, K.; Stock, J. M.; Fuis, G. S.; Hole, J. A.; Goldman, M.

2015-12-01

The strong influence of basin structure and crustal heterogeneities on seismic wave propagation suggests that these factors should be included in calculations of strong ground shaking. Knowledge of the shallow subsurface is thus essential for an accurate seismic hazard estimate for the densely populated Coachella Valley, the region north of the potential M7.8 rupture near the Salton Sea. Using SSIP data, we analyzed first arrivals from nine 65-911 kg explosive shots recorded along a profile in the Coachella Valley in order to evaluate the interpretation of our 2D tomographic results and give added details on the structural complexity of the shallow crust. The line extends 37 km from the Peninsular Ranges to the Little San Bernardino Mountains crossing the major strands of the San Andreas Fault Zone. We fit traveltime curves to our picks with forward modeling ray tracing, and determined 1D P-wave velocity models for traveltime arrivals east and west of each shot, and a 2D model for the line. We also inferred the geometry of near-vertical faults from the pre-stack line migration method of Bauer et al. (2013). In general, the 1D models east of individual shots have deeper basement contacts and lower apparent velocities, ~5 km/s at 4 km depth, whereas the models west of individual shots have shallower basement and velocities up to 6 km/s at 2 km depth. Mismatches in basement depths (assuming 5-6 km/s) between individual 1D models indicate a shallowly dipping basement, deepening eastward towards the Banning Fault and shoaling abruptly farther east. An east-dipping structure in the 2D model also gives a better fit than horizontal layers. Based on high velocity zones derived from traveltimes at 9-20 km from the western end of the line, we included an offset from ~2 km to 4 km depth near the middle of the line, which significantly improved the 2D model fit. If fault-related, this offset could represent the Garnet Hill Fault if it continues southward in the subsurface.

Buried shallow fault slip from the South Napa earthquake revealed by near-field geodesy

PubMed Central

Brooks, Benjamin A.; Minson, Sarah E.; Glennie, Craig L.; Nevitt, Johanna M.; Dawson, Tim; Rubin, Ron; Ericksen, Todd L.; Lockner, David; Hudnut, Kenneth; Langenheim, Victoria; Lutz, Andrew; Mareschal, Maxime; Murray, Jessica; Schwartz, David; Zaccone, Dana

2017-01-01

Earthquake-related fault slip in the upper hundreds of meters of Earth’s surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests. PMID:28782026

Buried shallow fault slip from the South Napa earthquake revealed by near-field geodesy.

PubMed

Brooks, Benjamin A; Minson, Sarah E; Glennie, Craig L; Nevitt, Johanna M; Dawson, Tim; Rubin, Ron; Ericksen, Todd L; Lockner, David; Hudnut, Kenneth; Langenheim, Victoria; Lutz, Andrew; Mareschal, Maxime; Murray, Jessica; Schwartz, David; Zaccone, Dana

2017-07-01

Earthquake-related fault slip in the upper hundreds of meters of Earth's surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests.

Buried shallow fault slip from the South Napa earthquake revealed by near-field geodesy

USGS Publications Warehouse

Brooks, Benjamin A.; Minson, Sarah E.; Glennie, Craig L.; Nevitt, Johanna; Dawson, Timothy E.; Rubin, Ron S.; Ericksen, Todd; Lockner, David A.; Hudnut, Kenneth W.; Langenheim, Victoria; Lutz, Andrew; Murray, Jessica R.; Schwartz, David P.; Zaccone, Dana

2017-01-01

Earthquake-related fault slip in the upper hundreds of meters of Earth’s surface has remained largely unstudied because of challenges measuring deformation in the near field of a fault rupture. We analyze centimeter-scale accuracy mobile laser scanning (MLS) data of deformed vine rows within ±300 m of the principal surface expression of the M (magnitude) 6.0 2014 South Napa earthquake. Rather than assuming surface displacement equivalence to fault slip, we invert the near-field data with a model that allows for, but does not require, the fault to be buried below the surface. The inversion maps the position on a preexisting fault plane of a slip front that terminates ~3 to 25 m below the surface coseismically and within a few hours postseismically. The lack of surface-breaching fault slip is verified by two trenches. We estimate near-surface slip ranging from ~0.5 to 1.25 m. Surface displacement can underestimate fault slip by as much as 30%. This implies that similar biases could be present in short-term geologic slip rates used in seismic hazard analyses. Along strike and downdip, we find deficits in slip: The along-strike deficit is erased after ~1 month by afterslip. We find no evidence of off-fault deformation and conclude that the downdip shallow slip deficit for this event is likely an artifact. As near-field geodetic data rapidly proliferate and will become commonplace, we suggest that analyses of near-surface fault rupture should also use more sophisticated mechanical models and subsurface geomechanical tests.

A fault‐based model for crustal deformation in the western United States based on a combined inversion of GPS and geologic inputs

USGS Publications Warehouse

Zeng, Yuehua; Shen, Zheng-Kang

2017-01-01

We develop a crustal deformation model to determine fault‐slip rates for the western United States (WUS) using the Zeng and Shen (2014) method that is based on a combined inversion of Global Positioning System (GPS) velocities and geological slip‐rate constraints. The model consists of six blocks with boundaries aligned along major faults in California and the Cascadia subduction zone, which are represented as buried dislocations in the Earth. Faults distributed within blocks have their geometrical structure and locking depths specified by the Uniform California Earthquake Rupture Forecast, version 3 (UCERF3) and the 2008 U.S. Geological Survey National Seismic Hazard Map Project model. Faults slip beneath a predefined locking depth, except for a few segments where shallow creep is allowed. The slip rates are estimated using a least‐squares inversion. The model resolution analysis shows that the resulting model is influenced heavily by geologic input, which fits the UCERF3 geologic bounds on California B faults and ±one‐half of the geologic slip rates for most other WUS faults. The modeled slip rates for the WUS faults are consistent with the observed GPS velocity field. Our fit to these velocities is measured in terms of a normalized chi‐square, which is 6.5. This updated model fits the data better than most other geodetic‐based inversion models. Major discrepancies between well‐resolved GPS inversion rates and geologic‐consensus rates occur along some of the northern California A faults, the Mojave to San Bernardino segments of the San Andreas fault, the western Garlock fault, the southern segment of the Wasatch fault, and other faults. Off‐fault strain‐rate distributions are consistent with regional tectonics, with a total off‐fault moment rate of 7.2×1018">7.2×1018 and 8.5×1018  N·m/year">8.5×1018  N⋅m/year for California and the WUS outside California, respectively.

A review of the rupture characteristics of the 2011 Tohoku-oki Mw 9.1 earthquake

NASA Astrophysics Data System (ADS)

Lay, Thorne

2018-05-01

The 2011 March 11 Tohoku-oki great (Mw 9.1) earthquake ruptured the plate boundary megathrust fault offshore of northern Honshu with estimates of shallow slip of 50 m and more near the trench. Non-uniform slip extended 220 km across the width and 400 km along strike of the subduction zone. Extensive data provided by regional networks of seismic and geodetic stations in Japan and global networks of broadband seismic stations, regional and global ocean bottom pressure sensors and sea level measurement stations, seafloor GPS/Acoustic displacement sites, repeated multi-channel reflection images, extensive coastal runup and inundation observations, and in situ sampling of the shallow fault zone materials and temperature perturbation, make the event the best-recorded and most extensively studied great earthquake to date. An effort is made here to identify the more robust attributes of the rupture as well as less well constrained, but likely features. Other issues involve the degree to which the rupture corresponded to geodetically-defined preceding slip-deficit regions, the influence of re-rupture of slip regions for large events in the past few centuries, and relationships of coseismic slip to precursory slow slip, foreshocks, aftershocks, afterslip, and relocking of the megathrust. Frictional properties associated with the slip heterogeneity and in situ measurements of frictional heating of the shallow fault zone support low stress during shallow sliding and near-total shear stress drop of 10-30 MPa in large-slip regions in the shallow megathrust. The roles of fault morphology, sediments, fluids, and dynamical processes in the rupture behavior continue to be examined; consensus has not yet been achieved. The possibility of secondary sources of tsunami excitation such as inelastic deformation of the sedimentary wedge or submarine slumping remains undemonstrated; dislocation models in an elastic continuum appear to sufficiently account for most mainshock observations, although afterslip and viscoelastic processes remain contested.

Links Between Earthquake Characteristics and Subducting Plate Heterogeneity in the 2016 Pedernales Ecuador Earthquake Rupture Zone

NASA Astrophysics Data System (ADS)

Bai, L.; Mori, J. J.

2016-12-01

The collision between the Indian and Eurasian plates formed the Himalayas, the largest orogenic belt on the Earth. The entire region accommodates shallow earthquakes, while intermediate-depth earthquakes are concentrated at the eastern and western Himalayan syntaxis. Here we investigate the focal depths, fault plane solutions, and source rupture process for three earthquake sequences, which are located at the western, central and eastern regions of the Himalayan orogenic belt. The Pamir-Hindu Kush region is located at the western Himalayan syntaxis and is characterized by extreme shortening of the upper crust and strong interaction of various layers of the lithosphere. Many shallow earthquakes occur on the Main Pamir Thrust at focal depths shallower than 20 km, while intermediate-deep earthquakes are mostly located below 75 km. Large intermediate-depth earthquakes occur frequently at the western Himalayan syntaxis about every 10 years on average. The 2015 Nepal earthquake is located in the central Himalayas. It is a typical megathrust earthquake that occurred on the shallow portion of the Main Himalayan Thrust (MHT). Many of the aftershocks are located above the MHT and illuminate faulting structures in the hanging wall with dip angles that are steeper than the MHT. These observations provide new constraints on the collision and uplift processes for the Himalaya orogenic belt. The Indo-Burma region is located south of the eastern Himalayan syntaxis, where the strike of the plate boundary suddenly changes from nearly east-west at the Himalayas to nearly north-south at the Burma Arc. The Burma arc subduction zone is a typical oblique plate convergence zone. The eastern boundary is the north-south striking dextral Sagaing fault, which hosts many shallow earthquakes with focal depth less than 25 km. In contrast, intermediate-depth earthquakes along the subduction zone reflect east-west trending reverse faulting.

Three-dimensional deformation caused by the Bam, Iran, earthquake and the origin of shallow slip deficit.

PubMed

Fialko, Yuri; Sandwell, David; Simons, Mark; Rosen, Paul

2005-05-19

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4-5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths (4-10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.

Origins and Driving Mechanisms for Shallow Methane Accumulations on the Svyatogor Ridge, Fram Strait

NASA Astrophysics Data System (ADS)

Waghorn, K. A.; Bunz, S.; Plaza-Faverola, A. A.; Westvig, I. M.; Johnson, J. E.

2015-12-01

The Svyatogor Ridge, located west of the Knipovich Spreading Ridge (KR) and south of the Molloy Transform Fault (MTF), is hypothesized to have once been the south tip of Vestnesa Ridge; a large sediment drift that was offset during the last 2 Ma along the MTF. The sedimentary cover across Svyatogor Ridge is limited, compared to Vestnesa Ridge, and basement outcrops are identified ~850 mbsf on the apex of the ridge. Despite the limited sedimentation, and its unique location at the intersection between the KR and MTF, Svyatogor Ridge has evidence of shallow gas accumulations; a strong BSR indicating a gas hydrate and underlying free gas system, and fluid flow pathways to the seafloor culminating in pockmarks. Using a high-resolution P-Cable 3D seismic survey, 2D seismic, and multibeam bathymetry data, we investigate how tectonic and sedimentary regimes have influenced the formation of a well-developed gas hydrate system. Sedimentation related with the Vestnesa drift on Svyatogor Ridge is interpreted to have begun ~2-3 Ma. The young age of the underlying oceanic crust, and subsequent synrift sediments below drift strata, suggests gas production from early Miocene aged hydrocarbon source identified in ODP Site 909 to the west, is unlikely in this region. Additionally, given the ultra-slow, magma limited spreading regime of the KR, we do not expect significant thermogenic methane generation from shallow magmatic sources. Therefore, in addition to some microbial gas production, Johnson et al. (2015) hypothesize a contribution from an abiotic source may explain the well-developed gas hydrate system. Large-scale basement faults identified in the seismic data are interpreted as detachment faults, which have exhumed relatively young ultramafic rocks. These detachment faults act as conduits for fluid flow, allowing circulation of seawater to drive serpentinization and subsequently act as pathways for fluids and abiotic methane to reach the shallow subsurface. This work aims to constrain the sedimentary and tectonic history of Svyatogor Ridge to determine 1) the relative interactions between basement detachment faults and overlying faults in the sedimentary cover, 2) the potential role of these faults as gas/fluid conduits and 3) how the underlying structural evolution has influenced the evolution of the gas hydrate system.

Moment magnitude, local magnitude and corner frequency of small earthquakes nucleating along a low angle normal fault in the Upper Tiber valley (Italy)

NASA Astrophysics Data System (ADS)

Munafo, I.; Malagnini, L.; Chiaraluce, L.; Valoroso, L.

2015-12-01

The relation between moment magnitude (MW) and local magnitude (ML) is still a debated issue (Bath, 1966, 1981; Ristau et al., 2003, 2005). Theoretical considerations and empirical observations show that, in the magnitude range between 3 and 5, MW and ML scale 1∶1. Whilst for smaller magnitudes this 1∶1 scaling breaks down (Bethmann et al. 2011). For accomplishing this task we analyzed the source parameters of about 1500 (30.000 waveforms) well-located small earthquakes occurred in the Upper Tiber Valley (Northern Apennines) in the range of -1.5≤ML≤3.8. In between these earthquakes there are 300 events repeatedly rupturing the same fault patch generally twice within a short time interval (less than 24 hours; Chiaraluce et al., 2007). We use high-resolution short period and broadband recordings acquired between 2010 and 2014 by 50 permanent seismic stations deployed to monitor the activity of a regional low angle normal fault (named Alto Tiberina fault, ATF) in the framework of The Alto Tiberina Near Fault Observatory project (TABOO; Chiaraluce et al., 2014). For this study the direct determination of MW for small earthquakes is essential but unfortunately the computation of MW for small earthquakes (MW < 3) is not a routine procedure in seismology. We apply the contributions of source, site, and crustal attenuation computed for this area in order to obtain precise spectral corrections to be used in the calculation of small earthquakes spectral plateaus. The aim of this analysis is to achieve moment magnitudes of small events through a procedure that uses our previously calibrated crustal attenuation parameters (geometrical spreading g(r), quality factor Q(f), and the residual parameter k) to correct for path effects. We determine the MW-ML relationships in two selected fault zones (on-fault and fault-hanging-wall) of the ATF by an orthogonal regression analysis providing a semi-automatic and robust procedure for moment magnitude determination within a region characterized by small to moderate seismicity. Finally, we present for a subset of data, corner frequency values computed by spectral analysis of S-waves, using data from three nearby shallow borehole stations sampled at 500 sps.

Acoustic stratigraphy of Bear Lake, Utah-Idaho: late Quaternary sedimentation patterns in a simple half-graben

USGS Publications Warehouse

Colman, Steven M.

2006-01-01

A 277-km network of high-resolution seismic-reflection profiles, supplemented with a sidescan-sonar mosaic of the lake floor, was collected in Bear Lake, Utah–Idaho, in order to explore the sedimentary framework of the lake's paleoclimate record. The acoustic stratigraphy is tied to a 120 m deep, continuously cored drill hole in the lake. Based on the age model for the drill core, the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka, although older sediments were locally imaged. The acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half-graben, with a steep normal-fault margin on the east and a flexural margin on the west. As expected for a basin controlled by a listric master fault, seismic reflections steepen and diverge toward the fault, bounding eastward-thickening sediment wedges. Secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age. Several faults cut the youngest sediments in the lake as well as the modern lake floor. The relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large (4 × 10 km) paleodelta of the Bear River. The delta overlies a horizon with an age of about 97 ka, outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments. A feature interpreted as a wave-cut bench occurs in many places on the western side of the lake. The base of this bench occurs at a depth (22–24 m) similar to that (20–25 m) of the distal surface of the paleodelta. Pinch-outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little Holocene sediment has accumulated in water depths of less than 30 m. On the steep eastern margin of the basin, sediments commonly onlap the hanging wall of the East Bear Lake Fault. However, no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20–25 m.

Structural Characterization of the Foliated-Layered Gabbro Transition in Wadi Tayin of the Samail Ophiolite, Oman; Oman Drilling Project Holes GT1A and GT2A

NASA Astrophysics Data System (ADS)

Deans, J. R.; Crispini, L.; Cheadle, M. J.; Harris, M.; Kelemen, P. B.; Teagle, D. A. H.; Matter, J. M.; Takazawa, E.; Coggon, J. A.

2017-12-01

Oman Drilling Project Holes GT1A and GT2A were drilled into the Wadi Tayin massif, Samail ophiolite and both recovered ca. 400 m of continuous core through a section of the layered gabbros and the foliated-layered gabbro transition. Hole GT1A is cut by a discrete fault system including localized thin ultracataclastic fault zones. Hole GT2A is cut by a wider zone of brittle deformation and incipient brecciation. Here we report the structural history of the gabbros reflecting formation at the ridge to later obduction. Magmatic and high temperature history- 1) Both cores exhibit a pervasive, commonly well-defined magmatic foliation delineated by plagioclase, olivine and in places clinopyroxene. Minor magmatic deformation is present. 2) The dip of the magmatic foliation varies cyclically, gradually changing dip by 30o from gentle to moderate over a 50 m wavelength. 3) Layering is present throughout both cores, is defined by changes in mode and grain size ranging in thickness from 2 cm to 3 m and is commonly sub-parallel to the foliation. 4) There are no high temperature crystal-plastic shear zones in the core. Key observations include: no simple, systematic shallowing of dip with depth across the foliated-layered gabbro transition and layering is continuous across this transition. Cyclic variation of magmatic foliation dip most likely reflects the process of plate separation at the ridge axis. Near-axis faulting- i) On or near-axis structures consist of epidote-amphibole bearing hydraulic breccias and some zones of intense cataclasis with intensely deformed epidote and seams of clay and chlorite accompanied by syntectonic alteration of the wall rock. Early veins are filled with amphibole, chlorite, epidote, and anhydrite. ii) The deformation ranges from brittle-ductile, causing local deflection of the magmatic foliation, to brittle offset of the foliation and core and mantle structures in anhydrite veins. iii) The prevalent sense of shear is normal and slickenfibers indicate oblique offset. Obduction related faulting- i) Low temperature brittle faults and veins with laumontite, clay, and gypsum crosscut all structures. ii) Faults show a reverse sense of shear and crosscut, possibly reactivate, normal faults. Our observations suggest formation of reverse faults and late veins during obduction of the ophiolite.

Faulting type classification of small earthquakes using a template approach and their hypocenter relocation along the Japan and Kuril trenches

NASA Astrophysics Data System (ADS)

Nakamura, W.; Uchida, N.; Matsuzawa, T.

2013-12-01

After the 2011 Tohoku-oki earthquake, the number of interplate earthquakes off Miyagi was dramatically decreased (e.g., Asano et al., 2011), while many normal faulting earthquakes occurred in the outer trench region (e.g., Obana et al., 2012). To understand the meaning of the seismicity change caused by the huge earthquake, it is essential to know faulting types of small offshore earthquakes which cannot be determined using conventional methods. In this study, we developed a method to classify focal mechanisms of small earthquakes by using template events whose focal mechanisms were known. Here, we made pairs of earthquakes with inter-event distances of less than 20 km and difference in magnitude of less than 1.0, and calculated their waveform cross-correlation coefficients (CCs) in 1.5 and 5.0 sec windows for P and S waves, respectively. We first calculated 3D minimum rotation angle (Kagan's angle; Kagan, 1991) for pairs whose focal mechanisms were listed in the F-net catalogue, to examine the relationships among the Kagan's angles, CCs and inter-event distances. The CCs decrease with increasing inter-event distances and Kagan's angles. We set a CC threshold of 0.8 for Tohoku (to the south of 40° N), and 0.7 for Hokkaido (to the north of 40° N) regions to judge whether the two events have the same focal mechanisms. This is because more than 90% of event pairs whose CCs are greater than the thresholds show Kagan's angles of less than 30° when we calculated them for the mechanism-known earthquakes (templates). In total, 4012 earthquakes from 2003 to 2012 are newly classified and 60% and 30% of them are of interplate and normal faulting types, respectively. In the area of large coseismic slip of the 2011 Tohoku-oki earthquake, we found no interplate earthquakes after the main shock, while many interplate earthquakes occurred around the M9 coseismic slip area. We also found many normal faulting earthquakes near the trench after the 2011 main shock. Along the Kuril trench, many interplate earthquakes occurred as aftershocks of the 2003 Tokachi-oki earthquake (M8.0). To verify the validity of the results and to examine the detail of the focal mechanism distribution, we relocated hypocenters by tomoFDD code (Zhang and Thurber. 2006) using a 3D velocity structure. Most of interplate-type earthquakes were located near the plate boundary except in the near trench-region, suggesting the correctness of mechanism and earthquake location. The hypocenters of normal faulting events that occurred after the 2011 Tohoku-oki earthquake off Miyagi were relocated within 20km from the surface of the Pacific plate. This result suggests the normal faulting event in the incoming Pacific plate occurred in a shallower part of the plate as suggested from OBS data analyses. Normal faulting earthquakes off Miyagi occurred not only in the outer trench region but also above the plate boundary near the coast. The focal mechanism classification method developed in the present study using waveform cross-correlations increases the number of classified earthquakes that show the temporal changes in the interplate coupling and stress field around the plate boundary.

Metasomatism, Fluid Overpressure and Brecciation at the Slab-Mantle Interface: Insights from the Livingstone Fault, New Zealand

NASA Astrophysics Data System (ADS)

Tarling, M.; Smith, S. A. F.; Scott, J.

2017-12-01

Juxtaposition of mantle peridotite and serpentinite against quartzofeldspathic and mafic schists occurs along the shallow slab-mantle interface in some subduction zones. This part of the subduction interface has been invoked as a possible source region of episodic tremor and slow slip, yet geological observations of fault zone structures and chemical reactions pertinent to this region are quite rare. The >1000 km long Livingstone Fault in New Zealand is a superbly exposed fault zone that provides a suitable analogue (both in terms of scale and the rock types involved) for the shallow slab-mantle interface. The fault is characterized by a foliated and highly sheared serpentinite mélange tens to several hundreds of meters wide that separates (partially serpentinised) peridotites from quartzofeldspathic schists. Talc- and tremolite-forming metasomatic reactions occurred along the margins of the mélange and around entrained pods due to mixing of serpentinite with silica- and calcium-rich fluids derived from the adjacent quartzofeldspathic schist. The metasomatic reactions generated significant volumes of water at the melange-schist contact that became trapped between the two relatively impermeable fault zone lithologies. On the schist side of the contact, brittle faulting was promoted by the formation of a laterally-continuous silicified zone up to tens of metres wide. On the melange side, a zone up to tens of metres wide of `crackle-breccias' containing veined stockworks of tremolite indicates periodic increases of pore pressure sufficient to cause hydraulic fracture of serpentinite. The crackle-breccias are multi-generational indicating that this process was episodic. Sr and Nd isotope data and permeability calculations suggest that the episodic brecciation process was critical to the transfer of fluids across the melange. Our observations suggest that fluid-producing metasomatic reactions along the shallow slab-mantle interface may contribute to the tremor signal by triggering brecciation events and promoting brittle failure in serpentinite and schist.

Structural Features of the Western Taiwan Foreland Basin in the Eastern Taiwan Strait since Late Miocene

NASA Astrophysics Data System (ADS)

WANG, J. H.; Liu, C. S.; Chang, J. H.; Yang, E. Y.

2017-12-01

The western Taiwan Foreland Basin lies on the eastern part of Taiwan Strait. The structures in this region are dominated by crustal stretch and a series of flexural normal faults have been developed since Late Miocene owing to the flexural of Eurasia Plate. Through deciphering multi-channel seismic data and drilling data, these flexural features are observed in the offshore Changhua coastal area. The flexure normal faults are important features to realize structural activity in the western Taiwan Foreland Basin. Yang et al. (2016) mention that the reactivated normal faults are found north of the Zhushuixi estuary. It should be a significant issue to decipher whether these faults are still active. In this study, we have analyzed all the available seismic reflections profiles in the central part of the Taiwan Strait, and have observed many pre-Pliocene normal faults that are mainly distributed in the middle of the Taiwan Strait to Changyun Rise, and we tentatively suggest that the formation of these faults may be associated with the formation of the foreland basal unconformity. Furthermore, we will map the distribution of these normal faults and examine whether the reactivated normal faults have extended to south of the Zhushuixi estuary. Finally, we discuss the relation between the reactivated normal faults in the Taiwan Strait and those faults onshore. Key words: Multichannel seismic reflection profile, Taiwan Strait, Foreland basin, normal fault.

Migrating tremors illuminate complex deformation beneath the seismogenic San Andreas fault

USGS Publications Warehouse

Shelly, David R.

2010-01-01

The San Andreas fault is one of the most extensively studied faults in the world, yet its physical character and deformation mode beneath the relatively shallow earthquake-generating portion remain largely unconstrained. Tectonic ‘non-volcanic’ tremor, a recently discovered seismic signal probably generated by shear slip on the deep extension of some major faults, can provide new insight into the deep fate of such faults, including that of the San Andreas fault near Parkfield, California. Here I examine continuous seismic data from mid-2001 to 2008, identifying tremor and decomposing the signal into different families of activity based on the shape and timing of the waveforms at multiple stations. This approach allows differentiation between activities from nearby patches of the deep fault and begins to unveil rich and complex patterns of tremor occurrence. I find that tremor exhibits nearly continuous migration, with the most extensive episodes propagating more than 20 kilometres along fault strike at rates of 15–80 kilometres per hour. This suggests that the San Andreas fault remains a localized through-going structure, at least to the base of the crust, in this area. Tremor rates and recurrence behaviour changed markedly in the wake of the 2004 magnitude-6.0 Parkfield earthquake, but these changes were far from uniform within the tremor zone, probably reflecting heterogeneous fault properties and static and dynamic stresses decaying away from the rupture. The systematic recurrence of tremor demonstrated here suggests the potential to monitor detailed time-varying deformation on this portion of the deep San Andreas fault, deformation which unsteadily loads the shallower zone that last ruptured in the 1857 magnitude-7.9 Fort Tejon earthquake.

Interpretation of shallow crustal structure of the Imperial Valley, California, from seismic reflection profiles

DOE Office of Scientific and Technical Information (OSTI.GOV)

Severson, L.K.

1987-05-01

Eight seismic reflection profiles (285 km total length) from the Imperial Valley, California, were provided to CALCRUST for reprocessing and interpretation. Two profiles were located along the western margin of the valley, five profiles were situated along the eastern margin and one traversed the deepest portion of the basin. These data reveal that the central basin contains a wedge of highly faulted sediments that thins to the east. Most of the faulting is strike-slip but there is evidence for block rotations on the scale of 5 to 10 kilometers within the Brawley Seismic Zone. These lines provide insight into themore » nature of the east and west edges of the Imperial Valley. The basement at the northwestern margin of the valley, to the north of the Superstition Hills, has been normal-faulted and blocks of basement material have ''calved'' into the trough. A blanket of sediments has been deposited on this margin. To the south of the Superstition Hills and Superstition Mountain, the top of the basement is a detachment surface that dips gently into the basin. This margin is also covered by a thick sequence sediments. The basement of the eastern margin consists of metamorphic rocks of the upper plate of the Chocolate Mountain Thrust system underlain by the Orocopia Schist. These rocks dip to the southeast and extend westward to the Sand Hills Fault but do not appear to cross it. Thus, the Sand Hills Fault is interpreted to be the southern extension of the San Andreas Fault. North of the Sand Hills Fault the East Highline Canal seismicity lineament is associated with a strike-slip fault and is probably linked to the Sand Hills Fault. Six geothermal areas crossed by these lines, in agreement with previous studies of geothermal reservoirs, are associated with ''faded'' zones, Bouguer gravity and heat flow maxima, and with higher seismic velocities than surrounding terranes.« less

Aseismic Deformation Associated with an Earthquake Swarm in the Northern Apennines (Italy)

NASA Astrophysics Data System (ADS)

Gualandi, A.; Nichele, C.; Serpelloni, E.; Chiaraluce, L.; Anderlini, L.; Latorre, D.; Belardinelli, M. E.; Avouac, J. P.

2017-12-01

Analyzing the displacement time series from continuous GPS (cGPS) with an Independent Component Analysis (ICA) we detect a transient deformation signal that correlates both in space and time with a seismic swarm activity (maximum Mw = 3.69 ± 0.09) occurred in the hanging wall of the Altotiberina normal fault (Northern Apennines, Italy) in 2013-2014. The geodetic transient lasted ˜6 months and produced a NW-SE trending extension of ˜ 5.3 mm, consistent with the regional tectonic regime. The seismicity and the geodetic signal are consistent with slip on two splay faults in the ATF hanging wall. Comparing the seismic moment associated with the geodetic transient and the seismic events, we observe that seismicity accounts for only a fraction of the measured geodetic deformation. The combined seismic and aseismic slip decreased the Coulomb stress on the locked shallow portion of the ATF, while the transition region to the creeping section has been loaded.

Vein deposits hosted by plutonic rocks in the Croesus Stock and Hailey gold belt mineralized areas, Blaine County, Idaho

USGS Publications Warehouse

Worl, Ronald G.; Lewis, Reed S.

2001-01-01

Mineral deposits in the Croesus and Hailey gold belt mineralized areas in Blaine County, south-central Idaho, are preciousand base-metal quartz veins that are part of a family of vein deposits spatially and temporally associated with the Idaho batholith. Historic production from these veins has been mainly gold and silver. Host rocks are older border phase plutons of the Idaho batholith that are characterized by more potassium and less sodium as compared to rocks from the main body of the batholith to the west. Host structures are reverse faults that have moderate to shallow dips to the northeast and high-angle normal faults that also strike northwest. The veins are characterized by several generations of quartz and generally sparse sulfide minerals; gold is associated with late-stage comb quartz. The precious-metal ore bodies are in a series of shoots, each of which is as much as 8 ft in width, 400 ft in breadth, and 1,000 ft in pitch length.

Application of ERTS images and image processing to regional geologic problems and geologic mapping in northern Arizona

NASA Technical Reports Server (NTRS)

Goetz, A. F. H. (Principal Investigator); Billingsley, F. C.; Gillespie, A. R.; Abrams, M. J.; Squires, R. L.; Shoemaker, E. M.; Lucchitta, I.; Elston, D. P.

1975-01-01

The author has identified the following significant results. Computer image processing was shown to be both valuable and necessary in the extraction of the proper subset of the 200 million bits of information in an ERTS image to be applied to a specific problem. Spectral reflectivity information obtained from the four MSS bands can be correlated with in situ spectral reflectance measurements after path radiance effects have been removed and a proper normalization has been made. A detailed map of the major fault systems in a 90,000 sq km area in northern Arizona was compiled from high altitude photographs and pre-existing published and unpublished map data. With the use of ERTS images, three major fault systems, the Sinyala, Bright Angel, and Mesa Butte, were identified and their full extent measured. A byproduct of the regional studies was the identification of possible sources of shallow ground water, a scarce commodity in these regions.

Early warning of freshwater salinization due to upward brine displacement by species transport simulations combined with a hydrochemical genesis model

NASA Astrophysics Data System (ADS)

Langer, Maria; Kühn, Michael

2016-04-01

Shallow groundwater resources could be possibly affected by intruding brines, which are displaced along hydraulically conductive faults as result of subsurface activities like CO2 injection. To avoid salinization of potable freshwater aquifers an early detection of intruding saline water is necessary, especially in regions where an initial geogenic salinization already exists. Our study is based on work of Tillner et al. [1] and Langer et al. [2] who investigated the influence of permeable fault systems on brine displacement for the prospective storage site Beeskow-Birkholz in the Northeast German Basin. With a 3D regional scale model considering the deep groundwater system, they demonstrated that the existence of hydraulically conductive faults is not necessarily an exclusion criterion for potential injection sites, because salinization of shallower aquifers strongly depends on the effective damage zone volume, the initial salinity distribution and overlying reservoirs [2], while permeability of fault zones does not influence salinization of shallower aquifers significantly [1]. Here we extracted a 2D cross section regarding the upper 220 m of the study area mainly represented by shallow freshwater aquifers, but also considering an initial geogenic salinization [3]. We took flow rates of the intruding brines from the previous studies [2] and implemented species transport simulations with the program code SHEMAT [4]. Results are investigated and interpreted with the hydrochemical genesis model GEBAH [5] which has been already applied as early warning of saltwater intrusions into freshwater aquifers and surface water [6]. GEBAH allows a categorization of groundwater by the ion ratios of the dissolved components and offers a first indicative determination for an existence and the intensity of saline water intrusion in shallow groundwater aquifer, independent of the concentration of the solution. With our model we investigated the migration of saline water through a fault or an erosional channel which both allows an exchange between the shallow freshwater and the deeper saline water complex. The salinization potential of a drinking water well in vicinity to the brine source was determined for different scenarios. [1] Tillner E., Kempka T., Nakaten B., Kühn M. (2013) Brine migration through fault zones: 3D numerical simulations for a prospective CO2 storage site in Northeast Germany. International Journal of Greenhouse Gas Control 19, 689-703. doi: 10.1016/ j.ijggc.2013.03.012 [2] Langer M., Tillner E., Kempka T., Kühn M. (2015) Effective damage zone volume of fault zones and initial salinity distribution determine intensity of shallow aquifer salinization in geological underground utilization. Hydrology and Earth System Sciences Discussion, 12, 5703-5748. doi: 10.5194/hessd-12-5703-2015 [3] Hotzan, G., and Voss, T. (2013): Complex hydrogeochemic-genetic mapping for evaluation of the endangerment of pleistocene and tertiary aquifers by saline waters in the region Storkow-Frankfurt (Oder)-Eisenhüttenstadt. Brandenburgische Geowissenschaftliche Beiträge, 20 (1/2), 62-82. (in German) [4] Clauser C. (2003) SHEMAT and Processing SHEMAT - Numerical simulation of reactive flow in hot aquifers, Springer Publishers, Heidelberg [5] Rechlin, B., Hoffknecht, A., Scholz, H., Helms, A. (2010): Genetic evaluation of analyses from the hydrosphere. Software GEBAH Vers. 1.1 LBGR/GCI, Cottbus, Königs Wusterhausen (in German) [6] Rechlin, B. (2008): A method for a concentration free early detection of saltwater intrusions into freshwater aquifers and surface water. Brandenburgische Geowissenschaftliche Beiträge, 15 (1/2), 57-68. (in German)

Maturity of nearby faults influences seismic hazard from hydraulic fracturing.

PubMed

Kozłowska, Maria; Brudzinski, Michael R; Friberg, Paul; Skoumal, Robert J; Baxter, Nicholas D; Currie, Brian S

2018-02-20

Understanding the causes of human-induced earthquakes is paramount to reducing societal risk. We investigated five cases of seismicity associated with hydraulic fracturing (HF) in Ohio since 2013 that, because of their isolation from other injection activities, provide an ideal setting for studying the relations between high-pressure injection and earthquakes. Our analysis revealed two distinct groups: ( i ) deeper earthquakes in the Precambrian basement, with larger magnitudes (M > 2), b-values < 1, and many post-shut-in earthquakes, versus ( ii ) shallower earthquakes in Paleozoic rocks ∼400 m below HF, with smaller magnitudes (M < 1), b-values > 1.5, and few post-shut-in earthquakes. Based on geologic history, laboratory experiments, and fault modeling, we interpret the deep seismicity as slip on more mature faults in older crystalline rocks and the shallow seismicity as slip on immature faults in younger sedimentary rocks. This suggests that HF inducing deeper seismicity may pose higher seismic hazards. Wells inducing deeper seismicity produced more water than wells with shallow seismicity, indicating more extensive hydrologic connections outside the target formation, consistent with pore pressure diffusion influencing seismicity. However, for both groups, the 2 to 3 h between onset of HF and seismicity is too short for typical fluid pressure diffusion rates across distances of ∼1 km and argues for poroelastic stress transfer also having a primary influence on seismicity.

Maturity of nearby faults influences seismic hazard from hydraulic fracturing

NASA Astrophysics Data System (ADS)

Kozłowska, Maria; Brudzinski, Michael R.; Friberg, Paul; Skoumal, Robert J.; Baxter, Nicholas D.; Currie, Brian S.

2018-02-01

Understanding the causes of human-induced earthquakes is paramount to reducing societal risk. We investigated five cases of seismicity associated with hydraulic fracturing (HF) in Ohio since 2013 that, because of their isolation from other injection activities, provide an ideal setting for studying the relations between high-pressure injection and earthquakes. Our analysis revealed two distinct groups: (i) deeper earthquakes in the Precambrian basement, with larger magnitudes (M > 2), b-values < 1, and many post–shut-in earthquakes, versus (ii) shallower earthquakes in Paleozoic rocks ˜400 m below HF, with smaller magnitudes (M < 1), b-values > 1.5, and few post–shut-in earthquakes. Based on geologic history, laboratory experiments, and fault modeling, we interpret the deep seismicity as slip on more mature faults in older crystalline rocks and the shallow seismicity as slip on immature faults in younger sedimentary rocks. This suggests that HF inducing deeper seismicity may pose higher seismic hazards. Wells inducing deeper seismicity produced more water than wells with shallow seismicity, indicating more extensive hydrologic connections outside the target formation, consistent with pore pressure diffusion influencing seismicity. However, for both groups, the 2 to 3 h between onset of HF and seismicity is too short for typical fluid pressure diffusion rates across distances of ˜1 km and argues for poroelastic stress transfer also having a primary influence on seismicity.

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

NASA Astrophysics Data System (ADS)

Hüpers, Andre; Torres, Marta E.; Owari, Satoko; McNeill, Lisa C.; Dugan, Brandon; Henstock, Timothy J.; Milliken, Kitty L.; Petronotis, Katerina E.; Backman, Jan; Bourlange, Sylvain; Chemale, Farid; Chen, Wenhuang; Colson, Tobias A.; Frederik, Marina C. G.; Guèrin, Gilles; Hamahashi, Mari; House, Brian M.; Jeppson, Tamara N.; Kachovich, Sarah; Kenigsberg, Abby R.; Kuranaga, Mebae; Kutterolf, Steffen; Mitchison, Freya L.; Mukoyoshi, Hideki; Nair, Nisha; Pickering, Kevin T.; Pouderoux, Hugo F. A.; Shan, Yehua; Song, Insun; Vannucchi, Paola; Vrolijk, Peter J.; Yang, Tao; Zhao, Xixi

2017-05-01

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records.

A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt

NASA Astrophysics Data System (ADS)

Ezzelarab, Mohamed; Ebraheem, Mohamed O.; Zahradník, Jiří

2018-03-01

The Mw 4.3 earthquake of September 2015 is the first felt earthquake since 1900 A.D in the northern part of the Western Desert, Egypt, south of the El-Alamein City. The available waveform data observed at epicentral distances 52-391 km was collected and carefully evaluated. Nine broad-band stations were selected to invert full waveforms for the centroid position (horizontal and vertical) and for the focal mechanism solution. The first-arrival travel times, polarities and low-frequency full waveforms (0.03-0.08 Hz) are consistently explained in this paper as caused by a shallow source of the strike-slip mechanism. This finding indicates causal relation of this earthquake to the W-E trending South El-Alamein fault, which developed in Late Cretaceous as dextral strike slip fault. Recent activity of this fault, proven by the studied rare earthquake, is of fundamental importance for future seismic hazard evaluations, underlined by proximity (∼65 km) of the source zone to the first nuclear power plant planned site in Egypt. Safe exploration and possible future exploitation of hydrocarbon reserves, reported around El-Alamein fault in the last decade, cannot be made without considering the seismic potential of this fault.

