Surface Rupture Map of the 2002 M7.9 Denali Fault Earthquake, Alaska: Digital Data

USGS Publications Warehouse

Haeussler, Peter J.

2009-01-01

The November 3, 2002, Mw7.9 Denali Fault earthquake produced about 340 km of surface rupture along the Susitna Glacier Thrust Fault and the right-lateral, strike-slip Denali and Totschunda Faults. Digital photogrammetric methods were primarily used to create a 1:500-scale, three-dimensional surface rupture map, and 1:6,000-scale aerial photographs were used for three-dimensional digitization in ESRI's ArcMap GIS software, using Leica's StereoAnalyst plug in. Points were digitized 4.3 m apart, on average, for the entire surface rupture. Earthquake-induced landslides, sackungen, and unruptured Holocene fault scarps on the eastern Denali Fault were also digitized where they lay within the limits of air photo coverage. This digital three-dimensional fault-trace map is superior to traditional maps in terms of relative and absolute accuracy, completeness, and detail and is used as a basis for three-dimensional visualization. Field work complements the air photo observations in locations of dense vegetation, on bedrock, or in areas where the surface trace is weakly developed. Seventeen km of the fault trace, which broke through glacier ice, were not digitized in detail due to time constraints, and air photos missed another 10 km of fault rupture through the upper Black Rapids Glacier, so that was not mapped in detail either.

Surface Rupture Characteristics and Rupture Mechanics of the Yushu Earthquake (Ms7.1), 14/04/2010

NASA Astrophysics Data System (ADS)

Pan, J.; Li, H.; Xu, Z.; Li, N.; Wu, F.; Guo, R.; Zhang, W.

2010-12-01

On April 14th 2010, a disastrous earthquake (Ms 7.1) struck Yushu County, Qinghai Province, China, killing thousands of people. This earthquake occurred as a result of sinistral strike-slip faulting on the western segment of the Xianshuihe Fault zone in eastern Tibetan Plateau. Our group conducted scientific investigation in the field on co-seismic surface rupture and active tectonics in the epicenter area immediately after the earthquake. Here, we introduce our preliminary results on the surface ruptures and rupture mechanics of the Yushu Earthquake. The surface rupture zone of Yushu earthquake, which is about 49 km-long, consists of 3 discontinuous left stepping rupture segments, which are 19 km, 22 km, and about 8 km, respectively, from west to east. Each segment consists of a series of right stepping en-echelon branch ruptures. The branch ruptures consist of interphase push-up and tension fissures or simply en-echelon tension fissures. The co-seismic displacements had been surveyed with a total station in detail on landmarks such as rivers, gullies, roads, farmlands, wire poles, and fences. The maximum offset measured is 2.3m, located near the Guoyangyansongduo Village. There are 3 offset peaks along the rupture zone corresponding to the 3 segments of the surface rupture zone. The maximum offsets in the west, central, and east segment rupture zones are 1.4m, 2.3m, and 1.6m respectively. The surface rupture zone of Yushu earthquake strikes in a 310°NW direction. The fault plane dips to the northeast and the dip angle is about 81°. The rupture zone is developed in transtension setting. Tension normal fault developed during the sinistral strike-slip process of the fault. The valley west of Yushu City and the Longbao Lake are both pull-apart basins formed during the transtension activity of the fault.

Fine-scale delineation of the location of and relative ground shaking within the San Andreas Fault zone at San Andreas Lake, San Mateo County, California

USGS Publications Warehouse

Catchings, R.D.; Rymer, M.J.; Goldman, M.R.; Prentice, C.S.; Sickler, R.R.

2013-01-01

The San Francisco Public Utilities Commission is seismically retrofitting the water delivery system at San Andreas Lake, San Mateo County, California, where the reservoir intake system crosses the San Andreas Fault (SAF). The near-surface fault location and geometry are important considerations in the retrofit effort. Because the SAF trends through highly distorted Franciscan mélange and beneath much of the reservoir, the exact trace of the 1906 surface rupture is difficult to determine from surface mapping at San Andreas Lake. Based on surface mapping, it also is unclear if there are additional fault splays that extend northeast or southwest of the main surface rupture. To better understand the fault structure at San Andreas Lake, the U.S. Geological Survey acquired a series of seismic imaging profiles across the SAF at San Andreas Lake in 2008, 2009, and 2011, when the lake level was near historical lows and the surface traces of the SAF were exposed for the first time in decades. We used multiple seismic methods to locate the main 1906 rupture zone and fault splays within about 100 meters northeast of the main rupture zone. Our seismic observations are internally consistent, and our seismic indicators of faulting generally correlate with fault locations inferred from surface mapping. We also tested the accuracy of our seismic methods by comparing our seismically located faults with surface ruptures mapped by Schussler (1906) immediately after the April 18, 1906 San Francisco earthquake of approximate magnitude 7.9; our seismically determined fault locations were highly accurate. Near the reservoir intake facility at San Andreas Lake, our seismic data indicate the main 1906 surface rupture zone consists of at least three near-surface fault traces. Movement on multiple fault traces can have appreciable engineering significance because, unlike movement on a single strike-slip fault trace, differential movement on multiple fault traces may exert compressive and extensional stresses on built structures within the fault zone. Such differential movement and resulting distortion of built structures appear to have occurred between fault traces at the gatewell near the southern end of San Andreas Lake during the 1906 San Francisco earthquake (Schussler, 1906). In addition to the three fault traces within the main 1906 surface rupture zone, our data indicate at least one additional fault trace (or zone) about 80 meters northeast of the main 1906 surface rupture zone. Because ground shaking also can damage structures, we used fault-zone guided waves to investigate ground shaking within the fault zones relative to ground shaking outside the fault zones. Peak ground velocity (PGV) measurements from our guided-wave study indicate that ground shaking is greater at each of the surface fault traces, varying with the frequency of the seismic data and the wave type (P versus S). S-wave PGV increases by as much as 5–6 times at the fault traces relative to areas outside the fault zone, and P-wave PGV increases by as much as 3–10 times. Assuming shaking increases linearly with increasing earthquake magnitude, these data suggest strong shaking may pose a significant hazard to built structures that extend across the fault traces. Similarly complex fault structures likely underlie other strike-slip faults (such as the Hayward, Calaveras, and Silver Creek Faults) that intersect structures of the water delivery system, and these fault structures similarly should be investigated.

Crystal plastic earthquakes in dolostones

NASA Astrophysics Data System (ADS)

Passelegue, Francois; Aubry, Jerome; Nicolas, Aurelien; Fondriest, Michele; Schubnel, Alexandre; Di Toro, Giulio

2017-04-01

Dolostone is the most dominant lithology of the seismogenic upper crust around the Mediterranean Sea. Understanding the internal mechanisms controlling fault friction is crucial for understanding seismicity along active faults. Displacement in such fault zones is frequently highlighted by highly reflective (mirror-like) slip surfaces, created by thin films of nanogranular fault rock. Using saw-cut dolostone samples coming from natural fault zones, we conducted friction experiments under triaxial loading conditions. To reproduce the natural conditions, experiments were conducted at 30, 60 and 90 MPa confining pressure at respectively 30, 65 and 100 degrees C. At 30 and 65 degrees C, only slow rupture was observed and the experimental fault exhibits frictional behaviour, i.e. a dependence of normal stress on peak shear stress. At 65 degrees C, a strengthening behaviour is observed after the main rupture, leading to a succession of slow rupture. At 100 degrees C, the macroscopic behaviour of the fault becomes ductile, and no dependence of pressure on the peak shear stress is observed. In addition, the increase of the confining pressure up to 60 and 90 MPa allow the transition from slow to fast rupture, highlighted by the records of acoustic activity and by dynamic stress drop occurring in a few tens of microseconds. Using strain gages located along the fault surface and acoustic transducers, we were able to measure the rupture velocities during slow and fast rupture. Slow ruptures propagated around 0.1 m/s, in agreement with natural observations. Fast ruptures propagated up the supershear velocities, i.e. faster than the shear wave speed (>3500 m/s). A complete study of the microstructures was realized before and after ruptures. Slow ruptures lead to the production of mirror-like surface driven by the production of nanograins due to dislocation processes. Fast ruptures induce the production of amorphous material along the fault surface, which may come from melting processes. We demonstrate that the transition from slow to dynamic instabilities is observed when the entire fault exhibits plastic processes, which increase the stiffness of the fault.

Analytic Study of Three-Dimensional Rupture Propagation in Strike-Slip Faulting with Analogue Models

NASA Astrophysics Data System (ADS)

Chan, Pei-Chen; Chu, Sheng-Shin; Lin, Ming-Lang

2014-05-01

Strike-slip faults are high angle (or nearly vertical) fractures where the blocks have moved along strike way (nearly horizontal). Overburden soil profiles across main faults of Strike-slip faults have revealed the palm and tulip structure characteristics. McCalpin (2005) has trace rupture propagation on overburden soil surface. In this study, we used different offset of slip sandbox model profiles to study the evolution of three-dimensional rupture propagation by strike -slip faulting. In strike-slip faults model, type of rupture propagation and width of shear zone (W) are primary affecting by depth of overburden layer (H), distances of fault slip (Sy). There are few research to trace of three-dimensional rupture behavior and propagation. Therefore, in this simplified sandbox model, investigate rupture propagation and shear zone with profiles across main faults when formation are affecting by depth of overburden layer and distances of fault slip. The investigators at the model included width of shear zone, length of rupture (L), angle of rupture (θ) and space of rupture. The surface results was follow the literature that the evolution sequence of failure envelope was R-faults, P-faults and Y-faults which are parallel to the basement fault. Comparison surface and profiles structure which were curved faces and cross each other to define 3-D rupture and width of shear zone. We found that an increase in fault slip could result in a greater width of shear zone, and proposed a W/H versus Sy/H relationship. Deformation of shear zone showed a similar trend as in the literature that the increase of fault slip resulted in the increase of W, however, the increasing trend became opposite after a peak (when Sy/H was 1) value of W was reached (small than 1.5). The results showed that the W width is limited at a constant value in 3-D models by strike-slip faulting. In conclusion, this study helps evaluate the extensions of the shear zone influenced regions for strike-slip faults.

Low footwall accelerations and variable surface rupture behavior on the Fort Sage Mountains fault, northeast California

USGS Publications Warehouse

Briggs, Richard W.; Wesnousky, Steven G.; Brune, James N.; Purvance, Matthew D.; Mahan, Shannon

2013-01-01

The Fort Sage Mountains fault zone is a normal fault in the Walker Lane of the western Basin and Range that produced a small surface rupture (L 5.6 earthquake in 1950. We investigate the paleoseismic history of the Fort Sage fault and find evidence for two paleoearthquakes with surface displacements much larger than those observed in 1950. Rupture of the Fort Sage fault ∼5.6  ka resulted in surface displacements of at least 0.8–1.5 m, implying earthquake moment magnitudes (Mw) of 6.7–7.1. An older rupture at ∼20.5  ka displaced the ground at least 1.5 m, implying an earthquake of Mw 6.8–7.1. A field of precariously balanced rocks (PBRs) is located less than 1 km from the surface‐rupture trace of this Holocene‐active normal fault. Ground‐motion prediction equations (GMPEs) predict peak ground accelerations (PGAs) of 0.2–0.3g for the 1950 rupture and 0.3–0.5g for the ∼5.6  ka paleoearthquake one kilometer from the fault‐surface trace, yet field tests indicate that the Fort Sage PBRs will be toppled by PGAs between 0.1–0.3g. We discuss the paleoseismic history of the Fort Sage fault in the context of the nearby PBRs, GMPEs, and probabilistic seismic hazard maps for extensional regimes. If the Fort Sage PBRs are older than the mid‐Holocene rupture on the Fort Sage fault zone, this implies that current GMPEs may overestimate near‐fault footwall ground motions at this site.

High-resolution electrical resistivity and aeromagnetic imaging reveal the causative fault of the 2009 Mw 6.0 Karonga, Malawi earthquake

NASA Astrophysics Data System (ADS)

Kolawole, F.; Atekwana, E. A.; Laó-Dávila, D. A.; Abdelsalam, M. G.; Chindandali, P. R.; Salima, J.; Kalindekafe, L.

2018-05-01

Seismic events of varying magnitudes have been associated with ruptures along unknown or incompletely mapped buried faults. The 2009 Mw 6.0 Karonga, Malawi earthquake caused a surface rupture length of 14-18 km along a single W-dipping fault [St. Mary Fault (SMF)] on the hanging wall of the North Basin of the Malawi Rift. Prior to this earthquake, there was no known surface expression or knowledge of the presence of this fault. Although the earthquake damage zone is characterized by surface ruptures and coseismic liquefaction-induced sand blows, the origin of the causative fault and the near-surface structure of the rupture zone are not known. We used high-resolution aeromagnetic and electrical resistivity data to elucidate the relationship between surface rupture locations and buried basement structures. We also acquired electrical resistivity tomography (ERT) profiles along and across the surface rupture zone to image the near-surface structure of the damaged zone. We applied mathematical derivative filters to the aeromagnetic data to enhance basement structures underlying the rupture zone and surrounding areas. Although several magnetic lineaments are visible in the basement, mapped surface ruptures align with a single 37 km long, 148°-162°—striking magnetic lineament, and is interpreted as the ruptured normal fault. Inverted ERT profiles reveal three regional geoelectric layers which consist of 15 m thick layer of discontinuous zones of high and low resistivity values, underlain by a 27 m thick zone of high electrical resistivity (up to 100 Ω m) and a basal layer of lower resistivity (1.0-6.0 Ω m) extending from 42 m depth downwards (the maximum achieved depth of investigation). The geoelectric layers are truncated by a zone of electrical disturbance (electrical mélange) coinciding with areas of coseismic surface rupturing and sediment liquefaction along the ruptured. Our study shows that the 2009 Karonga earthquake was associated with the partial rupture of the buried SMF, and illuminates other potential seismogenic buried faults within the Karonga area of the North Basin. Although our electrical surveys were conducted 6 yr after the 2009 Karonga earthquake, we observe that near-surface lenses of electrically conductive sediments imaged by our ERT profiles, coincide with zones of coseismic surface rupture and liquefaction sand blows. We suggest that the presence of these preserved near-surface lenses of potentially water-saturated sand pose potential hazard in the event of a future earthquake in the area. In addition, our ERT profiles reveal structures that could represent relics of previous earthquake events along the SMF. In addition, our study demonstrates that the integration of ERT and aeromagnetic data can be very useful in illuminating seismogenic buried faults, thereby significantly improving seismic hazard analysis in tectonically active areas.

Controls on Patterns of Repeated Fault Rupture: Examples From the Denali and Bear River Faults

NASA Astrophysics Data System (ADS)

Schwartz, D. P.; Hecker, S.

2013-12-01

A requirement for estimating seismic hazards is assigning magnitudes to earthquake sources. This relies on anticipating rupture length and slip along faults. Fundamental questions include whether lengths of past surface ruptures can be reasonably determined from fault zone characteristics and whether the variability in length and slip during repeated faulting can be constrained. To address these issues, we look at rupture characteristics and their possible controls from examples in very different tectonic settings: the high slip rate (≥15 mm/yr) Denali fault system, Alaska, and the recently activated Bear River normal fault, Wyoming-Utah. The 2002 rupture of the central Denali fault (CDF) is associated with two noteworthy geometric features. First, rupture initiated where the Susitna Glacier thrust fault (SG) intersects the CDF at depth, near the apex of a structurally complex restraining bend along the Denali. Paleoseismic data show that for the past 700 years the timing of large surface ruptures on the Denali fault west of the 2002 rupture has been distinct from those along the CDF. For the past ~6ka the frequency of SG to Denali ruptures has been ~1:12, indicating that this complexity of the 2002 rupture has not been common. Second, rupture propagated off of one strike-slip fault (CDF) onto another (the Totschunda fault, TF), an occurrence that seldom has been observed. LiDAR mapping of the intersection shows direct connectivity of the two faults--the CDF simply branches into both the TF and the eastern Denali fault (EDF). Differences in the timing of earthquakes during the past 700-800 years at sites surrounding this intersection, and estimates of accumulated slip from slip rates, indicate that for the 2002 rupture sufficient strain had accumulated on the TF to favor its failure. In contrast, the penultimate CDF rupture, with the same slip distribution as in 2002, appears to have stopped at or near the branch point, implying that neither the TF nor the EDF was stressed sufficiently to fail at that time. The Bear River fault zone (BRFZ) is a young normal fault along the eastern margin of basin-range extension that appears to have reactivated a ramp in the Laramide-age Darby-Hogsback thrust. The entire Cenozoic history of the BRFZ may consist of only two surface-rupturing events in the late Holocene (one at ~5 ka and the most recent at ~2.5 ka). The 40-km-long fault comprises synthetic and antithetic scarps extending across a zone up to 5 km wide. Remote sensing, including airborne LiDAR, and field studies show that, despite the complexity, the pattern of faulting was similar (in location and amount) for each of the two events and, at the south end, was strongly influenced by the east-west-trending Uinta Arch. Pre-existing structure clearly has exerted a first-order control on moment release on this immature fault. As shown by these examples, data on timing of surface ruptures, coseismic slip, slip rate, and fault geometry can provide a basis to constrain lengths of past and future earthquake ruptures, including possible alternative rupture scenarios. The difficult question for hazard analysis is whether the available data capture the full range of behavior and with what relative frequency do the alternatives occur?

Crystal plastic earthquakes in dolostones: from slow to fast ruptures.

NASA Astrophysics Data System (ADS)

Passelegue, F. X.; Aubry, J.; Nicolas, A.; Fondriest, M.; Schubnel, A.; Di Toro, G.

2017-12-01

Dolostone is the most dominant lithology of the seismogenic upper crust around the Mediterranean Sea. Understanding the internal mechanisms controlling fault friction is crucial for understanding seismicity along active faults. Displacement in such fault zones is frequently highlighted by highly reflective (mirror-like) slip surfaces, created by thin films of nanogranular fault rock. Using saw-cut dolostone samples coming from natural fault zones, we conducted stick-slip experiments under triaxial loading conditions at 30, 60 and 90 MPa confining pressure and temperature ranging from 30 to 100 degrees C. At 30 and 65 degrees C, only slow rupture was observed and the experimental fault exhibits frictional behaviour, i.e. a dependence of normal stress on peak shear stress. At 65 degrees C, a strengthening behaviour is observed after the main rupture, leading to a succession of slow rupture. At 100 degrees C, the macroscopic behaviour of the fault becomes ductile, and no dependence of pressure on the peak shear stress is observed. In addition, the increase of the confining pressure up to 60 and 90 MPa allow the transition from slow to fast rupture, highlighted by the records of acoustic activity and by dynamic stress drop occurring in a few tens of microseconds. Using strain gages located along the fault surface and acoustic transducers, we were able to measure the rupture velocities during slow and fast rupture. Slow ruptures propagated around 0.1 m/s, in agreement with natural observations. Fast ruptures propagated up to supershear velocities, i.e. faster than the shear wave speed (>3500 m/s). A complete study of the microstructures was realized before and after ruptures. Slow ruptures lead to the production of mirror-like surface driven by the production of nanograins due to dislocation processes. Fast ruptures induce the production of amorphous material along the fault surface, which may come from decarbonation and melting processes. We demonstrate that the transition from slow to fast instabilities is observed due to an increase of the fault stiffness with increasing both temperature and confining pressure. This increase in the stiffness leads to an increase of the slip velocity during the main instability, which allow flash weakening processes and fast propagation of the seismic rupture.

Earthquake rupture process recreated from a natural fault surface

USGS Publications Warehouse

Parsons, Thomas E.; Minasian, Diane L.

2015-01-01

What exactly happens on the rupture surface as an earthquake nucleates, spreads, and stops? We cannot observe this directly, and models depend on assumptions about physical conditions and geometry at depth. We thus measure a natural fault surface and use its 3D coordinates to construct a replica at 0.1 m resolution to obviate geometry uncertainty. We can recreate stick-slip behavior on the resulting finite element model that depends solely on observed fault geometry. We clamp the fault together and apply steady state tectonic stress until seismic slip initiates and terminates. Our recreated M~1 earthquake initiates at contact points where there are steep surface gradients because infinitesimal lateral displacements reduce clamping stress most efficiently there. Unclamping enables accelerating slip to spread across the surface, but the fault soon jams up because its uneven, anisotropic shape begins to juxtapose new high-relief sticking points. These contacts would ultimately need to be sheared off or strongly deformed before another similar earthquake could occur. Our model shows that an important role is played by fault-wall geometry, though we do not include effects of varying fluid pressure or exotic rheologies on the fault surfaces. We extrapolate our results to large fault systems using observed self-similarity properties, and suggest that larger ruptures might begin and end in a similar way, though the scale of geometrical variation in fault shape that can arrest a rupture necessarily scales with magnitude. In other words, fault segmentation may be a magnitude dependent phenomenon and could vary with each subsequent rupture.

Primary surface rupture associated with the Mw 7.1 16 October 1999 Hector Mine earthquake, San Bernardino County, California

USGS Publications Warehouse

Treiman, J.A.; Kendrick, K.J.; Bryant, W.A.; Rockwell, T.K.; McGill, S.F.

2002-01-01

The Mw 7.1 Hector Mine earthquake occurred within the Mojave Desert portion of the eastern California shear zone and was accompanied by 48 km of dextral surface rupture. Complex northward rupture began on two branches of the Lavic Lake fault in the northern Bullion Mountains and also propagated southward onto the Bullion fault. Lesser amounts of rupture occurred across two right steps to the south. Surface rupture was mapped using postearthquake, 1:10,000-scale aerial photography. Field mapping provided additional detail and more than 400 fault-rupture observations; of these, approximately 300 measurements were used to characterize the slip distribution. En echelon surface rupture predominated in areas of thick alluvium, whereas in the bedrock areas, rupture was more continuous and focused within a narrower zone. Measured dextral offsets were relatively symmetrical about the epicentral region, with a maximum displacement of 5.25 ?? 0.85 m. Vertical slip was a secondary component and was variable, with minor west-side-down displacements predominat.ing in the Bullion Mountains. Field and aerial photographic evidence indicates that most of the faults that ruptured in 1999 had had prior late-Quaternary displacement, although only limited sections of the rupture show evidence for prior Holocene displacement.

Tearing the terroir: Details and implications of surface rupture and deformation from the 24 August 2014 M6.0 South Napa earthquake, California

USGS Publications Warehouse

DeLong, Stephen B.; Donnellan, Andrea; Ponti, Daniel J.; Rubin, Ron S.; Lienkaemper, James J.; Prentice, Carol S.; Dawson, Timothy E.; Seitz, Gordon G.; Schwartz, David P.; Hudnut, Kenneth W.; Rosa, Carla M.; Pickering, Alexandra J; Parker, Jay W.

2016-01-01

The Mw 6.0 South Napa earthquake of 24 August 2014 caused slip on several active fault strands within the West Napa Fault Zone (WNFZ). Field mapping identified 12.5 km of surface rupture. These field observations, near-field geodesy and space geodesy, together provide evidence for more than ~30 km of surface deformation with a relatively complex distribution across a number of subparallel lineaments. Along a ~7 km section north of the epicenter, the surface rupture is confined to a single trace that cuts alluvial deposits, reoccupying a low-slope scarp. The rupture continued northward onto at least four other traces through subparallel ridges and valleys. Postseismic slip exceeded coseismic slip along much of the southern part of the main rupture trace with total slip 1 year postevent approaching 0.5 m at locations where only a few centimeters were measured the day of the earthquake. Analysis of airborne interferometric synthetic aperture radar data provides slip distributions along fault traces, indicates connectivity and extent of secondary traces, and confirms that postseismic slip only occurred on the main trace of the fault, perhaps indicating secondary structures ruptured as coseismic triggered slip. Previous mapping identified the WNFZ as a zone of distributed faulting, and this was generally borne out by the complex 2014 rupture pattern. Implications for hazard analysis in similar settings include the need to consider the possibility of complex surface rupture in areas of complex topography, especially where multiple potentially Quaternary-active fault strands can be mapped.

Evidence for surface rupture in 1868 on the Hayward Fault in North Oakland and major rupturing in prehistoric earthquakes

NASA Astrophysics Data System (ADS)

Lienkaemper, James J.; Williams, Patrick L.

1999-07-01

WGCEP90 estimated the Hayward fault to have a high probability (0.45 in 30 yr) of producing a future M7 Bay Area earthquake. This was based on a generic recurrence time and an unverified segmentation model, because there were few direct observations for the southern fault and none for the northern Hayward fault. To better constrain recurrence and segmentation of the northern Hayward fault, we trenched in north Oakland. Unexpectedly, we observed evidence of surface rupture probably from the M7 1868 earthquake. This extends the limit of that surface rupture 13 km north of the segmentation boundary used in the WGCEP90 model and forces serious re-evaluation of the current two-segment paradigm. Although we found that major prehistoric ruptures have occurred here, we could not radiocarbon date them. However, the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated AD 1640-1776.

Evidence for surface rupture in 1868 on the Hayward fault in north Oakland and major rupturing in prehistoric earthquakes

USGS Publications Warehouse

Lienkaemper, J.J.; Williams, P.L.

1999-01-01

WGCEP90 estimated the Hayward fault to have a high probability (0.45 in 30 yr) of producing a future M7 Bay Area earthquake. This was based on a generic recurrence time and an unverified segmentation model, because there were few direct observations for the southern fault and none for the northern Hayward fault. To better constrain recurrence and segmentation of the northern Hayward fault, we trenched in north Oakland. Unexpectedly, we observed evidence of surface rupture probably from the M7 1868 earthquake. This extends the limit of that surface rupture 13 km north of the segmentation boundary used in the WGCEP90 model and forces serious re-evaluation of the current two-segment paradigm. Although we found that major prehistoric ruptures have occurred here, we could not radiocarbon date them. However, the last major prehistoric event appears correlative with a recently recognized event 13 km to the north dated AD 1640-1776. Copyright 1999 by the American Geophysical Union.

Detailed Surface Rupture Geometry from the 2016 Amatrice Earthquake

NASA Astrophysics Data System (ADS)

Mildon, Z. K.; Iezzi, F.; Wedmore, L. N. J.; Gregory, L. C.; McCaffrey, K. J. W.; Wilkinson, M. W.; Faure Walker, J.; Roberts, G.; Livio, F.; Vittori, E.; Michetti, A.; Frigerio, C.; Ferrario, F.; Blumetti, A. M.; Guerrieri, L.; Di Manna, P.; Comerci, V.

2016-12-01

The Amatrice earthquake was generated by co-rupture of the Mt. Vettore and Laga faults at depth. Surface ruptures were observed for 5km along the Mt. Vettore fault, with no clear observations on the Laga fault reported to date. The surface rupture on Mt. Vettore manifests as a 15-20cm pale stripe at the base of a 60-80o dipping bedrock fault scarp and similar magnitude vertical offsets of colluvial deposits. We have measured the strike and dip of the fault alongside the coseismic throw, heave, and slip azimuth along the length of the rupture with high spatial resolution (c.2-6m, >2000 measurements). The slip azimuth is relatively constant between 210-270° even where the rupture faces uphill at its SE termination which is consistent with the regional NW-SE extension direction, defined by focal mechanisms and borehole break-out data. The simplest coseismic throw profile that would be expected is quasi-symmetric. However we found the highest values of throw (Inter Quartile Range 15-19.5cm) are skewed towards the NW end on a 1.7 km section of the fault that is oblique relative to the overall fault strike. In the centre of the rupture, orientated close to the overall fault strike, the throw is lower (IQR 7.5-13cm) and discontinuous along strike. We suggest that the skewed throw profile occurs because the strike, dip and throw must vary systematically in order to preserve the principal strain rate across a fault, in agreement with previous publications. The density of our measurements, crucially including the slip azimuth, allows us to resolve the regional debate over whether normal fault ruptures are primary tectonic features or landslides of hangingwall sediments. If the surface offsets are due to landslides, then the slip azimuth should correlate with the downslope direction of the hangingwall. We show using an available 10m DEM that this is not the case and hence the surface offsets described herein are a primary tectonic feature. This presentation offers new insights into rupture processes because of the high resolution of the dataset collected rapidly after the earthquake, but crucially because it includes the slip vector azimuth, allowing a full description of the kinematics of the faulting relative to the regional stress field and local topographic variations.

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.

Comparison of coseismic near-field and off-fault surface deformation patterns of the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes: Implications for controls on the distribution of surface strain

NASA Astrophysics Data System (ADS)

Milliner, C. W. D.; Dolan, J. F.; Hollingsworth, J.; Leprince, S.; Ayoub, F.

2016-10-01

Subpixel correlation of preevent and postevent air photos reveal the complete near-field, horizontal surface deformation patterns of the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine ruptures. Total surface displacement values for both earthquakes are systematically larger than "on-fault" displacements from geologic field surveys, indicating significant distributed, inelastic deformation occurred along these ruptures. Comparison of these two data sets shows that 46 ± 10% and 39 ± 22% of the total surface deformation were distributed over fault zones averaging 154 m and 121 m in width for the Landers and Hector Mine events, respectively. Spatial variations of distributed deformation along both ruptures show correlations with the type of near-surface lithology and degree of fault complexity; larger amounts of distributed shear occur where the rupture propagated through loose unconsolidated sediments and areas of more complex fault structure. These results have basic implications for geologic-geodetic rate comparisons and probabilistic seismic hazard analysis.

Precursory, Nucleation and Propagation of Ruptures Along Heterogeneously Loaded, Circular Experimental Faults

NASA Astrophysics Data System (ADS)

Reches, Z.; Zu, X.; Jeffers, J.

2017-12-01

We explored the evolution of dynamic rupture along a circular experimental fault composed of clear acrylic blocks. The ring-shaped fault surface has inner and outer diameters of 7.72 and 10.16 cm, respectively. An array of ten rossette strain-gauges is attached to the outer rim of one block that provide the 2D strain tensor in a plane normal to the fault. The 30 components of the gauges are monitored at 10^6 samples/second. One 3D miniature accelerometer is attached to the fault block. The initial asperities of the fault surface generated a non-uniform strain (=stress) distribution that was recorded, and indicated local deviations of ±30% from the mean stress. The mean normal stress was up to 3.5 MPa, the remotely applied velocity was up to .002 m/s, and the slip velocities during rupture were not measured. The rupture characteristics, namely propagation velocity and rupture front strain-field, were determined from strain-gauge outputs. The analysis of tens of stick-slip events revealed the following preliminary results: (1) The ruptures consistently nucleated at sites of high local strains (=stresses) that were formed by the pre-shear, normal stress loading. (2) The pre-rupture nucleation process was recognized a by temporal (< 0.1 s), local (<20 mm) reduction of the shear strain. (3) Commonly, the initiation of nucleation was associated with micro acoustic emissions, whereas the initiation of rupture was associated with intense acoustic activity. (4) Nucleation could occur quasi-simultaneously at two, highly stressed sites. (5) From the nucleation site, the ruptures propagated in two directions along the ring-shaped fault, and the collision between the two fronts led to rupture `shut-off'. (5) The strain-field of rupture fronts was well-recognized for ruptures propagating faster than 50 m/s, and the fastest fronts propagated at 1000 m/s. (7) It appears that the rupture front strain-field close to the nucleation site differs from the front strain-field far from nucleation site. (8) Post-shear examination of the fault surfaces revealed evidence of brittle wear of the acrylic including gouge formation, ploughing, and powder smearing. (9) Work in progress includes attempts to achieve faster dynamic ruptures, and the utilization of the existing monitoring system to rupture granite faults.

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

Inelastic off-fault response and three-dimensional dynamics of earthquake rupture on a strike-slip fault

USGS Publications Warehouse

Andrews, D.J.; Ma, Shuo

2010-01-01

Large dynamic stress off the fault incurs an inelastic response and energy loss, which contributes to the fracture energy, limiting the rupture and slip velocity. Using an explicit finite element method, we model three-dimensional dynamic ruptures on a vertical strike-slip fault in a homogeneous half-space. The material is subjected to a pressure-dependent Drucker-Prager yield criterion. Initial stresses in the medium increase linearly with depth. Our simulations show that the inelastic response is confined narrowly to the fault at depth. There the inelastic strain is induced by large dynamic stresses associated with the rupture front that overcome the effect of the high confining pressure. The inelastic zone increases in size as it nears the surface. For material with low cohesion (~5 MPa) the inelastic zone broadens dramatically near the surface, forming a "flowerlike" structure. The near-surface inelastic strain occurs in both the extensional and the compressional regimes of the fault, induced by seismic waves ahead of the rupture front under a low confining pressure. When cohesion is large (~10 MPa), the inelastic strain is significantly reduced near the surface and confined mostly to depth. Cohesion, however, affects the inelastic zone at depth less significantly. The induced shear microcracks show diverse orientations near the surface, owing to the low confining pressure, but exhibit mostly horizontal slip at depth. The inferred rupture-induced anisotropy at depth has the fast wave direction along the direction of the maximum compressive stress.

Mineralogical compositions of fault rocks from surface ruptures of Wenchuan earthquake and implication of mineral transformation during the seismic cycle along Yingxiu-Beichuan fault, Sichuan Province, China

NASA Astrophysics Data System (ADS)

Dang, Jiaxiang; Zhou, Yongsheng; He, Changrong; Ma, Shengli

2018-06-01

There are two co-seismic bedrock surface ruptures from the Mw 7.9 Wenchuan earthquake in the northern and central parts of the Beichuan-Yingxiu fault, Sichuan Province, southwest China. In this study, we report on the macrostructure of the fault rocks and results from X-ray powder diffraction analysis of minerals from rocks in the fault zone. The most recent fault gouge (the gouge produced by the most recent co-seismic fault movement) in all the studied outcrops is dark or grayish-black, totally unconsolidated and ultrafine-grained. Older fault gouges in the same outcrops are grayish or yellowish and weakly consolidated. X-ray powder diffraction analysis results show that mineral assemblages in both the old fault gouge and the new fault gouge are more complicated than the mineral assemblages in the bedrock as the fault gouge is rich in clay minerals. The fault gouge inherited its major rock-forming minerals from the parent rocks, but the clay minerals in the fault gouge were generated in the fault zone and are therefore authigenic and synkinematic. In profiles across the fault, clay mineral abundances increase as one traverses from the bedrock to the breccia to the old gouge and from the old gouge to the new gouge. Quartz and illite are found in all collected gouge samples. The dominant clay minerals in the new fault gouge are illite and smectite along the northern part of the surface rupture and illite/smectite mixed-layer clay in the middle part of the rupture. Illite/smectite mixed-layer clay found in the middle part of the rupture indicates that fault slip was accompanied by K-rich fluid circulation. The existence of siderite, anhydrite, and barite in the northern part of the rupture suggests that fault slip at this locality was accompanied by acidic fluids containing ions of Fe, Ca, and Ba.

Complementary Ruptures of Surface Ruptures and Deep Asperity during the 2014 Northern Nagano, Japan, Earthquake (MW 6.3)

NASA Astrophysics Data System (ADS)

Asano, K.; Iwata, T.; Kubo, H.

2015-12-01

A thrust earthquake of MW 6.3 occurred along the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) in the northern Nagano prefecture, central Japan, on November 22, 2014. This event was reported to be related to an active fault, the Kamishiro fault belonging to the ISTL (e.g., HERP, 2014). The surface rupture is observed along the Kamishiro fault (e.g., Lin et al., 2015; Okada et al., 2015). We estimated the kinematic source rupture process of this earthquake through the multiple time-window linear waveform inversion method (Hartzell and Heaton, 1983). We used velocity waveforms in 0.05-1 Hz from 12 strong motion stations of K-NET, KiK-net (NIED), JMA, and Nagano prefecture (SK-net, ERI). In order to enhance the reliability in Green's functions, we assumed one-dimensional velocity structure models different for the different stations, which were extracted from the nation-wide three-dimensional velocity structure model, Japan Integrated Velocity Structure Model (JIVSM, Koketsu et al., 2012). Considering the spatial distribution of aftershocks (Sakai et al., 2015) and surface ruptures, the assumed fault model consisted of two dip-bending fault segments with different dip angles between the northern and southern segments. The total length and width of the fault plane is 20 km and 13 km, relatively, and the fault model is divided into 260 subfaults of 1 km × 1 km in space and six smoothed ramp functions in time. An asperity or large slip area with a peak slip of 1.9 m was estimated in the lower plane of the northern segment in the approximate depth range of 4 to 8 km. The depth extent of this asperity is consistent with the seismogenic zone revealed by past studies (e.g., Panayotopoulos et al., 2014). In contrast, the slip in the southern segment is relatively concentrated in the shallow portion of the segment where the surface ruptures were found along the Kamishiro fault. The overall spatial rupture pattern of the source fault, in which the deep asperity was located on the northern segment and surface rupture was found on the southern segment, seems to be spatially consistent with the mapped active faults. These findings suggest characteristic and repeating features of fault ruptures along active faults where static offsets have accumulated over past events, and it would be a good constraint on earthquake scenarios along it.

Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement

USGS Publications Warehouse

Bonilla, M.G.; Mark, R.K.; Lienkaemper, J.J.

1984-01-01

In order to refine correlations of surface-wave magnitude, fault rupture length at the ground surface, and fault displacement at the surface by including the uncertainties in these variables, the existing data were critically reviewed and a new data base was compiled. Earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the Gutenberg methods and results, which necessarily make up much of the data base. Measurement errors were estimated for the three variables for 58 moderate to large shallow-focus earthquakes. Regression analyses were then made utilizing the estimated measurement errors. The regression analysis demonstrates that the relations among the variables magnitude, length, and displacement are stochastic in nature. The stochastic variance, introduced in part by incomplete surface expression of seismogenic faulting, variation in shear modulus, and regional factors, dominates the estimated measurement errors. Thus, it is appropriate to use ordinary least squares for the regression models, rather than regression models based upon an underlying deterministic relation with the variance resulting from measurement errors. Significant differences exist in correlations of certain combinations of length, displacement, and magnitude when events are qrouped by fault type or by region, including attenuation regions delineated by Evernden and others. Subdivision of the data results in too few data for some fault types and regions, and for these only regressions using all of the data as a group are reported. Estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating M with the logarithms of rupture length, fault displacement, or the product of length and displacement. Fault rupture area could be reliably estimated for about 20 of the events in the data set. Regression of MS on rupture area did not result in a marked improvement over regressions that did not involve rupture area. Because no subduction-zone earthquakes are included in this study, the reported results do not apply to such zones.

High-resolution seismic profiling reveals faulting associated with the 1934 Ms 6.6 Hansel Valley earthquake (Utah, USA)

USGS Publications Warehouse

Bruno, Pier Paolo G.; Duross, Christopher; Kokkalas, Sotirios

2017-01-01

The 1934 Ms 6.6 Hansel Valley, Utah, earthquake produced an 8-km-long by 3-km-wide zone of north-south−trending surface deformation in an extensional basin within the easternmost Basin and Range Province. Less than 0.5 m of purely vertical displacement was measured at the surface, although seismologic data suggest mostly strike-slip faulting at depth. Characterization of the origin and kinematics of faulting in the Hansel Valley earthquake is important to understand how complex fault ruptures accommodate regions of continental extension and transtension. Here, we address three questions: (1) How does the 1934 surface rupture compare with faults in the subsurface? (2) Are the 1934 fault scarps tectonic or secondary features? (3) Did the 1934 earthquake have components of both strike-slip and dip-slip motion? To address these questions, we acquired a 6.6-km-long, high-resolution seismic profile across Hansel Valley, including the 1934 ruptures. We observed numerous east- and west-dipping normal faults that dip 40°−70° and offset late Quaternary strata from within a few tens of meters of the surface down to a depth of ∼1 km. Spatial correspondence between the 1934 surface ruptures and subsurface faults suggests that ruptures associated with the earthquake are of tectonic origin. Our data clearly show complex basin faulting that is most consistent with transtensional tectonics. Although the kinematics of the 1934 earthquake remain underconstrained, we interpret the disagreement between surface (normal) and subsurface (strike-slip) kinematics as due to slip partitioning during fault propagation and to the effect of preexisting structural complexities. We infer that the 1934 earthquake occurred along an ∼3-km wide, off-fault damage zone characterized by distributed deformation along small-displacement faults that may be alternatively activated during different earthquake episodes.

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.

Structure, Frictional Melting and Fault Weakening during the 2008 Mw 7.9 Wenchuan Earthquake Slip: Observation from the WFSD Drilling Core Samples

NASA Astrophysics Data System (ADS)

Li, H.; Wang, H.; Li, C.; Zhang, J.; Sun, Z.; Si, J.; Liu, D.; Chevalier, M. L.; Han, L.; Yun, K.; Zheng, Y.

2015-12-01

The 2008 Mw7.9 Wenchuan earthquake produced two co-seismic surface ruptures along Yingxiu-Beichuan fault (~270 km) and the Guanxian-Anxian fault (~80 km) simultaneously in the Longmen Shan thrust belt. Besides, two surface rupture zones were tracked in the southern segment of the Yingxiu-Beichuan rupture zone, one along the Yingxiu fault, the other along the Shenxigou-Longchi fault, which both converged into one rupture zone at the Bajiaomiao village, Hongkou town, where one distinct fault plane with two striation orientations was exposed. The Wenchuan earthquake Fault Scientific Drilling project (WFSD) was carried out right after the earthquake to investigate its faulting mechanisms and rupture process. Six boreholes were drilled along the rupture zones with depths ranging from 600 to 2400 m. WFSD-1 and WFSD-2 are located at the Bajiaomiao area, the southern segment of the Yingxiu-Beichuan rupture zone, while WFSD-4 and WFSD-4S are in the Nanba town area, in the northern part of the rupture zone. Detailed research showed that ~1 mm thick Principal Slip Zone (PSZ) of the Wenchuan earthquake is located at ~589 m-depth in the WFSD-1 cores. Graphite present in the PSZ indicates a low fault strength. Long-term temperature monitoring shows an extremely low fault friction coefficient during the earthquake. Recently, another possible PSZ was found in WFSD-1 cores at ~732 m-depth, with a ~2 mm thick melt layer in the fault gouge, where feldspar was melted but quartz was not, indicating that the frictional melting temperature was 1230°C < T < 1720°C. These two PSZs at depth may correspond to the two co-seismic surface rupture zones. Besides, the Wenchuan earthquake PSZ was also recognized in the WFSD-4S cores, at ~1084 m-depth. About 200-400 μm thick melt layer (fault vein, mainly feldspar), as well as melt injection veins, were observed in the slip zone, where oblique distinct striations were visible on the slip surface. Therefore, there are two PSZs in the shallow crust at the southern segment along the Yingxiu-Beichuan fault, and another one along the northern segment. Melt and graphite in the PSZs indicate that the frictional melting and thermal pressurization are the main fault mechanisms during the Wenchuan earthquake. The melt and graphite can be considered as markers of large earthquakes.

Broadband Rupture Process of the 2001 Kunlun Fault (Mw 7.8) Earthquake

NASA Astrophysics Data System (ADS)

Antolik, M.; Abercrombie, R.; Ekstrom, G.

2003-04-01

We model the source process of the 14 November, 2001 Kunlun fault earthquake using broadband body waves from the Global Digital Seismographic Network (P, SH) and both point-source and distributed slip techniques. The point-source mechanism technique is a non-linear iterative inversion that solves for focal mechanism, moment rate function, depth, and rupture directivity. The P waves reveal a complex rupture process for the first 30 s, with smooth unilateral rupture toward the east along the Kunlun fault accounting for the remainder of the 120 s long rupture. The obtained focal mechanism for the main portion of the rupture is (strike=96o, dip=83o, rake=-8o) which is consistent with both the Harvard CMT solution and observations of the surface rupture. The seismic moment is 5.29×1020 Nm and the average rupture velocity is ˜3.5 km/s. However, the initial portion of the P waves cannot be fit at all with this mechanism. A strong pulse visible in the first 20 s can only be matched with an oblique-slip subevent (MW ˜ 6.8-7.0) involving a substantial normal faulting component, but the nodal planes of this mechanism are not well constrained. The first-motion polarities of the P waves clearly require a strike mechanism with a similar orientation as the Kunlun fault. Field observations of the surface rupture (Xu et al., SRL, 73, No. 6) reveal a small 26 km-long strike-slip rupture at the far western end (90.5o E) with a 45-km long gap and extensional step-over between this rupture and the main Kunlun fault rupture. We hypothesize that the initial fault break occurred on this segment, with release of the normal faulting energy as a continuous rupture through the extensional step, enabling transfer of the slip to the main Kunlun fault. This process is similar to that which occurred during the 2002 Denali fault (MW 7.9) earthquake sequence except that 11 days elapsed between the October 23 (M_W 6.7) foreshock and the initial break of the Denali earthquake along a thrust fault.

Geologic and structural controls on rupture zone fabric: A field-based study of the 2010 Mw 7.2 El Mayor–Cucapah earthquake surface rupture

USGS Publications Warehouse

Teran, Orlando; Fletcher, John L.; Oskin, Michael; Rockwell, Thomas; Hudnut, Kenneth W.; Spelz, Ronald; Akciz, Sinan; Hernandez-Flores, Ana Paula; Morelan, Alexander

2015-01-01

We systematically mapped (scales >1:500) the surface rupture of the 4 April 2010 Mw (moment magnitude) 7.2 El Mayor-Cucapah earthquake through the Sierra Cucapah (Baja California, northwestern Mexico) to understand how faults with similar structural and lithologic characteristics control rupture zone fabric, which is here defined by the thickness, distribution, and internal configuration of shearing in a rupture zone. Fault zone thickness and master fault dip are strongly correlated with many parameters of rupture zone fabric. Wider fault zones produce progressively wider rupture zones and both of these parameters increase systematically with decreasing dip of master faults, which varies from 20° to 90° in our dataset. Principal scarps that accommodate more than 90% of the total coseismic slip in a given transect are only observed in fault sections with narrow rupture zones (<25 m). As rupture zone thickness increases, the number of scarps in a given transect increases, and the scarp with the greatest relative amount of coseismic slip decreases. Rupture zones in previously undeformed alluvium become wider and have more complex arrangements of secondary fractures with oblique slip compared to those with pure normal dip-slip or pure strike-slip. Field relations and lidar (light detection and ranging) difference models show that as magnitude of coseismic slip increases from 0 to 60 cm, the links between kinematically distinct fracture sets increase systematically to the point of forming a throughgoing principal scarp. Our data indicate that secondary faults and penetrative off-fault strain continue to accommodate the oblique kinematics of coseismic slip after the formation of a thoroughgoing principal scarp. Among the widest rupture zones in the Sierra Cucapah are those developed above buried low angle faults due to the transfer of slip to widely distributed steeper faults, which are mechanically more favorably oriented. The results from this study show that the measureable parameters that define rupture zone fabric allow for testing hypotheses concerning the mechanics and propagation of earthquake ruptures, as well as for siting and designing facilities to be constructed in regions near active faults.

Effects of the 2016 Kumamoto earthquakes on the Aso volcanic edifice

NASA Astrophysics Data System (ADS)

Tajima, Yasuhisa; Hasenaka, Toshiaki; Torii, Masayuki

2017-05-01

Large earthquakes occurred in the central part of Kumamoto Prefecture on April 14-16, 2016, causing severe damage to the northern segment of the Hinagu faults and the eastern segment of the Futagawa faults. Earthquake surface ruptures appeared along these faults and on the Aso volcanic edifice, which in turn generated landslides. We conducted landform change analysis of the central cones of Aso volcano by using satellite and aerial photographs. First, we categorized the topographical changes as surface scarps, arc-shaped cracks, and linear cracks. Field survey indicated that landslides caused the scarps and arc-shaped cracks, whereas faulting caused the linear cracks. We discovered a surface rupture concentration zone (RCZ) formed three ruptures bands with many surface ruptures and landslides extending from the west foot to the center of the Aso volcanic edifice. The magmatic volcanic vents that formed during the past 10,000 years are located along the north margin of the RCZ. Moreover, the distribution and dip of the core of rupture concentration zone correspond with the Nakadake craters. We conclude that a strong relationship exists between the volcanic vents and fault structures in the central cones of Aso volcano.[Figure not available: see fulltext.

Source rupture process of the 2016 Kaikoura, New Zealand earthquake estimated from the kinematic waveform inversion of strong-motion data

NASA Astrophysics Data System (ADS)

Zheng, Ao; Wang, Mingfeng; Yu, Xiangwei; Zhang, Wenbo

2018-03-01

On 2016 November 13, an Mw 7.8 earthquake occurred in the northeast of the South Island of New Zealand near Kaikoura. The earthquake caused severe damages and great impacts on local nature and society. Referring to the tectonic environment and defined active faults, the field investigation and geodetic evidence reveal that at least 12 fault sections ruptured in the earthquake, and the focal mechanism is one of the most complicated in historical earthquakes. On account of the complexity of the source rupture, we propose a multisegment fault model based on the distribution of surface ruptures and active tectonics. We derive the source rupture process of the earthquake using the kinematic waveform inversion method with the multisegment fault model from strong-motion data of 21 stations (0.05-0.35 Hz). The inversion result suggests the rupture initiates in the epicentral area near the Humps fault, and then propagates northeastward along several faults, until the offshore Needles fault. The Mw 7.8 event is a mixture of right-lateral strike and reverse slip, and the maximum slip is approximately 19 m. The synthetic waveforms reproduce the characteristics of the observed ones well. In addition, we synthesize the coseismic offsets distribution of the ruptured region from the slips of upper subfaults in the fault model, which is roughly consistent with the surface breaks observed in the field survey.

Geologic Inheritance and Earthquake Rupture Processes: The 1905 M ≥ 8 Tsetserleg-Bulnay Strike-Slip Earthquake Sequence, Mongolia

NASA Astrophysics Data System (ADS)

Choi, Jin-Hyuck; Klinger, Yann; Ferry, Matthieu; Ritz, Jean-François; Kurtz, Robin; Rizza, Magali; Bollinger, Laurent; Davaasambuu, Battogtokh; Tsend-Ayush, Nyambayar; Demberel, Sodnomsambuu

2018-02-01

In 1905, 14 days apart, two M 8 continental strike-slip earthquakes, the Tsetserleg and Bulnay earthquakes, occurred on the Bulnay fault system, in Mongolia. Together, they ruptured four individual faults, with a total length of 676 km. Using submetric optical satellite images "Pleiades" with ground resolution of 0.5 m, complemented by field observation, we mapped in detail the entire surface rupture associated with this earthquake sequence. Surface rupture along the main Bulnay fault is 388 km in length, striking nearly E-W. The rupture is formed by a series of fault segments that are 29 km long on average, separated by geometric discontinuities. Although there is a difference of about 2 m in the average slip between the western and eastern parts of the Bulnay rupture, along-fault slip variations are overall limited, resulting in a smooth slip distribution, except for local slip deficit at segment boundaries. We show that damage, including short branches and secondary faulting, associated with the rupture propagation, occurred significantly more often along the western part of the Bulnay rupture, while the eastern part of the rupture appears more localized and thus possibly structurally simpler. Eventually, the difference of slip between the western and eastern parts of the rupture is attributed to this difference of rupture localization, associated at first order with a lateral change in the local geology. Damage associated to rupture branching appears to be located asymmetrically along the extensional side of the strike-slip rupture and shows a strong dependence on structural geologic inheritance.

The 2016 Kaikōura earthquake: Simultaneous rupture of the subduction interface and overlying faults

NASA Astrophysics Data System (ADS)

Wang, Teng; Wei, Shengji; Shi, Xuhua; Qiu, Qiang; Li, Linlin; Peng, Dongju; Weldon, Ray J.; Barbot, Sylvain

2018-01-01

The distribution of slip during an earthquake and how it propagates among faults in the subduction system play a major role in seismic and tsunami hazards, yet they are poorly understood because offshore observations are often lacking. Here we derive the slip distribution and rupture evolution during the 2016 Mw 7.9 Kaikōura (New Zealand) earthquake that reconcile the surface rupture, space geodetic measurements, seismological and tsunami waveform records. We use twelve fault segments, with eleven in the crust and one on the megathrust interface, to model the geodetic data and match the major features of the complex surface ruptures. Our modeling result indicates that a large portion of the moment is distributed on the subduction interface, making a significant contribution to the far field surface deformation and teleseismic body waves. The inclusion of local strong motion and teleseismic waveform data in the joint inversion reveals a unilateral rupture towards northeast with a relatively low averaged rupture speed of ∼1.5 km/s. The first 30 s of the rupture took place on the crustal faults with oblique slip motion and jumped between fault segments that have large differences in strike and dip. The peak moment release occurred at ∼65 s, corresponding to simultaneous rupture of both plate interface and the overlying splay faults with rake angle changes progressively from thrust to strike-slip. The slip on the Papatea fault produced more than 2 m of offshore uplift, making a major contribution to the tsunami at the Kaikōura station, while the northeastern end of the rupture can explain the main features at the Wellington station. Our inversions and simulations illuminate complex up-dip rupture behavior that should be taken into consideration in both seismic and tsunami hazard assessment. The extreme complex rupture behavior also brings new challenges to the earthquake dynamic simulations and understanding the physics of earthquakes.

Slip History of the 2008 Mw 7.9 Wenchuan Earthquake Constrained by Joint Inverting Seismic, Geodetic, and Geological Observations

NASA Astrophysics Data System (ADS)

Shao, G.; Ji, C.; Lu, Z.; Hudnut, K. W.; Liu, J.; Zhang, W.

2009-12-01

We study the kinematic rupture process of the 2008 Mw 7.9 Wenchuan earthquake using all geophysical and geological datasets that we are able to access, including the waveforms of teleseismic long period surface waves, broadband body waves and local strong motions, GPS vectors, interferometic radar (INSAR) images, and geological surface offsets. The relocated aftershock locations have also been included to constrain the potential fault geometry. These datasets have very different sensitivities to not only the slip on the fault but also the “a priori” information of the source inversions, such as the local velocity structure and the details of irregular fault surface. Effects have then been made to reconcile these datasets by reasonably perturbing the velocity structure and fault geometry, which are both poorly constrained. We have used two 1D velocity models, one for the Tibet plateau and the other for Sichuan basin, to calculate the static and dynamic earth responses; and developed a complex fault system including two irregular fault planes for Beichuan and Pengguan faults, respectively. The long wavelength errors of the INSAR LOS displacements have also been considered and been corrected simultaneously during the joint inversions. Our preferred model not only explains the geodetic and tele-seismic data very well, but also reasonably matches most strong motion waveforms. According to this result, the Wenchuan earthquake has an unprecedented complex rupture process. It initiated southwest of the town of Yingxiu at a depth of about 12 km, where the low-angle Pengguan fault and the high-angle Beichuan fault intersect. The rupture initiated on the low angle Pengguan fault and then later triggered the rupture on the high angle Beichuan fault. It then unilaterally ruptured northeastward for 270 km, mainly on the Beichuan fault. The entire rupture duration is over 95 seconds with an average rupture velocity of 3.0 km/s. Except for the region near the hypocenter and the region near the northeast end of the rupture, the majority of slip occurred at depths less than 12 km. The total seismic moment released by this earthquake was 1.02 x 1021 Nm, with ~36% on the Pengguan fault. Our analysis also indicates that the aftershock zone along the extension of the Xiaoyudong fault is consistent with the theory of static stress triggering due to the co-seismic rupture.

Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987

USGS Publications Warehouse

Sharp, R.V.

1989-01-01

The M6.2 Elmore Desert Ranch earthquake of 24 November 1987 was associated spatially and probably temporally with left-lateral surface rupture on many northeast-trending faults in and near the Superstition Hills in western Imperial Valley. Three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the Superstition Hills fault zone as far as 9km; the maximum observed surface slip, 12.5cm, was on the northern of the three, the Elmore Ranch fault, at a point near the epicenter. Twelve hours after the Elmore Ranch earthquake, the M6.6 Superstition Hills earthquake occurred near the northwest end of the right-lateral Superstition Hills fault zone. We measured displacements over 339 days at as many as 296 sites along the Superstition Hills fault zone, and repeated measurements at 49 sites provided sufficient data to fit with a simple power law. The overall distributions of right-lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture. The average estimated final right-lateral slip for the Superstition Hills fault zone is ~54cm. The average left-lateral slip for the conjugate faults trending northeastward is ~23cm. The southernmost ruptured member of the Superstition Hills fault zone, newly named the Wienert fault, extends the known length of the zone by about 4km. -from Authors

Statistical relations among earthquake magnitude, surface rupture length, and surface fault displacement

USGS Publications Warehouse

Bonilla, Manuel G.; Mark, Robert K.; Lienkaemper, James J.

1984-01-01

In order to refine correlations of surface-wave magnitude, fault rupture length at the ground surface, and fault displacement at the surface by including the uncertainties in these variables, the existing data were critically reviewed and a new data base was compiled. Earthquake magnitudes were redetermined as necessary to make them as consistent as possible with the Gutenberg methods and results, which make up much of the data base. Measurement errors were estimated for the three variables for 58 moderate to large shallow-focus earthquakes. Regression analyses were then made utilizing the estimated measurement errors.The regression analysis demonstrates that the relations among the variables magnitude, length, and displacement are stochastic in nature. The stochastic variance, introduced in part by incomplete surface expression of seismogenic faulting, variation in shear modulus, and regional factors, dominates the estimated measurement errors. Thus, it is appropriate to use ordinary least squares for the regression models, rather than regression models based upon an underlying deterministic relation in which the variance results primarily from measurement errors.Significant differences exist in correlations of certain combinations of length, displacement, and magnitude when events are grouped by fault type or by region, including attenuation regions delineated by Evernden and others.Estimates of the magnitude and the standard deviation of the magnitude of a prehistoric or future earthquake associated with a fault can be made by correlating Ms with the logarithms of rupture length, fault displacement, or the product of length and displacement.Fault rupture area could be reliably estimated for about 20 of the events in the data set. Regression of Ms on rupture area did not result in a marked improvement over regressions that did not involve rupture area. Because no subduction-zone earthquakes are included in this study, the reported results do not apply to such zones.

Width of the Surface Rupture Zone for Thrust Earthquakes and Implications for Earthquake Fault Zoning: Chi-Chi 1999 and Wenchuan 2008 Earthquakes

NASA Astrophysics Data System (ADS)

Boncio, P.; Caldarella, M.

2016-12-01

We analyze the zones of coseismic surface faulting along thrust faults, whit the aim of defining the most appropriate criteria for zoning the Surface Fault Rupture Hazard (SFRH) along thrust faults. Normal and strike-slip faults were deeply studied in the past, while thrust faults were not studied with comparable attention. We analyze the 1999 Chi-Chi, Taiwan (Mw 7.6) and 2008 Wenchuan, China (Mw 7.9) earthquakes. Several different types of coseismic fault scarps characterize the two earthquakes, depending on the topography, fault geometry and near-surface materials. For both the earthquakes, we collected from the literature, or measured in GIS-georeferenced published maps, data about the Width of the coseismic Rupture Zone (WRZ). The frequency distribution of WRZ compared to the trace of the main fault shows that the surface ruptures occur mainly on and near the main fault. Ruptures located away from the main fault occur mainly in the hanging wall. Where structural complexities are present (e.g., sharp bends, step-overs), WRZ is wider then for simple fault traces. We also fitted the distribution of the WRZ dataset with probability density functions, in order to define a criterion to remove outliers (e.g., by selecting 90% or 95% probability) and define the zone where the probability of SFRH is the highest. This might help in sizing the zones of SFRH during seismic microzonation (SM) mapping. In order to shape zones of SFRH, a very detailed earthquake geologic study of the fault is necessary. In the absence of such a very detailed study, during basic (First level) SM mapping, a width of 350-400 m seems to be recommended (95% of probability). If the fault is carefully mapped (higher level SM), one must consider that the highest SFRH is concentrated in a narrow zone, 50 m-wide, that should be considered as a "fault-avoidance (or setback) zone". These fault zones should be asymmetric. The ratio of footwall to hanging wall (FW:HW) calculated here ranges from 1:5 to 1:3.

The 2016-2017 central Italy coseismic surface ruptures and their meaning with respect to foreseen active fault systems segmentation

NASA Astrophysics Data System (ADS)

De Martini, P. M.; Pucci, S.; Villani, F.; Civico, R.; Del Rio, L.; Cinti, F. R.; Pantosti, D.

2017-12-01

In 2016-2017 a series of moderate to large normal faulting earthquakes struck central Italy producing severe damage in many towns including Amatrice, Norcia and Visso and resulting in 299 casualties and >20,000 homeless. The complex seismic sequence depicts a multiple activation of the Mt. Vettore-Mt. Bove (VBFS) and the Laga Mts. fault systems, which were considered in literature as independent segments characterizing a recent seismic gap in the region comprised between two modern seismic sequences: the 1997-1998 Colfiorito and the 2009 L'Aquila. We mapped in detail the coseismic surface ruptures following three mainshocks (Mw 6.0 on 24th August, Mw 5.9 and Mw 6.5 on 26th and 30th October, 2016, respectively). Primary surface ruptures were observed and recorded for a total length of 5.2 km, ≅10 km and ≅25 km, respectively, along closely-spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, in some cases rupturing repeatedly the same location. Some coseismic ruptures were mapped also along the Norcia Fault System, paralleling the VBFS about 10 km westward. We recorded geometric and kinematic characteristics of the normal faulting ruptures with an unprecedented detail thanks to almost 11,000 oblique photographs taken from helicopter flights soon after the mainshocks, verified and integrated with field data (more than 7000 measurements). We analyze the along-strike coseismic slip and slip vectors distribution to be observed in the context of the geomorphic expression of the disrupted slopes and their depositional and erosive processes. Moreover, we constructed 1:10.000 scale geologic cross-sections based on updated maps, and we reconstructed the net offset distribution of the activated fault system to be compared with the morphologic throws and to test a cause-effect relationship between faulting and first-order landforms. We provide a reconstruction of the 2016 coseismic rupture pattern as representative of the VBFS behavior, a discussion on the fault system boundaries persistence, and on the significance of the repeated surface faulting at same location.

Empirical Relationships Among Magnitude and Surface Rupture Characteristics of Strike-Slip Faults: Effect of Fault (System) Geometry and Observation Location, Dervided From Numerical Modeling

NASA Astrophysics Data System (ADS)

Zielke, O.; Arrowsmith, J.

2007-12-01

In order to determine the magnitude of pre-historic earthquakes, surface rupture length, average and maximum surface displacement are utilized, assuming that an earthquake of a specific size will cause surface features of correlated size. The well known Wells and Coppersmith (1994) paper and other studies defined empirical relationships between these and other parameters, based on historic events with independently known magnitude and rupture characteristics. However, these relationships show relatively large standard deviations and they are based only on a small number of events. To improve these first-order empirical relationships, the observation location relative to the rupture extent within the regional tectonic framework should be accounted for. This however cannot be done based on natural seismicity because of the limited size of datasets on large earthquakes. We have developed the numerical model FIMozFric, based on derivations by Okada (1992) to create synthetic seismic records for a given fault or fault system under the influence of either slip- or stress boundary conditions. Our model features A) the introduction of an upper and lower aseismic zone, B) a simple Coulomb friction law, C) bulk parameters simulating fault heterogeneity, and D) a fault interaction algorithm handling the large number of fault patches (typically 5,000-10,000). The joint implementation of these features produces well behaved synthetic seismic catalogs and realistic relationships among magnitude and surface rupture characteristics which are well within the error of the results by Wells and Coppersmith (1994). Furthermore, we use the synthetic seismic records to show that the relationships between magntiude and rupture characteristics are a function of the observation location within the regional tectonic framework. The model presented here can to provide paleoseismologists with a tool to improve magnitude estimates from surface rupture characteristics, by incorporating the regional and local structural context which can be determined in the field: Assuming a paleoseismologist measures the offset along a fault caused by an earthquake, our model can be used to determine the probability distribution of magnitudes which are capable of producing the observed offset, accounting for regional tectonic setting and observation location.

High Frequency Near-Field Ground Motion Excited by Strike-Slip Step Overs

NASA Astrophysics Data System (ADS)

Hu, Feng; Wen, Jian; Chen, Xiaofei

2018-03-01

We performed dynamic rupture simulations on step overs with 1-2 km step widths and present their corresponding horizontal peak ground velocity distributions in the near field within different frequency ranges. The rupture speeds on fault segments are determinant in controlling the near-field ground motion. A Mach wave impact area at the free surface, which can be inferred from the distribution of the ratio of the maximum fault-strike particle velocity to the maximum fault-normal particle velocity, is generated in the near field with sustained supershear ruptures on fault segments, and the Mach wave impact area cannot be detected with unsustained supershear ruptures alone. Sub-Rayleigh ruptures produce stronger ground motions beyond the end of fault segments. The existence of a low-velocity layer close to the free surface generates large amounts of high-frequency seismic radiation at step over discontinuities. For near-vertical step overs, normal stress perturbations on the primary fault caused by dipping structures affect the rupture speed transition, which further determines the distribution of the near-field ground motion. The presence of an extensional linking fault enhances the near-field ground motion in the extensional regime. This work helps us understand the characteristics of high-frequency seismic radiation in the vicinities of step overs and provides useful insights for interpreting the rupture speed distributions derived from the characteristics of near-field ground motion.

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.

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.

Late Quaternary Surface Displacement Across a Normal-Fault Structural Boundary on the Northern Lost River Fault Zone (Idaho, USA)

NASA Astrophysics Data System (ADS)

DuRoss, C. B.; Bunds, M. P.; Reitman, N. G.; Gold, R. D.; Personius, S. F.; Briggs, R. W.; Toke, N. A.; Johnson, K. L.; Lajoie, L. J.

2017-12-01

In 1983, about 36 km of the 130-km-long multisegment Lost River fault zone (LRFZ) (Idaho, USA) ruptured in the M 6.9 Borah Peak earthquake. Normal-faulting surface rupture propagated along the entire 24-km-long Thousand Springs section, then branched to the northwest along a 4-km-long fault (western section) that continues into the Willow Creek Hills, a prominent bedrock ridge that forms a structural boundary between the Thousand Springs section and Warms Springs section to the north. North of the Willow Creek Hills, the 1983 rupture continued onto the southern 8 km of the 16-km-long Warm Springs section. To improve our understanding of the Borah Peak earthquake and the role of structural boundaries in normal-fault rupture propagation, we acquired low-altitude aerial imagery of the southern 8 km of the Warm Springs section and northern 6 km of the Thousand Springs section, including the western section branch fault. Using 5-10-cm-pixel digital surface models generated from this dataset, we measured vertical surface offsets across both 1983 and prehistoric scarps. On the Warm Springs section, 1983 displacement is minor (mean of 0.3 m) compared to at least two prehistoric events having mean displacements of 1.1 m and 1.7 m inferred from displacement difference curves. Prehistoric scarps on the western section indicate rupture of this branch fault prior to 1983. Correcting for 1983 displacement, mean prehistoric displacement on the western section is 0.9 m compared to a mean of 0.7 m in 1983. Using these data and previous paleoseismic displacements, we evaluate the spatial distribution of cumulative and per-earthquake displacement. Our results suggest that at least one prehistoric rupture of the Thousand Springs section occurred with a similar length and displacement to that in 1983. Further, the 1983 spillover rupture from the Thousand Springs section to the southernmost Warm Springs section appears unique from larger displacement, prehistoric ruptures that may have spanned the majority of the Warm Springs section and possibly continued south into the Willow Creek Hills based on paleoseismic and surface-offset data. We conclude that the Willow Creek Hills structural boundary has likely moderated, but not completely impeded both prehistoric and 1983 ruptures of the northern LRFZ.

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.

High-resolution topography along surface rupture of the 16 October 1999 Hector Mine, California (Mw 7.1) from airborne laser swath mapping

USGS Publications Warehouse

Hudnutt, K.W.; Borsa, A.; Glennie, C.; Minster, J.-B.

2002-01-01

In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographic-mapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same flight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors.

Earthquake behavior of the Enriquillo fault zone, Haiti revealed by interactive terrain visualization

NASA Astrophysics Data System (ADS)

Cowgill, E.; Bernardin, T. S.; Oskin, M. E.; Bowles, C. J.; Yikilmaz, M. B.; Kreylos, O.; Elliott, A. J.; Bishop, M. S.; Gold, R. D.; Morelan, A.; Bawden, G. W.; Hamann, B.; Kellogg, L. H.

2010-12-01

The Mw 7.0 January 12, 2010 Haiti earthquake ended 240 years of relative quiescence following earthquakes that destroyed Port-au-Prince in 1751 and 1770. We place the 2010 rupture in the context of past earthquakes and future hazards by using remote analysis of airborne LiDAR to observe the topographic expression of active faulting and develop a new conceptual model for the earthquake behavior of the eastern Enriquillo fault zone (EFZ). In this model, the 2010 event occupies a long-lived segment boundary at a stepover within the EFZ separating fault segments that likely ruptured in 1751 and 1770, explaining both past clustering and the lack of 2010 surface rupture. Immediately following the 2010 earthquake, an airborne LiDAR point cloud containing over 2.7 billion point measurements of surface features was collected by the Rochester Inst. of Technology. To analyze these data, we capitalize on the human capacity to visually identify meaningful patterns embedded in noisy data by conducting interactive visual analysis of the entire 66.8 GB Haiti terrain data in a 4-sided, 800 ft3 immersive virtual-reality environment at the UC Davis KeckCAVES using the software tools LiDAR Viewer (to analyze point cloud data) and Crusta (for 3D surficial geologic mapping on DEM data). We discovered and measured landforms displaced by past surface-rupturing earthquakes and remotely characterized the regional fault geometry. Our analysis of the ~50 km long reach of EFZ spanning the 2010 epicenter indicates that geomorphic evidence of active faulting is clearer east of the epicenter than to the west. West of the epicenter, and in the region of the 2010 rupture, the fault is poorly defined along an embayed, low-relief range front, with little evidence of recent surface rupture. In contrast, landform offsets of 6 to 50 m along the reach of the EFZ east of the epicenter and closest to Port-au-Prince attest to repeated recent surface-rupturing earthquakes here. Specifically, we found and documented offset landforms including fluvial terrace risers near Dumay (6.3 +0.9/-1.3 m) and Chauffard/Jameau (32.2 +1.8/-3.1 m), a channel (52 +18/-13 m) ~500 m east of the Chauffard/Jameau site, and an alluvial fan near Fayette (8.6 +2.8/-2.5 m). Based on the fault-trace morphology and distribution of sites where we see 6-8 m offsets, we estimate the probable along-strike extent of past surface rupture was 40 to 60 km along this fault reach. Application of moment-rupture area relationships to these observations suggest that an earthquake similar to, or larger than the Mw 7.0 2010 event is possible along the Enriquillo fault near Port-au-Prince. We deduce that the 2010 earthquake was a relatively small event on a boundary between fault segments that ruptured in 1751 and 1770, based on new analysis of historical damage reports and the gap of well-defined fault-zone morphology where the 2010 earthquake occurred.

The history of late holocene surface-faulting earthquakes on the central segments of the Wasatch fault zone, Utah

USGS Publications Warehouse

Duross, Christopher; Personius, Stephen; Olig, Susan S; Crone, Anthony J.; Hylland, Michael D.; Lund, William R; Schwartz, David P.

2017-01-01

The Wasatch fault (WFZ)—Utah’s longest and most active normal fault—forms a prominent eastern boundary to the Basin and Range Province in northern Utah. To provide paleoseismic data for a Wasatch Front regional earthquake forecast, we synthesized paleoseismic data to define the timing and displacements of late Holocene surface-faulting earthquakes on the central five segments of the WFZ. Our analysis yields revised histories of large (M ~7) surface-faulting earthquakes on the segments, as well as estimates of earthquake recurrence and vertical slip rate. We constrain the timing of four to six earthquakes on each of the central segments, which together yields a history of at least 24 surface-faulting earthquakes since ~6 ka. Using earthquake data for each segment, inter-event recurrence intervals range from about 0.6 to 2.5 kyr, and have a mean of 1.2 kyr. Mean recurrence, based on closed seismic intervals, is ~1.1–1.3 kyr per segment, and when combined with mean vertical displacements per segment of 1.7–2.6 m, yield mean vertical slip rates of 1.3–2.0 mm/yr per segment. These data refine the late Holocene behavior of the central WFZ; however, a significant source of uncertainty is whether structural complexities that define the segments of the WFZ act as hard barriers to ruptures propagating along the fault. Thus, we evaluate fault rupture models including both single-segment and multi-segment ruptures, and define 3–17-km-wide spatial uncertainties in the segment boundaries. These alternative rupture models and segment-boundary zones honor the WFZ paleoseismic data, take into account the spatial and temporal limitations of paleoseismic data, and allow for complex ruptures such as partial-segment and spillover ruptures. Our data and analyses improve our understanding of the complexities in normal-faulting earthquake behavior and provide geological inputs for regional earthquake-probability and seismic hazard assessments.

Surface faulting along the inland Itozawa normal fault (eastern Japan) and relation to the 2011 Tohoku-oki megathrust earthquake

NASA Astrophysics Data System (ADS)

Ferry, Matthieu; Tsutsumi, Hiroyuki; Meghraoui, Mustapha; Toda, Shinji

2013-04-01

The 11 March 2011 Mw 9 Tohoku-oki earthquake ruptured ~500 km length of the Japan Trench along the coast of eastern Japan and significantly impacted the stress regime within the crust. The resulting change in seismicity over the Japan mainland was exhibited by the 11 April 2011 Mw 6.6 Iwaki earthquake that ruptured the Itozawa and Yunodake faults. Trending NNW and NW, respectively, these 70-80° W-dipping faults bound the Iwaki basin of Neogene age and have been reactivated simultaneously both along 15-km-long sections. Here, we present initial results from a paleoseismic excavation performed across the Itozawa fault within the Tsunagi Valley at the northern third of the observed surface rupture. At the Tsunagi site, the rupture affects a rice paddy, which provides an ideally horizontal initial state to collect detailed and accurate measurements. The surface break is composed of a continuous 30-to-40-cm-wide purely extensional crack that separates the uplifted block from a gently dipping 1-to-2-m-wide strip affected by right-stepping en-echelon cracks and locally bounded by a ~0.1-m-high reverse scarplet. Total station across-fault topographic profiles indicate the pre-earthquake ground surface was vertically deformed by ~0.6 m while direct field examinations reveal that well-defined rice paddy limits have been left-laterally offset by ~0.1 m. The 12-m-long, 3.5-m-deep trench exposes the 30-to-40-cm-thick cultivated soil overlaying a 1-m-thick red to yellow silt unit, a 2-m-thick alluvial gravel unit and a basal 0.1-1-m-thick organic-rich silt unit. Deformation associated to the 2011 rupture illustrates down-dip movement along a near-vertical fault with a well-expressed bending moment at the surface and generalized warping. On the north wall, the intermediate gravel unit displays a deformation pattern similar to granular flow with only minor discrete faulting and no splay to be continuously followed from the main fault to the surface. On the south wall, warping dominates as well but with some strain localization along two major splays that exhibit 15-20 cm of vertical offset. On both walls, the basal silt unit is vertically deformed by ~0.6 m, similarly to what is observed for the 2011 rupture. Furthermore, the base of said silt unit exhibits indication for secondary faulting prior to the 2011 event and that resemble cracks observed at the present-day surface. This suggests that the Itozawa fault has already ruptured in a similar fashion in the late Pleistocene). Hence, recent activity of the Itozawa fault may be controlled entirely by large to giant earthquakes along the Japan Trench.

Dynamics of delayed triggering in multi-segmented foreshock sequence: Evidence from the 2016 Kumamoto, Japan, earthquake

NASA Astrophysics Data System (ADS)

Arai, H.; Ando, R.; Aoki, Y.

2017-12-01

The 2016 Kumamoto earthquake sequence hit the SW Japan, from April 14th to 16th and its sequence includes two M6-class foreshocks and the main shock (Mw 7.0). Importantly, the detailed surface displacement caused solely by the two foreshocks could be captured by a SAR observation isolated from the mainshock deformation. The foreshocks ruptured the previously mapped Hinagu fault and their hypocentral locations and the aftershock distribution indicates the involvement of two different subparallel faults. Therefore we assumed that the 1st and the 2nd foreshocks respectively ruptured each of the subparallel faults (faults A and B). One of the interesting points of this earthquake is that the two major foreshocks had a temporal gap of 2.5 hours even though the fault A and B are quite close by each other. This suggests that the stress perturbation due to the 1st foreshock is not large enough to trigger the 2nd one right away but that it's large enough to bring about the following earthquake after a delay time.We aim to reproduce the foreshock sequence such as rupture jumping over the subparallel faults by using dynamic rupture simulations. We employed a spatiotemporal-boundary integral equation method accelerated by the Fast Domain Partitioning Method (Ando, 2016, GJI) since this method allows us to construct a complex fault geometry in 3D media. Our model has two faults and a free ground surface. We conducted rupture simulation with various sets of parameters to identify the optimal condition describing the observation.Our simulation results are roughly categorized into 3 cases with regard to the criticality for the rupture jumping. The case 1 (supercritical case) shows the fault A and B ruptured consecutively without any temporal gap. In the case 2 (nearly critical), the rupture on the fault B started with a temporal gap after the fault A finished rupturing, which is what we expected as a reproduction. In the case 3 (subcritical), only the fault A ruptured and its rupture did not transfer to the fault B. We succeed in reproducing rupture jumping over two faults with a temporal gap due to the nucleation by taking account of a velocity strengthening (direct) effect. With a detailed analysis of the case 2, we can constrain ranges of parameters strictly, and this gives us deeper insights into the physics underlying the delayed foreshock activity.

Transpressional Rupture Cascade of the 2016 Mw 7.8 Kaikoura Earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Xu, Wenbin; Feng, Guangcai; Meng, Lingsen; Zhang, Ailin; Ampuero, Jean Paul; Bürgmann, Roland; Fang, Lihua

2018-03-01

Large earthquakes often do not occur on a simple planar fault but involve rupture of multiple geometrically complex faults. The 2016 Mw 7.8 Kaikoura earthquake, New Zealand, involved the rupture of at least 21 faults, propagating from southwest to northeast for about 180 km. Here we combine space geodesy and seismology techniques to study subsurface fault geometry, slip distribution, and the kinematics of the rupture. Our finite-fault slip model indicates that the fault motion changes from predominantly right-lateral slip near the epicenter to transpressional slip in the northeast with a maximum coseismic surface displacement of about 10 m near the intersection between the Kekerengu and Papatea faults. Teleseismic back projection imaging shows that rupture speed was overall slow (1.4 km/s) but faster on individual fault segments (approximately 2 km/s) and that the conjugate, oblique-reverse, north striking faults released the largest high-frequency energy. We show that the linking Conway-Charwell faults aided in propagation of rupture across the step over from the Humps fault zone to the Hope fault. Fault slip cascaded along the Jordan Thrust, Kekerengu, and Needles faults, causing stress perturbations that activated two major conjugate faults, the Hundalee and Papatea faults. Our results shed important light on the study of earthquakes and seismic hazard evaluation in geometrically complex fault systems.

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.

How does damage affect rupture propagation across a fault stepover?

NASA Astrophysics Data System (ADS)

Cooke, M. L.; Savage, H. M.

2011-12-01

We investigate the potential for fault damage to influence earthquake rupture at fault step-overs using a mechanical numerical model that explicitly includes the generation of cracks around faults. We compare the off-fault fracture patterns and slip profiles generated along faults with a variety of frictional slip-weakening distances and step-over geometry. Models with greater damage facilitate the transfer of slip to the second fault. Increasing separation and decreasing the overlap distance reduces the transfer of slip across the step over. This is consistent with observations of rupture stopping at step-over separation greater than 4 km (Wesnousky, 2006). In cases of slip transfer, rupture is often passed to the second fault before the damage zone cracks of the first fault reach the second fault. This implies that stresses from the damage fracture tips are transmitted elastically to the second fault to trigger the onset of slip along the second fault. Consequently, the growth of damage facilitates transfer of rupture from one fault to another across the step-over. In addition, the rupture propagates along the damage-producing fault faster than along the rougher fault that does not produce damage. While this result seems counter to our understanding that damage slows rupture propagation, which is documented in our models with pre-existing damage, these model results are suggesting an additional process. The slip along the newly created damage may unclamp portions of the fault ahead of the rupture and promote faster rupture. We simulate the M7.1 Hector Mine Earthquake and compare the generated fracture patterns to maps of surface damage. Because along with the detailed damage pattern, we also know the stress drop during the earthquake, we may begin to constrain parameters like the slip-weakening distance along portions of the faults that ruptured in the Hector Mine earthquake.

Fault Branching and Long-Term Earthquake Rupture Scenario for Strike-Slip Earthquake

NASA Astrophysics Data System (ADS)

Klinger, Y.; CHOI, J. H.; Vallage, A.

2017-12-01

Careful examination of surface rupture for large continental strike-slip earthquakes reveals that for the majority of earthquakes, at least one major branch is involved in the rupture pattern. Often, branching might be either related to the location of the epicenter or located toward the end of the rupture, and possibly related to the stopping of the rupture. In this work, we examine large continental earthquakes that show significant branches at different scales and for which ground surface rupture has been mapped in great details. In each case, rupture conditions are described, including dynamic parameters, past earthquakes history, and regional stress orientation, to see if the dynamic stress field would a priori favor branching. In one case we show that rupture propagation and branching are directly impacted by preexisting geological structures. These structures serve as pathways for the rupture attempting to propagate out of its shear plane. At larger scale, we show that in some cases, rupturing a branch might be systematic, hampering possibilities for the development of a larger seismic rupture. Long-term geomorphology hints at the existence of a strong asperity in the zone where the rupture branched off the main fault. There, no evidence of throughgoing rupture could be seen along the main fault, while the branch is well connected to the main fault. This set of observations suggests that for specific configurations, some rupture scenarios involving systematic branching are more likely than others.

Surface rupture and slip distribution of the Denali and Totschunda faults in the 3 November 2002 M 7.9 earthquake, Alaska

USGS Publications Warehouse

Haeussler, Peter J.; Schwartz, David P.; Dawson, Timothy E.; Stenner, Heidi D.; Lienkaemper, James J.; Sherrod, Brian; Cinti, Francesca R.; Montone, Paola; Craw, Patricia; Crone, Anthony J.; Personius, Stephen F.

2004-01-01

The 3 November 2002 Denali fault, Alaska, earthquake resulted in 341 km of surface rupture on the Susitna Glacier, Denali, and Totschunda faults. The rupture proceeded from west to east and began with a 48-km-long break on the previously unknown Susitna Glacier thrust fault. Slip on this thrust averaged about 4 m (Crone et al., 2004). Next came the principal surface break, along 226 km of the Denali fault, with average right-lateral offsets of 4.5–5.1 m and a maximum offset of 8.8 m near its eastern end. The Denali fault trace is commonly left stepping and north side up. About 99 km of the fault ruptured through glacier ice, where the trace orientation was commonly influenced by local ice fabric. Finally, slip transferred southeastward onto the Totschunda fault and continued for another 66 km where dextral offsets average 1.6–1.8 m. The transition from the Denali fault to the Totschunda fault occurs over a complex 25-km-long transfer zone of right-slip and normal fault traces. Three methods of calculating average surface slip all yield a moment magnitude of Mw 7.8, in very good agreement with the seismologically determined magnitude of M 7.9. A comparison of strong-motion inversions for moment release with our slip distribution shows they have a similar pattern. The locations of the two largest pulses of moment release correlate with the locations of increasing steps in the average values of observed slip. This suggests that slip-distribution data can be used to infer moment release along other active fault traces.

Surface Rupture and Slip Distribution Resulting from the 2013 M7.7 Balochistan, Pakistan Earthquake

NASA Astrophysics Data System (ADS)

Reitman, N. G.; Gold, R. D.; Briggs, R. W.; Barnhart, W. D.; Hayes, G. P.

2014-12-01

The 24 September 2013 M7.7 earthquake in Balochistan, Pakistan, produced a ~200 km long left-lateral strike-slip surface rupture along a portion of the Hoshab fault, a moderately dipping (45-75º) structure in the Makran accretionary prism. The rupture is remarkably continuous and crosses only two (0.7 and 1.5 km wide) step-overs along its arcuate path through southern Pakistan. Displacements are dominantly strike-slip, with a minor component of reverse motion. We remotely mapped the surface rupture at 1:5,000 scale and measured displacements using high resolution (0.5 m) pre- and post-event satellite imagery. We mapped 295 laterally faulted stream channels, terrace margins, and roads to quantify near-field displacement proximal (±10 m) to the rupture trace. The maximum near-field left-lateral offset is 15±2 m (average of ~7 m). Additionally, we used pre-event imagery to digitize 254 unique landforms in the "medium-field" (~100-200 m from the rupture) and then measured their displacements compared to the post-event imagery. At this scale, maximum left-lateral offset approaches 17 m (average of ~8.5 m). The width (extent of observed surface faulting) of the rupture zone varies from ~1 m to 3.7 km. Near- and medium-field offsets show similar slip distributions that are inversely correlated with the width of the fault zone at the surface (larger offsets correspond to narrow fault zones). The medium-field offset is usually greater than the near-field offset. The along-strike surface slip distribution is highly variable, similar to the slip distributions documented for the 2002 Denali M7.9 earthquake and 2001 Kunlun M7.8 earthquake, although the Pakistan offsets are larger in magnitude. The 2013 Pakistan earthquake ranks among the largest documented continental strike-slip displacements, possibly second only to the 18+ m surface displacements attributed to the 1855 Wairarapa M~8.1 earthquake.

Pulsed strain release on the Altyn Tagh fault, northwest China

USGS Publications Warehouse

Gold, Ryan D.; Cowgill, Eric; Arrowsmith, J. Ramón; Friedrich, Anke M.

2017-01-01

Earthquake recurrence models assume that major surface-rupturing earthquakes are followed by periods of reduced rupture probability as stress rebuilds. Although purely periodic, time- or slip-predictable rupture models are known to be oversimplifications, a paucity of long records of fault slip clouds understanding of fault behavior and earthquake recurrence over multiple ruptures. Here, we report a 16 kyr history of fault slip—including a pulse of accelerated slip from 6.4 to 6.0 ka—determined using a Monte Carlo analysis of well-dated offset landforms along the central Altyn Tagh strike-slip fault (ATF) in northwest China. This pulse punctuates a median rate of 8.1+1.2/−0.9 mm/a and likely resulted from either a flurry of temporally clustered ∼Mw 7.5 ground-rupturing earthquakes or a single large >Mw 8.2 earthquake. The clustered earthquake scenario implies rapid re-rupture of a fault reach >195 km long and indicates decoupled rates of elastic strain energy accumulation versus dissipation, conceptualized as a crustal stress battery. If the pulse reflects a single event, slip-magnitude scaling implies that it ruptured much of the ATF with slip similar to, or exceeding, the largest documented historical ruptures. Both scenarios indicate fault rupture behavior that deviates from classic time- or slip-predictable models.

Coseismic surface displacements from optical and SAR image offset tracking, fault modeling and geomorphological analysis of the Sept. 24th, 2013 M7.7 Balochistan earthquake

NASA Astrophysics Data System (ADS)

Harrington, Jonathan; Wang, Teng; Feng, Guangcai; Akoglu, Ahmet; Jónsson, Sigurjón; Motagh, Mahdi

2014-05-01

The M 7.7 earthquake in the Balochistan province of Pakistan on September 24th, 2013 took place along a subsidiary fault in the transition area between the Makran accretionary prism and the Chaman transform fault. This tectonics of the Indian and Arabian plate collisions with Eurasia produce primarily oblique left-lateral strike slip in this region. In this work, measurements of displacement and mapping of the rupture trace are achieved through image correlation of Landsat 8 images and SAR offset tracking of TerraSAR-X data. Horizontal displacements from both methods and derived vertical displacements are used to constrain a fault rupture model for the earthquake. Preliminary results show a surprisingly uniform slip distribution with maximum displacement near the surface. The total fault rupture length is ~210 km, with up to 9 m of left-lateral strike-slip and 3 m of reverse faulting. Additionally, mapping of the rupture trace is made use of for geomorphological observations relating to slip rates and identification of transpressional and transtensional features. Our results indicate a mostly smooth rupture trace, with the presence of two restraining steps, a releasing bend and a 3 km long sliver where the surface rupture jumped from the foot of the range-front into the alluvial fans at their base. A small block at one of the restraining steps shows intermediate displacement in both data sets. At the southern end of the rupture we observe that displacement from the earthquake cuts across a fold-and-thrust belt of the Makran accretionary prism. Preliminary results show a minimum of 12 km of repeated section of the accretionary wedge, and within the southern repeated section we find an offset of 600 m between two parallel ridges across the rupture trace. We relate these observations to conceptual models of fault segmentation and growth.

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.

Mechanics of Multifault Earthquake Ruptures

NASA Astrophysics Data System (ADS)

Fletcher, J. M.; Oskin, M. E.; Teran, O.

2015-12-01

The 2010 El Mayor-Cucapah earthquake of magnitude Mw 7.2 produced the most complex rupture ever documented on the Pacific-North American plate margin, and the network of high- and low-angle faults activated in the event record systematic changes in kinematics with fault orientation. Individual faults have a broad and continuous spectrum of slip sense ranging from endmember dextral strike slip to normal slip, and even faults with thrust sense of dip slip were commonly observed in the aftershock sequence. Patterns of coseismic slip are consistent with three-dimensional constrictional strain and show that integrated transtensional shearing can be accommodated in a single earthquake. Stress inversions of coseismic surface rupture and aftershock focal mechanisms define two coaxial, but permuted stress states. The maximum (σ1) and intermediate (σ2) principal stresses are close in magnitude, but flip orientations due to topography- and density-controlled gradients in lithostatic load along the length of the rupture. Although most large earthquakes throughout the world activate slip on multiple faults, the mechanical conditions of their genesis remain poorly understood. Our work attempts to answer several key questions. 1) Why do complex fault systems exist? They must do something that simple, optimally-oriented fault systems cannot because the two types of faults are commonly located in close proximity. 2) How are faults with diverse orientations and slip senses prepared throughout the interseismic period to fail spontaneously together in a single earthquake? 3) Can a single stress state produce multi-fault failure? 4) Are variations in pore pressure, friction and cohesion required to produce simultaneous rupture? 5) How is the fabric of surface rupture affected by variations in orientation, kinematics, total geologic slip and fault zone architecture?

Paleoearthquakes at Frazier Mountain, California delimit extent and frequency of past San Andreas Fault ruptures along 1857 trace

USGS Publications Warehouse

Scharer, Katherine M.; Weldon, Ray; Streig, Ashley; Fumal, Thomas

2014-01-01

Large earthquakes are infrequent along a single fault, and therefore historic, well-characterized earthquakes exert a strong influence on fault behavior models. This is true of the 1857 Fort Tejon earthquake (estimated M7.7–7.9) on the southern San Andreas Fault (SSAF), but an outstanding question is whether the 330 km long rupture was typical. New paleoseismic data for six to seven ground-rupturing earthquakes on the Big Bend of the SSAF restrict the pattern of possible ruptures on the 1857 stretch of the fault. In conjunction with existing sites, we show that over the last ~650 years, at least 75% of the surface ruptures are shorter than the 1857 earthquake, with estimated rupture lengths of 100 to <300 km. These results suggest that the 1857 rupture was unusual, perhaps leading to the long open interval, and that a return to pre-1857 behavior would increase the rate of M7.3–M7.7 earthquakes.

Mw7.7 2013 Balochistan Earthquake. Slip-Distribution and Deformation Field in Oblique Tectonic Context

NASA Astrophysics Data System (ADS)

Klinger, Y.; Vallage, A.; Grandin, R.; Delorme, A.; Rosu, A. M.; Pierro-Deseilligny, M.

2014-12-01

The Mw7.7 2013 Balochistan earthquake ruptured 200 km of the Hoshab fault, the southern end of the Chaman fault. Azimuth of the fault changes by more than 30° along rupture, from a well-oriented strike-slip fault to a more thrust prone direction. We use the MicMac optical image software to correlate pairs of Landsat images taken before and after the earthquake to access to the horizontal displacement field associated with the earthquake. We combine the horizontal displacement with radar image correlation in range and radar interferometry to derive the co-seismic slip on the fault. The combination of these different datasets actually provides the 3D displacement field. We note that although the earthquake was mainly strike-slip all along the rupture length, some vertical motion patches exist, which locations seem to be controlled by kilometric-scale variations of the fault geometry. 5 pairs of SPOT images were also correlated to derive a 2.5m pixel-size horizontal displacement field, providing unique opportunity to look at deformation in the near field and to obtain high-resolution strike-slip and normal slip-distributions. We note a significant difference, especially in the normal component, between the slip localized at depth on the fault plane and the slip localized closer to the surface, with more apparent slip at the surface. A high-resolution map of ground rupture allows us to locate the distribution of the deformation over the whole rupture length. The rupture map also highlights multiple fault geometric complexities where we could quantify details of the slip distribution. At the rupture length-scale, the local azimuth variations between segments have a large impact on the expression of the localized slip at the surface. The combination of those datasets gives an overview of the large distribution of the deformation in the near field, corresponding to the co-seismic damage zone.

Holocene activity and seismogenic capability of intraplate thrusts: Insights from the Pampean Ranges, Argentina

NASA Astrophysics Data System (ADS)

Costa, Carlos H.; Owen, Lewis A.; Ricci, Walter R.; Johnson, William J.; Halperin, Alan D.

2018-07-01

Trench excavations across the El Molino fault in the southeastern Pampean Ranges of central-western Argentina have revealed a deformation zone composed of opposite-verging thrusts that deform a succession of Holocene sediments. The west-verging thrusts place Precambrian basement over Holocene proximal scarp-derived deposits, whereas the east-verging thrusts form an east-directed fault-propagation fold that deforms colluvium, fluvial and aeolian deposits. Ages for exposed fault-related deposits range from 7.1 ± 0.4 to 0.3 ka. Evidence of surface deformation suggests multiple rupture events with related scarp-derived deposits and a minimum of three surface ruptures younger than 7.1 ± 0.4 ka, the last rupture event being younger than 1 ka. Shortening rates of 0.7 ± 0.2 mm/a are near one order of magnitude higher than those estimated for the faults bounding neighboring crustal blocks and are considered high for this intraplate setting. These ground-rupturing crustal earthquakes are estimated to be of magnitude Mw ≥ 7.0, a significant discrepancy with the magnitudes Mw < 6.5 recorded in the seismic catalog of this region at present with low to moderate seismicity. Results highlight the relevance of identifying primary surface ruptures as well as the seismogenic potential of thrust faults in seemingly stable continental interiors.

The 1959 MW 7.3 Hebgen Lake earthquake revisited: morphology and mechanics from lidar

NASA Astrophysics Data System (ADS)

Johnson, K. L.; Nissen, E.; Lajoie, L. J.

2016-12-01

This study demonstrates how we can glean new information by revisiting an early instrumental earthquake with high-resolution topography and modern thinking about the mechanics of surface rupturing. The 1959 MW 7.3 Hebgen Lake earthquake is among the largest and most deadly historic earthquakes within the conterminous United States outside of California, and one of the largest normal faulting earthquakes on record globally. The earthquake ruptured the subparallel Hebgen and Red Canyon faults within the slowly extending ( 3 mm/yr) Centennial Mountain Belt, and is one of the first to be field mapped in detail, modeled from global seismograms, and surveyed geodetically. Here, we augment these early studies with an investigation of the surface rupture in its current state. We use a 50 cm-resolution airborne lidar digital terrain model collected by the National Center for Airborne Laser Mapping (NCALM) in 2014 to document the fault scarp morphology, constrain its evolution, and speculate on the mechanical rupture properties. Using a dense set of scarp profiles, we add >400 displacement measurements to the 143 published data points from early field work, allowing more rigorous quantification of along-strike slip variability and strain gradients. Evidence of off-fault deformation is sparse along most of the scarp, though damage zone width increases where the earthquake ruptured closely spaced sedimentary contacts rather than unconsolidated Quaternary deposits. In a few places, we can identify composite scarps from which we estimate the number of earthquakes that have offset Holocene surfaces. We assess the scarp's degraded state, including some sites that were surveyed in 1980 and 2009 and others that have not been revisited since the initial investigation. Where the rupture crosses unconsolidated surfaces, we compute local sediment diffusion coefficients and analyze their variability along strike. Lastly, we model subsurface fault geometry by fitting dipping planes to its surface trace, testing our best-fit fault dips against those recovered in seismic analyses; this reaffirms that both main rupture strands correspond to primary faulting rather than induced landsliding.

Insights into the relationship between surface and subsurface activity from mechanical modeling of the 1992 Landers M7.3 earthquake

NASA Astrophysics Data System (ADS)

Madden, E. H.; Pollard, D. D.

2009-12-01

Multi-fault, strike-slip earthquakes have proved difficult to incorporate into seismic hazard analyses due to the difficulty of determining the probability of these ruptures, despite collection of extensive data associated with such events. Modeling the mechanical behavior of these complex ruptures contributes to a better understanding of their occurrence by elucidating the relationship between surface and subsurface earthquake activity along transform faults. This insight is especially important for hazard mitigation, as multi-fault systems can produce earthquakes larger than those associated with any one fault involved. We present a linear elastic, quasi-static model of the southern portion of the 28 June 1992 Landers earthquake built in the boundary element software program Poly3D. This event did not rupture the extent of any one previously mapped fault, but trended 80km N and NW across segments of five sub-parallel, N-S and NW-SE striking faults. At M7.3, the earthquake was larger than the potential earthquakes associated with the individual faults that ruptured. The model extends from the Johnson Valley Fault, across the Landers-Kickapoo Fault, to the Homestead Valley Fault, using data associated with a six-week time period following the mainshock. It honors the complex surface deformation associated with this earthquake, which was well exposed in the desert environment and mapped extensively in the field and from aerial photos in the days immediately following the earthquake. Thus, the model incorporates the non-linearity and segmentation of the main rupture traces, the irregularity of fault slip distributions, and the associated secondary structures such as strike-slip splays and thrust faults. Interferometric Synthetic Aperture Radar (InSAR) images of the Landers event provided the first satellite images of ground deformation caused by a single seismic event and provide constraints on off-fault surface displacement in this six-week period. Insight is gained by comparing the density, magnitudes and focal plane orientations of relocated aftershocks for this time frame with the magnitude and orientation of planes of maximum Coulomb shear stress around the fault planes at depth.