Do scaly clays control seismicity on faulted shale rocks?

NASA Astrophysics Data System (ADS)

Orellana, Luis Felipe; Scuderi, Marco M.; Collettini, Cristiano; Violay, Marie

2018-04-01

One of the major challenges regarding the disposal of radioactive waste in geological formations is to ensure isolation of radioactive contamination from the environment and the population. Shales are suitable candidates as geological barriers. However, the presence of tectonic faults within clay formations put the long-term safety of geological repositories into question. In this study, we carry out frictional experiments on intact samples of Opalinus Clay, i.e. the host rock for nuclear waste storage in Switzerland. We report experimental evidence suggesting that scaly clays form at low normal stress (≤20 MPa), at sub-seismic velocities (≤300 μm/s) and is related to pre-existing bedding planes with an ongoing process where frictional sliding is the controlling deformation mechanism. We have found that scaly clays show a velocity-weakening and -strengthening behaviour, low frictional strength, and poor re-strengthening over time, conditions required to allow the potential nucleation and propagation of earthquakes within the scaly clays portion of the formation. The strong similarities between the microstructures of natural and experimental scaly clays suggest important implications for the slip behaviour of shallow faults in shales. If natural and anthropogenic perturbations modify the stress conditions of the fault zone, earthquakes might have the potential to nucleate within zones of scaly clays controlling the seismicity of the clay-rich tectonic system, thus, potentially compromising the long-term safeness of geological repositories situated in shales.

Submarine gas seepage in a mixed contractional and shear deformation regime: Cases from the Hikurangi oblique-subduction margin

NASA Astrophysics Data System (ADS)

Plaza-Faverola, Andreia; Pecher, Ingo; Crutchley, Gareth; Barnes, Philip M.; Bünz, Stefan; Golding, Thomas; Klaeschen, Dirk; Papenberg, Cord; Bialas, Joerg

2014-02-01

Gas seepage from marine sediments has implications for understanding feedbacks between the global carbon reservoir, seabed ecology, and climate change. Although the relationship between hydrates, gas chimneys, and seafloor seepage is well established, the nature of fluid sources and plumbing mechanisms controlling fluid escape into the hydrate zone and up to the seafloor remain one of the least understood components of fluid migration systems. In this study, we present the analysis of new three-dimensional high-resolution seismic data acquired to investigate fluid migration systems sustaining active seafloor seepage at Omakere Ridge, on the Hikurangi subduction margin, New Zealand. The analysis reveals at high resolution, complex overprinting fault structures (i.e., protothrusts, normal faults from flexural extension, and shallow (<1 km) arrays of oblique shear structures) implicated in fluid migration within the gas hydrate stability zone in an area of 2 × 7 km. In addition to fluid migration systems sustaining seafloor seepage on both sides of a central thrust fault, the data show seismic evidence for subseafloor gas-rich fluid accumulation associated with proto-thrusts and extensional faults. In these latter systems fluid pressure dissipation through time has been favored, hindering the development of gas chimneys. We discuss the elements of the distinct fluid migration systems and the influence that a complex partitioning of stress may have on the evolution of fluid flow systems in active subduction margins.

Fluid involvement in normal faulting

NASA Astrophysics Data System (ADS)

Sibson, Richard H.

2000-04-01

Evidence of fluid interaction with normal faults comes from their varied role as flow barriers or conduits in hydrocarbon basins and as hosting structures for hydrothermal mineralisation, and from fault-rock assemblages in exhumed footwalls of steep active normal faults and metamorphic core complexes. These last suggest involvement of predominantly aqueous fluids over a broad depth range, with implications for fault shear resistance and the mechanics of normal fault reactivation. A general downwards progression in fault rock assemblages (high-level breccia-gouge (often clay-rich) → cataclasites → phyllonites → mylonite → mylonitic gneiss with the onset of greenschist phyllonites occurring near the base of the seismogenic crust) is inferred for normal fault zones developed in quartzo-feldspathic continental crust. Fluid inclusion studies in hydrothermal veining from some footwall assemblages suggest a transition from hydrostatic to suprahydrostatic fluid pressures over the depth range 3-5 km, with some evidence for near-lithostatic to hydrostatic pressure cycling towards the base of the seismogenic zone in the phyllonitic assemblages. Development of fault-fracture meshes through mixed-mode brittle failure in rock-masses with strong competence layering is promoted by low effective stress in the absence of thoroughgoing cohesionless faults that are favourably oriented for reactivation. Meshes may develop around normal faults in the near-surface under hydrostatic fluid pressures to depths determined by rock tensile strength, and at greater depths in overpressured portions of normal fault zones and at stress heterogeneities, especially dilational jogs. Overpressures localised within developing normal fault zones also determine the extent to which they may reutilise existing discontinuities (for example, low-angle thrust faults). Brittle failure mode plots demonstrate that reactivation of existing low-angle faults under vertical σ1 trajectories is only likely if fluid overpressures are localised within the fault zone and the surrounding rock retains significant tensile strength. Migrating pore fluids interact both statically and dynamically with normal faults. Static effects include consideration of the relative permeability of the faults with respect to the country rock, and juxtaposition effects which determine whether a fault is transmissive to flow or acts as an impermeable barrier. Strong directional permeability is expected in the subhorizontal σ2 direction parallel to intersections between minor faults, extension fractures, and stylolites. Three dynamic mechanisms tied to the seismic stress cycle may contribute to fluid redistribution: (i) cycling of mean stress coupled to shear stress, sometimes leading to postfailure expulsion of fluid from vertical fractures; (ii) suction pump action at dilational fault jogs; and, (iii) fault-valve action when a normal fault transects a seal capping either uniformly overpressured crust or overpressures localised to the immediate vicinity of the fault zone at depth. The combination of σ2 directional permeability with fluid redistribution from mean stress cycling may lead to hydraulic communication along strike, contributing to the protracted earthquake sequences that characterise normal fault systems.

Shallow Aquifer Vulnerability From Subsurface Fluid Injection at a Proposed Shale Gas Hydraulic Fracturing Site

NASA Astrophysics Data System (ADS)

Wilson, M. P.; Worrall, F.; Davies, R. J.; Hart, A.

2017-11-01

Groundwater flow resulting from a proposed hydraulic fracturing (fracking) operation was numerically modeled using 91 scenarios. Scenarios were chosen to be a combination of hydrogeological factors that a priori would control the long-term migration of fracking fluids to the shallow subsurface. These factors were induced fracture extent, cross-basin groundwater flow, deep low hydraulic conductivity strata, deep high hydraulic conductivity strata, fault hydraulic conductivity, and overpressure. The study considered the Bowland Basin, northwest England, with fracking of the Bowland Shale at ˜2,000 m depth and the shallow aquifer being the Sherwood Sandstone at ˜300-500 m depth. Of the 91 scenarios, 73 scenarios resulted in tracked particles not reaching the shallow aquifer within 10,000 years and 18 resulted in travel times less than 10,000 years. Four factors proved to have a statistically significant impact on reducing travel time to the aquifer: increased induced fracture extent, absence of deep high hydraulic conductivity strata, relatively low fault hydraulic conductivity, and magnitude of overpressure. Modeling suggests that high hydraulic conductivity formations can be more effective barriers to vertical flow than low hydraulic conductivity formations. Furthermore, low hydraulic conductivity faults can result in subsurface pressure compartmentalization, reducing horizontal groundwater flow, and encouraging vertical fluid migration. The modeled worst-case scenario, using unlikely geology and induced fracture lengths, maximum values for strata hydraulic conductivity and with conservative tracer behavior had a particle travel time of 130 years to the base of the shallow aquifer. This study has identified hydrogeological factors which lead to aquifer vulnerability from shale exploitation.

Tectonic aspects of the guatemala earthquake of 4 february 1976.

PubMed

Plafker, G

1976-09-24

The locations of surface ruptures and the main shock epicenter indicate that the disastrous Guatemala earthquake of 4 February 1976 was tectonic in origin and generated mainly by slip on the Motagua fault, which has an arcuate roughly east-west trend across central Guatemala. Fault breakage was observed for 230 km. Displacement is predominantly horizontal and sinistral with a maximum measured offset of 340 cm and an average of about 100 cm. Secondary fault breaks trending roughly north-northeast to south-southwest have been found in a zone about 20 km long and 8 km wide extending from the western suburbs of Guatemala City to near Mixco, and similar faults with more subtle surface expression probably occur elsewhere in the Guatemalan Highlands. Displacements on the secondary faults are predominantly extensional and dip-slip, with as much as 15 cm vertical offset on a single fracture. The primary fault that broke during the earthquake involved roughly 10 percent of the length of the great transform fault system that defines the boundary between the Caribbean and North American plates. The observed sinistral displacement is striking confirmation of deductions regarding the late Cenozoic relative motion between these two crustal plates that were based largely on indirect geologic and geophysical evidence. The earthquake-related secondary faulting, together with the complex pattern of geologically young normal faults that occur in the Guatemalan Highlands and elsewhere in western Central America, suggest that the eastern wedge-shaped part of the Caribbean plate, roughly between the Motagua fault system and the volcanic arc, is being pulled apart in tension and left behind as the main mass of the plate moves relatively eastward. Because of their proximity to areas of high population density, shallow-focus earthquakes that originate on the Motagua fault system, on the system of predominantly extensional faults within the western part of the Caribbean plate, and in association with volcanism may pose a more serious seismic hazard than the more numerous (but generally more distant) earthquakes that are generated in the eastward-dipping subduction zone beneath Middle America.

10Be Erosion Rates Controlled by Normal Fault Slip Rates and Transient Incision

NASA Astrophysics Data System (ADS)

Roda-Boluda, D. C.; D'Arcy, M. K.; Whittaker, A. C.; Allen, P.; Gheorghiu, D. M.; Rodés, Á.

2016-12-01

Quantifying erosion rates, and how they compare to rock uplift rates, is fundamental for understanding the evolution of relief and the associated sediment supply from mountains to basins. The trade-off between uplift and erosion is well-represented by river incision, which is often accompanied by hillslope steepening and landsliding. However, characterizing the relation between these processes and the impact that these have on sediment delivered to basins, remains a major challenge in many tectonically-active areas. We use Southern Italy as a natural laboratory to address these questions, and quantify the interplay of tectonics, geomorphic response and sediment export. We present 15 new 10Be catchment-averaged erosion rates, collected from catchments along five active normal faults with excellent slip rate constraints. We find that erosion rates are strongly controlled by fault slip rates and the degree of catchment incision. Our data suggests that overall 70% of the rock uplifted by the faults is being eroded, offering new insights into the topographic balance of uplift and erosion in this area. None of the erosion rates are greater than local fault slip rates, so fault activity is effectively establishing an upper limit on erosion. However, eight 10Be samples from low relief, unincised areas within the catchments, collected above knickpoints, yield consistent erosion rates of 0.12 mm/yr. In contrast, samples collected below knickpoints and below the incised sectors of the channels, have erosion rates of 0.2-0.8 mm/yr. The comparison allows us to quantify the impact that transient incisional response has on erosion rates. We show that incision is associated with frequent, shallow landsliding, and we find that the volumes of landslides stored on the catchments are highly correlated with 10Be-derived sediment flux estimates, suggesting that landslides are likely to be a major contributor to sediment fluxes; and we examine the implications that this may have on 10Be concentrations. Finally, we examine the influence that these coupled landscape responses have on the sediment exported from the catchments, and we find that coarser grain size export is associated with deeper channel incision and greater 10Be-derived sediment fluxes.

Semi-automated fault system extraction and displacement analysis of an excavated oyster reef using high-resolution laser scanned data

NASA Astrophysics Data System (ADS)

Molnár, Gábor; Székely, Balázs; Harzhauser, Mathias; Djuricic, Ana; Mandic, Oleg; Dorninger, Peter; Nothegger, Clemens; Exner, Ulrike; Pfeifer, Norbert

2015-04-01

In this contribution we present a semi-automated method for reconstructing the brittle deformation field of an excavated Miocene oyster reef, in Stetten, Korneuburg Basin, Lower Austria. Oyster shells up to 80 cm in size were scattered in a shallow estuarine bay forming a continuous and almost isochronous layer as a consequence of a catastrophic event in the Miocene. This shell bed was preserved by burial of several hundred meters of sandy to silty sediments. Later the layers were tilted westward, uplifted and erosion almost exhumed them. An excavation revealed a 27 by 17 meters area of the oyster covered layer. During the tectonic processes the sediment volume suffered brittle deformation. Faults mostly with some centimeter normal component and NW-SE striking affected the oyster covered volume, dissecting many shells and the surrounding matrix as well. Faults and displacements due to them can be traced along the site typically at several meters long, and as fossil oysters are broken and parts are displaced due to the faulting, along some faults it is possible to follow these displacements in 3D. In order to quantify these varying displacements and to map the undulating fault traces high-resolution scanning of the excavated and cleaned surface of the oyster bed has been carried out using a terrestrial laser scanner. The resulting point clouds have been co-georeferenced at mm accuracy and a 1mm resolution 3D point cloud of the surface has been created. As the faults are well-represented in the point cloud, this enables us to measure the dislocations of the dissected shell parts along the fault lines. We used a semi-automatic method to quantify these dislocations. First we manually digitized the fault lines in 2D as an initial model. In the next step we estimated the vertical (i.e. perpendicular to the layer) component of the dislocation along these fault lines comparing the elevations on two sides of the faults with moving averaging windows. To estimate the strike-slip dislocation component, the surface points of the dissected shells on both sides of the fault planes were compared and displacement vectors were derived. The exact orientation of the fault planes cannot be accurately extracted automatically, so the distinction between normal and reverse fault is difficult. This makes the third component of the dislocation to be estimated inaccurately. These derived dislocation values are regarded as components of the dislocation vectors and were transformed back to the real world spatial coordinate system. Interpolating these dislocation vectors along fault lines we calculated and visualized the deformation field along the whole surface of the oyster reef. Although this deformation field is only a 2D section of the real 3D deformation field, its elaboration reveals the spatial variability of the deformation according to sediment inhomogeneity. The project is supported by the Austrian Science Fund (FWF P 25883-N29).

Seismic imaging of the Main Frontal Thrust in Nepal reveals a shallow décollement and blind thrusting

NASA Astrophysics Data System (ADS)

Almeida, Rafael V.; Hubbard, Judith; Liberty, Lee; Foster, Anna; Sapkota, Soma Nath

2018-07-01

Because great earthquakes in the Himalaya have an average recurrence interval exceeding 500 yr, most of what we know about past earthquakes comes from paleoseismology and tectonic geomorphology studies of the youngest fault system there, the Main Frontal Thrust (MFT). However, these data are sparse relative to fault segmentation and length, and interpretations are often hard to validate in the absence of information about fault geometry. Here, we image the upper two km of strata in the vicinity of the fault tip of the MFT in central Nepal (around the town of Bardibas) applying a pre-stack migration approach to two new seismic reflection profiles that we interpret using quantitative fault-bend folding theory. Our results provide direct evidence that a shallow décollement produces both emergent (Patu thrust) and blind (Bardibas thrust) fault strands. We show that the décollement lies about 2 km below the land surface near the fault tip, and steps down to a regional 5 km deep décollement level to the north. This implies that there is significant variation in the depth of the décollement. We demonstrate that some active faults do not reach the surface, and therefore paleoseismic trenching alone cannot characterize the earthquake history at these locations. Although blind, these faults have associated growth strata that allow us to infer their most recent displacement history. We present the first direct evidence of fault dip on two fault strands of the MFT at depth that can allow terrace uplift measurements to be more accurately converted to fault slip. We identify a beveled erosional surface buried beneath Quaternary sediments, indicating that strath surface formation is modulated by both climate-related base level changes and tectonics. Together, these results indicate that subsurface imaging, in conjunction with traditional paleoseismological tools, can best characterize the history of fault slip in the Himalaya and other similar thrust fault systems.

Regional-scale advective, diffusive, and eruptive dynamics of CO2 and brine leakage through faults and wellbores

NASA Astrophysics Data System (ADS)

Jung, Na-Hyun; Han, Weon Shik; Han, Kyungdoe; Park, Eungyu

2015-05-01

Regional-scale advective, diffusive, and eruptive transport dynamics of CO2 and brine within a natural analogue in the northern Paradox Basin, Utah, were explored by integrating numerical simulations with soil CO2 flux measurements. Deeply sourced CO2 migrates through steeply dipping fault zones to the shallow aquifers predominantly as an aqueous phase. Dense CO2-rich brine mixes with regional groundwater, enhancing CO2 dissolution. Linear stability analysis reveals that CO2 could be dissolved completely within only 500 years. Assigning lower permeability to the fault zones induces fault-parallel movement, feeds up-gradient aquifers with more CO2, and impedes down-gradient fluid flow, developing anticlinal CO2 traps at shallow depths (<300 m). The regional fault permeability that best reproduces field spatial CO2 flux variation is estimated 1 × 10-17 ≤ kh < 1 × 10-16 m2 and 5 × 10-16 ≤ kv < 1 × 10-15 m2. The anticlinal trap serves as an essential fluid source for eruption at Crystal Geyser. Geyser-like discharge sensitively responds to varying well permeability, radius, and CO2 recharge rate. The cyclic behavior of wellbore CO2 leakage decreases with time.

Thinning Mechanism of the South China Sea Crust: New Insight from the Deep Crustal Images

NASA Astrophysics Data System (ADS)

Chang, S. P.; Pubellier, M. F.; Delescluse, M.; Qiu, Y.; Liang, Y.; Chamot-Rooke, N. R. A.; Nie, X.; Wang, J.

2017-12-01

The passive margin in the South China Sea (SCS) has experienced a long-lived extension period from Paleocene to late Miocene, as well as an extreme stretching which implies an unusual fault system to accommodate the whole amount of extension. Previous interpretations of the fault system need to be revised to explain the amount of strain. We study a long multichannel seismic profile crossing the whole rifted margin in the southwest of SCS, using 6 km- and 8 km-long streamers. After de-multiple processing by SRME, Radon and F-K filtering, an enhanced image of the crustal geometry, especially on the deep crust, allows us to illustrate two levels of detachment at depth. The deeper detachment is around 7-8 sec TWT in the profile. The faults rooting at this detachment are characterized by large offset and are responsible for thicker synrift sediment. A few of these faults appear to reach the Moho. The geometry of the acoustic basement between these boundary faults suggests gentle tilting with a long wavelength ( 200km), and implies some internal deformation. The shallower detachment is located around 4-5 sec TWT. The faults rooting at this detachment represent smaller offset, a shorter wavelength of the basement and thinner packages of synrift sediment. Two detachments separate the crust into upper, middle and lower crust. If the lower crust shows ductile behavior, the upper and middle crust is mostly brittle and form large wavelength boudinage structure, and the internal deformation of the boudins might imply low friction detachments at shallower levels. The faults rooting to deep detachment have activated during the whole rifting period until the breakup. Within the upper and middle crust, the faults resulted in important tilting of the basement at shallow depth, and connect to the deep detachment at some places. The crustal geometry illustrates how the two detachments are important for the thinning process, and also constitute a pathway for the following magmatic activity from the mantle to the surface.

Fault fluid evolution at the outermost edges of the southern Apennines fold-and-thrust belt, Italy

NASA Astrophysics Data System (ADS)

Agosta, Fabrizio; Belviso, Claudia; Cavalcante, Francesco; Vita Petrullo, Angela

2017-04-01

This work focuses on the structural architecture and mineralization of a high-angle, extensional fault zone that crosscuts the Middle Pleistocene tuffs and pyroclastites of the Vulture Volcano, southern Italy. This fault zone is topped by a few m-thick travertine deposit formed by precipitation, in a typical lacustrine depositional environment, from a fault fluid that included a mixed, biogenic- and mantle-derived CO2. The detailed analysis of its different mineralization can shed new lights into the shallow crustal fluid flow that took place during deformation of the outer edge of the southern Apennines fold-and-thrust belt. In fact, the study fault zone is interpreted as a shallow-seated, tear fault associated with a shallow thrust fault displacing the most inner portion of the Bradano foredeep basin infill, and was thus active during the latest stages of contractional deformation. Far from the fault zone, the fracture network is made up of three high-angle joint sets striking N-S, E-W and NW-SE, respectively. The former two sets can be interpreted as the older structural elements that pre-dated the latter one, which is likely due to the current stress state that affects the whole Italian peninsula. In the vicinity of the fault zone, a fourth joint high-angle set striking NE-SW is also present, which becomes the most dominant fracture set within the study footwall fault damage zone. Detailed X-ray diffraction analysis of the powder obtained from hand specimens representative of the multiple mineralization present within the fault zone, and in the surrounding volcanites, are consistent with circulation of a fault fluid that modified its composition with time during the latest stages of volcanic activity and contractional deformation. Specifically, veins infilled with and slickenside coated by jarosite, Opal A and/or goethite are found in the footwall fault damage zone. Based upon the relative timing of formation of the aforementioned joint sets, deciphered after an accurate analysis of their abutting and crosscutting relationships, we envision that the fault fluid was first likely derived from a deep-seated, acid fluid, which interacted with either Triassic or Messinian in age evaporitic rocks during its ascendance from depth. From such a fluid, jarosite precipitated within N-S and NE-SW joints and sheared joints located both away and within the fault damage zone. Then, very warm fluids similar to the lahars that were channeled along the eastern flank of the Vulture Volcano caused the precipitation of Opal A within the dense fracture network of the footwall damage zone, likely causing its hydraulic fracturing, and in the N-S striking veins present in the vicinity of the fault zone. Finally, gotheite coated the major slickensides and sealed the NE-SW fractures, postdating all previous mineralization. Gothetite precipitate from a fault fluid, meteoric in origin, which interacted with the volcanic aquifer causing oxidation of the iron-rich minerals.

High-frequency Born synthetic seismograms based on coupled normal modes

USGS Publications Warehouse

Pollitz, F.

2011-01-01

High-frequency and full waveform synthetic seismograms on a 3-D laterally heterogeneous earth model are simulated using the theory of coupled normal modes. The set of coupled integral equations that describe the 3-D response are simplified into a set of uncoupled integral equations by using the Born approximation to calculate scattered wavefields and the pure-path approximation to modulate the phase of incident and scattered wavefields. This depends upon a decomposition of the aspherical structure into smooth and rough components. The uncoupled integral equations are discretized and solved in the frequency domain, and time domain results are obtained by inverse Fourier transform. Examples show the utility of the normal mode approach to synthesize the seismic wavefields resulting from interaction with a combination of rough and smooth structural heterogeneities. This approach is applied to an ~4 Hz shallow crustal wave propagation around the site of the San Andreas Fault Observatory at Depth (SAFOD). ?? The Author Geophysical Journal International ?? 2011 RAS.

Can compliant fault zones be used to measure absolute stresses in the upper crust?

NASA Astrophysics Data System (ADS)

Hearn, E. H.; Fialko, Y.

2009-04-01

Geodetic and seismic observations reveal long-lived zones with reduced elastic moduli along active crustal faults. These fault zones localize strain from nearby earthquakes, consistent with the response of a compliant, elastic layer. Fault zone trapped wave studies documented a small reduction in P and S wave velocities along the Johnson Valley Fault caused by the 1999 Hector Mine earthquake. This reduction presumably perturbed a permanent compliant structure associated with the fault. The inferred changes in the fault zone compliance may produce a measurable deformation in response to background (tectonic) stresses. This deformation should have the same sense as the background stress, rather than the coseismic stress change. Here we investigate how the observed deformation of compliant zones in the Mojave Desert can be used to constrain the fault zone structure and stresses in the upper crust. We find that gravitational contraction of the coseismically softened zones should cause centimeters of coseismic subsidence of both the compliant zones and the surrounding region, unless the compliant fault zones are shallow and narrow, or essentially incompressible. We prefer the latter interpretation because profiles of line of sight displacements across compliant zones cannot be fit by a narrow, shallow compliant zone. Strain of the Camp Rock and Pinto Mountain fault zones during the Hector Mine and Landers earthquakes suggests that background deviatoric stresses are broadly consistent with Mohr-Coulomb theory in the Mojave upper crust (with μ ≥ 0.7). Large uncertainties in Mojave compliant zone properties and geometry preclude more precise estimates of crustal stresses in this region. With improved imaging of the geometry and elastic properties of compliant zones, and with precise measurements of their strain in response to future earthquakes, the modeling approach we describe here may eventually provide robust estimates of absolute crustal stress.

Structure and Tectonics of the Andaman Subduction Zone from Modeling of Seismological and Gravity Data

NASA Astrophysics Data System (ADS)

Nemalikanti, P. R.; Rao, N.; Hazarika, P.; Tiwari, V. M.; Mangalampally, R.; Singh, A.

2012-12-01

The 10 August 2009 Andaman earthquake of Mw 7.5 occurred to the north of the Andaman and Nicobar Islands at 14o N and 93o E which interestingly, coincides with the northern periphery of the rupture of the Sumatra-Andaman giant mega-thrust earthquake of Mw 9.1 that occurred on 26 December 2004. The event was followed by aftershocks with a peculiar vertical distribution at the same location which was earlier devoid of any significant seismicity. Waveform modeling of five of these events recorded by ISLANDS - the broadband seismic network deployed along the Andaman and Nicobar Islands, indicates that the main shock and two of its aftershocks have a normal fault mechanism with shallow focal depths within 18 km while two others have a strike-slip mechanism occurring deeper, down to 26 km. The computed Bouger gravity anomalies in this region indicate the steepest gradient of 1.5 mgal/km exactly centered over this zone of vertical seismic distribution that characterizes a region of lithospheric split or tear which is devoid of a subducting slab. This is in contrast to a clear subduction trend visible in the southern Andaman and Sunda arcs further south, as evidenced by tomographic images. Joint inversion of waveforms of these five events simultaneously, provides the best fitting P wave velocity structure of this region, given by a Moho at a depth of 30 km and a high crustal Vp/Vs ratio of 1.81. We infer an oceanic double crustal column corresponding to a thickness of about 21 km of Burmese crust including a 5 km thick sedimentary column, underlain by a thinner Indian crust which apparently has a thickness of about 9 km, a model that is also confirmed independently by gravity modeling. We interpret the mechanism of shallow normal fault earthquakes as an intra-plate relaxation phenomenon following the buckling of the overriding Burmese plate in the accretionary wedge of the fore-arc basin, in response to the 2004 mega-thrust subduction event. The deeper strike slip events correspond to an intra-plate phenomenon within the subducting Indian lithospheric plate representing left-lateral faulting across the Andaman arc, due to uneven convergence along the subduction front. Such strike-slip movements are seen all over the Indian Ocean diffuse deformation zone and represent strain accommodation in the Indian crust in response to a grosser mechanism of wrench fault tectonics of the Indo-Australian subduction beneath the Burma-Sunda plate.

Deformation associated with continental normal faults

NASA Astrophysics Data System (ADS)

Resor, Phillip G.

Deformation associated with normal fault earthquakes and geologic structures provide insights into the seismic cycle as it unfolds over time scales from seconds to millions of years. Improved understanding of normal faulting will lead to more accurate seismic hazard assessments and prediction of associated structures. High-precision aftershock locations for the 1995 Kozani-Grevena earthquake (Mw 6.5), Greece image a segmented master fault and antithetic faults. This three-dimensional fault geometry is typical of normal fault systems mapped from outcrop or interpreted from reflection seismic data and illustrates the importance of incorporating three-dimensional fault geometry in mechanical models. Subsurface fault slip associated with the Kozani-Grevena and 1999 Hector Mine (Mw 7.1) earthquakes is modeled using a new method for slip inversion on three-dimensional fault surfaces. Incorporation of three-dimensional fault geometry improves the fit to the geodetic data while honoring aftershock distributions and surface ruptures. GPS Surveying of deformed bedding surfaces associated with normal faulting in the western Grand Canyon reveals patterns of deformation that are similar to those observed by interferometric satellite radar interferometry (InSAR) for the Kozani Grevena earthquake with a prominent down-warp in the hanging wall and a lesser up-warp in the footwall. However, deformation associated with the Kozani-Grevena earthquake extends ˜20 km from the fault surface trace, while the folds in the western Grand Canyon only extend 500 m into the footwall and 1500 m into the hanging wall. A comparison of mechanical and kinematic models illustrates advantages of mechanical models in exploring normal faulting processes including incorporation of both deformation and causative forces, and the opportunity to incorporate more complex fault geometry and constitutive properties. Elastic models with antithetic or synthetic faults or joints in association with a master normal fault illustrate how these secondary structures influence the deformation in ways that are similar to fault/fold geometry mapped in the western Grand Canyon. Specifically, synthetic faults amplify hanging wall bedding dips, antithetic faults reduce dips, and joints act to localize deformation. The distribution of aftershocks in the hanging wall of the Kozani-Grevena earthquake suggests that secondary structures may accommodate strains associated with slip on a master fault during postseismic deformation.

Fault and anthropogenic processes in central California constrained by satellite and airborne InSAR and in-situ observations

NASA Astrophysics Data System (ADS)

Liu, Zhen; Lundgren, Paul

2016-07-01

The San Andreas Fault (SAF) system is the primary plate boundary in California, with the central SAF (CSAF) lying adjacent to the San Joaquin Valley (SJV), a vast structural trough that accounts for about one-sixth of the United Sates' irrigated land and one-fifth of its extracted groundwater. The CSAF displays a range of fault slip behavior with creeping in its central segment that decreases towards its northwest and southeast ends, where the fault transitions to being fully locked. At least six Mw ~6.0 events since 1857 have occurred near the Parkfield transition, most recently in 2004. Large earthquakes also occurred on secondary faults parallel to the SAF, the result of distributed deformation across the plate boundary zone. Recent studies have revealed the complex interaction between anthropogenic related groundwater depletion and the seismic activity on adjacent faults through stress interaction. Despite recent progress, many questions regarding fault and anthropogenic processes in the region still remain. For example, how is the relative plate motion accommodated between the CSAF and off-fault deformation? What is the distribution of fault creep and slip deficit at shallow depths? What are the spatiotemporal variations of fault slip? What are the spatiotemporal characteristics of anthropogenic and lithospheric processes and how do they interact with each other? To address these, we combine satellite InSAR and NASA airborne UAVSAR data to image on and off-fault deformation. The UAVSAR data cover fault perpendicular swaths imaged from opposing look directions and fault parallel swaths since 2009. The much finer spatial resolution and optimized viewing geometry provide important constraints on near fault deformation and fault slip at very shallow depth. We performed a synoptic InSAR time series analysis using ERS-1/2, Envisat, ALOS and UAVSAR interferograms. The combined C-band ERS-1/2 and Envisat data provide a long time interval of SAR data over the region, but are subject to severe decorrelation. The L-band ALOS and UAVSAR SAR sensors provide improved coherence compared to the shorter wavelength radar data. Joint analysis of UAVSAR and ALOS interferometry measurements show clear variability in deformation along the fault strike, suggesting variable fault creep and locking at depth and along strike. Modeling selected fault transects reveals a distinct change in surface creep and shallow slip deficit from the central creeping section towards the Parkfield transition. In addition to fault creep, the L-band ALOS, and especially ALOS-2 ScanSAR interferometry, show large-scale ground subsidence in the SJV due to over-exploitation of groundwater. Groundwater related deformation is spatially and temporally variable and is composed of both recoverable elastic and non-recoverable inelastic components. InSAR time series are compared to GPS and well-water hydraulic head in-situ time series to understand water storage processes and mass loading changes. We are currently developing poroelastic finite element method models to assess the influence of anthropogenic processes on surface deformation and fault mechanics. Ongoing work is to better constrain both tectonic and non-tectonic processes and understand their interaction and implication for regional earthquake hazard.

Steep-dip seismic imaging of the shallow San Andreas Fault near Parkfield

USGS Publications Warehouse

Hole, J.A.; Catchings, R.D.; St. Clair, K.C.; Rymer, M.J.; Okaya, D.A.; Carney, B.J.

2001-01-01

Seismic reflection and refraction images illuminate the San Andreas Fault to a depth of 1 kilometer. The prestack depth-migrated reflection image contains near-vertical reflections aligned with the active fault trace. The fault is vertical in the upper 0.5 kilometer, then dips about 70° to the southwest to at least 1 kilometer subsurface. This dip reconciles the difference between the computed locations of earthquakes and the surface fault trace. The seismic velocity cross section shows strong lateral variations. Relatively low velocity (10 to 30%), high electrical conductivity, and low density indicate a 1-kilometer-wide vertical wedge of porous sediment or fractured rock immediately southwest of the active fault trace.

High-Precision Locations and the Stress Field from Instrumental Seismicity, Moment Tensors, and Short-Period Mechanisms through the Mina Deflection, Central Walker Lane

NASA Astrophysics Data System (ADS)

Ruhl, C. J.; Smith, K. D.

2012-12-01

The Mina Deflection (MD) region of the central Walker Lane of eastern California and western Nevada, is a complex zone of northeast-trending normal, and primarily left-lateral strike-slip to oblique-slip faulting that separates the Southern Walker Lane (SWL) from a series of east-tilted normal fault blocks in the Central Walker Lane (CWL) (Faulds and Henry, 2008; Surpless, 2008). The MD accommodates the transfer of right-lateral strike-slip motion from northwest-striking faults in the SWL to a series of left-stepping northwest-striking right-lateral strike-slip faults in the CWL, east of the Wassuk Range near Hawthorne, NV. The ~50 km wide ~80 km long right-step is a distinct transition in regional physiography that has been attributed to strain accommodation through pre-Cenozoic lithospheric structures. Several slip transfer mechanisms have been proposed within the MD, from clockwise rotation of high-angle fault blocks (Wesnousky, 2005), to low-angle displacement within the Silver Peak-Lone Mountain complex (Oldow et al., 2001), and curved fault arrays associated with localized basins and tectonic depressions (Ferranti et al., 2009). The region has been a regular source of M4+ events, the most recent being an extended sequence that included twenty-seven M 3.5+ earthquakes (largest event M 4.6) south of Hawthorne in 2011. These earthquakes (< 5 km depth) define shallow W-dipping (dip ~56°) and NW-dipping (dip ~70°) normal faulting constrained by moment tensor (MT) solutions and earthquake relocations. Temporary stations deployed in the source area provide good control. A distributed sequence in 2004, between Queen Valley and Mono Lake, primarily associated with the Huntoon Valley fault, included three M 5+ left-lateral strike-slip faulting events. A 1997 sequence in northern Fish Lake Valley (east of the White Mountains), with mainshock Mw 5.3 (Ichinose et al., 2003), also showed high-angle northeast-striking left-lateral strike-slip motion. Historical events include the 1934 M 6.5 Excelsior Mountains event south of Mina, NV, and the 1932 M 7.1 Cedar Mountains earthquake east of the Pilot Mountains. Another persistent feature in the seismicity is an ~40 km long arcuate distribution of activity extending from approximately Queen Valley, north of the White Mountains, to Mono Lake that appears to reflect a southwestern boundary to northeast-striking structures in the MD. Here we develop high-precision relocations of instrumental seismicity in the MD from 1984 through 2012, including relocations of the 2004 sequence, and account for the historical seismic record. MT solutions from published reports and computed from recent M 3.5+ earthquakes as well as available and developed short-period focal mechanisms are compiled to evaluate the stress field to assess mechanisms of slip accommodation. Based on the complex distribution of fault orientations, the stress field varies locally northward from the SWL throughout the MD; however, in many cases, fault plane alignments can be isolated from high-precision locations, providing better constraints on stress and slip orientations.

Geophysical basin structure of the Cotonou (Dahomey/Benin) basin, West African Gulf of Guinea

DOE Office of Scientific and Technical Information (OSTI.GOV)

Babalola, O.O.

1990-05-01

The frontier Cotonou basin (or Dahomey/Benin embayment), situated west of the prolific Niger Delta basin, appears from seismic, gravity, and aeromagnetic interpretation, as a series of grabens and troughs confined on the west and east by the Romanche and the Chain fracture zones, respectively. The Keta trough of the western basin rim was formed by a 2700-m southeasterly downthrow of the Adina fault. This trough is separated by a north-northeasterly fault from the Lome-Anecho gravity high. Eastward, the arcuate Allada-Adjohon trough is abutted on its southern flank by the northwest-trending Nokue-Afowo trough and separated from the northwesterly Ikorodu trough bymore » the 50-km-wide aeromagnetically inferred ro-Otta ridge. The Ikorodu trough is adjoined on the northwest by the Aiyetoro trough and on the southeast by the Yemoja offshore graben trending east northeast as the Seme oil-field structural trend. North of the regional northeasterly axial, gravity positive, structural divide (the continental precursor of the Charcot fracture zone) a series of half-grabens (notably the Aplahoue, Bohicon, and Keiou troughs), normal faulted eastward and downthrown in the west, dominate the landward western rim of the Cotonou basin. Graben-bounding faults control the upper valleys of the basin drainage, converge toward the regional intrabasin structural trend and continue into the Fenyi-koe fault and the Charcot fracture zone. These faults resulted from brittle dextral shear of continental crust oblique to local, preexisting north-northeast structural trends. In the eastern basin rim, preexisting north-northwest structural trends influenced the shearing stress regime to generate small, shallow, structurally bounded, east-northeast- and north-northwest trending grabens.« less

Origin and significance of clay-coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)

USGS Publications Warehouse

Schleicher, A.M.; van der Pluijm, B.A.; Solum, J.G.; Warr, L.N.

2006-01-01

The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X-ray-diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca-K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Namontmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault. Copyright 2006 by the American Geophysical Union.

Source characteristics of 2000 small earthquakes nucleating on the Alto Tiberina fault system (central Italy).

NASA Astrophysics Data System (ADS)

Munafo, I.; Malagnini, L.; Tinti, E.; Chiaraluce, L.; Di Stefano, R.; Valoroso, L.

2014-12-01

The Alto Tiberina Fault (ATF) is a 60 km long east-dipping low-angle normal fault, located in a sector of the Northern Apennines (Italy) undergoing active extension since the Quaternary. The ATF has been imaged by analyzing the active source seismic reflection profiles, and the instrumentally recorded persistent background seismicity. The present study is an attempt to separate the contributions of source, site, and crustal attenuation, in order to focus on the mechanics of the seismic sources on the ATF, as well on the synthetic and the antithetic structures within the ATF hanging-wall (i.e. Colfiorito fault, Gubbio fault and Umbria Valley fault). In order to compute source spectra, we perform a set of regressions over the seismograms of 2000 small earthquakes (-0.8 < ML< 4) recorded between 2010 and 2014 at 50 permanent seismic stations deployed in the framework of the Alto Tiberina Near Fault Observatory project (TABOO) and equipped with three-components seismometers, three of which located in shallow boreholes. Because we deal with some very small earthquakes, we maximize the signal to noise ratio (SNR) with a technique based on the analysis of peak values of bandpass-filtered time histories, in addition to the same processing performed on Fourier amplitudes. We rely on a tool called Random Vibration Theory (RVT) to completely switch from peak values in the time domain to Fourier spectral amplitudes. Low-frequency spectral plateau of the source terms are used to compute moment magnitudes (Mw) of all the events, whereas a source spectral ratio technique is used to estimate the corner frequencies (Brune spectral model) of a subset of events chosen over the analysis of the noise affecting the spectral ratios. So far, the described approach provides high accuracy over the spectral parameters of earthquakes of localized seismicity, and may be used to gain insights into the underlying mechanics of faulting and the earthquake processes.