Near-field fault slip of the 2016 Vettore Mw 6.6 earthquake (Central Italy) measured using low-cost GNSS.

PubMed

Wilkinson, Maxwell W; McCaffrey, Ken J W; Jones, Richard R; Roberts, Gerald P; Holdsworth, Robert E; Gregory, Laura C; Walters, Richard J; Wedmore, Luke; Goodall, Huw; Iezzi, Francesco

2017-07-04

The temporal evolution of slip on surface ruptures during an earthquake is important for assessing fault displacement, defining seismic hazard and for predicting ground motion. However, measurements of near-field surface displacement at high temporal resolution are elusive. We present a novel record of near-field co-seismic displacement, measured with 1-second temporal resolution during the 30 th October 2016 M w 6.6 Vettore earthquake (Central Italy), using low-cost Global Navigation Satellite System (GNSS) receivers located in the footwall and hangingwall of the Mt. Vettore - Mt. Bove fault system, close to new surface ruptures. We observe a clear temporal and spatial link between our near-field record and InSAR, far-field GPS data, regional measurements from the Italian Strong Motion and National Seismic networks, and field measurements of surface ruptures. Comparison of these datasets illustrates that the observed surface ruptures are the propagation of slip from depth on a surface rupturing (i.e. capable) fault array, as a direct and immediate response to the 30 th October earthquake. Large near-field displacement ceased within 6-8 seconds of the origin time, implying that shaking induced gravitational processes were not the primary driving mechanism. We demonstrate that low-cost GNSS is an accurate monitoring tool when installed as custom-made, short-baseline networks.

Active faulting in the central Betic Cordillera (Spain): Palaeoseismological constraint of the surface-rupturing history of the Baza Fault (Central Betic Cordillera, Iberian Peninsula)

NASA Astrophysics Data System (ADS)

Castro, J.; Martin-Rojas, I.; Medina-Cascales, I.; García-Tortosa, F. J.; Alfaro, P.; Insua-Arévalo, J. M.

2018-06-01

This paper on the Baza Fault provides the first palaeoseismic data from trenches in the central sector of the Betic Cordillera (S Spain), one of the most tectonically active areas of the Iberian Peninsula. With the palaeoseismological data we constructed time-stratigraphic OxCal models that yield probability density functions (PDFs) of individual palaeoseismic event timing. We analysed PDF overlap to quantitatively correlate the walls and site events into a single earthquake chronology. We assembled a surface-rupturing history of the Baza Fault for the last ca. 45,000 years. We postulated six alternative surface rupturing histories including 8-9 fault-wide earthquakes. We calculated fault-wide earthquake recurrence intervals using Monte Carlo. This analysis yielded a 4750-5150 yr recurrence interval. Finally, compared our results with the results from empirical relationships. Our results will provide a basis for future analyses of more of other active normal faults in this region. Moreover, our results will be essential for improving earthquake-probability assessments in Spain, where palaeoseismic data are scarce.

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.

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.

3D fault curvature and fractal roughness: Insights for rupture dynamics and ground motions using a Discontinous Galerkin method

NASA Astrophysics Data System (ADS)

Ulrich, Thomas; Gabriel, Alice-Agnes

2017-04-01

Natural fault geometries are subject to a large degree of uncertainty. Their geometrical structure is not directly observable and may only be inferred from surface traces, or geophysical measurements. Most studies aiming at assessing the potential seismic hazard of natural faults rely on idealised shaped models, based on observable large-scale features. Yet, real faults are wavy at all scales, their geometric features presenting similar statistical properties from the micro to the regional scale. Dynamic rupture simulations aim to capture the observed complexity of earthquake sources and ground-motions. From a numerical point of view, incorporating rough faults in such simulations is challenging - it requires optimised codes able to run efficiently on high-performance computers and simultaneously handle complex geometries. Physics-based rupture dynamics hosted by rough faults appear to be much closer to source models inverted from observation in terms of complexity. Moreover, the simulated ground-motions present many similarities with observed ground-motions records. Thus, such simulations may foster our understanding of earthquake source processes, and help deriving more accurate seismic hazard estimates. In this presentation, the software package SeisSol (www.seissol.org), based on an ADER-Discontinuous Galerkin scheme, is used to solve the spontaneous dynamic earthquake rupture problem. The usage of tetrahedral unstructured meshes naturally allows for complicated fault geometries. However, SeisSol's high-order discretisation in time and space is not particularly suited for small-scale fault roughness. We will demonstrate modelling conditions under which SeisSol resolves rupture dynamics on rough faults accurately. The strong impact of the geometric gradient of the fault surface on the rupture process is then shown in 3D simulations. Following, the benefits of explicitly modelling fault curvature and roughness, in distinction to prescribing heterogeneous initial stress conditions on a planar fault, is demonstrated. Furthermore, we show that rupture extend, rupture front coherency and rupture speed are highly dependent on the initial amplitude of stress acting on the fault, defined by the normalized prestress factor R, the ratio of the potential stress drop over the breakdown stress drop. The effects of fault complexity are particularly pronounced for lower R. By low-pass filtering a rough fault at several cut-off wavelengths, we then try to capture rupture complexity using a simplified fault geometry. We find that equivalent source dynamics can only be obtained using a scarcely filtered fault associated with a reduced stress level. To investigate the wavelength-dependent roughness effect, the fault geometry is bandpass-filtered over several spectral ranges. We show that geometric fluctuations cause rupture velocity fluctuations of similar length scale. The impact of fault geometry is especially pronounced when the rupture front velocity is near supershear. Roughness fluctuations significantly smaller than the rupture front characteristic dimension (cohesive zone size) affect only macroscopic rupture properties, thus, posing a minimum length scale limiting the required resolution of 3D fault complexity. Lastly, the effect of fault curvature and roughness on the simulated ground-motions is assessed. Despite employing a simple linear slip weakening friction law, the simulated ground-motions compare well with estimates from ground motions prediction equations, even at relatively high frequencies.

Geometry, slip distribution, and kinematics of surface rupture on the Sakarya fault segment during the 17 August 1999 İzmit, Turkey, earthquake

USGS Publications Warehouse

Langridge, R.M.; Stenner, Heidi D.; Fumal, T.E.; Christofferson, S.A.; Rockwell, T.K.; Hartleb, R.D.; Bachhuber, J.; Barka, A.A.

2002-01-01

The Mw 7.4 17 August 1999 İzmit earthquake ruptured five major fault segments of the dextral North Anatolian Fault Zone. The 26-km-long, N86°W-trending Sakarya fault segment (SFS) extends from the Sapanca releasing step-over in the west to near the town of Akyazi in the east. The SFS emerges from Lake Sapanca as two distinct fault traces that rejoin to traverse the Adapazari Plain to Akyazi. Offsets were measured across 88 cultural and natural features that cross the fault, such as roads, cornfield rows, rows of trees, walls, rails, field margins, ditches, vehicle ruts, a dike, and ground cracks. The maximum displacement observed for the İzmit earthquake (∼5.1 m) was encountered on this segment. Dextral displacement for the SFS rises from less than 1 m at Lake Sapanca to greater than 5 m near Arifiye, only 3 km away. Average slip decreases uniformly to the east from Arifiye until the fault steps left from Sagir to Kazanci to the N75°W, 6-km-long Akyazi strand, where slip drops to less than 1 m. The Akyazi strand passes eastward into the Akyazi Bend, which consists of a high-angle bend (18°-29°) between the Sakarya and Karadere fault segments, a 6-km gap in surface rupture, and high aftershock energy release. Complex structural geometries exist between the İzmit, Düzce, and 1967 Mudurnu fault segments that have arrested surface ruptures on timescales ranging from 30 sec to 88 days to 32 yr. The largest of these step-overs may have acted as a rupture segmentation boundary in previous earthquake cycles.

The Surface faulting produced by the 30 October 2016 Mw 6.5 Central Italy earthquake: the Open EMERGEO Working Group experience

NASA Astrophysics Data System (ADS)

Pantosti, Daniela

2017-04-01

The October 30, 2016 (06:40 UTC) Mw 6.5 earthquake occurred about 28 km NW of Amatrice village as the result of upper crust normal faulting on a nearly 30 km-long, NW-SE oriented, SW dipping fault system in the Central Apennines. This earthquake is the strongest Italian seismic event since the 1980 Mw 6.9 Irpinia earthquake. The Mw 6.5 event was the largest shock of a seismic sequence, which began on August 24 with a Mw 6.0 earthquake and also included a Mw 5.9 earthquake on October 26, about 9 and 35 km NW of Amatrice village, respectively. Field surveys of coseismic geological effects at the surface started within hours of the mainshock and were carried out by several national and international teams of earth scientists (about 120 people) from different research institutions and universities coordinated by the EMERGEO Working Group of the Istituto Nazionale di Geofisica e Vulcanologia. This collaborative effort was focused on the detailed recognition and mapping of: 1) the total extent of the October 30 coseismic surface ruptures, 2) their geometric and kinematic characteristics, 3) the coseismic displacement distribution along the activated fault system, including subsidiary and antithetic ruptures. The huge amount of collected data (more than 8000 observation points of several types of coseismic effects at the surface) were stored, managed and shared using a specifically designed spreadsheet to populate a georeferenced database. More comprehensive mapping of the details and extent of surface rupture was facilitated by Structure-from-Motion photogrammetry surveys by means of several helicopter flights. An almost continuous alignment of ruptures about 30 km long, N150/160 striking, mainly SW side down was observed along the already known active Mt. Vettore - Mt. Bove fault system. The mapped ruptures occasionally overlapped those of the August 24 Mw 6.0 and October 26 Mw 5.9 shocks. The coincidence between the observed surface ruptures and the trace of active normal faults mapped in the available geological literature is noteworthy. The field data collected suggest a complex coseismic surface faulting pattern along closely-spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays. The cumulative surface faulting length has been estimated in about 40 km. The maximum vertical offset is significant, locally exceeding 2 meters along the Mt. Vettore Fault, measured both along bedrock fault planes and free-faces affecting unconsolidated deposits. This enormous collaborative experience has a twofold relevance, on the one side allowed to document in high detail the earthquake ruptures before Winter would destroy them, on the other represent the first large European experience for coseismic effects survey that we should use a leading case to establish a coseismic effects European team to get ready to respond to future seismic crises at the European level.

Surface faulting along the inland Itozawa normal fault (eastern Japan) and relation to the 2011 Tohoku-oki megathrust earthquake

NASA Astrophysics Data System (ADS)

Ferry, M.; Tsutsumi, H.; Meghraoui, M.; Toda, S.

2012-12-01

The 11 March 2011 Mw 9 Tohoku-oki earthquake ruptured ~500 km length of the Japan Trench along the coast of eastern Japan and significantly impacted the stress regime within the crust. The resulting change in seismicity over the Japan mainland was exhibited by the 11 April 2011 Mw 6.6 Iwaki earthquake that ruptured the Itozawa and Yunodake faults. Trending NNW and NW, respectively, these 70-80° W-dipping faults bound the Iwaki basin of Neogene age and have been reactivated simultaneously both along 15-km-long sections. Here, we present initial results from a paleoseismic excavation performed across the Itozawa fault within the Tsunagi Valley at the northern third of the observed surface rupture. At the Tsunagi site, the rupture affects a rice paddy, which provides an ideally horizontal initial state to collect detailed and accurate measurements. The surface break is composed of a continuous 30-to-40-cm-wide purely extensional crack that separates the uplifted block from a gently dipping 1-to-2-m-wide strip affected by right-stepping en-echelon cracks and locally bounded by a ~0.1-m-high reverse scarplet. Total station across-fault topographic profiles indicate the pre-earthquake ground surface was vertically deformed by ~0.6 m while direct field examinations reveal that well-defined rice paddy limits have been left-laterally offset by ~0.1 m. The 12-m-long, 3.5-m-deep trench exposes the 30-to-40-cm-thick cultivated soil overlaying a 1-m-thick red to yellow silt unit, a 2-m-thick alluvial gravel unit and a basal 0.1-1-m-thick organic-rich silt unit. Deformation associated to the 2011 rupture illustrates down-dip movement along a near-vertical fault with a well-expressed bending moment at the surface and generalized warping. On the north wall, the intermediate gravel unit displays a deformation pattern similar to granular flow with only minor discrete faulting and no splay to be continuously followed from the main fault to the surface. On the south wall, warping dominates as well but with some strain localization along two major splays that exhibit 15-20 cm of vertical offset. On both walls, the basal silt unit is vertically deformed by ~0.6 m, similarly to what is observed for the 2011 rupture. Furthermore, the base of said silt unit exhibits indication for secondary faulting prior to the 2011 event and that resemble cracks observed at the present-day surface. This suggests that the Itozawa fault has already ruptured in a similar fashion; probably in the late Pleistocene-early Holocene (radiocarbon samples are being processed). Hence, recent activity of the Itozawa fault may be controlled entirely by large to giant earthquakes along the Japan Trench.

Evidence for two surface ruptures in the past 500 years on the San Andreas fault at Frazier Mountain, California

USGS Publications Warehouse

Lindvall, S.C.; Rockwell, T.K.; Dawson, T.E.; Helms, J.G.; Bowman, K.W.

2002-01-01

We conducted paleoseismic studies in a closed depression along the San Andreas fault on the north flank of Frazier Mountain near Frazier Park, California. We recognized two earthquake ruptures in our trench exposure and interpreted the most recent rupture, event 1, to represent the historical 1857 earthquake. We also exposed evidence of an earlier surface rupture, event 2, along an older group of faults that did not rerupture during event 1. Radiocarbon dating of the stratigraphy above and below the earlier event constrains its probable age to between A.D. 1460 and 1600. Because we documented continuous, unfaulted stratigraphy between the earlier event horizon and the youngest event horizon in the portion of the fault zone exposed, we infer event 2 to be the penultimate event. We observed no direct evidence of an 1812 earthquake in our exposures. However, we cannot preclude the presence of this event at our site due to limited age control in the upper part of the section and the possibility of other fault strands beyond the limits of our exposures. Based on overlapping age ranges, event 2 at Frazier Mountain may correlate with event B at the Bidart fan site in the Carrizo Plain to the northwest and events V and W4 at Pallett Creek and Wrightwood, respectively, to the southeast. If the events recognized at these multiple sites resulted from the same surface rupture, then it appears that the San Andreas fault has repeatedly failed in large ruptures similar in extent to 1857.

New Field Observations About 19 August 1966 Varto earthquake, Eastern Turkey

NASA Astrophysics Data System (ADS)

Gurboga, S.

2013-12-01

Some destructive earthquakes in the past and even in the recent have several mysteries. For example, magnitude, epicenter location, faulting type and source fault of an earthquake have not been detected yet. One of these mysteries events is 19 August 1966 Varto earthquake in Turkey. 19 August 1966 Varto earthquake (Ms = 6.8) was an extra ordinary event at the 40 km east of junction between NAFS and EAFS which are two seismogenic system and active structures shaping the tectonics of Turkey. This earthquake sourced from Varto fault zone which are approximately 4 km width and 43 km length. It consists of faults which have parallel to sub-parallel, closely-spaced, north and south-dipping up to 85°-88° dip amount. Although this event has 6.8 (Ms) magnitude that is big enough to create a surface rupture, there was no clear surface deformation had been detected. This creates the controversial issue about the source fault and the mechanism of the earthquake. According to Wallace (1968) the type of faulting is right-lateral. On the other hand, McKenzie (1972) proposed right-lateral movement with thrust component by using the focal mechanism solution. The recent work done by Sançar et al. (2011) claimed that type of faulting is pure right-lateral strike-slip and there is no any surface rupture during the earthquake. Furthermore, they suggested that Varto segment in the Varto Fault Zone was most probably not broken in 1966 earthquake. This study is purely focused on the field geology and trenching survey for the investigation of 1966 Varto earthquake. Four fault segments have been mapped along the Varto fault zone: Varto, Sazlica, Leylekdağ and Çayçati segments. Because of the thick volcanic cover on the area around Varto, surface rupture has only been detected by trenching survey. Two trenching survey have been applied along the Yayikli and Ağaçalti faults in the Varto fault zone. Consequently, detailed geological work in the field and trenching survey indicate that a) source of 1966 earthquake is Varto segment in Varto Fault Zone, b) many of the surface deformations observed just after the earthquake is lateral-spreading and small landslides, c) surface rupture was created with 10 cm displacement at the surface with thrust component. Because of the volcanic cover and activation of many faults, ground surface rupture could not be seen clearly which has been expected after 6.8 magnitude earthquake, d) faulting type is right-lateral component with thrust component. Keywords: 1966 Varto earthquake, paleoseismology, right-lateral fault with thrust component.

Effect of off-fault low-velocity elastic inclusions on supershear rupture dynamics

NASA Astrophysics Data System (ADS)

Ma, Xiao; Elbanna, A. E.

2015-10-01

Heterogeneous velocity structures are expected to affect fault rupture dynamics. To quantitatively evaluate some of these effects, we examine a model of dynamic rupture on a frictional fault embedded in an elastic full space, governed by plane strain elasticity, with a pair of off-fault inclusions that have a lower rigidity than the background medium. We solve the elastodynamic problem using the Finite Element software Pylith. The fault operates under linear slip-weakening friction law. We initiate the rupture by artificially overstressing a localized region near the left edge of the fault. We primarily consider embedded soft inclusions with 20 per cent reduction in both the pressure wave and shear wave speeds. The embedded inclusions are placed at different distances from the fault surface and have different sizes. We show that the existence of a soft inclusion may significantly shorten the transition length to supershear propagation through the Burridge-Andrews mechanism. We also observe that supershear rupture is generated at pre-stress values that are lower than what is theoretically predicted for a homogeneous medium. We discuss the implications of our results for dynamic rupture propagation in complex velocity structures as well as supershear propagation on understressed faults.

Rupture Dynamics and Ground Motion from Earthquakes in Heterogeneous Media

NASA Astrophysics Data System (ADS)

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

2012-12-01

Heterogeneities in the material properties of Earth's crust scatter propagating seismic waves. The effects of scattered waves are reflected in the seismic coda and depend on the relative strength of the heterogeneities, spatial arrangement, and distance from source to receiver. In the vicinity of the fault, scattered waves influence the rupture process by introducing fluctuations in the stresses driving propagating ruptures. Further variability in the rupture process is introduced by naturally occurring geometric complexity of fault surfaces, and the stress changes that accompany slip on rough surfaces. We have begun a modeling effort to better understand the origin of complexity in the earthquake source process, and to quantify the relative importance of source complexity and scattering along the propagation path in causing incoherence of high frequency ground motion. To do this we extended our two-dimensional high order finite difference rupture dynamics code to accommodate material heterogeneities. We generate synthetic heterogeneous media using Von Karman correlation functions and their associated power spectral density functions. We then nucleate ruptures on either flat or rough faults, which obey strongly rate-weakening friction laws. Preliminary results for flat faults with uniform frictional properties and initial stresses indicate that off-fault material heterogeneity alone can lead to a complex rupture process. Our simulations reveal the excitation of high frequency bursts of waves, which radiate energy away from the propagating rupture. The average rupture velocity is thus reduced relative to its value in simulations employing homogeneous material properties. In the coming months, we aim to more fully explore parameter space by varying the correlation length, Hurst exponent, and amplitude of medium heterogeneities, as well as the statistical properties characterizing fault roughness.

Dynamic rupture modeling of the transition from thrust to strike-slip motion in the 2002 Denali fault earthquake, Alaska

USGS Publications Warehouse

Aagaard, Brad T.; Anderson, G.; Hudnut, K.W.

2004-01-01

We use three-dimensional dynamic (spontaneous) rupture models to investigate the nearly simultaneous ruptures of the Susitna Glacier thrust fault and the Denali strike-slip fault. With the 1957 Mw 8.3 Gobi-Altay, Mongolia, earthquake as the only other well-documented case of significant, nearly simultaneous rupture of both thrust and strike-slip faults, this feature of the 2002 Denali fault earthquake provides a unique opportunity to investigate the mechanisms responsible for development of these large, complex events. We find that the geometry of the faults and the orientation of the regional stress field caused slip on the Susitna Glacier fault to load the Denali fault. Several different stress orientations with oblique right-lateral motion on the Susitna Glacier fault replicate the triggering of rupture on the Denali fault about 10 sec after the rupture nucleates on the Susitna Glacier fault. However, generating slip directions compatible with measured surface offsets and kinematic source inversions requires perturbing the stress orientation from that determined with focal mechanisms of regional events. Adjusting the vertical component of the principal stress tensor for the regional stress field so that it is more consistent with a mixture of strike-slip and reverse faulting significantly improves the fit of the slip-rake angles to the data. Rotating the maximum horizontal compressive stress direction westward appears to improve the fit even further.

Timing of late Holocene surface rupture of the Wairau Fault, Marlborough, New Zealand

USGS Publications Warehouse

Zachariasen, J.; Berryman, K.; Langridge, Rob; Prentice, C.; Rymer, M.; Stirling, M.; Villamor, P.

2006-01-01

Three trenches excavated across the central portion of the right-lateral strike-slip Wairau Fault in South Island, New Zealand, exposed a complex set of fault strands that have displaced a sequence of late Holocene alluvial and colluvial deposits. Abundant charcoal fragments provide age control for various stratigraphic horizons dating back to c. 5610 yr ago. Faulting relations from the Wadsworth trench show that the most recent surface rupture event occurred at least 1290 yr and at most 2740 yr ago. Drowned trees in landslide-dammed Lake Chalice, in combination with charcoal from the base of an unfaulted colluvial wedge at Wadsworth trench, suggest a narrower time bracket for this event of 1811-2301 cal. yr BP. The penultimate faulting event occurred between c. 2370 and 3380 yr, and possibly near 2680 ?? 60 cal. yr BP, when data from both the Wadsworth and Dillon trenches are combined. Two older events have been recognised from Dillon trench but remain poorly dated. A probable elapsed time of at least 1811 yr since the last surface rupture, and an average slip rate estimate for the Wairau Fault of 3-5 mm/yr, suggests that at least 5.4 m and up to 11.5 m of elastic shear strain has accumulated since the last rupture. This is near to or greater than the single-event displacement estimates of 5-7 m. The average recurrence interval for surface rupture of the fault determined from the trench data is 1150-1400 yr. Although the uncertainties in the timing of faulting events and variability in inter-event times remain high, the time elapsed since the last event is in the order of 1-2 times the average recurrence interval, implying that the Wairau Fault is near the end of its interseismic period. ?? The Royal Society of New Zealand 2006.

Dynamic rupture models of earthquakes on the Bartlett Springs Fault, Northern California

USGS Publications Warehouse

Lozos, Julian C.; Harris, Ruth A.; Murray, Jessica R.; Lienkaemper, James J.

2015-01-01

The Bartlett Springs Fault (BSF), the easternmost branch of the northern San Andreas Fault system, creeps along much of its length. Geodetic data for the BSF are sparse, and surface creep rates are generally poorly constrained. The two existing geodetic slip rate inversions resolve at least one locked patch within the creeping zones. We use the 3-D finite element code FaultMod to conduct dynamic rupture models based on both geodetic inversions, in order to determine the ability of rupture to propagate into the creeping regions, as well as to assess possible magnitudes for BSF ruptures. For both sets of models, we find that the distribution of aseismic creep limits the extent of coseismic rupture, due to the contrast in frictional properties between the locked and creeping regions.

Coseismic rupturing stopped by Aso volcano during the 2016 Mw 7.1 Kumamoto earthquake, Japan.

PubMed

Lin, A; Satsukawa, T; Wang, M; Mohammadi Asl, Z; Fueta, R; Nakajima, F

2016-11-18

Field investigations and seismic data show that the 16 April 2016 moment magnitude (M w ) 7.1 Kumamoto earthquake produced a ~40-kilometer-long surface rupture zone along the northeast-southwest-striking Hinagu-Futagawa strike-slip fault zone and newly identified faults on the western side of Aso caldera, Kyushu Island, Japan. The coseismic surface ruptures cut Aso caldera, including two volcanic cones inside it, but terminate therein. The data show that northeastward propagation of coseismic rupturing terminated in Aso caldera because of the presence of magma beneath the Aso volcanic cluster. The seismogenic faults of the 2016 Kumamoto earthquake may require reassessment of the volcanic hazard in the vicinity of Aso volcano. Copyright © 2016, American Association for the Advancement of Science.

On- and off-fault deformation associated with the September 2013 Mw7.7 Balochistan earthquake: implications for geologic slip rate measurements

USGS Publications Warehouse

Gold, Ryan D.; Reitman, Nadine G.; Briggs, Richard; Barnhart, William; Hayes, Gavin; Wilson, Earl M.

2015-01-01

The 24 September 2013 Mw7.7 Balochistan, Pakistan earthquake ruptured a ~ 200 km-long stretch of the Hoshab fault in southern Pakistan and produced the second-largest lateral surface displacement observed for a continental strike-slip earthquake. We remotely measured surface deformation associated with this event using high-resolution (0.5 m) pre- and post-event satellite optical imagery. We document left lateral, near-field, on-fault offsets (10 m from fault) using 309 laterally offset piercing points, such as streams, terrace risers, and roads. Peak near-field displacement is 13.6 + 2.5/− 3.4 m. We characterize off-fault deformation by measuring medium- (< 350 m from fault) and far-field (> 350 m from fault) displacement using manual (259 measurements) and automated image cross-correlation methods, respectively. Off-fault peak lateral displacement values are ~ 15 m and exceed on-fault displacement magnitudes for ~ 85% of the rupture length. Our observations suggest that for this rupture, coseismic surface displacement typically increases with distance away from the surface trace of the fault; however, nearly 100% of total surface displacement occurs within a few hundred meters of the primary fault trace. Furthermore, off-fault displacement accounts for, on average, 28% of the total displacement but exhibits a highly heterogeneous along-strike pattern. The best agreement between near-field and far-field displacements generally corresponds to the narrowest fault zone widths. Our analysis demonstrates significant and heterogeneous mismatches between on- and off-fault coseismic deformation, and we conclude that this phenomenon should be considered in hazard models based on geologically determined on-fault slip rates.

On- and off-fault deformation associated with the September 2013 Mw 7.7 Balochistan earthquake: Implications for geologic slip rate measurements

NASA Astrophysics Data System (ADS)

Gold, Ryan D.; Reitman, Nadine G.; Briggs, Richard W.; Barnhart, William D.; Hayes, Gavin P.; Wilson, Earl

2015-10-01

The 24 September 2013 Mw7.7 Balochistan, Pakistan earthquake ruptured a ~ 200 km-long stretch of the Hoshab fault in southern Pakistan and produced the second-largest lateral surface displacement observed for a continental strike-slip earthquake. We remotely measured surface deformation associated with this event using high-resolution (0.5 m) pre- and post-event satellite optical imagery. We document left lateral, near-field, on-fault offsets (10 m from fault) using 309 laterally offset piercing points, such as streams, terrace risers, and roads. Peak near-field displacement is 13.6 + 2.5/- 3.4 m. We characterize off-fault deformation by measuring medium- (< 350 m from fault) and far-field (> 350 m from fault) displacement using manual (259 measurements) and automated image cross-correlation methods, respectively. Off-fault peak lateral displacement values are ~ 15 m and exceed on-fault displacement magnitudes for ~ 85% of the rupture length. Our observations suggest that for this rupture, coseismic surface displacement typically increases with distance away from the surface trace of the fault; however, nearly 100% of total surface displacement occurs within a few hundred meters of the primary fault trace. Furthermore, off-fault displacement accounts for, on average, 28% of the total displacement but exhibits a highly heterogeneous along-strike pattern. The best agreement between near-field and far-field displacements generally corresponds to the narrowest fault zone widths. Our analysis demonstrates significant and heterogeneous mismatches between on- and off-fault coseismic deformation, and we conclude that this phenomenon should be considered in hazard models based on geologically determined on-fault slip rates.

From slow to fast rupture during laboratory earthquakes in dolostones

NASA Astrophysics Data System (ADS)

Passelegue, F. X.; Fondriest, M.; Nicolas, A.; Aubry, J.; Schubnel, A.; Di Toro, G.

2016-12-01

Dolostones are the dominant lithology of the shallow portions of many seismically active regions (e.g., Italian Apennines). Displacement in natural fault zones cutting dolostones and exhumed from < 3-4 km depth is frequently localized on highly reflective (mirror-like) slip surfaces, coated with thin films of nano-granular fault rock. Using saw-cut dolostone samples, we conducted stick-slip experiments under upper crustal stress conditions (confining pressures and temperatures of 30, 60 and 90 MPa at 30, 65 and 100 °C, respectively). Samples were equipped with 15 piezoelectric transducers allowing the record of acoustic activity. At 30 and 65 °C, only slow ruptures (Vr < 200 m/s) were observed and the experimental faults exhibited ductile behaviour. At 65 °C, a slip strengthening behaviour was observed after the main slow rupture, leading to a succession of slow ruptures. At T = 100 °C and 30 MPa confining pressure, fault strengthening increased after each rupture, allowing, while the rupture processes remained slow (no acoustic activity), a sequence of slow stick-slip events. Instead, at the same ambient temperature but under larger confining pressures (60 and 90 MPa), we observed the transition from slow to fast rupture events (up to supershear rupture velocities), associated to clusters of acoustic activity and dynamic stress drop occurring in few tens of microseconds. In all experiments, mirror-like surfaces and nanoparticles were observed under the scanning electron microscope as a result of slow and fast ruptures. Clearly, mirror-like surfaces and nano powders are not representative of seismic slip events in cohesive dolostones. Instead, the transition from slow to fast ruptures (and generation of acoustic emissions) was related to a flash weakening processes, enhanced at 100° C, which allowed the experimental fault to weaken with slip faster than the rate at which the elastic strain was released from the surrounding medium.

Study on conditional probability of surface rupture: effect of fault dip and width of seismogenic layer

NASA Astrophysics Data System (ADS)

Inoue, N.

2017-12-01

The conditional probability of surface ruptures is affected by various factors, such as shallow material properties, process of earthquakes, ground motions and so on. Toda (2013) pointed out difference of the conditional probability of strike and reverse fault by considering the fault dip and width of seismogenic layer. This study evaluated conditional probability of surface rupture based on following procedures. Fault geometry was determined from the randomly generated magnitude based on The Headquarters for Earthquake Research Promotion (2017) method. If the defined fault plane was not saturated in the assumed width of the seismogenic layer, the fault plane depth was randomly provided within the seismogenic layer. The logistic analysis was performed to two data sets: surface displacement calculated by dislocation methods (Wang et al., 2003) from the defined source fault, the depth of top of the defined source fault. The estimated conditional probability from surface displacement indicated higher probability of reverse faults than that of strike faults, and this result coincides to previous similar studies (i.e. Kagawa et al., 2004; Kataoka and Kusakabe, 2005). On the contrary, the probability estimated from the depth of the source fault indicated higher probability of thrust faults than that of strike and reverse faults, and this trend is similar to the conditional probability of PFDHA results (Youngs et al., 2003; Moss and Ross, 2011). The probability of combined simulated results of thrust and reverse also shows low probability. The worldwide compiled reverse fault data include low fault dip angle earthquake. On the other hand, in the case of Japanese reverse fault, there is possibility that the conditional probability of reverse faults with less low dip angle earthquake shows low probability and indicates similar probability of strike fault (i.e. Takao et al., 2013). In the future, numerical simulation by considering failure condition of surface by the source fault would be performed in order to examine the amount of the displacement and conditional probability quantitatively.

Magnitude and Surface Rupture Length of Prehistoric Upper Crustal Earthquakes in the Puget Lowland, Washington State

NASA Astrophysics Data System (ADS)

Sherrod, B. L.; Styron, R. H.

2016-12-01

Paleoseismic studies documented prehistoric earthquakes after the last glaciation ended 15 ka on 13 upper-crustal fault zones in the Cascadia fore arc. These fault zones are a consequence of north-directed fore arc block migration manifesting as a series of bedrock uplifts and intervening structural basins in the southern Salish Sea lowland between Vancouver, B.C. to the north and Olympia, WA to the south, and bounded on the east and west by the Cascade Mountains and Olympic Mountains, respectively. Our dataset uses published information and includes 27 earthquakes tabulated from observations of postglacial deformation at 63 sites. Stratigraphic offsets along faults consist of two types of measurements: 1) vertical separation of strata along faults observed in fault scarp excavations, and 2) estimates from coastal uplift and subsidence. We used probabilistic methods to estimate past rupture magnitudes and surface rupture length (SRL), applying empirical observations from modern earthquakes and point measurements from paleoseismic sites (Biasi and Weldon, 2006). Estimates of paleoearthquake magnitude ranged between M 6.5 and M 7.5. SRL estimates varied between 20 and 90 km. Paleoearthquakes on the Seattle fault zone and Saddle Mountain West fault about 1100 years ago were outliers in our analysis. Large offsets observed for these two earthquakes implies a M 7.8 and 200 km SRL, given the average observed ratio of slip/SRL in modern earthquakes. The actual mapped traces of these faults are less than 200km, implying these earthquakes had an unusually high static stress drop or, in the case of the Seattle fault, splay faults may have accentuated uplift in the hanging wall. Refined calculations incorporating fault area may change these magnitude and SRL estimates. Biasi, G.P., and Weldon, R.J., 2006, Estimating Surface Rupture Length and Magnitude of Paleoearthquakes from Point Measurements of Rupture Displacement: B. Seismol. Soc. Am., 96, 1612-1623.

Quantifying near-field and off-fault deformation patterns of the 1992 Mw 7.3 Landers earthquake

NASA Astrophysics Data System (ADS)

Milliner, Christopher W. D.; Dolan, James F.; Hollingsworth, James; Leprince, Sebastien; Ayoub, Francois; Sammis, Charles G.

2015-05-01

Coseismic surface deformation in large earthquakes is typically measured using field mapping and with a range of geodetic methods (e.g., InSAR, lidar differencing, and GPS). Current methods, however, either fail to capture patterns of near-field coseismic surface deformation or lack preevent data. Consequently, the characteristics of off-fault deformation and the parameters that control it remain poorly understood. We develop a standardized method to fully measure the surface, near-field, coseismic deformation patterns at high resolution using the COSI-Corr program by correlating pairs of aerial photographs taken before and after the 1992 Mw 7.3 Landers earthquake. COSI-Corr offers the advantage of measuring displacement across the entire zone of surface deformation and over a wider aperture than that available to field geologists. For the Landers earthquake, our measured displacements are systematically larger than the field measurements, indicating the presence of off-fault deformation. We show that 46% of the total surface displacement occurred as off-fault deformation, over a mean deformation width of 154 m. The magnitude and width of off-fault deformation along the rupture is primarily controlled by the macroscopic structural complexity of the fault system, with a weak correlation with the type of near-surface materials through which the rupture propagated. Both the magnitude and width of distributed deformation are largest in stepovers, bends, and at the southern termination of the surface rupture. We find that slip along the surface rupture exhibits a consistent degree of variability at all observable length scales and that the slip distribution is self-affine fractal with dimension of 1.56.

Holocene surface-faulting earthquakes at the Spring Lake and North Creek Sites on the Wasatch Fault Zone: Evidence for complex rupture of the Nephi Segment

USGS Publications Warehouse

Duross, Christopher; Hylland, Michael D.; Hiscock, Adam; Personius, Stephen; Briggs, Richard; Gold, Ryan D.; Beukelman, Gregg; McDonald, Geg N; Erickson, Ben; McKean, Adam; Angster, Steve; King, Roselyn; Crone, Anthony J.; Mahan, Shannon

2017-01-01

The Nephi segment of the Wasatch fault zone (WFZ) comprises two fault strands, the northern and southern strands, which have evidence of recurrent late Holocene surface-faulting earthquakes. We excavated paleoseismic trenches across these strands to refine and expand their Holocene earthquake chronologies; improve estimates of earthquake recurrence, displacement, and fault slip rate; and assess whether the strands rupture separately or synchronously in large earthquakes. Paleoseismic data from the Spring Lake site expand the Holocene record of earthquakes on the northern strand: at least five to seven earthquakes ruptured the Spring Lake site at 0.9 ± 0.2 ka (2σ), 2.9 ± 0.7 ka, 4.0 ± 0.5 ka, 4.8 ± 0.8 ka, 5.7 ± 0.8 ka, 6.6 ± 0.7 ka, and 13.1 ± 4.0 ka, yielding a Holocene mean recurrence of ~1.2–1.5 kyr and vertical slip rate of ~0.5–0.8 mm/yr. Paleoseismic data from the North Creek site help refine the Holocene earthquake chronology for the southern strand: at least five earthquakes ruptured the North Creek site at 0.2 ± 0.1 ka (2σ), 1.2 ± 0.1 ka, 2.6 ± 0.9 ka, 4.0 ± 0.1 ka, and 4.7 ± 0.7 ka, yielding a mean recurrence of 1.1–1.3 kyr and vertical slip rate of ~1.9–2.0 mm/yr. We compare these Spring Lake and North Creek data with previous paleoseismic data for the Nephi segment and report late Holocene mean recurrence intervals of ~1.0–1.2 kyr for the northern strand and ~1.1–1.3 kyr for the southern strand. The northern and southern strands have similar late Holocene earthquake histories, which allow for models of both independent and synchronous rupture. However, considering the earthquake timing probabilities and per-event vertical displacements, we have the greatest confidence in the simultaneous rupture of the strands, including rupture of one strand with spillover rupture to the other. Ultimately, our results improve the surface-faulting earthquake history of the Nephi segment and enhance our understanding of how structural barriers influence normal-fault rupture.

Modeling of fault reactivation and induced seismicity during hydraulic fracturing of shale-gas reservoirs

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

Rutqvist, Jonny; Rinaldi, Antonio P.; Cappa, Frédéric

2013-07-01

We have conducted numerical simulation studies to assess the potential for injection-induced fault reactivation and notable seismic events associated with shale-gas hydraulic fracturing operations. The modeling is generally tuned towards conditions usually encountered in the Marcellus shale play in the Northeastern US at an approximate depth of 1500 m (~;;4,500 feet). Our modeling simulations indicate that when faults are present, micro-seismic events are possible, the magnitude of which is somewhat larger than the one associated with micro-seismic events originating from regular hydraulic fracturing because of the larger surface area that is available for rupture. The results of our simulations indicatedmore » fault rupture lengths of about 10 to 20 m, which, in rare cases can extend to over 100 m, depending on the fault permeability, the in situ stress field, and the fault strength properties. In addition to a single event rupture length of 10 to 20 m, repeated events and aseismic slip amounted to a total rupture length of 50 m, along with a shear offset displacement of less than 0.01 m. This indicates that the possibility of hydraulically induced fractures at great depth (thousands of meters) causing activation of faults and creation of a new flow path that can reach shallow groundwater resources (or even the surface) is remote. The expected low permeability of faults in producible shale is clearly a limiting factor for the possible rupture length and seismic magnitude. In fact, for a fault that is initially nearly-impermeable, the only possibility of larger fault slip event would be opening by hydraulic fracturing; this would allow pressure to penetrate the matrix along the fault and to reduce the frictional strength over a sufficiently large fault surface patch. However, our simulation results show that if the fault is initially impermeable, hydraulic fracturing along the fault results in numerous small micro-seismic events along with the propagation, effectively preventing larger events from occurring. Nevertheless, care should be taken with continuous monitoring of induced seismicity during the entire injection process to detect any runaway fracturing along faults.« less

3-D kinematics analysis of surface ruptures on an active creeping fault at Chihshang, Eastern Taiwan

NASA Astrophysics Data System (ADS)

Lee, J.; Angelier, J.; Chen, H.; Chu, H.; Hu, J.

2003-12-01

The Chihshang fault is one of the most active segments of the Longitudinal Valley Fault, the plate suture between the converging Philippine and Eurasian plates. A destructive earthquake of M 7.1 with substantial surface scarps resulted from rupturing of the Chihshang fault in 1951. From that on, no big earthquake greater than M 5.5 occurred in this area. Instead, the Chihshang fault reveals a creeping behavior at a rapid rate of about 20 mm/yr at least during the past 20 years. The surface breaks of the creeping Chihshang fault can be observed at the several places. A typical feature is reverse-fault-like fractures on the retaining wall. We deployed small geodetic networks across the fault zone at five sites. Each network comprises of 5 to 15 benchmarks. Trilateration measurements including angles and distances as well as leveling among the benchmarks have been carried out on an annual basis or twice a year since 1998. Compared to previous other measurements which have shown the first order creep rate for the entire fault zone, the present geodetic data provides the detailed information of the surface movements across the fault zone which usually composed of more than one fault strands and folds structures. According to our data from the local geodetic networks, we are able to reconstruct the 3-D kinematics of surface deformation across the Chihshang fault zone. Multiple fault strands are common along the Chihshang fault. Oblique shortening occurred at all sites and was characterized by a combination of thrusts, backthrust and surface warps. Strike-slip motion can also be distinguished on some fault strands. It is worth to note that the cultural feature, such as concrete basement of strong resistance, sometimes acted as deflection of surface ruptures. It should be taken into consideration for mitigation against seismic hazards.

Direct measurement of the breakdown slip from near-fault strong motion data

NASA Astrophysics Data System (ADS)

Cruz-Atienza, V. M.; Olsen, K. B.; Dalguer, L. A.

2007-12-01

Obtaining reliable estimates of the frictional behaviour on earthquake faults is a fundamental task, particularly the breakdown slip Dc, which has an important role on rupture propagation through the earthquake energy budget. Several studies have attempted to estimate Dc indirectly from kinematical analysis of fault ruptures (e.g., Ide and Takeo, JGR, 1997). However, such estimates are complicated because of both the limited band-width of the observed seismograms used to image the rupture process and the rapid decay of high frequencies with distance from the fault. Mikumo et al. (BSSA, 2003) proposed a method to estimate Dc on the fault plane as the slip at the time of the peak sliprate function (Dc'). Fukuyama and Mikumo (GRL, 2007) proposed to extend this method beyond the fault plane, by estimating Dc as twice the rake-parallel particle displacement at the time of the peak particle velocity. The factor of two arises from an equal amount of opposite displacement on either side of the fault. They concluded that such method allows reliable Dc' estimates with negligible dependence on the perpendicular distance from the fault, and used it to obtain Dc' estimates for the 2000 M6.6 Tottori (0.3 m) and the 2002 M7.9 Denali (2.5 m) earthquakes. The study by Fukuyama and Mikumo was based on simple two-dimensional Green's functions in a homogeneous full space for an anti-plane kinematic crack, and suffers from three fundamental omissions: 1) the free surface and heterogeneous structure, 2) the finiteness of the rupture surface and 3) the dynamic rupture complexity of real 3D earthquakes. Here, we re-examine the methodology proposed by Fukuyama and Mikumo by means of a more realistic approach. We use spontaneous rupture propagation simulated by a recently developed and highly accurate approach, namely the staggered-grid split-node (SGSN) method in a fourth-order staggered- grid finite difference method (Dalguer and Day, JGR, 2007). We assume a vertical strike-slip fault governed by both linear and non-linear slip-weakening friction laws. Our results show that both the free surface and the stopping phases strongly affect Dc estimates. The particle motion recorded by surface instruments is amplified roughly by a factor of two due to the presence of the free surface. As a consequence, the method by Fukuyama and Mikumo over-estimates Dc when applied to strong motion data recorded on the earth's surface. Moreover, contrary to the results by Fukuyama and Mikumo, we observe a strong distance-dependence of the Dc estimates perpendicular to the fault. This variation includes a minimum near the fault, increasing up to about 140% of the target Dc value at a distance 2-3 km from the fault. At further distances from the fault the Dc estimate decreases to about 60% of the target value 10 km away. This distance dependence of the Dc estimate is presumably caused mainly by stopping phases propagating from the fault boundaries. Simulations in heterogeneous media including a low-velocity layer, intrinsic attenuation (Q) and stochastic initial stress conditions allow us to asses the reliability and uncertainty involved in the method proposed by Fukuyama and Mikumo. Dc estimates under these realistic conditions are important but remain below a factor of two in most of the cases we have analyzed. In summary, the accuracy of the method is strongly affected by the presence of the free surface, finite fault extent, and likely by complexity in the velocity structure and rupture propagation.

Nonlinear interaction of strong S-waves with the rupture front in the shallow subsurface

NASA Astrophysics Data System (ADS)

Sleep, N. H.

2017-12-01

Shallow deformation in moderate to large earthquakes is sometimes distributed rather than being concentrated on a single fault plane. Strong high-frequency S-waves interact with the rupture front to produce this effect. For strike-slip faults, the rupture propagation velocity is a fraction of the S-wave velocity. The rupture propagation vector refracts essentially vertically in the low (S-wave) velocity shallow subsurface. So does the propagation direction of S-waves. The shallow rupture front is essentially mode 3 near the surface. Strong S-waves arrive before the rupture front. They continue to arrive for several seconds in a large event. There are simple scaling relationships. The dynamic Coulomb stress ratio of horizontal stress on horizontal planes from S-waves is the normalized acceleration in g's. For fractured rock and gravel, frictional failure occurs when the normalized acceleration exceeds the effective coefficient of friction. Acceleration tends to saturate at that level as the anelastic strain rate increases rapidly with stress. For muddy materials, failure begins at a low normalized acceleration but increases slowly with dynamic stress. Dynamic accelerations sometimes exceed 1 g. In both cases, the rupture tip finds the shallow subsurface already in nonlinear failure down to a few to tens of meters depth. The material does not distinguish between S-wave and rupture tip stresses. Both stresses add to the stress invariant and hence to the anelastic strain rate tensor. Surface anelastic strain from fault slip is thus distributed laterally over a distance scaling to the depth of nonlinearity from S-waves. The environs of the fault anelastically accommodate the fault slip at depth. This process differs from blind faults where the shallow coseismic strain is mostly elastic and interseismic anelastic processes accommodate the long-term shallow deformation.

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.

Rupture processes of the 2010 Canterbury earthquake and the 2011 Christchurch earthquake inferred from InSAR, strong motion and teleseismic datasets

NASA Astrophysics Data System (ADS)

Yun, S.; Koketsu, K.; Aoki, Y.

2014-12-01

The September 4, 2010, Canterbury earthquake with a moment magnitude (Mw) of 7.1 is a crustal earthquake in the South Island, New Zealand. The February 22, 2011, Christchurch earthquake (Mw=6.3) is the biggest aftershock of the 2010 Canterbury earthquake that is located at about 50 km to the east of the mainshock. Both earthquakes occurred on previously unrecognized faults. Field observations indicate that the rupture of the 2010 Canterbury earthquake reached the surface; the surface rupture with a length of about 30 km is located about 4 km south of the epicenter. Also various data including the aftershock distribution and strong motion seismograms suggest a very complex rupture process. For these reasons it is useful to investigate the complex rupture process using multiple data with various sensitivities to the rupture process. While previously published source models are based on one or two datasets, here we infer the rupture process with three datasets, InSAR, strong-motion, and teleseismic data. We first performed point source inversions to derive the focal mechanism of the 2010 Canterbury earthquake. Based on the focal mechanism, the aftershock distribution, the surface fault traces and the SAR interferograms, we assigned several source faults. We then performed the joint inversion to determine the rupture process of the 2010 Canterbury earthquake most suitable for reproducing all the datasets. The obtained slip distribution is in good agreement with the surface fault traces. We also performed similar inversions to reveal the rupture process of the 2011 Christchurch earthquake. Our result indicates steep dip and large up-dip slip. This reveals the observed large vertical ground motion around the source region is due to the rupture process, rather than the local subsurface structure. To investigate the effects of the 3-D velocity structure on characteristic strong motion seismograms of the two earthquakes, we plan to perform the inversion taking 3-D velocity structure of this region into account.

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.

Detailed ground surface displacement and fault ruptures of the 2016 Kumamoto Earthquake revealed by SAR and GNSS data

NASA Astrophysics Data System (ADS)

Kobayashi, T.; Yarai, H.; Morishita, Y.; Kawamoto, S.; Fujiwara, S.; Nakano, T.

2016-12-01

We report ground displacement associated with the 2016 Kumamoto Earthquake obtained by ALOS-2 SAR and GNSS data. For the SAR analyses, we applied InSAR, MAI, and pixel offset methods, which has successfully provided a 3D displacement field showing the widely- and locally-distributed deformation. The obtained displacement field shows clear displacement boundaries linearly along the Futagawa, the Hinagu, and the Denokuchi faults across which the sign of displacement component turns to be opposite, suggesting that the fault ruptures occurred there. Our fault model for the main shock suggests that the main rupture occurred on the Futagawa fault with a right-lateral motion including a slight normal fault motion. Due to the normal faulting movement, the northern side of the active fault subsides with approximately 2 m. The rupture on the Futagawa fault extends into the Aso caldera with slightly shifting the position northward. Of note, the fault plane oppositely dips toward southeast. It may be a conjugate fault against the main fault. In the western side of the Futagawa fault, the slip on the Hinagu fault, in which the Mj6.5 and Mj6.4 foreshocks occurred with a pure right-lateral motion, is also deeply involved with the main shock. This fault rupture released the amount of approximately 30 percent of the total seismic moment. The hypocenter is determined near the fault and its focal mechanism is consistent with the estimated slip motion of this fault plane, maybe suggesting that the rupture started at this fault and proceeded toward the Futagawa fault eastward. Acknowledgements: ALOS-2 data were provided from the Earthquake Working Group under a cooperative research contract with JAXA (Japan Aerospace Exploration Agency). The ownership of ALOS-2 data belongs to JAXA.