Reconnaissance study of late quaternary faulting along cerro GoDen fault zone, western Puerto Rico

USGS Publications Warehouse

Mann, P.; Prentice, C.S.; Hippolyte, J.-C.; Grindlay, N.R.; Abrams, L.J.; Lao-Davila, D.

2005-01-01

The Cerro GoDen fault zone is associated with a curvilinear, continuous, and prominent topographic lineament in western Puerto Rico. The fault varies in strike from northwest to west. In its westernmost section, the fault is ???500 m south of an abrupt, curvilinear mountain front separating the 270- to 361-m-high La CaDena De San Francisco range from the Rio A??asco alluvial valley. The Quaternary fault of the A??asco Valley is in alignment with the bedrock fault mapped by D. McIntyre (1971) in the Central La Plata quadrangle sheet east of A??asco Valley. Previous workers have postulated that the Cerro GoDen fault zone continues southeast from the A??asco Valley and merges with the Great Southern Puerto Rico fault zone of south-central Puerto Rico. West of the A??asco Valley, the fault continues offshore into the Mona Passage (Caribbean Sea) where it is characterized by offsets of seafloor sediments estimated to be of late Quaternary age. Using both 1:18,500 scale air photographs taken in 1936 and 1:40,000 scale photographs taken by the U.S. Department of Agriculture in 1986, we iDentified geomorphic features suggestive of Quaternary fault movement in the A??asco Valley, including aligned and Deflected drainages, apparently offset terrace risers, and mountain-facing scarps. Many of these features suggest right-lateral displacement. Mapping of Paleogene bedrock units in the uplifted La CaDena range adjacent to the Cerro GoDen fault zone reveals the main tectonic events that have culminated in late Quaternary normal-oblique displacement across the Cerro GoDen fault. Cretaceous to Eocene rocks of the La CaDena range exhibit large folds with wavelengths of several kms. The orientation of folds and analysis of fault striations within the folds indicate that the folds formed by northeast-southwest shorTening in present-day geographic coordinates. The age of Deformation is well constrained as late Eocene-early Oligocene by an angular unconformity separating folDed, Deep-marine middle Eocene rocks from transgressive, shallow-marine rocks of middle-upper Oligocene age. Rocks of middle Oligocene-early Pliocene age above unconformity are gently folDed about the roughly east-west-trending Puerto Rico-Virgin Islands arch, which is well expressed in the geomorphology of western Puerto Rico. Arching appears ongoing because onshore and offshore late Quaternary oblique-slip faults closely parallel the complexly Deformed crest of the arch and appear to be related to exTensional strains focused in the crest of the arch. We estimate ???4 km of vertical throw on the Cerro GoDen fault based on the position of the carbonate cap north of the fault in the La CaDena De San Francisco and its position south of the fault inferred from seismic reflection data in Mayaguez Bay. Based on these observations, our interpretation of the kinematics and history of the Cerro GoDen fault zone incluDes two major phases of motion: (1) Eocene northeast-southwest shorTening possibly accompanied by left-lateral shearing as Determined by previous workers on the Great Southern Puerto Rico fault zone; and (2) post-early Pliocene regional arching of Puerto Rico accompanied by normal offset and right-lateral shear along faults flanking the crest of the arch. The second phase of Deformation accompanied east-west opening of the Mona rift and is inferred to continue to the present day. ?? 2005 Geological Society of America.

The Earth isn't flat: The (large) influence of topography on geodetic fault slip imaging.

NASA Astrophysics Data System (ADS)

Thompson, T. B.; Meade, B. J.

2017-12-01

While earthquakes both occur near and generate steep topography, most geodetic slip inversions assume that the Earth's surface is flat. We have developed a new boundary element tool, Tectosaur, with the capability to study fault and earthquake problems including complex fault system geometries, topography, material property contrasts, and millions of elements. Using Tectosaur, we study the model error induced by neglecting topography in both idealized synthetic fault models and for the cases of the MW=7.3 Landers and MW=8.0 Wenchuan earthquakes. Near the steepest topography, we find the use of flat Earth dislocation models may induce errors of more than 100% in the inferred slip magnitude and rake. In particular, neglecting topographic effects leads to an inferred shallow slip deficit. Thus, we propose that the shallow slip deficit observed in several earthquakes may be an artefact resulting from the systematic use of elastic dislocation models assuming a flat Earth. Finally, using this study as an example, we emphasize the dangerous potential for forward model errors to be amplified by an order of magnitude in inverse problems.

Seismic imaging beneath an InSAR anomaly in eastern Washington State: Shallow faulting associated with an earthquake swarm in a low-hazard area

USGS Publications Warehouse

Stephenson, William J.; Odum, Jackson K.; Wicks, Chuck; Pratt, Thomas L.; Blakely, Richard J.

2016-01-01

In 2001, a rare swarm of small, shallow earthquakes beneath the city of Spokane, Washington, caused ground shaking as well as audible booms over a five‐month period. Subsequent Interferometric Synthetic Aperture Radar (InSAR) data analysis revealed an area of surface uplift in the vicinity of the earthquake swarm. To investigate the potential faults that may have caused both the earthquakes and the topographic uplift, we collected ∼3  km of high‐resolution seismic‐reflection profiles to image the upper‐source region of the swarm. The two profiles reveal a complex deformational pattern within Quaternary alluvial, fluvial, and flood deposits, underlain by Tertiary basalts and basin sediments. At least 100 m of arching on a basalt surface in the upper 500 m is interpreted from both the seismic profiles and magnetic modeling. Two west‐dipping faults deform Quaternary sediments and project to the surface near the location of the Spokane fault defined from modeling of the InSAR data.

High density of structurally controlled, shallow to deep water fluid seep indicators imaged offshore Costa Rica

NASA Astrophysics Data System (ADS)

Kluesner, Jared W.; Silver, Eli A.; Bangs, Nathan L.; McIntosh, Kirk D.; Gibson, James; Orange, Daniel; Ranero, Cesar R.; von Huene, Roland

2013-03-01

We used high-resolution mapping to document 161 sites of potential fluid seepage on the shelf and slope regions where no geophysical seep indicators had been reported. Identified potential seabed seepage sites show both high-backscatter anomalies and bathymetric expressions, such as pockmarks, mounds, and ridges. Almost all identified seabed features are associated with bright spots and flat spots beneath, as mapped within the 3-D seismic grid. We obtained EM122 multi-beam data using closely spaced receiver beams and 4-5 times overlapping multi-beam swaths, which greatly improved the sounding density and geologic resolvability of the data. At least one location shows an acoustic plume in the water column on a 3.5 kHz profile, and this plume is located along a fault trace and above surface and subsurface seepage indicators. Fluid indicators are largely associated with folds and faults within the sediment section, and many of the faults continue into and offset the reflective basement. A dense pattern of normal faults is seen on the outer shelf in the multi-beam bathymetry, backscatter, and 3-D seismic data, and the majority of fluid seepage indicators lie along mapped fault traces. Furthermore, linear mounds, ridges, and pockmark chains are found on the upper, middle, and lower slope regions. The arcuate shape of the shelf edge, projection of the Quepos Ridge, and high density of potential seep sites suggest that this area may be a zone of former seamount/ridge subduction. These results demonstrate a much greater potential seep density and distribution than previously reported across the Costa Rican margin.

The IODP NanTroSEIZE Transect: Accomplishments and Future Plans

NASA Astrophysics Data System (ADS)

Tobin, H. J.; Kinoshita, M.; Araki, E.; Byrne, T. B.; Kimura, G.; McNeill, L. C.; Moore, G. F.; Saffer, D. M.; Underwood, M.; Saito, S.

2009-12-01

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a decade-long project to investigate the processes and properties that determine the nature of frictional locking, creep and other fault behavior governing seismogenic rupture and tsunamigenesis on a major plate boundary where great subduction earthquakes occur. The main goal of the science plan is to sample and instrument the key faults in several locations across the transition from those dominated by frictionally stable, aseismic processes vs. those hypothesized to be frictionally locked (seismogenic) faults of the megathrust system. The transect includes primary drill sites from the incoming plate, across the outer accretionary complex of the lower slope, to the Kumano forearc basin and underlying up-dip end of the likely locked plate interface. The scale of this project required a division into multiple stages of operations, spanning a number of years and IODP expeditions. From September 2007 through October 2009, the NanTroSEIZE science team has achieved many of its primary goals during 5 expeditions. Completed drill sites to date include penetrations ranging from ~200 m to ~1600 m below the sea floor that have documented the faults and wall rocks of both the frontal thrust and out-of-sequence splay faults in the accretionary system, the sedimentary section of the subducting plate, and the thick forearc basin sedimentary record and underlying older subduction complex in the hanging wall of the main plate interface. Major results include characterization of: the fault zone geology, strain localization, and physical properties shallower than ~ 1 km, the distribution of ambient (and paleo-) stress orientations across the transect, the absence of evidence for focused fluid channeling along the principal shallow fault systems, and the tectonic history of the subduction system. Extensive downhole measurements and a 2-ship VSP have further documented stress, pressure, rock strength, and elastic properties around the boreholes. The first temporary long-term monitoring instruments are now in place in one sealed borehole, recording pore pressure and temperature. The most ambitious aspect of the NanTroSEIZE project remains for the now-scheduled next stage: drilling to ~ 7000 m below the sea bed across the faults of the main plate boundary, then placing long-term monitoring instruments into both deep and shallow sealed borehole observatories - all to test hypotheses of locking, strain accumulation, and interseismic fault processes.

Displaced rocks, strong motion, and the mechanics of shallow faulting associated with the 1999 Hector Mine, California, earthquake

USGS Publications Warehouse

Michael, Andrew J.; Ross, Stephanie L.; Stenner, Heidi D.

2002-01-01

The paucity of strong-motion stations near the 1999 Hector Mine earthquake makes it impossible to make instrumental studies of key questions about near-fault strong-motion patterns associated with this event. However, observations of displaced rocks allow a qualitative investigation of these problems. By observing the slope of the desert surface and the frictional coefficient between these rocks and the desert surface, we estimate the minimum horizontal acceleration needed to displace the rocks. Combining this information with observations of how many rocks were displaced in different areas near the fault, we infer the level of shaking. Given current empirical shaking attenuation relationships, the number of rocks that moved is slightly lower than expected; this implies that slightly lower than expected shaking occurred during the Hector Mine earthquake. Perhaps more importantly, stretches of the fault with 4 m of total displacement at the surface displaced few nearby rocks on 15?? slopes, suggesting that the horizontal accelerations were below 0.2g within meters of the fault scarp. This low level of shaking suggests that the shallow parts of this rupture did not produce strong accelerations. Finally, we did not observe an increased incidence of displaced rocks along the fault zone itself. This suggests that, despite observations of fault-zone-trapped waves generated by aftershocks of the Hector Mine earthquake, such waves were not an important factor in controlling peak ground acceleration during the mainshock.

An approach to secure weather and climate models against hardware faults

NASA Astrophysics Data System (ADS)

Düben, Peter D.; Dawson, Andrew

2017-03-01

Enabling Earth System models to run efficiently on future supercomputers is a serious challenge for model development. Many publications study efficient parallelization to allow better scaling of performance on an increasing number of computing cores. However, one of the most alarming threats for weather and climate predictions on future high performance computing architectures is widely ignored: the presence of hardware faults that will frequently hit large applications as we approach exascale supercomputing. Changes in the structure of weather and climate models that would allow them to be resilient against hardware faults are hardly discussed in the model development community. In this paper, we present an approach to secure the dynamical core of weather and climate models against hardware faults using a backup system that stores coarse resolution copies of prognostic variables. Frequent checks of the model fields on the backup grid allow the detection of severe hardware faults, and prognostic variables that are changed by hardware faults on the model grid can be restored from the backup grid to continue model simulations with no significant delay. To justify the approach, we perform model simulations with a C-grid shallow water model in the presence of frequent hardware faults. As long as the backup system is used, simulations do not crash and a high level of model quality can be maintained. The overhead due to the backup system is reasonable and additional storage requirements are small. Runtime is increased by only 13 % for the shallow water model.

An approach to secure weather and climate models against hardware faults

NASA Astrophysics Data System (ADS)

Düben, Peter; Dawson, Andrew

2017-04-01

Enabling Earth System models to run efficiently on future supercomputers is a serious challenge for model development. Many publications study efficient parallelisation to allow better scaling of performance on an increasing number of computing cores. However, one of the most alarming threats for weather and climate predictions on future high performance computing architectures is widely ignored: the presence of hardware faults that will frequently hit large applications as we approach exascale supercomputing. Changes in the structure of weather and climate models that would allow them to be resilient against hardware faults are hardly discussed in the model development community. We present an approach to secure the dynamical core of weather and climate models against hardware faults using a backup system that stores coarse resolution copies of prognostic variables. Frequent checks of the model fields on the backup grid allow the detection of severe hardware faults, and prognostic variables that are changed by hardware faults on the model grid can be restored from the backup grid to continue model simulations with no significant delay. To justify the approach, we perform simulations with a C-grid shallow water model in the presence of frequent hardware faults. As long as the backup system is used, simulations do not crash and a high level of model quality can be maintained. The overhead due to the backup system is reasonable and additional storage requirements are small. Runtime is increased by only 13% for the shallow water model.

Internal structure of the San Jacinto fault zone in the trifurcation area southeast of Anza, California, from data of dense seismic arrays

NASA Astrophysics Data System (ADS)

Qin, L.; Ben-Zion, Y.; Qiu, H.; Share, P.-E.; Ross, Z. E.; Vernon, F. L.

2018-04-01

We image the internal structure of the San Jacinto fault zone (SJFZ) in the trifurcation area southeast of Anza, California, with seismic records from dense linear and rectangular arrays. The examined data include recordings from more than 20 000 local earthquakes and nine teleseismic events. Automatic detection algorithms and visual inspection are used to identify P and S body waves, along with P- and S-types fault zone trapped waves (FZTW). The location at depth of the main branch of the SJFZ, the Clark fault, is identified from systematic waveform changes across lines of sensors within the dense rectangular array. Delay times of P arrivals from teleseismic and local events indicate damage asymmetry across the fault, with higher damage to the NE, producing a local reversal of the velocity contrast in the shallow crust with respect to the large-scale structure. A portion of the damage zone between the main fault and a second mapped surface trace to the NE generates P- and S-types FZTW. Inversions of high-quality S-type FZTW indicate that the most likely parameters of the trapping structure are width of ˜70 m, S-wave velocity reduction of 60 per cent, Q value of 60 and depth of ˜2 km. The local reversal of the shallow velocity contrast across the fault with respect to large-scale structure is consistent with preferred propagation of earthquake ruptures in the area to the NW.

InSAR Time Series Analysis of Dextral Strain Partitioning Across the Burma Plate

NASA Astrophysics Data System (ADS)

Reitman, N. G.; Wang, Y.; Lin, N.; Lindsey, E. O.; Mueller, K. J.

2017-12-01

Oblique convergence between the India and Sunda plates creates partitioning of strike-slip and compressional strain across the Burma plate. GPS data indicate up to 40 mm/yr (Steckler et al 2016) of dextral strain exists between the India and Sunda plates. The Sagaing fault in Myanmar accommodates 20 mm/yr at the eastern boundary of the Burma plate, but the location and magnitude of dextral strain on other faults remains an open question, as does the relative importance of seismic vs aseismic processes. The remaining 20 mm/yr of dextral strain may be accommodated on one or two faults or widely distributed on faults across the Burma plate, scenarios that have a major impact on seismic hazard. However, the dense GPS data necessary for precise determination of which faults accommodate how much strain do not exist yet. Previous studies using GPS data ascribe 10-18 mm/yr dextral strain on the Churachandpur Mao fault in India (Gahaluat et al 2013, Steckler et al 2016) and 18-22 mm/yr on the northern Sagaing fault (Maurin et al 2010, Steckler et al 2016), leaving up to 10 mm/yr unconstrained. Several of the GPS results are suggestive of shallow aseismic slip along parts of these faults, which, if confirmed, would have a significant impact on our understanding of hazard in the area. Here, we use differential InSAR analyzed in time series to investigate dextral strain on the Churachandpur Mao fault and across the Burma plate. Ascending ALOS-1 imagery spanning 2007-2010 were processed in time series for three locations. Offsets in phase and a strong gradient in line-of-sight deformation rate are observed across the Churachandpur Mao fault, and work is ongoing to determine if these are produced by shallow fault movement, topographic effects, or both. The results of this study will provide further constraints for strain rate on the Churachandpur Mao fault, and yield a more complete understanding of strain partitioning across the Burma plate.

Influence of low-angle normal faulting on radial fracture pattern associated to pluton emplacement in Tuscany, Italy

NASA Astrophysics Data System (ADS)

Balsamo, F.; Rossetti, F.; Salvini, F.

2003-04-01

Fault-related fracture distribution significantly influences fluid flow in the sub-surface. Fault zone can act either as barriers or conduits to fluid migration, or as mixed conduit/barrier systems, depending on several factors that include the enviromental condition of deformation (pore fluid pressure, regional stress fields, overburden etc.), the kinematics of the fault and its geometry, and the rock type. The aim of this study is to estimate the boundary conditions of deformation along the Boccheggiano Fault, in the central Appennines. Seismic and deep well data are avaible for the Boccheggiano area, where a fossil geothermal system is exposed. The dominant structural feature of the studied area is a NW-SE trending low-angle detachment fault (Boccheggiano fault, active since the upper Miocene times), separating non-metamorphic sedimentary sequences of the Tuscan meso-cenozoic pelagiac succession and oceanic-derived Ligurids in the hangingwall, from green-schists facies metamorphic rocks of Paleozoic age in the footwall. Gouge-bearing mineralized damage zone (about 100 m thick) is present along the fault. The deep geometry of the Boccheggiano Fault is well imaged in the seismic profiles. The fault is shallow-dipping toward NE and flattens at the top of a magmatic intrusion, which lies at about 1000 m below the ground-level. Geometrical relationships indicate syn-tectonic pluton emplacement at the footwall of the Boccheggiano fault. Statistical analysis of fracture distribution pointed out a strong control of both azimuth and frequency by their position with respect to the Boccheggiano Fault: (i) a NW-SE trending fracture set within the fault zone, (ii) a radial pattern associated away from fault zone. Interpretation of structural and seismic data suggest an interplay between the near-field deformation associated with the rising intrusion during its emplacement (radial fracturing) and the NE-SW far-field extensional tectonic regime (NW-SE fractures) recognized in the area, responsible for the fault development. The 3-D geometry of the Boccheggiano Fault was simulated in a numerical tool specifically designed to model the 3-D distribution of fractures (joints and solution surfaces) along fault. Comparison between the actual fracture distribution and the predicted ones at different boundary conditions allowed to estimate the resulting stress field (both far field and near field) and the pore fluid pressure acting during fault motion and co-eval pluton emplacement. Numerical modelling predictions indicate transfer segments along the main fault as more permeable sectors. This justify the location intense mineralisation zones and abandoned mines.

Footwall progradation in syn-rift carbonate platform-slope systems (Early Jurassic, Northern Apennines, Italy)

NASA Astrophysics Data System (ADS)

Fabbi, Simone; Santantonio, Massimo

2012-12-01

The so-called Umbria-Marche Domain of Northern Apennines represents a vast depositional system, also stretching across the Adriatic Sea subsurface, that was characterized by dominantly pelagic sedimentation through most of its Jurassic to Oligocene/Early Miocene history. The pelagic succession is underlain by Hettangian shallow-water carbonates (Calcare Massiccio Fm.), constituting a regional carbonate platform that was subjected to tectonic extension due to rifting of the Adria/African Plate in the earliest Jurassic. While tectonic subsidence of the hangingwalls drove the drowning of the platform around the Hettangian/Sinemurian boundary, the production of benthic carbonate on footwall blocks continued parallel to faulting, through a sequence of facies that was abruptly terminated by drowning and development of condensed pelagites in the early Pliensbachian. By then rifting had ceased, so that the Pliensbachian to Early Cretaceous hangingwall deposits represent a post-rift basin-fill succession onlapping the tectonically-generated escarpment margins of the highs. During the early phases of syndepositional faulting, the carbonate factories of footwall blocks were still temporarily able to fill part of the accommodation space produced by the normal faults by prograding into the incipient basins. In this paper we describe for the first time a relatively low-angle (< 10°) clinoform bed package documenting such an ephemeral phase of lateral growth of a carbonate factory. The clinoforms are sigmoidal, and form low-relief (maximum 5-7 m) bodies representing a shallow-water slope that was productive due to development of a Lithocodium-dominated factory. Continued faulting and hangingwall subsidence then decoupled the slope from the platform top, halting the growth of clinoforms and causing the platform margin to switch from accretionary to bypass mode as the pre-rift substrate became exposed along a submarine fault escarpment. The downfaulted clinoform slope was then buried by base-of-escarpment proximal turbidites, forming a bypass wedge. Such a contact would be imaged along a seismic section as an unconformity, suggestive of shut-off of the local carbonate factory and onlap by pelagic mud. The composition of the turbidites, however, at least initially duplicates that of the clinoforms, indicating that the footwall top was still productive, yet the mechanisms of sediment shedding into the basin had changed due to the modifications of submarine topography induced by synsedimentary tectonics.

Structure of Kilauea's southwest rift zone and western south flank defined by relocated earthquakes

NASA Astrophysics Data System (ADS)

Rinard, Bethany D.

This study is the first detailed seismic investigation of the southwest rift and western south flank of Kilauea Volcano. Earthquakes outline the tectonic and magmatic systems of the volcano. In this study, more than 4800 earthquakes from the years 1981--2001 were relocated with a double-difference method, and almost 500 were relocated with cross-correlation. The result is a much-improved image of Kilauea's south flank structure. The shallowest of the earthquakes on Kilauea (<5km) are usually related to magma movement, and occur almost exclusively in the actively intruded rift. The few tectonic earthquakes that occur at this depth are along the Koae and Hilina Fault systems. Focal mechanisms indicate that the shallow events on the Hilina system have [normal, right-lateral] oblique-slip motion. Beneath the entire south flank are earthquakes that occur on a decollement, located at a depth of 7--10km. The inland-dipping decollement structure is clearly imaged with this new data set. Earthquakes on the volcano's south flank normal faults appear to extend downward to the decollement. Earthquakes at intermediate depths image the decollement, a plane that dips inland. This is the boundary between the volcano and the old oceanic crust beneath it. Movement on faults at decollement depths of 7--10km have [right-lateral thrust] oblique-slip motion. When intrusions occur in the rift zones, the flank is forced seaward along the decollement. Since the decollement dips inland, the south flank must move up an incline as it slides seaward. Hawaii also experiences deep (>25km) earthquakes, which are the most intriguing events in this study. These earthquakes are significant because the Moho is located at a depth of 13--15km, so they are clearly occurring in the mantle. The deep events examined in this study are tectonic earthquakes, not attributable to melt migration. A high strain rate in the mantle, largely due to the geologically rapid formation of the island that has quickly increased the load on the underlying mantle, may account for the occurrence of these deep earthquakes. Focal mechanisms indicate [normal, right-lateral] oblique-slip motion on faults below 25km depth.

Factors affecting the recognition of faults exposed in exploratory trenches

USGS Publications Warehouse

Bonilla, Manuel G.; Lienkaemper, James J.

1991-01-01

Trenching-a widely used method for evaluating fault activity-has limitations that can mislead investigators. Some segments of fault strands in trench walls may not be visible, and this nonvisibility can lead to incorrect interpretations of time of most recent displacement and recurrence intervals on a fault. We examined the logs of 163 trench exposures and tabulated data on more than 1,200 fault strands to investigate three categories of nonvisibility: (1) strands with obscure (invisible or poorly visible) segments, (2) strands that die out upward, and (3) strands that die out downward. About 14 percent of all the strands have obscure segments. Of the 143 strands on which it is possible to recognize dieout up (limited to strands for which position of ground surface at time of faulting is known), 45 percent do die out upward, and the fraction exceeds 70 percent for strike-slip and reverse faults. Thus a fault strand overlain by an apparently undisturbed deposit is not necessarily older than the deposit. More than 30 percent of all the strands die out downward, providing more evidence that fault strands can end for reasons other than being covered by deposits younger than the fault. Analysis of trench-log data revealed various relations between geologic factors and nonvisibility of fault strands. For example, fault type affects the incidence of nonvisibility, which is generally most common on strike-slip faults, less common on reverse faults, and least common on normal fau Its. The type of material penetrated by the fault also influences nonvisibility, which tends to be more common in soil horizons and sand, and less common in gravel. Dieout down is weakly influenced by fault displacement, decreasing in frequency with increase in displacement; the frequencies of obscure segments and dieout up do not vary consistently with fault displacement. Frequency of obscure segments generally decreases with increase in length of obscure segments, and frequency of dieout up generally decreases with depth of dieout up. Length of obscure segments and depth of dieout up are typically less than the effective thickness of associated beds. On the basis of few data, obscure segments seem to be more common on faults with younger, rather than older, ages of latest displacement. Our study revealed additional relations not directly related to nonvisibility. For example, the median widths of faults crossed by the trenches vary by fault type, strike-slip faults being narrower than dip-slip faults. In the shallow and mostly unconsolidated materials cut by the trenches, fault widths show only an erratic and, at best, weak relationship to fault displacements. Hanging walls are deformed more frequently than footwalls in dip-slip faults, but both walls are deformed at more than 30 percent of the exposures. We tabulated several phenomena that may indicate faulting or provide evidence of prehistorical earthquakes. Rotation of pebbles was identified in 41 percent of the exposures having gravel in the fault zone; type of fault has no strong influence on the incidence of pebble rotation. Fissures were recorded at 52 percent of the exposures and were more common in strike-slip and normal faults than in reverse fau Its. Gouge was reported at 1 5 percent of the exposures; fault type has no significant influence on its frequency. Slickensides were noted at 10 percent of the exposures, and fault type has an unknown influence on their incidence. Slickensides in unconsolidated materials were restricted to clay, silt, and gouge. Other mechanical or hydrologic effects related to faulting or earthquakesrubble, breccia, mixing, crushing, polishing, water barriers, c;ind probable liquefaction effects-were reported at fewer than 1 0 percent of the exposures.

Karst geomorphology and hydrology of the Shenandoah Valley near Harrisonburg, Virginia

USGS Publications Warehouse

Doctor, Daniel H.; Orndorff, Wil; Maynard, Joel; Heller, Matthew J.; Casile, Gerolamo C.

2014-01-01

The karst of the central Shenandoah Valley has characteristics of both shallow and deep phreatic formation. This field guide focuses on the region around Harrisonburg, Virginia, where a number of these karst features and their associated geologic context can be examined. Ancient, widespread alluvial deposits cover much of the carbonate bedrock on the western side of the valley, where shallow karstification has resulted in classical fluviokarst development. However, in upland exposures of carbonate rock, isolated caves exist atop hills not affected by surface processes other than exposure during denudation. The upland caves contain phreatic deposits of calcite and fine-grained sediments. They lack any evidence of having been invaded by surface streams. Recent geologic mapping and LIDAR (light detection and ranging) elevation data have enabled interpretive association between bedrock structure, igneous intrusions, silicification and brecciation of host carbonate bedrock, and the location of several caves and karst springs. Geochemistry, water quality, and water temperature data support the broad categorization of springs into those affected primarily by shallow near-surface recharge, and those sourced deeper in the karst aquifer. The deep-seated karst formation occurred in the distant past where subvertical fracture and fault zones intersect thrust faults and/or cross-strike faults, enabling upwelling of deep-circulating meteoric groundwater. Most caves formed in such settings have been overprinted by later circulation of shallow groundwater, thus removing evidence of the history of earliest inception; however, several caves do preserve evidence of an earlier formation.

Depth-Dependent Earthquake Properties Beneath Long-Beach, CA: Implications for the Rheology at the Brittle-Ductile Transition Zone

NASA Astrophysics Data System (ADS)

Inbal, A.; Clayton, R. W.; Ampuero, J. P.

2015-12-01

Except for a few localities, seismicity along faults in southern California is generally confined to depths shallower than 15 km. Among faults hosting deep seismicity, the Newport-Inglewood Fault (NIF), which traverses the Los-Angeles basin, has an exceptionally mild surface expression and low deformation rates. Moreover, the NIF structure is not as well resolved as other, less well instrumented faults because of poor signal-to-noise ratio. Here we use data from three temporary dense seismic arrays, which were deployed for exploration purposes and contain up to several thousands of vertical geophones, to investigate the properties of deep seismicity beneath Long-Beach (LB), Compton and Santa-Fe Springs (SFS). The latter is located 15 km northeast of the NIF, presumably above a major detachment fault underthrusting the basin.Event detection is carried out using a new approach for microseismic multi-channel picking, in which downward-continued data are back-projected onto the volume beneath the arrays, and locations are derived from statistical analysis of back-projection images. Our technique reveals numerous, previously undetected events along the NIF, and confirms the presence of an active shallow structure gently dipping to the north beneath SFS. Seismicity characteristics vary along the NIF strike and dip. While LB seismicity is uncorrelated with the mapped trace of the NIF, Compton seismicity illuminates a sub-vertical fault that extends down to about 20 km. This result, along with the reported high flux of mantle Helium along the NIF (Boles et al., 2015), suggests that the NIF is deeply rooted and acts as a major conduit for mantle fluids. We find that the LB size distribution obeys the typical power-law at shallow depths, but falls off exponentially for events occurring below 20 km. Because deep seismicity occurs uniformly beneath LB, this transition is attributed to a reduction in seismic asperity density with increasing depth, consistent with a transition to a diffuse deformation regime.

Thin-skinned tectonics of the Upper Ojai Valley and Sulphur Mountain area, Ventura basin, California

DOE Office of Scientific and Technical Information (OSTI.GOV)

Huftile, G.J.

1991-08-01

By integrating surface mapping with subsurface well data and drawing cross sections and subsurface maps, the geometry of shallow structures and their geologic history of the Upper Ojai Valley of California can be reconstructed. The geometry of shallow structures, the geologic history, and the location of earthquake foci then offer constraints on the deep structure of this complex area. The Upper Ojai Valley is a tectonic depression between opposing reverse faults. Its northern border is formed by the active, north-dipping San Cayetano fault, which has 6.0 km of stratigraphic separation in the Silverthread area of the Ojai oil field andmore » 2.6 km of stratigraphic separation west of Sisar Creek. The fault dies out farther west in Ojai Valley, where the south-vergent shortening is transferred to a blind thrust. The southern border of the Upper Ojai Valley is formed by the Quaternary Lion fault set, which dips south and merges into the Sisar decollement within the south-dipping, ductile, lower Miocene Rincon formation. By the middle Pleistocene, the Sulphur Mountain anticlinorium and the Big Canyon syncline began forming as a fault-propagation fold; the fault-propagation fold is rooted in the Sisar decollement, a passive backthrust rising from a blind thrust at depth. The formation of the Sulphur Mountain anticlinorium was followed closely by the ramping of the south-dipping Lion fault set to the surface over the nonmarine upper Pleistocene Saugus Formation. To the east, the San Cayetano fault overrides and folds the Lion Fault set near the surface. Area-balancing of the deformation shows shortening of 15.5 km, and suggests a 17 km depth to the brittle-ductile transition.« less

Preliminary geologic map of Black Canyon and surrounding region, Nevada and Arizona

USGS Publications Warehouse

Felger, Tracey J.; Beard, L. Sue; Anderson, Zachary W.; Fleck, Robert J.; Wooden, Joseph L.; Seixas, Gustav B.

2014-01-01

Thermal springs in Black Canyon of the Colorado River, downstream of Hoover Dam, are important recreational, ecological, and scenic features of the Lake Mead National Recreation Area. This report presents the results from a U.S. Geological Survey study of the geologic framework of the springs. The study was conducted in cooperation with the National Park Service and funded by both the National Park Service and National Cooperative Geologic Mapping Program of the U.S. Geological Survey. The report has two parts: A, a 1:48,000-scale geologic map created from existing geologic maps and augmented by new geologic mapping and geochronology; and B, an interpretive report that presents results based on a collection of fault kinematic data near springs within Black Canyon and construction of 1:100,000-scale geologic cross sections that extend across the western Lake Mead region. Exposures in Black Canyon are mostly of Miocene volcanic rocks, underlain by crystalline basement composed of Miocene plutonic rocks or Proterozoic metamorphic rocks. The rocks are variably tilted and highly faulted. Faults strike northwest to northeast and include normal and strike-slip faults. Spring discharge occurs along faults intruded by dacite dikes and plugs; weeping walls and seeps extend away from the faults in highly fractured rock or relatively porous volcanic breccias, or both. Results of kinematic analysis of fault data collected along tributaries to the Colorado River indicate two episodes of deformation, consistent with earlier studies. The earlier episode formed during east-northeast-directed extension, and the later during east-southeast-directed extension. At the northern end of the study area, pre-existing fault blocks that formed during the first episode were rotated counterclockwise along the left-lateral Lake Mead Fault System. The resulting fault pattern forms a complex arrangement that provides both barriers and pathways for groundwater movement within and around Black Canyon. Regional cross sections in this report show that thick Paleozoic carbonate aquifer rocks of east-central Nevada do not extend into the Black Canyon area and generally are terminated to the south at a major tectonic boundary defined by the northeast-striking Lake Mead Fault System and the northwest-striking Las Vegas Valley shear zone. Faults to the west of Black Canyon strike dominantly north-south and form a complicated pattern that may inhibit easterly groundwater movement from Eldorado Valley. To the east of Black Canyon, crystalline Proterozoic rocks locally overlain by Tertiary volcanic rocks in the Black Mountains are bounded by steep north-south normal faults. These faults may also inhibit westerly groundwater movement from Detrital Valley toward Black Canyon. Finally, the cross sections show clearly that Proterozoic basement rocks and (or) Tertiary plutonic rocks are shallow in the Black Canyon area (at the surface to a few hundred meters depth) and are cut by several major faults that discharge most of the springs in the Black Canyon. Therefore, the faults most likely provide groundwater pathways to sufficient depths that the groundwater is heated to the observed temperatures of up to 55 °C.

Geologic framework of thermal springs, Black Canyon, Nevada and Arizona

USGS Publications Warehouse

Beard, L. Sue; Anderson, Zachary W.; Felger, Tracey J.; Seixas, Gustav B.

2014-01-01

Thermal springs in Black Canyon of the Colorado River, downstream of Hoover Dam, are important recreational, ecological, and scenic features of the Lake Mead National Recreation Area. This report presents the results from a U.S. Geological Survey study of the geologic framework of the springs. The study was conducted in cooperation with the National Park Service and funded by both the National Park Service and National Cooperative Geologic Mapping Program of the U.S. Geological Survey. The report has two parts: A, a 1:48,000-scale geologic map created from existing geologic maps and augmented by new geologic mapping and geochronology; and B, an interpretive report that presents results based on a collection of fault kinematic data near springs within Black Canyon and construction of 1:100,000-scale geologic cross sections that extend across the western Lake Mead region. Exposures in Black Canyon are mostly of Miocene volcanic rocks, underlain by crystalline basement composed of Miocene plutonic rocks or Proterozoic metamorphic rocks. The rocks are variably tilted and highly faulted. Faults strike northwest to northeast and include normal and strike-slip faults. Spring discharge occurs along faults intruded by dacite dikes and plugs; weeping walls and seeps extend away from the faults in highly fractured rock or relatively porous volcanic breccias, or both. Results of kinematic analysis of fault data collected along tributaries to the Colorado River indicate two episodes of deformation, consistent with earlier studies. The earlier episode formed during east-northeast-directed extension, and the later during east-southeast-directed extension. At the northern end of the study area, pre-existing fault blocks that formed during the first episode were rotated counterclockwise along the left-lateral Lake Mead Fault System. The resulting fault pattern forms a complex arrangement that provides both barriers and pathways for groundwater movement within and around Black Canyon. Regional cross sections in this report show that thick Paleozoic carbonate aquifer rocks of east-central Nevada do not extend into the Black Canyon area and generally are terminated to the south at a major tectonic boundary defined by the northeast-striking Lake Mead Fault System and the northwest-striking Las Vegas Valley shear zone. Faults to the west of Black Canyon strike dominantly north-south and form a complicated pattern that may inhibit easterly groundwater movement from Eldorado Valley. To the east of Black Canyon, crystalline Proterozoic rocks locally overlain by Tertiary volcanic rocks in the Black Mountains are bounded by steep north-south normal faults. These faults may also inhibit westerly groundwater movement from Detrital Valley toward Black Canyon. Finally, the cross sections show clearly that Proterozoic basement rocks and (or) Tertiary plutonic rocks are shallow in the Black Canyon area (at the surface to a few hundred meters depth) and are cut by several major faults that discharge most of the springs in the Black Canyon. Therefore, the faults most likely provide groundwater pathways to sufficient depths that the groundwater is heated to the observed temperatures of up to 55 °C.

Novel Coupled Thermochronometric and Geochemical Investigation of Blind Geothermal Resources in Fault-Controlled Dilational Corners

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stockli, Daniel

Geothermal plays in extensional and transtensional tectonic environments have long been a major target in the exploration of geothermal resources and the Dixie Valley area has served as a classic natural laboratory for this type of geothermal plays. In recent years, the interactions between normal faults and strike-slip faults, acting either as strain relay zones have attracted significant interest in geothermal exploration as they commonly result in fault-controlled dilational corners with enhanced fracture permeability and thus have the potential to host blind geothermal prospects. Structural ambiguity, complications in fault linkage, etc. often make the selection for geothermal exploration drilling targetsmore » complicated and risky. Though simplistic, the three main ingredients of a viable utility-grade geothermal resource are heat, fluids, and permeability. Our new geological mapping and fault kinematic analysis derived a structural model suggest a two-stage structural evolution with (a) middle Miocene N -S trending normal faults (faults cutting across the modern range), - and tiling Olio-Miocene volcanic and sedimentary sequences (similar in style to East Range and S Stillwater Range). NE-trending range-front normal faulting initiated during the Pliocene and are both truncating N-S trending normal faults and reactivating some former normal faults in a right-lateral fashion. Thus the two main fundamental differences to previous structural models are (1) N-S trending faults are pre-existing middle Miocene normal faults and (2) these faults are reactivated in a right-later fashion (NOT left-lateral) and kinematically linked to the younger NE-trending range-bounding normal faults (Pliocene in age). More importantly, this study provides the first constraints on transient fluid flow through the novel application of apatite (U-Th)/He (AHe) and 4He/ 3He thermochronometry in the geothermally active Dixie Valley area in Nevada.« less

Methane seeps along boundaries of receding glaciers in Alaska and Greenland

NASA Astrophysics Data System (ADS)

Walter Anthony, K. M.; Anthony, P. M.; Grosse, G.; Chanton, J.

2012-12-01

Glaciers, ice sheets, and permafrost form a 'cryosphere cap' that traps methane formed in the subsurface, restricting its flow to the Earth's surface and atmosphere. Despite model predictions that glacier melt and degradation of permafrost open conduits for methane's escape, there has been a paucity of field evidence for 'subcap' methane seepage to the atmosphere as a direct result of cryosphere disintegration in the terrestrial Arctic. Here, we document for the first time the release of sub-cryosphere methane to lakes, rivers, shallow marine fjords and the atmosphere from abundant gas seeps concentrated along boundaries of receding glaciers and permafrost thaw in Alaska and Greenland. Through aerial and ground surveys of 6,700 lakes and fjords in Alaska we mapped >150,000 gas seeps identified as bubbling-induced open holes in seasonal ice. Using gas flow rates, stable isotopes, and radiocarbon dating, we distinguished recent ecological methane from subcap, geologic methane. Subcap seeps had anomalously high bubbling rates, 14C-depletion, and stable isotope values matching microbial sources associated with sedimentary deposits and coal beds as well as thermogenic methane accumulations in Alaska. Since differential ice loading can overpressurize fluid reservoirs and cause sediment fracturing beneath ice sheets, and since the loss of glacial ice reduces normal stress on ground, opens joints, and activates faults and fissures, thereby increasing permeability of the crust to fluid flow, we hypothesized that in the previously glaciated region of Southcentral Alaska, where glacial wastage continues presently, subcap seeps should be disproportionately associated with neotectonic faults. Geospatial analysis confirmed that subcap seep sites were associated with faults within a 7 km belt from the modern glacial extent. The majority of seeps were located in areas affected by seismicity from isostatic rebound associated with deglaciation following the Little Ice Age (LIA; ca. 1650-1850 C.E.). Across Alaska, we found a relationship between methane stable isotopes, radiocarbon age, and distance to faults. Faults appear to allow the escape of deeper, more 14C-depleted methane to the atmosphere, whereas seeps away from faults entrained 14C-enriched methane formed in shallower sediments from microbial decomposition of younger organic matter. Additionally, we observed younger subcap methane seeps in lakes of Greenland's Sondrestrom Fjord that were associated with ice-sheet retreat since the LIA. These correlations suggest that in a warming climate, continued disintegration of glaciers, permafrost, and parts of the polar ice sheets will weaken subsurface seals and further open conduits, allowing a transient expulsion of methane currently trapped by the cryosphere cap.

Fluid Pressure in the Shallow Plate Interface at the Nankai Trough Subduction Zone

NASA Astrophysics Data System (ADS)

Tobin, H. J.; Saffer, D.

2003-12-01

The factors controlling the occurrence, magnitude, and other characteristics of great earthquakes is a fundamental outstanding question in fault physics. Pore fluid pressure is perhaps the most critical yet poorly known parameter governing the strength and seismogenic character of plate boundary faults, but unfortunately cannot be directly inferred through available geophysical sensing methods. Moreover, true in situ fluid pressure has proven difficult to measure even in boreholes. At the Nankai Trough, several hundred meters of sediment are subducted beneath the frontal portion of the accretionary prism. The up-dip portion of the plate interface is therefore hosted in these fine-grained marine sedimentary rocks. ODP Leg 190 and 196 showed that these rapidly-loaded underthrust sediments are significantly overpressured near the deformation front. Here, we attempt to quantitatively infer porosity, pore pressure, and effective normal stress at the plate interface at depths currently inaccessible to drilling. Using seismic reflection interval velocity calibrated at the boreholes to porosity, we quantitatively infer pore pressure to ˜ 20 km down-dip of the deformation front, to a plate interface depth of ˜ 6 km. We have developed a Nankai-specific velocity-porosity transform using ODP cores and logs. We use this function to derive a porosity profile for each of two down-dip seismic sections extracted from a 3-D dataset from the Cape Muroto region. We then calculate pore fluid pressure and effective vertical (fault-normal) stress for the underthrust sediment section using a compaction disequilibrium approach and core-based consolidation test data. Because the pore fluid pressure at the fault interface is likely controlled by that of the top of the underthrust section, this calculation represents a quantitative profile of effective stress and pore pressure at the plate interface. Results show that seismic velocity and porosity increase systematically downdip in the underthrust section, but the increase is suppressed relative to that expected from normally consolidating sediments. The computed pore pressure increases landward from an overpressure ratio (λ * = hydrostatic pressure divided by the lithostatic overburden) of ˜ 0.6 at the deformation front to ˜ 0.77 where sediments have been subducted 15 km. The results of this preliminary analysis suggest that a 3-dimensional mapping of predicted effective normal stress in the seismic data volume is possible.

Possible cause for an improbable earthquake: The 1997 MW 4.9 southern Alabama earthquake and hydrocarbon recovery

USGS Publications Warehouse

Gomberg, J.; Wolf, L.

1999-01-01

Circumstantial and physical evidence indicates that the 1997 MW 4.9 earthquake in southern Alabama may have been related to hydrocarbon recovery. Epicenters of this earthquake and its aftershocks were located within a few kilometers of active oil and gas extraction wells and two pressurized injection wells. Main shock and aftershock focal depths (2-6 km) are within a few kilometers of the injection and withdrawal depths. Strain accumulation at geologic rates sufficient to cause rupture at these shallow focal depths is not likely. A paucity of prior seismicity is difficult to reconcile with the occurrence of an earthquake of MW 4.9 and a magnitude-frequency relationship usually assumed for natural earthquakes. The normal-fault main-shock mechanism is consistent with reactivation of preexisting faults in the regional tectonic stress field. If the earthquake were purely tectonic, however, the question arises as to why it occurred on only the small fraction of a large, regional fault system coinciding with active hydrocarbon recovery. No obvious temporal correlation is apparent between the earthquakes and recovery activities. Although thus far little can be said quantitatively about the physical processes that may have caused the 1997 sequence, a plausible explanation involves the poroelastic response of the crust to extraction of hydrocarbons.

Seismicity in South Carolina

USGS Publications Warehouse

Shedlock, K.M.

1988-01-01

The largest historical earthquake in South Carolina, and in the southeastern US, occurred in the Coastal Plain province, probably northwest of Charleston, in 1886. Locations for aftershocks associated with this earthquake, estimated using intensities based on newspaper accounts, defined a northwest trending zone about 250 km long that was at least 100 km wide in the Coastal Plain but widened to a northeast trending zone in the Piedmont. The subsequent historical and instrumentally recorded seismicity in South Carolina images the 1886 aftershock zone. Instrumentally recorded seismicity in the Coastal Plain province occurs in 3 seismic zones or clusters: Middleton Place-Summervile (MPSSZ), Adams Run (ARC), and Bowman (BSZ). Approximately 68% of the Coastal Plain earthquakes occur in the MPSSZ, a north trending zone about 22 km long and 12 km wide, lying about 20 km northwest of Charleston. The hypocenters of MPSSZ earthquakes range in depth from near the surface to almost 12 km. Thrust, strike-slip, and some normal faulting are indicated by the fault plane solutions for Coastal Plain earthquakes. The maximum horizontal compressive stress, inferred from the P-axes of the fault plane solutions, is oriented NE-SW in the shallow crust (<9 km deep) but appears to be diffusely E-W between 9 to 12 km deep. -from Author

Late extension in compressional wedges above a weak, viscous décollement: results from analogue modeling

NASA Astrophysics Data System (ADS)

Borderie, Sandra; Vendeville, Bruno C.; Graveleau, Fabien; Witt, César

2016-04-01

Extension during convergence is a structural process commonly encountered in different geodynamic settings, such as accretionary wedges subjected to tectonic erosion, or mountain belts undergoing post-orogenic collapse. This has been investigated with experimental models at the scale of doubly-vergent wedges (Haq and Davis 2008; Bonini et al. 2000, Buck and Sokoutis 1994) but not thoroughly at the scale of fold-and-thrust belts. During an experimental investigation carried out on the behavior of segmented fold-and-thrust belts induced by stratigraphic inheritance in the foreland series (Borderie et al., EGU this session), unexpected shallow normal faulting occurred. The models comprised one basal frictional décollement (glass microbeads) and one upper viscous décollement embedded in the cover (silicone polymer). Extension took place during the late stages of the experiments and it was localized at the transition zone between the rear domain of the wedge and the frontal fold-and-thrust belt that detached on the upper viscous décollement. Normal faults strike parallel to the compressional structures and mainly dip toward the foreland. They root in the viscous décollement. Through a series of parametrized experiments dedicated to constrain the timing of formation of these extensional structures, we could evidence that these normal faults appear once the bulk shortening in the rear domain has created enough uplift of the internal zone by antiformal stacking and enough forelandward tilting of the upper viscous décollement. These two latter mechanisms are direct consequences of the whole wedge dynamics that links the thrust fault dynamics in the upper shallow sedimentary sequence and the thrust dynamics of the deep subsalt basement. The occurrence of this extension depends on the initial position of the upper viscous décollement and notably the position of the internal pinchout relative to the position of the backstop. Additional tests have also demonstrated that this extension is prevented by surface processes and notably sedimentation. We compare our experimental findings with natural examples of extensional features in various fold-and-thrust belts and accretionary features across the world (e.g. the Mediterranean ridge). References: Bonini, Marco, Dimitrios Sokoutis, Genene Mulugeta, and Emmanouil Katrivanos. 2000. "Modelling Hanging Wall Accommodation above Rigid Thrust Ramps." Journal of Structural Geology 22 (8): 1165-79. Borderie, Sandra, Fabien Graveleau, Cesar Witt and Bruno C. Vendeville. 2016. "Analogue modeling of 3-D structural segmentation in fold-and-thrust belts: interactions between frictional and viscous provinces in foreland basins." Gephys. Res. Abstr., 18, EGU2016-Vienne. Buck, W Roger, and Dimitrios Sokoutis. 1994. "Analogue Model of Gravitational Collapse and Surface Extension during Continental Convergence." Nature 369: 737-40. Haq, Saad SB, and Dan M. Davis. 2008. "Extension during Active Collision in Thin-Skinned Wedges: Insights from Laboratory Experiments." Geology 36 (6): 475-78.

Potential for a large earthquake near Los Angeles inferred from the 2014 La Habra earthquake.

PubMed

Donnellan, Andrea; Grant Ludwig, Lisa; Parker, Jay W; Rundle, John B; Wang, Jun; Pierce, Marlon; Blewitt, Geoffrey; Hensley, Scott

2015-09-01

Tectonic motion across the Los Angeles region is distributed across an intricate network of strike-slip and thrust faults that will be released in destructive earthquakes similar to or larger than the 1933  M 6.4 Long Beach and 1994  M 6.7 Northridge events. Here we show that Los Angeles regional thrust, strike-slip, and oblique faults are connected and move concurrently with measurable surface deformation, even in moderate magnitude earthquakes, as part of a fault system that accommodates north-south shortening and westerly tectonic escape of northern Los Angeles. The 28 March 2014 M 5.1 La Habra earthquake occurred on a northeast striking, northwest dipping left-lateral oblique thrust fault northeast of Los Angeles. We present crustal deformation observation spanning the earthquake showing that concurrent deformation occurred on several structures in the shallow crust. The seismic moment of the earthquake is 82% of the total geodetic moment released. Slip within the unconsolidated upper sedimentary layer may reflect shallow release of accumulated strain on still-locked deeper structures. A future M 6.1-6.3 earthquake would account for the accumulated strain. Such an event could occur on any one or several of these faults, which may not have been identified by geologic surface mapping.

Potential for a large earthquake near Los Angeles inferred from the 2014 La Habra earthquake

PubMed Central

Grant Ludwig, Lisa; Parker, Jay W.; Rundle, John B.; Wang, Jun; Pierce, Marlon; Blewitt, Geoffrey; Hensley, Scott

2015-01-01

Abstract Tectonic motion across the Los Angeles region is distributed across an intricate network of strike‐slip and thrust faults that will be released in destructive earthquakes similar to or larger than the 1933 M6.4 Long Beach and 1994 M6.7 Northridge events. Here we show that Los Angeles regional thrust, strike‐slip, and oblique faults are connected and move concurrently with measurable surface deformation, even in moderate magnitude earthquakes, as part of a fault system that accommodates north‐south shortening and westerly tectonic escape of northern Los Angeles. The 28 March 2014 M5.1 La Habra earthquake occurred on a northeast striking, northwest dipping left‐lateral oblique thrust fault northeast of Los Angeles. We present crustal deformation observation spanning the earthquake showing that concurrent deformation occurred on several structures in the shallow crust. The seismic moment of the earthquake is 82% of the total geodetic moment released. Slip within the unconsolidated upper sedimentary layer may reflect shallow release of accumulated strain on still‐locked deeper structures. A future M6.1–6.3 earthquake would account for the accumulated strain. Such an event could occur on any one or several of these faults, which may not have been identified by geologic surface mapping. PMID:27981074

Earthquake Clustering on Normal Faults: Insight from Rate-and-State Friction Models

NASA Astrophysics Data System (ADS)

Biemiller, J.; Lavier, L. L.; Wallace, L.

2016-12-01

Temporal variations in slip rate on normal faults have been recognized in Hawaii and the Basin and Range. The recurrence intervals of these slip transients range from 2 years on the flanks of Kilauea, Hawaii to 10 kyr timescale earthquake clustering on the Wasatch Fault in the eastern Basin and Range. In addition to these longer recurrence transients in the Basin and Range, recent GPS results there also suggest elevated deformation rate events with recurrence intervals of 2-4 years. These observations suggest that some active normal fault systems are dominated by slip behaviors that fall between the end-members of steady aseismic creep and periodic, purely elastic, seismic-cycle deformation. Recent studies propose that 200 year to 50 kyr timescale supercycles may control the magnitude, timing, and frequency of seismic-cycle earthquakes in subduction zones, where aseismic slip transients are known to play an important role in total deformation. Seismic cycle deformation of normal faults may be similarly influenced by its timing within long-period supercycles. We present numerical models (based on rate-and-state friction) of normal faults such as the Wasatch Fault showing that realistic rate-and-state parameter distributions along an extensional fault zone can give rise to earthquake clusters separated by 500 yr - 5 kyr periods of aseismic slip transients on some portions of the fault. The recurrence intervals of events within each earthquake cluster range from 200 to 400 years. Our results support the importance of stress and strain history as controls on a normal fault's present and future slip behavior and on the characteristics of its current seismic cycle. These models suggest that long- to medium-term fault slip history may influence the temporal distribution, recurrence interval, and earthquake magnitudes for a given normal fault segment.

Evolving transpressional strain fields along the San Andreas fault in southern California: implications for fault branching, fault dip segmentation and strain partitioning

NASA Astrophysics Data System (ADS)

Bergh, Steffen; Sylvester, Arthur; Damte, Alula; Indrevær, Kjetil

2014-05-01

The San Andreas fault in southern California records only few large-magnitude earthquakes in historic time, and the recent activity is confined primarily on irregular and discontinuous strike-slip and thrust fault strands at shallow depths of ~5-20 km. Despite this fact, slip along the San Andreas fault is calculated to c. 35 mm/yr based on c.160 km total right lateral displacement for the southern segment of the fault in the last c. 8 Ma. Field observations also reveal complex fault strands and multiple events of deformation. The presently diffuse high-magnitude crustal movements may be explained by the deformation being largely distributed along more gently dipping reverse faults in fold-thrust belts, in contrast to regions to the north where deformation is less partitioned and localized to narrow strike-slip fault zones. In the Mecca Hills of the Salton trough transpressional deformation of an uplifted segment of the San Andreas fault in the last ca. 4.0 My is expressed by very complex fault-oblique and fault-parallel (en echelon) folding, and zones of uplift (fold-thrust belts), basement-involved reverse and strike-slip faults and accompanying multiple and pervasive cataclasis and conjugate fracturing of Miocene to Pleistocene sedimentary strata. Our structural analysis of the Mecca Hills addresses the kinematic nature of the San Andreas fault and mechanisms of uplift and strain-stress distribution along bent fault strands. The San Andreas fault and subsidiary faults define a wide spectrum of kinematic styles, from steep localized strike-slip faults, to moderate dipping faults related to oblique en echelon folds, and gently dipping faults distributed in fold-thrust belt domains. Therefore, the San Andreas fault is not a through-going, steep strike-slip crustal structure, which is commonly the basis for crustal modeling and earthquake rupture models. The fault trace was steep initially, but was later multiphase deformed/modified by oblique en echelon folding, renewed strike-slip movements and contractile fold-thrust belt structures. Notably, the strike-slip movements on the San Andreas fault were transformed outward into the surrounding rocks as oblique-reverse faults to link up with the subsidiary Skeleton Canyon fault in the Mecca Hills. Instead of a classic flower structure model for this transpressional uplift, the San Andreas fault strands were segmented into domains that record; (i) early strike-slip motion, (ii) later oblique shortening with distributed deformation (en echelon fold domains), followed by (iii) localized fault-parallel deformation (strike-slip) and (iv) superposed out-of-sequence faulting and fault-normal, partitioned deformation (fold-thrust belt domains). These results contribute well to the question if spatial and temporal fold-fault branching and migration patterns evolving along non-vertical strike-slip fault segments can play a role in the localization of earthquakes along the San Andreas fault.

Characterization of intrabasin faulting and deformation for earthquake hazards in southern Utah Valley, Utah, from high-resolution seismic imaging

USGS Publications Warehouse

Stephenson, William J.; Odum, Jack K.; Williams, Robert A.; McBride, John H.; Tomlinson, Iris

2012-01-01

We conducted active and passive seismic imaging investigations along a 5.6-km-long, east–west transect ending at the mapped trace of the Wasatch fault in southern Utah Valley. Using two-dimensional (2D) P-wave seismic reflection data, we imaged basin deformation and faulting to a depth of 1.4 km and developed a detailed interval velocity model for prestack depth migration and 2D ground-motion simulations. Passive-source microtremor data acquired at two sites along the seismic reflection transect resolve S-wave velocities of approximately 200 m/s at the surface to about 900 m/s at 160 m depth and confirm a substantial thickening of low-velocity material westward into the valley. From the P-wave reflection profile, we interpret shallow (100–600 m) bedrock deformation extending from the surface trace of the Wasatch fault to roughly 1.5 km west into the valley. The bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front. Further west in the valley, the P-wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the Wasatch fault. Another inferred fault about 4 km west of the mapped Wasatch fault displaces horizons within the valley to as shallow as 100 m depth. The overall deformational pattern imaged in our data is consistent with the Wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults, as part of the evolution of an inferred 20-km-wide half-graben structure within Utah Valley. Finite-difference 2D modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m.

Spatiotemporal analysis of Quaternary normal faults in the Northern Rocky Mountains, USA

NASA Astrophysics Data System (ADS)

Davarpanah, A.; Babaie, H. A.; Reed, P.

2010-12-01

The mid-Tertiary Basin-and-Range extensional tectonic event developed most of the normal faults that bound the ranges in the northern Rocky Mountains within Montana, Wyoming, and Idaho. The interaction of the thermally induced stress field of the Yellowstone hot spot with the existing Basin-and-Range fault blocks, during the last 15 my, has produced a new, spatially and temporally variable system of normal faults in these areas. The orientation and spatial distribution of the trace of these hot-spot induced normal faults, relative to earlier Basin-and-Range faults, have significant implications for the effect of the temporally varying and spatially propagating thermal dome on the growth of new hot spot related normal faults and reactivation of existing Basin-and-Range faults. Digitally enhanced LANDSAT 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 4 and 5 Thematic Mapper (TM) bands, with spatial resolution of 30 m, combined with analytical GIS and geological techniques helped in determining and analyzing the lineaments and traces of the Quaternary, thermally-induced normal faults in the study area. Applying the color composite (CC) image enhancement technique, the combination of bands 3, 2 and 1 of the ETM+ and TM images was chosen as the best statistical choice to create a color composite for lineament identification. The spatiotemporal analysis of the Quaternary normal faults produces significant information on the structural style, timing, spatial variation, spatial density, and frequency of the faults. The seismic Quaternary normal faults, in the whole study area, are divided, based on their age, into four specific sets, which from oldest to youngest include: Quaternary (>1.6 Ma), middle and late Quaternary (>750 ka), latest Quaternary (>15 ka), and the last 150 years. A density map for the Quaternary faults reveals that most active faults are near the current Yellowstone National Park area (YNP), where most seismically active faults, in the past 1.6 my, are located. The GIS based autocorrelation method, applied to the trace orientation, length, frequency, and spatial distribution for each age-defined fault set, revealed spatial homogeneity for each specific set. The results of the method of Moran`sI and Geary`s C show no spatial autocorrelation among the trend of the fault traces and their location. Our results suggest that while lineaments of similar age define a clustered pattern in each domain, the overall distribution pattern of lineaments with different ages seems to be non-uniform (random). The directional distribution analysis reveals a distinct range of variation for fault traces of different ages (i.e., some displaying ellipsis behavior). Among the Quaternary normal fault sets, the youngest lineament set (i.e., last 150 years) defines the greatest ellipticity (eccentricity) and the least lineaments distribution variation. The frequency rose diagram for the entire Quaternary normal faults, shows four major modes (around 360o, 330o, 300o, and 270o), and two minor modes (around 235 and 205).

Shallow velocity structure of the Alaska Peninsula subduction zone and implications for controls on seismic behavior

NASA Astrophysics Data System (ADS)

Li, J.; Shillington, D. J.; Becel, A.; Nedimovic, M. R.; Kuehn, H.; Webb, S. C.; Abers, G. A.; Keranen, K. M.; Saffer, D. M.

2014-12-01

Downdip and along-strike variations in the seismic behavior of subduction zone megathrust faults are thought to be strongly controlled by changes in the material properties along the plate boundary. Roughness and hydration of the incoming plate, fluid pressure and lithology in the subducting sediment channel are likely to control the distribution of shallower rupture. Here, we focus on the subduction zone offshore of the Alaska Peninsula. In 2011, the ALEUT program acquired deep penetration multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data across the apparently freely sliding Shumagin Gap, the locked Semidi segment that last ruptured in 1938 M8.2 earthquake, and the locked western Kodiak asperity, which ruptured in the 1964 M9.2 earthquake. Seismic reflection data from the ALEUT cruise reveal significant variability in the thickness of sediment on the incoming plate and entering the trench, and the roughness and degree of hydration of the incoming plate. Oceanic crust entering the trench in the Shumagin gap is rugged with extensive faults and only a thin layer of sediment (<0.5 km thick). Farther east in the Semidi segment, the subducting plate has a smoother surface with thicker sediments (~1 km thick) and less faulting/hydration. To better constrain the properties of the accretionary prism and shallow part of the plate boundary, we are undertaking travel time tomography using reflection/refraction phases in OBS and MCS data, and constraints on the interface geometry from MCS images to estimate the detailed shallow velocity structure, with particular focus on properties within the shallow subduction channel. We observe refractions and reflections in OBS data from the shallow part of the subduction zone in both the Shumagin Gap and Semidi segment, including reflections off the top and base of what appears to be a layer of subducting sediment, which can be used for this work. We plan to present initial models of the shallow part of the subduction zone from both segments and discuss comparisons between the two.

Seismotectonics of Northeastern Sicily and Southern Calabria (Italy): New constraints on the tectonic structures featuring in a crucial sector for the Central Mediterranean geodynamics

NASA Astrophysics Data System (ADS)

Scarfì, Luciano; Barberi, Graziella; Musumeci, Carla; Patanè, Domenico

2016-04-01

The purpose of this study is to gain a better understanding on the tectonic structures featuring in a crucial sector of central Mediterranean area, including the Aeolian Islands, southern Calabria and northeastern Sicily, where the convergence between Eurasian and African plates has given rise to a complicated collisional/subduction complex. A high quality dataset of about 3000 earthquakes has been exploited for local earthquake tomography and focal mechanisms computation. Results depict undiscovered details of a network of faults which enables the contemporary existence of adjacent compressional and extensional domains. In particular, tomographic images, seismic events distribution and focal mechanisms pinpoint the geometry and activity of a lithospheric-scale tear faults system which, with a NW-SE trend through Sicily and the Tyrrhenian and Ionian Seas, represents the southern edge of the Ionian subduction trench zone. At crustal depth, this tearing is well highlighted by a rotation of the maximum horizontal stress, moving across the area from west toward east. In addition, the shallow normal fault regime, characterising the northeastern Sicily mainland, south of the NW-SE lineament, changes in the deeper part of the crust. Indeed, a NE-SW earthquake distribution, NW gently dipping, and inverse fault solutions indicate a still active contractional deformation in the eastern Sicily, caused by the Africa-Eurasia convergence and well framed with the current compressive regime along the southern Tyrrhenian zone and at the front of the Sicilian Chain-Foreland.

Rupture Characteristics of the 25 November 2016 Aketao Earthquake ( M w 6.6) in Eastern Pamir Revealed by GPS and Teleseismic Data

NASA Astrophysics Data System (ADS)

Li, Jie; Liu, Gang; Qiao, Xuejun; Xiong, Wei; Wang, Xiaoqiang; Liu, Daiqin; Sun, Jianing; Yushan, Ailixiati; Yusan, Sulitan; Fang, Wei; Wang, Qi

2018-02-01

The 25 November 2016 Aketao, Xinjiang earthquake occurred on the northeastern margin of the Pamir plateau, rupturing the Muji fault on the northern segment of the Kongur Extensional System. We collected coseismic offsets at 7 GPS sites, which show that the fault experienced significate dextral slip with a near-field geodetic displacement of up to 12 cm along the strike. The joint inversion of GPS data and teleseismic P waveforms suggests a complex rupture pattern characterized by the unilateral propagation slip from the epicenter to the southeast for 60 km with a total seismic moment of 1.3 × 1019 Nm, corresponding to a magnitude of M w 6.7 earthquake. Our model of slip distribution shows two major slip patches with a slip amplitude up to 0.6 m, one located at shallow depths of 0-8 km close to the hypocenter with apparent surface breaks and the other, 40 km to the southeast, buried at a greater depth of 12 km. The rupture is dominated by a right-lateral strike slip with significant normal-slip components. The near-field GPS data enhances the spatial resolution of source model. Based on the preferred slip model, the static Coulomb Failure Stress change caused by 2016 Aketao earthquake suggests that the unzipped western and eastern ends of Muji fault and the northern segment of Kungai Fault are significantly promoted.

Seismic identification and origin of shallow gas in the Baiyun Sag Northern South China Sea

NASA Astrophysics Data System (ADS)

Qin, Q.

2016-12-01

The analytics of three dimensional high resolution seismic data and multi-beam data gotten from Baiyun Sag(BYS), the northern South China Sea(SCS) reveals varieties of shallow gas indicators. Such indicators include gas chimneys, enhanced reflections, bright spots, pipes and acoustic blanking. Among them, the enhanced reflections suggest that the free gas has been presented. And, there are also some very high amplitude reflections and they have occurred in both deep and shallow sedimentary sections. Gas chimneys are dominant and pipes (line zones of big faults) also have been observed in much of the surveyed area if observing at 31 lines. Gas chimneys and pipes in the study area can be associated with some known faults that would act as migration pathways from deep fluids. There are some columnar zones of acoustic blanking in the survey area. This suggests that the observed structures in Baiyun sag sediments allow the emission of gases which might be for a large share of source rocks or deep gas reservoir, and there are abundant shallow gas in the Baiyun Sag. As we all know, the obvious characteristics of shallow gas are high pressure and highly dangerous. So our results are very essential to explore resources (hydrocarbon and gas hydrate) in such a petroliferous basin.

Elemental Geochemistry of Samples From Fault Segments of the San Andreas Fault Observatory at Depth (SAFOD) Drill Hole

NASA Astrophysics Data System (ADS)

Tourscher, S. N.; Schleicher, A. M.; van der Pluijm, B. A.; Warr, L. N.

2006-12-01

Elemental geochemistry of mudrock samples from phase 2 drilling of the San Andreas Fault Observatory at Depth (SAFOD) is presented from bore hole depths of 3066 m to 3169 m and from 3292 m to 3368 m, which contain a creeping section and main trace of the fault, respectively. In addition to preparation and analysis of whole rock sample, fault grains with neomineralized, polished surfaces were hand picked from well-washed whole rock samples, minimizing the potential contamination from drilling mud and steel shavings. The separated fractions were washed in deionized water, powdered using a mortar and pestle, and analyzed using an Inductively Coupled Plasma- Optical Emission Spectrometer for major and minor elements. Based on oxide data results, systematic differences in element concentrations are observed between the whole rock and fault rock. Two groupings of data points are distinguishable in the regions containing the main trace of the fault, a shallow part (3292- 3316 m) and a deeper section (3320-3368 m). Applying the isocon method, assuming Zr and Ti to be immobile elements in these samples, indicates a volume loss of more than 30 percent in the shallow part and about 23 percent in the deep part of the main trace. These changes are minimum estimates of fault-related volume loss, because the whole rock from drilling samples contains variable amount of fault rock as well. Minimum estimates for volume loss in the creeping section of the fault are more than 50 percent when using the isocon method, comparing whole rock to plucked fault rock. The majority of the volume loss in the fault rocks is due to the dissolution and loss of silica, potassium, aluminum, sodium and calcium, whereas (based on oxide data) the mineralized surfaces of fractures appear to be enriched in Fe and Mg. The large amount of element mobility within these fault traces suggests extensive circulation of hydrous fluids along fractures that was responsible for progressive dissolution and leaching of the wall rock during faulting.

Role of extensional structures on the location of folds and thrusts during tectonic inversion (northern Iberian Chain, Spain)

NASA Astrophysics Data System (ADS)

Cortés, Angel L.; Liesa, Carlos L.; Soria, Ana R.; Meléndez, Alfonso

1999-03-01

The Aguilón Subbasin (NE Spain) was originated daring the Late Jurassic-Early Cretaceous rifting due to the action of large normal faults, probably inherited from Late Variscan fracturing. WNW-ESE normal faults limit two major troughs filled by continental deposits (Valanginian to Early Barremian). NE-SW faults control the location of subsidiary depocenters within these troughs. These basins were weakly inverted during the Tertiary with folds and thrusts striking E-W to WNW-ESE involving the Mesozoic-Tertiary cover with a maximum estimated shortening of about 12 %. Tertiary compression did not produce the total inversion of the Mesozoic basin but extensional structures are responsible for the location of major Tertiary folds. Shortening of the cover during the Tertiary involved both reactivation of some normal faults and development of folds and thrusts nucleated on basement extensional steps. The inversion style depends mainly on the occurrence and geometry of normal faults limiting the basin. Steep normal faults were not reactivated but acted as buttresses to the cover translation. Around these faults, affecting both basement and cover, folds and thrusts were nucleated due to the stress rise in front of major faults. Within the cover, the buttressing against normal faults consists of folding and faulting implying little shortening without development of ceavage or other evidence of internal deformation.

A 3-D view of field-scale fault-zone cementation from geologically ground-truthed electrical resistivity

NASA Astrophysics Data System (ADS)

Barnes, H.; Spinelli, G. A.; Mozley, P.

2015-12-01

Fault-zones are an important control on fluid flow, affecting groundwater supply, hydrocarbon/contaminant migration, and waste/carbon storage. However, current models of fault seal are inadequate, primarily focusing on juxtaposition and entrainment effects, despite the recognition that fault-zone cementation is common and can dramatically reduce permeability. We map the 3D cementation patterns of the variably cemented Loma Blanca fault from the land surface to ~40 m depth, using electrical resistivity and induced polarization (IP). The carbonate-cemented fault zone is a region of anomalously low normalized chargeability, relative to the surrounding host material. Zones of low-normalized chargeability immediately under the exposed cement provide the first ground-truth that a cemented fault yields an observable IP anomaly. Low-normalized chargeability extends down from the surface exposure, surrounded by zones of high-normalized chargeability, at an orientation consistent with normal faults in the region; this likely indicates cementation of the fault zone at depth, which could be confirmed by drilling and coring. Our observations are consistent with: 1) the expectation that carbonate cement in a sandstone should lower normalized chargeability by reducing pore-surface area and bridging gaps in the pore space, and 2) laboratory experiments confirming that calcite precipitation within a column of glass beads decreases polarization magnitude. The ability to characterize spatial variations in the degree of fault-zone cementation with resistivity and IP has exciting implications for improving predictive models of the hydrogeologic impacts of cementation within faults.

Late Quaternary activity of the Grote Brogel fault, NE Belgium

NASA Astrophysics Data System (ADS)

Vanneste, Kris; Deckers, Jef; Van Noten, Koen; Schiltz, Marco; Lecocq, Thomas

2017-04-01

The Grote Brogel fault (GBF) is a WNW-ESE striking normal fault that is part of the western border fault system of the Roer Valley Graben in NE Belgium. It is one of three faults branching NW-ward from the main border fault (Geleen fault) near Bree, but its orientation diverges 22° from the general NW-SE orientation of the graben, causing a wide left step. Unlike the Geleen fault, the surface expression of the GBF has not been investigated in detail so far. We studied the Quaternary activity of the GBF and its effects on the local hydrology based on a high-resolution LiDAR digital terrain model (DTM), and geophysical and geological surveying at two sites, combining Electrical Resistivity Tomography (ERT), Cone Penetration Tests (CPTs) and boreholes. The GBF defines the northern edge of the Campine Plateau, an elevated area covered by the late Early to Middle Pleistocene Main Terrace of the Meuse River. Cumulative vertical offset since deposition of this terrace has resulted in a distinct 10-km-long fault scarp, the height of which decreases from 11 m near Bree in the east to less than 5 m near Grote Brogel in the west. The along-strike evolution of offset suggests that the GBF does not define an individual rupture segment, but is likely contiguous with the Geleen fault. DTM analysis indicates that scarps are only preserved in a few isolated places, and that the surface trace is rather complex, consisting of a series of short, relatively straight sections with strikes varying between 255° and 310°, arranged in a generally left-stepping pattern. At both investigated sites, ERT profiles clearly demonstrate the presence of fault splays in the shallow subsurface (< 50 m) underneath the identified scarps evidenced by a sudden increase in depth and thickness of a high-resistivity unit on top of a lower-resistivity unit. Boreholes and CPTs allow correlating the high-resistivity unit with the medium to coarse gravel-bearing sands of the Meuse Group, and the lower-resistivity unit below with the finer sands of the Pliocene Mol Formation. From the ERT profiles, we estimate vertical offsets of the base of the Meuse deposits of 13 m at the eastern site, and 6 m in the west. These are only slightly larger than the topographic offsets, indicating that most of the offset post-dates deposition of the Meuse Group. Earlier fault activity is attested by a change in facies (and related resistivity values on the ERT-profiles) in the Mol Fm, but cannot be quantified. Water level measurements in the boreholes and CPT holes indicate that the GBF acts as a hydrologic boundary that prevents groundwater flow from the elevated footwall towards the hanging wall, resulting in hydraulic heads of up to 12.7 m. At both investigated sites, the hydraulic head correlates with the topographic offset. At the eastern site, the shallow groundwater table in the footwall has given rise to a wet zone that is indicated on soil moisture maps and is also expressed by darker tones on aerial maps. The extent of this wet zone appears largely influenced by a local stepover that we could image in pseudo-3D using a series of closely spaced ERT profiles.

Elasto-plastic deformation and plate weakening due to normal faulting in the subducting plate along the Mariana Trench

NASA Astrophysics Data System (ADS)

Zhou, Zhiyuan; Lin, Jian

2018-06-01

We investigated variations in the elasto-plastic deformation of the subducting plate along the Mariana Trench through an analysis of flexural bending and normal fault characteristics together with geodynamic modeling. Most normal faults were initiated at the outer-rise region and grew toward the trench axis with strikes mostly subparallel to the local trench axis. The average trench relief and maximum fault throws were measured to be significantly greater in the southern region (5 km and 320 m, respectively) than the northern and central regions (2 km and 200 m). The subducting plate was modeled as an elasto-plastic slab subjected to tectonic loading at the trench axis. The calculated strain rates and velocities revealed an array of normal fault-like shear zones in the upper plate, resulting in significant faulting-induced reduction in the deviatoric stresses. We then inverted for solutions that best fit the observed flexural bending and normal faulting characteristics, revealing normal fault penetration to depths of 21, 20, and 32 km beneath the seafloor for the northern, central, and southern regions, respectively, which is consistent with the observed depths of the relocated normal faulting earthquakes in the central Mariana Trench. The calculated deeper normal faults of the southern region might lead to about twice as much water being carried into the mantle per unit trench length than the northern and central regions. We further calculated that normal faulting has reduced the effective elastic plate thickness Te by up to 52% locally in the southern region and 33% in both the northern and central regions. The best-fitting solutions revealed a greater apparent angle of the pulling force in the southern region (51-64°) than in the northern (22-35°) and central (20-34°) regions, which correlates with a general southward increase in the seismically-determined dip angle of the subducting slab along the Mariana Trench.

Shallow slip amplification and enhanced tsunami hazard unravelled by dynamic simulations of mega-thrust earthquakes

PubMed Central

Murphy, S.; Scala, A.; Herrero, A.; Lorito, S.; Festa, G.; Trasatti, E.; Tonini, R.; Romano, F.; Molinari, I.; Nielsen, S.

2016-01-01

The 2011 Tohoku earthquake produced an unexpected large amount of shallow slip greatly contributing to the ensuing tsunami. How frequent are such events? How can they be efficiently modelled for tsunami hazard? Stochastic slip models, which can be computed rapidly, are used to explore the natural slip variability; however, they generally do not deal specifically with shallow slip features. We study the systematic depth-dependence of slip along a thrust fault with a number of 2D dynamic simulations using stochastic shear stress distributions and a geometry based on the cross section of the Tohoku fault. We obtain a probability density for the slip distribution, which varies both with depth, earthquake size and whether the rupture breaks the surface. We propose a method to modify stochastic slip distributions according to this dynamically-derived probability distribution. This method may be efficiently applied to produce large numbers of heterogeneous slip distributions for probabilistic tsunami hazard analysis. Using numerous M9 earthquake scenarios, we demonstrate that incorporating the dynamically-derived probability distribution does enhance the conditional probability of exceedance of maximum estimated tsunami wave heights along the Japanese coast. This technique for integrating dynamic features in stochastic models can be extended to any subduction zone and faulting style. PMID:27725733

Analysis of the similar epicenter earthquakes on 22 January 2013 and 01 June 2013, Central Gulf of Suez, Egypt

NASA Astrophysics Data System (ADS)

Toni, Mostafa; Barth, Andreas; Ali, Sherif M.; Wenzel, Friedemann

2016-09-01

On 22 January 2013 an earthquake with local magnitude ML 4.1 occurred in the central part of the Gulf of Suez. Six months later on 1 June 2013 another earthquake with local magnitude ML 5.1 took place at the same epicenter and different depths. These two perceptible events were recorded and localized by the Egyptian National Seismological Network (ENSN) and additional networks in the region. The purpose of this study is to determine focal mechanisms and source parameters of both earthquakes to analyze their tectonic relation. We determine the focal mechanisms by applying moment tensor inversion and first motion analysis of P- and S-waves. Both sources reveal oblique focal mechanisms with normal faulting and strike-slip components on differently oriented faults. The source mechanism of the larger event on 1 June in combination with the location of aftershock sequence indicates a left-lateral slip on N-S striking fault structure in 21 km depth that is in conformity with the NE-SW extensional Shmin (orientation of minimum horizontal compressional stress) and the local fault pattern. On the other hand, the smaller earthquake on 22 January with a shallower hypocenter in 16 km depth seems to have happened on a NE-SW striking fault plane sub-parallel to Shmin. Thus, here an energy release on a transfer fault connecting dominant rift-parallel structures might have resulted in a stress transfer, triggering the later ML 5.1 earthquake. Following Brune's model and using displacement spectra, we calculate the dynamic source parameters for the two events. The estimated source parameters for the 22 January 2013 and 1 June 2013 earthquakes are fault length (470 and 830 m), stress drop (1.40 and 2.13 MPa), and seismic moment (5.47E+21 and 6.30E+22 dyn cm) corresponding to moment magnitudes of MW 3.8 and 4.6, respectively.

Polygonal deformation bands in sandstone

NASA Astrophysics Data System (ADS)

Antonellini, Marco; Nella Mollema, Pauline

2017-04-01

We report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are dm-wide zones of shear deformation bands that developed under shallow burial conditions in the lower portion of the Jurassic Entrada Fm (Utah, USA). The edges of the polygons are 1 to 5 meters long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain < 0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. Density inversion, that takes place where under-compacted and over-pressurized layers (Carmel Fm) lay below normally compacted sediments (Entrada Sandstone), may be an important process for polygonal deformation bands formation. The gravitational sliding and soft sediment structures typically observed within the Carmel Fm support this hypothesis. Soft sediment deformation may induce polygonal faulting in the section of the Entrada Sandstone just above the Carmel Fm. The permeability of the polygonal deformation bands is approximately 10-14 to 10-13 m2, which is less than the permeability of the host, Entrada Sandstone (range 10-12 to 10-11 m2). The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes.