Dynamic rupture simulations of the 2016 Mw7.8 Kaikōura earthquake: a cascading multi-fault event

NASA Astrophysics Data System (ADS)

Ulrich, T.; Gabriel, A. A.; Ampuero, J. P.; Xu, W.; Feng, G.

2017-12-01

The Mw7.8 Kaikōura earthquake struck the Northern part of New Zealand's South Island roughly one year ago. It ruptured multiple segments of the contractional North Canterbury fault zone and of the Marlborough fault system. Field observations combined with satellite data suggest a rupture path involving partly unmapped faults separated by large stepover distances larger than 5 km, the maximum distance usually considered by the latest seismic hazard assessment methods. This might imply distant rupture transfer mechanisms generally not considered in seismic hazard assessment. We present high-resolution 3D dynamic rupture simulations of the Kaikōura earthquake under physically self-consistent initial stress and strength conditions. Our simulations are based on recent finite-fault slip inversions that constrain fault system geometry and final slip distribution from remote sensing, surface rupture and geodetic data (Xu et al., 2017). We assume a uniform background stress field, without lateral fault stress or strength heterogeneity. We use the open-source software SeisSol (www.seissol.org) which is based on an arbitrary high-order accurate DERivative Discontinuous Galerkin method (ADER-DG). Our method can account for complex fault geometries, high resolution topography and bathymetry, 3D subsurface structure, off-fault plasticity and modern friction laws. It enables the simulation of seismic wave propagation with high-order accuracy in space and time in complex media. We show that a cascading rupture driven by dynamic triggering can break all fault segments that were involved in this earthquake without mechanically requiring an underlying thrust fault. Our prefered fault geometry connects most fault segments: it does not features stepover larger than 2 km. The best scenario matches the main macroscopic characteristics of the earthquake, including its apparently slow rupture propagation caused by zigzag cascading, the moment magnitude and the overall inferred slip distribution. We observe a high sensitivity of cascading dynamics on fault-step over distance and off-fault energy dissipation.

On- and off-fault coseismic surface deformation associated with the September 2013 M7.7 Balochistan, Pakistan earthquake measured from mapping and automated pixel correlation

NASA Astrophysics Data System (ADS)

Gold, R. D.; Reitman, N. G.; Briggs, R. W.; Barnhart, W. D.; Hayes, G. P.

2014-12-01

The 24 September 2013 Mw7.7 Balochistan, Pakistan earthquake ruptured a ~200 km-long stretch of the Hoshab fault in southern Pakistan. We remotely measured the coseismic surface deformation field using high-resolution (0.5 m) pre- and post-event satellite imagery. We measured ~300 near-field (0-10 m from fault) laterally offset piercing points (streams, terrace risers, roads, etc.) and find peak left-lateral offsets of ~12-15 m. We characterized the far-field (0-10 km from fault) displacement field using manual (~250 measurements) and automated image cross-correlation methods (e.g., pixel tracking) and find peak displacement values of ~16 m, which commonly exceed the on-fault displacement magnitudes. Our preliminary observations suggest the following: (1) coseismic surface displacement typically increases with distance away from the surface trace of the fault (e.g., highest displacement values in the far field), (2) for certain locations along the fault rupture, as little as 50% of the coseismic displacement field occurred in the near-field; and (3) the magnitudes of individual displacements are inversely correlated to the width of the surface rupture zone (e.g., largest displacements where the fault zone is narrowest). This analysis highlights the importance of identifying field study sites spanning fault sections with narrow deformation zones in order to capture the entire deformation field. For regions of distributed deformation, these results would predict that geologic slip rate studies underestimate a fault's complete slip rate.

Geomorphic features of surface ruptures associated with the 2016 Kumamoto earthquake in and around the downtown of Kumamoto City, and implications on triggered slip along active faults

NASA Astrophysics Data System (ADS)

Goto, Hideaki; Tsutsumi, Hiroyuki; Toda, Shinji; Kumahara, Yasuhiro

2017-02-01

The 30-km-long surface ruptures associated with the M w 7.0 ( M j 7.3) earthquake at 01:25 JST on April 16 in Kumamoto Prefecture appeared along the previously mapped 100-km-long active fault called the Futagawa-Hinagu fault zone (FHFZ). The surface ruptures appeared to have extended further west out of the main FHFZ into the Kumamoto Plain. Although InSAR analysis by Geospatial Information Authority of Japan (GSI) indicated coseismic surface deformation in and around the downtown of Kumamoto City, the surface ruptures have not been clearly mapped in the central part of the Kumamoto Plain, and whether there are other active faults other than the Futagawa fault in the Kumamoto Plain remained unclear. We produced topographical stereo images (anaglyph) from 5-m-mesh digital elevation model of GSI, which was generated from light detection and ranging data. We interpreted them and identified that several SW-sloping river terraces formed after the deposition of the pyroclastic flow deposits related to the latest large eruption of the Aso caldera (86.8-87.3 ka) are cut and deformed by several NW-trending flexure scarps down to the southwest. These 5.4-km-long scarps that cut across downtown Kumamoto were identified for the first time, and we name them as the Suizenji fault zone. Surface deformation such as continuous cracks, tilts, and monoclinal folding associated with the main shock of the 2016 Kumamoto earthquake was observed in the field along the fault zone. The amount of vertical deformation ( 0.1 m) along this fault associated with the 2016 Kumamoto earthquake was quite small compared to the empirically calculated coseismic slip (0.5 m) based on the fault length. We thus suggest that the slip on this fault zone was triggered by the Kumamoto earthquake, but the fault zone has potential to generate an earthquake with larger slip that poses a high seismic risk in downtown Kumamoto area.[Figure not available: see fulltext.

Numerical simulation of the 1976 Ms7.8 Tangshan Earthquake

NASA Astrophysics Data System (ADS)

Li, Zhengbo; Chen, Xiaofei

2017-04-01

An Ms 7.8 earthquake happened in Tangshan in 1976, causing more than 240000 people death and almost destroying the whole city. Numerous studies indicated that the surface rupture zone extends 8 to 11 km in the south of Tangshan City. The fault system is composed with more than ten NE-trending right-lateral strike-slip left-stepping echelon faults, with a general strike direction of N30°E. However, recent scholars proposed that the surface ruptures appeared in a larger area. To simulate the rupture process closer to the real situation, the curvilinear grid finite difference method presented by Zhang et al. (2006, 2014) which can handle the free surface and the complex geometry were implemented to investigate the dynamic rupture and ground motion of Tangshan earthquake. With the data from field survey, seismic section, borehole and trenching results given by different studies, several fault geometry models were established. The intensity, the seismic waveform and the displacement resulted from the simulation of different models were compared with the observed data. The comparison of these models shows details of the rupture process of the Tangshan earthquake and implies super-shear may occur during the rupture, which is important for better understanding of this complicated rupture process and seismic hazard distributions of this earthquake.

Neotectonics and paleoseismology of the Limón and pedro miguel faults in Panamá: earthquake hazard to the Panamá canal

USGS Publications Warehouse

Rockwell, Thomas; Gath, Edon; Gonzalez, Tania; Madden, Chris; Verdugo, Danielle; Lippincott, Caitlin; Dawson, Tim; Owen, Lewis A.; Fuchs, Markus; Cadena, Ana; Williams, Pat; Weldon, Elise; Franceschi, Pastora

2010-01-01

We present new geologic, tectonic geomorphic, and geochronologic data on the slip rate, timing, and size of past surface ruptures for the right-lateral Lim??n and Pedro Miguel faults in central Panam??. These faults are part of a system of conjugate faults that accommodate the internal deformation of Panam?? resulting from the ongoing collision of Central and South America. There have been at least three surface ruptures on the Lim??n fault in the past 950-1400 years, with the most recent during the past 365 years. Displacement in this young event is at least 1.2 m (based on trenching) and may be 1.6-2 m (based on small channel offsets). Awell-preserved 4.2 m offset suggests that the penultimate event also sustained significant displacement. The Holocene slip rate has averaged about 6 mm=yr, based on a 30-m offset terrace riser incised into a 5-ka abandoned channel. The Pedro Miguel fault has sustained three surface ruptures in the past 1600 years, the most recent being the 2 May 1621 earthquake that partially destroyed Panam?? Viejo. At least 2.1 m of slip occurred in this event near the Canal, with geomorphic offsets suggesting 2.5-3 m. The historic Camino de Cruces is offset 2.8 m, indicating multimeter displacement over at least 20 km of fault length. Channel offsets of 100-400 m, together with a climate-induced incision model, suggest a Late Quaternary slip rate of about 5 mm=yr, which is consistent with the paleoseismic results. Comparison of the timing of surface ruptures between the Lim??n and Pedro Miguel faults suggests that large earthquakes may rupture both faults with 2-3 m of displacement for over 40 km, such as is likely in earthquakes in the M 7 range. Altogether, our observations indicate that the Lim??n and Pedro Miguel faults represent a significant seismic hazard to central Panam?? and, specifically, to the Canal and Panam?? City.

Surface rupture of the 1933 M 7.5 Diexi earthquake in eastern Tibet: implications for seismogenic tectonics

NASA Astrophysics Data System (ADS)

Ren, Junjie; Xu, Xiwei; Zhang, Shimin; Yeats, Robert S.; Chen, Jiawei; Zhu, Ailan; Liu, Shao

2018-03-01

The 1933 M 7.5 Diexi earthquake is another catastrophic event with the loss of over 10 000 lives in eastern Tibet comparable to the 2008 Mw 7.9 Wenchuan earthquake. Because of its unknown surface rupture, the seismogenic tectonics of the 1933 earthquake remains controversial. We collected unpublished reports, literatures and old photos associated with the 1933 earthquake and conducted field investigations based on high-resolution Google Earth imagery. Combined with palaeoseismological analysis, radiocarbon dating and relocated earthquakes, our results demonstrate that the source of the 1933 earthquake is the northwest-trending Songpinggou fault. This quake produced a > 30 km long normal-faulting surface rupture with the coseismic offset of 0.9-1.7 m. Its moment magnitude (Mw) is ˜6.8. The Songpinggou fault undergoes an average vertical slip rate of ˜0.25 mm yr-1 and has a recurrence interval of ˜6700 yr of large earthquakes. The normal-faulting surface rupture of this quake is probably the reactivation of the Mesozoic Jiaochang tectonic belt in gravitational adjustment of eastern Tibet. Besides the major boundary faults, minor structures within continental blocks may take a role in strain partitioning of eastern Tibet and have the potential of producing large earthquake. This study contributes to a full understanding of seismotectonics of large earthquakes and strain partitioning in eastern Tibet.

Rupture Dynamics and Ground Motion from Earthquakes on Rough Faults in Heterogeneous Media

NASA Astrophysics Data System (ADS)

Bydlon, S. A.; Kozdon, J. E.; Duru, K.; Dunham, E. M.

2013-12-01

Heterogeneities in the material properties of Earth's crust scatter propagating seismic waves. The effects of scattered waves are reflected in the seismic coda and depend on the amplitude of the heterogeneities, spatial arrangement, and distance from source to receiver. In the vicinity of the fault, scattered waves influence the rupture process by introducing fluctuations in the stresses driving propagating ruptures. Further variability in the rupture process is introduced by naturally occurring geometric complexity of fault surfaces, and the stress changes that accompany slip on rough surfaces. Our goal is to better understand the origin of complexity in the earthquake source process, and to quantify the relative importance of source complexity and scattering along the propagation path in causing incoherence of high frequency ground motion. Using a 2D high order finite difference rupture dynamics code, we nucleate ruptures on either flat or rough faults that obey strongly rate-weakening friction laws. These faults are embedded in domains with spatially varying material properties characterized by Von Karman autocorrelation functions and their associated power spectral density functions, with variations in wave speed of approximately 5 to 10%. Flat fault simulations demonstrate that off-fault material heterogeneity, at least with this particular form and amplitude, has only a minor influence on the rupture process (i.e., fluctuations in slip and rupture velocity). In contrast, ruptures histories on rough faults in both homogeneous and heterogeneous media include much larger short-wavelength fluctuations in slip and rupture velocity. We therefore conclude that source complexity is dominantly influenced by fault geometric complexity. To examine contributions of scattering versus fault geometry on ground motions, we compute spatially averaged root-mean-square (RMS) acceleration values as a function of fault perpendicular distance for a homogeneous medium and several heterogeneous media characterized by different statistical properties. We find that at distances less than ~6 km from the fault, RMS acceleration values from simulations with homogeneous and heterogeneous media are similar, but at greater distances the RMS values associated with heterogeneous media are larger than those associated with homogeneous media. The magnitude of this divergence increases with the amplitude of the heterogeneities. For instance, for a heterogeneous medium with a 10% standard deviation in material property values relative to mean values, RMS accelerations are ~50% larger than for a homogeneous medium at distances greater than 6 km. This finding is attributed to the scattering of coherent pulses into multiple pulses of decreased amplitude that subsequently arrive at later times. In order to understand the robustness of these results, an extension of our dynamic rupture and wave propagation code to 3D is underway.

Co-seismic ruptures of the 12 May 2008, Ms 8.0 Wenchuan earthquake, Sichuan: East-west crustal shortening on oblique, parallel thrusts along the eastern edge of Tibet

USGS Publications Warehouse

Liu-Zeng, J.; Zhang, Z.; Wen, L.; Tapponnier, P.; Sun, Jielun; Xing, X.; Hu, G.; Xu, Q.; Zeng, L.; Ding, L.; Ji, C.; Hudnut, K.W.; van der Woerd, J.

2009-01-01

The Ms 8.0, Wenchuan earthquake, which devastated the mountainous western rim of the Sichuan basin in central China, produced a surface rupture over 200??km-long with oblique thrust/dextral slip and maximum scarp heights of ~ 10??m. It thus ranks as one of the world's largest continental mega-thrust events in the last 150??yrs. Field investigation shows clear surface breaks along two of the main branches of the NE-trending Longmen Shan thrust fault system. The principal rupture, on the NW-dipping Beichuan fault, displays nearly equal amounts of thrust and right-lateral slip. Basin-ward of this rupture, another continuous surface break is observed for over 70??km on the parallel, more shallowly NW-dipping Pengguan fault. Slip on this latter fault was pure thrusting, with a maximum scarp height of ~ 3.5??m. This is one of the very few reported instances of crustal-scale co-seismic slip partitioning on parallel thrusts. This out-of-sequence event, with distributed surface breaks on crustal mega-thrusts, highlights regional, ~ EW-directed, present day crustal shortening oblique to the Longmen Shan margin of Tibet. The long rupture and large offsets with strong horizontal shortening that characterize the Wenchuan earthquake herald a re-evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau, and require a re-assessment of seismic hazard along potentially under-rated active faults across the densely populated western Sichuan basin and mountains. ?? 2009 Elsevier B.V.

A method and example of seismically imaging near‐surface fault zones in geologically complex areas using Vp, Vs, and their ratios

USGS Publications Warehouse

Catchings, Rufus D.; Rymer, Michael J.; Goldman, Mark R.; Sickler, Robert R.; Criley, Coyn J.

2014-01-01

The determination of near‐surface (vadose zone and slightly below) fault locations and geometries is important because assessment of ground rupture, strong shaking, geologic slip rates, and rupture histories occurs at shallow depths. However, seismic imaging of fault zones at shallow depths can be difficult due to near‐surface complexities, such as weathering, groundwater saturation, massive (nonlayered) rocks, and vertically layered strata. Combined P‐ and S‐wave seismic‐refraction tomography data can overcome many of the near‐surface, fault‐zone seismic‐imaging problems because of differences in the responses of elastic (bulk and shear) moduli of P and S waves to shallow‐depth, fault‐zone properties. We show that high‐resolution refraction tomography images of P‐ to S‐wave velocity ratios (VP/VS) can reliably identify near‐surface faults. We demonstrate this method using tomography images of the San Andreas fault (SAF) surface‐rupture zone associated with the 18 April 1906 ∼M 7.9 San Francisco earthquake on the San Francisco peninsula in California. There, the SAF cuts through Franciscan mélange, which consists of an incoherent assemblage of greywacke, chert, greenstone, and serpentinite. A near‐vertical zone (∼75° northeast dip) of high P‐wave velocities (up to 3000  m/s), low S‐wave velocities (∼150–600  m/s), high VP/VS ratios (4–8.8), and high Poisson’s ratios (0.44–0.49) characterizes the main surface‐rupture zone to a depth of about 20 m and is consistent with nearby trench observations. We suggest that the combined VP/VSimaging approach can reliably identify most near‐surface fault zones in locations where many other seismic methods cannot be applied.

Effect of thermal pressurization on dynamic rupture propagation under depth-dependent stress

NASA Astrophysics Data System (ADS)

Urata, Y.; Kuge, K.; Kase, Y.

2009-12-01

Fluid and pore pressure evolution can affect dynamic propagation of earthquake ruptures owing to thermal pressurization (e.g., Mase and Smith, 1985). We investigate dynamic rupture propagation with thermal pressurization on a fault subjected to depth-dependent stress, on the basis of 3-D numerical simulations for spontaneous dynamic ruptures. We put a vertical strike-slip rectangular fault in a semi-infinite, homogenous, and elastic medium. The length and width of the fault are 8 and 3 km, respectively. We assume a depth-dependent stress estimated by Yamashita et al. (2004). The numerical algorithm is based on the finite-difference method by Kase and Kuge (2001). A rupture is initiated by increasing shear stress in a small patch at the bottom of the fault, and then proceeds spontaneously, governed by a slip-weakening law with the Coulomb failure criteria. Coefficients of friction and Dc are homogeneous on the fault. On a fault with thermal pressurization, we allow effective normal stress to vary with pore pressure change due to frictional heating by the formulation of Bizzarri and Cocco (2006). When thermal pressurization does not work, tractions drop in the same way everywhere and rupture velocity is subshear except near the free surface. Due to thermal pressurization, dynamic friction on the fault decreases and is heterogeneous not only vertically but horizontally, slip increases, and rupture velocity along the strike direction becomes supershear. As a result, plural peaks of final slip appear, as observed in the case of undrained dip-slip fault by Urata et al. (2008). We found in this study that the early stage of rupture growth under the depth-dependent stress is affected by the location of an initial crack. When a rupture is initiated at the center of the fault without thermal pressurization, the rupture cannot propagate and terminates. Thermal pressurization can help such a powerless rupture to keep propagating.

Misbheaving Faults: The Expanding Role of Geodetic Imaging in Unraveling Unexpected Fault Slip Behavior

NASA Astrophysics Data System (ADS)

Barnhart, W. D.; Briggs, R.

2015-12-01

Geodetic imaging techniques enable researchers to "see" details of fault rupture that cannot be captured by complementary tools such as seismology and field studies, thus providing increasingly detailed information about surface strain, slip kinematics, and how an earthquake may be transcribed into the geological record. For example, the recent Haiti, Sierra El Mayor, and Nepal earthquakes illustrate the fundamental role of geodetic observations in recording blind ruptures where purely geological and seismological studies provided incomplete views of rupture kinematics. Traditional earthquake hazard analyses typically rely on sparse paleoseismic observations and incomplete mapping, simple assumptions of slip kinematics from Andersonian faulting, and earthquake analogs to characterize the probabilities of forthcoming ruptures and the severity of ground accelerations. Spatially dense geodetic observations in turn help to identify where these prevailing assumptions regarding fault behavior break down and highlight new and unexpected kinematic slip behavior. Here, we focus on three key contributions of space geodetic observations to the analysis of co-seismic deformation: identifying near-surface co-seismic slip where no easily recognized fault rupture exists; discerning non-Andersonian faulting styles; and quantifying distributed, off-fault deformation. The 2013 Balochistan strike slip earthquake in Pakistan illuminates how space geodesy precisely images non-Andersonian behavior and off-fault deformation. Through analysis of high-resolution optical imagery and DEMs, evidence emerges that a single fault map slip as both a strike slip and dip slip fault across multiple seismic cycles. These observations likewise enable us to quantify on-fault deformation, which account for ~72% of the displacements in this earthquake. Nonetheless, the spatial distribution of on- and off-fault deformation in this event is highly spatially variable- a complicating factor for comparisons of geologic and geodetic slip rates. As such, detailed studies such as this will play a continuing vital role in the accurate assessment of short- and long-term fault slip kinematics.

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.

Fault geometry and slip distribution of the 2008 Mw 7.9 Wenchuan, China earthquake, inferred from GPS and InSAR measurements

NASA Astrophysics Data System (ADS)

Wan, Yongge; Shen, Zheng-Kang; Bürgmann, Roland; Sun, Jianbao; Wang, Min

2017-02-01

We revisit the problem of coseismic rupture of the 2008 Mw7.9 Wenchuan earthquake. Precise determination of the fault structure and slip distribution provides critical information about the mechanical behaviour of the fault system and earthquake rupture. We use all the geodetic data available, craft a more realistic Earth structure and fault model compared to previous studies, and employ a nonlinear inversion scheme to optimally solve for the fault geometry and slip distribution. Compared to a homogeneous elastic half-space model and laterally uniform layered models, adopting separate layered elastic structure models on both sides of the Beichuan fault significantly improved data fitting. Our results reveal that: (1) The Beichuan fault is listric in shape, with near surface fault dip angles increasing from ˜36° at the southwest end to ˜83° at the northeast end of the rupture. (2) The fault rupture style changes from predominantly thrust at the southwest end to dextral at the northeast end of the fault rupture. (3) Fault slip peaks near the surface for most parts of the fault, with ˜8.4 m thrust and ˜5 m dextral slip near Hongkou and ˜6 m thrust and ˜8.4 m dextral slip near Beichuan, respectively. (4) The peak slips are located around fault geometric complexities, suggesting that earthquake style and rupture propagation were determined by fault zone geometric barriers. Such barriers exist primarily along restraining left stepping discontinuities of the dextral-compressional fault system. (5) The seismic moment released on the fault above 20 km depth is 8.2×1021 N m, corresponding to an Mw7.9 event. The seismic moments released on the local slip concentrations are equivalent to events of Mw7.5 at Yingxiu-Hongkou, Mw7.3 at Beichuan-Pingtong, Mw7.2 near Qingping, Mw7.1 near Qingchuan, and Mw6.7 near Nanba, respectively. (6) The fault geometry and kinematics are consistent with a model in which crustal deformation at the eastern margin of the Tibetan plateau is decoupled by differential motion across a decollement in the mid crust, above which deformation is dominated by brittle reverse faulting and below which deformation occurs by viscous horizontal shortening and vertical thickening.

Slip Model of the 2015 Mw 7.8 Gorkha (Nepal) Earthquake from Inversions of ALOS-2 and GPS Data

NASA Astrophysics Data System (ADS)

Wang, K.; Fialko, Y. A.

2015-12-01

We use surface deformation measurements including Interferometric Synthetic Aperture Radar (InSAR) data acquired by the ALOS-2 mission of the Japanese Aerospace Exploration Agency (JAXA) and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 Mw 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the of Main Frontal Thrust fault (MFT) between 84.34E and 86.19E, the best-fitting model suggests a dip angle of 7 degrees. The moment calculated from the slip model is 6.17*1020 Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150-km long zone 50 to 100 km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of ~6 m at a depth of ~ 8 km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar-Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the East.

Rupture Propagation of the 2013 Mw7.7 Balochistan, Pakistan, Earthquake Affected by Poroelastic Stress Changes

NASA Astrophysics Data System (ADS)

He, J.; Wang, W.; Xiao, J.

2015-12-01

The 2013 Mw7.7 Balochistan, Pakistan, earthquake occurred on the curved Hoshab fault. This fault connects with the north-south trending Chaman strike-slip fault to northeast, and with the west-east trending Makran thrust fault system to southwest. Teleseismic waveform inversion, incorporated with coseismic ground surface deformation data, show that the rupture of this earthquake nucleated around northeast segment of the fault, and then propagated southwestward along the northwest dipping Hoshab fault about 200 km, with the maximum coseismic displacement, featured mainly by purely left-lateral strike-slip motion, about 10 meters. In context of the India-Asia collision frame, associating with the fault geometry around this region, the rupture propagation of this earthquake seems to not follow an optimal path along the fault segment, because after nucleation of this event the Hoshab fault on the southwest of hypocenter of this earthquake is clamped by elastic stress change. Here, we build a three-dimensional finite-element model to explore the evolution of both stress and pore-pressure during the rupturing process of this earthquake. In the model, the crustal deformation is treated as undrained poroelastic media as described by Biot's theory, and the instantaneous rupture process is specified with split-node technique. By testing a reasonable range of parameters, including the coefficient of friction, the undrained Poisson's ratio, the permeability of the fault zone and the bulk crust, numerical results have shown that after the nucleation of rupture of this earthquake around the northeast of the Hoshab fault, the positive change of normal stress (clamping the fault) on the fault plane is greatly reduced by the instantaneous increase of pore pressure (unclamping the fault). This process could result in the change of Coulomb failure stress resolved on the Hoshab fault to be hastened, explaining the possible mechanism for southwestward propagation of rupture of the Mw7.7 Balochistan earthquake along the Hoshab fault.

Aftershocks of the june 20, 1978, Greece earthquake: A multimode faulting sequence

USGS Publications Warehouse

Carver, D.; Bollinger, G.A.

1981-01-01

A 10-station portable seismograph network was deployed in northern Greece to study aftershocks of the magnitude (mb) 6.4 earthquake of June 20, 1978. The main shock occurred (in a graben) about 25 km northeast of the city of Thessaloniki and caused an east-west zone of surface rupturing 14 km long that splayed to 7 km wide at the west end. The hypocenters for 116 aftershocks in the magnitude range from 2.5 to 4.5 were determined. The epicenters for these events cover an area 30 km (east-west) by 18 km (north-south), and focal depths ranges from 4 to 12 km. Most of the aftershocks in the east half of the aftershock zone are north of the surface rupture and north of the graben. Those in the west half are located within the boundaries of the graben. Composite focalmechanism solutions for selected aftershocks indicate reactivation of geologically mapped normal faults in the area. Also, strike-slip and dip-slip faults that splay off the western end of the zone of surface ruptures may have been activated. The epicenters for four large (M ??? 4.8) foreshocks and the main shock were relocated using the method of joint epicenter determination. Collectively, those five epicenters form an arcuate pattern convex southward, that is north of and 5 km distant from the surface rupturing. The 5-km separation, along with a focal depth of 8 km (average aftershock depth) or 16 km (NEIS main-shock depth), implies that the fault plane dips northward 58?? or 73??, respectively. A preferred nodal-plane dip of 36?? was determined by B.C. Papazachos and his colleagues in 1979 from a focal-mechanism solution for the main shock. If this dip is valid for the causal fault and that fault projects to the zone of surface rupturing, a decrease of dip with depth is required. ?? 1981.

Logs and Geologic Data from a Paleoseismic Investigation of the Susitna Glacier fault, Central Alaska Range, Alaska

USGS Publications Warehouse

Personius, Stephen F.; Crone, Anthony J.; Burns, Patricia A.C.; Beget, James E.; Seitz, Gordon G.; Bemis, Sean P.

2010-01-01

This report contains field and laboratory data from a paleoseismic study of the Susitna Glacier fault, Alaska. The initial M 7.2 subevent of the November 3, 2002, M 7.9 Denali fault earthquake sequence produced a 48-km-long set of complex fault scarps, folds, and aligned landslides on the previously unknown, north-dipping Susitna Glacier thrust fault along the southern margin of the Alaska Range in central Alaska. Most of the 2002 folds and fault scarps are 1-3 m high, implying dip-slip thrust offsets (assuming a near-surface fault dip of approximately 20 degrees)of 3-5 m. Locally, some of the 2002 ruptures were superimposed on preexisting scarps that have as much as 5-10 m of vertical separation and are evidence of previous surface-rupturing earthquakes on the Susitna Glacier fault. In 2003-2005, we focused follow-up studies on several of the large scarps at the 'Wet fan' site in the central part of the 2002 rupture to determine the pre-2002 history of large surface-rupturing earthquakes on the fault. We chose this site for several reasons: (1) the presence of pre-2002 thrust- and normal-fault scarps on a gently sloping, post-glacial alluvial fan; (2) nearby natural exposures of underlying fan sediments revealed fine-grained fluvial silts with peat layers and volcanic ash beds useful for chronological control; and (3) a lack of permafrost to a depth of more than 1 m. Our studies included detailed mapping, fault-scarp profiling, and logging of three hand-excavated trenches. We were forced to restrict our excavations to 1- to 2-m-high splay faults and folds because the primary 2002 ruptures mostly were superimposed on such large scarps that it was impossible to hand dig through the hanging wall to expose the fault plane. Additional complications are the pervasive effects of cryogenic processes (mainly solifluction) that can mask or mimic tectonic deformation. The purpose of this report is to present photomosaics, trench logs, scarp profiles, and fault slip, radiocarbon, tephrochronologic, and unit description data obtained during this investigation. We do not attempt to use the data presented herein to construct a paleoseismic history of the Susitna Glacier fault; that history will be the subject of a future report. When completed, our results will be used to compare the Susitna Glacier fault paleoseismic record with results of similar studies on the nearby Denali fault to determine if the simultaneous rupture of these two faults during the 2002 Denali fault earthquake sequence is typical or atypical of their long-term interaction.

Late Pleistocene - Holocene ruptures of the Lima Reservoir fault, SW Montana

NASA Astrophysics Data System (ADS)

Anastasio, David J.; Majerowicz, Christina N.; Pazzaglia, Frank J.; Regalla, Christine A.

2010-12-01

Active tectonics within the northern Basin and Range province provide a natural laboratory for the study of normal fault growth, linkage, and interaction. Here, we present new geologic mapping and morphologic fault-scarp modeling within the Centennial Valley, Montana to characterize Pleistocene - Holocene ruptures of the young and active Lima Reservoir fault. Geologic relationships and rupture ages indicate Middle Pleistocene activity on the Henry Gulch (>50 ka and 23-10 ka), Trail Creek (>43 ka and ˜13 ka), and reservoir (˜23 ka) segments. Offset Quaternary deposits also record Holocene rupture of the reservoir segment (˜8 ka), but unfaulted modern streams show that no segments of the Lima Reservoir fault have experienced a large earthquake in at least several millennia. The clustered pattern of rupture ages on the Lima Reservoir fault segments suggests a seismogenic linkage though segment length and spacing make a physical connection at depth unlikely. Trail Creek and reservoir segment slip rates were non-steady and appear to be increasing. The fault helps accommodate differential horizontal surface velocity measured by GPS geodesy across the boundary between the northern Basin and Range province and the Snake River Plain.

Seismic constraints and coulomb stress changes of a blind thrust fault system, 1: Coalinga and Kettleman hills, California

USGS Publications Warehouse

Lin, Jian; Stein, Ross S.

2006-01-01

This report reviews the seismicity and surface ruptures associated with the 1982-1985 earthquake sequence in the Coalinga region in California, and the role of Coulomb stress in triggering the mainshock sequence and aftershocks. The 1982-1985 New Idria, Coalinga, and Kettleman Hills earthquakes struck on a series of west-dipping, en echelon blind thrust faults. Each earthquake was accompanied by uplift of a Quaternary anticline atop the fault, and each was accompanied by a vigorous aftershock sequence. Aftershocks were widely dispersed, and are seen above and below the thrust fault, as well as along the up-dip and down-dip projection of the main thrust fault. For the Coalinga and Kettleman Hills earthquakes, high-angle reverse faults in the core of the anticlines are evident in seismic reflection profiles, and many of these faults are associated with small aftershocks. The shallowest aftershocks extended to within 3-4 km of the ground surface. There is no compelling evidence for aftershocks associated with flexural slip faulting. No secondary surface rupture was found on any of the anticlines. In contrast, the 1983 Nu?ez rupture struck on a high-angle reverse fault 10 km west of the Coalinga epicenter, and over a 40-80-day period, up to 1 m of oblique surface slip occurred. The slip on this Holocene fault likely extended from the ground surface to a depth of 8-10 km. We argue that both the Nu?ez and Kettleman earthquakes were triggered by stresses imparted by the Coalinga mainshock, which was the largest of the four events in the sequence.

Persistent rupture terminations at a restraining bend from slip rates on the eastern Altyn Tagh fault

NASA Astrophysics Data System (ADS)

Elliott, A. J.; Oskin, M. E.; Liu-zeng, J.; Shao, Y.-X.

2018-05-01

Restraining double-bends along strike-slip faults inhibit or permit throughgoing ruptures depending on bend angle, length, and prior rupture history. Modeling predicts that for mature strike-slip faults in a regional stress regime characterized by simple shear, a restraining bend of >18° and >4 km length impedes propagating rupture. Indeed, natural evidence shows that the most recent rupture(s) of the Xorkoli section (90°-93°E) of the eastern Altyn Tagh fault (ATF) ended at large restraining bends. However, when multiple seismic cycles are considered in numerical dynamic rupture modeling, heterogeneous residual stresses enable some ruptures to propagate further, modulating whether the bends persistently serve as barriers. These models remain to be tested using observations of the cumulative effects of multiple earthquake ruptures. Here we investigate whether a large restraining double-bend on the ATF serves consistently as a barrier to rupture by measuring long-term slip rates around the terminus of its most recent surface rupture at the Aksay bend. Our results show a W-E decline in slip as the SATF enters the bend, as would be predicted from repeated rupture terminations there. Prior work demonstrated no Holocene slip on the central, most misoriented portion of the bend, while 19-79 m offsets suggest that multiple ruptures have occurred on the west side of the bend during the Holocene. Thus we conclude the gradient in the SATF's slip rate results from the repeated termination of earthquake ruptures there. However, a finite slip rate east of the bend represents the transmission of some slip, suggesting that a small fraction of ruptures may fully traverse or jump the double-bend. This agreement between natural observations of slip accumulation and multi-cycle models of fault rupture enables us to translate observed slip rates into insight about the dynamic rupture process of individual earthquakes as they encounter geometric complexities along faults.

Eastern Denali Fault surface trace map, eastern Alaska and Yukon, Canada

USGS Publications Warehouse

Bender, Adrian M.; Haeussler, Peter J.

2017-05-04

We map the 385-kilometer (km) long surface trace of the right-lateral, strike-slip Denali Fault between the Totschunda-Denali Fault intersection in Alaska, United States and the village of Haines Junction, Yukon, Canada. In Alaska, digital elevation models based on light detection and ranging and interferometric synthetic aperture radar data enabled our fault mapping at scales of 1:2,000 and 1:10,000, respectively. Lacking such resources in Yukon, we developed new structure-from-motion digital photogrammetry products from legacy aerial photos to map the fault surface trace at a scale of 1:10,000 east of the international border. The section of the fault that we map, referred to as the Eastern Denali Fault, did not rupture during the 2002 Denali Fault earthquake (moment magnitude 7.9). Seismologic, geodetic, and geomorphic evidence, along with a paleoseismic record of past ground-rupturing earthquakes, demonstrate Holocene and contemporary activity on the fault, however. This map of the Eastern Denali Fault surface trace complements other data sets by providing an openly accessible digital interpretation of the location, length, and continuity of the fault’s surface trace based on the accompanying digital topography dataset. Additionally, the digitized fault trace may provide geometric constraints useful for modeling earthquake scenarios and related seismic hazard.

Dynamic ruptures on faults of complex geometry: insights from numerical simulations, from large-scale curvature to small-scale fractal roughness

NASA Astrophysics Data System (ADS)

Ulrich, T.; Gabriel, A. A.

2016-12-01

The geometry of faults is subject to a large degree of uncertainty. As buried structures being not directly observable, their complex shapes may only be inferred from surface traces, if available, or through geophysical methods, such as reflection seismology. As a consequence, most studies aiming at assessing the potential hazard of faults rely on idealized fault models, based on observable large-scale features. Yet, real faults are known to be wavy at all scales, their geometric features presenting similar statistical properties from the micro to the regional scale. The influence of roughness on the earthquake rupture process is currently a driving topic in the computational seismology community. From the numerical point of view, rough faults problems are challenging problems that require optimized codes able to run efficiently on high-performance computing infrastructure and simultaneously handle complex geometries. Physically, simulated ruptures hosted by rough faults appear to be much closer to source models inverted from observation in terms of complexity. Incorporating fault geometry on all scales may thus be crucial to model realistic earthquake source processes and to estimate more accurately seismic hazard. In this study, we use the software package SeisSol, based on an ADER-Discontinuous Galerkin scheme, to run our numerical simulations. SeisSol allows solving the spontaneous dynamic earthquake rupture problem and the wave propagation problem with high-order accuracy in space and time efficiently on large-scale machines. In this study, the influence of fault roughness on dynamic rupture style (e.g. onset of supershear transition, rupture front coherence, propagation of self-healing pulses, etc) at different length scales is investigated by analyzing ruptures on faults of varying roughness spectral content. In particular, we investigate the existence of a minimum roughness length scale in terms of rupture inherent length scales below which the rupture ceases to be sensible. Finally, the effect of fault geometry on ground-motions, in the near-field, is considered. Our simulations feature a classical linear slip weakening on the fault and a viscoplastic constitutive model off the fault. The benefits of using a more elaborate fast velocity-weakening friction law will also be considered.

Influence of Fault Surface Heterogeneity on Apparent Frictional Strength, Slip Mode and Rupture Mode: Insights from Meter-Scale Rock Friction Experiments

NASA Astrophysics Data System (ADS)

Xu, S.; Fukuyama, E.; Yamashita, F.; Mizoguchi, K.; Takizawa, S.; Kawakata, H.

2016-12-01

Influence of fault zone heterogeneity on the behavior of fault motion has been studied in many aspects, such as strain partitioning, heat generation, slip mode, rupture mode, and effective friction law. However, a multi-scale investigation of fault behavior due to heterogeneity was difficult in nature, because of the limited access to natural fault zones at the seismogenic depth and the lack of in situ high-resolution observations. To overcome these difficulties, we study the behavior of a meter-scale synthetic fault made of Indian metagabbro during laboratory direct shear experiments, utilizing high-density arrays of strain gauges mounted close to the fault. We focus on two target experiments that are loaded under the same normal stress of 6.7 MPa and loading rate of 0.01 mm/s, but with different initial surface conditions. To change the surface condition, we applied a fast loading experiment under a rate of 1 mm/s between the two target experiments. It turned out the fast loading activated many foreshocks before the mainshock and caused a roaming of the mainshock nucleation site. These features were closely related to the re-distribution of the real contact area and surface wear, which together reflected a more heterogeneous state of the surface condition. During the first target experiment before the fast loading, the synthetic fault moved in a classic stick-slip fashion and the typical rupture mode was subshear within the range of the fault length. However, during the second target experiment, the synthetic fault inherited the heterogeneous features generated from the previous fast loading, showing a macroscopic creep-like behavior that actually consisted of many small stick-slip events. The apparent frictional strength increased while the recurrence interval and the stress drop decreased, compared to the levels seen in the first target experiment. The rupture mode became more complicated; supershear phases sometimes emerged but may only exist transiently. Their occurrence or termination showed a strong correlation with the local stress field characterized by short-range coherence. These observations highlight the role of surface heterogeneity in influencing fault motion, both macroscopically and locally, and have important implications for understanding the behavior of natural faults.

Faulting, Seismicity and Stress Interaction in the Salton Sea Region of Southern California

NASA Astrophysics Data System (ADS)

Kilb, D. L.; Brothers, D. S.; Lin, G.; Kent, G.; Newman, R. L.; Driscoll, N.

2009-12-01

The Salton Sea region in southern California provides an ideal location to study the relationship between transcurrent and extensional motion in the northern Gulf of California margin, allowing us to investigate the spatial and temporal interaction of faults in the area and better understand their kinematics. In this region, the San Andreas Fault (SAF) and Imperial Fault present two major transform faults separated by the Salton Sea transtensional domain. Earthquakes over magnitude 4 in this area almost always have associated aftershock sequences. Recent seismic reflection surveys in the Salton Sea reveal that the majority of faults under the southern Salton Sea trend ~N15°E, appear normal-dominant and have very minimal associated microseismicity. These normal faults rupture every 100-300 years in large earthquakes and most of the nearby microseismicity locates east of the mapped surface traces. For example, there is profuse microseismicity in the Brawley Seismic Zone (BSZ), which is coincident with the southern terminus of the SAF as it extends offshore into the Salton Sea. Earthquakes in the BSZ are dominantly swarm-like, occurring along short (<5 km) ~N45°E oriented sinistral and N35°W oriented dextral fault planes. This mapped seismicity makes a rung-and-ladder pattern. In an effort to reconcile differences between processes at the surface and those at seismogenic depths we integrate near surface fault kinematics, geometry and paleoseismic history with seismic data. We identify linear and planer trends in these data (20 near surface faults, >20,000 relocated earthquakes and >2,000 earthquake focal mechanisms) and when appropriate estimate the fault strike and dip using principal component analysis. With our more detailed image of the fault structure we assess how static stress changes imparted by magnitude ~6.0 ruptures along N15E oriented normal faults beneath the Salton Sea can modulate the stress field in the BSZ and along the SAF. These tests include exploring sensitivity of the results to parameter uncertainties. In general, we find rupture of the normal faults produces a butterfly pattern of static stress changes on the SAF with decreases along the southernmost portion below latitude 33.3±0.1 and increases on segments above these latitudes. Additionally, simulated ruptures on the normal faults predict optimally oriented sinistral faults that align with the “rungs” in the BSZ and optimally oriented dextral faults that are parallel to the SAF. Given these observations and results, we favor the scenario that normal faults beneath the Salton Sea accommodate most of the strain budget, rupturing as magnitude ~6.0-6.6 events every 100 years or so, and the consequent stress field generated within the relatively weak crust shapes the orientation of the short faults in the BSZ.

Estimation of source processes of the 2016 Kumamoto earthquakes from strong motion waveforms

NASA Astrophysics Data System (ADS)

Kubo, H.; Suzuki, W.; Aoi, S.; Sekiguchi, H.

2016-12-01

In this study, we estimated the source processes for two large events of the 2016 Kumamoto earthquakes (the M7.3 event at 1:25 JST on April 16, 2016 and the M6.5 event at 21:26 JST on April 14, 2016) from strong motion waveforms using multiple-time-window linear waveform inversion (Hartzell and Heaton 1983; Sekiguchi et al. 2000). Based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model was developed for the source-process analysis of the M7.3 event. The source model obtained for the M7.3 event with a seismic moment of 5.5 × 1019 Nm (Mw 7.1) had two significant ruptures. One rupture propagated toward the northeastern shallow region at 4 s after rupture initiation, and continued with large slips to approximately 16 s. This rupture caused a large slip region with a peak slip of 3.8 m that was located 10-30 km northeast of the hypocenter and reached the caldera of Mt. Aso. The contribution of the large slip region to the seismic waveforms was large at many stations. Another rupture propagated toward the surface from the hypocenter at 2-6 s, and then propagated toward the northeast along the near surface at 6-10 s. This rupture largely contributed to the seismic waveforms at the stations south of the fault and close to the hypocenter. A comparison with the results obtained using a single fault plane model demonstrate that the use of the curved fault model led to improved waveform fit at the stations south of the fault. The extent of the large near-surface slips in this source model for the M7.3 event is roughly consistent with the extent of the observed large surface ruptures. The source model obtained for the M6.5 event with a seismic moment of 1.7 × 1018 Nm (Mw 6.1) had large slips in the region around the hypocenter and in the shallow region north-northeast of the hypocenter, both of which had a maximum slip of 0.7 m. The rupture of the M6.5 event propagated from the former region to the latter region at 1-6 s after rupture initiation, which is expected to have caused the strong ground motions due to the forward directivity effect at KMMH16 and surroundings. The occurrence of the near-surface large slips in this source model for the M6.5 event is consistent with the appearance of small surface cracks, which were observed by some residents.

Interactive terrain visualization enables virtual field work during rapid scientific response to the 2010 Haiti earthquake

USGS Publications Warehouse

Cowgill, Eric; Bernardin, Tony S.; Oskin, Michael E.; Bowles, Christopher; Yikilmaz, M. Burak; Kreylos, Oliver; Elliott, Austin J.; Bishop, Scott; Gold, Ryan D.; Morelan, Alexander; Bawden, Gerald W.; Hamann, Bernd; Kellogg, Louise

2012-01-01

The moment magnitude (Mw) 7.0 12 January 2010 Haiti earthquake is the first major earthquake for which a large-footprint LiDAR (light detection and ranging) survey was acquired within several weeks of the event. Here, we describe the use of virtual reality data visualization to analyze massive amounts (67 GB on disk) of multiresolution terrain data during the rapid scientific response to a major natural disaster. In particular, we describe a method for conducting virtual field work using both desktop computers and a 4-sided, 22 m3 CAVE immersive virtual reality environment, along with KeckCAVES (Keck Center for Active Visualization in the Earth Sciences) software tools LiDAR Viewer, to analyze LiDAR point-cloud data, and Crusta, for 2.5 dimensional surficial geologic mapping on a bare-earth digital elevation model. This system enabled virtual field work that yielded remote observations of the topographic expression of active faulting within an ∼75-km-long section of the eastern Enriquillo–Plantain Garden fault spanning the 2010 epicenter. Virtual field observations indicated that the geomorphic evidence of active faulting and ancient surface rupture varies along strike. Landform offsets of 6–50 m along the Enriquillo–Plantain Garden fault east of the 2010 epicenter and closest to Port-au-Prince attest to repeated recent surface-rupturing earthquakes there. In the west, the fault trace is well defined by displaced landforms, but it is not as clear as in the east. The 2010 epicenter is within a transition zone between these sections that extends from Grand Goâve in the west to Fayette in the east. Within this transition, between L'Acul (lat 72°40′W) and the Rouillone River (lat 72°35′W), the Enriquillo–Plantain Garden fault is undefined along an embayed low-relief range front, with little evidence of recent surface rupture. Based on the geometry of the eastern and western faults that show evidence of recent surface rupture, we propose that the 2010 event occurred within a stepover that appears to have served as a long-lived boundary between rupture segments, explaining the lack of 2010 surface rupture. This study demonstrates how virtual reality–based data visualization has the potential to transform rapid scientific response by enabling virtual field studies and real-time interactive analysis of massive terrain data sets.

Rupture preparation process controlled by surface roughness on meter-scale laboratory fault

NASA Astrophysics Data System (ADS)

Yamashita, Futoshi; Fukuyama, Eiichi; Xu, Shiqing; Mizoguchi, Kazuo; Kawakata, Hironori; Takizawa, Shigeru

2018-05-01

We investigate the effect of fault surface roughness on rupture preparation characteristics using meter-scale metagabbro specimens. We repeatedly conducted the experiments with the same pair of rock specimens to make the fault surface rough. We obtained three experimental results under the same experimental conditions (6.7 MPa of normal stress and 0.01 mm/s of loading rate) but at different roughness conditions (smooth, moderately roughened, and heavily roughened). During each experiment, we observed many stick-slip events preceded by precursory slow slip. We investigated when and where slow slip initiated by using the strain gauge data processed by the Kalman filter algorithm. The observed rupture preparation processes on the smooth fault (i.e. the first experiment among the three) showed high repeatability of the spatiotemporal distributions of slow slip initiation. Local stress measurements revealed that slow slip initiated around the region where the ratio of shear to normal stress (τ/σ) was the highest as expected from finite element method (FEM) modeling. However, the exact location of slow slip initiation was where τ/σ became locally minimum, probably due to the frictional heterogeneity. In the experiment on the moderately roughened fault, some irregular events were observed, though the basic characteristics of other regular events were similar to those on the smooth fault. Local stress data revealed that the spatiotemporal characteristics of slow slip initiation and the resulting τ/σ drop for irregular events were different from those for regular ones even under similar stress conditions. On the heavily roughened fault, the location of slow slip initiation was not consistent with τ/σ anymore because of the highly heterogeneous static friction on the fault, which also decreased the repeatability of spatiotemporal distributions of slow slip initiation. These results suggest that fault surface roughness strongly controls the rupture preparation process, and generally increases its complexity with the degree of roughness.

Dynamic rupture models of subduction zone earthquakes with off-fault plasticity

NASA Astrophysics Data System (ADS)

Wollherr, S.; van Zelst, I.; Gabriel, A. A.; van Dinther, Y.; Madden, E. H.; Ulrich, T.

2017-12-01

Modeling tsunami-genesis based on purely elastic seafloor displacement typically underpredicts tsunami sizes. Dynamic rupture simulations allow to analyse whether plastic energy dissipation is a missing rheological component by capturing the complex interplay of the rupture front, emitted seismic waves and the free surface in the accretionary prism. Strike-slip models with off-fault plasticity suggest decreasing rupture speed and extensive plastic yielding mainly at shallow depths. For simplified subduction geometries inelastic deformation on the verge of Coulomb failure may enhance vertical displacement, which in turn favors the generation of large tsunamis (Ma, 2012). However, constraining appropriate initial conditions in terms of fault geometry, initial fault stress and strength remains challenging. Here, we present dynamic rupture models of subduction zones constrained by long-term seismo-thermo-mechanical modeling (STM) without any a priori assumption of regions of failure. The STM model provides self-consistent slab geometries, as well as stress and strength initial conditions which evolve in response to tectonic stresses, temperature, gravity, plasticity and pressure (van Dinther et al. 2013). Coseismic slip and coupled seismic wave propagation is modelled using the software package SeisSol (www.seissol.org), suited for complex fault zone structures and topography/bathymetry. SeisSol allows for local time-stepping, which drastically reduces the time-to-solution (Uphoff et al., 2017). This is particularly important in large-scale scenarios resolving small-scale features, such as the shallow angle between the megathrust fault and the free surface. Our dynamic rupture model uses a Drucker-Prager plastic yield criterion and accounts for thermal pressurization around the fault mimicking the effect of pore pressure changes due to frictional heating. We first analyze the influence of this rheology on rupture dynamics and tsunamigenic properties, i.e. seafloor displacement, in 2D. Finally, we use the same rheology in a large-scale 3D scenario of the 2004 Sumatra earthquake to shed light to the source process that caused the subsequent devastating tsunami.