Miocene extension and extensional folding in an anticlinal segment of the Black Mountains accommodation zone, Colorado River extensional corridor, southwestern United States

USGS Publications Warehouse

Varga, R.J.; Faulds, J.E.; Snee, L.W.; Harlan, S.S.; Bettison-Varga, L.

2004-01-01

Recent studies demonstrate that rifts are characterized by linked tilt domains, each containing a consistent polarity of normal faults and stratal tilt directions, and that the transition between domains is typically through formation of accommodation zones and generally not through production of throughgoing transfer faults. The mid-Miocene Black Mountains accommodation zone of southern Nevada and western Arizona is a well-exposed example of an accommodation zone linking two regionally extensive and opposing tilt domains. In the southeastern part of this zone near Kingman, Arizona, east dipping normal faults of the Whipple tilt domain and west dipping normal faults of the Lake Mead domain coalesce across a relatively narrow region characterized by a series of linked, extensional folds. The geometry of these folds in this strike-parallel portion of the accommodation zone is dictated by the geometry of the interdigitating normal faults of opposed polarity. Synclines formed where normal faults of opposite polarity face away from each other whereas anticlines formed where the opposed normal faults face each other. Opposed normal faults with small overlaps produced short folds with axial trends at significant angles to regional strike directions, whereas large fault overlaps produce elongate folds parallel to faults. Analysis of faults shows that the folds are purely extensional and result from east/northeast stretching and fault-related tilting. The structural geometry of this portion of the accommodation zone mirrors that of the Black Mountains accommodation zone more regionally, with both transverse and strike-parallel antithetic segments. Normal faults of both tilt domains lose displacement and terminate within the accommodation zone northwest of Kingman, Arizona. However, isotopic dating of growth sequences and crosscutting relationships show that the initiation of the two fault systems in this area was not entirely synchronous and that west dipping faults of the Lake Mead domain began to form between 1 m.y. to 0.2 m.y. prior to east dipping faults of the Whipple domain. The accommodation zone formed above an active and evolving magmatic center that, prior to rifting, produced intermediate-composition volcanic rocks and that, during rifting, produced voluminous rhyolite and basalt magmas. Copyright 2004 by the American Geophysical Union.

Shallow-depth location and geometry of the Piedmont Reverse splay of the Hayward Fault, Oakland, California

USGS Publications Warehouse

Catchings, Rufus D.; Goldman, Mark R.; Trench, David; Buga, Michael; Chan, Joanne H.; Criley, Coyn J.; Strayer, Luther M.

2017-04-18

The Piedmont Thrust Fault, herein referred to as the Piedmont Reverse Fault (PRF), is a splay of the Hayward Fault that trends through a highly populated area of the City of Oakland, California (fig. 1A). Although the PRF is unlikely to generate a large-magnitude earthquake, slip on the PRF or high-amplitude seismic energy traveling along the PRF may cause considerable damage during a large earthquake on the Hayward Fault. Thus, it is important to determine the exact location, geometry (particularly dip), and lateral extent of the PRF within the densely populated Oakland area. In the near surface, the PRF juxtaposes Late Cretaceous sandstone (of the Franciscan Complex Novato Quarry terrane of Blake and others, 1984) and an older Pleistocene alluvial fan unit along much of its mapped length (fig. 1B; Graymer and others, 1995). The strata of the Novato Quarry unit vary greatly in strike (NW, NE, and E), dip direction (NE, SW, E, and NW), dip angle (15° to 85°), and lithology (shale and sandstone), and the unit has been intruded by quartz diorite in places. Thus, it is difficult to infer the structure of the fault, particularly at depth, with conventional seismic reflection imaging methods. To better determine the location and shallow-depth geometry of the PRF, we used high-resolution seismic imaging methods described by Catchings and others (2014). These methods involve the use of coincident P-wave (compressional wave) and S-wave (shear wave) refraction tomography and reflection data, from which tomographic models of P- and S-wave velocity and P-wave reflection images are developed. In addition, the coincident P-wave velocity (VP) and S-wave velocity (VS) data are used to develop tomographic models of VP/VS ratios and Poisson’s ratio, which are sensitive to shallow-depth faulting and groundwater. In this study, we also compare measurements of Swave velocities determined from surface waves with those determined from refraction tomography. We use the combination of seismic methods to infer the fault location, dip, and the National Earthquake Hazards Reduction Program (NEHRP) site classification along the seismic profile. Our seismic study is a smaller part of a larger study of the PRF by Trench and others (2016).

Transfer fault earthquake in compressionally reactivated back-arc failed rift: 1948 Fukui earthquake (M7.1), Japan

NASA Astrophysics Data System (ADS)

Ishiyama, Tatsuya; Kato, Naoko; Sato, Hiroshi; Koshiya, Shin

2017-04-01

Back-arc rift structures in many subduction zones are recognized as mechanically and thermally weak zones that possibly play important roles in strain accommodation at later post-rift stages within the overriding plates. In case of Miocene back-arc failed rift structures in the Sea of Japan in the Eurasian-Pacific subduction system, the mechanical contrasts between the crustal thrust wedges of the pre-rift continental crust and high velocity lower crust have fundamentally controlled the styles of post-rift, Quaternary active deformation (Ishiyama et al. 2016). In this study, we show a possibility that strike-slip M>7 devastating earthquakes in this region have been gregion enerated by reactivation of transfer faults highly oblique to the rift axes. The 1948 Fukui earthquake (M7.1), onshore shallow seismic event with a strike-slip faulting mechanism (Kanamori, 1973), resulted in more than 3,500 causalities and destructive damages on the infrastructures. While geophysical analyses on geodetic measurements based on leveling and triangulation networks clearly show coseismic left-lateral fault slip on a NNW striking vertical fault plane beneath the Fukui plain (Sagiya, 1999), no evidence for coseismic surface rupture has been identified based on both post-earthquake intensive fieldwork and recent reexamination of stereopair interpretations using 1/3,000 aerial photographs taken in 1948 (Togo et al., 2000). To find recognizable fault-related structures that deform Neogene basin fill sediments, we collected new 9.6-km-long high-resolution seismic reflection data across the geodetically estimated fault plane and adjacent subparallel active strike slip faults, using 925 offline recorders and Envirovib truck as a seismic source. A depth-converted section to 1.5 km depth contains discontinuous seismic reflectors correlated to Miocene volcaniclastic deposits and depression of the overlying Plio-Pleistocene sediments above the geodetically determined fault plane. We interpreted these structural features as negative flower structures related to the strike-slip fault activated during the 1948 seismic event. Locations of these strike-slip faults are consistent with Miocene transfer faults that offset syn- and post-rift sediments and underlying crustal wedges, suggesting that reactivation of transfer faults resulted in active strike-slip faulting including the 1948 seismic event. These findings demonstrate that not only rift-related normal faults but also transfer faults have strong structural inheritances and played essential roles on their active reactivation and seismicity during the post-rift stress regime.

Late Quaternary faulting in the Cabo San Lucas-La Paz Region, Baja California

NASA Astrophysics Data System (ADS)

Busch, M.; Arrowsmith, J. R.; Umhoefer, P. J.; Gutiérrez, G. M.; Toke, N.; Brothers, D.; Dimaggio, E.; Maloney, S.; Zielke, O.; Buchanan, B.

2006-12-01

While Baja California drifts, active deformation on and just offshore indicates that spreading is not completely localized to the rift axis in the Gulf of California. Using on and offshore data, we characterize normal faulting- related deformation in the Cabo San Lucas-La Paz area. We mapped sections of the north trending faults in a 150 km long left-stepping fault array. Starting in the south, the San Jose del Cabo fault (east dipping) bounds the ~2 km high Sierra La Laguna. It is >70 km long with well defined 1-10 meter fault scarps cutting the youngest late Quaternary geomorphic surfaces. Our preliminary mapping along the north central section exhibits extensive late Quaternary terraces with riser heights of tens of meters above Holocene terraces. The San Jose del Cabo fault trace becomes diffuse and terminates in the area of Los Barriles. Moving northward, the fault system steps to the west, apparently transferring slip to the faults of San Juan de Los Planes and Saltito, which then step left again across the La Paz basin to the NNW trending Carrizal Fault. It has an on shore length of > 60 km. We produced a 25 km detailed strip map along the northern segment. It is embayed by convex east arcs several km long and 100 m deep. In the south, few-m-high scarps cut a pediment of thin Quaternary cover over tertiary volcanic rocks. The escarpment along the fault is hundreds of meters high and scarps 1-10 m high where it goes offshore in the north. Near Bonfil, a quarry cut exposes the fault zone. It comprises a 5-10 m wide bedrock shear zone with sheared tertiary volcanic units. On the footwall, the lower silty and sandy units have moderately well developed pedogenic carbonate, whereas the upper coarse gravel does not. These late Quaternary units appear to be faulted by one to three earthquakes. Finally, we mapped the Saltito fault zone NNE of La Paz. It is a NW trending structure with well developed 5- 10 meter high bedrock scarps defining its NW 5 km and slightly concave east with a 500 m left. Along all the fault zones studied, offset geomorphic surfaces indicate late Pleistocene to Holocene offset. These surfaces can be exploited to determine slip rates and produce a regional chronosequence to test for synchroneity of climatically modulated variations in sediment supply and transport capacity. In addition, a shallow marine geophysics and coring extends our mapping and provides important age control and improved stratigraphic assessment of fault activity.

Morphogenesis of the SW Balearic continental slope and adjacent abyssal plain, Western Mediterranean Sea

NASA Astrophysics Data System (ADS)

Camerlenghi, Angelo; Accettella, Daniela; Costa, Sergio; Lastras, Galderic; Acosta, Juan; Canals, Miquel; Wardell, Nigel

2009-06-01

We present the seafloor morphology and shallow seismic structure of the continental slope south-east of the Balearic promontory and of the adjacent Algero-Balearic abyssal plain from multibeam and chirp sonar data. The main purpose of this research was to identify the sediment pathways from the Balearic promontory to the Algero-Balearic deep basin from the Early Pliocene to the Present. The morphology of the southern Balearic margin is controlled by a SW-NE structural trend, whose main expressions are the Emile Baudot Escarpment transform fault, and a newly discovered WSW-ENE trend that affects the SW end of the escarpment and the abyssal plain. We relate the two structural trends to right-lateral simple shear as a consequence of the Miocene westward migration of the Gibraltar Arc. Newly discovered steep and narrow volcanic ridges were probably enabled to grow by local transtension along the transform margin. Abyssal plain knolls and seahills relate to the subsurface deformation of early stage halokinetic structures such as salt rollers, salt anticlines, and salt pillows. The limited thickness of the overburden and the limited amount of deformation in the deep basin prevent the formation of more mature halokinetic structures such as diapirs, salt walls, bulbs, and salt extrusions. The uppermost sediment cover is affected by a dense pattern of sub-vertical small throw normal faults resulting from extensional stress induced in the overburden by subsurface salt deformation structures. Shallow gas seismic character and the possible presence of an active polygonal fault system suggest upward fluid migration and fluid and sediment expulsion at the seafloor through a probable mud volcano and other piercement structures. One large debris flow deposit, named Formentera Debris Flow, has been identified on the lower slope and rise of the south Formentera margin. Based on current observations, we hypothesize that the landslide originating the Formentera Debris Flow occurred in the Holocene, perhaps in historical times.

Geophysical characterization of Range-Front Faults, Snake Valley, Nevada

USGS Publications Warehouse

Asch, Theodore H.; Sweetkind, Donald S.

2010-01-01

In September 2009, the U.S. Geological Survey, in cooperation with the National Park Service, collected audiomagnetotelluric (AMT) data along two profiles on the eastern flank of the Snake Range near Great Basin National Park to refine understanding of the subsurface geology. Line 1 was collected along Baker Creek, was approximately 6.7-km long, and recorded subsurface geologic conditions to approximately 800-m deep. Line 2, collected farther to the southeast in the vicinity of Kious Spring, was 2.8-km long, and imaged to depths of approximately 600 m. The two AMT lines are similar in their electrical response and are interpreted to show generally similar subsurface geologic conditions. The geophysical response seen on both lines may be described by three general domains of electrical response: (1) a shallow (mostly less than 100-200-m deep) domain of highly variable resistivity, (2) a deep domain characterized by generally high resistivity that gradually declines eastward to lower resistivity with a steeply dipping grain or fabric, and (3) an eastern domain in which the resistivity character changes abruptly at all depths from that in the western domain. The shallow, highly variable domain is interpreted to be the result of a heterogeneous assemblage of Miocene conglomerate and incorporated megabreccia blocks overlying a shallowly eastward-dipping southern Snake Range detachment fault. The deep domain of generally higher resistivity is interpreted as Paleozoic sedimentary rocks (Pole Canyon limestone and Prospect Mountain Quartzite) and Mesozoic and Cenozoic plutonic rocks occurring beneath the detachment surface. The range of resistivity values within this deep domain may result from fracturing adjacent to the detachment, the presence of Paleozoic rock units of variable resistivities that do not crop out in the vicinity of the lines, or both. The eastern geophysical domain is interpreted to be a section of Miocene strata at depth, overlain by Quaternary alluvial fill. These deposits lie east of a steeply east-dipping normal fault that cuts all units and has about 100 m of east-side-down offset.

High-resolution seismic characterization of the gas and gas hydrate system at Green Canyon 955, Gulf of Mexico, USA

USGS Publications Warehouse

Haines, Seth S.; Hart, Patrick E.; Collett, Timothy S.; Shedd, William; Frye, Matthew; Weimer, Paul; Boswell, Ray

2017-01-01

The Pliocene and Pleistocene sediments at lease block Green Canyon 955 (GC955) in the Gulf of Mexico include sand-rich strata with high saturations of gas hydrate; these gas hydrate accumulations and the associated geology have been characterized over the past decade using conventional industry three-dimensional (3D) seismic data and dedicated logging-while-drilling (LWD) borehole data. To improve structural and stratigraphic characterization and to address questions of gas flow and reservoir properties, in 2013 the U.S. Geological Survey acquired high-resolution two-dimensional (2D) seismic data at GC955. Combined analysis of all available data improves our understanding of the geological evolution of the study area, which includes basin-scale migration of the Mississippi River sediment influx as well as local-scale shifting of sedimentary channels at GC955 in response to salt-driven uplift, structural deformation associated with the salt uplift, and upward gas migration from deeper sediments that charges the main gas hydrate reservoir and shallower strata. The 2D data confirm that the sand-rich reservoir is composed principally of sediments deposited in a proximal levee setting and that episodes of channel scour, interspersed with levee deposition, have resulted in an assemblage of many individual proximal levee deposit “pods” each with horizontal extent up to several hundred meters. Joint analysis of the 2D and 3D data reveals new detail of a complex fault network that controls the fluid-flow system; large east-west trending normal faults allow fluid flow through the reservoir-sealing fine-grained unit, and smaller north-south oriented faults provide focused fluid-flow pathways (chimneys) through the shallower sediments. This system has enabled the flow of gas from the main reservoir to the seafloor throughout the recent history at GC955, and its intricacies help explain the distributed occurrences of gas hydrate in the intervening strata.

Active arc-continent collision: Earthquakes, gravity anomalies, and fault kinematics in the Huon-Finisterre collision zone, Papua New Guinea

NASA Astrophysics Data System (ADS)

Abers, Geoffrey A.; McCaffrey, Robert

1994-04-01

The Huon-Finisterre island arc terrane is actively colliding with the north edge of the Australian continent. The collision provides a rare opportunity to study continental accretion while it occurs. We examine the geometry and kinematics of the collision by comparing earthquake source parameters to surface fault geometries and plate motions, and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies. Waveform inversion is used to constrain focal mechanisms for 21 shallow earthquakes that occurred between 1966 and 1992 (seismic moment 1017 to 3 × 1020 N m). Twelve earthquakes show thrust faulting at 22-37 km depth. The largest thrust events are on the north side of the Huon Peninsula and are consistent with slip on the Ramu-Markham thrust fault zone, the northeast dipping thrust fault system that bounds the Huon-Finisterre terrane. Thus much of the terrane's crust but little of its mantle is presently being added to the Australian continent. The large thrust earthquakes also reveal a plausible mechanism for the uplift of Pleistocene coral terraces on the north side of the Huon Peninsula. Bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the Huon Peninsula. With such forces, plate configurations are found that are consistent with observed gravity and basin geometry. Other earthquakes give evidence of deformation above and below the Ramu-Markham thrust system. Four thrust events, 22-27 km depth directly below the Ramu-Markham fault outcrop, are too deep to be part of a planar Ramu-Markham thrust system and may connect to the north dipping Highlands thrust system farther south. Two large strike-slip faulting earthquakes and their aftershocks, in 1970 and 1987, show faulting within the upper plate of the thrust system. The inferred fault planes show slip vectors parallel to those on nearby thrust faults, and may represent small offsets in the overriding plate. These faults, along with small normal-faulting earthquakes beneath the Huon-Finisterre ranges and a 25° along-strike rotation of slip vectors, demonstrate the presence of along-strike extension of the accreting terrane and along-strike compression of the lower plate.

Seismic stratigraphy, tectonics and depositional history in the Halk el Menzel region, NE Tunisia

NASA Astrophysics Data System (ADS)

Sebei, Kawthar; Inoubli, Mohamed Hédi; Boussiga, Haïfa; Tlig, Said; Alouani, Rabah; Boujamaoui, Mustapha

2007-01-01

In the Halk el Menzel area, the proximal- to pelagic platform transition and related tectonic events during the Upper Cretaceous-Lower Miocene have not been taken into adequate consideration. The integrated interpretation of outcrop and subsurface data help define a seismic stratigraphic model and clarify the geodynamic evolution of the Halk el Menzel block. The sedimentary column comprises marls and limestones of the Campanian to Upper Eocene, overlain by Oligocene to Lower Miocene aged siliciclastics and carbonates. Well to well correlations show sedimentary sequences vary considerably in lithofacies and thicknesses over short distances with remarkable gaps. The comparison of sedimentary sequences cut by borehole and seismic stratigraphic modelling as well help define ten third order depositional sequences (S1-S10). Sequences S1 through S6 (Campanian-Paleocene) are mainly characterized by oblique to sigmoid configurations with prograding sedimentary structures, whereas, sequences S7-S10 (Ypresian to Middle Miocene) are organized in shallow water deposits with marked clinoform ramp geometry. Sedimentary discontinuities developed at sequence boundaries are thought to indicate widespread fall in relative sea level. Angular unconformities record a transpressive tectonic regime that operated from the Campanian to Upper Eocene. The geometry of sequences with reduced thicknesses, differential dipping of internal seismic reflections and associated normal faulting located westerly in the area, draw attention to a depositional sedimentary system developed on a gentle slope evolving from a tectonically driven steepening towards the Northwest. The seismic profiles help delimit normal faulting control environments of deposition. In contrast, reef build-ups in the Eastern parts occupy paleohighs NE-SW in strike with bordering Upper Maastrichtian-Ypresian seismic facies onlapping Upper Cretaceous counterparts. During the Middle-Upper Eocene, transpressive stress caused reactivation of faults from normal to reverse play. This has culminated in propagation folds located to the west; whereas, the eastern part of the block has suffered progressive subsidence. Transgressive carbonate depositional sequences have predominated during the Middle Miocene and have sealed pre-existing tectonic structures.

Long Valley caldera and the UCERF depiction of Sierra Nevada range-front faults

USGS Publications Warehouse

Hill, David P.; Montgomery-Brown, Emily K.

2015-01-01

Long Valley caldera lies within a left-stepping offset in the north-northwest-striking Sierra Nevada range-front normal faults with the Hilton Creek fault to the south and Hartley Springs fault to the north. Both Uniform California Earthquake Rupture Forecast (UCERF) 2 and its update, UCERF3, depict slip on these major range-front normal faults as extending well into the caldera, with significant normal slip on overlapping, subparallel segments separated by ∼10  km. This depiction is countered by (1) geologic evidence that normal faulting within the caldera consists of a series of graben structures associated with postcaldera magmatism (intrusion and tumescence) and not systematic down-to-the-east displacements consistent with distributed range-front faulting and (2) the lack of kinematic evidence for an evolving, postcaldera relay ramp structure between overlapping strands of the two range-front normal faults. The modifications to the UCERF depiction described here reduce the predicted shaking intensity within the caldera, and they are in accord with the tectonic influence that underlapped offset range-front faults have on seismicity patterns within the caldera associated with ongoing volcanic unrest.

Laboratory observations of fault strength in response to changes in normal stress

USGS Publications Warehouse

Kilgore, Brian D.; Lozos, Julian; Beeler, Nicholas M.; Oglesby, David

2012-01-01

Changes in fault normal stress can either inhibit or promote rupture propagation, depending on the fault geometry and on how fault shear strength varies in response to the normal stress change. A better understanding of this dependence will lead to improved earthquake simulation techniques, and ultimately, improved earthquake hazard mitigation efforts. We present the results of new laboratory experiments investigating the effects of step changes in fault normal stress on the fault shear strength during sliding, using bare Westerly granite samples, with roughened sliding surfaces, in a double direct shear apparatus. Previous experimental studies examining the shear strength following a step change in the normal stress produce contradictory results: a set of double direct shear experiments indicates that the shear strength of a fault responds immediately, and then is followed by a prolonged slip-dependent response, while a set of shock loading experiments indicates that there is no immediate component, and the response is purely gradual and slip-dependent. In our new, high-resolution experiments, we observe that the acoustic transmissivity and dilatancy of simulated faults in our tests respond immediately to changes in the normal stress, consistent with the interpretations of previous investigations, and verify an immediate increase in the area of contact between the roughened sliding surfaces as normal stress increases. However, the shear strength of the fault does not immediately increase, indicating that the new area of contact between the rough fault surfaces does not appear preloaded with any shear resistance or strength. Additional slip is required for the fault to achieve a new shear strength appropriate for its new loading conditions, consistent with previous observations made during shock loading.

Crustal Deformation across the Jericho Valley Section of the Dead Sea Fault as Resolved by Detailed Field and Geodetic Observations

NASA Astrophysics Data System (ADS)

Hamiel, Yariv; Piatibratova, Oksana; Mizrahi, Yaakov; Nahmias, Yoav; Sagy, Amir

2018-04-01

Detailed field and geodetic observations of crustal deformation across the Jericho Fault section of the Dead Sea Fault are presented. New field observations reveal several slip episodes that rupture the surface, consist with strike slip and extensional deformation along a fault zone width of about 200 m. Using dense Global Positioning System measurements, we obtain the velocities of new stations across the fault. We find that this section is locked for strike-slip motion with a locking depth of 16.6 ± 7.8 km and a slip rate of 4.8 ± 0.7 mm/year. The Global Positioning System measurements also indicate asymmetrical extension at shallow depths of the Jericho Fault section, between 0.3 and 3 km. Finally, our results suggest the vast majority of the sinistral slip along the Dead Sea Fault in southern Jorden Valley is accommodated by the Jericho Fault section.

Quantifying the Variation in Shear Zone Character with Depth: a Case Study from the Simplon Shear Zone, Central Alps

NASA Astrophysics Data System (ADS)

Cawood, T. K.; Platt, J. P.

2017-12-01

A widely-accepted model for the rheology of crustal-scale shear zones states that they comprise distributed strain at depth, in wide, high-temperature shear zones, which narrow to more localized, high-strain zones at lower temperature and shallower crustal levels. We test and quantify this model by investigating how the width, stress, temperature and deformation mechanisms change with depth in the Simplon Shear Zone (SSZ). The SSZ marks a major tectonic boundary in the central Alps, where normal-sense motion and rapid exhumation of the footwall have preserved evidence of older, deeper deformation in rocks progressively further into the currently-exposed footwall. As such, microstructures further from the brittle fault (which represents the most localized, most recently-active part of the SSZ) represent earlier, higher- temperature deformation from deeper crustal levels, while rocks closer to the fault have been overprinted by successively later, cooler deformation at shallower depths. This study uses field mapping and microstructural studies to identify zones representing deformation at various crustal levels, and characterize each in terms of zone width (representing width of the shear zone at that time and depth) and dominant deformation mechanism. In addition, quartz- (by Electron Backscatter Diffraction, EBSD) and feldspar grain size (measured optically) piezometry are used to calculate the flow stress for each zone, while the Ti-in-quartz thermometer (TitaniQ) is used to calculate the corresponding temperature of deformation. We document the presence of a broad zone in which quartz is recrystallized by the Grain Boundary Migration (GBM) mechanism and feldspar by Subgrain Rotation (SGR), which represents the broad, deep zone of deformation occurring at relatively high temperatures and low stresses. In map view, this transitions to successively narrower zones, respectively characterized by quartz SGR and feldspar Bulge Nucleation (BLG); quartz BLG and brittle deformation of feldspar; and finally, a zone of generally brittle deformation. These zones represent deformation in progressively narrower regions at shallower depths, under lower temperatures and higher stresses.

Geology and ore deposits of the McDermitt Caldera, Nevada-Oregon

USGS Publications Warehouse

Rytuba, James J.

1976-01-01

The McDermitt caldera is a Miocene collapse structure along the Nevada-Oregon border. The oval-shaped caldera is bounded by arcuate normal faults on the north and south and by rhyolite ring domes on the west. Precollapse ash-flow tuffs exposed within the south caldera rim consist of three cooling units and are peralkaline in composition. Refractive indexes of nonhydrated glasses from basal vitrophyres of the. units range from 1.493 to 1.503 and are typical of comendites. Post-collapse intracaldera rocks consist of tuffaceous lake sediments, rhyolite flows and domes, and ash-flow tuffs. Within the caldera are the mercury mines of Bretz, Cordero, McDermitt, Opalite, and Ruja and the Moonlight uranium mine. The mercury mines are adjacent to ring fracture faults, and the uranium mine and other uranium occurrences are located within rhyolite ring domes. Fluid inclusions in quartz indicate a deposition temperature of 340?C for the uranium deposit and 200?C for the mercury deposits. The mercury deposits formed at shallow depth by replacement of lakebed sediments and volcanic rocks.

Interseismic Coupling on the Quito Fault System in Ecuador Using New GPS and InSAR Data and Its Implication on Seismic Hazard Assessment.

NASA Astrophysics Data System (ADS)

Mariniere, J.; Champenois, J.; Nocquet, J. M.; Beauval, C. M.; Audin, L.; Baize, S.; Alvarado, A. P.; Yepes, H. A.; Jomard, H.

2017-12-01

Quito, the capital of Ecuador hosting two million inhabitants lies on an active reverse fault system within the Andes. Regular moderate size earthquakes (M 5) occur on these faults, widely felt within the city and its surrounding. Despite a relatively small magnitude of Mw 5.1, the 2014 August 12 earthquake triggered landslides that killed 4 people, cut off one of the main highways for several weeks and caused the temporary shutdown of the airport. Quantifying the seismic potential of the Quito fault system is therefore crucial for a better preparation and mitigation to seismic risk. Previous work using a limited GPS data set found that the Quito fault accommodates 4 mm/yr of EW shortening (Alvarado et al., 2014) at shallow locking depths (3-7 km). We combine GPS and new InSAR data to extend the previous analysis and better quantify the spatial distribution of locking of the Quito fault. GPS dataset includes new continuous sites operating since 2013. 18 ERS SAR scenes, spanning the 1993-2000 time period and covering an area of 85 km by 30 km, were processed using a Permanent Scatter strategy. We perform a joint inversion of both data set (GPS and InSAR) to infer a new and better-constrained kinematic model of the fault to determine both the slip rate and the locking distribution at depth. We find a highly variable level of locking which changes along strike. At some segments, sharp displacement gradients observed both for GPS and InSAR suggest that the fault is creeping up to the surface, while shallow locking is found for other segments. Previous Probabilistic Seismic Hazard Assessment studies have shown that the Quito fault fully controls the hazard in Quito city (Beauval et al. 2014). The results will be used to improve the forecast of earthquakes on the Quito fault system for PSHA studies.

Shallow Faulting in Morelia, Mexico, Based on Seismic Tomography and Geodetically Detected Land Subsidence

NASA Astrophysics Data System (ADS)

Cabral-Cano, E.; Arciniega-Ceballos, A.; Vergara-Huerta, F.; Chaussard, E.; Wdowinski, S.; DeMets, C.; Salazar-Tlaczani, L.

2013-12-01

Subsidence has been a common occurrence in several cities in central Mexico for the past three decades. This process causes substantial damage to the urban infrastructure and housing in several cities and it is a major factor to be considered when planning urban development, land-use zoning and hazard mitigation strategies. Since the early 1980's the city of Morelia in Central Mexico has experienced subsidence associated with groundwater extraction in excess of natural recharge from rainfall. Previous works have focused on the detection and temporal evolution of the subsidence spatial distribution. The most recent InSAR analysis confirms the permanence of previously detected rapidly subsiding areas such as the Rio Grande Meander area and also defines 2 subsidence patches previously undetected in the newly developed suburban sectors west of Morelia at the Fraccionamiento Del Bosque along, south of Hwy. 15 and another patch located north of Morelia along Gabino Castañeda del Rio Ave. Because subsidence-induced, shallow faulting develops at high horizontal strain localization, newly developed a subsidence areas are particularly prone to faulting and fissuring. Shallow faulting increases groundwater vulnerability because it disrupts discharge hydraulic infrastructure and creates a direct path for transport of surface pollutants into the underlying aquifer. Other sectors in Morelia that have been experiencing subsidence for longer time have already developed well defined faults such as La Colina, Central Camionera, Torremolinos and La Paloma faults. Local construction codes in the vicinity of these faults define a very narrow swath along which housing construction is not allowed. In order to better characterize these fault systems and provide better criteria for future municipal construction codes we have surveyed the La Colina and Torremolinos fault systems in the western sector of Morelia using seismic tomographic techniques. Our results indicate that La Colina Fault include secondary faults at depths up to 4-8m below the surface and located up to 24m away from the main fault trace. The Torremolinos fault system includes secondary faults, which are present up to 8m deep and 12-18m away from the main fault trace. Even though the InSAR analysis provides an unsurpassed synoptic view, a higher temporal resolution observation of fault movement has been pursued using the MOIT continuously operating GPS station, which is located within 100 m from the La Colina main fault trace. GPS data is also particularly useful to decompose horizontal and vertical motion in the absence of both ascending and descending SAR data acquisitions. Observations since July 2009 show a total general displacement trend of -39mm/yr and a total horizontal differential motion of 41.8 mm/yr and -4.7mm/yr in its latitudinal and Longitudinal components respectively in respect to the motion observed at the MOGA GPS station located 5.0 km to the SSE within an area which is not affected by subsidence. In addition to the overall trend, high amplitude excursions at the MOIT station with individual residual amplitudes up to 20mm, 25mm, and 60mm in its latitudinal, longitudinal and vertical components respectively vertical are observed. The correlation of fault motion excursions in relationship to the rainfall records will be analyzed.

Seasonal variability of soil-gas radon concentration in central California

USGS Publications Warehouse

King, C.-Y.; Minissale, A.

1994-01-01

Radon concentrations in soil gas were measured by the track-etch method in 60 shallow holes, each 70 cm deep and supported by a capped plastic tube, along several major faults in central California during 1975-1985. This set of data was analyzed to investigate the seasonal variability of soil-gas radon concentration in an area which has various geological conditions but similar climate. The results show several different patterns of seasonal variations, but all of which can be largely attributed to the water-saturation and moisture-retention characteristics of the shallow part of the soil. During the rainy winter and spring seasons, radon tended to be confined underground by the water-saturated surface soil which had much reduced gas permeability, while during the sunny summer and autumn seasons, it exhaled more readily as the soil became drier and more permeable. At several sites located on creeping faults, the radon-variation patterns changed with time, possibly because of disturbance of site condition by fault movement. ?? 1994.

Growth and linkage of the quaternary Ubrique Normal Fault Zone, Western Gibraltar Arc: role on the along-strike relief segmentation

NASA Astrophysics Data System (ADS)

Jiménez-Bonilla, Alejandro; Balanya, Juan Carlos; Exposito, Inmaculada; Diaz-Azpiroz, Manuel; Barcos, Leticia

2015-04-01

Strain partitioning modes within migrating orogenic arcs may result in arc-parallel stretching that produces along-strike structural and topographic discontinuities. In the Western Gibraltar Arc, arc-parallel stretching has operated from the Lower Miocene up to recent times. In this study, we have reviewed the Colmenar Fault, located at the SW end of the Subbetic ranges, previously interpreted as a Middle Miocene low-angle normal fault. Our results allow to identify younger normal fault segments, to analyse their kinematics, growth and segment linkage, and to discuss its role on the structural and relief drop at regional scale. The Colmenar Fault is folded by post-Serravallian NE-SW buckle folds. Both the SW-dipping fault surfaces and the SW-plunging fold axes contribute to the structural relief drop toward the SW. Nevertheless, at the NW tip of the Colmenar Fault, we have identified unfolded normal faults cutting quaternary soils. They are grouped into a N110˚E striking brittle deformation band 15km long and until 3km wide (hereafter Ubrique Normal Fault Zone; UNFZ). The UNFZ is divided into three sectors: (a) The western tip zone is formed by normal faults which usually dip to the SW and whose slip directions vary between N205˚E and N225˚E. These segments are linked to each other by left-lateral oblique faults interpreted as transfer faults. (b) The central part of the UNFZ is composed of a single N115˚E striking fault segment 2,4km long. Slip directions are around N190˚E and the estimated throw is 1,25km. The fault scarp is well-conserved reaching up to 400m in its central part and diminishing to 200m at both segment terminations. This fault segment is linked to the western tip by an overlap zone characterized by tilted blocks limited by high-angle NNE-SSW and WNW-ESE striking faults interpreted as "box faults" [1]. (c) The eastern tip zone is formed by fault segments with oblique slip which also contribute to the downthrown of the SW block. This kinematic pattern seems to be related to other strike-slip fault systems developed to the E of the UNFZ. The structural revision together with updated kinematic data suggest that the Colmenar Fault is cut and downthrown by a younger normal fault zone, the UNFZ, which would have contributed to accommodate arc-parallel stretching until the Quaternary. This stretching provokes along-strike relief segmentation, being the UNFZ the main fault zone causing the final drop of the Subbetic ranges towards the SW within the Western Gibraltar Arc. Our results show displacement variations in each fault segment of the UNFZ, diminishing to their tips. This suggests fault segment linkage finally evolved to build the nearly continuous current fault zone. The development of current large through-going faults linked inside the UNFZ is similar to those ones simulated in some numerical modelling of rift systems [2]. Acknowledgements: RNM-415 and CGL-2013-46368-P [1]Peacock, D.C.P., Knipe, R.J., Sanderson, D.J., 2000. Glossary of normal faults. Journal Structural Geology, 22, 291-305. [2]Cowie, P.A., Gupta, S., Dawers, N.H., 2000. Implications of fault array evolution for synrift depocentre development: insights from a numerical fault growth model. Basin Research, 12, 241-261.

Subduction and vertical coastal motions in the eastern Mediterranean

NASA Astrophysics Data System (ADS)

Howell, Andy; Jackson, James; Copley, Alex; McKenzie, Dan; Nissen, Ed

2017-10-01

Convergence in the eastern Mediterranean of oceanic Nubia with Anatolia and the Aegean is complex and poorly understood. Large volumes of sediment obscure the shallow structure of the subduction zone, and since much of the convergence is accommodated aseismically, there are limited earthquake data to constrain its kinematics. We present new source models for recent earthquakes, combining these with field observations, published GPS velocities and reflection-seismic data to investigate faulting in three areas: the Florence Rise, SW Turkey and the Pliny and Strabo Trenches. The depths and locations of earthquakes reveal the geometry of the subducting Nubian plate NE of the Florence Rise, a bathymetric high that is probably formed by deformation of sediment at the surface projection of the Anatolia-Nubia subduction interface. In SW Turkey, the presence of a strike-slip shear zone has often been inferred despite an absence of strike-slip earthquakes. We show that the GPS-derived strain-rate field is consistent with extension on the orthogonal systems of normal faults observed in the region and that strike-slip faulting is not required to explain observed GPS velocities. Further SW, the Pliny and Strabo Trenches are also often interpreted as strike-slip shear zones, but almost all nearby earthquakes have either reverse-faulting or normal-faulting focal mechanisms. Oblique convergence across the trenches may be accommodated either by a partitioned system of strike-slip and reverse faults or by oblique slip on the Aegean-Nubia subduction interface. The observed late-Quaternary vertical motions of coastlines close to the subduction zone are influenced by the interplay between: (1) thickening of the material overriding the subduction interface associated with convergence, which promotes coastal uplift; and (2) subsidence due to extension and associated crustal thinning. Long-wavelength gravity data suggest that some of the observed topographic contrasts in the eastern Mediterranean are supported by mantle convection. However, whether the convection is time dependent and whether its pattern moves relative to Nubia are uncertain, and its contribution to present-day rates of vertical coastal motions is therefore hard to constrain. The observed extension of the overriding material in the subduction system is probably partly related to buoyancy forces arising from topographic contrasts between the Aegean, Anatolia and the Mediterranean seafloor, but the reasons for regional variations are less clear.

Ground Motion Synthetics For Spontaneous Versus Prescribed Rupture On A 45(o) Thrust Fault

NASA Astrophysics Data System (ADS)

Gottschämmer, E.; Olsen, K. B.

We have compared prescribed (kinematic) and spontaneous dynamic rupture propaga- tion on a 45(o) dipping thrust fault buried up to 5 km in a half-space model, as well as ground motions on the free surface for frequencies less than 1 Hz. The computa- tions are carried out using a 3D finite-difference method with rate-and-state friction on a planar, 20 km by 20 km fault. We use a slip-weakening distance of 15 cm and a slip- velocity weakening distance of 9.2 cm/s, similar to those for the dynamic study for the 1994 M6.7 Northridge earthquake by Nielsen and Olsen (2000) which generated satis- factory fits to selected strong motion data in the San Fernando Valley. The prescribed rupture propagation was designed to mimic that of the dynamic simulation at depth in order to isolate the dynamic free-surface effects. In this way, the results reflect the dy- namic (normal-stress) interaction with the free surface for various depths of burial of the fault. We find that the moment, peak slip and peak sliprate for the rupture breaking the surface are increased by up to 60%, 80%, and 10%, respectively, compared to the values for the scenario buried 5 km. The inclusion of these effects increases the peak displacements and velocities above the fault by factors up 3.4 and 2.9 including the increase in moment due to normal-stress effects at the free surface, and up to 2.1 and 2.0 when scaled to a Northridge-size event with surface rupture. Similar differences were found by Aagaard et al. (2001). Significant dynamic effects on the ground mo- tions include earlier arrival times caused by super-shear rupture velocities (break-out phases), in agreement with the dynamic finite-element simulations by Oglesby et al. (1998, 2000). The presence of shallow low-velocity layers tend to increase the rup- ture time and the sliprate. In particular, they promote earlier transitions to super-shear velocities and decrease the rupture velocity within the layers. Our results suggest that dynamic interaction with the free surface can significantly affect the ground motion for faults buried less than 1-3 km. We therefore recommend that strong ground motion for these scenarios be computed including such dynamic rupture effects.

Stress triggering in thrust and subduction earthquakes and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults

USGS Publications Warehouse

Lin, J.; Stein, R.S.