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.

Potential for larger earthquakes in the East San Francisco Bay Area due to the direct connection between the Hayward and Calaveras Faults

NASA Astrophysics Data System (ADS)

Chaussard, E.; Bürgmann, R.; Fattahi, H.; Nadeau, R. M.; Taira, T.; Johnson, C. W.; Johanson, I.

2015-04-01

The Hayward and Calaveras Faults, two strike-slip faults of the San Andreas System located in the East San Francisco Bay Area, are commonly considered independent structures for seismic hazard assessment. We use Interferometric Synthetic Aperture RADAR to show that surface creep on the Hayward Fault continues 15 km farther south than previously known, revealing new potential for rupture and damage south of Fremont. The extended trace of the Hayward Fault, also illuminated by shallow repeating micro-earthquakes, documents a surface connection with the Calaveras Fault. At depths greater than 3-5 km, repeating micro-earthquakes located 10 km north of the surface connection highlight the 3-D wedge geometry of the junction. Our new model of the Hayward and Calaveras Faults argues that they should be treated as a single system with potential for earthquake ruptures generating events with magnitudes greater than 7, posing a higher seismic hazard to the East San Francisco Bay Area than previously considered.

Ground Surface Deformation in Unconsolidated Sediments Caused by Bedrock Fault Movements: Dip-Slip and Strike-Slip Fault Model Test and Field Survey

NASA Astrophysics Data System (ADS)

Ueta, K.; Tani, K.

2001-12-01

Sandbox experiments were performed to investigate ground surface deformation in unconsolidated sediments caused by dip-slip and strike-slip motion on bedrock faults. A 332.5 cm long, 200 cm high, and 40 cm wide sandbox was used in a dip-slip fault model test. In the strike-slip fault test, a 600 cm long, 250 cm wide, and 60 cm high sandbox and a 170 cm long, 25 cm wide, 15 cm high sandbox were used. Computerized X-ray tomography applied to the sandbox experiments made it possible to analyze the kinematic evolution, as well as the three-dimensional geometry, of the faults. The fault type, fault dip, fault displacement, thickness and density of sandpack and grain size of the sand were varied for different experiments. Field survey of active faults in Japan and California were also made to investigate the deformation of unconsolidated sediments overlying bedrock faults. A comparison of the experimental results with natural cases of active faults reveals the following: (1) In the case of dip-slip faulting, the shear bands are not shown as one linear plane but as en echelon pattern. Thicker and finer unconsolidated sediments produce more shear bands and clearer en echelon shear band patterns. (2) In the case of left-lateral strike-slip faulting, the deformation of the sand pack with increasing basement displacement is observed as follows. a) In three dimensions, the right-stepping shears that have a "cirque" / "shell" / "ship body" shape develop on both sides of the basement fault. The shears on one side of the basement fault join those on the other side, resulting in helicoidal shaped shear surfaces. Shears reach the surface of the sand near or above the basement fault and en echelon Riedel shears are observed at the surface of the sand. b) Right-stepping pressure ridges develop within the zone defined by the Riedel shears. c) Lower-angle shears generally branch off from the first Riedel shears. d) Right-stepping helicoidal shaped lower-angle shears offset Riedel shears and pressure ridges, and left-stepping and right-stepping pressure ridges are observed. d) With displacement concentrated on the central throughgoing fault zone, a "Zone of shear band" (ZSB) developed directly above the basement fault. The geometry of the ZSB shows a strong resemblance to linear ridge and trough geomorphology associated with active strike-slip faulting. (3) In the case of normal faulting, the location of the surface fault rupture is just above the bedrock faults, which have no relationship with the fault dip. On the other hand, the location of the surface rupture of the reverse fault has closely relationship with the fault dip. In the case of strike-slip faulting, the width of the deformation zone in dense sand is wider than that in loose sand. (4) The horizontal distance of surface rupture from the bedrock fault normalized by the height of sand mass (W/H) does not depend on the height of sand mass and grain size of sand. The values of W/H from the test agree well with those of earthquake faults. (5) The normalized base displacement required to propagate the shear rupture zone to the ground surface (D/H), in the case of normal faulting, is lower than those for reverse faulting and strike-slip faulting.

Active Fault Topography and Fault Outcrops in the Central Part of the Nukumi fault, the 1891 Nobi Earthquake Fault System, Central Japan

NASA Astrophysics Data System (ADS)

Sasaki, T.; Ueta, K.; Inoue, D.; Aoyagi, Y.; Yanagida, M.; Ichikawa, K.; Goto, N.

2010-12-01

It is important to evaluate the magnitude of earthquake caused by multiple active faults, taking into account the simultaneous effects. The simultaneity of adjacent active faults are often decided on the basis of geometric distances except for known these paleoseismic records. We have been studied the step area between the Nukumi fault and the Neodani fault, which appeared as consecutive ruptures in the 1891 Nobi earthquake, since 2009. The purpose of this study is to establish innovation in valuation technique of the simultaneity of adjacent active faults in addition to the paleoseismic record and the geometric distance. Geomorphological, geological and reconnaissance microearthquake surveys are concluded. The present work is intended to clarify the distribution of tectonic geomorphology along the Nukumi fault and the Neodani fault by high-resolution interpretations of airborne LiDAR DEM and aerial photograph, and the field survey of outcrops and location survey. The study area of this work is the southeastern Nukumi fault and the northwestern Neodani fault. We interpret DEM using shaded relief map and stereoscopic bird's-eye view made from 2m mesh DEM data which is obtained by airborne laser scanner of Kokusai Kogyo Co., Ltd. Aerial photographic survey is for confirmation of DEM interpretation using 1/16,000 scale photo. As a result of topographic survey, we found consecutive tectonic topography which is left lateral displacement of ridge and valley lines and reverse scarplets along the Nukumi fault and the Neodani fault . From Ogotani 2km southeastern of Nukumi pass which is located at the southeastern end of surface rupture along the Nukumi fault by previous study to Neooppa 9km southeastern of Nukumi pass, we can interpret left lateral topographies and small uphill-facing fault scarps on the terrace surface by detail DEM investigation. These topographies are unrecognized by aerial photographic survey because of heavy vegetation. We have found several new outcrops in this area where the surface ruptures of the 1891 Nobi earthquake have not been known. These outcrops have active fault which cut the layer of terrace deposit and slope deposit to the bottom of present soil layer in common. At the locality of Ogotani outcrop, the humic layer which age is from14th century to 15th century by 14C age dating is deformed by the active fault. The vertical displacement of the humic layer is 0.8-0.9m and the terrace deposit layer below the humic layer is ca. 1.3m. For this reason and the existence of fain grain deposit including AT tephra (28ka) in the footwall of the fault, this fault movement occurred more than once since the last glacial age. We conclude that the surface rupture of Nukumi fault in the 1891 Nobi earthquake is continuous to 9km southeast of Nukumi pass. In other words, these findings indicate that there is 10km parallel overlap zone between the surface rupture of the southeastern end of Nukumi fault and the northwestern end of Neodani fault.

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.

Structure of the San Andreas Fault Zone in the Salton Trough Region of Southern California: A Comparison with San Andreas Fault Structure in the Loma Prieta Area of Central California

NASA Astrophysics Data System (ADS)

Fuis, G. S.; Catchings, R.; Scheirer, D. S.; Goldman, M.; Zhang, E.; Bauer, K.

2016-12-01

The San Andreas fault (SAF) in the northern Salton Trough, or Coachella Valley, in southern California, appears non-vertical and non-planar. In cross section, it consists of a steeply dipping segment (75 deg dip NE) from the surface to 6- to 9-km depth, and a moderately dipping segment below 6- to 9-km depth (50-55 deg dip NE). It also appears to branch upward into a flower-like structure beginning below about 10-km depth. Images of the SAF zone in the Coachella Valley have been obtained from analysis of steep reflections, earthquakes, modeling of potential-field data, and P-wave tomography. Review of seismological and geodetic research on the 1989 M 6.9 Loma Prieta earthquake, in central California (e.g., U.S. Geological Survey Professional Paper 1550), shows several features of SAF zone structure similar to those seen in the northern Salton Trough. Aftershocks in the Loma Prieta epicentral area form two chief clusters, a tabular zone extending from 18- to 9-km depth and a complex cluster above 5-km depth. The deeper cluster has been interpreted to surround the chief rupture plane, which dips 65-70 deg SW. When double-difference earthquake locations are plotted, the shallower cluster contains tabular subclusters that appear to connect the main rupture with the surface traces of the Sargent and Berrocal faults. In addition, a diffuse cluster may surround a steep to vertical fault connecting the main rupture to the surface trace of the SAF. These interpreted fault connections from the main rupture to surface fault traces appear to define a flower-like structure, not unlike that seen above the moderately dipping segment of the SAF in the Coachella Valley. But importantly, the SAF, interpreted here to include the main rupture plane, appears segmented, as in the Coachella Valley, with a moderately dipping segment below 9-km depth and a steep to vertical segment above that depth. We hope to clarify fault-zone structure in the Loma Prieta area by reanalyzing active-source data collected after the earthquake for steep reflections.

SURFACE RUPTURE OF THE NORMAL SEISMIC FAULTS AND SLOPE FAILURES APPEARED IN APRIL 11th, 2011 FUKUSHIMA-PREFECTURE HAMADOORI EARTHQUAKE

NASA Astrophysics Data System (ADS)

Kazmi, Zaheer Abbas; Konagai, Kazuo; Kyokawa, Hiroyuki; Tetik, Cigdem

On April 11th, 2011, Iwaki region of Fukushima prefecture was jolted by Fukushima-Prefecture Hamadoori Earthquake. Surface ruptures were observed along causative Idosawa and Yunotake normal faults. In addition to numerous small slope failures, a coherent landslide and building structures of Tabito Junior High School, bisected by Idosawa Fault, were found along the causative faults. A precise digital elevation model of the coherent landslide was obtained through the ground and air-born LiDAR surveys. The measurements of perimeters of the gymnasium building and the swimming pool of Tabito Junior High School have shown that ground undergoes a slow and steady/continual deformation.

Reevaluation of 1935 M 7.0 earthquake fault, Miaoli-Taichung Area, western Taiwan: a DEM and field study

NASA Astrophysics Data System (ADS)

Lin, Y. N.; Chen, Y.; Ota, Y.

2003-12-01

A large earthquake (M 7.0) took place in Miaoli area, western Taiwan on April 21st, 1935. Right to its south is the 1999 Chi-Chi earthquake fault, indicating it is not only tectonically but seismically active. As the previous study, the study area is located in the mature zone of a tectonic collision that occurred between Philippine sea Plate and Eurasia continental Plate. The associated surface ruptures of 1935 earthquake daylighted Tungtsichiao Fault, a tear fault trending NE in the south and Chihhu Fault, a back thrust trending N-S in the north, but no ruptures occurred in between. Strike-slip component was identified by the horizontal offset observed along Tungtsichiao Fault; however, there are still disputes on the reported field evidence. Our purposes are (1) to identify the structural behaviors of these two faults, (2) to find out what the seismogenic structure is, and (3) to reconstruct the regional geology by information given by this earthquake. By DEM interpretation and field survey, we can clearly recognize a lot of the 1935 associated features. In the west of Chihhu Fault, a series of N-S higher terraces can be identified with eastward tilted surfaces and nearly 200 m relative height. Another lower terrace is also believed being created during the 1935 earthquake, showing an east-facing scarp with a height of ca. 1.5~2 m. Outcrop investigation reveals that the late-Miocene bedrock has been easterly thrusted over the Holocene conglomerates, indicating a west-dipping fault plane. The Tungtsichiao Fault cuts through a lateritic terrace at Holi, which is supposed developed in Pleistocene. The fault scarp is only discernible in the northeastern ending. Other noticeable features are the fault related antiforms that line up along the surface rupture. There is no outcrop to show the fault geometry among bedrocks. We re-interpret the northern Chihhu Fault as the back thrust generated from a main subsurface detachment, which may be the actual seismogenic fault. Due to the bend geometry normally existing between ramp and detachment, stress accumulated and earthquake happened right on it. The fault tip of this main thrust may be blind on land or break out offshore, which explains why no surface ruptures related to the main thrust were found.

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.

Rupture process of the M 7.9 Denali fault, Alaska, earthquake: Subevents, directivity, and scaling of high-frequency ground motions

USGS Publications Warehouse

Frankel, A.

2004-01-01

Displacement waveforms and high-frequency acceleration envelopes from stations at distances of 3-300 km were inverted to determine the source process of the M 7.9 Denali fault earthquake. Fitting the initial portion of the displacement waveforms indicates that the earthquake started with an oblique thrust subevent (subevent # 1) with an east-west-striking, north-dipping nodal plane consistent with the observed surface rupture on the Susitna Glacier fault. Inversion of the remainder of the waveforms (0.02-0.5 Hz) for moment release along the Denali and Totschunda faults shows that rupture proceeded eastward on the Denali fault, with two strike-slip subevents (numbers 2 and 3) centered about 90 and 210 km east of the hypocenter. Subevent 2 was located across from the station at PS 10 (Trans-Alaska Pipeline Pump Station #10) and was very localized in space and time. Subevent 3 extended from 160 to 230 km east of the hypocenter and had the largest moment of the subevents. Based on the timing between subevent 2 and the east end of subevent 3, an average rupture velocity of 3.5 km/sec, close to the shear wave velocity at the average rupture depth, was found. However, the portion of the rupture 130-220 km east of the epicenter appears to have an effective rupture velocity of about 5.0 km/ sec, which is supershear. These two subevents correspond approximately to areas of large surface offsets observed after the earthquake. Using waveforms of the M 6.7 Nenana Mountain earthquake as empirical Green's functions, the high-frequency (1-10 Hz) envelopes of the M 7.9 earthquake were inverted to determine the location of high-frequency energy release along the faults. The initial thrust subevent produced the largest high-frequency energy release per unit fault length. The high-frequency envelopes and acceleration spectra (>0.5 Hz) of the M 7.9 earthquake can be simulated by chaining together rupture zones of the M 6.7 earthquake over distances from 30 to 180 km east of the hypocenter. However, the inversion indicates that there was relatively little high-frequency energy generated along the 60-km portion of the Totschunda fault on the east end of the rupture.

Surface faulting and paleoseismic history of the 1932 Cedar Mountain earthquake area, west-central Nevada, and implications for modern tectonics of the Walker Lane

USGS Publications Warehouse

Bell, J.W.; DePolo, C.M.; Ramelli, A.R.; Sarna-Wojcicki, A. M.; Meyer, C.E.

1999-01-01

The 1932 Cedar Mountain earthquake (Ms 7.2) was one of the largest historical events in the Walker Lane region of western Nevada, and it produced a complicated strike-slip rupture pattern on multiple Quaternary faults distributed through three valleys. Primary, right-lateral surface ruptures occurred on north-striking faults in Monte Cristo Valley; small-scale lateral and normal offsets occurred in Stewart Valley; and secondary, normal faulting occurred on north-northeast-striking faults in the Gabbs Valley epicentral region. A reexamination of the surface ruptures provides new displacement and fault-zone data: maximum cumulative offset is estimated to be 2.7 m, and newly recognized faults extend the maximum width and end-to-end length of the rupture zone to 17 and 75 km, respectively. A detailed Quaternary allostratigraphic chronology based on regional alluvialgeomorphic relationships, tephrochronology, and radiocarbon dating provides a framework for interpreting the paleoseismic history of the fault zone. A late Wisconsinan alluvial-fan and piedmont unit containing a 32-36 ka tephra layer is a key stratigraphic datum for paleoseismic measurements. Exploratory trenching and radiocarbon dating of tectonic stratigraphy provide the first estimates for timing of late Quaternary faulting along the Cedar Mountain fault zone. Three trenches display evidence for six faulting events, including that in 1932, during the past 32-36 ka. Radiocarbon dating of organic soils interstratified with tectonically ponded silts establishes best-fit ages of the pre-1932 events at 4, 5,12,15, and 18 ka, each with ??2 ka uncertainties. On the basis of an estimated cumulative net slip of 6-12 m for the six faulting events, minimum and maximum late Quaternary slip rates are 0.2 and 0.7 mm/yr, respectively, and the preferred rate is 0.4-0.5 mm/yr. The average recurrence (interseismic) interval is 3600 yr. The relatively uniform thickness of the ponded deposits suggests that similar-size, characteristic rupture events may characterize late Quaternary slip on the zone. A comparison of event timing with the average late Quaternary recurrence interval indicates that slip has been largely regular (periodic) rather than temporally clustered. To account for the spatial separation of the primary surface faulting in Monte Cristo Valley from the epicenter and for a factor-of-two-to-three disparity between the instrumentally and geologically determined seismic moments associated with the earthquake, we hypothesize two alternative tectonic models containing undetected subevents. Either model would adequately account for the observed faulting on the basis of wrench-fault kinematics that may be associated with the Walker Lane. The 1932 Cedar Mountain earthquake is considered an important modern analogue for seismotectonic modeling and estimating seismic hazard in the Walker Lane region. In contrast to most other historical events in the Basin and Range province, the 1932 event did not occur along a major range-bounding fault, and no single, throughgoing basement structure can account for the observed rupture pattern. The 1932 faulting supports the concept that major earthquakes in the Basin and Range province can exhibit complicated distributive rupture patterns and that slip rate may not be a reliable criterion for modeling seismic hazard.

Near-Fault Strong Ground Motions during the 2016 Kumamoto, Japan, Earthquake

NASA Astrophysics Data System (ADS)

Iwata, T.; Asano, K.

2016-12-01

The 2016 Kumamoto mainshock (Mw7.0) produced a surface ruptured fault of about 20km long with maximum 2m offset, and identified as a surface ruptured event. Two strong motion records were observed near the surface ruptured fault at Mashiki town hall and Nishihara village hall. We investigated characteristics of those strong ground motions. As the acceleration records consisted of the baseline errors caused by nonzero initial acceleration and tilting of the accelerograph, we carefully removed the baseline errors (c.f. Chiu, 2001, Boore and Bommer, 2005) so as to obtain velocity and displacements. The observed permanent displacements were about 1.2m in horizontal direction and about 0.7m sinking in vertical direction at Mashiki town hall, and about 1.7m and 1.8m, respectively, at Nishihara village hall. Those permanent displacements almost coincide to results by GNSS and InSAR analysis (e.g., GSI, 2016). It takes about only 3 s to reach the permanent displacement. Somerville (2003) pointed out that ground motions from earthquakes producing large surface ruptures appeared to have systematically weaker ground motions than ground motions from earthquakes whose rupture were confined to the subsurface using the Ground Motion Prediction Equation (GMPE) for response spectra (Abrahamson and Silva, 1997). We calculated the response spectra of those records, compared them to the GMPE with the same manner and found two records were systematically larger than the expected from the GMPE in the period range of 0.3 s to 5 s. We need to re-consider the working hypothesis that the near-fault ground motions are weaker and to separate the near-fault and site effects on ground motions. Strong motions in the longer period range would be mainly caused by the near-fault (near-field term) effect.We used the acceleration data of the Kumamoto seismic intensity information network, provided by JMA.

Analysis and selection of magnitude relations for the Working Group on Utah Earthquake Probabilities

USGS Publications Warehouse

Duross, Christopher; Olig, Susan; Schwartz, David

2015-01-01

Prior to calculating time-independent and -dependent earthquake probabilities for faults in the Wasatch Front region, the Working Group on Utah Earthquake Probabilities (WGUEP) updated a seismic-source model for the region (Wong and others, 2014) and evaluated 19 historical regressions on earthquake magnitude (M). These regressions relate M to fault parameters for historical surface-faulting earthquakes, including linear fault length (e.g., surface-rupture length [SRL] or segment length), average displacement, maximum displacement, rupture area, seismic moment (Mo ), and slip rate. These regressions show that significant epistemic uncertainties complicate the determination of characteristic magnitude for fault sources in the Basin and Range Province (BRP). For example, we found that M estimates (as a function of SRL) span about 0.3–0.4 units (figure 1) owing to differences in the fault parameter used; age, quality, and size of historical earthquake databases; and fault type and region considered.

Paleoearthquakes and Eolian-dominated fault sedimentation along the Hubbell Spring fault zone near Albuquerque, New Mexico

USGS Publications Warehouse

Personius, S.F.; Mahan, S.A.

2003-01-01

The Hubbell Spring fault zone forms the modern eastern margin of the Rio Grande rift in the Albuquerque basin of north-central New Mexico. Knowledge of its seismic potential is important because the fault zone transects Kirtland Air Force Base/Sandia National Laboratories and underlies the southern Albuquerque metropolitan area. No earthquakes larger than ML 5.5 have been reported in the last 150 years in this region, so we excavated the first trench across this fault zone to determine its late Quaternary paleoseismic history. Our trench excavations revealed a complex, 16-m-wide fault zone overlain by four tapered blankets of mixed eolian sand and minor colluvium that we infer were deposited after four large-magnitude, surface-rupturing earthquakes. Although the first (oldest) rupture event is undated, we used luminescence (thermoluminescence and infrared-stimulated luminescence) ages to determine that the subsequent three rupture events occurred about 56 ?? 6, 29 ?? 3, and 12 ?? 1 ka. These ages yield recurrence intervals of 27 and 17 k.y. between events and an elapsed time of 12 k.y. since the latest surface-rupturing paleoearthquake. Slip rates are not well constrained, but our preferred average slip rate since rupture event 2 (post-56 ka) is 0.05 mm/yr, and interval slip rates between the last three events are 0.06 and 0.09 mm/yr, respectively. Vertical displacements of 1-2 m per event and probable rupture lengths of 34-43 km indicate probable paleoearthquake magnitudes (Ms or Mw) of 6.8-7.1. Future earthquakes of this size likely would cause strong ground motions in the Albuquerque metropolitan area.

Verification of SORD, and Application to the TeraShake Scenario

NASA Astrophysics Data System (ADS)

Ely, G. P.; Day, S.; Minster, J.

2007-12-01

The Support Operator Rupture Dynamics (SORD) code provides a highly scalable (up to billions of nodes) computational tool for modeling spontaneous rupture on a non-planar fault surface embedded in a heterogeneous medium with surface topography. SORD successfully performs the SCEC Rupture Dynamics Code Validation Project tests, and we have undertaken further dynamic rupture tests assessing the effects of distorted hexahedral meshes on code accuracy. We generate a family of distorted meshes by simple shearing (applied both parallel and normal to the fault plane) of an initially Cartesian mesh. For shearing normal to the fault, shearing angle was varied, up to a maximum of 73-degrees. For SCEC Validation Problem 3, grid-induced errors increase with mesh-shear angle, with the logarithm of error approximately proportional to angle over the range tested. At 73-degrees, RMS misfits are about 10% for peak slip rate, and 0.5% for both rupture time and total slip, indicating that the method--which up to now we have applied mainly to near-vertical strike-slip faulting-- also is capable of handling geometries appropriate to low-angle surface-rupturing thrust earthquakes. The SORD code was used to reexamine the TeraShake 2 dynamics simulations of a M7.7 earthquake on the southern San Andreas Fault. Relative to the original (Olsen et al, 2007) TeraShake 2 simulations, our spontaneous rupture models find decreased peak ground velocities in the Los Angles basin, principally due to a shallower eastward connecting basin chain in the SCEC Velocity Model Version 4 (used in our simulations) compared to Version 3 (used by Olsen et al.). This is partially offset by including the effects of surface topography (which was not included in the Olsen et al. models) in the simulation, which increases PGV at some basin sites by as much as a factor of two. Some non-basin sites showed comparable decreases in PGV. These predicted topographic effects are quite large, so it is important to quantify SORD accuracy in the presence of non-planar free surface geometry. We test the case of a semi-circular canyon to an incident P wave, and find close agreement with boundary element methods, for surface amplification at wavelengths comparable to the canyon width.

Pulverization provides a mechanism for the nucleation of earthquakes at low stress on strong faults

USGS Publications Warehouse

Felzer, Karen R.

2014-01-01

An earthquake occurs when rock that has been deformed under stress rebounds elastically along a fault plane (Gilbert, 1884; Reid, 1911), radiating seismic waves through the surrounding earth. Rupture along the entire fault surface does not spontaneously occur at the same time, however. Rather the rupture starts in one tiny area, the rupture nucleation zone, and spreads sequentially along the fault. Like a row of dominoes, one bit of rebounding fault triggers the next. This triggering is understood to occur because of the large dynamic stresses at the tip of an active seismic rupture. The importance of these crack tip stresses is a central question in earthquake physics. The crack tip stresses are minimally important, for example, in the time predictable earthquake model (Shimazaki and Nakata, 1980), which holds that prior to rupture stresses are comparable to fault strength in many locations on the future rupture plane, with bits of variation. The stress/strength ratio is highest at some point, which is where the earthquake nucleates. This model does not require any special conditions or processes at the nucleation site; the whole fault is essentially ready for rupture at the same time. The fault tip stresses ensure that the rupture occurs as a single rapid earthquake, but the fact that fault tip stresses are high is not particularly relevant since the stress at most points does not need to be raised by much. Under this model it should technically be possible to forecast earthquakes based on the stress-renewaql concept, or estimates of when the fault as a whole will reach the critical stress level, a practice used in official hazard mapping (Field, 2008). This model also indicates that physical precursors may be present and detectable, since stresses are unusually high over a significant area before a large earthquake.

The geometry of slip surfaces in the hanging-wall of the Sierra Madre fault, La-Canada, California

NASA Astrophysics Data System (ADS)

Dor, O.; Sammis, C. G.; Ben-Zion, Y.

2009-12-01

Fault-slip data from the granitic hanging-wall of the Sierra Madre fault near La-Canada, California, show a steeply dipping conjugate set of cm- to decimeter scale slip surfaces (115 data samples) with moderate to strong inclinations of slip vectors. These off-fault damage elements may be associated with Mohr-Coulomb slip in the stress field of a propagating earthquake rupture. At the microscale, we identified two dominant fracture orientations. The first appears both near and far from the fault and is compatible with Andersonian failure on the main fault. The second appears only within meters from the fault and may be associated with the formation of the slip surfaces. Characterization of damage fabric in the microscale suggests that in-situ failure of grains under tension with minimal strain immediately above the fault plane may be associated with an opening mode of rupture. We conclude that the architecture of the slip surfaces was developed during slip events over a finite displacement history with fairly stable faulting conditions, and that with continuing displacement, as the rock mass approached the surface, a dynamic opening mode could have led to the shattering of grains in the immediate vicinity of the slip zone.

Slip model of the 2015 Mw 7.8 Gorkha (Nepal) earthquake from inversions of ALOS-2 and GPS data

NASA Astrophysics Data System (ADS)

Wang, Kang; Fialko, Yuri

2015-09-01

We use surface deformation measurements including Interferometric Synthetic Aperture Radar data acquired by the ALOS-2 mission of the Japanese Aerospace Exploration Agency and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 Mw 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the Main Frontal Thrust (MFT) fault between 84.34°E and 86.19°E, the best fitting model suggests a dip angle of 7°. The moment calculated from the slip model is 6.08 × 1020 Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150 km long zone 50 to 100 km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of ˜ 5.8 m at a depth of ˜8 km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar-Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the east.

The 2002 Denali fault earthquake, Alaska: A large magnitude, slip-partitioned event

USGS Publications Warehouse

Eberhart-Phillips, D.; Haeussler, Peter J.; Freymueller, J.T.; Frankel, A.D.; Rubin, C.M.; Craw, P.; Ratchkovski, N.A.; Anderson, G.; Carver, G.A.; Crone, A.J.; Dawson, T.E.; Fletcher, H.; Hansen, R.; Harp, E.L.; Harris, R.A.; Hill, D.P.; Hreinsdottir, S.; Jibson, R.W.; Jones, L.M.; Kayen, R.; Keefer, D.K.; Larsen, C.F.; Moran, S.C.; Personius, S.F.; Plafker, G.; Sherrod, B.; Sieh, K.; Sitar, N.; Wallace, W.K.

2003-01-01

The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.

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.

Paleoseismic investigations in the Santa Cruz mountains, California: Implications for recurrence of large-magnitude earthquakes on the San Andreas fault

USGS Publications Warehouse

Schwartz, D.P.; Pantosti, D.; Okumura, K.; Powers, T.J.; Hamilton, J.C.

1998-01-01

Trenching, microgeomorphic mapping, and tree ring analysis provide information on timing of paleoearthquakes and behavior of the San Andreas fault in the Santa Cruz mountains. At the Grizzly Flat site alluvial units dated at 1640-1659 A.D., 1679-1894 A.D., 1668-1893 A.D., and the present ground surface are displaced by a single event. This was the 1906 surface rupture. Combined trench dates and tree ring analysis suggest that the penultimate event occurred in the mid-1600s, possibly in an interval as narrow as 1632-1659 A.D. There is no direct evidence in the trenches for the 1838 or 1865 earthquakes, which have been proposed as occurring on this part of the fault zone. In a minimum time of about 340 years only one large surface faulting event (1906) occurred at Grizzly Flat, in contrast to previous recurrence estimates of 95-110 years for the Santa Cruz mountains segment. Comparison with dates of the penultimate San Andreas earthquake at sites north of San Francisco suggests that the San Andreas fault between Point Arena and the Santa Cruz mountains may have failed either as a sequence of closely timed earthquakes on adjacent segments or as a single long rupture similar in length to the 1906 rupture around the mid-1600s. The 1906 coseismic geodetic slip and the late Holocene geologic slip rate on the San Francisco peninsula and southward are about 50-70% and 70% of their values north of San Francisco, respectively. The slip gradient along the 1906 rupture section of the San Andreas reflects partitioning of plate boundary slip onto the San Gregorio, Sargent, and other faults south of the Golden Gate. If a mid-1600s event ruptured the same section of the fault that failed in 1906, it supports the concept that long strike-slip faults can contain master rupture segments that repeat in both length and slip distribution. Recognition of a persistent slip rate gradient along the northern San Andreas fault and the concept of a master segment remove the requirement that lower slip sections of large events such as 1906 must fill in on a periodic basis with smaller and more frequent earthquakes.

Fault segmentation: New concepts from the Wasatch Fault Zone, Utah, USA

USGS Publications Warehouse

Duross, Christopher; Personius, Stephen F.; Crone, Anthony J.; Olig, Susan S.; Hylland, Michael D.; Lund, William R.; Schwartz, David P.

2016-01-01

The question of whether structural segment boundaries along multisegment normal faults such as the Wasatch fault zone (WFZ) act as persistent barriers to rupture is critical to seismic hazard analyses. We synthesized late Holocene paleoseismic data from 20 trench sites along the central WFZ to evaluate earthquake rupture length and fault segmentation. For the youngest (<3 ka) and best-constrained earthquakes, differences in earthquake timing across prominent primary segment boundaries, especially for the most recent earthquakes on the north-central WFZ, are consistent with segment-controlled ruptures. However, broadly constrained earthquake times, dissimilar event times along the segments, the presence of smaller-scale (subsegment) boundaries, and areas of complex faulting permit partial-segment and multisegment (e.g., spillover) ruptures that are shorter (~20–40 km) or longer (~60–100 km) than the primary segment lengths (35–59 km). We report a segmented WFZ model that includes 24 earthquakes since ~7 ka and yields mean estimates of recurrence (1.1–1.3 kyr) and vertical slip rate (1.3–2.0 mm/yr) for the segments. However, additional rupture scenarios that include segment boundary spatial uncertainties, floating earthquakes, and multisegment ruptures are necessary to fully address epistemic uncertainties in rupture length. We compare the central WFZ to paleoseismic and historical surface ruptures in the Basin and Range Province and central Italian Apennines and conclude that displacement profiles have limited value for assessing the persistence of segment boundaries but can aid in interpreting prehistoric spillover ruptures. Our comparison also suggests that the probabilities of shorter and longer ruptures on the WFZ need to be investigated.

Surface deformation associated with the 2013 Mw7.7 Balochistan earthquake: Geologic slip rates may significantly underestimate strain release

NASA Astrophysics Data System (ADS)

Gold, Ryan; Reitman, Nadine; Briggs, Richard; Barnhart, William; Hayes, Gavin

2015-04-01

The 24 September 2013 Mw7.7 Balochistan, Pakistan earthquake ruptured a ~200 km-long stretch of the 60° ± 15° northwest-dipping Hoshab fault in southern Pakistan. The earthquake is notable because it produced the second-largest lateral surface displacement observed for a continental strike-slip earthquake. Surface displacements and geodetic and teleseismic inversions indicate that peak slip occurred within the upper 0-3 km of the crust. To explore along-strike and fault-perpendicular surface deformation patterns, we remotely mapped the surface trace of the rupture and measured its surface deformation using high-resolution (0.5 m) pre- and post-event satellite imagery. Post-event images were collected 7-114 days following the earthquake, so our analysis captures the sum of both the coseismic and post-seismic (e.g., after slip) deformation. We document peak left-lateral offset of ~15 m using 289 near-field (±10 m from fault) laterally offset piercing points, such as streams, terrace risers, and roads. We characterize off-fault deformation by measuring the medium- (±200 m from fault) and far-field (±10 km from fault) displacement using manual (242 measurements) and automated image cross-correlation methods. Off-fault peak lateral displacement values (medium- and far-field) are ~16 m and commonly exceed the on-fault displacement magnitudes. Our observations suggest that coseismic surface displacement typically increases with distance away from the surface trace of the fault; however, the majority of surface displacement is within 100 m of the primary fault trace and is most localized on sections of the rupture exhibiting narrow (<5 m) zones of observable surface deformation. Furthermore, the near-field displacement measurements account for, on average, only 73% of the total coseismic displacement field and the pattern is highly heterogeneous. This analysis highlights the importance of identifying paleoseismic field study sites (e.g. trenches) that span fault sections with narrow deformation zones in order to capture the full deformation field. Our results imply that hazard analyses based on geologically-determined fault slip rates (e.g., near-field) should consider the significant and heterogeneous mismatch we document between on- and off-fault coseismic deformation.

The source parameters, surface deformation and tectonic setting of three recent earthquakes: thessalonki (Greece), tabas-e-golshan (iran) and carlisle (u.k.).

PubMed

King, G; Soufleris, C; Berberian, M

1981-03-01

Abstract- Three earthquakes have been studied. These are the Thessaloniki earthquake of 20th June 1978 (Ms = 6.4, Normal faulting), the Tabase-Golshan earthquake of 16th September 1978 (Ms = 7.7 Thrust faulting) and the Carlisle earth-quake of 26th December 1979 (Mb = 5.0, Thrust faulting). The techniques employed to determine source parameters included field studies of SUP face deformation, fault breaks, locations of locally recorded aftershocks and teleseismic studies including joint hypocentral location, first motion methods and waveform modelling. It is clear that these techniques applied together provide more information than the same methods used separately. The moment of the Thessaloniki earthquake determined teleseismically (Force moment 5.2 times 10(25) dyne cm. Geometric moment 1.72 times 10(8) m(3) ) is an order of magnitude greater than that determined using field data (surface ruptures and aftershock depths) (Force moment 4.5 times 10(24) dyne cm. Geometric moment 0.16 times 10(8) m(3) ). It is concluded that for this earthquake the surface rupture only partly reflects the processes on the main rupture plane. This view i s supported by a distribution of aftershocks and damage which extends well outside the region of ground rupture. However, the surface breaks consistently have the same slip vector direction as the fault plane solutions suggesting that they are in this respect related to to the main faulting and are not superficial slumping. Both field studies and waveform studies suggest a low stress drop which may explain the relatively little damage and loss of life as a result of the Thessaloniki earthquake. In contrast, the teleseismic moment of the Tabas-e-Golshan earthquake (Force moment 4.4 times 10(26) dyne cm. Geometric moment 1.5 times 10(9) m(3) ) is similar t o that determined from field studies (Force moment 10.2 times 10(26) dyne cm. Geometric moment 3.4 times 10(9) m(3) ) and the damage and after-shock distributions clearly relate to the surface faulting. It h a s also been observed that high aftershock activity appears beneath gaps in the surface rupture system. The Carlisle earthquake (Force moment 9 times 10(23) dyne cm. Geometric moment 3 times 10(6) m(3) ) produced no surface ruptures. However, dislocation model-ling suggests that surface deformation will be visible on a first order levelling line which passes very close t o the epicentre. A well controlled fault plane solution, the first in the British Isles, derived from an aftershock study shows north-south compression. All three studied earthquakes occurred along major faults which had been reactivated in geological times. The fault on which the Tabas-e-Golshan earthquake occurred could have been identified a s active from evidence of Quaternary motion and previous smaller earthquakes. However, there were no perceptible events in the 12 months preceeding the catastrophic earthquake. In both Thessaloniki and Carlisle, significant foreshocks did occur within 6 months prior to the main shock*

Intra-caldera active fault: An example from the Mw 7.0 2016 Kumamoto, Japan, earthquake

NASA Astrophysics Data System (ADS)

Toda, S.; Murakami, T.; Takahashi, N.

2017-12-01

A NE-trending 30-km-long surface rupture with up to 2.4 m dextral slip emerged during the Mw=7.0 16 April 2016 Kumamoto earthquake along the previously mapped Futagawa and northern Hinagu fault systems. The 5-km-long portion of the northeast rupture end, which was previously unidentified, crossed somma and extended to the 20-km-diameter Aso Caldera, one of the major active volcanoes, central Kyushu. We here explore geologic exposures of interplays of active faulting and active volcanism, and then argue the Futagawa fault system has been influenced by the ring fault system associated with the caldera forming gigantic eruptions since 270 ka, last of which occurred 90 ka ejecting a huge amount of ignimbrite. To understand the interplays, together with the mapping of the 2016 rupture, we employed an UAV to capture numerous photos of the exposures along the canyon and developed 3D orthochromatic topographic model using PhotoScan. One-hundred-meter-deep Kurokawa River canyon by the Aso Caldera rim exposes two lava flow units of 50 ka vertically offset by 10 m by the Futatawa fault system. Reconstructions of the collapsed bridges across the Kurokawa River also reveal cross sections of a 30-meter-high tectonic bulge and 10-m-scale negative flower structure deformed by the frequent fault movements. We speculate two fault developing models across the Aso Caldera. One is that the NE edge of the Futagawa fault system was cut and reset by the caldera forming ring fault, which indicates the 3-km-long rupture extent within the Aso Caldera would be a product of the fault growth since the last Aso-4 eruption of 90 ka. It enables us to estimate the 33 mm/yr of the fault propagation speed. An alternative model is that subsurface rupture of the Kumamoto earthquake extended further to the NE rim, the other side of the caldera edge, which is partially supported by the geodetic and seismic inversions. With respect to the model, the clear surface rupture of the 2016 Kumamoto earthquake was invisible due to the lava and fallout layers younger than 4ka that probably experienced only one or two events and do not have the pre-existing weak and sharp fault plane yet.

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.

Fault Rupture Model of the 2016 Gyeongju, South Korea, Earthquake and Its Implication for the Underground Fault System

NASA Astrophysics Data System (ADS)

Uchide, Takahiko; Song, Seok Goo

2018-03-01

The 2016 Gyeongju earthquake (ML 5.8) was the largest instrumentally recorded inland event in South Korea. It occurred in the southeast of the Korean Peninsula and was preceded by a large ML 5.1 foreshock. The aftershock seismicity data indicate that these earthquakes occurred on two closely collocated parallel faults that are oblique to the surface trace of the Yangsan fault. We investigate the rupture properties of these earthquakes using finite-fault slip inversion analyses. The obtained models indicate that the ruptures propagated NNE-ward and SSW-ward for the main shock and the large foreshock, respectively. This indicates that these earthquakes occurred on right-step faults and were initiated around a fault jog. The stress drops were up to 62 and 43 MPa for the main shock and the largest foreshock, respectively. These high stress drops imply high strength excess, which may be overcome by the stress concentration around the fault jog.

Modeling 3D Dynamic Rupture on Arbitrarily-Shaped faults by Boundary-Conforming Finite Difference Method

NASA Astrophysics Data System (ADS)

Zhu, D.; Zhu, H.; Luo, Y.; Chen, X.

2008-12-01

We use a new finite difference method (FDM) and the slip-weakening law to model the rupture dynamics of a non-planar fault embedded in a 3-D elastic media with free surface. The new FDM, based on boundary- conforming grid, sets up the mapping equations between the curvilinear coordinate and the Cartesian coordinate and transforms irregular physical space to regular computational space; it also employs a higher- order non-staggered DRP/opt MacCormack scheme which is of low dispersion and low dissipation so that the high accuracy and stability of our rupture modeling are guaranteed. Compared with the previous methods, not only we can compute the spontaneous rupture of an arbitrarily shaped fault, but also can model the influence of the surface topography on the rupture process of earthquake. In order to verify the feasibility of this method, we compared our results and other previous results, and found out they matched perfectly. Thanks to the boundary-conforming FDM, problems such as dynamic rupture with arbitrary dip, strike and rake over an arbitrary curved plane can be handled; and supershear or subshear rupture can be simulated with different parameters such as the initial stresses and the critical slip displacement Dc. Besides, our rupture modeling is economical to be implemented owing to its high efficiency and does not suffer from displacement leakage. With the help of inversion data of rupture by field observations, this method is convenient to model rupture processes and seismograms of natural earthquakes.

Historic surface slip along the San Andreas Fault near Parkfield, California

USGS Publications Warehouse

Lienkaemper, J.J.; Prescott, W.H.

1989-01-01

The Parkfield Earthquake Prediction Experiment is focusing close attention on the 44-km-long section of the San Andreas fault that last ruptured seismically in 1966 (Ms 6.0). The 20-km-long central segment of the 1966 Parkfield rupture, extending from the mainshock epicenter at Middle Mountain southeastward to Gold Hill, forms a 1- to 2-km salient northeastward away from the dominant N40??W strike. Following the 1966 earthquake afterslip, aseismic slip has been nearly constant. Moderate Parkfield earthquakes have recurred on average every 21 years since 1857, when a great earthquake (M ~ 8) ruptured at least as far north as the southern Parkfield segment. Many measurements of slip have been made near Parkfield since 1966. Nevertheless, much of the history of surface slip remained uncertain, especially the total amount associated with the 1966 event. In 1985 we measured accumulated slip on the four oldest cultural features offset by the fault along the 1966 Parkfield rupture segment. -from Authors

Surface Ruptures and Building Damage of the 2003 Bam, Iran, Earthquake Mapped by Satellite Synthetic Aperture Radar Interferometric Correlation

NASA Technical Reports Server (NTRS)

Fielding, Eric J.; Talebian, M.; Rosen, P. A.; Nazari, H.; Jackson, J. A.; Ghorashi, M.; Walker, R.

2005-01-01

We use the interferometric correlation from Envisat synthetic aperture radar (SAR) images to map the details of the surface ruptures related to the 26 December 2003 earthquake that devastated Bam, Iran. The main strike-slip fault rupture south of the city of Bam has a series of four segments with left steps shown by a narrow line of low correlation in the coseismic interferogram. This also has a clear expression in the field because of the net extension across the fault. Just south of the city limits, the surface strain becomes distributed over a width of about 500 m, probably because of a thicker layer of soft sedimentary material.

The Elizabeth Lake paleoseismic site: Rupture pattern constraints for the past ~800 years for the Mojave section of the south-central San Andreas Fault

USGS Publications Warehouse

Bemis, Sean; Scharer, Katherine M.; Dolan, James F.; Rhodes, Ed

2016-01-01

The southern San Andreas Fault in California has hosted two historic surface-rupturing earthquakes, the ~M7 1812 Wrightwood earthquake and the ~M7.9 1857 Fort Tejon earthquake (e.g., Sieh, 1978; Jacoby et al., 1988). Numerous paleoseismic studies have established chronologies of historic and prehistoric earthquakes at sites along the full length of the 1857 rupture (e.g., Sieh, 1978; Scharer et al., 2014). These studies provide an unparalleled opportunity to examine patterns of recent ruptures; however, at least two significant spatial gaps in high-quality paleoseismic sites remain. At ~100 km long each, these gaps contribute up to 100 km of uncertainty to paleo-rupture lengths and could also permit a surface rupture from an earthquake up to ~M7.2 to go undetected [using scaling relationships of Wells and Coppersmith (1994)]. Given the known occurrence of an ~M7 earthquake on this portion of the SAF (1812), it is critical to fill these gaps in order to better constrain paleo-rupture lengths and to increase the probability of capturing the full spatial record of surface rupturing earthquakes.   In this study, we target a new site within the 100 km long stretch of the San Andreas Fault between the Frazier Mountain and Pallett Creek paleoseismic sites (Figure 1), near Elizabeth Lake, California. Prior excavations at the site during 1998-1999 encountered promising stratigraphy but these studies were hindered by shallow groundwater throughout the site. We began our current phase of investigations in 2012, targeting the northwestern end of a 40 x 350 m fault-parallel depression that defines the site (Figure 2). Subsequent investigations in 2013 and 2014 focused on the southeastern end of the depression where the fault trace is constrained between topographic highs and is proximal to an active drainage. In total, our paleoseismic investigations consist of 10 fault-perpendicular trenches that cross the depression (Figure 2) and expose a >2000 year depositional record. These trenches reveal that the thickest section of young stratigraphy occurs at the southeastern end of the site where the fault zone projects through an area of relatively continuous sediment accumulation from a northeast-flowing drainage. This portion of the site contains a 3-m-wide pop-up structure within the fault zone that separates alternating alluvial and paludal deposits south of the fault zone from a thick organic-rich loam on the north side of the fault zone. Faults, fissures, and tilted blocks provide evidence for 4 to 5 paleoearthquakes since ca. 1250 A.D. Radiocarbon dating established that the site has a significant component of detrital charcoal producing an age spread of up to 500 years. To supplement our age chronology we incorporated ages from collections of micro-scale organic fractions and post-IR infrared stimulated luminescence dating in order to better estimate true layer ages.

The Gabbs Valley, Nevada, geothermal prospect: Exploring for a potential blind geothermal resource

NASA Astrophysics Data System (ADS)

Payne, J.; Bell, J. W.; Calvin, W. M.

2012-12-01

The Gabbs Valley prospect in west-central Nevada is a potential blind geothermal resource system. Possible structural controls on this system were investigated using high-resolution LiDAR, low sun-angle aerial (LSA) photography, exploratory fault trenching and a shallow temperature survey. Active Holocene faults have previously been identified at 37 geothermal systems with indication of temperatures greater than 100° C in the western Nevada region. Active fault controls in Gabbs Valley include both Holocene and historical structures. Two historical earthquakes occurring in 1932 and 1954 have overlapping surface rupture patterns in Gabbs Valley. Three active fault systems identified through LSA and LiDAR mapping have characteristics of Basin and Range normal faulting and Walker Lane oblique dextral faulting. The East Monte Cristo Mountains fault zone is an 8.5 km long continuous NNE striking, discrete fault with roughly 0.5 m right-normal historic motion and 3 m vertical Quaternary separation. The Phillips Wash fault zone is an 8.2 km long distributed fault system striking NE to N, with Quaternary fault scarps of 1-3 m vertical separation and a 500 m wide graben adjacent to the Cobble Cuesta anticline. This fault displays ponded drainages, an offset terrace riser and right stepping en echelon fault patterns suggestive of left lateral offset, and fault trenching exposed non-matching stratigraphy typical of a significant component of lateral offset. The unnamed faults of Gabbs Valley are a 10.6 km long system of normal faults striking NNE and Quaternary scarps are up to 4 m high. These normal faults largely do not have historic surface rupture, but a small segment of 1932 rupture has been identified. A shallow (2 m deep) temperature survey of 80 points covering roughly 65 square kilometers was completed. Data were collected over approximately 2 months, and continual base station temperature measurements were used to seasonally correct temperature measurements. A 2.5 km long temperature anomaly greater than 3° C above background temperatures forms west-northwest trending zone between terminations of the Phillips Wash fault zone and unnamed faults of Gabbs Valley to the south. Rupture segments of two young active faults bracket the temperature anomaly. The temperature anomaly may be due to several possible causes. 1. Increases in stress near the rupture segments or tip-lines of these faults, or where multiple fault splays exist, can increase fault permeability. The un-ruptured segments of these faults may be controlling the location of the Gabbs Valley thermal anomaly between ruptured segments of the 1932 Cedar Mountain and 1954 Fairview Peak earthquakes. 2. Numerous unnamed normal faults may interact and the hanging wall of these faults is hosting the thermal anomaly. The size and extent of the anomaly may be due to its proximity to a flat playa and not the direct location of the shallow heat anomaly. 3. The linear northwest nature of the thermal anomaly may reflect a hydrologic barrier in the subsurface controlling where heated fluids rise. A concealed NW- striking fault is possible, but has not been identified in previous studies or in the LiDAR or LSA fault mapping.