2004-01-01

We argue that key features of thrust earthquake triggering, inhibition, and clustering can be explained by Coulomb stress changes, which we illustrate by a suite of representative models and by detailed examples. Whereas slip on surface-cutting thrust faults drops the stress in most of the adjacent crust, slip on blind thrust faults increases the stress on some nearby zones, particularly above the source fault. Blind thrusts can thus trigger slip on secondary faults at shallow depth and typically produce broadly distributed aftershocks. Short thrust ruptures are particularly efficient at triggering earthquakes of similar size on adjacent thrust faults. We calculate that during a progressive thrust sequence in central California the 1983 Mw = 6.7 Coalinga earthquake brought the subsequent 1983 Mw = 6.0 Nunez and 1985 Mw = 6.0 Kettleman Hills ruptures 10 bars and 1 bar closer to Coulomb failure. The idealized stress change calculations also reconcile the distribution of seismicity accompanying large subduction events, in agreement with findings of prior investigations. Subduction zone ruptures are calculated to promote normal faulting events in the outer rise and to promote thrust-faulting events on the periphery of the seismic rupture and its downdip extension. These features are evident in aftershocks of the 1957 Mw = 9.1 Aleutian and other large subduction earthquakes. We further examine stress changes on the rupture surface imparted by the 1960 Mw = 9.5 and 1995 Mw = 8.1 Chile earthquakes, for which detailed slip models are available. Calculated Coulomb stress increases of 2-20 bars correspond closely to sites of aftershocks and postseismic slip, whereas aftershocks are absent where the stress drops by more than 10 bars. We also argue that slip on major strike-slip systems modulates the stress acting on nearby thrust and strike-slip faults. We calculate that the 1857 Mw = 7.9 Fort Tejon earthquake on the San Andreas fault and subsequent interseismic slip brought the Coalinga fault ???1 bar closer to failure but inhibited failure elsewhere on the Coast Ranges thrust faults. The 1857 earthquake also promoted failure on the White Wolf reverse fault by 8 bars, which ruptured in the 1952 Mw = 7.3 Kern County shock but inhibited slip on the left-lateral Garlock fault, which has not ruptured since 1857. We thus contend that stress transfer exerts a control on the seismicity of thrust faults across a broad spectrum of spatial and temporal scales. Copyright 2004 by the American Geophysical Union.

Toward resolving an earthquake ground motion mystery in west Seattle, Washington State: Shallow seismic focusing may cause anomalous chimney damage

USGS Publications Warehouse

Stephenson, W.J.; Frankel, A.D.; Odum, J.K.; Williams, R.A.; Pratt, T.L.

2006-01-01

A shallow bedrock fold imaged by a 1.3-km long high-resolution shear-wave seismic reflection profile in west Seattle focuses seismic waves arriving from the south. This focusing may cause a pocket of amplified ground shaking and the anomalous chimney damage observed in earthquakes of 1949, 1965 and 2001. The 200-m bedrock fold at ???300-m depth is caused by deformation across an inferred fault within the Seattle fault zone. Ground motion simulations, using the imaged geologic structure and northward-propagating north-dipping plane wave sources, predict a peak horizontal acceleration pattern that matches that observed in strong motion records of the 2001 Nisqually event. Additionally, a pocket of chimney damage reported for both the 1965 and the 2001 earthquakes generally coincides with a zone of simulated amplification caused by focusing. This study further demonstrates the significant impact shallow (<1km) crustal structures can have on earthquake ground-motion variability.

The structural inventory of a small complex impact crater: Jebel Waqf as Suwwan, Jordan

NASA Astrophysics Data System (ADS)

Kenkmann, Thomas; Sturm, Sebastian; Krüger, Tim; Salameh, Elias; Al-Raggad, Marwan; Konsul, Khalil

2017-07-01

The investigation of terrestrial impact structures is crucial to gain an in-depth understanding of impact cratering processes in the solar system. Here, we use the impact structure Jebel Waqf as Suwwan, Jordan, as a representative for crater formation into a layered sedimentary target with contrasting rheology. The complex crater is moderately eroded (300-420 m) with an apparent diameter of 6.1 km and an original rim fault diameter of 7 km. Based on extensive field work, IKONOS imagery, and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis. Parametric scaling indicates that the structural uplift (250-350 m) and the depth of the ring syncline (<200 m) are anomalously low. The very shallow relief of the crater along with a NE vergence of the asymmetric central uplift and the enhanced deformations in the up-range and down-range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one ( 20°). One of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred-meter scale. The crater rim is defined by a circumferential normal fault dipping mostly toward the crater. Footwall strata beneath the rim fault are bent-up in the down-range sector but appear unaffected in the up-range sector. The hanging wall displays various synthetic and antithetic rotations in the down-range sector but always shows antithetic block rotation in the up-range sector. At greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage.

Moderate rates of late Quaternary slip along the northwestern margin of the Basin and Range Province, Surprise Valley fault, northeastern California

USGS Publications Warehouse

Personius, Stephen F.; Crone, Anthony J.; Machette, Michael N.; Mahan, Shannon; Lidke, David J.

2009-01-01

The 86-km-long Surprise Valley normal fault forms part of the active northwestern margin of the Basin and Range province in northeastern California. We use trench mapping and radiocarbon, luminescence, and tephra dating to estimate displacements and timing of the past five surface-rupturing earthquakes on the central part of the fault near Cedarville. A Bayesian OxCal analysis of timing constraints indicates earthquake times of 18.2 ± 2.6, 10.9 ± 3.2, 8.5 ± 0.5, 5.8 ± 1.5, and 1.2 ± 0.1 ka. These data yield recurrence intervals of 7.3 ± 4.1, 2.5 ± 3.2, 2.7 ± 1.6, and 4.5 ± 1.5 ka and an elapsed time of 1.2 ± 0.1 ka since the latest surface-rupturing earthquake. Our best estimate of latest Quaternary vertical slip rate is 0.6 ?? 0.1 mm/a. This late Quaternary rate is remarkably similar to long-term (8-14 Ma) minimum vertical slip rates (>0.4-0.5 ± 0.3 mm/a) calculated from recently acquired seismic reflection and chronologic and structural data in Surprise Valley and the adjacent Warner Mountains. However, our slip rate yields estimates of extension that are lower than recent campaign GPS determinations by factors of 1.5-4 unless the fault has an unusually shallow (30°-35°) dip as suggested by recently acquired seismic reflection data. Coseismic displacements of 2-4.5 ± 1 m documented in the trench and probable rupture lengths of 53-65 km indicate a history of latest Quaternary earthquakes of M 6.8-7.3 on the central part of the. Surprise Valley fault.

A synthetic GMPE based on deterministic simulated ground motion data obtained from dynamic rupture models

NASA Astrophysics Data System (ADS)

Dalguer, L. A.; Baumann, C.; Cauzzi, C.

2013-12-01

Empirical ground motion prediction in the very near-field and for large magnitudes is often based on extrapolation of ground motion prediction equations (GMPEs) outside the range where they are well constrained by recorded data. With empirical GMPEs it is also difficult to capture source-dominated ground motion patterns, such as the effects of velocity pulses induced by subshear and supershear rupture directivity, buried and surface-rupturing, hanging-wall and foot-wall, weak shallow layers, complex geometry faults and stress drop. A way to cope at least in part with these shortcomings is to augment the calibration datasets with synthetic ground motions. To this aim, physics-based dynamic rupture models - where the physical bases involved in the fault rupture are explicitly considered - appear to be a suitable approach to produce synthetic ground motions. In this contribution, we first perform an assessment of a database of synthetic ground motions generated by a suite of dynamic rupture simulations to verify compatibility of the peak ground amplitudes with current GMPEs. The synthetic data-set is composed by 360 earthquake scenarios with moment magnitudes in the range of 5.5-7, for three mechanisms of faulting (reverse, normal and strike-slip) and for both buried faults and surface rupturing faults. Second, we parameterise the synthetic dataset through a GMPE. For this purpose, we identify the basic functional forms by analyzing the variation of the synthetic peak ground motions and spectral ordinates as a function of different explanatory variables related to the earthquake source characteristics, in order to account for some of the source effects listed above. We argue that this study provides basic guidelines for the developments of future GMPEs including data from physics-based numerical simulations.

Dynamic stresses, coulomb failure, and remote triggering: corrected

USGS Publications Warehouse

Hill, David P.

2012-01-01

Dynamic stresses associated with crustal surface waves with 15–30 s periods and peak amplitudes <1  MPa are capable of triggering seismicity at sites remote from the generating mainshock under appropriate conditions. Coulomb failure models based on a frictional strength threshold offer one explanation for instances of rapid‐onset triggered seismicity that develop during the surface‐wave peak dynamic stressing. Evaluation of the triggering potential of surface‐wave dynamic stresses acting on critically stressed faults using a Mohr’s circle representation together with the Coulomb failure criteria indicates that Love waves should have a higher triggering potential than Rayleigh waves for most fault orientations and wave incidence angles. That (1) the onset of triggered seismicity often appears to begin during the Rayleigh wave rather than the earlier arriving Love wave, and (2) Love‐wave amplitudes typically exceed those for Rayleigh waves suggests that the explanation for rapid‐onset dynamic triggering may not reside solely with a simple static‐threshold friction mode. The results also indicate that normal faults should be more susceptible to dynamic triggering by 20‐s Rayleigh‐wave stresses than thrust faults in the shallow seismogenic crust (<10  km) while the advantage tips in favor of reverse faults greater depths. This transition depth scales with wavelength and coincides roughly with the transition from retrograde‐to‐prograde particle motion. Locally elevated pore pressures may have a role in the observed prevalence of dynamic triggering in extensional regimes and geothermal/volcanic systems. The result is consistent with the apparent elevated susceptibility of extensional or transtensional tectonic regimes to remote triggering by Rayleigh‐wave dynamic stresses than compressional or transpressional regimes.

McGee Mountain Shallow (2m) Temperature Survey, Humboldt County, Nevada 2009

DOE Data Explorer

Richard Zehner

2009-01-01

This shapefile contains location and attribute data for a shallow (2 meter) temperature survey conducted by Geothermal Technical Partners, Inc. during late 2008 and early 2009. Temperatures at 2m depth were measured at 192 separate points as outlined by Coolbaugh et al., 2007. The purpose of the survey was to try and detect a shallow thermal anomaly associated with the McGee Mountain geothermal area as discovered by Phillips Petroleum and Earth Power Resources in the late 1970’s. Drilling identified ~120oC temperatures at ~100m depth. This 2-meter survey delineated what was interpreted as a steam-heated fault zone centered along a range front fault in the vicinity of the drilled holes and fumaroles. Coolbaugh, M.F., Sladek, C., Faulds, J.E., Zehner, R.E., and Oppliger, G.L., 2007, Use of rapid temperature measurements at a 2-meter depth to augment deeper temperature gradient drilling: Proceedings, 32nd Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, Jan. 22-24, 2007, p. 109-116. Zehner, R., Tullar, K., and Rutledge, E., 2012, Effectiveness of 2-Meter and geoprobe shallow temperature surveys in early stage geothermal exploration: Geothermal Resources Council Transactions, v. 36, in press.

On abrupt transpression to transtension transition in the South Baikal rift system (Tunka - South Baikal segment)

NASA Astrophysics Data System (ADS)

Sankov, Vladimir; Parfeevets, Anna; Lukhnev, Andrey; Miroshnitchenko, Andrey; Ashurkov, Sergey; Sankov, Alexey; Usynin, Leonid; Eskin, Alexander; Bryzhak, Evgeny

2013-04-01

This work addresses to relation of transpression and extension stress-strain conditions in intracontinental rift system. In our investigation we use a new structural, shallow geophysics, GPS geodetic data and paleostress reconstructions. The surroundings of southern tip of Siberian platform is the region of three Late Cenozoic structures conjugation: sublatitudinal Obruchev fault (OF) controlling the northern boundary of the South Baikal basin, NW trending Main Sayan fault (MSF) as the strike-slip boundary between Siberian platform and East Sayan block and WNW trending eastern segment of Tunka fault (TF) as part of the Tunka basins system northern boundary. A new evidences of superposition of compression and extension fault structures were revealed near the southern extremity of Baikal lake. We've find a very close vicinity of Late Pleistocene - Holocene strike-slip, thrust and normal faulting in the MSF and OF junction zone. The on-land Holocene normal faulting can be considered as secondary fault paragenesis within the main strike-slip zone (Sankov et al., 2009). Active strike-slip, thrust and reverse faulting characterize the MSF and TF junction zone. The transpression conditions are replaced very sharply by transtension and extension ones in eastern direction from zone of structures conjugation - the active normal faulting is dominated within the South Baikal basin. The Bystraya rift basin located in the west shows the tectonic inversion since Middle Pleistocene as a result of the strike-slip movements partitioning between TF and MSF and inset of edition compression stress. The active strike-slip and intrabasin extension conditions are dominated father to the west in Tunka basin. The results of our GPS measurements show the present day convergence and east movements of Khamar-Daban block and eastern Tunka basins relative to Siberian platform along MSF and TF with NE-SW shortening domination. The clear NW-SE divergence across Baikal basin is documented. Holocene and present-day left lateral relative motions of about 3 mm/yr (Sankov et al., 2004) between of Siberian platform and its mounting frame are accommodated along south-eastern segment of MSF. We consider two main factors of sharp transition between transpression and transtension to extension conditions in Tunka-South Baikal segment of Baikal rift system. The first one is the influence of geometry of southern tip of Siberian platform as a first order ancient lithosphere heterogeneity in agreement with (Petit et al., 1996). The second factor is the interaction in this region of two tectonic forces driving the Cenozoic geodynamics. The initial opening of the Tunka and South Baikal basins since Oligocene time as well as father Baikal rift system development caused by long lived asthenosphere flow along NW-SE direction (Sankov et al., 2011). The addition NE-SW compression started during Pliocene (Parfeevets, Sankov, 2006) as the result of the Hindustan and Eurasia convergence. The former caused transpression deformations and clockwise horizontal block rotations along south-western boundary of the platform with their SE movements to the "free space" opened by the divergence of Siberian platform and Transbaikal block (Sankov et al., 2002, 2005).

Alternative model of thrust-fault propagation

NASA Astrophysics Data System (ADS)

Eisenstadt, Gloria; de Paor, Declan G.

1987-07-01

A widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory. We propose an alternative model that applies Gretener's beam-failure mechanism to a multilayered sequence. Invoking compatibility conditions, which demand that a thrust propagate both upsection and downsection, we suggest that ramps form first, at shallow levels, and are subsequently connected by flat faults. This hypothesis also explains the formation of many minor structures associated with thrusts, such as backthrusts, wedge structures, pop-ups, and duplexes, and provides a unified conceptual framework in which to evaluate field observations.

Geometry and kinematics of the eastern Lake Mead fault system in the Virgin Mountains, Nevada and Arizona

USGS Publications Warehouse

Beard, Sue; Campagna, David J.; Anderson, R. Ernest

2010-01-01

The Lake Mead fault system is a northeast-striking, 130-km-long zone of left-slip in the southeast Great Basin, active from before 16 Ma to Quaternary time. The northeast end of the Lake Mead fault system in the Virgin Mountains of southeast Nevada and northwest Arizona forms a partitioned strain field comprising kinematically linked northeast-striking left-lateral faults, north-striking normal faults, and northwest-striking right-lateral faults. Major faults bound large structural blocks whose internal strain reflects their position within a left step-over of the left-lateral faults. Two north-striking large-displacement normal faults, the Lakeside Mine segment of the South Virgin–White Hills detachment fault and the Piedmont fault, intersect the left step-over from the southwest and northeast, respectively. The left step-over in the Lake Mead fault system therefore corresponds to a right-step in the regional normal fault system.Within the left step-over, displacement transfer between the left-lateral faults and linked normal faults occurs near their junctions, where the left-lateral faults become oblique and normal fault displacement decreases away from the junction. Southward from the center of the step-over in the Virgin Mountains, down-to-the-west normal faults splay northward from left-lateral faults, whereas north and east of the center, down-to-the-east normal faults splay southward from left-lateral faults. Minimum slip is thus in the central part of the left step-over, between east-directed slip to the north and west-directed slip to the south. Attenuation faults parallel or subparallel to bedding cut Lower Paleozoic rocks and are inferred to be early structures that accommodated footwall uplift during the initial stages of extension.Fault-slip data indicate oblique extensional strain within the left step-over in the South Virgin Mountains, manifested as east-west extension; shortening is partitioned between vertical for extension-dominated structural blocks and south-directed for strike-slip faults. Strike-slip faults are oblique to the extension direction due to structural inheritance from NE-striking fabrics in Proterozoic crystalline basement rocks.We hypothesize that (1) during early phases of deformation oblique extension was partitioned to form east-west–extended domains bounded by left-lateral faults of the Lake Mead fault system, from ca. 16 to 14 Ma. (2) Beginning ca. 13 Ma, increased south-directed shortening impinged on the Virgin Mountains and forced uplift, faulting, and overturning along the north and west side of the Virgin Mountains. (3) By ca. 10 Ma, initiation of the younger Hen Spring to Hamblin Bay fault segment of the Lake Mead fault system accommodated westward tectonic escape, and the focus of south-directed shortening transferred to the western Lake Mead region. The shift from early partitioned oblique extension to south-directed shortening may have resulted from initiation of right-lateral shear of the eastern Walker Lane to the west coupled with left-lateral shear along the eastern margin of the Great Basin.

Imaging the concealed section of the Whakatane fault below Whakatane city, New Zealand, with a shear wave land streamer system

NASA Astrophysics Data System (ADS)

Polom, Ulrich; Mueller, Christof; Krawczyk, CharLotte M.

2016-04-01

The Mw 7.1 Darfield Earthquake in September 2010 ruptured the surface along the Greendale Fault that was not known prior to the earthquake. The subsequent Mw 6.3 Christchurch earthquake in February 2011 demonstrated that concealed active faults have a significant risk potential for urban infrastructure and human life in New Zealand if they are located beneath or close to such areas. Mapping exposures and analysis of active faults incorporated into the National Seismic Hazard Model (NSHM) suggests that several thousands of these active structures are yet to be identified and have the potential to generate moderate to large magnitude earthquakes (i.e. magnitudes >5). Geological mapping suggests that active faults pass beneath, or within many urban areas in New Zealand, including Auckland, Blenheim, Christchurch, Hastings/Napier, Nelson, Rotorua, Taupo, Wellington, and Whakatane. Since no established methodology for routinely locating and assessing the earthquake hazard posed by concealed active faults is available, the principal objective of the presented study was to evaluate the usefulness of high-resolution shear wave seismic reflection profiling using a land streamer to locate buried faults in urban areas of New Zealand. During the survey carried out in the city of Whakatane in February 2015, the method was first tested over a well known surface outcrop of the Edgecumbe Fault 30 km south-west of Whakatane city. This allowed further to investigate the principle shear wave propagation characteristics in the unknown sediments, consisting mainly of effusive rock material of the Taupo volcanic zone mixed with marine transgression units. Subsequently the survey was continued within Whakatane city using night operation time slots to reduce the urban noise. In total, 11 profiles of 5.7 km length in high data quality were acquired, which clearly show concealed rupture structures of obviously different age in the shallow sediments down to 100 m depth. Subject to depth verification by drillings normal fault displacements of up to 15 m are visible in depths of 20-40 m, deeper rupture structures show displacements of up to 20 m. Furthermore, indications of strike-slip fault activities are visible. The concealed rupture structures found are not aligned along former estimated fault lineaments or main surface structures like the Whakatane river bed. Correlations exist with small topographic variations detected by LIDAR imaging and surface signatures on a historic map of 1867.

The permeability of fault zones in the upper continental crust: statistical analysis from 460 datasets, updated depth-trends, and permeability contrasts between fault damage zones and protoliths.

NASA Astrophysics Data System (ADS)

Scibek, J.; Gleeson, T. P.; Ingebritsen, S.; McKenzie, J. M.

2017-12-01

Fault zones are an important part of the hydraulic structure of the Earth's crust and influence a wide range of Earth processes and a large amount of test data has been collected over the years. We conducted a meta-analysis of global of fault zone permeabilities in the upper brittle continental crust, using about 10,000 published research items from a variety of geoscience and engineering disciplines. Using 460 datasets at 340 localities, the in-situ bulk permeabilities (>10's meters scale, including macro-fractures) and matrix permeabilities (drilled core samples or outcrop spot tests) are separated, analyzed, and compared. The values have log-normal distributions and we analyze the log-permeability values. In the fault damage zones of plutonic and metamorphic rocks the mean bulk permeability was 1x10-14m2, compared to matrix mean of 1x10-16m2. In sedimentary siliciclastic rocks the mean value was the same for bulk and matrix permeability (4x10-14m2). More useful insights were determined from the regression analysis of paired permeability data at all sites (fault damage zone vs. protolith). Much of the variation in fault permeability is explained by the permeability of protolith: in relatively weak volcaniclastic and clay-rich rocks up to 70 to 88% of the variation is explained, and only 20-30% in plutonic and metamorphic rocks. We propose a revision at shallow depths for previously published upper-bound curves for the "fault-damaged crust " and the geothermal-metamorphic rock assemblage outside of major fault zones. Although the bounding curves describe the "fault-damaged crust" permeability parameter space adequately, the only statistically significant permeability-depth trend is for plutonic and metamorphic rocks (50% of variation explained). We find a depth-dependent systematic variation of the permeability ratio (fault damage zone / protolith) from the in-situ bulk permeability global data. A moving average of the log-permeability ratio value is 2 to 2.5 (global mean is 2.2). Although the data is unevenly distributed with depth, the present evidence is that the permeability ratio is at a maximum at depths 1 to 2 kilometers, decreases with depth below 2km, and is also lower near the ground surface.

Land-level changes from a late Holocene earthquake in the northern Puget lowland, Washington

USGS Publications Warehouse

Kelsey, H.M.; Sherrod, B.; Johnson, S.Y.; Dadisman, S.V.

2004-01-01

An earthquake, probably generated on the southern Whidbey Island fault zone, caused 1-2 m of ground-surface uplift on central Whidbey Island ???2800-3200 yr ago. The cause of the uplift is a fold that grew coseismically above a blind fault that was the earthquake source. Both the fault and the fold at the fault's tip are imaged on multichannel seismic refection profiles in Puget Sound immediately east of the central Whidbey Island site. Uplift is documented through contrasting histories of relative sea level at two coastal marshes on either side of the fault. Late Holocene shallow-crustal earthquakes of Mw = 6.5-7 pose substantial seismic hazard to the northern Puget Lowland. ?? 2004 Geological Society of America.

Crustal Deformation at the Arabian Plate-Boundary observed by InSAR

NASA Astrophysics Data System (ADS)

Jonsson, S.; Cavalié, O.; Akoglu, A. M.; Wang, T.; Xu, W.; Feng, G.; Dutta, R.; Abdullin, A. K.

2013-12-01

The Arabian plate is bounded by a variety of active plate boundaries, with extension in the Red Sea and Gulf of Aden to the south, compression in Turkey and Iran to the north, and transform faults to the west and to the east. Internally, however, the Arabian plate has been shown to be tectonically rather stable, despite evidence of recent volcanism and earthquake faulting. We use InSAR observations to study recent tectonic and volcanic activity at several locations at the Arabian plate boundary as well within the plate itself. The region near the triple junction between the Arabian, Eurasian, and Anatolian plates has often been the focus of studies on continental deformation behavior and interseismic deformation. Here we use large-scale InSAR data processing to map the deformation near the triple junction and find the deformation to be focused on major faults with little intra-plate deformation. The eastern part of the East Anatolian Fault appears to have a very shallow locking depth with limited fault-normal deformation. Several major earthquakes that have occurred in recent years on the Arabian plate boundary, including the 2011 magnitude 7.1 Van earthquake in eastern Turkey. It occurred as a result of convergence of the Arabian plate towards Eurasia and caused significant surface deformation that we have analyzed with multiple coseismic InSAR, GPS, and coastal uplift observations. We use high-resolution Cosmo-Skymed and TerraSAR-X data to derive 3D coseismic displacements from offsets alone, as some of the interferograms are almost completely incoherent. By identifying point-like targets within the images, we were able to derive accurate pixel offsets between SAR sub-images containing such targets, which we used to estimate the 3D coseismic displacements. The derived 3D displacement field helped in constraining the causative northward dipping thrust-fault. The Qadimah fault is a recently discovered fault located on the Red Sea coast north of Jeddah and under the King Abdullah Economic City, a planned $50 billion harbor city. The fault is a normal fault, parallel to the Red Sea, but it is unclear if the fault is still active and poses significant hazard to the new city. We use MERIS-corrected Envisat InSAR data to study the limited interseismic deformation across the fault and the results suggest that more investigations will be needed to assess the activity of the fault. Several volcanic events have taken place in the region during the past several years, including the 2007-8 Jebel at Tair island (Red Sea) eruption, the 2009 Harrat Lunayyir (western Saudi Arabia) magmatic intrusion, and the 2011-12 Zubair islands (Red Sea) eruption. All these three volcanic events were fed by dike intrusions whose geometry we constrain using the InSAR and optical data. The derived dike orientations provide information about extensional stress field in and around the Red Sea, although on Tair island the upper-most part of the feeder dike was controlled by local stresses within the volcanic edifice.

The 2015 M w 6.0 Mt. Kinabalu earthquake: an infrequent fault rupture within the Crocker fault system of East Malaysia

NASA Astrophysics Data System (ADS)

Wang, Yu; Wei, Shengji; Wang, Xin; Lindsey, Eric O.; Tongkul, Felix; Tapponnier, Paul; Bradley, Kyle; Chan, Chung-Han; Hill, Emma M.; Sieh, Kerry

2017-12-01

The M w 6.0 Mt. Kinabalu earthquake of 2015 was a complete (and deadly) surprise, because it occurred well away from the nearest plate boundary in a region of very low historical seismicity. Our seismological, space geodetic, geomorphological, and field investigations show that the earthquake resulted from rupture of a northwest-dipping normal fault that did not reach the surface. Its unilateral rupture was almost directly beneath 4000-m-high Mt. Kinabalu and triggered widespread slope failures on steep mountainous slopes, which included rockfalls that killed 18 hikers. Our seismological and morphotectonic analyses suggest that the rupture occurred on a normal fault that splays upwards off of the previously identified normal Marakau fault. Our mapping of tectonic landforms reveals that these faults are part of a 200-km-long system of normal faults that traverse the eastern side of the Crocker Range, parallel to Sabah's northwestern coastline. Although the tectonic reason for this active normal fault system remains unclear, the lengths of the longest fault segments suggest that they are capable of generating magnitude 7 earthquakes. Such large earthquakes must occur very rarely, though, given the hitherto undetectable geodetic rates of active tectonic deformation across the region.

3D geometries of normal faults in a brittle-ductile sedimentary cover: Analogue modelling

NASA Astrophysics Data System (ADS)

Vasquez, Lina; Nalpas, Thierry; Ballard, Jean-François; Le Carlier De Veslud, Christian; Simon, Brendan; Dauteuil, Olivier; Bernard, Xavier Du

2018-07-01

It is well known that ductile layers play a major role in the style and location of deformation. However, at the scale of a single normal fault, the impact of rheological layering is poorly constrained and badly understood, and there is a lack of information regarding the influence of several décollement levels within a sedimentary cover on the single fault geometry under purely extensive deformation. We present small-scale experiments that were built with interbedded layers of brittle and ductile materials and with minimum initial constraints (only a velocity discontinuity at the base of the experiment) on the normal fault geometry in order to investigate the influence of controlled parameters such as extension velocity, rate of extension, ductile thickness and varying stratigraphy on the 3D fault geometry. These experiments showed a broad-spectrum of tectonic features such as grabens, ramp-flat-ramp normal faults and reverse faults. Forced folds are associated with fault flats that develop in the décollement levels (refraction of the fault angle). One of the key points is that the normal fault geometry displays large variations in both direction and dip, despite the imposed homogeneous extension. This result is exclusively related to the presence of décollement levels, and is not associated with any global/regional variation in extension direction and/or inversion.

The Role of Coseismic Coulomb Stress Changes in Shaping the Hard Link Between Normal Fault Segments

NASA Astrophysics Data System (ADS)

Hodge, M.; Fagereng, Å.; Biggs, J.

2018-01-01

The mechanism and evolution of fault linkage is important in the growth and development of large faults. Here we investigate the role of coseismic stress changes in shaping the hard links between parallel normal fault segments (or faults), by comparing numerical models of the Coulomb stress change from simulated earthquakes on two en echelon fault segments to natural observations of hard-linked fault geometry. We consider three simplified linking fault geometries: (1) fault bend, (2) breached relay ramp, and (3) strike-slip transform fault. We consider scenarios where either one or both segments rupture and vary the distance between segment tips. Fault bends and breached relay ramps are favored where segments underlap or when the strike-perpendicular distance between overlapping segments is less than 20% of their total length, matching all 14 documented examples. Transform fault linkage geometries are preferred when overlapping segments are laterally offset at larger distances. Few transform faults exist in continental extensional settings, and our model suggests that propagating faults or fault segments may first link through fault bends or breached ramps before reaching sufficient overlap for a transform fault to develop. Our results suggest that Coulomb stresses arising from multisegment ruptures or repeated earthquakes are consistent with natural observations of the geometry of hard links between parallel normal fault segments.

The 2000 Nemuro-Hanto-Oki earthquake, off eastern Hokkaido, Japan, and the high intraslab seismic activity in the southwestern Kuril Trench

USGS Publications Warehouse

Takahashi, H.; Hirata, K.

2003-01-01

The 2000 Nemuro-Hanto-Oki earthquake (Mw6.8) occurred in the southwestern part of the Kuril Trench. The hypocenter was located close to the aftershock region of the great 1994 Kuril earthquake (Mw8.3), named "the 1994 Hokkaido-Toho-Oki earthquake" by the Japan Meteorological Agency, for which the fault plane is still in debate. Analysis of the 2000 event provides a clue to resolve the fault plane issue for the 1994 event. The hypocenters of the 2000 main shock and aftershocks are determined using arrival times from a combination of nearby inland and submarine seismic networks with an improved azimuthal coverage. They clearly show that the 2000 event was an intraslab event occurring on a shallow-dipping fault plane between 55 and 65 km in depth. The well-focused aftershock distribution of the 2000 event, the relative location of the 1994 event with respect to the 2000 event, and the similarity between their focal mechanisms strongly suggest that the faulting of the great 1994 earthquake also occurred on a shallow-dipping fault plane in the subducting slab. The recent hypocenter distribution around the 1994 aftershock region also supports this result. Large intraslab earthquakes occuring to the southeast of Hokkaido may occur due to a strong coupling on the plate boundary, which generates relatively large stress field within the subducting Pacific plate.

How material contrast around subduction faults may control coseismic slip and rupture dynamics: tsunami applications for the case study of Tohoku

NASA Astrophysics Data System (ADS)

Scala, Antonio; Murphy, Shane; Romano, Fabrizio; Lorito, Stefano; Festa, Gaetano; Volpe, Manuela; Piatanesi, Alessio

2017-04-01

Recent megathrust tsunamigenic events, e.g. Maule 2010 (M8.8) and Tohoku 2011 (M9.0), generated huge tsunami waves as a consequence of high slip in the shallow part of the respective subduction zone. Other events, (e.g. the recent Mentawai 2010, M7.8, or the historical Meiji 1896, M8.2), referred to as tsunami earthquakes, produced unexpectedly large tsunami waves, probably due to large slip at shallow depth over longer rupture durations compared to deeper thrust events. Subduction zone earthquakes originate and propagate along bimaterial interfaces separating materials having different elastic properties, e.g. continental and oceanic crust, a stiffer deep mantle wedge, shallow compliant accretionary prism etc. Bimaterial interfaces have been showed, through observations (seismological and laboratory) and theoretical studies, to affect the rupture: introducing a preferred rupture direction as well as asymmetric rupture velocities and shear stress redistributions. Such features are predominantly due to the break of symmetry between the two sides of the interface in turn ascribable to the complex coupling between the frictional interfacial sliding and the slip-induced normal stress perturbations. In order to examine the influence of material contrast on a fault plane on the seismic source and tsunami waves, we modelled a Tohoku-like subduction zone to perform a large number of 2D along-dip rupture dynamics simulations in the framework of linear slip weakening both for homogeneous and bimaterial fault. In this latter model, the rupture acts as the interface between the subducting oceanic crust and the overriding layers (accretionary prism, continental crust and mantle wedge), varying the position of the shear stress asperity acting as nucleation patch. Initial results reveal that ruptures in homogeneous media produce earthquakes with large slip at depth compared to the case where bi-material interface is included. However the opposite occurs for events nucleating at intermediate depths: the compliant accretionary prism favours slip up to the free surface leading to larger events compared to the homogeneous case. These preliminary findings will be further investigated considering different material contrasts between the slab and the overriding accretionary prism to mimic the slowness of the sedimentary wedge. This will contribute to assess the influence of these contrasts in more realistic environment on the seismic source features and, in turn, on the conditional probability of exceedance for maximum tsunami wave height for a M9 event. Several source parameters, such as coseismic slip, rupture duration, rupture velocity and stress conditions, derived from the numerical simulations will be compared to those inferred from real events using existing finite fault catalogues (e.g. USGS, SRCMOD, etc.).

Holocene surface ruptures of the Rurrand Fault, Germany—insights from palaeoseismology, remote sensing and shallow geophysics

NASA Astrophysics Data System (ADS)

Grützner, Christoph; Fischer, Peter; Reicherter, Klaus

2016-03-01

The Lower Rhine Embayment in Central Europe hosts a rift system that has very low deformation rates. The faults in this area have slip rates of less than 0.1 mm yr-1, which does not allow to investigate ongoing tectonic deformation with geodetic techniques, unless they cover very long time spans. Instrumental seismicity does only cover a small fraction of the very long earthquake recurrence intervals of several thousands of years. Palaeoseismological studies are needed to constrain slip rates and the earthquake history of such faults. Destructive earthquakes are rare in the study area, but did occur in historic times. In 1755/1756, a series of strong earthquakes caused significant destruction in the city of Düren (Germany) and the surrounding areas. In this study we document palaeoseismological data from the nearby Rurrand Fault. In contrast to earlier studies on the same fault, we found evidence for a surface rupturing earthquake in the Holocene, and we identified at least one more surface rupturing event. Our study shows that the Rurrand Fault currently accommodates deformation in earthquakes rather than by creeping. The coseismic offsets were determined to be between less than 0.5 m per event. We assign maximum possible magnitudes of Mw 5.9-6.8 for the Rurrand Fault and a slip rate of at least 0.02-0.03 mm yr-1 for the last ˜130-50 kyr. The surface ruptures did not occur at the main fault trace that has a clear morphological expression due to older tectonic motions, but on a younger fault strand in the hanging wall of the main fault. Terrain analyses based on 1 m resolution airborne LiDAR data have been used to image the subtle morphological expression of this young fault zone. Georadar and electric resistivity tomography were applied to image the fault zone at depth and to test if these shallow geophysical methods can be used to identify and trace the fault zone. Georadar failed to produce reliable results, but geoelectrics were successfully applied and allowed us to retrieve slip rate estimates. Our results indicate that the Düren 1755/1756 earthquakes did not produce surface ruptures at the Rurrand Fault, either because they did not rupture the surface at all, or because they occurred at another, neighbouring fault.

Interaction between the Dauki and the Indo-Burman convergence boundaries from teleseismic and locally recorded earthquake data

NASA Astrophysics Data System (ADS)

Howe, M.; Moulik, P.; Seeber, L.; Kim, W.; Steckler, M. S.

2012-12-01

The Himalayan and the Burma Arcs converge onto the Indian plate from opposite sides near their syntaxial juncture and have reduced it to a sliver. Both geology and seismicity point to recent internal deformation and high seismogenic potential within this sliver. Large historical earthquakes, including the Great Indian earthquake of 1897 (Mw ~8.1), along with the recent seismicity, suggest that the cratonic blocks in the region are bounded by active faults. The most prominent is the E-W trending Dauki Fault, a deeply-rooted, north-dipping thrust fault, situated between the Shillong massif to the north and the Sylhet Basin to the south. Along the Burma Arc, the subducted seismogenic slab of the Indian plate is continuous north to the syntaxis. Yet the Naga and Tripura segments of the accretionary fold belt, respectively north and south of the easterly extrapolation of the Dauki fault, are distinct. Accretion has advanced far westward into the foredeep of the Dauki structure along the front of the Tripura segment, while it has remained stunted facing the uplifted Shillong massif along the Naga segment. Moreover, the Dauki topographic front can be traced eastwards across the Burma Arc separating the two segments. Recent earthquakes support the hypothesis that the Dauki convergence structure continues below the Burma accretionary belt. Using teleseismic and regional data from the deployment of a local network, we explore the interaction of the Dauki thrust fault with the Burma Arc subduction zone. Preliminary observations include: While seismicity is concentrated in the slab at the eastward extrapolation of the Dauki fault, shallow seismicity is diffuse and does not illuminate the Dauki fault itself. P-axes in moment-tensor solutions of earthquakes within the Indian plate tend to be directed N-S and are locally parallel to the India-Burma boundary, particularly in the slab. T-axes tend to be oriented E-W with a strong tendency to follow the slab down dip. This pattern is remarkably consistent, despite the scattered seismicity, and suggests that the stress in the Indian plate, including the subducted oceanic portion of the plate, is still primarily controlled by the Himalayan collision to the north and down-dip pull by the Burma slab. Moment tensor solutions for some of the shallow earthquakes along the fold belt are consistent with geodetic results, showing partitioning of the oblique India-Burma convergence between belt-parallel dextral faults and belt-normal shortening by thrust faults. Relocations of the events using the double-difference algorithm may provide additional constraints on the geometry of the slab. In addition to the analysis of teleseismic data, a network of six seismic stations was also installed in Bangladesh in the region surrounding Sylhet, south of the Shillong Plateau during 2007-2008. Over 200 regional and local events are detected and located by the Sylhet array. About a dozen events are large enough allowing us to determine focal depths and mechanisms that will augment the catalog of the teleseismic events, providing additional insights into the tectonics in the region.

A study of the Herald-Phillipstown fault in the Wabash Valley using drillhole and 3-D seismic reflection data

NASA Astrophysics Data System (ADS)

Kroenke, Samantha E.

In June 2009, a 2.2 square mile 3-D high resolution seismic reflection survey was shot in southeastern Illinois in the Phillipstown Consolidated oilfield. A well was drilled in the 3-D survey area to tie the seismic to the geological data with a synthetic seismogram from the sonic log. The objectives of the 3-D seismic survey were three-fold: (1) To image and interpret faulting of the Herald-Phillipstown Fault using drillhole-based geological and seismic cross-sections and structural contour maps created from the drillhole data and seismic reflection data, (2) To test the effectiveness of imaging the faults by selected seismic attributes, and (3) To compare spectral decomposition amplitude maps with an isochron map and an isopach map of a selected geologic interval (VTG interval). Drillhole and seismic reflection data show that various formation offsets increase near the main Herald-Phillipstown fault, and that the fault and its large offset subsidiary faults penetrate the Precambrian crystalline basement. A broad, northeast-trending 10,000 feet wide graben is consistently observed in the drillhole data. Both shallow and deep formations in the geological cross-sections reveal small horst and graben features within the broad graben created possibly in response to fault reactivations. The HPF faults have been interpreted as originally Precambrian age high-angle, normal faults reactivated with various amounts and types of offset. Evidence for strike-slip movement is also clear on several faults. Changes in the seismic attribute values in the selected interval and along various time slices throughout the whole dataset correlate with the Herald-Phillipstown faults. Overall, seismic attributes could provide a means of mapping large offset faults in areas with limited or absent drillhole data. Results of the spectral decomposition suggest that if the interval velocity is known for a particular formation or interval, high-resolution 3-D seismic reflection surveys could utilize these amplitudes as an alternative seismic interpretation method for estimating formation thicknesses. A VTG isopach map was compared with an isochron map and a spectral decomposition amplitude map. The results reveal that the isochron map strongly correlates with the isopach map as well as the spectral decomposition map. It was also found that thicker areas in the isopach correlated with higher amplitude values in the spectral decomposition amplitude map. Offsets along the faults appear sharper in these amplitudes and isochron maps than in the isopach map, possibly as a result of increased spatial sampling.