Imaging the M7.9 Denali Fault Earthquake 2002 rupture at the Delta River using LiDAR, RADAR, and SASW Surface Wave Geophysics

NASA Astrophysics Data System (ADS)

Kayen, R.; Barnhardt, W.; Carkin, B.; Collins, B. D.; Grossman, E. E.; Minasian, D.; Thompson, E.

2004-12-01

The Mw 7.9 Denali fault earthquake of 3 November 2002 resulted in approximately 5.5 meters of right-lateral offset and sub-meter (0.6m average) up-to-the north vertical displacement of alluvial deposits of the Delta River. We characterize the surface rupture and shallow fault structure of the Denali fault zone at the Delta River in order to better understand these most recent displacements and to estimate the total vertical offset of alluvium above glacially scoured bedrock. To analyze deformations along the fault-trace, we performed tripod-mounted ground-based LiDAR surveys, and Spectral analysis of Surface Wave (SASW) and Ground Penetrating RADAR (GPR) geophysical investigations. These studies were performed between the Trans-Alaska Pipeline (TAPS) corridor on the terrace deposits of the eastern flanks of the Delta River valley and the steeply sloping bedrock surface on the western side of the river. To produce digital terrain models (DTM) of the surface break we used a Riegl Z210i Laser-scanner to image eight independent LiDAR scans, and ISite3D modeling software to merge these scans into three DTM surfaces. We find that using a rotating scanning-laser allows us to produce ultra-high resolution quantitative DTMs for geomorphic analysis that can be used to resolve features and detect topographic changes on a fine-scale (0.9-2.5cm). Local geo-referencing control points are established using fixed auto reflectors. The near subsurface alluvium was imaged using reflection-based (GPR). A suite of parallel and orthogonal GPR reflection lines were measured to develop block models of the surface rupture at two locations. Radar imagery clearly delineates a plane of chaotic reflectors across the rupture zone. To characterize the depth of alluvium over bedrock on either side of the fault, we used the spectral analysis of surface waves (SASW) approach to invert the near-surface shear wave velocity profile. An Alyeska Co. Catepillar D9N track-mounted dozer was used as a high-energy random-wave source for the SASW test. This source allowed us to profile to depths in excess of 200 meters on either side of the fault. We found the combination of LiDAR and GPR allows us to analyze the surface and near-surface characteristics of a complex oblique rupture across the braid bars of the Delta River. SASW-based shear wave velocity profiles on either side of the fault indicate total up-to-the north uplift on the Denali fault of between 60-90 meters since Pleistocene (?) deglaciation. This investigation is the product of a collaborative research and development agreement between the Alyeska Pipeline Services Company, Pacific Gas and Electric Company and the U.S. Geological Survey.

Co-seismic strike-slip surface rupture and displacement produced by the 2010 Mw 6.9 Yushu earthquake, China, and implications for Tibetan tectonics

NASA Astrophysics Data System (ADS)

Lin, A.; Rao, G.; Jia, D.; Wu, X.; Yan, B.; Ren, Z.

2010-12-01

The magnitude (Mw) 6.9 (Ms 7.1) Yushu earthquake occurred on 14 April 2010 in the Yushu area, central Tibetan Plateau, killing approximately 3000 people (including 270 missing) and causing widespread damage in the high mountain regions of the central Tibetan Plateau. The Yushu earthquake is comparable with the 1997 Mw 7.6 Manyi earthquake, the 2001 Mw 7.8 Kunlun earthquake, and the 2008 Mw 7.9 Wenchuan earthquake, which all occurred in the northern and eastern Tibetan Plateau, in terms of their magnitude and seismotectonic environment, related to the eastward extrusion of the Tibetan Plateau in response to continental collision between the Indian and Eurasian plates. Although some prompt reports related to ground deformation and the focal mechanism were published in the Chinese literature soon after the Yushu earthquake, there are scarce data related to the nature of co-seismic strike-slip rupturing structures and displacement distributions because the co-seismic surface ruptures were produced mainly in remote, high mountain regions of the Tibetan Plateau (average elevation >4000 m) and roads to the epicentral area were damaged, which made it difficult to gain access to the area and to undertake fieldwork immediately after the earthquake. Field investigations reveal that the earthquake produced a 33-km-long surface rupture zone, with dominantly left-lateral strike-slip along the Yushu Fault of the pre-existing strike-slip Ganzi-Yushu Fault Zone. The co-seismic surface ruptures are characterized by discontinuous shear faults, right-stepping en echelon tensional cracks, and left-stepping mole track structures that indicate a left-lateral strike-slip shear sense for the seismic fault. Field measurements indicate co-seismic left-lateral strike-slip displacements of approximately 0.3-3.2 m (typically 1-2 m), accompanied by a minor vertical component of <0.6 m. The present results show that (i) the Yushu earthquake occurred upon the pre-existing active Ganzi-Yushu Fault Zone, which controlled the spatial distribution of co-seismic surface ruptures and displacements; (ii) the left-lateral strike-slip motion indicates that the Ganzi-Yushu Fault Zone partitions deformation into eastward extrusion and northeastward shortening of the central Tibetan Plateau to accommodate the continuing penetration of the Indian plate into the Eurasian plate. Our findings confirm that present-day strain energy related to continental deformation in the central Tibetan Plateau, generated by collision between the Indian and Eurasian plates, is mainly released by strike-slip faulting along active strike-slip faults, and that the Ganzi-Yushu Fault Zone plays an important role in this crustal deformation, generating strong earthquakes that help to release the accumulated strain energy.

Character and Significance of Surface Rupture Near the Intersection of the Denali and Totschunda Faults, M7.9 Denali Fault Earthquake, Alaska, November 3, 2002

NASA Astrophysics Data System (ADS)

Wallace, W. K.; Sherrod, B. L.; Dawson, T. E.

2002-12-01

Preliminary observations suggest that right-lateral strike-slip on the Denali fault is transferred to the Totschunda fault via an extensional bend in the Little Tok River valley. Most of the surface rupture during the Denali fault earthquake was along an east- to east-southeast striking, gently curved segment of the Denali fault. However, in the Little Tok River valley, rupture transferred to the southeast-striking Totschunda fault and continued to the southeast for another 75 km. West of the Little Tok River valley, 5-7 m of right-lateral slip and up to 2 m of vertical offset occurred on the main strand of the Denali fault, but no apparent displacement occurred on the Denali fault east of the valley. Rupture west of the intersection also occurred on multiple discontinuous strands parallel to and south of the main strand of the Denali fault. In the Little Tok River valley, the northern part of the Totschunda fault system consists of multiple discontinuous southeast-striking strands that are connected locally by south-striking stepover faults. Faults of the northern Totschunda system display 0-2.5 m of right-lateral slip and 0-2.75 m of vertical offset, with the largest vertical offset on a dominantly extensional stepover fault. The strands of the Totschunda system converge southeastward to a single strand that had up to 2 m of slip. Complex and discontinuous faulting may reflect in part the immaturity of the northern Totschunda system, which is known to be younger and have much less total slip than the Denali. The Totschunda fault forms an extensional bend relative to the dominantly right-lateral Denali fault to the west. The fault geometry and displacements at the intersection suggest that slip on the Denali fault during the earthquake was accommodated largely by extension in the northern Totschunda fault system, allowing a significant decrease in strike-slip relative to the Denali fault. Strands to the southwest in the area of the bend may represent shortcut faults that have reduced the curvature at the intersection of the two fault systems.

Geologic and paleoseismic study of the Lavic Lake fault at Lavic Lake Playa, Mojave Desert, Southern California

USGS Publications Warehouse

Rymer, M.J.; Seitz, G.G.; Weaver, K.D.; Orgil, A.; Faneros, G.; Hamilton, J.C.; Goetz, C.

2002-01-01

Paleoseismic investigations of the Lavic Lake fault at Lavic Lake playa place constraints on the timing of a possible earlier earthquake along the 1999 Hector Mine rupture trace and reveal evidence of the timing of the penultimate earthquake on a strand of the Lavic Lake fault that did not rupture in 1999. Three of our four trenches, trenches A, B, and C, were excavated across the 1999 Hector Mine rupture; a fourth trench, D, was excavated across a vegetation lineament that had only minor slip at its southern end in 1999. Trenches A-C exposed strata that are broken only by the 1999 rupture; trench D exposed horizontal bedding that is locally warped and offset by faults. Stratigraphic evidence for the timing of an earlier earthquake along the 1999 rupture across Lavic Lake playa was not exposed. Thus, an earlier event, if there was one along that rupture trace, predates the lowest stratigraphic level exposed in our trenches. Radiocarbon dating of strata near the bottom of trenches constrains a possible earlier event to some time earlier than about 4950 B.C. Buried faults revealed in trench D are below a vegetation lineament at the ground surface. A depositional contact about 80 cm below the ground surface acts as the upward termination of fault breaks in trench D. Thus, this contact may be the event horizon for a surface-rupturing earthquake prior to 1999-the penultimate earthquake on the Lavic Lake fault. Radiocarbon ages of detrital charcoal samples from immediately below the event horizon indicate that the earthquake associated with the faulting occurred later than A.D. 260. An approximately 1300-year age difference between two samples at about the same stratigraphic level below the event horizon suggests the potential for a long residence time of detrital charcoal in the area. Coupled with a lack of bioturbation that could introduce young organic material into the stratigraphic section, the charcoal ages provide only a maximum bounding age; thus, the recognized event may be younger. There is abundant, subtle evidence for pre-1999 activity of the Lavic Lake fault in the playa area, even though the fault was not mapped near the playa prior to the Hector Mine earthquake. The most notable indicators for long-term presence of the fault are pronounced, persistent vegetation lineaments and uplifted basalt exposures. Primary and secondary slip occurred in 1999 on two southern vegetation lineaments, and minor slip locally formed on a northern lineament; trench exposures across the northern vegetation lineament revealed the post-A.D. 260 earthquake, and a geomorphic trough extends northward into alluvial fan deposits in line with this lineament. The presence of two basalt exposures in Lavic lake playa indicates the presence of persistent compressional steps and uplift along the fault. Fault-line scarps are additional geomorphic markers of repeated slip events in basalt exposures.

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.

Mapping of the surface rupture induced by the M 7.3 Kumamoto Earthquake along the Eastern segment of Futagawa fault using image correlation techniques

NASA Astrophysics Data System (ADS)

Ekhtari, N.; Glennie, C. L.; Fielding, E. J.; Liang, C.

2016-12-01

Near field surface deformation is vital to understanding the shallow fault physics of earthquakes but near-field deformation measurements are often sparse or not reliable. In this study, we use the Co-seismic Image Correlation (COSI-Corr) technique to map the near-field surface deformation caused by the M 7.3 April 16, 2016 Kumamoto Earthquake, Kyushu, Japan. The surface rupture around the Eastern segment of Futagawa fault is mapped using a pair of panchromatic 1.5 meter resolution SPOT 7 images. These images were acquired on January 16 and April 29, 2016 (3 months before and 13 days after the earthquake respectively) with close to nadir (less than 1.5 degree off nadir) viewing angle. The two images are ortho-rectified using SRTM Digital Elevation Model and further co-registered using tie points far away from the rupture field. Then the COSI-Corr technique is utilized to produce an estimated surface displacement map, and a horizontal displacement vector field is calculated which supplies a seamless estimate of near field displacement measurements along the Eastern segment of the Futagawa fault. The COSI-Corr estimated displacements are then compared to other existing displacement observations from InSAR, GPS and field observations.

The 1999 Hector Mine Earthquake, Southern California: Vector Near-Field Displacements from ERS InSAR

NASA Technical Reports Server (NTRS)

Sandwell, David T.; Sichoix, Lydie; Smith, Bridget

2002-01-01

Two components of fault slip are uniquely determined from two line-of-sight (LOS) radar interferograms by assuming that the fault-normal component of displacement is zero. We use this approach with ascending and descending interferograms from the ERS satellites to estimate surface slip along the Hector Mine earthquake rupture. The LOS displacement is determined by visually counting fringes to within 1 km of the outboard ruptures. These LOS estimates and uncertainties are then transformed into strike- and dip-slip estimates and uncertainties; the transformation is singular for a N-S oriented fault and optimal for an E-W oriented fault. In contrast to our previous strike-slip estimates, which were based only on a descending interferogram, we now find good agreement with the geological measurements, except at the ends of the rupture. The ascending interferogram reveals significant west-sidedown dip-slip (approximately 1.0 m) which reduces the strike-slip estimates by 1 to 2 m, especially along the northern half of the rupture. A spike in the strike-slip displacement of 6 m is observed in central part of the rupture. This large offset is confirmed by subpixel cross correlation of features in the before and after amplitude images. In addition to strike slip and dip slip, we identify uplift and subsidence along the fault, related to the restraining and releasing bends in the fault trace, respectively. Our main conclusion is that at least two look directions are required for accurate estimates of surface slip even along a pure strike-slip fault. Models and results based only on a single look direction could have major errors. Our new estimates of strike slip and dip slip along the rupture provide a boundary condition for dislocation modeling. A simple model, which has uniform slip to a depth of 12 km, shows good agreement with the observed ascending and descending interferograms.

Seismotectonics of the Loma Prieta, California, region determined from three-dimensional Vp, Vp/Vs, and seismicity

USGS Publications Warehouse

Eberhart-Phillips, D.; Michael, A.J.

1998-01-01

Three-dimensional Vp and Vp/Vs velocity models for the Loma Prieta region were developed from the inversion of local travel time data (21,925 P arrivals and 1,116 S arrivals) from earthquakes, refraction shots, and blasts recorded on 1700 stations from the Northern California Seismic Network and numerous portable seismograph deployments. The velocity and density models and microearthquake hypocenters reveal a complex structure that includes a San Andreas fault extending to the base of the seismogenic layer. A body with high Vp extends the length of the rupture and fills the 5 km wide volume between the Loma Prieta mainshock rupture and the San Andreas and Sargent faults. We suggest that this body controls both the pattern of background seismicity on the San Andreas and Sargent faults and the extent of rupture during the mainshock, thus explaining how the background seismicity outlined the along-strike and depth extent of the mainshock rupture on a different fault plane 5 km away. New aftershock focal mechanisms, based on three-dimensional ray tracing through the velocity model, support a heterogeneous postseismic stress field and can not resolve a uniform fault normal compression. The subvertical (or steeply dipping) San Andreas fault and the fault surfaces that ruptured in the 1989 Loma Prieta earthquake are both parts of the San Andreas fault zone and this section of the fault zone does not have a single type of characteristic event.

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 support-operator method for 3-D rupture dynamics

NASA Astrophysics Data System (ADS)

Ely, Geoffrey P.; Day, Steven M.; Minster, Jean-Bernard

2009-06-01

We present a numerical method to simulate spontaneous shear crack propagation within a heterogeneous, 3-D, viscoelastic medium. Wave motions are computed on a logically rectangular hexahedral mesh, using the generalized finite-difference method of Support Operators (SOM). This approach enables modelling of non-planar surfaces and non-planar fault ruptures. Our implementation, the Support Operator Rupture Dynamics (SORD) code, is highly scalable, enabling large-scale, multiprocessors calculations. The fault surface is modelled by coupled double nodes, where rupture occurs as dictated by the local stress conditions and a frictional failure law. The method successfully performs test problems developed for the Southern California Earthquake Center (SCEC)/U.S. Geological Survey (USGS) dynamic earthquake rupture code validation exercise, showing good agreement with semi-analytical boundary integral method results. We undertake further dynamic rupture tests to quantify numerical errors introduced by shear deformations to the hexahedral mesh. We generate a family of meshes distorted by simple shearing, in the along-strike direction, up to a maximum of 73°. For SCEC/USGS validation problem number 3, grid-induced errors increase with mesh shear angle, with the logarithm of error approximately proportional to angle over the range tested. At 73°, rms misfits are about 10 per cent for peak slip rate, and 0.5 per cent for both rupture time and total slip, indicating that the method (which, up to now, we have applied mainly to near-vertical strike-slip faulting) is also capable of handling geometries appropriate to low-angle surface-rupturing thrust earthquakes. Additionally, we demonstrate non-planar rupture effects, by modifying the test geometry to include, respectively, cylindrical curvature and sharp kinks.

A New Perspective on Fault Geometry and Slip Distribution of the 2009 Dachaidan Mw 6.3 Earthquake from InSAR Observations

PubMed Central

Liu, Yang; Xu, Caijun; Wen, Yangmao; Fok, Hok Sum

2015-01-01

On 28 August 2009, the northern margin of the Qaidam basin in the Tibet Plateau was ruptured by an Mw 6.3 earthquake. This study utilizes the Envisat ASAR images from descending Track 319 and ascending Track 455 for capturing the coseismic deformation resulting from this event, indicating that the earthquake fault rupture does not reach to the earth’s surface. We then propose a four-segmented fault model to investigate the coseismic deformation by determining the fault parameters, followed by inverting slip distribution. The preferred fault model shows that the rupture depths for all four fault planes mainly range from 2.0 km to 7.5 km, comparatively shallower than previous results up to ~13 km, and that the slip distribution on the fault plane is complex, exhibiting three slip peaks with a maximum of 2.44 m at a depth between 4.1 km and 4.9 km. The inverted geodetic moment is 3.85 × 1018 Nm (Mw 6.36). The 2009 event may rupture from the northwest to the southeast unilaterally, reaching the maximum at the central segment. PMID:26184210

A New Perspective on Fault Geometry and Slip Distribution of the 2009 Dachaidan Mw 6.3 Earthquake from InSAR Observations.

PubMed

Liu, Yang; Xu, Caijun; Wen, Yangmao; Fok, Hok Sum

2015-07-10

On 28 August 2009, the northern margin of the Qaidam basin in the Tibet Plateau was ruptured by an Mw 6.3 earthquake. This study utilizes the Envisat ASAR images from descending Track 319 and ascending Track 455 for capturing the coseismic deformation resulting from this event, indicating that the earthquake fault rupture does not reach to the earth's surface. We then propose a four-segmented fault model to investigate the coseismic deformation by determining the fault parameters, followed by inverting slip distribution. The preferred fault model shows that the rupture depths for all four fault planes mainly range from 2.0 km to 7.5 km, comparatively shallower than previous results up to ~13 km, and that the slip distribution on the fault plane is complex, exhibiting three slip peaks with a maximum of 2.44 m at a depth between 4.1 km and 4.9 km. The inverted geodetic moment is 3.85 × 10(18) Nm (Mw 6.36). The 2009 event may rupture from the northwest to the southeast unilaterally, reaching the maximum at the central segment.

Ball-and-socket tectonic rotation during the 2013 Mw 7.7 Balochistan earthquake

NASA Astrophysics Data System (ADS)

Barnhart, W. D.; Hayes, G. P.; Briggs, R. W.; Gold, R. D.; Bilham, R.

2014-10-01

The September 2013 Mw 7.7 Balochistan earthquake ruptured a ∼200-km-long segment of the curved Hoshab fault in southern Pakistan with 10 ± 0.2 m of peak sinistral and ∼ 1.7 ± 0.8 m of dip slip. This rupture is unusual because the fault dips 60 ± 15° towards the focus of a small circle centered in northwest Pakistan, and, despite a 30° increase in obliquity along strike, the ratios of strike and dip slip remain relatively uniform. Surface displacements and geodetic and teleseismic source inversions quantify a bilateral rupture that propagated rapidly at shallow depths from a transtensional jog near the northern end of the rupture. Static friction prior to rupture was unusually weak (μ < 0.05), and friction may have approached zero during dynamic rupture. Here we show that the inward-dipping Hoshab fault defines the northern rim of a structural unit in southeast Makran that rotates - akin to a 2-D ball-and-socket joint - counter-clockwise in response to India's penetration into the Eurasian plate. This rotation accounts for complexity in the Chaman fault system and, in principle, reduces seismic potential near Karachi; nonetheless, these findings highlight deficiencies in strong ground motion equations and tectonic models that invoke Anderson-Byerlee faulting predictions.

Ball-and-socket tectonic rotation during the 2013 Mw7.7 Balochistan earthquake

USGS Publications Warehouse

Barnhart, William D.; Hayes, Gavin P.; Briggs, Richard W.; Gold, Ryan D.; Bilham, R.

2014-01-01

The September 2013 Mw7.7 Balochistan earthquake ruptured a ∼200-km-long segment of the curved Hoshab fault in southern Pakistan with 10±0.2 m of peak sinistral and ∼1.7±0.8 m of dip slip. This rupture is unusual because the fault dips 60±15° towards the focus of a small circle centered in northwest Pakistan, and, despite a 30° increase in obliquity along strike, the ratios of strike and dip slip remain relatively uniform. Surface displacements and geodetic and teleseismic source inversions quantify a bilateral rupture that propagated rapidly at shallow depths from a transtensional jog near the northern end of the rupture. Static friction prior to rupture was unusually weak (μ<0.05), and friction may have approached zero during dynamic rupture. Here we show that the inward-dipping Hoshab fault defines the northern rim of a structural unit in southeast Makran that rotates – akin to a 2-D ball-and-socket joint – counter-clockwise in response to India's penetration into the Eurasian plate. This rotation accounts for complexity in the Chaman fault system and, in principle, reduces seismic potential near Karachi; nonetheless, these findings highlight deficiencies in strong ground motion equations and tectonic models that invoke Anderson–Byerlee faulting predictions.

Complex rupture process of the Mw 7.8, 2016, Kaikoura earthquake, New Zealand, and its aftershock sequence

NASA Astrophysics Data System (ADS)

Cesca, S.; Zhang, Y.; Mouslopoulou, V.; Wang, R.; Saul, J.; Savage, M.; Heimann, S.; Kufner, S.-K.; Oncken, O.; Dahm, T.

2017-11-01

The M7.8 Kaikoura Earthquake that struck the northeastern South Island, New Zealand, on November 14, 2016 (local time), is one of the largest ever instrumentally recorded earthquakes in New Zealand. It occurred at the southern termination of the Hikurangi subduction margin, where the subducting Pacific Plate transitions into the dextral Alpine transform fault. The earthquake produced significant distributed uplift along the north-eastern part of the South Island, reaching a peak amplitude of ∼8 m, which was accompanied by large (≥10 m) horizontal coseismic displacements at the ground surface along discrete active faults. The seismic waveforms' expression of the main shock indicate a complex rupture process. Early automated centroid moment tensor solutions indicated a strong non-double-couple term, which supports a complex rupture involving multiple faults. The hypocentral distribution of aftershocks, which appears diffuse over a broad region, clusters spatially along lineaments with different orientations. A key question of global interest is to shed light on the mechanism with which such a complex rupture occurred, and whether the underlying plate-interface was involved in the rupture. The consequences for seismic hazard of such a distributed, shallow faulting is important to be assessed. We perform a broad seismological analysis, combining regional and teleseismic seismograms, GPS and InSAR, to determine the rupture process of the main shock and moment tensors of 118 aftershocks down to Mw 4.2. The joint interpretation of the main rupture and aftershock sequence allow reconstruction of the geometry, and suggests sequential activation and slip distribution on at least three major active fault domains. We find that the rupture nucleated as a weak strike-slip event along the Humps Fault, which progressively propagated northward onto a shallow reverse fault, where most of the seismic moment was released, before it triggered slip on a second set of strike-slip faults at the northern end of the rupture. The northern and southern strike-slip fault domains have the same orientation but are spatially separated by >15 km. In our model, the low angle splay thrust fault is located above the slab and connects the strike-slip faults kinematically. During the aftershock phase, the entire fault system remained active.

The Non-Regularity of Earthquake Recurrence in California: Lessons From Long Paleoseismic Records in Simple vs Complex Fault Regions (Invited)

NASA Astrophysics Data System (ADS)

Rockwell, T. K.

2010-12-01

A long paleoseismic record at Hog Lake on the central San Jacinto fault (SJF) in southern California documents evidence for 18 surface ruptures in the past 3.8-4 ka. This yields a long-term recurrence interval of about 210 years, consistent with its slip rate of ~16 mm/yr and field observations of 3-4 m of displacement per event. However, during the past 3800 years, the fault has switched from a quasi-periodic mode of earthquake production, during which the recurrence interval is similar to the long-term average, to clustered behavior with the inter-event periods as short as a few decades. There are also some periods as long as 450 years during which there were no surface ruptures, and these periods are commonly followed by one to several closely-timed ruptures. The coefficient of variation (CV) for the timing of these earthquakes is about 0.6 for the past 4000 years (17 intervals). Similar behavior has been observed on the San Andreas Fault (SAF) south of the Transverse Ranges where clusters of earthquakes have been followed by periods of lower seismic production, and the CV is as high as 0.7 for some portions of the fault. In contrast, the central North Anatolian Fault (NAF) in Turkey, which ruptured in 1944, appears to have produced ruptures with similar displacement at fairly regular intervals for the past 1600 years. With a CV of 0.16 for timing, and close to 0.1 for displacement, the 1944 rupture segment near Gerede appears to have been both periodic and characteristic. The SJF and SAF are part of a broad plate boundary system with multiple parallel strands with significant slip rates. Additional faults lay to the east (Eastern California shear zone) and west (faults of the LA basin and southern California Borderland), which makes the southern SAF system a complex and broad plate boundary zone. In comparison, the 1944 rupture section of the NAF is simple, straight and highly localized, which contrasts with the complex system of parallel faults in southern California. These observations suggest that the complexity of the southern California fault network is partly responsible for the apparent increase in “noise” and non-periodic behavior, perhaps resulting from stress transfer to adjacent faults after a large earthquake on one fault. The simplicity of the central NAF may account for its relatively simple behavior. If correct, the study of simple plate boundary faults may provide new insights into the constitutive elements of fault zones, and may aid in identifying those components that are critical for better forecasting future seismicity in complex systems.

The 2013 Mw 6.2 Khaki-Shonbe (Iran) Earthquake: Seismic Shortening of the Zagros Sedimentary Cover

NASA Astrophysics Data System (ADS)

Elliott, J. R.; Bergman, E.; Copley, A.; Ghods, A.; Nissen, E.; Oveisi, B.; Walters, R. J.

2014-12-01

The 2013 Mw 6.2 Khaki-Shonbe earthquake occurred in the Simply Folded Belt of the Zagros Mountains, Iran. This is the largest earthquake in the Zagros since the November 1990 Mw 6.4 Furg (Hormozgan) thrust faulting event, and therefore the largest in the period for which dense InSAR ground displacements are available. It is also the biggest seismic event to have occurred in the Simply Folded Belt since the March 1977 Mw 6.7 Khurgu earthquake. This earthquake therefore potentially provides valuable insights into a range of controversies: (1) the preponderance of earthquake faulting in the crystalline basement versus the sedimentary cover and the potential importance of lithology in controlling and limiting seismic rupture; (2) the nature of surface folding and whether or not there is a one-to-one relationship between buried reverse faults and surface anticlines; and (3) the presence or absence of large pulses of aseismic slip triggered by mainshock rupture. We combine seismological solutions and aftershock relocations with satellite interferometric ground displacements and observations from the field to determine the geometry of faulting and its relationship with the structure, stratigraphy and tectonics of the Central Zagros. The earthquake rupture involved reverse slip on two along-strike southwest dipping fault segments, the rupture initiating at the northern and bottom end of the larger north-west segment. These faults verge away from the foreland and towards the high range interior, contrary to the fault geometries depicted in many structural cross-sections of the Zagros. The slip measured on the reverse segments occurred over two mutually exclusive depth ranges, 10-5 km and 4-2 km, resulting in long (16 km), narrow (7 km) rupture segments. Conversely, aftershocks are found to cluster in the depth range 8-16 km, beneath the main rupture segment. This indicates only significant reverse slip and coseismic shortening in the sedimentary cover, with the slip distribution likely to be lithologically controlled in depth by the Hormuz salt at the base of the sedimentary cover, and the Kazhdumi Formation mudrocks at upper-levels (5 km), and aftershocks constrained largely beneath the main coseismic rupture planes.

A review of recently active faults in Taiwan

USGS Publications Warehouse

Bonilla, Manuel G.

1975-01-01

Six faults associated with five large earthquakes produced surface displacements ranging from 1 to 3 m in the period 1906 through 1951. Four of the ruptures occurred in the western coastal plain and foothills, and two occurred in the Longitudinal Valley of eastern Taiwan. Maps are included showing the locations and dimensions of the displacements. The published geological literature probably would not lead one to infer the existence of a fault along most of the 1906 rupture, except for descriptions of the rupture itself. Over most of its length the 1935 rupture on the Chihhu fault is parallel to but more than 0.5 km from nearby faults shown on geologic maps published in 1969 and 1971; only about 1.5 km of its 15 km length coincides with a mapped fault. The coastal plain part of the Tuntzuchio fault which ruptured in 1935 is apparently not revealed by landforms, and only suggested by other data. Part of the 1946 Hsinhua faulting coincides with a fault identified in the subsurface by seismic work but surface indications of the fault are obscure. The 1951 Meilun faulting occurred along a conspicuous pre-1951 scarp and the 1951 Yuli faulting occurred near or in line with pre-1951 scarps. More than 40 faults which, according to the published literature, have had Pleistocene or later movement are shown on a small-scale map. Most of these faults are in the densely-populated western part of Taiwan. The map and text calls attention to faults that may be active and therefore may be significant in planning important structures. Equivocal evidence suggestive of fault creep was found on the Yuli fault and the Hsinhua fault. Fault creep was not found at several places examined along the 1906 fault trace. Tectonic uplift has occurred in Taiwan in the last 10,000 years and application of eustatic sea level curves to published radiocarbon dates shows that the minimum rate of uplift is considerably different in different parts of the island. Incomplete data indicate that the rate is high near Hualien, where an uplift of at least 0.6 m and probably more than 1 m occurred in the 1951 earthquake, and near and south of the 1946 faulting. Sudden uplifts can have serious consequences for installations near the shore. Investigation of this process, study of recently active faults, and continuing study of seismicity are necessary parts of a practical earthquake-hazard reduction program.

The 1999 Izmit, Turkey, earthquake: A 3D dynamic stress transfer model of intraearthquake triggering

USGS Publications Warehouse

Harris, R.A.; Dolan, J.F.; Hartleb, R.; Day, S.M.

2002-01-01

Before the August 1999 Izmit (Kocaeli), Turkey, earthquake, theoretical studies of earthquake ruptures and geological observations had provided estimates of how far an earthquake might jump to get to a neighboring fault. Both numerical simulations and geological observations suggested that 5 km might be the upper limit if there were no transfer faults. The Izmit earthquake appears to have followed these expectations. It did not jump across any step-over wider than 5 km and was instead stopped by a narrower step-over at its eastern end and possibly by a stress shadow caused by a historic large earthquake at its western end. Our 3D spontaneous rupture simulations of the 1999 Izmit earthquake provide two new insights: (1) the west- to east-striking fault segments of this part of the North Anatolian fault are oriented so as to be low-stress faults and (2) the easternmost segment involved in the August 1999 rupture may be dipping. An interesting feature of the Izmit earthquake is that a 5-km-long gap in surface rupture and an adjacent 25° restraining bend in the fault zone did not stop the earthquake. The latter observation is a warning that significant fault bends in strike-slip faults may not arrest future earthquakes.

Horizontal surface-slip distribution through several seismic cycles: The Eastern Bogd fault, Gobi-Altai, Mongolia

NASA Astrophysics Data System (ADS)

Kurtz, R.; Klinger, Y.; Ferry, M.; Ritz, J.-F.

2018-06-01

The 1957, MW 8.1, Gobi-Altai earthquake, Southern Mongolia, produced a 360-km-long surface rupture along the Eastern Bogd fault. Cumulative offsets of geomorphic features suggest that the Eastern Bogd fault might produce characteristic slip over the last seismic cycles. Using orthophotographs derived from a dataset of historical aerial photographs acquired in 1958, we measured horizontal offsets along two thirds ( 170 km) of the 1957 left-lateral strike-slip surface rupture. We propose a new empirical methodology to extract the average slip for each past earthquake that could be recognized along successive fault segments, to determine the slip distribution associated with successive past earthquakes. Our results suggest that the horizontal slip distribution of the 1957 Gobi-Altai earthquake is fairly flat, with an average offset of 3.5 m ± 1.3 m. A combination of our lateral measurements with vertical displacements derived from the literature, allows us to re-assess the magnitude of the Gobi-Altai earthquake to be between MW 7.8 and MW 8.2, depending on the depth of the rupture, and related value of the shear modulus. When comparing this magnitude to magnitudes derived from seismic data, it suggests that the rupture may have extended deeper than the 15 km to 20 km usually considered for the seismogenic crust. We observe that some fault segments are more likely than others to record seismic deformation through several seismic cycles, depending on the local rupture complexity and geomorphology. Additionally, our results allow us to model the horizontal slip function for the 1957 Gobi-Altai earthquake and for three previous paleoseismic events along 70% of the studied area. Along about 50% of the fault sections where we could recognize three past earthquakes, our results suggest that the slip per event was similar for each earthquake.

LiDAR and field observations of slip distribution for the most recent surface ruptures along the central San Jacinto fault

USGS Publications Warehouse

J.B. Salisbury,; T.K. Rockwell,; T.J. Middleton,; Hudnut, Kenneth W.

2012-01-01

We measured offsets on tectonically displaced geomorphic features along 80 km of the Clark strand of the San Jacinto fault (SJF) to estimate slip‐per‐event for the past several surface ruptures. We identify 168 offset features from which we make over 490 measurements using B4 light detection and ranging (LiDAR) imagery and field observations. Our results suggest that LiDAR technology is an exemplary supplement to traditional field methods in slip‐per‐event studies. Displacement estimates indicate that the most recent surface‐rupturing event (MRE) produced an average of 2.5–2.9 m of right‐lateral slip with maximum slip of nearly 4 m at Anza, a Mw 7.2–7.5 earthquake. Average multiple‐event offsets for the same 80 kms are ∼5.5  m, with maximum values of 3 m at Anza for the penultimate event. Cumulative displacements of 9–10 m through Anza suggest the third event was also similar in size. Paleoseismic work at Hog Lake dates the most recent surface rupture event at ca. 1790. A poorly located, large earthquake occurred in southern California on 22 November 1800; we relocate this event to the Clark fault based on the MRE at Hog Lake. We also recognize the occurrence of a younger rupture along ∼15–20  km of the fault in Blackburn Canyon with ∼1.25  m of average displacement. We attribute these offsets to the 21 April 1918 Mw 6.9 event. These data argue that much or all of the Clark fault, and possibly also the Casa Loma fault, fail together in large earthquakes, but that shorter sections may fail in smaller events.

Wasatch fault zone, Utah - segmentation and history of Holocene earthquakes

USGS Publications Warehouse

Machette, Michael N.; Personius, Stephen F.; Nelson, Alan R.; Schwartz, David P.; Lund, William R.

1991-01-01

The Wasatch fault zone (WFZ) forms the eastern boundary of the Basin and Range province and is the longest continuous, active normal fault (343 km) in the United States. It underlies an urban corridor of 1.6 million people (80% of Utah's population) representing the largest earthquake risk in the interior of the western United States. The authors have used paleoseismological data to identify 10 discrete segments of the WFZ. Five are active, medial segments with Holocene slip rates of 1-2 mm a-1, recurrence intervals of 2000-4000 years and average lengths of about 50 km. Five are less active, distal segments with mostly pre-Holocene surface ruptures, late Quaternary slip rates of <0.5 mm a-1, recurrence intervals of ???10,000 years and average lengths of about 20 km. Surface-faulting events on each of the medial segments of the WFZ formed 2-4-m-high scarps repeatedly during the Holocene. Paleoseismological records for the past 6000 years indicate that a major surface-rupturing earthquake has occurred along one of the medial segments about every 395 ?? 60 years. However, between about 400 and 1500 years ago, the WFZ experienced six major surface-rupturing events, an average of one event every 220 years, or about twice as often as expected from the 6000-year record. Evidence has been found that surface-rupturing events occurred on the WFZ during the past 400 years, a time period which is twice the average intracluster recurrence interval and equal to the average Holocene recurrence interval.

Static Rupture Model of the 2010 M7.2 El Mayor-Cucapah Earthquake from ALOS, ENVISAT, SPOT and GPS Data

NASA Astrophysics Data System (ADS)

Fialko, Y.; Gonzalez, A.; Gonzalez-Garcia, J. J.; Barbot, S.; Leprince, S.; Sandwell, D. T.; Agnew, D. C.

2010-12-01

The April 4, 2010 "Easter Sunday" earthquake on the US-Mexico border was the largest event to strike Southern California in the last 18 years. The earthquake occurred on a northwest trending fault close to, but not coincident with the identified 1892 Laguna Salada rupture. We investigate coseismic deformation due to the 2010 El Mayor-Cucapah earthquake using Synthetic Aperture Radar (SAR) imagery form ENVISAT and ALOS satellites, optical imagery from SPOT-5 satellite, and continuous and campaign GPS data. The earliest campaign postseismic GPS survey was conducted within days after the earthquake, and provided the near-field cosesmic offsets. Along-track SAR interferograms and amplitude cross-correlation of optical images reveal a relatively simple continuous fault trace with maximum offsets of the order of 3 meters. This is in contrast to the results of geological mapping that portrayed a complex broad zone of distributed faulting. Also, SAR data indicate that the rupture propagated bi-laterally from the epicenter near the town of Durango both to the North-West into the Cucapah mountains and to the South-East into the Mexically valley. The inferred South-East part of the rupture was subsequently field-checked and associated with several fresh scarps, although overall the earthquake fault does not have a conspicuous surface trace South-East of the hypocenter. It is worth noting that the 2010 earthquake propagated into stress shadows of prior events - the Laguna Salada earthquake that ruptured the North-West part of the fault in 1892, and several M6+ earthquakes that ruptured the South-East part of the fault over the last century. Analysis of the coseismic displacement field at the Earth's surface (in particular, the full 3-component displacement field retrieved from SAR and optical imagery) shows a pronounced asymmetry in horizontal displacements across both nodal planes. The maximum displacements are observed in the North-Eastern and South-Western quadrants. This pattern cannot be explained by oblique slip on a quasi-planar fault. Multi-parametric inversions of the space geodetic data suggest that the El Mayor-Cucapah earthquake occurred on a helix-shaped rupture, with Eastward dip in the Northern section and Westward dip in the Southern section. This interpretation is consistent with field observations of the surface rupture and aftershock data, and provides an explanation for a strong non-double-couple component suggested by the seismic moment tensor solution. The total geodetic moment of our best-fitting model is in a good agreement with the seismic moment. We will also discuss effects of the elastic structure on the inferred static rupture model, and observations of early postseismic deformation.

Near-Field Deformation Associated with the M6.0 South Napa Earthquake Surface Rupture

NASA Astrophysics Data System (ADS)

Brooks, B. A.; Hudnut, K. W.; Glennie, C. L.; Ericksen, T.

2014-12-01

We characterize near-field deformation associated with the surface rupture of the M6.0 South Napa earthquake from repeat mobile laser scanning (MLS) surveys. Starting the day after the main shock, we operated, sometime simultaneously, short (~75 m range) and medium (~400m range) range laser scanners on a truck or backpack. We scanned most of the length of the principal and secondary surface ruptures at speeds less than 10 km/hr. Scanning occurred primarily in either suburban subdivisions or cultivated vineyards of varying varietals with differing leaf patterns and stages of maturity. Spot-spacing is dense enough (100s of points/m^2) to permit creation of 10-25cm digital elevation models of much of the surface rupture. Scanned features of the right-lateral rupture include classic mole tracks through a variety of soil types, en echelon cracks, offset vine rows, and myriad types of pavement-related deformation. We estimate coseismic surface displacements ranging from 5 to 45 cm by examining offset cultural features and vine rows and by comparing the MLS data with preexisting airborne laser scans from 2003 using point-cloud and solid-modeling methodologies. Additionally, we conducted repeat MLS scans to measure the magnitude and spatial variation of fault afterslip, exceeding 20 cm in some places, particularly in the southern portion of the rupture zone. We anticipate these data sets, in conjunction with independently collected ground-based alinement arrays and space-based geodetic data will contribute significant insight into topics of current debate including assessing the most appropriate material models for shallow fault zones and how shallow and deeper fault slip relate to one another.

Active faults system and related potential seismic events near Ulaanbaatar, capital of Mongolia.

NASA Astrophysics Data System (ADS)

Schlupp, Antoine; Ferry, Matthieu; Munkhuu, Ulziibat; Sodnomsambuu, Demberel; Al-Ashkar, Abeer

2013-04-01

The region of Ulaanbaatar lies several hundred kilometers from large known active faults that produced magnitude 6 to 8+ earthquakes during the last century. Beside the Hustai fault, which displays a clear morphological expression, no active fault was previously described less than 100 km from the city. In addition, no large historical (i.e. more recent than the 16th c.) earthquakes are known in this region. However, since 2005 a very dense seismic activity has developed over the Emeelt Township area, a mere 10 km from Ulaanbaatar. The activity is characterized by numerous low magnitude events (M<2.8), which are distributed linearly along several tens of kilometers where no active fault has been identified. This raises several questions: Is this seismicity associated to a -yet- unknown active fault? If so, are there other unknown active faults near Ulaanbaatar? Hence, we deployed a multi-disciplinary approach including morpho-tectonic, near-surface geophysical and paleoseismological investigations. We describe four large active faults west and south of Ulaanbaatar, three of them are newly discovered (Emeelt, Sharai, Avdar), one was previously known (Hustai) but without precise study on its seismic potential. The Emeelt seismicity can be mapped over 35 km along N150 and corresponds in the field to a smoothed, but clear, active fault morphology that can be mapped along a 10-km-long section. The fault dips at ~30° NE (GPR and surface morphology observations) and uplifts the eastern block. The age of the last surface rupture observed in trenches is about 10 ka (preliminary OSL dating). Considering a rupture length of 35 km, a full segment rupture would be comparable to the 1967 Mogod earthquake with a magnitude as large as Mw 7. It has to be considered today as a possible scenario for the seismic risk of Ulaanbaatar. The 90-km-long Hustai Range Fault System, oriented WSW-ENE and located about 10 km west of Ulaanbaatar, displays continuous microseismicity with five light to moderate (M 4 - 5.4) earthquakes over the last 40 years. The last surface-rupturing earthquake occurred about 1000 years ago (OSL dating). Alluvial fans affected by the fault suggest the rate of deformation (left lateral with normal component) along the main segment ranges from 0.3 to 0.4 mm/year for the last 120 000 years. Hence, the average recurrence interval for a full-segment M 7-7.5 is likely in the order of 10 ky. However, if the Hustai fault also releases strain during partial ruptures along its strongly segmented trace, a Mw 6.5 event may be expected anytime. However, only the main central fault segment has been investigated in terms of paleoseismicity. The Sharai and Avdar faults, oriented NNE-SSW, were mapped along ~50-km-long sections. Each of these faults was the site of earthquakes of magnitude 6 and more in the past as suggested by morphology and trench observations. Full-segment-ruptures could produce events as large as M 7.2. The precise relationship and interactions between these faults as well as associated earthquakes have to be clarified by collecting more data. They are the key of the seismic hazard and risk of Ulaanbaatar.

Comparison of the November 2002 Denali and November 2001 Kunlun Earthquakes

NASA Astrophysics Data System (ADS)

Bufe, C. G.

2002-12-01

Major earthquakes occurred in Tibet on the central Kunlun fault (M 7.8) on November 14, 2001 (Lin and others, 2002) and in Alaska on the central Denali fault (M 7.9) on November 3, 2002. Both earthquakes generated large surface waves (Kunlun Ms 8.0 (USGS) and Denali Ms 8.5). Each event occurred on east-west-trending strike-slip faults and exhibited nearly unilateral rupture propagating several hundred kilometers from west to east. Surface rupture length estimates were about 400 km for Kunlun, 300 km for Denali. Maximum surface faulting and moment release were observed far to the east of the points of rupture initiation. Harvard moment centroids were located east of USGS epicenters by 182 km (Kunlun) and by 126 km (Denali). Maximum surface faulting was observed near 240 km (Kunlun, 16 m left lateral) and near 175 km (Denali, 9 m right lateral) east of the USGS epicenters. Significant thrust components were observed in the initiation of the Denali event (ERI analysis and mapped thrust) and in the termination of the Kunlun rupture, as evidenced by thrust mechanisms of the largest aftershocks which occurred near the eastern part of the Kunlun rupture. In each sequence the largest aftershock was about 2 orders of magnitude smaller than the mainshock. Moment release along the ruptured segments was examined for the 25-year periods preceding the main shocks. The Denali zone shows precursory accelerating moment release with the dominant events occurring on October 22, 1996 (M 5.8) and October 23, 2002 (M 6.7). The Kunlun zone shows nearly constant moment release over time with the last significant event before the main shock occurring on November 26, 2000 (M 5.4). Moment release data are consistent with previous observations of annual periodicity preceding major earthquakes, possibly due to the evolution of a critical state with seasonal and tidal triggering (Varnes and Bufe, 2001). Annual periodicity is also evident for the larger events in the greater San Francisco Bay region over several decades preceding the 1906 San Francisco earthquake (M 7.8). Both the Kunlun and the Denali mainshocks occurred at new moon.

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.

Investigating the deformation of upper crustal faults at the N-Chilean convergent plate boundary at different scales using high-resolution topography datasets and creepmeter measurements

NASA Astrophysics Data System (ADS)

Ewiak, O.; Victor, P.; Ziegenhagen, T.; Oncken, O.

2012-04-01

The Chilean convergent plate boundary is one of the tectonically most active regions on earth and prone to large megathrust earthquakes as e. g. the 2010 Mw 8.8 Maule earthquake which ruptured a mature seismic gap in south-central Chile. In northern Chile historical data suggests the existence of a seismic gap between Arica and Mejillones Peninsula (MP), which has not ruptured since 1877. Further south, the 1995 Mw 8.0 Antofagasta earthquake ruptured the subduction interface between MP and Taltal. In this study we investigate the deformation at four active upper plate faults (dip-slip and strike-slip) located above the coupling zone of the subduction interface. The target faults (Mejillones Fault - MF, Salar del Carmen Fault - SCF, Cerro Fortuna Fault - CFF, Chomache Fault - CF) are situated in forearc segments, which are in different stages of the megathrust seismic cycle. The main question of this study is how strain is accumulated in the overriding plate, what is the response of the target faults to the megathrust seismic cycle and what are the mechanisms / processes involved. The hyper arid conditions of the Atacama desert and the extremely low erosion rates enable us to investigate geomorphic markers, e .g. fault scarps and knickpoints, which serve as a record for upper crustal deformation and fault activity about ten thousands years into the past. Fault scarp data has been acquired with Differential-GPS by measuring high-resolution topographic profiles perpendicular to the fault scarps and along incised gullies. The topographic data show clear variations between the target faults which possibly result from their position within the forearc. The surveyed faults, e. g. the SCF, exhibit clear along strike variations in the morphology of surface ruptures attributed to seismic events and can be subdivided into individual segments. The data allows us to distinguish single, composite and multiple fault scarps and thus to detect differences in fault growth initiated either by seismic rupture or fault creep. Additional information on the number of seismic events responsible for the cumulative displacement can be derived from the mapping of knickpoints. By reconstructing the stress field responsible for the formation of identified seismic surface ruptures we can determine stress conditions for failure of upper crustal faults. Comparing these paleo stress conditions with the recent forearc stresses (interseismic / coseismic) we can derive information about a possible activation of upper crustal faults during the megathrust seismic cycle. In addition to the morphotectonic surveys we explore the recent deformation of the target faults by analyzing time series of displacements recorded with micron precision by an array of creepmeters at the target faults for over three years. Total displacement is composed of steady state creep, creep events and sudden displacement events (SDEs) related to seismic rupture. The percentage of SDEs accounts for >50 % (SCF) to 90 % (CFF) of the cumulative displacement. This result very well reflects the field observation that a considerable amount of the total displacement has been accumulated during multiple seismic events.

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.

Rupture history of the 2008 Mw 7.9 Wenchuan, China, earthquake: Evaluation of separate and joint inversions of geodetic, teleseismic, and strong-motion data

USGS Publications Warehouse

Hartzell, Stephen; Mendoza, Carlos; Ramírez-Guzmán, Leonardo; Zeng, Yuesha; Mooney, Walter

2013-01-01

An extensive data set of teleseismic and strong-motion waveforms and geodetic offsets is used to study the rupture history of the 2008 Wenchuan, China, earthquake. A linear multiple-time-window approach is used to parameterize the rupture. Because of the complexity of the Wenchuan faulting, three separate planes are used to represent the rupturing surfaces. This earthquake clearly demonstrates the strengths and limitations of geodetic, teleseismic, and strong-motion data sets. Geodetic data (static offsets) are valuable for determining the distribution of shallower slip but are insensitive to deeper faulting and reveal nothing about the timing of slip. Teleseismic data in the distance range 30°–90° generally involve no modeling difficulties because of simple ray paths and can distinguish shallow from deep slip. Teleseismic data, however, cannot distinguish between different slip scenarios when multiple fault planes are involved because steep takeoff angles lead to ambiguity in timing. Local strong-motion data, on the other hand, are ideal for determining the direction of rupture from directivity but can easily be over modeled with inaccurate Green’s functions, leading to misinterpretation of the slip distribution. We show that all three data sets are required to give an accurate description of the Wenchuan rupture. The moment is estimated to be approximately 1.0 × 1021 N · m with the slip characterized by multiple large patches with slips up to 10 m. Rupture initiates on the southern end of the Pengguan fault and proceeds unilaterally to the northeast. Upon reaching the cross-cutting Xiaoyudong fault, rupture of the adjacent Beichuan fault starts at this juncture and proceeds bilaterally to the northeast and southwest.