Identification of the Polaris Fault using lidar and shallow geophysical methods

USGS Publications Warehouse

Hunter, Lewis E.; Powers, Michael H.; Burton, Bethany L.

2017-01-01

As part of the U.S. Army Corps of Engineers' (USACE) Dam Safety Assurance Program, Martis Creek Dam near Truckee, CA, is under evaluation for earthquake and seepage hazards. The investigations to date have included LiDAR (Light Detection and Ranging) and a wide range of geophysical surveys. The LiDAR data led to the discovery of an important and previously unknown fault tracing very near and possibly under Martis Creek Dam. The geophysical surveys of the dam foundation area confirm evidence of the fault in the area.

Study on the Evaluation Method for Fault Displacement: Probabilistic Approach Based on Japanese Earthquake Rupture Data - Principal fault displacements -

NASA Astrophysics Data System (ADS)

Kitada, N.; Inoue, N.; Tonagi, M.

2016-12-01

The purpose of Probabilistic Fault Displacement Hazard Analysis (PFDHA) is estimate fault displacement values and its extent of the impact. There are two types of fault displacement related to the earthquake fault: principal fault displacement and distributed fault displacement. Distributed fault displacement should be evaluated in important facilities, such as Nuclear Installations. PFDHA estimates principal fault and distributed fault displacement. For estimation, PFDHA uses distance-displacement functions, which are constructed from field measurement data. We constructed slip distance relation of principal fault displacement based on Japanese strike and reverse slip earthquakes in order to apply to Japan area that of subduction field. However, observed displacement data are sparse, especially reverse faults. Takao et al. (2013) tried to estimate the relation using all type fault systems (reverse fault and strike slip fault). After Takao et al. (2013), several inland earthquakes were occurred in Japan, so in this time, we try to estimate distance-displacement functions each strike slip fault type and reverse fault type especially add new fault displacement data set. To normalized slip function data, several criteria were provided by several researchers. We normalized principal fault displacement data based on several methods and compared slip-distance functions. The normalized by total length of Japanese reverse fault data did not show particular trend slip distance relation. In the case of segmented data, the slip-distance relationship indicated similar trend as strike slip faults. We will also discuss the relation between principal fault displacement distributions with source fault character. According to slip distribution function (Petersen et al., 2011), strike slip fault type shows the ratio of normalized displacement are decreased toward to the edge of fault. However, the data set of Japanese strike slip fault data not so decrease in the end of the fault. This result indicates that the fault displacement is difficult to appear at the edge of the fault displacement in Japan. This research was part of the 2014-2015 research project `Development of evaluating method for fault displacement` by the Secretariat of Nuclear Regulation Authority (NRA), Japan.

The influence of joint parameters on normal fault evolution and geometry: a parameter study using analogue modeling

NASA Astrophysics Data System (ADS)

Kettermann, Michael; von Hagke, Christoph; Urai, Janos L.

2017-04-01

Dilatant faults often form in rocks containing pre-existing joints, but the effects of joints on fault segment linkage and fracture connectivity is not well understood. Studying evolution of dilatancy and influence of fractures on fault development provides insights into geometry of fault zones in brittle rocks and will eventually allow for predicting their subsurface appearance. In an earlier study we recognized the effect of different angles between strike direction of vertical joints and a basement fault on the geometry of a developing fault zone. We now systematically extend the results by varying geometric joint parameters such as joint spacing and vertical extent of the joints and measuring fracture density and connectivity. A reproducibility study shows a small error-range for the measurements, allowing for a confident use of the experimental setup. Analogue models were carried out in a manually driven deformation box (30x28x20 cm) with a 60° dipping pre-defined basement fault and 4.5 cm of displacement. To produce open joints prior to faulting, sheets of paper were mounted in the box to a depth of 5 cm at a spacing of 2.5 cm. We varied the vertical extent of the joints from 5 to 50 mm. Powder was then sieved into the box, embedding the paper almost entirely (column height of 19 cm), and the paper was removed. During deformation we captured structural information by time-lapse photography that allows particle imaging velocimetry analyses (PIV) to detect localized deformation at every increment of displacement. Post-mortem photogrammetry preserves the final 3-dimensional structure of the fault zone. A counterintuitive result is that joint depth is of only minor importance for the evolution of the fault zone. Even very shallow joints form weak areas at which the fault starts to form and propagate. More important is joint spacing. Very large joint spacing leads to faults and secondary fractures that form subparallel to the basement fault. In contrast, small joint spacing results in fault strands that only localize at the pre-existing joints, and secondary fractures that are oriented at high angles to the pre-existing joints. With this new set of experiments we can now quantitatively constrain how (i) the angle between joints and basement fault, (ii) the joint depth and (iii) the joint spacing affect fault zone parameters such as (1) the damage zone width, (2) the density of secondary fractures, (3) map-view area of open gaps or (4) the fracture connectivity. We apply these results to predict subsurface geometries of joint-fault networks in cohesive rocks, e.g. basaltic sequences in Iceland and sandstones in the Canyonlands NP, USA.

Temporal evolution of surface rupture deduced from coseismic multi-mode secondary fractures: Insights from the October 8, 2005 (Mw 7.6) Kashmir earthquake, NW Himalaya

NASA Astrophysics Data System (ADS)

Sayab, Mohammad; Khan, Muhammad Asif

2010-10-01

Detailed rupture-fracture analyses of some of the well-studied earthquakes have revealed that the geometrical arrangement of secondary faults and fractures can be used as a geological tool to understand the temporal evolution of slip produced during the mainshock. The October 8, 2005 Mw 7.6 Kashmir earthquake, NW Himalaya, surface rupture provides an opportunity to study a complex network of secondary fractures developed on the hanging wall of the fault scarp. The main fault scarp is clearly thrust-type, rupture length is ~ 75 ± 5 km and the overall trend of the rupture is NW-SE. We present the results of our detailed structural mapping of secondary faults and fractures at 1:100 scale, on the hanging wall of the southern end of the rupture in the vicinity of the Sar Pain. Secondary ruptures can be broadly classified as two main types, 1) normal faults and, (2) right-lateral strike-slip 'Riedel' fractures. The secondary normal faults are NW-SE striking, with a maximum 3.3 meter vertical displacement and 2.5 meter horizontal displacement. Estimated total horizontal extension across the secondary normal faults is 3.1-3.5%. We propose that the bending-moment and coseismic stress relaxation can explain the formation of secondary normal faults on the hanging wall of the thrust fault. The strike-slip 'Riedel' fractures form distinct sets of tension (T) and shear fractures (R', R, Y) with right-lateral displacement. Field observations revealed that the 'Riedel' fractures (T) cut the secondary normal faults. In addition, there is kinematic incompatibility and magnitude mismatch between the secondary normal faults and strike-slip 'Riedel' fractures. The cross-cutting relationship, geometric and magnitude incoherence implies a temporal evolution of slip from dip- to strike-slip during the mainshock faulting. The interpretation is consistent with the thrust fault plane solution with minor right-lateral strike-slip component.

Inherited discontinuities and fault kinematics of a multiphase, non-colinear extensional setting: Subsurface observations from the South Flank of the Golfo San Jorge basin, Patagonia

NASA Astrophysics Data System (ADS)

Paredes, José Matildo; Aguiar, Mariana; Ansa, Andrés; Giordano, Sergio; Ledesma, Mario; Tejada, Silvia

2018-01-01

We use three-dimensional (3D) seismic reflection data to analyze the structural style, fault kinematics and growth fault mechanisms of non-colinear normal fault systems in the South Flank of the Golfo San Jorge basin, central Patagonia. Pre-existing structural fabrics in the basement of the South Flank show NW-SE and NE-SW oriented faults. They control the location and geometry of wedge-shaped half grabens from the "main synrift phase" infilled with Middle Jurassic volcanic-volcaniclastic rocks and lacustrine units of Late Jurassic to Early Cretaceous age. The NE-striking, basement-involved normal faults resulted in the rapid establishment of fault lenght, followed by gradual increasing in displacement, and minor reactivation during subsequent extensional phases; NW-striking normal faults are characterized by fault segments that propagated laterally during the "main rifting phase", being subsequently reactivated during succesive extensional phases. The Aptian-Campanian Chubut Group is a continental succession up to 4 km thick associated to the "second rifting stage", characterized by propagation and linkage of W-E to WNW-ESE fault segments that increase their lenght and displacement in several extensional phases, recognized by detailed measurement of current throw distribution of selected seismic horizons along fault surfaces. Strain is distributed in an array of sub-parallel normal faults oriented normal to the extension direction. A Late Cretaceous-Paleogene (pre-late Eocene) extensional event is characterized by high-angle, NNW-SSE to NNE-SSW grabens coeval with intraplate alkali basaltic volcanism, evidencing clockwise rotation of the stress field following a ∼W-E extension direction. We demonstrate differences in growth fault mechanisms of non-colinear fault populations, and highlight the importance of follow a systematic approach to the analysis of fault geometry and throw distribution in a fault network, in order to understand temporal-spatial variations in the coeval topography, potential structural traps, and distribution of oil-bearing sandstone reservoirs.

Research in Seismology

DTIC Science & Technology

1975-01-01

edge of the Persian Gulf, isolatina the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably...Fault-plane solutions for these events (Fitch, 1970; Nowroozi, 1972) confirm the mechanisms to be shallow underthrusting. The Bouguer gravity anomaly...at 40 km behind the subduction zone. A Bouguer gravity anomaly has also been calculated for this model by assuming that the structure is flat and

Co- and post-seismic shallow fault physics from near-field geodesy, seismic tomography, and mechanical modeling

NASA Astrophysics Data System (ADS)

Nevitt, J.; Brooks, B. A.; Catchings, R.; Goldman, M.; Criley, C.; Chan, J. H.; Glennie, C. L.; Ericksen, T. L.; Madugo, C. M.

2017-12-01

The physics governing near-surface fault slip and deformation are largely unknown, introducing significant uncertainty into seismic hazard models. Here we combine near-field measurements of surface deformation from the 2014 M6.0 South Napa earthquake with high-resolution seismic imaging and finite element models to investigate the effects of rupture speed, elastic heterogeneities, and plasticity on shallow faulting. We focus on two sites that experienced either predominantly co-seismic or post-seismic slip. We measured surface deformation with mobile laser scanning of deformed vine rows within 300 m of the fault at 1 week and 1 month after the event. Shear strain profiles for the co- and post-seismic sites are similar, with maxima of 0.012 and 0.013 and values exceeding 0.002 occurring within 26 m- and 18 m-wide zones, respectively. That the rupture remained buried at the two sites and produced similar deformation fields suggests that permanent deformation due to dynamic stresses did not differ significantly from the quasi-static case, which might be expected if the rupture decelerated as it approached the surface. Active-source seismic surveys, 120 m in length with 1 m geophone/shot spacing, reveal shallow compliant zones of reduced shear modulus. For the co- and post-seismic sites, the tomographic anomaly (Vp/Vs > 5) at 20 m depth has a width of 80 m and 50 m, respectively, much wider than the observed surface displacement fields. We investigate this discrepancy with a suite of finite element models in which a planar fault is buried 5 m below the surface. The model continuum is defined by either homogeneous or heterogeneous elastic properties, with or without Drucker-Prager plastic yielding, with properties derived from lab testing of similar near-surface materials. We find that plastic yielding can greatly narrow the surface displacement zone, but that the width of this zone is largely insensitive to changes in the elastic structure (i.e., the presence of a compliant zone).

Evidence for cross rift structural controls on deformation and seismicity at a continental rift caldera

NASA Astrophysics Data System (ADS)

Lloyd, Ryan; Biggs, Juliet; Wilks, Matthew; Nowacki, Andy; Kendall, J.-Michael; Ayele, Atalay; Lewi, Elias; Eysteinsson, Hjálmar

2018-04-01

In continental rifts structural heterogeneities, such as pre-existing faults and foliations, are thought to influence shallow crustal processes, particularly the formation of rift faults, magma reservoirs and surface volcanism. We focus on the Corbetti caldera, in the southern central Main Ethiopian Rift. We measure the surface deformation between 22nd June 2007 and 25th March 2009 using ALOS and ENVISAT SAR interferograms and observe a semi-circular pattern of deformation bounded by a sharp linear feature cross-cutting the caldera, coincident with the caldera long axis. The signal reverses in sign but is not seasonal: from June to December 2007 the region south of this structure moves upwards 3 cm relative to the north, while from December 2007 until November 2008 it subsides by 2 cm. Comparison of data taken from two different satellite look directions show that the displacement is primarily vertical. We discuss potential mechanisms and conclude that this deformation is associated with pressure changes within a shallow (<1 km) fault-bounded hydrothermal reservoir prior to the onset of a phase of caldera-wide uplift. Analysis of the distribution of post-caldera vents and cones inside the caldera shows their locations are statistically consistent with this fault structure, indicating that the fault has also controlled the migration of magma from a reservoir to the surface over tens of thousands of years. Spatial patterns of seismicity are consistent with a cross-rift structure that extents outside the caldera and to a depth of ∼30 km, and patterns of seismic anisotropy suggests stress partitioning occurs across the structure. We discuss the possible nature of this structure, and conclude that it is most likely associated with the Goba-Bonga lineament, which cross-cuts and pre-dates the current rift. Our observations show that pre-rift structures play an important role in magma transport and shallow hydrothermal processes, and therefore they should not be neglected when discussing these processes.

Geophysical Investigations of the Smoke Creek Desert and their Geologic Implications, Northwest Nevada and Northeast California

USGS Publications Warehouse

Ponce, David A.; Glen, Jonathan M.G.; Tilden, Janet E.

2006-01-01

The Smoke Creek Desert is a large basin about 100 km (60 mi) north of Reno near the California-Nevada border, situated along the northernmost parts of the Walker Lane Belt, a physiographic region defined by diverse topographic expression consisting of northweststriking topographic features and strike-slip faulting. Because geologic and geophysical framework studies play an important role in understanding the hydrogeology of the Smoke Creek Desert, a geophysical effort was undertaken to help determine basin geometry, infer structural features, and estimate depth to basement. In the northernmost parts of the Smoke Creek Desert basin, along Squaw Creek Valley, geophysical data indicate that the basin is shallow and that granitic rocks are buried at shallow depths throughout the valley. These granitic rocks are faulted and fractured and presumably permeable, and thus may influence ground-water resources in this area. The Smoke Creek Desert basin itself is composed of three large oval sub-basins, all of which reach depths to basement of up to about 2 km (1.2 mi). In the central and southern parts of the Smoke Creek Desert basin, magnetic anomalies form three separate and narrow EW-striking features. These features consist of high-amplitude short-wavelength magnetic anomalies and probably reflect Tertiary basalt buried at shallow depth. In the central part of the Smoke Creek Desert basin a prominent EW-striking gravity and magnetic prominence extends from the western margin of the basin to the central part of the basin. Along this ridge, probably composed of Tertiary basalt, overlying unconsolidated basin-fill deposits are relatively thin (< 400 m). The central part of the Smoke Creek Desert basin is also characterized by the Mid-valley fault, a continuous geologic and geophysical feature striking NS and at least 18-km long, possibly connecting with faults mapped in the Terraced Hills and continuing southward to Pyramid Lake. The Mid-valley fault may represent a lateral (east-west) barrier to ground-water flow. In addition, the Mid-valley fault may also be a conduit for along-strike (north-south) ground-water flow, channeling flow to the southernmost parts of the basin and the discharge areas north of Sand Pass.

Geodynamic Evolution of the Banda Sea Region

NASA Astrophysics Data System (ADS)

Kaymakci, N.; Decker, J.; Orange, D.; Teas, P.; Van Heiningen, P.

2013-12-01

We've carried out a large on- and offshore study in Eastern Indonesia to characterize the major structures and to provide constraints on the Neogene geodynamic evolution of the Banda Sea region. The onshore portion utilized remote sensing data and published geology. We tied the onshore to the offshore using recently acquired high resolution bathymetric data (16m and 25m bin size) and 2D seismic profiles that extend from Sulawesi in the west to Irian Jaya in the east across the northern part of the Banda Arc. We interpret the northern boundary of the 'Birds Head' (BH) of Papua, the Sorong Fault, to be a sinistral strike-slip fault zone with a minimum of 48 km displacement over the last few million years. The western boundary fault of Cendrawasih Basin defines the eastern boundary of BH and corresponds to the Wandamen Peninsula which comprises high pressure metamorphic rocks, including eclogite and granulite facies rocks, with exhumation ages from 4 to 1 Ma. Earthquake focal mechanism solutions indicate that the eastern boundary of BH is linked with a large scale offshore normal fault which we suggest may be related to the exhumation of the Wandamen Peninsula. The eastern boundary of Cendrawasih Basin is defined by a large transpressive belt along which BH is decoupled from the rest of Papua / Irian Jaya. This interpretation is supported by recent GPS studies. We propose that the BH and the Pacific plate are coupled, and therefore the Birds Head is therefore completely detached from Irian Jaya. Furthermore, Aru Basin, located at the NE corner of Banda Arc, is a Fault-Fault-Transform (FFT) type triple junction. According to available literature information the Banda Sea includes three distinct basins with different geologic histories; the North Banda Sea Basin (NBSB) was opened during 12-7 Ma, Wetar-Damar Basin (WDB) during 7-3.5 Ma and Weber Basin (WB) 3-0 Ma. Our bathymetric and seismic data indicated that the NBSB and Weber Basin lack normal oceanic crust and are probably floored by exhumed mantle, while WDB seems to have normal oceanic crust. These basins thought to be developed sequentially from north to south, possibly due to back arc extension resulting from trench retreat and roll-back of the northwards subducting Indo-Australian oceanic plate below the SE Eurasian margin along the Sunda-Banda subduction zone. We suggest that a trench-perpendicular tear in the subducting slab extends from the southwestern corner of Celebes Sea to the northeastern corner of Seram Island. It defines the southern boundary of the Banggai-Sula and Bird's Head (BH) blocks and northern boundary of Banda Sea micro-plate. The dominant character of this structure is sinistral strike-slip fault zone that eastward gradually become transpressional to ultimately thrusting at the tip of the tear east of Seram Island. Here, deformation results in a large accretionary wedge, the Seram Accretionary Belt (SAB) that is partitioned by intensely sheared strike-slip faults. The deformation mechanisms within the SAB is difficult to interpret due to poor seismic imaging below a shallow (Pliocene?) unconformity and the inferred complexity of the deformation within the belt. However, geometries of faults and fault blocks are very well pronounced on bathymetric data which provide hints for the deformation style of the belt.

Active faulting induced by the slip partitioning in the Lesser Antilles arc

NASA Astrophysics Data System (ADS)

Leclerc, Frédérique; Feuillet, Nathalie

2010-05-01

AGUADOMAR marine cruise data acquired 11 years ago allowed us to identified and map two main sets of active faults within the Lesser Antilles arc (Feuillet et al., 2002; 2004). The faults belonging to the first set, such as Morne-Piton in Guadeloupe, bound up to 100km-long and 50km-wide arc-perpendicular graben or half graben that disrupt the fore-arc reef platforms. The faults of the second set form right-stepping en echelon arrays, accommodating left-lateral slip along the inner, volcanic islands. The two fault systems form a sinistral horsetail east of the tip of the left-lateral Puerto Rico fault zone that takes up the trench-parallel component of convergence between the North-American and Caribbean plates west of the Anegada passage. In other words, they together accommodate large-scale slip partitioning along the northeastern arc, consistent with recent GPS measurements (Lopez et al., 2006). These intraplate faults are responsible for a part of the shallow seismicity in the arc and have produce damaging historical earthquakes. Two magnitude 6.3 events occurred in the last 25 years along the inner en echelon faults, the last one on November 21 2004 in Les Saintes in the Guadeloupe archipelago. To better constrain the seismic hazard related to the inner arc faults and image the ruptures and effects on the seafloor of Les Saintes 2004 earthquake, we acquired new marine data between 23 February and 25 March 2009 aboard the French R/V le Suroît during the GWADASEIS cruise. We present here the data (high-resolution 72 channel and very high-resolution chirp 3.5 khz seismic reflection profiles, EM300 multibeam bathymetry, Küllenberg coring and SAR imagery) and the first results. We identified, mapped and characterized in detail several normal to oblique fault systems between Martinique and Saba. They offset the seafloor by several hundred meters and crosscut all active volcanoes, among them Nevis Peak, Soufriere Hills, Soufriere de Guadeloupe and Montagne Pelée. Some faults, located between Guadeloupe and Montserrat have throws up to thousand meters. Between St Lucia and Martinique, the St Lucia channel is crosscut by several normal faults with scarps up to 100m-high. These faults extend onshore and cut the southern shore of Martinique. Given their length (~20km), they could produce magnitude 6 or more earthquakes in the most tourist towns of the island (St Anne, St Lucie). Recent coseismic offsets could be identified along most faults in the chirp profiles. Turbidite deposits recognized in the Küllenberg cores could be related to damaging earthquakes. High resolution SAR imagery (25 cm) reveals several coseismic scarps in Les Saintes channel along the faults that ruptured in 2004. References: Feuillet, N., I. Manighetti, and P. Tapponnier, Arc parallel extension and localization of volcanic complexes in guadeloupe, lesser antilles, Journal of Geophysical Research, 107, 2002. Feuillet, N., P. Tapponnier, I. Manighetti, B. Villemant, and G. C. P. King, Differential uplift and tilt of pleistocene reef platforms and quaternary slip Lopez, A.M., S. Stein, T. Dixon, G. Sella, E. Calais, P. Jansma, J. Weber, and P. La Femina, Is there a northern lesser antilles forearc block ?, Geophysical Research Letters, 33, 2006.

Tectonic Constraints on the Evolution of Geothermal Systems in the Central Andean Volcanic Zone (CAVZ)

NASA Astrophysics Data System (ADS)

Veloso, E. E.; Tardani, D.; Aron, F.; Elizalde, J. D.; Sanchez-Alfaro, P.; Godoy, B.

2017-12-01

South of 19°S, geothermal fields and Pliocene-to-Holocene volcanic centers of the Central Andean Volcanic Zone are spatially associated with distinct, large-scale fault systems disrupting the volcanic arc, which control the architecture and dynamics of the fluids reservoirs at shallow crustal levels. Based on an extensive compilation of structural, lithological and isotopic data, and satellite imagery band-ratio analyses, we produced detailed maps of 13 areas comprising 19 identified and/or potential geothermal fields, to examine if particular local-scale tectonic configurations are associated to fluids migrating from different crustal levels. We defined three main tectonic environments according to the specific, kilometer-scale structural arrangement and its spatial relation to the geothermal surface manifestations. T1, dominated by left-lateral, pure strike-slip motion on a NW-trending duplex-like geometry with geothermal fields located along the faults - in turn distributed into five major subparallel zones cutting across the orogenic belt between ca. 20° and 27°S. T2, dominated by shortening on a series of N-trending thrust faults and fault-propagated folds, cut and displaced by the above mentioned NW-trending faults, with geothermal fields hosted at fault intersections and at fold hinges. And T3, characterized by transtension accommodated by NW-to-WNW-trending left-lateral/normal faults, with hot-springs lying along the fault traces. Interestingly, each of the independently defined tectonic environments has distinctive helium (in fluids) and strontium (in lavas) isotopic signatures and estimated geothermal reservoir temperatures. T1 shows a large 4He contribution, low 87Sr/86Sr ratio and temperatures varying between ca. 220°-310°C; T3 low 4He and high 87Sr/86Sr ratio and temperature (260°-320°C); T2 isotopic values fall between T1 and T3, yet showing the lowest (130°-250°C) temperatures. We suggest that these particular isotopic signatures are due to a strong structural control on the hot reservoir location and meteoric water content, T3 allowing deeper hot fluid provenances and T1 more meteoric influx.

A Lower Carboniferous two-stage extensional basin along the Avalon-Meguma terrane boundary: Evidence from southeastern Isle Madame, Nova Scotia

USGS Publications Warehouse

Force, E.R.; Barr, S.M.

2006-01-01

Anomalously thick and coarse clastic sedimentary successions, including over 5000 m of conglomerate, are exposed on Isle Madame off the southern coast of Cape Breton Island. Two steeply to moderately dipping stratigraphic packages are recognized: one involving Horton and lower Windsor groups (Tournasian-Visean); the other involving upper Windsor and Mabou (Visean-Namurian) groups. Also anomalous on Isle Madame are three long narrow belts of "basement" rocks, together with voluminous chloritic microbreccia and minor semi-ductile mylonite, which are separated from the conglomerate-dominated successions by faults. The angular relations between the cataclastic rocks and the conglomerate units, combined with the presence of cataclasite clasts in the conglomerate units and evidence of dip-slip faults within the basin, suggest an extensional setting, where listric normal faults outline detachment allochthons. Allochthon geometry requires two stages of extension, the older stage completed in early Windsor Group time and including most of the island, and the more local younger stage completed in Mabou Group time. Domino-style upper-plate faulting in the younger stage locally repeated the older detachment relation of basement and conglomerate to form the observed narrow belts. Re-rotation of older successions in the younger stage also locally overturned the Horton Group. These features developed within a broad zone of Carboniferous dextral transcurrent faulting between already-docked Avalon and Meguma terranes. Sites of transpression and transtension alternated along the Cobequid-Chedabucto fault zone that separated these terranes. The earlier extensional features in Isle Madame likely represent the northern headwall and associated clastic debris of a pull-apart or other type of transtensional basin developed along part of this fault zone that had become listric; they were repeated and exposed by being up-ended in the second stage of extension, also on listric faults. The two-stage history on Isle Madame exposes the deeper parts of one of the Horton-age extensional basins of the Maritimes, others of which have been described as half-grabens based on their shallower exposures.

Deep neural networks: A promising tool for fault characteristic mining and intelligent diagnosis of rotating machinery with massive data

NASA Astrophysics Data System (ADS)

Jia, Feng; Lei, Yaguo; Lin, Jing; Zhou, Xin; Lu, Na

2016-05-01

Aiming to promptly process the massive fault data and automatically provide accurate diagnosis results, numerous studies have been conducted on intelligent fault diagnosis of rotating machinery. Among these studies, the methods based on artificial neural networks (ANNs) are commonly used, which employ signal processing techniques for extracting features and further input the features to ANNs for classifying faults. Though these methods did work in intelligent fault diagnosis of rotating machinery, they still have two deficiencies. (1) The features are manually extracted depending on much prior knowledge about signal processing techniques and diagnostic expertise. In addition, these manual features are extracted according to a specific diagnosis issue and probably unsuitable for other issues. (2) The ANNs adopted in these methods have shallow architectures, which limits the capacity of ANNs to learn the complex non-linear relationships in fault diagnosis issues. As a breakthrough in artificial intelligence, deep learning holds the potential to overcome the aforementioned deficiencies. Through deep learning, deep neural networks (DNNs) with deep architectures, instead of shallow ones, could be established to mine the useful information from raw data and approximate complex non-linear functions. Based on DNNs, a novel intelligent method is proposed in this paper to overcome the deficiencies of the aforementioned intelligent diagnosis methods. The effectiveness of the proposed method is validated using datasets from rolling element bearings and planetary gearboxes. These datasets contain massive measured signals involving different health conditions under various operating conditions. The diagnosis results show that the proposed method is able to not only adaptively mine available fault characteristics from the measured signals, but also obtain superior diagnosis accuracy compared with the existing methods.

Widespread afterslip and triggered slow slip events following the M7.8 Kaikoura earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Wallace, L. M.; Hreinsdottir, S.; Hamling, I. J.; D'Anastasio, E.; Bartlow, N. M.

2017-12-01

Just after midnight on 14 Nov 2016 (NZ Local time), the M7.8 Kaikoura earthquake ruptured a complex sequence of strike-slip and reverse faults over an approximately 150 km length in the northeastern South Island of New Zealand (Hamling et al., 2017, Science). In the months following the earthquake, time-dependent inversions of InSAR observations and continuous and semi-continuous GPS measurements reveal up to 0.5 m of afterslip on the subduction interface beneath the northern South Island underlying the region of large coseismic slip on crustal faults in the M7.8 earthquake. The geodetic data also require significant afterslip on a subset of the crustal faults that ruptured in the earthquake, including the Needles, Jordan Thrust, and Kekerengu faults. Our best-fitting models also suggest significant afterslip on an offshore reverse fault, in a similar position to one inferred by Clark et al. (2017, EPSL) from coseismic coastal uplift data. The M7.8 earthquake also triggered widespread slow slip occurring over much of the Hikurangi subduction zone beneath the North Island. Immediately following the earthquake, continuous GPS sites operated by GeoNet (www.geonet.org.nz) along the North Island's east coast (above the Hikurangi subduction zone) detected several to 30 mm of eastward motion over the two-week period immediately following the M7.8 event. These sites are located 350-650 km from the M7.8 earthquake. Such large eastward motion along the North Island's east coast following the earthquake is consistent with the initiation of a large slow slip event along the shallow, offshore portion of the Hikurangi subduction zone. In addition to shallow slow slip (<15 km depth) triggered offshore the east coast, we also observe deeper slow slip (>30 km depth) triggered in the Kapiti region at the southern Hikurangi margin. The Kapiti SSE was still ongoing as of August 2017, although we expect it to finish before the end of 2017. Given the large distance of the shallow east coast SSE from the M7.8 earthquake, we suggest that the shallow SSE was more likely to be triggered by dynamic stress changes, while the deeper SSEs closer to the Mw 7.8 were more likely triggered by static stress changes.

Rupture behaviors of the 2010 Jiashian and 2016 Meinong Earthquakes: Implication for interaction of two asperities on the Chishan Transfer Fault Zone in SW Taiwan.

NASA Astrophysics Data System (ADS)

Jian, P. R.; Hung, S. H.; Chen, Y. L.; Meng, L.; Tseng, T. L.

2017-12-01

After about 45 years of seismic quiescence, southwest Taiwan was imperiled by two strong earthquakes, the 2010 Mw 6.2 Jiashian and deadly 2016 Mw 6.4 Meinong earthquakes in the last decade. The focal mechanisms and their aftershock distributions imply that both events occurred on NW-SE striking, shallow-dipping fault planes but at different depths of 21 and 16 km, respectively. Here we present the MUSIC back projection images using high-frequency P- and sP-waves recorded in the European and Australian seismic networks, the directivity analysis using global teleseismic P waves and relocated aftershocks to characterize the rupture behaviors of the two mainshocks and explore the potential connection between them. The results for the Meinong event indicate a unilateral, subhorizontal rupture propagating NW-ward 17 km and lasting for 6-7 s [Jian et al., 2017]. For the Jiashian event, the rupture initiated at a greater depth of 21 km and then propagated both NW-ward and up-dip ( 16o) on the fault plane, with a shorter rupture length of 10 km and duration of 4-5 s. The up-dip propagation is corroborated by the 3-D directivity analysis that leads to the widths of P-wave pulses increasing linearly with the directivity parameter. Moreover, relocation of aftershocks reveals that the Jiashian sequence is confined in a NW-SE elongated zone extending 15 km and 5 km shallower than the hypocenter. The Meinong aftershock sequence shows three clusters: one surrounding the mainshock hypocenter, another one distributed northwestern and deeper (>20 km) off the rupture plane beneath Tainan, and the other distant shallow-focus one (<10 km) beneath the southern Central Mountain Range. As evidenced by similar focal mechanism, rupture behaviors, as well as the spatial configuration of the mainshock rupture zones and aftershock distributions, we attribute the Jiashian and Meinong earthquakes to two asperities on a buried oblique fault that has been reactivated recently, the NW-SE striking Chishan Transfer Fault Zone as a likely candidate rupture plane. In 2010, the Jiasian earthquake initiated at the deeper NE asperity and propagated NW-ward and up-dip. Six years later, the stronger shallower asperity responsible for the Meinong event was statically triggered, which consequently caused the ruinous destruction in SW Taiwan.

Stress Study on Southern Segment of Longmenshan Fault Constrained by Focal Mechanism Data

NASA Astrophysics Data System (ADS)

Yang, Y.; Liang, C.; Su, J.; Zhou, L.

2016-12-01

The Longmenshan fault (LMSF) lies at the eastern margin of Tibetan plateau and constitutes the boundary of the active Bayankala block and rigid Sichuan basin. This fault was misinterpreted as an inactive fault before the great Wenchuan earthquake. Five years after the devastating event, the Lushan MS 7.0 stroke the southern segment of the LMSF but fractured in a very limited scale and formed a seismic gap between the two earthquakes. In this study, we determined focal mechanisms of earthquakes with magnitude M≥3 from Jan 2008 to July 2014 in the southern segment of LMSF, and then applied the damped linear inversion to derive the regional stress field based on the focal mechanisms. Focal mechanisms of 755 earthquakes in total were determined. We further used a damped linear inversion technique to produce a 2D stress map in upper crust in the study region. A dominant thrust regime is determined south of the seismic gap, with a horizontal maximum compression oriented in NWW-SEE. But in the area to the north of the seismic gap is characterized as a much more complex stress environment. To the west of the Dujiangyan city, there appear to be a seismic gap in the Pengguan complex. The maximum compressions show the anti-clockwise and clockwise patterns to the south and north of this small gap. Thus the small gap seems to be an asperity that causes the maximum compression to rotate around it. While combined the maximum compression pattern with the focal solutions of strong earthquakes (Mw≥5) in this region, two of those strong earthquakes located near the back-range-fault have strikes parallel to the Miyaluo fault. Considering a large amount of earthquakes in Lixian branch, the Miyaluo fault may be extended to LMSF following the great Wenchuan earthquake. Investigations on the stress field of different depths indicate complex spatial variations. The Pengguan complex is almost aseismic in shallow depth in its central part. In deeper depth, the maximum compressions show the NNW-SSE and NE-SW directions to the north and south of the seismic gap respectively, this are surprisingly different from that of the shallower depth. Thus the maximum compressions vary with depth may imply the movement in depth is decoupled from the movement in shallow depth. This work was partially supported by National Natural Science Foundation of China (41340009).

The transtensional offshore portion of the northern San Andreas fault: Fault zone geometry, late Pleistocene to Holocene sediment deposition, shallow deformation patterns, and asymmetric basin growth

USGS Publications Warehouse

Beeson, Jeffrey W.; Johnson, Samuel Y.; Goldfinger, Chris

2017-01-01

We mapped an ~120 km offshore portion of the northern San Andreas fault (SAF) between Point Arena and Point Delgada using closely spaced seismic reflection profiles (1605 km), high-resolution multibeam bathymetry (~1600 km2), and marine magnetic data. This new data set documents SAF location and continuity, associated tectonic geomorphology, shallow stratigraphy, and deformation. Variable deformation patterns in the generally narrow (∼1 km wide) fault zone are largely associated with fault trend and with transtensional and transpressional fault bends.We divide this unique transtensional portion of the offshore SAF into six sections along and adjacent to the SAF based on fault trend, deformation styles, seismic stratigraphy, and seafloor bathymetry. In the southern region of the study area, the SAF includes a 10-km-long zone characterized by two active parallel fault strands. Slip transfer and long-term straightening of the fault trace in this zone are likely leading to transfer of a slice of the Pacific plate to the North American plate. The SAF in the northern region of the survey area passes through two sharp fault bends (∼9°, right stepping, and ∼8°, left stepping), resulting in both an asymmetric lazy Z–shape sedimentary basin (Noyo basin) and an uplifted rocky shoal (Tolo Bank). Seismic stratigraphic sequences and unconformities within the Noyo basin correlate with the previous 4 major Quaternary sea-level lowstands and record basin tilting of ∼0.6°/100 k.y. Migration of the basin depocenter indicates a lateral slip rate on the SAF of 10–19 mm/yr for the past 350 k.y.Data collected west of the SAF on the south flank of Cape Mendocino are inconsistent with the presence of an offshore fault strand that connects the SAF with the Mendocino Triple Junction. Instead, we suggest that the SAF previously mapped onshore at Point Delgada continues onshore northward and transitions to the King Range thrust.

Seismotectonics of northeastern Sicily and southern Calabria (Italy): New constraints on the tectonic structures featuring in a crucial sector for the central Mediterranean geodynamics

NASA Astrophysics Data System (ADS)

Scarfı, L.; Barberi, G.; Musumeci, C.; Patanè, D.

2016-03-01

The purpose of this study is to gain a better understanding on the tectonic structures featuring in a crucial sector of central Mediterranean area, including the Aeolian Islands, southern Calabria, and northeastern Sicily, where the convergence between Eurasian and African Plates has given rise to a complicated collisional/subduction complex. A high-quality data set of about 3000 earthquakes has been exploited for local earthquake tomography and focal mechanisms computation together with available source mechanisms from published catalogues. The results depict new details of a network of faults which enables the concurrent existence of adjacent compressional and extensional domains. In particular, tomographic images, seismic events distribution, and focal mechanisms pinpoint the geometry and activity of a lithospheric-scale tear faults system which, with a NW-SE trend through Sicily and the Tyrrhenian and Ionian Seas, represents the southern edge of the Ionian subduction trench zone. At crustal depth, this tearing is well highlighted by a rotation of the maximum horizontal stress, moving across the area from west toward east. In addition, the shallow normal fault regime, characterizing the southern Calabria and northeastern Sicily mainland, south of the NW-SE lineament, changes in the deeper part of the crust. Indeed, a NE-SW earthquake distribution, gently dipping NW, and inverse fault solutions indicate a still active contractional deformation in eastern Sicily, caused by the Africa-Eurasia convergence and well framed with the current compressive regime along the southern Tyrrhenian zone and at the front of the Sicilian Chain-Foreland.

Geological and structural interpretation of Peninsular Malaysia by marine and aeromagnetic data: Some preliminary results

NASA Astrophysics Data System (ADS)

Bahrudin, Nurul Fairuz Diyana Binti; Hamzah, Umar

2016-11-01

Magnetic data were processed to interpret the geology of Peninsular Malaysia especially in delineating the igneous bodies and structural lineament trends by potential field geophysical method. A total of about 32000 magnetic intensity data were obtained from Earth Magnetic Anomaly Grid (EMAG2) covering an area of East Sumatra to part of South China Sea within 99° E to 105° E Longitude and 1° N to 7°N Latitude. These data were used in several processing stages in generating the total magnetic intensity (TMI), reduce to equator (RTE), total horizontal derivative (THD) and total vertical derivative (TVD). Values of the possible surface and subsurface magnetic sources associated to the geological features of the study area. The magnetic properties are normally corresponding to features like igneous bodies and faults structures. The anomalies obtained were then compared to the geological features of the area. In general, the high magnetic anomalies of the TMI-RTE are closely matched with major igneous intrusion of Peninsular Malaysia such as the Main Range, Eastern Belt and the Mersing-Johor Bahru stretch. More dense lineaments of magnetic structures were observed in the THD and TVD results indicating the presence of more deep and shallow magnetic rich geological features. The positions of Bukit Tinggi, Mersing and Lepar faults are perfectly matched with the magnetic highs while the presence of Lebir and Bok Bak faults are not clearly observed in the magnetic results. The high magnetic values of igneous bodies may have concealed and obscured the magnetic values representing these faults.