Rare normal faulting earthquake induced by subduction megaquake: example from 2011 Tohoku-oki earthquake

NASA Astrophysics Data System (ADS)

Ishiyama, T.; Sugito, N.; Echigo, T.; Sato, H.; Suzuki, T.

2012-04-01

A month after March 11 gigantic M9.0 Tohoku-oki earthquake, M7.0 intraplate earthquake occurred at a depth of 5 km on April 11 beneath coastal area of near Iwaki city, Fukushima prefecture. Focal mechanism of the mainshock indicates that this earthquake is a normal faulting event. Based on field reconnaissance and LIDAR mapping by Geospatial Information Authority of Japan, we recognized coseismic surface ruptures, presumably associated with the main shock. Coseismic surface ruptures extend NNW for about 11 km in a right-stepping en echelon manner. Geomorphic expressions of these ruptures commonly include WWS-facing normal fault scarps and/or drape fold scarp with open cracks on their crests, on the hanging wall sides of steeply west-dipping normal fault planes subparallel to Cretaceous metamorphic rocks. Highest topographic scarp height is about 2.3 m. In this study we introduce preliminary results of a trenching survey across the coseismic surface ruptures at Shionohira site, to resolve timing of paleoseismic events along the Shionohira fault. Trench excavations were carried out at two sites (Ichinokura and Shionohira sites) in Iwaki, Fukushima. At Shionohira site a 2-m-deep trench was excavated across the coseismic fault scarp emerged on the alluvial plain on the eastern flank of the Abukuma Mountains. On the trench walls we observed pairs of steeply dipping normal faults that deform Neogene to Paleogene conglomerates and unconformably overlying, late Quaternary to Holocene fluvial units. Sense of fault slip observed on the trench walls (large dip-slip with small sinistral component) is consistent with that estimated from coseismic surface ruptures. Fault throw estimated from separation of piercing points on lower Unit I and vertical structural relief on folded upper Unit I is consistent with topographic height of the coseismic fault scarp at the trench site. In contrast, vertical separation of Unit II, unconformably overlain by Unit I, is measured as about 1.5 m, twice as large as coseismic vertical component of slip, indicative of penultimate seismic event prior to the 2011 earthquake. Abrupt thickening of overlying Unit I may also suggest preexisting topographic relief prior to its deposition. Radiocarbon dating of charred materials included in event horizons and tephrostratigraphy at two sites indicate that penultimate event prior to the 2011 event might occurred at about 40 ka. This normal fault earthquake is in contrast to compressional or neutral stress regimes in Tohoku region before the 2011 megaquake and rarity of the normal faulting earthquake inferred from these paleoseismic studies may reflect its mechanical relation to the gigantic megathrust earthquakes, such as unusual, enhanced extensional stress on the hangingwall block induced by mainshock and/or postseismic creep after the M~9 earthquake.

Strain release along ocean transform faults

NASA Astrophysics Data System (ADS)

Stewart, L. M.

A global study of the nature of seismic rupture along oceanic transform faults (TFs) is presented, and many aspects of fault behavior and Mid-Ocean Ridge processes are discussed. A classification of TF earthquakes is developed based on their relative excitation of short period body waves to long period surface waves. Since the ways in which transform faults release their accumulated strain varies, for more than 50 earthquakes occurring on 30 TFs since 1963 form the database for a comparison of rupture processes. The variation of TF rupture processes is not related to spreading rate or TF offset. A study of seismicity of the Eltanin Fracture Zone system shows that unlike many TFs, the Eltanin FZ realizes more than 90% of its slip aseismically. This identifies a major portion of plate boundary whose motion persists undetected by seismic instruments. The global variations in rupture patterns are discussed in terms of current models of fault behavior. The versatility of the asperity model accommodates the entire range of observed patterns. Variations in physical properties within TF contact zones (asperities) are documented in the petrology and geochemistry of rocks from ophiolite sections and TFs.

Paleo-earthquake Analysis from the Morphologic Features of Unconsolidated-sediment Fault Scarp: An Example from Dushanzi Thrust Fault in the Northern Tianshan, China

NASA Astrophysics Data System (ADS)

Wei, Z.; He, H.

2016-12-01

Fault scarp is important specific tectonic landform caused by surface-rupture earthquake. The morphology of the fault scarp in unconsolidated sediment could evolve in a predictable, time-dependent diffusion model. As a result, the investigation of fault-generated fault scarps is a prevalent technique used to study fault activity, geomorphic evolution, and the recurrence of faulting events. Addition to obtainment of cumulative displacement, gradient changes, i.e. slope breaks, in the morphology of fault scarps could indicate multiple rupture events along an active fault. In this study, we exacted a large set of densely spaced topographic profiles across fault scarp from LiDAR-derive DEM to detect subtle changes in the fault scarp geometry at the Dushanzi trust fault in the Northern Tianshan, China. Several slope breaks in topographic profiles can be identified, which may represent repeated rupture at the investigated fault. The number of paleo-earthquakes derived from our analysis is 4-3, well in agreement with the investigation results from the paleoseismological trenches. Statistical analysis results show that the scarp height of fault scarp with one slope break is 0.75±0.12 (mean value ±1 standard deviation) m representing the last incremental displacement during earthquakes; the height of fault scarp with two slope breaks is 1.86±0.32 m, and the height of fault scarp with three-four slope break is 6.45±1.44 m. Our approach enables us to obtain paleo-earthquake information from geomorphological analysis of fault scarps, and to assess the multiple rupture history of a complex fault system.

Surface slip and off-fault deformation patterns in the 2013 MW 7.7 Balochistan, Pakistan earthquake: Implications for controls on the distribution of near-surface coseismic slip

NASA Astrophysics Data System (ADS)

Zinke, Robert; Hollingsworth, James; Dolan, James F.

2014-12-01

Comparison of 398 fault offsets measured by visual analysis of WorldView high-resolution satellite imagery with deformation maps produced by COSI-Corr subpixel image correlation of Landsat-8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 Mw 7.7 Balochistan, Pakistan earthquake. Average slip along the main trace of the fault was 4.2 m, with local maximum offsets up to 11.4 m. Comparison of slip measured from offset geomorphic features, which record localized slip along the main strand of the fault, to the total displacement across the entire width of the surface deformation zone from COSI-Corr reveals ˜45% off-fault deformation. While previous studies have shown that the structural maturity of the fault exerts a primary control on the total percentage of off-fault surface deformation, large along-strike variations in the percentage of strain localization observed in the 2013 rupture imply the influence of important secondary controls. One such possible secondary control is the type of near-surface material through which the rupture propagated. We therefore compared the percentage off-fault deformation to the type of material (bedrock, old alluvium, and young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (a proxy for sediment thickness along this hybrid strike slip/reverse slip fault). We find significantly more off-fault deformation in younger and/or thicker sediments. Accounting for and predicting such off-fault deformation patterns has important implications for the interpretation of geologic slip rates, especially for their use in probabilistic seismic hazard assessments, the behavior of near-surface materials during coseismic deformation, and the future development of microzonation protocols for the built environment.

Tectonic position and geological manifestations of the Mogod (Central Mongolia), January 5, 1967, earthquake (a view after 40 years)

USGS Publications Warehouse

Rogozhin, E.A.; Imaev, V.S.; Smekalin, O.P.; Schwartz, D.P.

2008-01-01

The earthquake source, reaching the surface in the form of an extended system of faults, encompassed the N-S and NW-SE planes of two large faults near their juncture zone. A revised seismotectonic study of the system of coseismic ruptures performed after many years revealed a complex structure of primary coseismic ruptures in the juncture area of fault branches of different directions. In addition to the two major faults, the juncture zone consists of intersecting or parallel branches of both structural directions. The trench study and detailed mapping of the shallow structure of the seismic rupture characterizes it as a right-lateral-thrust fault on the N-S branch and a strike-slip-reverse fault on the NW-SE branch. Results of our paleoseismogeological study indicate that equally strong earthquakes are likely to have occurred in the same seismic source in the past (about 8000 and 160 years ago). ?? Pleiades Publishing, Ltd. 2008.

Modeling Of Spontaneous Multiscale Roughening And Branching of Ruptures Propagating On A Slip-Weakening Frictional Fault

NASA Astrophysics Data System (ADS)

Elbanna, A. E.

2013-12-01

Numerous field and experimental observations suggest that faults surfaces are rough at multiple scales and tend to produce a wide range of branch sizes ranging from micro-branching to large scale secondary faults. The development and evolution of fault roughness and branching is believed to play an important role in rupture dynamics and energy partitioning. Previous work by several groups has succeeded in determining conditions under which a main rupture may branch into a secondary fault. Recently, there great progress has been made in investigating rupture propagation on rough faults with and without off-fault plasticity. Nonetheless, in most of these models the heterogeneity, whether the roughness profile or the secondary faults orientation, was built into the system from the beginning and consequently the final outcome depends strongly on the initial conditions. Here we introduce an adaptive mesh technique for modeling mode-II crack propagation on slip weakening frictional interfaces. We use a Finite Element Framework with random mesh topology that adapts to crack dynamics through element splitting and sequential insertion of frictional interfaces dictated by the failure criterion. This allows the crack path to explore non-planar paths and develop the roughness profile that is most compatible with the dynamical constraints. It also enables crack branching at different scales. We quantify energy dissipation due to the roughening process and small scale branching. We compare the results of our model to a reference case for propagation on a planar fault. We show that the small scale processes of roughening and branching influence many characteristics of the rupture propagation including the energy partitioning, rupture speed and peak slip rates. We also estimate the fracture energy required for propagating a crack on a planar fault that will be required to produce comparable results. We anticipate that this modeling approach provides an attractive methodology that complements the current efforts in modeling off-fault plasticity and damage.

Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand.

PubMed

Hamling, Ian J; Hreinsdóttir, Sigrún; Clark, Kate; Elliott, John; Liang, Cunren; Fielding, Eric; Litchfield, Nicola; Villamor, Pilar; Wallace, Laura; Wright, Tim J; D'Anastasio, Elisabetta; Bannister, Stephen; Burbidge, David; Denys, Paul; Gentle, Paula; Howarth, Jamie; Mueller, Christof; Palmer, Neville; Pearson, Chris; Power, William; Barnes, Philip; Barrell, David J A; Van Dissen, Russ; Langridge, Robert; Little, Tim; Nicol, Andrew; Pettinga, Jarg; Rowland, Julie; Stirling, Mark

2017-04-14

On 14 November 2016, northeastern South Island of New Zealand was struck by a major moment magnitude ( M w ) 7.8 earthquake. Field observations, in conjunction with interferometric synthetic aperture radar, Global Positioning System, and seismology data, reveal this to be one of the most complex earthquakes ever recorded. The rupture propagated northward for more than 170 kilometers along both mapped and unmapped faults before continuing offshore at the island's northeastern extent. Geodetic and field observations reveal surface ruptures along at least 12 major faults, including possible slip along the southern Hikurangi subduction interface; extensive uplift along much of the coastline; and widespread anelastic deformation, including the ~8-meter uplift of a fault-bounded block. This complex earthquake defies many conventional assumptions about the degree to which earthquake ruptures are controlled by fault segmentation and should motivate reevaluation of these issues in seismic hazard models. Copyright © 2017, American Association for the Advancement of Science.

Geotechnical reconnaissance of the 2002 Denali fault, Alaska, earthquake

USGS Publications Warehouse

Kayen, R.; Thompson, E.; Minasian, D.; Moss, R.E.S.; Collins, B.D.; Sitar, N.; Dreger, D.; Carver, G.

2004-01-01

The 2002 M7.9 Denali fault earthquake resulted in 340 km of ruptures along three separate faults, causing widespread liquefaction in the fluvial deposits of the alpine valleys of the Alaska Range and eastern lowlands of the Tanana River. Areas affected by liquefaction are largely confined to Holocene alluvial deposits, man-made embankments, and backfills. Liquefaction damage, sparse surrounding the fault rupture in the western region, was abundant and severe on the eastern rivers: the Robertson, Slana, Tok, Chisana, Nabesna and Tanana Rivers. Synthetic seismograms from a kinematic source model suggest that the eastern region of the rupture zone had elevated strong-motion levels due to rupture directivity, supporting observations of elevated geotechnical damage. We use augered soil samples and shear-wave velocity profiles made with a portable apparatus for the spectral analysis of surface waves (SASW) to characterize soil properties and stiffness at liquefaction sites and three trans-Alaska pipeline pump station accelerometer locations. ?? 2004, Earthquake Engineering Research Institute.

Persistent fine-scale fault structures control rupture development in Parkfield, CA.

NASA Astrophysics Data System (ADS)

Perrin, C.; Waldhauser, F.; Scholz, C. H.

2016-12-01

We investigate the fine-scale geometry and structure of the San Andreas Fault (SAF) near Parkfield, CA, and their role in the development of the 1966 and 2004 M6 earthquakes. Both events broke the fault mainly unilaterally with similar length ( 30 km) but in opposite directions. Seismic slip occurred in a narrow zone between 5 and 10 km depth, as outlined by the concentration of aftershocks along the edge of the slip area. Across fault distribution of the 2004 aftershocks show a rapid decrease of event density away from the fault core. The damage zone is narrower in the Parkfield section (few 100 meters) than in the creeping section ( 1 km). We observe a similar but broader distribution during the interseismic periods. This implies that stress accumulates in a volume around the fault during interseismic periods, whereas coseismic deformation is more localized on the mature SAF. Large aftershocks are concentrated at both rupture tips, characterized by strong heterogeneities in the fault structure at the surface and at depth: i) in the south near Gold Hill-Cholame, a large releasing bend (>25°) separates the Parkfield section from the southern section of the SAF; ii) in the north at Middle Mountain, the surface fault trace goes through an ancient restraining step-over connecting the Parkfield and creeping sections. Fine-scale analysis of the 2004 aftershocks reveals a change in the fault dip and local variations of the fault strike (up to 25°) beneath Middle Mountain, in good agreement with focal mechanisms, which show oblique normal and reverse faulting. We observe these variations during the interseismic periods before and after the 2004 event, suggesting that the structural heterogeneities persisted through at least two earthquake cycles. These heterogeneities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, but also as stress concentrations where rupture initiates.

Evidence for slip partitioning and bimodal slip behavior on a single fault: Surface slip characteristics of the 2013 Mw7.7 Balochistan, Pakistan earthquake

USGS Publications Warehouse

Barnhart, William; Briggs, Richard; Reitman, Nadine G.; Gold, Ryan D.; Hayes, Gavin

2015-01-01

Deformation is commonly accommodated by strain partitioning on multiple, independent strike-slip and dip-slip faults in continental settings of oblique plate convergence. As a corollary, individual faults tend to exhibit one sense of slip – normal, reverse, or strike-slip – until whole-scale changes in boundary conditions reactivate preexisting faults in a new deformation regime. In this study, we show that a single continental fault may instead partition oblique strain by alternatively slipping in a strike-slip or a dip-slip sense during independent fault slip events. We use 0.5 m resolution optical imagery and sub-pixel correlation analysis of the 200+ km 200+km"> 2013 Mw7.7 Balochistan, Pakistan earthquake to document co-seismic surface slip characteristics and Quaternary tectonic geomorphology along the causative Hoshab fault. We find that the 2013 earthquake, which involved a ∼6:1 strike-slip to dip-slip ratio, ruptured a structurally segmented fault. Quaternary geomorphic indicators of gross fault-zone morphology reveal both reverse-slip and strike-slip deformation in the rupture area of the 2013 earthquake that varies systematically along fault strike despite nearly pure strike-slip motion in 2013. Observations of along-strike variations in range front relief and geomorphic offsets suggest that the Hoshab fault accommodates a substantial reverse component of fault slip in the Quaternary, especially along the southern section of the 2013 rupture. We surmise that Quaternary bimodal slip along the Hoshab fault is promoted by a combination of the arcuate geometry of the Hoshab fault, the frictional weakness of the Makran accretionary prism, and time variable loading conditions from adjacent earthquakes and plate interactions.

Evidence for slip partitioning and bimodal slip behavior on a single fault: Surface slip characteristics of the 2013 Mw7.7 Balochistan, Pakistan earthquake

NASA Astrophysics Data System (ADS)

Barnhart, W. D.; Briggs, R. W.; Reitman, N. G.; Gold, R. D.; Hayes, G. P.

2015-06-01

Deformation is commonly accommodated by strain partitioning on multiple, independent strike-slip and dip-slip faults in continental settings of oblique plate convergence. As a corollary, individual faults tend to exhibit one sense of slip - normal, reverse, or strike-slip - until whole-scale changes in boundary conditions reactivate preexisting faults in a new deformation regime. In this study, we show that a single continental fault may instead partition oblique strain by alternatively slipping in a strike-slip or a dip-slip sense during independent fault slip events. We use 0.5 m resolution optical imagery and sub-pixel correlation analysis of the 200 + km 2013 Mw7.7 Balochistan, Pakistan earthquake to document co-seismic surface slip characteristics and Quaternary tectonic geomorphology along the causative Hoshab fault. We find that the 2013 earthquake, which involved a ∼6:1 strike-slip to dip-slip ratio, ruptured a structurally segmented fault. Quaternary geomorphic indicators of gross fault-zone morphology reveal both reverse-slip and strike-slip deformation in the rupture area of the 2013 earthquake that varies systematically along fault strike despite nearly pure strike-slip motion in 2013. Observations of along-strike variations in range front relief and geomorphic offsets suggest that the Hoshab fault accommodates a substantial reverse component of fault slip in the Quaternary, especially along the southern section of the 2013 rupture. We surmise that Quaternary bimodal slip along the Hoshab fault is promoted by a combination of the arcuate geometry of the Hoshab fault, the frictional weakness of the Makran accretionary prism, and time variable loading conditions from adjacent earthquakes and plate interactions.

Broadband ground-motion simulation using a hybrid approach

USGS Publications Warehouse

Graves, R.W.; Pitarka, A.

2010-01-01

This paper describes refinements to the hybrid broadband ground-motion simulation methodology of Graves and Pitarka (2004), which combines a deterministic approach at low frequencies (f 1 Hz). In our approach, fault rupture is represented kinematically and incorporates spatial heterogeneity in slip, rupture speed, and rise time. The prescribed slip distribution is constrained to follow an inverse wavenumber-squared fall-off and the average rupture speed is set at 80% of the local shear-wave velocity, which is then adjusted such that the rupture propagates faster in regions of high slip and slower in regions of low slip. We use a Kostrov-like slip-rate function having a rise time proportional to the square root of slip, with the average rise time across the entire fault constrained empirically. Recent observations from large surface rupturing earthquakes indicate a reduction of rupture propagation speed and lengthening of rise time in the near surface, which we model by applying a 70% reduction of the rupture speed and increasing the rise time by a factor of 2 in a zone extending from the surface to a depth of 5 km. We demonstrate the fidelity of the technique by modeling the strong-motion recordings from the Imperial Valley, Loma Prieta, Landers, and Northridge earthquakes.

Joint inversion of GNSS and teleseismic data for the rupture process of the 2017 M w6.5 Jiuzhaigou, China, earthquake

NASA Astrophysics Data System (ADS)

Li, Qi; Tan, Kai; Wang, Dong Zhen; Zhao, Bin; Zhang, Rui; Li, Yu; Qi, Yu Jie

2018-02-01

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is 8.0 × 1018 N·m (M w ≈ 6.5), and the centroid depth is 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5-15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 M w7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake.

Joint inversion of GNSS and teleseismic data for the rupture process of the 2017 M w6.5 Jiuzhaigou, China, earthquake

NASA Astrophysics Data System (ADS)

Li, Qi; Tan, Kai; Wang, Dong Zhen; Zhao, Bin; Zhang, Rui; Li, Yu; Qi, Yu Jie

2018-05-01

The spatio-temporal slip distribution of the earthquake that occurred on 8 August 2017 in Jiuzhaigou, China, was estimated from the teleseismic body wave and near-field Global Navigation Satellite System (GNSS) data (coseismic displacements and high-rate GPS data) based on a finite fault model. Compared with the inversion results from the teleseismic body waves, the near-field GNSS data can better restrain the rupture area, the maximum slip, the source time function, and the surface rupture. The results show that the maximum slip of the earthquake approaches 1.4 m, the scalar seismic moment is 8.0 × 1018 N·m ( M w ≈ 6.5), and the centroid depth is 15 km. The slip is mainly driven by the left-lateral strike-slip and it is initially inferred that the seismogenic fault occurs in the south branch of the Tazang fault or an undetectable fault, a NW-trending left-lateral strike-slip fault, and belongs to one of the tail structures at the easternmost end of the eastern Kunlun fault zone. The earthquake rupture is mainly concentrated at depths of 5-15 km, which results in the complete rupture of the seismic gap left by the previous four earthquakes with magnitudes > 6.0 in 1973 and 1976. Therefore, the possibility of a strong aftershock on the Huya fault is low. The source duration is 30 s and there are two major ruptures. The main rupture occurs in the first 10 s, 4 s after the earthquake; the second rupture peak arrives in 17 s. In addition, the Coulomb stress study shows that the epicenter of the earthquake is located in the area where the static Coulomb stress change increased because of the 12 May 2017 M w7.9 Wenchuan, China, earthquake. Therefore, the Wenchuan earthquake promoted the occurrence of the 8 August 2017 Jiuzhaigou earthquake.

3-D Spontaneous Rupture Simulations of the 2016 Kumamoto, Japan, Earthquake

NASA Astrophysics Data System (ADS)

Urata, Yumi; Yoshida, Keisuke; Fukuyama, Eiichi

2017-04-01

We investigated the M7.3 Kumamoto, Japan, earthquake to illuminate why and how the rupture of the main shock propagated successfully by 3-D dynamic rupture simulations, assuming a complicated fault geometry estimated based on the distributions of aftershocks. The M7.3 main shock occurred along the Futagawa and Hinagu faults. A few days before, three M6-class foreshocks occurred. Their hypocenters were located along by the Hinagu and Futagawa faults and their focal mechanisms were similar to those of the main shock; therefore, an extensive stress shadow can have been generated on the fault plane of the main shock. First, we estimated the geometry of the fault planes of the three foreshocks as well as that of the main shock based on the temporal evolution of relocated aftershock hypocenters. Then, we evaluated static stress changes on the main shock fault plane due to the occurrence of the three foreshocks assuming elliptical cracks with constant stress drops on the estimated fault planes. The obtained static stress change distribution indicated that the hypocenter of the main shock is located on the region with positive Coulomb failure stress change (ΔCFS) while ΔCFS in the shallow region above the hypocenter was negative. Therefore, these foreshocks could encourage the initiation of the main shock rupture and could hinder the rupture propagating toward the shallow region. Finally, we conducted 3-D dynamic rupture simulations of the main shock using the initial stress distribution, which was the sum of the static stress changes by these foreshocks and the regional stress field. Assuming a slip-weakening law with uniform friction parameters, we conducted 3-D dynamic rupture simulations by varying the friction parameters and the values of the principal stresses. We obtained feasible parameter ranges to reproduce the rupture propagation of the main shock consistent with those revealed by seismic waveform analyses. We also demonstrated that the free surface encouraged the slip evolution of the main shock.

Paleoseismic evidence for dynamic surface rupturing in an intraplate setting (Lower Rhine Embayment, central Europe)

NASA Astrophysics Data System (ADS)

Kuebler, S.; Friedrich, A. M.; Strecker, M. R.

2011-12-01

One of the most enigmatic problems in intraplate earthquake geology is the spatio-temporal recurrence pattern of large earthquakes. Intraplate regions such as the New Madrid seismic zone or the central European rift system are subject to considerable seismic hazards, because fault activity is highly disparate in space and time and our knowledge about the recurrence of large earthquakes is still rudimentary. The current debate in central Europe ranges from slip dominated by repeated large coseismic events to slip dominated by aseismic creep. Here, field evidence in support of the former is sparse, and hence, some authors concluded that many faults move by slow aseismic creep rather than by ground rupturing earthquakes. We report new results from a paleoseismic study carried out in the Lower Rhine Embayment across a subsidiary normal fault in the area of Germany's largest historical earthquake (1756 AD, ML 6.2±0.2) that clearly revealed field evidence of dynamic surface faulting. At the trench site, the fault is covered by <5 m-thick Holocene fluvial gravel and flood deposits overlaying Devonian shale. We mapped a surface offset of ~1 m and a ~10 m wide zone of localized deformation expressed by abundant fractures with aligned and broken clasts extending vertically throughout the entire gravel. Mapping of 237 fractured clasts and the long-axis orientation of ~10.000 clasts defines a deformation zone coinciding with the surface offset and two offset markers within the gravel layers. We interpret these features as the result of coseismic deformation at the near-surface end of the rupture. We rule out alternative processes which may lead to fracturing of pebbles such as freeze-thaw weathering or sediment loading effects, since both the gravel fabric and fracture planes coincide well with the fault orientation. We preclude slow deformation due to aseismic creep as governing process to cause rupturing of pebbles this close to the surface, as this would require an overburden stress of several hundreds of meters according to modelling results (e. g. Eidelmann, 1992, Geology). With a significantly smaller overburden, as in this study, a high differential acceleration force, such as a shock wave produced by an earthquake rupture or a seismic wave would be needed to overcome the pebble's shear resistance. Preliminary radiocarbon data bracket the youngest event horizon to Latest Holocene age. In conclusion, we identified coseismic deformation at the trench site, because special conditions produced a number of features not usually observed in other fault exposures. The thin sedimentary cover (<5 m) above basement rocks and the high groundwater table, which may reduce the shear strength of the pebbles, may have played an important role in producing this deformation pattern. Our results imply that large surface rupturing earthquakes in low-strain intraplate regions may be more common than previously thought.

The 2016-2017 Central Italy Seismic Sequence: Source Complexity Inferred from Rupture Models.

NASA Astrophysics Data System (ADS)

Scognamiglio, L.; Tinti, E.; Casarotti, E.; Pucci, S.; Villani, F.; Cocco, M.; Magnoni, F.; Michelini, A.

2017-12-01

The Apennines have been struck by several seismic sequences in recent years, showing evidence of the activation of multiple segments of normal fault systems in a variable and, relatively short, time span, as in the case of the 1980 Irpinia earthquake (three shocks in 40 s), the 1997 Umbria-Marche sequence (four main shocks in 18 days) and the 2009 L'Aquila earthquake having three segments activated within a few weeks. The 2016-2017 central Apennines seismic sequence begin on August 24th with a MW 6.0 earthquake, which strike the region between Amatrice and Accumoli causing 299 fatalities. This earthquake ruptures a nearly 20 km long normal fault and shows a quite heterogeneous slip distribution. On October 26th, another main shock (MW 5.9) occurs near Visso extending the activated seismogenic area toward the NW. It is a double event rupturing contiguous patches on the fault segment of the normal fault system. Four days after the second main shock, on October 30th, a third earthquake (MW 6.5) occurs near Norcia, roughly midway between Accumoli and Visso. In this work we have inverted strong motion waveforms and GPS data to retrieve the source model of the MW 6.5 event with the aim of interpreting the rupture process in the framework of this complex sequence of moderate magnitude earthquakes. We noted that some preliminary attempts to model the slip distribution of the October 30th main shock using a single fault plane oriented along the Apennines did not provide convincing fits to the observed waveforms. In addition, the deformation pattern inferred from satellite observations suggested the activation of a multi-fault structure, that is coherent to the complexity and the extension of the geological surface deformation. We investigated the role of multi-fault ruptures and we found that this event revealed an extraordinary complexity of the rupture geometry and evolution: the coseismic rupture propagated almost simultaneously on a normal fault and on a blind fault, possibly inherited from compressional tectonics. These earthquakes raise serious concerns on our understanding of fault segmentation and seismicity evolution during sequences of normal faulting earthquakes. Finally, the retrieved rupture history has important implications on seismic hazard assessment and on the maximum expected magnitude in a given tectonic area.

Active fault mapping in Karonga-Malawi after the December 19, 2009 Ms 6.2 seismic event

NASA Astrophysics Data System (ADS)

Macheyeki, A. S.; Mdala, H.; Chapola, L. S.; Manhiça, V. J.; Chisambi, J.; Feitio, P.; Ayele, A.; Barongo, J.; Ferdinand, R. W.; Ogubazghi, G.; Goitom, B.; Hlatywayo, J. D.; Kianji, G. K.; Marobhe, I.; Mulowezi, A.; Mutamina, D.; Mwano, J. M.; Shumba, B.; Tumwikirize, I.

2015-02-01

The East African Rift System (EARS) has natural hazards - earthquakes, volcanic eruptions, and landslides along the faulted margins, and in response to ground shaking. Strong damaging earthquakes have been occurring in the region along the EARS throughout historical time, example being the 7.4 (Ms) of December 1910. The most recent damaging earthquake is the Karonga earthquake in Malawi, which occurred on 19th December, 2009 with a magnitude of 6.2 (Ms). The earthquake claimed four lives and destroyed over 5000 houses. In its effort to improve seismic hazard assessment in the region, Eastern and Southern Africa Seismological Working Group (ESARSWG) under the sponsorship of the International Program on Physical Sciences (IPPS) carried out a study on active fault mapping in the region. The fieldwork employed geological and geophysical techniques. The geophysical techniques employed are ground magnetic, seismic refraction and resistivity surveys but are reported elsewhere. This article gives findings from geological techniques. The geological techniques aimed primarily at mapping of active faults in the area in order to delineate presence or absence of fault segments. Results show that the Karonga fault (the Karonga fault here referred to as the fault that ruptured to the surface following the 6th-19th December 2009 earthquake events in the Karonga area) is about 9 km long and dominated by dip slip faulting with dextral and insignificant sinistral components and it is made up of 3-4 segments of length 2-3 km. The segments are characterized by both left and right steps. Although field mapping show only 9 km of surface rupture, maximum vertical offset of about 43 cm imply that the surface rupture was in little excess of 14 km that corresponds with Mw = 6.4. We recommend the use or integration of multidisciplinary techniques in order to better understand the fault history, mechanism and other behavior of the fault/s for better urban planning in the area.

Slip history and dynamic implications of the 1999 Chi-Chi, Taiwan, earthquake

USGS Publications Warehouse

Ji, C.; Helmberger, D.V.; Wald, D.J.; Ma, K.-F.

2003-01-01

We investigate the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using extensive near-source observations, including three-component velocity waveforms at 36 strong motion stations and 119 GPS measurements. A three-plane fault geometry derived from our previous inversion using only static data [Ji et al., 2001] is applied. The slip amplitude, rake angle, rupture initiation time, and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm [Ji et al., 2002b]. The inversion results are validated by the forward prediction of an independent data set, the teleseismic P and SH ground velocities, with notable agreement. The results show that the total seismic moment release of this earthquake is 2.7 ?? 1020 N m and that most of the slip occured in a triangular-shaped asperity involving two fault segments, which is consistent with our previous static inversion. The rupture front propagates with an average rupture velocity of ???2.0 km s-1, and the average slip duration (risetime) is 7.2 s. Several interesting observations related to the temporal evolution of the Chi-Chi earthquake are also investigated, including (1) the strong effect of the sinuous fault plane of the Chelungpu fault on spatial and temporal variations in slip history, (2) the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation, and (3 the observation that the peak slip velocity near the surface is, in general, higher than on the deeper portion of the fault plane, as predicted by dynamic modeling.

Stress drop inferred from dynamic rupture simulations consistent with Moment-Rupture area empirical scaling models: Effects of week shallow zone

NASA Astrophysics Data System (ADS)

Dalguer, L. A.; Miyake, H.; Irikura, K.; Wu, H., Sr.

2016-12-01

Empirical scaling models of seismic moment and rupture area provide constraints to parameterize source parameters, such as stress drop, for numerical simulations of ground motion. There are several scaling models published in the literature. The effect of the finite width seismogenic zone and the free-surface have been attributed to cause the breaking of the well know self-similar scaling (e.g. Dalguer et al, 2008) given origin to the so called L and W models for large faults. These models imply the existence of three-stage scaling relationship between seismic moment and rupture area (e.g. Irikura and Miyake, 2011). In this paper we extend the work done by Dalguer et al 2008, in which these authors calibrated fault models that match the observations showing that the average stress drop is independent of earthquake size for buried earthquakes, but scale dependent for surface-rupturing earthquakes. Here we have developed additional sets of dynamic rupture models for vertical strike slip faults to evaluate the effect of the weak shallow layer (WSL) zone for the calibration of stress drop. Rupture in the WSL zone is expected to operate with enhanced energy absorption mechanism. The set of dynamic models consists of fault models with width 20km and fault length L=20km, 40km, 60km, 80km, 100km, 120km, 200km, 300km and 400km and average stress drop values of 2.0MPa, 2.5MPa, 3.0MPa, 3.5MPa, 5.0MPa and 7.5MPa. For models that break the free-surface, the WSL zone is modeled assuming a 2km width with stress drop 0.0MPa or -2.0 MPa. Our results show that depending on the characterization of the WSL zone, the average stress drop at the seismogenic zone that fit the empirical models changes. If WSL zone is not considered, that is, stress drop at SL zone is the same as the seismogenic zone, average stress drop is about 20% smaller than models with WSL zone. By introducing more energy absorption at the SL zone, that could be the case of large mature faults, the average stress drop in the seismogenic zone increases. Suggesting that large earthquakes need higher stress drop to break the fault than buried and moderate earthquakes. Therefore, the value of the average stress drop for large events that break the free-source depend on the definition of the WSL. Suggesting that the WSL plays an important role on the prediction of final slip and fault displacement.

Spatio-temporal mapping of plate boundary faults in California using geodetic imaging

USGS Publications Warehouse

Donnellan, Andrea; Arrowsmith, Ramon; DeLong, Stephen B.

2017-01-01

The Pacific–North American plate boundary in California is composed of a 400-km-wide network of faults and zones of distributed deformation. Earthquakes, even large ones, can occur along individual or combinations of faults within the larger plate boundary system. While research often focuses on the primary and secondary faults, holistic study of the plate boundary is required to answer several fundamental questions. How do plate boundary motions partition across California faults? How do faults within the plate boundary interact during earthquakes? What fraction of strain accumulation is relieved aseismically and does this provide limits on fault rupture propagation? Geodetic imaging, broadly defined as measurement of crustal deformation and topography of the Earth’s surface, enables assessment of topographic characteristics and the spatio-temporal behavior of the Earth’s crust. We focus here on crustal deformation observed with continuous Global Positioning System (GPS) data and Interferometric Synthetic Aperture Radar (InSAR) from NASA’s airborne UAVSAR platform, and on high-resolution topography acquired from lidar and Structure from Motion (SfM) methods. Combined, these measurements are used to identify active structures, past ruptures, transient motions, and distribution of deformation. The observations inform estimates of the mechanical and geometric properties of faults. We discuss five areas in California as examples of different fault behavior, fault maturity and times within the earthquake cycle: the M6.0 2014 South Napa earthquake rupture, the San Jacinto fault, the creeping and locked Carrizo sections of the San Andreas fault, the Landers rupture in the Eastern California Shear Zone, and the convergence of the Eastern California Shear Zone and San Andreas fault in southern California. These examples indicate that distribution of crustal deformation can be measured using interferometric synthetic aperture radar (InSAR), Global Navigation Satellite System (GNSS), and high-resolution topography and can improve our understanding of tectonic deformation and rupture characteristics within the broad plate boundary zone.

Modeling Earthquake Rupture and Corresponding Tsunamis Along a Segment of the Alaskan-Aleutian Megathrust

NASA Astrophysics Data System (ADS)

Ryan, K. J.; Geist, E. L.; Oglesby, D. D.; Kyriakopoulos, C.

2016-12-01

Motivated by the 2011 Mw 9 Tohoku-Oki event, we explore the effects of realistic fault dynamics on slip, free surface deformation, and the resulting tsunami generation and local propagation from a hypothetical Mw 9 megathrust earthquake along the Alaskan-Aleutian (A-A) Megathrust. We demonstrate three scenarios: a spatially-homogenous prestress and frictional parameter model and two models with rate-strengthening-like friction (e.g., Dieterich, 1992). We use a dynamic finite element code to model 3-D ruptures, using time-weakening friction (Andrews, 2004) as a proxy for rate-strengthening friction, along a portion of the A-A subduction zone. Given geometric, material, and plate-coupling data along the A-A megathrust assembled from the Science Application for Risk Reduction (SAFRR) team (e.g., Bruns et al., 1987; Hayes et al., 2012; Johnson et al., 2004; Santini et al., 2003; Wells at al., 2003), we are able to dynamically model rupture. Adding frictional-strengthening to a region of the fault reduces both average slip and free surface displacement above the strengthening zone, with the magnitude of the reductions depending on the strengthening zone location. Corresponding tsunami models, which use a finite difference method to solve the long-wave equations (e.g., Liu et al., 1995; Satake, 2002; Shuto, 1991), match sea floor displacement, in time, to the free surface displacement from the rupture models. Tsunami models show changes in local peak amplitudes and beaming patterns for each slip distribution. Given these results, other heterogeneous parameterizations, with respect to prestress and friction, still need to be examined. Additionally, a more realistic fault geometry will likely affect the rupture dynamics. Thus, future work will incorporate stochastic stress and friction distributions as well as a more complex fault geometry based on Slab 1.0 (Hayes et al., 2012).

Detailed Mapping of Historical and Preinstrumental Earthquake Ruptures in Central Asia Using Multi-Scale, Multi-Platform Photogrammetry

NASA Astrophysics Data System (ADS)

Elliott, A. J.; Walker, R. T.; Parsons, B.; Ren, Z.; Ainscoe, E. A.; Abdrakhmatov, K.; Mackenzie, D.; Arrowsmith, R.; Gruetzner, C.

2016-12-01

In regions of the planet with long historical records, known past seismic events can be attributed to specific fault sources through the identification and measurement of single-event scarps in high-resolution imagery and topography. The level of detail captured by modern remote sensing is now sufficient to map and measure complete earthquake ruptures that were originally only sparsely mapped or overlooked entirely. We can thus extend the record of mapped earthquake surface ruptures into the preinstrumental period and capture the wealth of information preserved in the numerous historical earthquake ruptures throughout regions like Central Asia. We investigate two major late 19th and early 20th century earthquakes that are well located macroseismically but whose fault sources had proved enigmatic in the absence of detailed imagery and topography. We use high-resolution topographic models derived from photogrammetry of satellite, low-altitude, and ground-based optical imagery to map and measure the coseismic scarps of the 1889 M8.3 Chilik, Kazakhstan and 1932 M7.6 Changma, China earthquakes. Measurement of the scarps on the combined imagery and topography reveals the extent and slip distribution of coseismic rupture in each of these events, showing both earthquakes involved multiple faults with variable kinematics. We use a 1-m elevation model of the Changma fault derived from Pleiades satellite imagery to map the changing kinematics of the 1932 rupture along strike. For the 1889 Chilik earthquake we use 1.5-m SPOT-6 satellite imagery to produce a regional elevation model of the fault ruptures, from which we identify three distinct, intersecting fault systems that each have >20 km of fresh, single-event scarps. Along sections of each of these faults we construct high resolution (330 points per sq m) elevation models using quadcopter- and helikite-mounted cameras. From the detailed topography we measure single-event oblique offsets of 6-10 m, consistent with the large inferred magnitude of the 1889 Chilik event. High resolution, photogrammetric topography offers a low-cost, effective way to thoroughly map rupture traces and measure coseismic displacements for past fault ruptures, extending our record of coseismic displacements into a past rich with formerly sparsely documented ruptures.

Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore Southern California

USGS Publications Warehouse

Marlow, M. S.; Gardner, J.V.; Normark, W.R.

2000-01-01

Recently acquired high-resolution multibeam bathymetric data reveal several linear traces that are the surficial expressions of seafloor rupture of Holocene faults on the upper continental slope southeast of the Palos Verdes Peninsula. High-resolution multichannel and boomer seismic-reflection profiles show that these linear ruptures are the surficial expressions of Holocene faults with vertical to steep dips. The most prominent fault on the multibeam bathymetry is about 10 km to the west of the mapped trace of the Palos Verdes fault and extends for at least 14 km between the shelf edge and the base of the continental slope. This fault is informally called the Avalon Knoll fault for the nearby geographic feature of that name. Seismic-reflection profiles show that the Avalon Knoll fault is part of a northwest-trending complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam bathymetry. This fault complex may extend onshore and contribute to the missing balance of Quaternary uplift determined for the Palos Verdes Hills and not accounted for by vertical uplift along the onshore Palos Verdes fault. We investigate the extent of the newly located offshore Avalon Knoll fault and use this mapped fault length to estimate likely minimum magnitudes for events along this fault.

Multi-Fault Rupture Scenarios in the Brawley Seismic Zone

NASA Astrophysics Data System (ADS)

Kyriakopoulos, C.; Oglesby, D. D.; Rockwell, T. K.; Meltzner, A. J.; Barall, M.

2017-12-01

Dynamic rupture complexity is strongly affected by both the geometric configuration of a network of faults and pre-stress conditions. Between those two, the geometric configuration is more likely to be anticipated prior to an event. An important factor in the unpredictability of the final rupture pattern of a group of faults is the time-dependent interaction between them. Dynamic rupture models provide a means to investigate this otherwise inscrutable processes. The Brawley Seismic Zone in Southern California is an area in which this approach might be important for inferring potential earthquake sizes and rupture patterns. Dynamic modeling can illuminate how the main faults in this area, the Southern San Andreas (SSAF) and Imperial faults, might interact with the intersecting cross faults, and how the cross faults may modulate rupture on the main faults. We perform 3D finite element modeling of potential earthquakes in this zone assuming an extended array of faults (Figure). Our results include a wide range of ruptures and fault behaviors depending on assumptions about nucleation location, geometric setup, pre-stress conditions, and locking depth. For example, in the majority of our models the cross faults do not strongly participate in the rupture process, giving the impression that they are not typically an aid or an obstacle to the rupture propagation. However, in some cases, particularly when rupture proceeds slowly on the main faults, the cross faults indeed can participate with significant slip, and can even cause rupture termination on one of the main faults. Furthermore, in a complex network of faults we should not preclude the possibility of a large event nucleating on a smaller fault (e.g. a cross fault) and eventually promoting rupture on the main structure. Recent examples include the 2010 Mw 7.1 Darfield (New Zealand) and Mw 7.2 El Mayor-Cucapah (Mexico) earthquakes, where rupture started on a smaller adjacent segment and later cascaded into a larger event. For that reason, we are investigating scenarios of a moderate rupture on a cross fault, and determining conditions under which the rupture will propagate onto the adjacent SSAF. Our investigation will provide fundamental insights that may help us interpret faulting behaviors in other areas, such as the complex Mw 7.8 2016 Kaikoura (New Zealand) earthquake.

Expectable Earthquakes and their ground motions in the Van Norman Reservoirs Area

USGS Publications Warehouse

Wesson, R.L.; Page, R.A.; Boore, D.M.; Yerkes, R.F.

1974-01-01

The upper and lower Van Norman dams, in northwesternmost San Fernando Valley about 20 mi (32 km) northwest of downtown Los Angeles, were severely damaged during the 1971 San Fernando earthquake. An investigation of the geologic-seismologic setting of the Van Norman area indicates that an earthquake of at least M 7.7 may be expected in the Van Norman area. The expectable transitory effects in the Van Norman area of such an earthquake are as follows: peak horizontal acceleration of at least 1.15 g, peak velocity of displacement of 4.43 ft/sec (135 cm/sec), peak displacement of 2.3 ft (70 cm), and duration of shaking at accelerations greater than 0.05 g, 40 sec. A great earthquake (M 8+) on the San Andreas fault, 25 mi distant, also is expectable. Transitory effects in the Van Norman area from such an earthquake are estimated as follows: peak horizontal acceleration of 0.5 g, peak velocity of 1.97 ft/sec (60 cm/sec), displacement of 1.31 ft (40 cm), and duration of shaking at accelerations greater than 0.05 g, 80 sec. The permanent effects of the expectable local earthquake could include simultaneous fault movement at the lower damsite, the upper damsite, and the site proposed for a replacement dam halfway between the upper and lower dams. The maximum differential displacements due to such movements are estimated at 16.4 ft (5 m) at the lower damsite and about 9.6 ft (2.93 m) at the upper and proposed damsites. The 1971 San Fernando earthquake (M 6?) was accompanied by the most intense ground motions ever recorded instrumentally for a natural earthquake. At the lower Van Norman dam, horizontal accelerations exceeded 0.6 g, and shaking greater than 0.25 g lasted for about 13 see; at Pacoima dam, 6 mi (10 km) northeast of the lower dam, high-frequency peak horizontal accelerations of 1.25 g were recorded in two directions, and shaking greater than 0.25 g lasted for about 7 sec. Permanent effects of the earthquake include slope failures in the embankments of the upper and lower Van Norman dams, rupturing of the ground surface by faulting along parts of the zone of old faults that extends easterly through the reservoir area and across the northern part of the valley, folding or arching of the ground surface, and differential horizontal displacement of the terrane north and south of the fault zone. Although a zone of old faults extends through the reservoir area, the 1971 surface ruptures apparently did not; however, arching and horizontal displacements caused small relative displacements of the abutment areas of each of the three damsites. The 1971 arching coincided with preexisting topographic highs, and the surface ruptures coincided with eroded fault scarps and a buried ground-water impediment formed by pre-1971 faulting in young valley fill. This coincidence with evidence of past deformation indicates that the 1971 deformations were the result of a continuing geologic process that is expected to produce similar deformations during future events. The 1971 San Fernando earthquake probably was not the largest that has occurred in this area during the last approximately 200 years, as indicated by a buried fault like scarp about 200 years old that is higher than, and aligned with, 1971 fault scarps. In addition, the San Fernando zone of 1971 ruptures is part of a regional tectonic system that includes the San Andreas and associated faults; one of these, the White Wolf fault north of the San Andreas, is symmetrical in structural attitude with the San Fernando zone and ruptured the ground surface during the 1952 Kern County earthquake (M 7.7). Other large earthquakes associated with surface rupturing on faults of this system include the 1857 Fort Tejon earthquake (M 8+) and possibly the 1852 Big Pine earthquake. Several other historic earthquakes in this general area are not known to be associated with surface ruptures, but were large enough to cause damage in the northern San Fernando Valley. The Van Norman rese

Geologic environment of the Van Norman Reservoirs area

USGS Publications Warehouse

Yerkes, R.F.; Bonilla, M.G.; Youd, T.L.; Sims, J.D.

1974-01-01

The upper and lower Van Norman dams, in northwesternmost San Fernando Valley about 20 mi (32 km) northwest of downtown Los Angeles, were severely damaged during the 1971 San Fernando earthquake. An investigation of the geologic-seismologic setting of the Van Norman area indicates that an earthquake of at least M 7.7 may be expected in the Van Norman area. The expectable transitory effects in the Van Norman area of such an earthquake are as follows: peak horizontal acceleration of at least 1.15 g, peak velocity of displacement of 4.43 ft/sec (135 cm/sec), peak displacement of 2.3 ft (70 cm), and duration of shaking at accelerations greater than 0.05 g, 40 sec. A great earthquake (M 8+) on the San Andreas fault, 25 mi distant, also is expectable. Transitory effects in the Van Norman area from such an earthquake are estimated as follows: peak horizontal acceleration of 0.5 g, peak velocity of 1.97 ft/sec (60 cm/sec), displacement of 1.31 ft (40 cm), and duration of shaking at accelerations greater than 0.05 g, 80 sec. The permanent effects of the expectable local earthquake could include simultaneous fault movement at the lower damsite, the upper damsite, and the site proposed for a replacement dam halfway between the upper and lower dams. The maximum differential displacements due to such movements are estimated at 16.4 ft (5 m) at the lower damsite and about 9.6 ft (2.93 m) at the upper and proposed damsites. The 1971 San Fernando earthquake (M 6?) was accompanied by the most intense ground motions ever recorded instrumentally for a natural earthquake. At the lower Van Norman dam, horizontal accelerations exceeded 0.6 g, and shaking greater than 0.25 g lasted for about 13 see; at Pacoima dam, 6 mi (10 km) northeast of the lower dam, high-frequency peak horizontal accelerations of 1.25 g were recorded in two directions, and shaking greater than 0.25 g lasted for about 7 sec. Permanent effects of the earthquake include slope failures in the embankments of the upper and lower Van Norman dams, rupturing of the ground surface by faulting along parts of the zone of old faults that extends easterly through the reservoir area and across the northern part of the valley, folding or arching of the ground surface, and differential horizontal displacement of the terrane north and south of the fault zone. Although a zone of old faults extends through the reservoir area, the 1971 surface ruptures apparently did not; however, arching and horizontal displacements caused small relative displacements of the abutment areas of each of the three damsites. The 1971 arching coincided with preexisting topographic highs, and the surface ruptures coincided with eroded fault scarps and a buried ground-water impediment formed by pre-1971 faulting in young valley fill. This coincidence with evidence of past deformation indicates that the 1971 deformations were the result of a continuing geologic process that is expected to produce similar deformations during future events. The 1971 San Fernando earthquake probably was not the largest that has occurred in this area during the last approximately 200 years, as indicated by a buried fault like scarp about 200 years old that is higher than, and aligned with, 1971 fault scarps. In addition, the San Fernando zone of 1971 ruptures is part of a regional tectonic system that includes the San Andreas and associated faults; one of these, the White Wolf fault north of the San Andreas, is symmetrical in structural attitude with the San Fernando zone and ruptured the ground surface during the 1952 Kern County earthquake (M 7.7). Other large earthquakes associated with surface rupturing on faults of this system include the 1857 Fort Tejon earthquake (M 8+) and possibly the 1852 Big Pine earthquake. Several other historic earthquakes in this general area are not known to be associated with surface ruptures, but were large enough to cause damage in the northern San Fernando Valley. The Van Norman rese

Earthquake geology and paleoseismology of major strands of the San Andreas fault system: Chapter 38

USGS Publications Warehouse

Rockwell, Thomas; Scharer, Katherine M.; Dawson, Timothy E.