Stress Models of the Annual Hydrospheric, Atmospheric, Thermal, and Tidal Loading Cycles on California Faults: Perturbation of Background Stress and Changes in Seismicity

NASA Astrophysics Data System (ADS)

Johnson, Christopher W.; Fu, Yuning; Bürgmann, Roland

2017-12-01

Stresses in the lithosphere arise from multiple natural loading sources that include both surface and body forces. The largest surface loads include near-surface water storage, snow and ice, atmosphere pressure, ocean loading, and temperature changes. The solid Earth also deforms from celestial body interactions and variations in Earth's rotation. We model the seasonal stress changes in California from 2006 through 2014 for seven different loading sources with annual periods to produce an aggregate stressing history for faults in the study area. Our modeling shows that the annual water loading, atmosphere, temperature, and Earth pole tides are the largest loading sources and should each be evaluated to fully describe seasonal stress changes. In California we find that the hydrological loads are the largest source of seasonal stresses. We explore the seasonal stresses with respect to the background principal stress orientation constrained with regional focal mechanisms and analyze the modulation of seismicity. Our results do not suggest a resolvable seasonal variation for the ambient stress orientation in the shallow crust. When projecting the seasonal stresses into the background stress orientation we find that the timing of microseismicity modestly increases from an 8 kPa seasonal mean-normal-stress perturbation. The results suggest that faults in California are optimally oriented with the background stress field and respond to subsurface pressure changes, possibly due to processes we have not considered in this study. At any time a population of faults are near failure as evident from earthquakes triggered by these slight seasonal stress perturbations.

Deciphering Past and Present Tectonics of the Rio Grande Rift in New Mexico Utilizing Apatite Fission Track Thermochronology, Geochronology, Quaternary Faulting, and Cross-Section Restoration

NASA Astrophysics Data System (ADS)

Ricketts, J. W.; Karlstrom, K. E.; Kelley, S. A.; Priewisch, A.; Crossey, L. J.; Asmerom, Y.; Polyak, V.; Selmi, M.

2011-12-01

The Rio Grande rift provides an excellent laboratory for understanding styles and processes of extensional tectonics, and their driving forces. We apply apatite fission track (AFT) thermochronology, geochronology, fracture analysis, and cross-section restoration to decipher past and present tectonics of the Rio Grande rift. AFT data has been compiled from rift flank uplifts along the Rio Grande rift in an attempt to recognize long wavelength spatial and temporal patterns. AFT ages record time of cooling of rocks below ~110°C and, when cooling is due to exhumation, age elevation traverses can record upward advection of rocks through paleo 110°C isotherms. The relatively passive sides of half-grabens (e.g. Manzanos and Santa Fe Range) preserve Laramide AFT ages ranging from 45-70 Ma, indicating they were cooled during the Laramide Orogeny and have remained cooler than 110°C since then. Rift flanks on the tectonically active sides of half-grabens, (e.g. Sierra Ladrones, Sandias, Taos Range, and Sierra Blanca) have AFT ages that range from 35 Ma to <10 Ma, and record cooling that initiated with the Oligocene ignimbrite flare-up and continues through the Neogene. Our analysis tracks the approximate elevation of paleo 110°C isotherms in 10 Ma intervals from the Laramide to the present and shows that reconstructed paleoisotherms have been differentially uplifted, warped, and faulted since their time of formation, and hence serve as markers of uplift history and its mechanisms. AFT data at Ladron Peak, an active rift flank along the western margin of the Rio Grande rift in central New Mexico, indicates that it was rapidly unroofed between 20-10 Ma. Preliminary apatite helium data gives a similar age vs. elevation trend, but apatites have highly radiogenically damaged lattices and hence have corrected closure temperatures tens of degrees higher than AFT ages. The style of faulting at Ladron Peak is unusual because it is bounded by the anomalously low-angle (~15°) Jeter fault. In order to understand the evolution of faulting in this region, a balanced cross-section was constructed and restored to its pre-rift geometry. Our working hypothesis is that the low angle of the Jeter fault is most adequately explained by a rolling hinge model, where isostatic uplift causes progressive rotation of an initially steep (~60°) normal fault to shallower dips. Thirty km north of Ladron along the west side of the rift, Quaternary extensional faulting is evident in large travertine deposits at the Belen Quarry. Extensional fractures and cm-scale displacement normal faults at 4 locations give average paleostress orientations of 087, 112, 116, 127. A U-series age of 312 ka on faulted upper layers in one quarry indicates post-312 ka slip that we interpret to reflect surface manifestations of microseismicity above the Socorro magma body.

Anatomy of an Active Seismic Source: the Interplay between Present-Day Seismic Activity and Inherited Fault Zone Architecture (Central Apennines, Italy)

NASA Astrophysics Data System (ADS)

Fondriest, M.; Demurtas, M.; Bistacchi, A.; Fabrizio, B.; Storti, F.; Valoroso, L.; Di Toro, G.

2017-12-01

The mechanics and seismogenic behaviour of fault zones are strongly influenced by their internal structure, in terms of both fault geometry and fault rock constitutive properties. In recent years high-resolution seismological techniques yielded new constraints on the geometry and velocity structure of seismogenic faults down to 10s meters length scales. This reduced the gap between geophysical imaging of active seismic sources and field observations of exhumed fault zones. Nevertheless fundamental questions such as the origin of geometrical and kinematic complexities associated to seismic faulting remain open. We addressed these topics by characterizing the internal structure of the Vado di Corno Fault Zone, an active seismogenic normal fault cutting carbonates in the Central Apennines of Italy and comparing it with the present-day seismicity of the area. The fault footwall block, which was exhumed from < 2 km depth, was mapped with high detail (< 1 m spatial resolution) for 2 km of exposure along strike, combining field structural data and photogrammetric surveys in a three dimensional structural model. Three main structural units separated by principal fault strands were recognized: (i) cataclastic unit (20-100 m thick), (ii) damage zone (≤ 300 m thick), (iii) breccia unit ( 20 thick). The cataclastic unit lines the master fault and represents the core of the normal fault zone. In-situ shattering together with evidence of extreme (possibly coseismic) shear strain localization (e.g., mirror-like faults with truncated clasts, ultrafine-grained sheared veins) was recognized. The breccia unit is an inherited thrust zone affected by pervasive veining and secondary dolomitization. It strikes subparallel to the active normal fault and is characterized by a non-cylindrical geometry with 10-100 m long frontal and lateral ramps. The cataclastic unit cuts through thrust flats within the breccia unit, whereas normal to oblique inversion occur on frontal and lateral ramps. A comparable structural setting was imaged South-West of the study area, during the 2009 L'Aquila seismic sequence. Here at 2 km depth, the master normal fault cross-cuts a 10 km long flat structure and clear lateral ramps are illuminated, suggesting the superposition of normal seismic faulting on inherited compressional structures.

Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern "prong" of New Madrid seismic zone

USGS Publications Warehouse

McBride, J.H.; Pugin, Andre J.M.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.; Devera, J.A.; Denny, F.B.; Woolery, E.W.

2003-01-01

High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters ("prongs") in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ. ?? 2003 Elsevier Science B.V. All rights reserved.

Porosity variations in and around normal fault zones: implications for fault seal and geomechanics

NASA Astrophysics Data System (ADS)

Healy, David; Neilson, Joyce; Farrell, Natalie; Timms, Nick; Wilson, Moyra

2015-04-01

Porosity forms the building blocks for permeability, exerts a significant influence on the acoustic response of rocks to elastic waves, and fundamentally influences rock strength. And yet, published studies of porosity around fault zones or in faulted rock are relatively rare, and are hugely dominated by those of fault zone permeability. We present new data from detailed studies of porosity variations around normal faults in sandstone and limestone. We have developed an integrated approach to porosity characterisation in faulted rock exploiting different techniques to understand variations in the data. From systematic samples taken across exposed normal faults in limestone (Malta) and sandstone (Scotland), we combine digital image analysis on thin sections (optical and electron microscopy), core plug analysis (He porosimetry) and mercury injection capillary pressures (MICP). Our sampling includes representative material from undeformed protoliths and fault rocks from the footwall and hanging wall. Fault-related porosity can produce anisotropic permeability with a 'fast' direction parallel to the slip vector in a sandstone-hosted normal fault. Undeformed sandstones in the same unit exhibit maximum permeability in a sub-horizontal direction parallel to lamination in dune-bedded sandstones. Fault-related deformation produces anisotropic pores and pore networks with long axes aligned sub-vertically and this controls the permeability anisotropy, even under confining pressures up to 100 MPa. Fault-related porosity also has interesting consequences for the elastic properties and velocity structure of normal fault zones. Relationships between texture, pore type and acoustic velocity have been well documented in undeformed limestone. We have extended this work to include the effects of faulting on carbonate textures, pore types and P- and S-wave velocities (Vp, Vs) using a suite of normal fault zones in Malta, with displacements ranging from 0.5 to 90 m. Our results show a clear lithofacies control on the Vp-porosity and the Vs-Vp relationships for faulted limestones. Using porosity patterns quantified in naturally deformed rocks we have modelled their effect on the mechanical stability of fluid-saturated fault zones in the subsurface. Poroelasticity theory predicts that variations in fluid pressure could influence fault stability. Anisotropic patterns of porosity in and around fault zones can - depending on their orientation and intensity - lead to an increase in fault stability in response to a rise in fluid pressure, and a decrease in fault stability for a drop in fluid pressure. These predictions are the exact opposite of the accepted role of effective stress in fault stability. Our work has provided new data on the spatial and statistical variation of porosity in fault zones. Traditionally considered as an isotropic and scalar value, porosity and pore networks are better considered as anisotropic and as scale-dependent statistical distributions. The geological processes controlling the evolution of porosity are complex. Quantifying patterns of porosity variation is an essential first step in a wider quest to better understand deformation processes in and around normal fault zones. Understanding porosity patterns will help us to make more useful predictive tools for all agencies involved in the study and management of fluids in the subsurface.

Analysis of P and Pdiff Coda Arrivals for Water Reverberations to Evaluate Shallow Slip Extent in Large Megathrust Earthquakes

NASA Astrophysics Data System (ADS)

Rhode, A.; Lay, T.

2017-12-01

Determining the up-dip rupture extent of large megathrust ruptures is important for understanding their tsunami excitation, frictional properties of the shallow megathrust, and potential for separate tsunami earthquake occurrence. On land geodetic data have almost no resolution of the up-dip extent of faulting and teleseismic observations have limited resolution that is strongly influenced by typically poorly known shallow seismic velocity structure near the toe of the accretionary prism. The increase in ocean depth as slip on the megathrust approaches the trench has significant influence on the strength and azimuthal distribution of water reverberations in the far-field P wave coda. For broadband P waves from large earthquakes with dominant signal periods of about 10 s, water reverberations generated by shallow fault slip under deep water may persist for over a minute after the direct P phases have passed, giving a clear signal of slip near the trench. As the coda waves can be quickly evaluated following the P signal, recognition of slip extending to the trench and associated enhanced tsunamigenic potential could be achieved within a few minutes after the P arrival, potentially contributing to rapid tsunami hazard assessment. We examine the broadband P wave coda at distances from 80 to 120° for a large number of recent major and great earthquakes with independently determined slip distributions and known tsunami excitation to evaluate the prospect for rapidly constraining up-dip rupture extent of large megathrust earthquakes. Events known to have significant shallow slip, at least locally extending to the trench (e.g., 2016 Illapel, Chile; 2010 Maule, 2010 Mentawai) do have relatively enhanced coda levels at all azimuths, whereas events that do not rupture the shallow megathrust (e.g., 2007 Sumatra, 2014 Iquique, 2003 Hokkaido) do not. Some events with slip models lacking shallow slip show strong coda generation, raising questions about the up-dip resolution of slip of their finite-fault models, and others show strong azimuthal patterns in coda strength that suggest propagation from the slip zone to the deep near-trench environments is involved rather than slip near the trench. The various behaviors will be integrated into an assessment of this approach.

Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

USGS Publications Warehouse

Eberhart-Phillips, D.; Christensen, D.H.; Brocher, T.M.; Hansen, R.; Ruppert, N.A.; Haeussler, Peter J.; Abers, G.A.

2006-01-01

In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike-slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three-dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low-velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high-velocity subducted slab and a low-velocity mantle wedge are observed, and high Vp/Vs beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low-velocity, high-Vp/Vs, crust. High-velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the Mw 9.2 great 1964 earthquake. Copyright 2006 by the American Geophysical Union.

Source parameters controlling the generation and propagation of potential local tsunamis along the cascadia margin

USGS Publications Warehouse

Geist, E.; Yoshioka, S.

1996-01-01

The largest uncertainty in assessing hazards from local tsunamis along the Cascadia margin is estimating the possible earthquake source parameters. We investigate which source parameters exert the largest influence on tsunami generation and determine how each parameter affects the amplitude of the local tsunami. The following source parameters were analyzed: (1) type of faulting characteristic of the Cascadia subduction zone, (2) amount of slip during rupture, (3) slip orientation, (4) duration of rupture, (5) physical properties of the accretionary wedge, and (6) influence of secondary faulting. The effect of each of these source parameters on the quasi-static displacement of the ocean floor is determined by using elastic three-dimensional, finite-element models. The propagation of the resulting tsunami is modeled both near the coastline using the two-dimensional (x-t) Peregrine equations that includes the effects of dispersion and near the source using the three-dimensional (x-y-t) linear long-wave equations. The source parameters that have the largest influence on local tsunami excitation are the shallowness of rupture and the amount of slip. In addition, the orientation of slip has a large effect on the directivity of the tsunami, especially for shallow dipping faults, which consequently has a direct influence on the length of coastline inundated by the tsunami. Duration of rupture, physical properties of the accretionary wedge, and secondary faulting all affect the excitation of tsunamis but to a lesser extent than the shallowness of rupture and the amount and orientation of slip. Assessment of the severity of the local tsunami hazard should take into account that relatively large tsunamis can be generated from anomalous 'tsunami earthquakes' that rupture within the accretionary wedge in comparison to interplate thrust earthquakes of similar magnitude. ?? 1996 Kluwer Academic Publishers.

Coseismic and postseismic deformation associated with the 2016 Mw 7.8 Kaikoura earthquake, New Zealand: fault movement investigation and seismic hazard analysis

NASA Astrophysics Data System (ADS)

Jiang, Zhongshan; Huang, Dingfa; Yuan, Linguo; Hassan, Abubakr; Zhang, Lupeng; Yang, Zhongrong

2018-04-01

The 2016 moment magnitude (Mw) 7.8 Kaikoura earthquake demonstrated that multiple fault segments can undergo rupture during a single seismic event. Here, we employ Global Positioning System (GPS) observations and geodetic modeling methods to create detailed images of coseismic slip and postseismic afterslip associated with the Kaikoura earthquake. Our optimal geodetic coseismic model suggests that rupture not only occurred on shallow crustal faults but also to some extent at the Hikurangi subduction interface. The GPS-inverted moment release during the earthquake is equivalent to a Mw 7.9 event. The near-field postseismic deformation is mainly derived from right-lateral strike-slip motions on shallow crustal faults. The afterslip did not only significantly extend northeastward on the Needles fault but also appeared at the plate interface, slowly releasing energy over the past 6 months, equivalent to a Mw 7.3 earthquake. Coulomb stress changes induced by coseismic deformation exhibit complex patterns and diversity at different depths, undoubtedly reflecting multi-fault rupture complexity associated with the earthquake. The Coulomb stress can reach several MPa during coseismic deformation, which can explain the trigger mechanisms of afterslip in two high-slip regions and the majority of aftershocks. Based on the deformation characteristics of the Kaikoura earthquake, interseismic plate coverage, and historical earthquakes, we conclude that Wellington is under higher seismic threat after the earthquake and great attention should be paid to potential large earthquake disasters in the near future.[Figure not available: see fulltext.

Along-strike variations in fault frictional properties along the San Andreas Fault near Cholame, California from joint earthquake and low-frequency earthquake relocations

USGS Publications Warehouse

Harrington, Rebecca M.; Cochran, Elizabeth S.; Griffiths, Emily M.; Zeng, Xiangfang; Thurber, Clifford H.

2016-01-01

Recent observations of low‐frequency earthquakes (LFEs) and tectonic tremor along the Parkfield–Cholame segment of the San Andreas fault suggest slow‐slip earthquakes occur in a transition zone between the shallow fault, which accommodates slip by a combination of aseismic creep and earthquakes (<15  km depth), and the deep fault, which accommodates slip by stable sliding (>35  km depth). However, the spatial relationship between shallow earthquakes and LFEs remains unclear. Here, we present precise relocations of 34 earthquakes and 34 LFEs recorded during a temporary deployment of 13 broadband seismic stations from May 2010 to July 2011. We use the temporary array waveform data, along with data from permanent seismic stations and a new high‐resolution 3D velocity model, to illuminate the fine‐scale details of the seismicity distribution near Cholame and the relation to the distribution of LFEs. The depth of the boundary between earthquakes and LFE hypocenters changes along strike and roughly follows the 350°C isotherm, suggesting frictional behavior may be, in part, thermally controlled. We observe no overlap in the depth of earthquakes and LFEs, with an ∼5  km separation between the deepest earthquakes and shallowest LFEs. In addition, clustering in the relocated seismicity near the 2004 Mw 6.0 Parkfield earthquake hypocenter and near the northern boundary of the 1857 Mw 7.8 Fort Tejon rupture may highlight areas of frictional heterogeneities on the fault where earthquakes tend to nucleate.

Late Quaternary Normal Faulting and Hanging Wall Basin Evolution of the Southwestern Rift Margin from Gravity and Geology, B.C.S., MX and Exploring the Influence of Text-Figure Format on Introductory Geology Learning

ERIC Educational Resources Information Center

Busch, Melanie M. D.

2011-01-01

An array of north-striking, left-stepping, active normal faults is situated along the southwestern margin of the Gulf of California. This normal fault system is the marginal fault system of the oblique-divergent plate boundary within the Gulf of California. To better understand the role of upper-crustal processes during development of an obliquely…

Tectonic implications of the 2017 Ayvacık (Çanakkale) earthquakes, Biga Peninsula, NW Turkey

NASA Astrophysics Data System (ADS)

Özden, Süha; Över, Semir; Poyraz, Selda Altuncu; Güneş, Yavuz; Pınar, Ali

2018-04-01

The west to southwestward motion of the Anatolian block results from the relative motions between the Eurasian, Arabian and African plates along the right-lateral North Anatolian Fault Zone in the north and left-lateral East Anatolian Fault Zone in the east. The Biga Peninsula is tectonically influenced by the Anatolian motion originating along the North Anatolian Fault Zone which splits into two main (northern and southern) branches in the east of Marmara region: the southern branch extends towards the Biga Peninsula which is characterized by strike-slip to oblique normal faulting stress regime in the central to northern part. The southernmost part of peninsula is characterized by a normal to oblique faulting stress regime. The analysis of both seismological and structural field data confirms the change of stress regime from strike-slip character in the center and north to normal faulting character in the south of peninsula where the earthquake swarm recently occurred. The earthquakes began on 14 January 2017 (Mw: 4.4) on Tuzla Fault and migrated southward along the Kocaköy and Babakale's stepped-normal faults of over three months. The inversion of focal mechanisms yields a normal faulting stress regime with an approximately N-S (N4°E) σ3 axis. The inversion of earthquakes occurring in central and northern Biga Peninsula and the north Aegean region gives a strike-slip stress regime with approximately WNW-ESE (N85°W) σ1 and NNE-SSW (N17°E) σ3 axis. The strike-slip stress regime is attributed to westward Anatolian motion, while the normal faulting stress regime is attributed to both the extrusion of Anatolian block and the slab-pull force of the subducting African plate along the Hellenic arc.

Marine forearc extension in the Hikurangi Margin: New insights from high-resolution 3D seismic data

NASA Astrophysics Data System (ADS)

Böttner, Christoph; Gross, Felix; Geersen, Jacob; Mountjoy, Joshu; Crutchley, Gareth; Krastel, Sebastian

2017-04-01

In subduction zones upper-plate normal faults have long been considered a tectonic feature primarily associated with erosive margins. However, increasing data coverage has proven that similar features also occur in accretionary margins, such as Cascadia, Makran, Nankai or Central Chile, where kinematics are dominated by compression. Considering their wide distribution there is, without doubt, a significant lack of qualitative and quantitative knowledge regarding the role and importance of normal faults and zones of extension for the seismotectonic evolution of accretionary margins. We use a high-resolution 3D P-Cable seismic volume from the Hikurangi Margin acquired in 2014 to analyze the spatial distribution and mechanisms of upper-plate normal faulting. The study area is located at the upper continental slope in the area of the Tuaheni landslide complex. In detail we aim to (1) map the spatial distribution of normal faults and characterize their vertical throws, strike directions, and dip angles; (2) investigate their possible influence on fluid migration in an area, where gas hydrates are present; (3) discuss the mechanisms that may cause extension of the upper-slope in the study area. Beneath the Tuaheni Landslide Complex we mapped about 200 normal faults. All faults have low displacements (<15 m) and dip at high (> 65°) angles. About 71% of the faults dip landward. We found two main strike directions, with the majority of faults striking 350-10°, parallel to the deformation front. A second group of faults strikes 40-60°. The faults crosscut the BSR, which indicates the base of the gas hydrate zone. In combination with seismically imaged bright-spots and pull-up structures, this indicates that the normal faults effectively transport fluids vertically across the base of the gas hydrate zone. Localized uplift, as indicated by the presence of the Tuaheni Ridge, might support normal faulting in the study area. In addition, different subduction rates across the margin may also favor extension between the segments. Future work will help to further untangle the mechanisms that cause extension of the upper continental slope.

Retardations in fault creep rates before local moderate earthquakes along the San Andreas fault system, central California

USGS Publications Warehouse

Burford, R.O.

1988-01-01

Records of shallow aseismic slip (fault creep) obtained along parts of the San Andreas and Calaveras faults in central California demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes. An immediate postearthquake increase followed by gradual, long-term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep. This phenomenon, identified as aseismic afterslip, usually is characterized by above-average creep rates for several months to a few years. In several cases, minor step-like movements, called coseismic slip events, have occurred at or near the times of mainshocks. One extreme case of coseismic slip, recorded at Cienega Winery on the San Andreas fault 17.5 km southeast of San Juan Bautista, consisted of 11 mm of sudden displacement coincident with earthquakes of ML=5.3 and ML=5.2 that occurred 2.5 minutes apart on 9 April 1961. At least one of these shocks originated on the main fault beneath the winery. Creep activity subsequently stopped at the winery for 19 months, then gradually returned to a nearly steady rate slightly below the previous long-term average. The phenomena mentioned above can be explained in terms of simple models consisting of relatively weak material along shallow reaches of the fault responding to changes in load imposed by sudden slip within the underlying seismogenic zone. In addition to coseismic slip and afterslip phenomena, however, pre-earthquake retardations in creep rates also have been observed. Onsets of significant, persistent decreases in creep rates have occurred at several sites 12 months or more before the times of moderate earthquakes. A 44-month retardation before the 1979 ML=5.9 Coyote Lake earthquake on the Calaveras fault was recorded at the Shore Road creepmeter site 10 km northwest of Hollister. Creep retardation on the San Andreas fault near San Juan Bautista has been evident in records from one creepmeter site for the past 5 years. Retardations with durations of 21 and 19 months also occurred at Shore Road before the 1974 and 1984 earthquakes of ML=5.2 and ML=6.2, respectively. Although creep retardation remains poorly understood, several possible explanations have been discussed previously. (1) Certain onsets of apparent creep retardation may be explained as abrupt terminations of afterslip generated from previous moderate-mainshock sequences. (2) Retardations may be related to significant decreases in the rate of seismic and/or aseismic slip occurring within or beneath the underlying seismogenic zone. Such decreases may be caused by changes in local conditions related to growth of asperities, strain hardening, or dilatancy, or perhaps by passage of stress-waves or other fluctuations in driving stresses. (3) Finally, creep rates may be lowered (or increased) by stresses imposed on the fault by seismic or aseismic slip on neighboring faults. In addition to causing creep-rate increases or retardations, such fault interactions occasionally may trigger earthquakes. Regardless of the actual mechanisms involved and the current lack of understanding of creep retardation, it appears that shallow fault creep is sensitive to local and regional effects that promote or accompany intermediate-term preparation stages leading to moderate earthquakes. A strategy for more complete monitoring of fault creep, wherever it is known to occur, therefore should be assigned a higher priority in our continuing efforts to test various hypotheses concerning the mechanical relations between seismic and aseismic slip. ?? 1988 Birkha??user Verlag.

Retardations in fault creep rates before local moderate earthquakes along the San Andreas fault system, central California

NASA Astrophysics Data System (ADS)

Burford, Robert O.

1988-06-01

Records of shallow aseismic slip (fault creep) obtained along parts of the San Andreas and Calaveras faults in central California demonstrate that significant changes in creep rates often have been associated with local moderate earthquakes. An immediate postearthquake increase followed by gradual, long-term decay back to a previous background rate is generally the most obvious earthquake effect on fault creep. This phenomenon, identified as aseismic afterslip, usually is characterized by above-average creep rates for several months to a few years. In several cases, minor step-like movements, called coseismic slip events, have occurred at or near the times of mainshocks. One extreme case of coseismic slip, recorded at Cienega Winery on the San Andreas fault 17.5 km southeast of San Juan Bautista, consisted of 11 mm of sudden displacement coincident with earthquakes of M L =5.3 and M L =5.2 that occurred 2.5 minutes apart on 9 April 1961. At least one of these shocks originated on the main fault beneath the winery. Creep activity subsequently stopped at the winery for 19 months, then gradually returned to a nearly steady rate slightly below the previous long-term average. The phenomena mentioned above can be explained in terms of simple models consisting of relatively weak material along shallow reaches of the fault responding to changes in load imposed by sudden slip within the underlying seismogenic zone. In addition to coseismic slip and afterslip phenomena, however, pre-earthquake retardations in creep rates also have been observed. Onsets of significant, persistent decreases in creep rates have occurred at several sites 12 months or more before the times of moderate earthquakes. A 44-month retardation before the 1979 M L =5.9 Coyote Lake earthquake on the Calaveras fault was recorded at the Shore Road creepmeter site 10 km northwest of Hollister. Creep retardation on the San Andreas fault near San Juan Bautista has been evident in records from one creepmeter site for the past 5 years. Retardations with durations of 21 and 19 months also occurred at Shore Road before the 1974 and 1984 earthquakes of M L =5.2 and M L =6.2, respectively. Although creep retardation remains poorly understood, several possible explanations have been discussed previously. (1) Certain onsets of apparent creep retardation may be explained as abrupt terminations of afterslip generated from previous moderate-mainshock sequences. (2) Retardations may be related to significant decreases in the rate of seismic and/or aseismic slip occurring within or beneath the underlying seismogenic zone. Such decreases may be caused by changes in local conditions related to growth of asperities, strain hardening, or dilatancy, or perhaps by passage of stress-waves or other fluctuations in driving stresses. (3) Finally, creep rates may be lowered (or increased) by stresses imposed on the fault by seismic or aseismic slip on neighboring faults. In addition to causing creep-rate increases or retardations, such fault interactions occasionally may trigger earthquakes. Regardless of the actual mechanisms involved and the current lack of understanding of creep retardation, it appears that shallow fault creep is sensitive to local and regional effects that promote or accompany intermediate-term preparation stages leading to moderate earthquakes. A strategy for more complete monitoring of fault creep, wherever it is known to occur, therefore should be assigned a higher priority in our continuing efforts to test various hypotheses concerning the mechanical relations between seismic and aseismic slip.

Drainage Asperities on Subduction Megathrusts

NASA Astrophysics Data System (ADS)

Sibson, R. H.

2012-12-01

Geophysical observations coupled with force-balance analyses suggest that the seismogenic shear zone interface of subduction megathrusts is generally fluid-overpressured to near-lithostatic values (λv = Pf/σv > 0.9) below the forearc hanging-wall, strongly modulating the profile of frictional shear resistance. Fluid sources include the accretionary prism at shallow levels and, with increasing depth, metamorphic dehydration of material entrained within the subduction shear zone together with progressive metamorphism of oceanic crust in the downgoing slab. Solution transfer in fine-grained material contained within the deeper subduction shear zone (150 < T < 350°C) likely contributes to hydrothermal sealing of fractures. A dramatic difference may therefore exist between low prefailure permeability surrounding the megathrust and high postfailure fracture permeability along the rupture zone and adjacent areas of aftershock activity. Observed postseismic changes in the velocity structure of the fore-arc hanging-wall led Husen and Kissling (2001) to propose massive fluid loss across the subduction interface following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture. Such trans-megathrust discharges represent a variant of 'fault-valve' action in which the subduction interface itself acts as a seal trapping overpressured fluids derived from metamorphic dehydration beneath. In low-permeability assemblages the maximum sustainable overpressure is limited by the activation or reactivation of brittle faults and fractures under the prevailing stress state. Highest overpressures tend to occur at low differential stress in compressional stress regimes. Loci for fluid discharge are likely determined by stress heterogeneities along the megathrust (e.g. the hangingwall of the rupture at its downdip termination). Discharge sites may be defined by swarm aftershocks defining activated fault-fracture meshes. However, fluid loss across a subduction interface will be enhanced when the stress-state in the forearc hanging-wall switches from compressional reverse-slip faulting before failure to extensional normal-slip faulting postfailure, as occurred during the 2011 Mw9.0 Tohoku megathrust rupture. Mean stress and fault-normal stress then change from being greater than vertical stress prefailure, to less than vertical stress postfailure. Postfailure reductions in overpressure are expected from a combination of poroelastic effects and fluid loss through fault-fracture networks, enhancing vertical permeability. Mineralised fault-fracture meshes in exhumed fore-arc assemblages (e.g. the Alaska-Juneau Au-quartz vein swarm) testify to the episodic discharge of substantial volumes of hydrothermal fluid (< tens of km3). Localized drainage from the subduction interface shear zone increases frictional strength significantly, giving rise to a postfailure strength asperities. Anticipated strength increases from such fluid discharge depends on the magnitude of the drop in overpressure but are potentially large (< hundreds of MPa). Time to the subsequent failure is then governed by reaccumulation of fluid overpressure as well as shear stress along the subduction interface.

The influence of a reverse-reactivated normal fault on natural fracture geometries and relative chronologies at Castle Cove, Otway Basin

NASA Astrophysics Data System (ADS)

Debenham, Natalie; King, Rosalind C.; Holford, Simon P.

2018-07-01

Despite the ubiquity of normal faults that have undergone compressional inversion, documentation of the structural history of natural fractures around these structures is limited. In this paper, we investigate the geometries and relative chronologies of natural fractures adjacent to a reverse-reactivated normal fault, the Castle Cove Fault in the Otway Basin, southeast Australia. Local variations in strain resulted in greater deformation within the fault damage zone closer to the fault. Structural mapping within the damage zone reveals a complex tectonic history recording both regional and local perturbations in stress and a total of 11 fracture sets were identified, with three sets geometrically related to the Castle Cove Fault. The remaining fracture sets formed in response to local stresses at Castle Cove. Rifting in the late Cretaceous resulted in normal movement of the Castle Cove Fault and associated rollover folding, and the formation of the largest fracture set. Reverse-reactivation of the fault and associated anticlinal folding occurred during late Miocene to Pliocene compression. Rollover folding may have provided structural traps if seals were not breached by fractures, however anticlinal folding likely post-dated the main episodes of hydrocarbon generation and migration in the region. This study highlights the need to conduct careful reconstruction of the structural histories of fault zones that experienced complex reactivation histories when attempting to define off-fault fluid flow properties.

Late Paleogene rifting along the Malay Peninsula thickened crust

NASA Astrophysics Data System (ADS)

Sautter, Benjamin; Pubellier, Manuel; Jousselin, Pierre; Dattilo, Paolo; Kerdraon, Yannick; Choong, Chee Meng; Menier, David

2017-07-01

Sedimentary basins often develop above internal zones of former orogenic belts. We hereafter consider the Malay Peninsula (Western Sunda) as a crustal high separating two regions of stretched continental crust; the Andaman/Malacca basins in the western side and the Thai/Malay basins in the east. Several stages of rifting have been documented thanks to extensive geophysical exploration. However, little is known on the correlation between offshore rifted basins and the onshore continental core. In this paper, we explore through mapping and seismic data, how these structures reactivate pre-existing Mesozoic basement heterogeneities. The continental core appears to be relatively undeformed after the Triassic Indosinian orogeny. The thick crustal mega-horst is bounded by complex shear zones (Ranong, Klong Marui and Main Range Batholith Fault Zones) initiated during the Late Cretaceous/Early Paleogene during a thick-skin transpressional deformation and later reactivated in the Late Paleogene. The extension is localized on the sides of this crustal backbone along a strip where earlier Late Cretaceous deformation is well expressed. To the west, the continental shelf is underlain by three major crustal steps which correspond to wide crustal-scale tilted blocks bounded by deep rooted counter regional normal faults (Mergui Basin). To the east, some pronounced rift systems are also present, with large tilted blocks (Western Thai, Songkhla and Chumphon basins) which may reflect large crustal boudins. In the central domain, the extension is limited to isolated narrow N-S half grabens developed on a thick continental crust, controlled by shallow rooted normal faults, which develop often at the contact between granitoids and the host-rocks. The outer limits of the areas affected by the crustal boudinage mark the boundary towards the large and deeper Andaman basin in the west and the Malay and Pattani basins in the east. At a regional scale, the rifted basins resemble N-S en-echelon structures along large NW-SE shear bands. The rifting is accommodated by large low angle normal faults (LANF) running along crustal morphostructures such as broad folds and Mesozoic batholiths. The deep Andaman, Malay and Pattani basins seem to sit on weaker crust inherited from Gondwana-derived continental blocks (Burma, Sibumasu, and Indochina). The set of narrow elongated basins in the core of the Region (Khien Sa, Krabi, and Malacca basins) suffered from a relatively lesser extension.

Tectonic Evolution of the Terceira Rift (Azores)

NASA Astrophysics Data System (ADS)

Stratmann, Sjard; Huebscher, Christian; Terrinha, Pedro; Ornelas Marques, Fernando; Weiß, Benedik

2017-04-01

The Azores Plateau is located in the Central Atlantic at the Eurasian, Nubian and North-American plates (RRT) Azores Triple Junction. The Terceira Rift (TR) connects the Mid-Atlantic Ridge with the Gloria Fault, hence establishing a transtensional-transform present day plate boundary between the Eurasian and the Nubian plates. Three volcanic islands arose along the TR, Graciosa, Terceira and Sao Miguel. In the geological past, the plate boundary in the Azores area between the Eurasian and Nubian plates was located further south at the East Azores Fracture Zone. The timing of the plate boundary jump, which marks the onset of rifting along the TR, is heavily disputed. Published ages vary from 36 to 1 Ma. Based on bathymetric data and high-resolution marine 2D multi-channel seismic data acquired during M113 cruise of R/V Meteor in 2014/2015 we discuss the structural evolution of the TR and address the question whether the divergence between both plates is entirely accommodated by the TR. The central TR between São Miguel and Terceira, also known as Hirondelle Basin, is up to 70 km wide. Rifting created two asymmetric graben sections separated by a rift parallel horst. The north-eastern and south-western graben sections are ca. 4 km and 3 km deep, respectively, and the corresponding graben floors are tilted towards the central horst. Volcanic cones emerged on the central horst and rift shoulders. Bright spots in the basin fill deposits indicate fluid flow out of the volcanic basement. The seafloor is displaced by faults which suggest recent fault displacement. In the Eastern Graciosa Basin between Terceira and Graciosa Islands the rift narrows to ca. 40 km and shallows to ca. 3200 m water depth. The central horst is no longer detectable. Instead, a buried normal fault and a small escarpment are observed. Shallow faults and block rotation are less pronounced compared to the basins to the south-east and north-west. The Western Graciosa Basin is about 30 km wide and ca. 3050 m deep. The floor of the wider and deeper north-eastern rift valley dips to the northeast. The southwestern basin is represented by tilted fault blocks. The relatively undisturbed rift valley between Terceira and Graciosa (Eastern Graciosa Basin) is consistent with a rather low earthquake activity compared to the other TR segments. We therefore conclude that the TR west of Terceira does not accommodate the entire Nubia-Eurasia plate motion. In fact, we assume that tectonic stress is also dissipated in a seismically active area south of the TR where the lineaments of Pico and São Jorge Island are located. Consequently, the new seismic data support the assumption of a diffuse plate boundary in the western half of the TR. Estimating the age of the TR on the basis of fault geometry and present day extension rates supports all those previous studies which suggested a TR age of 1-3 Ma.

Structural Controls of the Tuscarora Geothermal Field, Elko County, Nevada

NASA Astrophysics Data System (ADS)

Dering, Gregory M.

Detailed geologic mapping, structural analysis, and well data have been integrated to elucidate the stratigraphic framework and structural setting of the Tuscarora geothermal area. Tuscarora is an amagmatic geothermal system that lies in the northern part of the Basin and Range province, ˜15 km southeast of the Snake River Plain and ˜90 km northwest of Elko, Nevada. The Tuscarora area is dominated by late Eocene to middle Miocene volcanic and sedimentary rocks, all overlying Paleozoic metasedimentary rocks. A geothermal power plant was constructed in 2011 and currently produces 18 MWe from an ˜170°C reservoir in metasedimentary rocks at a depth of 1740 m. Analysis of drill core reveals that the subsurface geology is dominated to depths of ˜700-1000 m by intracaldera deposits of the Eocene Big Cottonwood Canyon caldera, including blocks of basement-derived megabreccia. Furthermore, the Tertiary-Paleozoic nonconformity within the geothermal field has been recognized as the margin of this Eocene caldera. Structural relations combined with geochronologic data from previous studies indicate that Tuscarora has undergone extension since the late Eocene, with significant extension in the late Miocene-Pliocene to early Pleistocene. Kinematic analysis of fault slip data reveal an east-west-trending least principal paleostress direction, which probably reflects an earlier episode of Miocene extension. Two distinct structural settings at different scales appear to control the geothermal field. The regional structural setting is a 10-km wide complexly faulted left step or relay ramp in the west-dipping range-bounding Independence-Bull Run Mountains normal fault system. Geothermal activity occurs within the step-over where sets of east- and west-dipping normal faults overlap in a northerly trending accommodation zone. The distribution of hot wells and hydrothermal surface features, including boiling springs, fumaroles, and siliceous sinter, indicate that the geothermal system is restricted to the narrow (< 1 km) axial part of the accommodation zone, where permeability is maintained at depth around complex fault intersections. Shallow up-flow appears to be focused along several closely spaced steeply west-dipping north-northeast-striking normal faults within the axial part of the accommodation zone. These faults are favorably oriented for extension and fluid flow under the present-day northwest-trending regional extension direction indicated by previous studies of GPS geodetic data, earthquake focal mechanisms, and kinematic data from late Quaternary faults. The recognition of the axial part of an accommodation zone as a favorable structural setting for geothermal activity may be a useful exploration tool for development of drilling targets in extensional terranes, as well as for developing geologic models of known geothermal fields. Preliminary analysis of broad step-overs similar to Tuscarora reveals that geothermal activity occurs in a variety of subsidiary structural settings within these regions. In addition, the presence of several high-temperature systems in northeastern Nevada demonstrates the viability of electrical-grade geothermal activity in this region despite low present-day strain rates as indicated by GPS geodetic data. Geothermal exploration potential in northeastern Nevada may therefore be higher than previously recognized.