2016-01-01

The San Andreas fault system in California is one of the best-studied faults in the world, both in terms of the long-term geologic history and paleoseismic study of past surface ruptures. In this paper, we focus on the Quaternary to historic data that have been collected from the major strands of the San Andreas fault system, both on the San Andreas Fault itself, and the major subparallel strands that comprise the plate boundary, including the Calaveras-Hayward- Rogers Creek-Maacama fault zone and the Concord-Green Valley-Bartlett Springs fault zone in northern California, and the San Jacinto and Elsinore faults in southern California. The majority of the relative motion between the Pacific and North American lithospheric plates is accommodated by these faults, with the San Andreas slipping at about 34 mm/yr in central California, decreasing to about 20 mm/yr in northern California north of its juncture with the Calaveras and Concord faults. The Calaveras-Hayward-Rogers Creek-Maacama fault zone exhibits a slip rate of 10-15 mm/yr, whereas the rate along the Concord-Green Valley-Bartlett Springs fault zone is lower at about 5 mm/yr. In southern California, the San Andreas exhibits a slip rate of about 35 mm/yr along the Mojave section, decreasing to as low as 10-15 mm/yr along its juncture with the San Jacinto fault, and about 20 mm/yr in the Coachella Valley. The San Jacinto and Elsinore fault zones exhibit rates of about 15 and 5 mm/yr, respectively. The average recurrence interval for surface-rupturing earthquakes along individual elements of the San Andreas fault system range from 100-500 years and is consistent with slip rate at those sites: higher slip rates produce more frequent or larger earthquakes. There is also evidence of short-term variations in strain release (slip rate) along various fault sections, as expressed as “flurries” or clusters of earthquakes as well as periods of relatively fewer surface ruptures in these relatively short records. This is reflected by non-periodic coefficients of variation in earthquake recurrence of 0.4 to 0.7 for the various paleoseismic sites.

Radiated energy and the rupture process of the Denali fault earthquake sequence of 2002 from broadband teleseismic body waves

USGS Publications Warehouse

Choy, G.L.; Boatwright, J.

2004-01-01

Displacement, velocity, and velocity-squared records of P and SH body waves recorded at teleseismic distances are analyzed to determine the rupture characteristics of the Denali fault, Alaska, earthquake of 3 November 2002 (MW 7.9, Me 8.1). Three episodes of rupture can be identified from broadband (???0.1-5.0 Hz) waveforms. The Denali fault earthquake started as a MW 7.3 thrust event. Subsequent right-lateral strike-slip rupture events with centroid depths of 9 km occurred about 22 and 49 sec later. The teleseismic P waves are dominated by energy at intermediate frequencies (0.1-1 Hz) radiated by the thrust event, while the SH waves are dominated by energy at lower frequencies (0.05-0.2 Hz) radiated by the strike-slip events. The strike-slip events exhibit strong directivity in the teleseismic SH waves. Correcting the recorded P-wave acceleration spectra for the effect of the free surface yields an estimate of 2.8 ?? 1015 N m for the energy radiated by the thrust event. Correcting the recorded SH-wave acceleration spectra similarly yields an estimate of 3.3 ?? 10 16 N m for the energy radiated by the two strike-slip events. The average rupture velocity for the strike-slip rupture process is 1.1??-1.2??. The strike-slip events were located 90 and 188 km east of the epicenter. The rupture length over which significant or resolvable energy is radiated is, thus, far shorter than the 340-km fault length over which surface displacements were observed. However, the seismic moment released by these three events, 4 ?? 1020 N m, was approximately half the seismic moment determined from very low-frequency analyses of the earthquake. The difference in seismic moment can be reasonably attributed to slip on fault segments that did not radiate significant or coherent seismic energy. These results suggest that very large and great strike-slip earthquakes can generate stress pulses that rapidly produce substantial slip with negligible stress drop and little discernible radiated energy on fault segments distant from the initial point of nucleation. The existence of this energy-deficient rupture mode has important implications for the evaluation of the seismic hazard of very large strike-slip earthquakes.

The Surface Displacement Field of the November 8, 1997, Mw7.6 Manyi (Tibet) Earthquake Observed with ERS InSAR Data

NASA Technical Reports Server (NTRS)

Peltzer, G.; Crampe, F.

1998-01-01

ERS2 radar data acquired before and after the Mw7.6, Manyi (Tibet) earthquake of November 8, 1997, provide geodetic information about the surface displacement produced by the earthquake in two ways. (1) The sub-pixel geometric adjustment of the before and after images provides a two dimensional offset field with a resolution of approx, 1m in both the range (radar line of sight) and azimuth (satellite track) directions. Comparison of offsets in azimuth and range indicates that the displacement along the fault is essentially strike-slip and in a left-lateral sense. The offset map reveals a relatively smooth and straight, N78E surface rupture that exceeds 150 km in length, consistent with the EW plane of the Harvard CMT solution. The rupture follows the trace of a quaternary fault visible on satellite imagery (Tapponnier and Molnar, 1978; Wan Der Woerd, pers. comm.). (2) Interferometric processing of the SAR data provides a range displacement map with a precision of a few millimeters. The slip distribution along the rupture reconstructed from the range change map is a bell-shaped curve in the 100-km long central section of the fault with smaller, local maxima near both ends. The curve shows that the fault slip exceeds 2.2 m in range, or 6.2 in strike-slip, along a 30-km long section of the fault and remains above 1 m in range, approx. 3 m strike-slip, along most of its length. Preliminary forward modeling of the central section of the rupture, assuming a uniform slip distribution with depth, indicates that the slip occur-red essentially between 0 and the depth of 10 km, consistent with a relatively shallow event (Velasco et al., 1998).

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

N. Seth Carpenter; Suzette J. Payne; Annette L. Schafer

We recognize a discrepancy in magnitudes estimated for several Basin and Range, U.S.A. faults. For example, magnitudes predicted for the Wasatch (Utah), Lost River (Idaho), and Lemhi (Idaho) faults from fault segment lengths (L{sub seg}) where lengths are defined between geometrical, structural, and/or behavioral discontinuities assumed to persistently arrest rupture, are consistently less than magnitudes calculated from displacements (D) along these same segments. For self-similarity, empirical relationships (e.g. Wells and Coppersmith, 1994) should predict consistent magnitudes (M) using diverse fault dimension values for a given fault (i.e. M {approx} L{sub seg}, should equal M {approx} D). Typically, the empirical relationshipsmore » are derived from historical earthquake data and parameter values used as input into these relationships are determined from field investigations of paleoearthquakes. A commonly used assumption - grounded in the characteristic-earthquake model of Schwartz and Coppersmith (1984) - is equating L{sub seg} with surface rupture length (SRL). Many large historical events yielded secondary and/or sympathetic faulting (e.g. 1983 Borah Peak, Idaho earthquake) which are included in the measurement of SRL and used to derive empirical relationships. Therefore, calculating magnitude from the M {approx} SRL relationship using L{sub seg} as SRL leads to an underestimation of magnitude and the M {approx} L{sub seg} and M {approx} D discrepancy. Here, we propose an alternative approach to earthquake magnitude estimation involving a relationship between moment magnitude (Mw) and length, where length is L{sub seg} instead of SRL. We analyze seven historical, surface-rupturing, strike-slip and normal faulting earthquakes for which segmentation of the causative fault and displacement data are available and whose rupture included at least one entire fault segment, but not two or more. The preliminary Mw {approx} L{sub seg} results are strikingly consistent with Mw {approx} D calculations using paleoearthquake data for the Wasatch, Lost River, and Lemhi faults, demonstrating self-similarity and implying that the Mw {approx} L{sub seg} relationship should supplant M {approx} SRL relationships currently employed in seismic hazard analyses. The relationship will permit reliable use of L{sub seg} data from field investigations and proper use and weighting of multiple-segment-rupture scenarios in seismic hazard analyses, and eliminate the need to reconcile the Mw {approx} SRL and Mw {approx} D differences in a multiple-parameter relationship for segmented faults.« less

Geotechnical Reconnaissance of the 3 November 2002, Mw 7.9, Denali- Earthquake, Alaska

NASA Astrophysics Data System (ADS)

Kayen, R.; Sitar, N.; Carver, G.; Collins, B.; Moss, R.

2002-12-01

Following the Mw 7.9 earthquake on the Denali and Totschunda faults on 3 November 2002, we conducted a reconnaissance of the region to investigate geotechnical and surface rupture features of the event. The focus of our investigation was to characterize the spatial extent and amplitude of ground failures and fault displacements, and assess damage to structures. As a first step, our team flew along the Denali fault from the Black Rapids Glacier, west of the Richardson Highway, to the Glenn Highway (Tok Cut-off). We also conducted a brief air reconnaissance of the southern part of the Totschunda fault northwest of the Nabesna River, and brief ground surveys where the fault intersected the highways and the TAPS pipeline. The most noteworthy aerial observations were that geotechnical and structural damages appeared to be focused towards the eastern end of the Denali- fault rupture area. For example, liquefaction features in the bars of the Tanana River, north of the fault-break, are sparsely located from Fairbanks to Delta, but are pervasive throughout the eastern area of the break to Northway Junction, the eastern limit of our survey. Likewise, for the four glacier-proximal rivers draining toward the north, little or no liquefaction was observed on the western Delta and Johnson Rivers whereas, the eastern Johnson and Tok Rivers and, especially, the Nabesna River had observable-to-abundant fissures and sand vents. Another curious aspect of the apparent differences in strong motion along and across the fault was the abundance of landslide and rock avalanche features on the south side of the fault and a dearth of these features on the northern side. Ice on frozen lakes and ponds were shattered within about 30-40 km of the fault along the western part of the surface rupture and to the east became more widespread. In the Northway region ice on most lakes was broken at distances of more than 100 km. The surface rupture was very linear, continuous, and confined to a relatively narrow zone composed over much of its length by closely spaced en-echelin breaks. Few significant branches or splays were observed. The apparent slip on the Denali Fault was also observed to increase to the east from Black Rapids Glacier toward the Mentasta Village area. , On the Totschunda fault, the rupture decreased in slip before dying out approximately 5 kilometers northwest of the Nabesna River. Where the fault crossed the trans-Alaska pipeline, dislocation occurred along a series of en echelon fissures. One of these en echelon breaks intersected the end of one of the Teflon surfaced skids (sleepers) that supports the pipe in the fault zone, displacing it about a meter but not damaging the pipe. Strong shaking and movement of the pipe resulted in damage to 8 horizontal support members, and 9 anchored supports near the fault crossing. These affects were not critical to the integrity of the pipeline, which performed well during the event. This reconnaissance was supported by the National Science Foundation (NSF) and the US Geological Survey (USGS).

The 7.9 Denali Fault, Alaska Earthquake of November 3, 2002: Aftershock Locations, Moment Tensors and Focal Mechanisms from the Regional Seismic Network Data

NASA Astrophysics Data System (ADS)

Ratchkovski, N. A.; Hansen, R. A.; Kore, K. R.

2003-04-01

The largest earthquake ever recorded on the Denali fault system (magnitude 7.9) struck central Alaska on November 3, 2002. It was preceded by a magnitude 6.7 earthquake on October 23. This earlier earthquake and its zone of aftershocks were located ~20 km to the west of the 7.9 quake. Aftershock locations and surface slip observations from the 7.9 quake indicate that the rupture was predominately unilateral in the eastward direction. The geologists mapped a ~300-km-long rupture and measured maximum offsets of 8.8 meters. The 7.9 event ruptured three different faults. The rupture began on the northeast trending Susitna Glacier Thrust fault, a splay fault south of the Denali fault. Then the rupture transferred to the Denali fault and propagated eastward for 220 km. At about 143W the rupture moved onto the adjacent southeast-trending Totschunda fault and propagated for another 55 km. The cumulative length of the 6.7 and 7.9 aftershock zones along the Denali and Totschunda faults is about 380 km. The earthquakes were recorded and processed by the Alaska Earthquake Information Center (AEIC). The AEIC acquires and processes data from the Alaska Seismic Network, consisting of over 350 seismograph stations. Nearly 40 of these sites are equipped with the broad-band sensors, some of which also have strong motion sensors. The rest of the stations are either 1 or 3-component short-period instruments. The data from these stations are collected, processed and archived at the AEIC. The AEIC staff installed a temporary seismic network of 6 instruments following the 6.7 earthquake and an additional 20 stations following the 7.9 earthquake. Prior to the 7.9 Denali Fault event, the AEIC was locating 35 to 50 events per day. After the event, the processing load increased to over 300 events per day during the first week following the event. In this presentation, we will present and interpret the aftershock location patterns, first motion focal mechanism solutions, and regional seismic moment tensors for the larger events. We used the double difference method to relocate aftershocks of both the 6.7 and 7.9 events. The relocated aftershocks indicate complex faulting along the rupture zone. The aftershocks are located not only along the main rupture zone, but also illuminate multiple splay faults north and south of the Denali fault. We calculated principal stress directions along the Denali fault both before and after the 7.9 event from the focal mechanisms. The stress orientations before and after the event are nearly identical. The maximum horizontal compressive stress is nearly normal to the trace of the Denali fault and rotates gradually from NW orientation at the western end of the rupture zone to NE orientation near the junction with the Totschunda fault.

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.

Cohesive zone length of metagabbro at supershear rupture velocity

NASA Astrophysics Data System (ADS)

Fukuyama, Eiichi; Xu, Shiqing; Yamashita, Futoshi; Mizoguchi, Kazuo

2016-10-01

We investigated the shear strain field ahead of a supershear rupture. The strain array data along the sliding fault surfaces were obtained during the large-scale biaxial friction experiments at the National Research Institute for Earth Science and Disaster Resilience. These friction experiments were done using a pair of meter-scale metagabbro rock specimens whose simulated fault area was 1.5 m × 0.1 m. A 2.6-MPa normal stress was applied with loading velocity of 0.1 mm/s. Near-fault strain was measured by 32 two-component semiconductor strain gauges installed at an interval of 50 mm and 10 mm off the fault and recorded at an interval of 1 MHz. Many stick-slip events were observed in the experiments. We chose ten unilateral rupture events that propagated with supershear rupture velocity without preceding foreshocks. Focusing on the rupture front, stress concentration was observed and sharp stress drop occurred immediately inside the ruptured area. The temporal variation of strain array data is converted to the spatial variation of strain assuming a constant rupture velocity. We picked up the peak strain and zero-crossing strain locations to measure the cohesive zone length. By compiling the stick-slip event data, the cohesive zone length is about 50 mm although it scattered among the events. We could not see any systematic variation at the location but some dependence on the rupture velocity. The cohesive zone length decreases as the rupture velocity increases, especially larger than √{2} times the shear wave velocity. This feature is consistent with the theoretical prediction.

New Data on Quaternary Surface Offset and Slip Rates of the Oquirrh Fault (Utah, USA) from DSMs made with Structure-from-Motion Methods

NASA Astrophysics Data System (ADS)

Bunds, M. P.; Andreini, J.; Larsen, K.; Fletcher, A.; Arnold, M.; Toke, N. A.

2016-12-01

We generated two high-resolution digital surface models (DSMs) using imagery collected with inexpensive quadcopters and processed with structure-from-motion software to measure offsets of pluvial Lake Bonneville shorelines along the Oquirrh Fault in Utah, USA. The Oquirrh Fault is a west-dipping normal fault that bounds the populous Tooele Valley and is likely contiguous with the East Great Salt Lake Fault to the north and Southern Oquirrh and Topliff Hill Faults to the south, forming a fault system >200 km long, the second longest in Utah. However, knowledge of the fault's parameters is based primarily on one trenching study on the northern section of the fault (Olig et al., 1996). The two DSMs were made using a 24 Mpixel Sony A5100 and 12 Mpixel GoPro camera, have 5 and 10 cm pixels, and span 3.9 km of the fault's trace at the boundary between its central and southern sections. Vertical RMS error of the DSMs relative to bare-ground checkpoints is 5.8 and 9.5 cm for the Sony and GoPro-derived DSMs, respectively. Shoreline features interpreted to have formed < 14,000, 18,000-23,000, and > 23,000 ybp (Godsey et al., 2011; Oviatt, 2015) are offset 2.8-3.0, 5.6-6.7, and 8.1-9.3 m, respectively. From these offsets we infer three surface-rupturing earthquakes with displacements of 2.8-3.0, 2.6-3.8, and 1.3-3.8 m, and estimate the slip rate to be 0.24 - 0.37 mm/yr. These results are consistent with those of the prior study to the north, suggesting co-rupturing of the northern, central and northernmost part of the southern section of the fault. In addition, the inferred large single event displacements suggest even longer surface ruptures. We have used the same methods to construct 5 cm pixel DSMs up to 4.4 km2 in area to support several additional paleoseismological, paleotsunami, and neotectonic investigations, which highlights the many benefits to geoscience research of the capacity to quickly produce accurate, high resolution DSMs from inexpensive equipment.

The 2011 Mw 7.1 Van (Eastern Turkey) earthquake

USGS Publications Warehouse

Elliot, John R.; Copley, Alex C.; Holley, R.; Scharer, Katherine M.; Parsons, Barry

2013-01-01

We use interferometric synthetic aperture radar (InSAR), body wave seismology, satellite imagery, and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse-slip earthquake, in the Turkish-Iranian plateau. Distributed slip models from elastic dislocation modeling of the InSAR surface displacements from ENVISAT and COSMO-SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40 °–54 ° dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8–9 km of the surface, consistent with the lack of significant ground rupture. The up-dip extension of this modeled WSW striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock on the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent.

The 2013, Mw 7.7 Balochistan earthquake, energetic strike-slip reactivation of a thrust fault

NASA Astrophysics Data System (ADS)

Avouac, Jean-Philippe; Ayoub, Francois; Wei, Shengji; Ampuero, Jean-Paul; Meng, Lingsen; Leprince, Sebastien; Jolivet, Romain; Duputel, Zacharie; Helmberger, Don

2014-04-01

We analyse the Mw 7.7 Balochistan earthquake of 09/24/2013 based on ground surface deformation measured from sub-pixel correlation of Landsat-8 images, combined with back-projection and finite source modeling of teleseismic waveforms. The earthquake nucleated south of the Chaman strike-slip fault and propagated southwestward along the Hoshab fault at the front of the Kech Band. The rupture was mostly unilateral, propagated at 3 km/s on average and produced a 200 km surface fault trace with purely strike-slip displacement peaking to 10 m and averaging around 6 m. The finite source model shows that slip was maximum near the surface. Although the Hoshab fault is dipping by 45° to the North, in accordance with its origin as a thrust fault within the Makran accretionary prism, slip was nearly purely strike-slip during that earthquake. Large seismic slip on such a non-optimally oriented fault was enhanced possibly due to the influence of the free surface on dynamic stresses or to particular properties of the fault zone allowing for strong dynamic weakening. Strike-slip faulting on thrust fault within the eastern Makran is interpreted as due to eastward extrusion of the accretionary prism as it bulges out over the Indian plate. Portions of the Makran megathrust, some thrust faults in the Kirthar range and strike-slip faults within the Chaman fault system have been brought closer to failure by this earthquake. Aftershocks cluster within the Chaman fault system north of the epicenter, opposite to the direction of rupture propagation. By contrast, few aftershocks were detected in the area of maximum moment release. In this example, aftershocks cannot be used to infer earthquake characteristics.

Constraining the slip distribution and fault geometry of the Mw 7.9, 3 November 2002, Denali fault earthquake with Interferometric Synthetic Aperture Radar and Global Positioning System data

USGS Publications Warehouse

Wright, Tim J.; Lu, Z.; Wicks, Charles

2004-01-01

The Mw 7.9, Denali fault earthquake (DFE) is the largest continental strike-slip earthquake to occur since the development of Interferometric Synthetic Aperture Radar (InSAR). We use five interferograms, constructed using radar images from the Canadian Radarsat-1 satellite, to map the surface deformation at the western end of the fault rupture. Additional geodetic data are provided by displacements observed at 40 campaign and continuous Global Positioning System (GPS) sites. We use the data to determine the geometry of the Susitna Glacier fault, thrusting on which initiated the DFE, and to determine a slip model for the entire event that is consistent with both the InSAR and GPS data. We find there was an average of 7.3 ± 0.4 m slip on the Susitna Glacier fault, between 1 and 9.5 km depth on a 29 km long fault that dips north at 41 ± 0.7° and has a surface projection close to the mapped rupture. On the Denali fault, a simple model with large slip patches finds a maximum of 8.7 ± 0.7 m of slip between the surface and 14.3 ± 0.2 km depth. A more complex distributed slip model finds a peak of 12.5 ± 0.8 m in the upper 4 km, significantly higher than the observed surface slip. We estimate a geodetic moment of 670 ± 10 × 1018 N m (Mw 7.9), consistent with seismic estimates. Lack of preseismic data resulted in an absence of InSAR coverage for the eastern half of the DFE rupture. A dedicated geodetic InSAR mission could obviate coverage problems in the future.

Microstructural and mineral analysis on the fault gouge in the coseismic shear zone of the 2008 M w 7.9 Wenchuan earthquake

NASA Astrophysics Data System (ADS)

Yuan, Ren-mao; Zhang, Bing-liang; Xu, Xi-wei; Lin, Chuan-yong; Han, Zhu-jun

2015-07-01

The 2008 M w 7.9 Wenchuan earthquake formed two coseismic surface rupture zones with the trend of N35°E, known as the Beichuan-Yingxiu rupture and the Pengguan rupture. The Beichuan-Yingxiu rupture is the principle one with abundant fault gouge development along its length. In the exploratory trench at the Saba village along the Beichuan-Yingxiu rupture, the new fault gouge zone is only ~3 mm wide, which suggests that fault slip was constrained in a very narrow zone. In this study, we thus carried out detailed microstructural and mineral component analysis on the oriented fault gouge samples from the Saba exploratory trench to understand their features and geological implication. The results show that different microstructures of localized brittle deformation can be observed in the fault gouges, including Y-shear, R1-shear, R2-shear, P-shear as well as tension fracture, bookshelf glided structure and so on. These microstructures are commonly recognized as the product of seismic fault slipping. Furthermore, within the area between two parallel Y-shears of the fault gouge, a few of microstructures of distributed ductile deformations were developed, such as P-foliation, elongation and asymmetrical trailing structure of detrital particles. The microstructure features of fault gouges implicate the thrust movement of the fault during the Wenchuan earthquake. In addition, the fault gouge has less quartz and feldspar and more clay than the surrounding rocks, which indicates that some quartz and feldspar in the surrounding rocks were transformed into clay, whereas the fault gouge has more illite and less illite/montmorillonite mixed layers than the surrounding rocks, which shows that the illite/montmorillonite mixed layer was partly converted into illite due to temperature increasing induced by coseismic fault slipping friction (also being affected partly by the chemical action of solutions). Such microstructures features and mineral component changes recorded the information of fault slip and provide criterions for discussing the genesis of fault gouge and recognition of the direction of fault movement.

Dynamic earthquake rupture simulation on nonplanar faults embedded in 3D geometrically complex, heterogeneous Earth models

NASA Astrophysics Data System (ADS)

Duru, K.; Dunham, E. M.; Bydlon, S. A.; Radhakrishnan, H.

2014-12-01

Dynamic propagation of shear ruptures on a frictional interface is a useful idealization of a natural earthquake.The conditions relating slip rate and fault shear strength are often expressed as nonlinear friction laws.The corresponding initial boundary value problems are both numerically and computationally challenging.In addition, seismic waves generated by earthquake ruptures must be propagated, far away from fault zones, to seismic stations and remote areas.Therefore, reliable and efficient numerical simulations require both provably stable and high order accurate numerical methods.We present a numerical method for:a) enforcing nonlinear friction laws, in a consistent and provably stable manner, suitable for efficient explicit time integration;b) dynamic propagation of earthquake ruptures along rough faults; c) accurate propagation of seismic waves in heterogeneous media with free surface topography.We solve the first order form of the 3D elastic wave equation on a boundary-conforming curvilinear mesh, in terms of particle velocities and stresses that are collocated in space and time, using summation-by-parts finite differences in space. The finite difference stencils are 6th order accurate in the interior and 3rd order accurate close to the boundaries. Boundary and interface conditions are imposed weakly using penalties. By deriving semi-discrete energy estimates analogous to the continuous energy estimates we prove numerical stability. Time stepping is performed with a 4th order accurate explicit low storage Runge-Kutta scheme. We have performed extensive numerical experiments using a slip-weakening friction law on non-planar faults, including recent SCEC benchmark problems. We also show simulations on fractal faults revealing the complexity of rupture dynamics on rough faults. We are presently extending our method to rate-and-state friction laws and off-fault plasticity.

Constraints on behaviour of a mining‐induced earthquake inferred from laboratory rock mechanics experiments

USGS Publications Warehouse

McGarr, Arthur F.; Johnston, Malcolm J.; Boettcher, M.; Heesakkers, V.; Reches, Z.

2013-01-01

On December 12, 2004, an earthquake of magnitude 2.2, located in the TauTona Gold Mine at a depth of about 3.65 km in the ancient Pretorius fault zone, was recorded by the in-mine borehole seismic network, yielding an excellent set of ground motion data recorded at hypocentral distances of several km. From these data, the seismic moment tensor, indicating mostly normal faulting with a small implosive component, and the radiated energy were measured; the deviatoric component of the moment tensor was estimated to be M0 = 2.3×1012 N·m and the radiated energy ER = 5.4×108 J. This event caused extensive damage along tunnels within the Pretorius fault zone. What rendered this earthquake of particular interest was the underground investigation of the complex pattern of exposed rupture surfaces combined with laboratory testing of rock samples retrieved from the ancient fault zone (Heesakkers et al.2011a, 2011b). Event 12/12 2004 was the result of fault slip across at least four nonparallel fault surfaces; 25 mm of slip was measured at one location on the rupture segment that is most parallel with a fault plane inferred from the seismic moment tensor, suggesting that this segment accounted for much of the total seismic deformation. By applying a recently developed technique based on biaxial stick-slip friction experiments (McGarr2012, 2013) to the seismic results, together with the 25 mm slip observed underground, we estimated a maximum slip rate of at least 6.6 m/s, which is consistent with the observed damage to tunnels in the rupture zone. Similarly, the stress drop and apparent stress were found to be correspondingly high at 21.9 MPa and 6.6 MPa, respectively. The ambient state of stress, measured at the approximate depth of the earthquake but away from the influence of mining, in conjunction with laboratory measurements of the strength of the fault zone cataclasites, indicates that during rupture of the M 2.2 event, the normal stress acting on the large-slip fault segment was about 260 MPa, the yield stress was 172 MPa and the seismic efficiency was 0.05. Thus, for event 12/12 2004, 5% of the energy released by the earthquake was radiated and the remaining 95% was consumed in overcoming fault friction and expanding the zone of rupture.

High-resolution lidar topography of the Puget Lowland, Washington - A bonanza for earth science

USGS Publications Warehouse

Haugerud, R.A.; Harding, D.J.; Johnson, S.Y.; Harless, J.L.; Weaver, C.S.; Sherrod, B.L.

2003-01-01

More than 10,000 km2 of high-resolution, public-domain topography acquired by the Puget Sound Lidar Consortium is revolutionizing investigations of active faulting, continental glaciation, landslides, and surficial processes in the seismically active Puget Lowland. The Lowland-the population and economic center of the Pacific Northwest-presents special problems for hazards investigations, with its young glacial topography, dense forest cover, and urbanization. Lidar mapping during leaf-off conditions has led to a detailed digital model of the landscape beneath the forest canopy. The surface thus revealed contains a rich and diverse record of previously unknown surface-rupturing faults, deep-seated landslides, uplifted Holocene and Pleistocene beaches, and subglacial and periglacial features. More than half a dozen suspected postglacial fault scarps have been identified to date. Five scarps that have been trenched show evidence of large, Holocene, surface-rupturing earthquakes.

Geodetically inferred coseismic and postseismic slip due to the M 5.4 31 October 2007 Alum Rock earthquake

USGS Publications Warehouse

Murray-Moraleda, J. R.; Simpson, R.W.

2009-01-01

On 31 October 2007 the M 5.4 Alum Rock earthquake occurred near the junction between the Hayward and Calaveras faults in the San Francisco Bay Area, producing coseismic and postseismic displacements recorded by 10 continuously operating Global Positioning System (GPS) instruments. The cumulative postseismic displacements over the four months following the earthquake are linearly related to the cumulative number of aftershocks and are comparable in magnitude to the coseis mic displacements. The postseismic signal suggests that, in addition to afterslip at seismogenic depths, localized right-lateral/reverse slip occurred on dipping shallow fault surfaces southwest of the Calaveras. The spatial distribution of slip inferred by inverting the GPS data is compatible with a model in which moderate Calaveras fault earthquakes rupture locked patches surrounded by areas of creep, afterslip, and microseismicity (Oppenheimer et al., 1990). If this model and existing Calaveras fault slip rate estimates are correct, a slip deficit remains on the 2007 Alum Rock rupture patch that may be made up by aseismic slip or slip in larger earthquakes. Recent studies (e.g., Manaker et al., 2005) suggest that at depth the Hayward and central Calaveras faults connect via a simple continuous surface illuminated by the Mission Seismic Trend (MST), implying that a damaging earthquake rupture could involve both faults (Graymer et al., 2008). If this geometry is correct, the combined coseismic and postseismic slip we infer for the 2007 Alum Rock event predicts static Coulomb stress increases of ???0:6 bar on the MST surface and on the northern Calaveras fault ???5 km northwest of the Alum Rock hypocenter.

The effect of segmented fault zones on earthquake rupture propagation and termination

NASA Astrophysics Data System (ADS)

Huang, Y.

2017-12-01

A fundamental question in earthquake source physics is what can control the nucleation and termination of an earthquake rupture. Besides stress heterogeneities and variations in frictional properties, damaged fault zones (DFZs) that surround major strike-slip faults can contribute significantly to earthquake rupture propagation. Previous earthquake rupture simulations usually characterize DFZs as several-hundred-meter-wide layers with lower seismic velocities than host rocks, and find earthquake ruptures in DFZs can exhibit slip pulses and oscillating rupture speeds that ultimately enhance high-frequency ground motions. However, real DFZs are more complex than the uniform low-velocity structures, and show along-strike variations of damages that may be correlated with historical earthquake ruptures. These segmented structures can either prohibit or assist rupture propagation and significantly affect the final sizes of earthquakes. For example, recent dense array data recorded at the San Jacinto fault zone suggests the existence of three prominent DFZs across the Anza seismic gap and the south section of the Clark branch, while no prominent DFZs were identified near the ends of the Anza seismic gap. To better understand earthquake rupture in segmented fault zones, we will present dynamic rupture simulations that calculate the time-varying rupture process physically by considering the interactions between fault stresses, fault frictional properties, and material heterogeneities. We will show that whether an earthquake rupture can break through the intact rock outside the DFZ depend on the nucleation size of the earthquake and the rupture propagation distance in the DFZ. Moreover, material properties of the DFZ, stress conditions along the fault, and friction properties of the fault also have a critical impact on rupture propagation and termination. We will also present scenarios of San Jacinto earthquake ruptures and show the parameter space that is favorable for rupture propagation through the Anza seismic gap. Our results suggest that a priori knowledge of properties of segmented fault zones is of great importance for predicting sizes of future large earthquakes on major faults.

Kinematic ground motion simulations on rough faults including effects of 3D stochastic velocity perturbations

USGS Publications Warehouse

Graves, Robert; Pitarka, Arben

2016-01-01

We describe a methodology for generating kinematic earthquake ruptures for use in 3D ground‐motion simulations over the 0–5 Hz frequency band. Our approach begins by specifying a spatially random slip distribution that has a roughly wavenumber‐squared fall‐off. Given a hypocenter, the rupture speed is specified to average about 75%–80% of the local shear wavespeed and the prescribed slip‐rate function has a Kostrov‐like shape with a fault‐averaged rise time that scales self‐similarly with the seismic moment. Both the rupture time and rise time include significant local perturbations across the fault surface specified by spatially random fields that are partially correlated with the underlying slip distribution. We represent velocity‐strengthening fault zones in the shallow (<5  km) and deep (>15  km) crust by decreasing rupture speed and increasing rise time in these regions. Additional refinements to this approach include the incorporation of geometric perturbations to the fault surface, 3D stochastic correlated perturbations to the P‐ and S‐wave velocity structure, and a damage zone surrounding the shallow fault surface characterized by a 30% reduction in seismic velocity. We demonstrate the approach using a suite of simulations for a hypothetical Mw 6.45 strike‐slip earthquake embedded in a generalized hard‐rock velocity structure. The simulation results are compared with the median predictions from the 2014 Next Generation Attenuation‐West2 Project ground‐motion prediction equations and show very good agreement over the frequency band 0.1–5 Hz for distances out to 25 km from the fault. Additionally, the newly added features act to reduce the coherency of the radiated higher frequency (f>1  Hz) ground motions, and homogenize radiation‐pattern effects in this same bandwidth, which move the simulations closer to the statistical characteristics of observed motions as illustrated by comparison with recordings from the 1979 Imperial Valley earthquake.

Kinematic Ground-Motion Simulations on Rough Faults Including Effects of 3D Stochastic Velocity Perturbations

DOE PAGES

Graves, Robert; Pitarka, Arben

2016-08-23

Here, we describe a methodology for generating kinematic earthquake ruptures for use in 3D ground–motion simulations over the 0–5 Hz frequency band. Our approach begins by specifying a spatially random slip distribution that has a roughly wavenumber–squared fall–off. Given a hypocenter, the rupture speed is specified to average about 75%–80% of the local shear wavespeed and the prescribed slip–rate function has a Kostrov–like shape with a fault–averaged rise time that scales self–similarly with the seismic moment. Both the rupture time and rise time include significant local perturbations across the fault surface specified by spatially random fields that are partially correlatedmore » with the underlying slip distribution. We represent velocity–strengthening fault zones in the shallow (<5 km) and deep (>15 km) crust by decreasing rupture speed and increasing rise time in these regions. Additional refinements to this approach include the incorporation of geometric perturbations to the fault surface, 3D stochastic correlated perturbations to the P– and S–wave velocity structure, and a damage zone surrounding the shallow fault surface characterized by a 30% reduction in seismic velocity. We demonstrate the approach using a suite of simulations for a hypothetical Mw 6.45 strike–slip earthquake embedded in a generalized hard–rock velocity structure. The simulation results are compared with the median predictions from the 2014 Next Generation Attenuation–West2 Project ground–motion prediction equations and show very good agreement over the frequency band 0.1–5 Hz for distances out to 25 km from the fault. Additionally, the newly added features act to reduce the coherency of the radiated higher frequency (f>1 Hz) ground motions, and homogenize radiation–pattern effects in this same bandwidth, which move the simulations closer to the statistical characteristics of observed motions as illustrated by comparison with recordings from the 1979 Imperial Valley earthquake.« less

Interaction of the subduction process and forearc tectonics: An example from the active N - Chilean margin

NASA Astrophysics Data System (ADS)

Victor, P.; Sobiesiak, M.

2005-12-01

Convergent plate boundaries at continental margins belong to the tectonically most active areas on earth and are endangered by devastating earthquakes and tsunamis. The north Chilean margin is a high strain continental margin driven by fast plate convergence rate. The greatest amount of strain is accommodated along the subduction interface. Nevertheless there is extensive crustal deformation obvious by surface ruptures along reactivated segments of large fault systems and vertical surface motions reflecting the interaction between subducting and overriding plates. The historical seismicity record indicates that great earthquakes affect the Chilean Forearc with recurrence intervals of about 112+/- 21 y . The last great event in northern Chile occurred in 1995 near Antofagasta. The Mw= 8.0 event ruptured the subduction interface 180 km along strike with an average slip of about 5m in the depth interval between 10-50 km. From careful evaluation of the aftershock sequence by examining the different catagories of aftershock focal mechanisms we can define three segments of the seismogenic zone affected by the Antofagasta main shock. The non-ruptured northern segment beneath Mejillones Peninsula is seperated by a broad transition zone from the central segment which hosts the earthquakes' rupture plane. The southern fault plane boundary is identified by linear alignment of all apparent aftershock mechanisms. Along this southern boundary the strike slip mechanisms are exclusively left lateral whereas the strike slip mechanisms along the northern transition zone are right lateral. The orientations of summed moment tensors calculated from aftershock fault plane solutions on the northern segment and in the northern transition zone differ from the orientations exhibited by moment tensors on the central segment. This might indicate a rotational component in the coseismic movement of the ruptured segment relative to the non-ruptured segment. The observed segmentation of the downgoing plate correlates well with changes in the coseismic surface displacement field and coseismic rotations derived from GPS data (Allmendinger et al. in press). We can localize a transition zone at Mejillones peninsula (23,5°S) striking approximately N 80°E dominated by clockwise vertical axis rotations also marked by rotations of the summed moment tensors on the downgoing plate. The calculated strain tensor for this transition zone does not correspond with long term surface deformation, implying that coseismic as well as early postseismic effects on the subduction interface do not contribute to long term deformation of crustal fault zones. The Antofagasta earthquake took place just south of the large 1877 gap which extends from southern Peru to Mejillones Peninsula, being the surface expression of a barrier seperating the Antofagasta fault plane from the expected future fault plane. From our studies of the Antofagasta subduction zone and the surface displacement field we hope to find evidences for interface-crust-surface interactions which can be extrapolated also to the 1877 gap.

Evidence of Multiple Ground-rupturing Earthquakes in the Past 4000 Years along the Pasuruan Fault, East Java, Indonesia

NASA Astrophysics Data System (ADS)

Marliyani, G. I.; Arrowsmith, R.; Helmi, H.

2015-12-01

Instrumental and historical records of earthquakes, supplemented by paleoeseismic constraints can help reveal the earthquake potential of an area. The Pasuruan fault is a high angle normal fault with prominent youthful scarps cutting young deltaic sediments in the north coast of East Java, Indonesia and may pose significant hazard to the densely populated region. This fault has not been considered a significant structure, and mapped as a lineament with no sense of motion. Information regarding past earthquakes along this fault is not available. The fault is well defined both in the imagery and in the field as a ~13km long, 2-50m-high scarp. Open and filled fractures and natural exposures of the south-dipping fault plane indicate normal sense of motion. We excavated two fault-perpendicular trenches across a relay ramp identified during our surface mapping. Evidence for past earthquakes (documented in both trenches) includes upward fault termination with associated fissure fills, colluvial wedges and scarp-derived debris, folding, and angular unconformities. The ages of the events are constrained by 23 radiocarbon dates on detrital charcoal. We calibrated the dates using IntCal13 and used Oxcal to build the age model of the events. Our preliminary age model indicates that since 2006±134 B.C., there has been at least five ground rupturing earthquakes along the fault. The oldest event identified in the trench however, is not well-dated. Our modeled 95th percentile ranges of the next four earlier earthquakes (and their mean) are A.D. 1762-1850 (1806), A.D. 1646-1770 (1708), A.D. 1078-1648 (1363), and A.D. 726-1092 (909), yielding a rough recurrence rate of 302±63 yrs. These new data imply that Pasuruan fault is more active than previously thought. Additional well-dated earthquakes are necessary to build a solid earthquake recurrence model. Rupture along the whole section implies a minimum earthquake magnitude of 6.3, considering 13km as the minimum surface rupture length.

The Constantine (northeast Algeria) earthquake of October 27, 1985: surface ruptures and aftershock study

NASA Astrophysics Data System (ADS)

Bounif, A.; Haessler, H.; Meghraoui, M.

1987-10-01

An earthquake of magnitude Ms = 6.0 (CSEM, Strasbourg) occurred at Constantine (Algeria) on 27 October 1985. This seismic event is the strongest felt in the Tellian Atlas since the El Asnam seismic crisis of October 10, 1980. A team from the Centre de Recherche d'Astronomie, d'Astrophysique et de Géophysique (CRAAG, Algeria), utilising 8 portable stations, registered the activity a few days after the main shock. The aftershocks follow a N045° direction, and show the existence of three ruptured segments. Cross sections display a remarkable vertical fault plane and suggest asperities in the rupture process. Surface breaks were found affecting Quaternary deposits. The principal segment is about 3.8 km long showing “enéchelon” cracks with left-lateral displacement while the main direction of the rupture is N055°. Although the vertical motion is small, the northwestern block shows a normal component of the main surface faulting, while the left-lateral displacement is about 10 cm. The strike-slip focal mechanism solution determined from the global seismic network and field observations are in good agreement.

Pore pressure may control rupture propagation of the 2001 Mw=7.8 Kokoxili earthquake from the Kunlun fault to the Kunlun Pass fault

NASA Astrophysics Data System (ADS)

Xiao, J.; Wang, W.; He, J.

2016-12-01

The 2001 Mw=7.8 Kokoxili earthquake nucleated on the west-east tending Kunlun strike-slip fault in center of the Tibetan plateau. When the rupture propagated eastward near the Xidatan segment of the Kunlun fault, this earthquake jumped to the Kunlun Pass fault, a less matured fault that, due to the geometric orientation, was obviously clamped by the coseismic deformation before its rupture. To investigate the possible mechanism for the rupture jump, we updated the coseismic rupture model from a joint inversion of the geological, geodetic and seismic wave data. Constrained with the rupture process, a three-dimensional finite element model was developed to calculate the failure stress from elastic and poroelastic deformation of the crust during the rupture propagation. Results show that just before the rupture reached the conjunction of the Xidatan segment and the Kunlun Pass fault, the failure stress induced by elastic deformation is indeed larger on Xidatan segment of the Kunlun fault than on the Kunlun Pass fault. However, if the pore pressure resulted from undrained poroelastic deformation was invoked, the failure stress is significantly increased on the Kunlun Pass fault. Given a reasonable bound on fault friction and on poroelastic parameters, it can be seen that the poroelastic failure stress is 0.3-0.9 Mpa greater on the Kunlun Pass fault than on Xidatan segment of the Kunlun fault. We therefore argue that during the rupture process of the 2001 Mw=7.8 Kokoxili earthquake, pore pressure may play an important role on controlling the rupture propagation from the Kunlun fault to the Kunlun Pass fault.

3-D dynamic rupture simulations of the 2016 Kumamoto, Japan, earthquake

NASA Astrophysics Data System (ADS)

Urata, Yumi; Yoshida, Keisuke; Fukuyama, Eiichi; Kubo, Hisahiko

2017-11-01

Using 3-D dynamic rupture simulations, we investigated the 2016 Mw7.1 Kumamoto, Japan, earthquake to elucidate why and how the rupture of the main shock propagated successfully, assuming a complicated fault geometry estimated on the basis of the distributions of the aftershocks. The Mw7.1 main shock occurred along the Futagawa and Hinagu faults. Within 28 h before the main shock, three M6-class foreshocks occurred. Their hypocenters were located along the Hinagu and Futagawa faults, and their focal mechanisms were similar to that of the main shock. Therefore, an extensive stress shadow should have been generated on the fault plane of the main shock. First, we estimated the geometry of the fault planes of the three foreshocks as well as that of the main shock based on the temporal evolution of the relocated aftershock hypocenters. We then evaluated the static stress changes on the main shock fault plane that were due to the occurrence of the three foreshocks, assuming elliptical cracks with constant stress drops on the estimated fault planes. The obtained static stress change distribution indicated that Coulomb failure stress change (ΔCFS) was positive just below the hypocenter of the main shock, while the ΔCFS in the shallow region above the hypocenter was negative. Therefore, these foreshocks could encourage the initiation of the main shock rupture and could hinder the propagation of the rupture toward the shallow region. Finally, we conducted 3-D dynamic rupture simulations of the main shock using the initial stress distribution, which was the sum of the static stress changes caused by these foreshocks and the regional stress field. Assuming a slip-weakening law with uniform friction parameters, we computed 3-D dynamic rupture by varying the friction parameters and the values of the principal stresses. We obtained feasible parameter ranges that could reproduce the characteristic features of the main shock rupture revealed by seismic waveform analyses. We also observed that the free surface encouraged the slip evolution of the main shock.[Figure not available: see fulltext.

Scale-Dependent Friction and Damage Interface law: implications for effective earthquake rupture dynamics and radiation

NASA Astrophysics Data System (ADS)

Festa, Gaetano; Vilotte, Jean-Pierre; Raous, Michel; Henninger, Carole

2010-05-01

Propagation and radiation of an earthquake rupture is commonly considered as a friction dominated process on fault surfaces. Friction laws, such as the slip weakening and the rate-and-state laws are widely used in the modeling of the earthquake rupture process. These laws prescribe the traction evolution versus slip, slip rate and potentially other internal variables. They introduce a finite cohesive length scale over which the fracture energy is released. However faults are finite-width interfaces with complex internal structures, characterized by highly damaged zones embedding a very thin principal slip interface where most of the dynamic slip localizes. Even though the rupture process is generally investigated at wavelengths larger than the fault zone thickness, which should justify a formulation based upon surface energy, a consistent homogeneization, a very challenging problem, is still missing. Such homogeneization is however be required to derive the consistent form of an effective interface law, as well as the appropriate physical variables and length scales, to correctly describe the coarse-grained dissipation resulting from surface and volumetric contributions at the scale of the fault zone. In this study, we investigate a scale-dependent law, introduced by Raous et al. (1999) in the context of adhesive material interfaces, that takes into account the transition between a damage dominated and a friction dominated state. Such a phase-field formalism describes this transition through an order parameter. We first compare this law to standard slip weakening friction law in terms of the rupture nucleation. The problem is analyzed through the representation of the solution of the quasi-static elastic problem onto the Chebyshev polynomial basis, generalizing the Uenishi-Rice solution. The nucleation solutions, at the onset of instability, are then introduced as initial conditions for the study of the dynamic rupture propagation, in the case of in-plane rupture, using high-order Spectral Element Methods and non-smooth contact mechanics. In particular, we investigate the implications of this new interface law in terms of the rupture propagation and arrest. Special attention is focused on radiation and supershear transition. Comparison with the classical slip weakening friction law is provided. Finally, first results toward a dynamic consistent homogeneization of damaged fault zones will be discussed. Raous, M., Cangémi, L. and Cocou, M. (1999). A consistent model coupling adhesion, friction and unilateral contact', Computer Methods in Applied Mechanics and Engineering, Vol. 177, pp.383-399.

Influence of fault steps on rupture termination of strike-slip earthquake faults

NASA Astrophysics Data System (ADS)

Li, Zhengfang; Zhou, Bengang

2018-03-01

A statistical analysis was completed on the rupture data of 29 historical strike-slip earthquakes across the world. The purpose of this study is to examine the effects of fault steps on the rupture termination of these events. The results show good correlations between the type and length of steps with the seismic rupture and a poor correlation between the step number and seismic rupture. For different magnitude intervals, the smallest widths of the fault steps (Lt) that can terminate the rupture propagation are variable: Lt = 3 km for Ms 6.5 6.9, Lt = 4 km for Ms 7.0 7.5, Lt = 6 km for Ms 7.5 8.0, and Lt = 8 km for Ms 8.0 8.5. The dilational fault step is easier to rupture through than the compression fault step. The smallest widths of the fault step for the rupture arrest can be used as an indicator to judge the scale of the rupture termination of seismic faults. This is helpful for research on fault segmentation, as well as estimating the magnitude of potential earthquakes, and is thus of significance for the assessment of seismic risks.

Finite-fault inversion of the Mw 5.9 2012 Emilia-Romagna earthquake (Northern Italy) using aftershocks as near-field Green's function approximations

NASA Astrophysics Data System (ADS)

Causse, Mathieu; Cultrera, Giovanna; Herrero, André; Courboulex, Françoise; Schiappapietra, Erika; Moreau, Ludovic

2017-04-01

On May 29, 2012 occurred a Mw 5.9 earthquake in the Emilia-Romagna region (Po Plain) on a thrust fault system. This shock, as well as hundreds of aftershocks, were recorded by 10 strong motion stations located less than 10 km away from the rupture plane, with 4 stations located within the surface rupture projection. The Po Plain is a very large EW trending syntectonic alluvial basin, delimited by the Alps and Apennines chains to the North and South. The Plio-Quaternary sedimentary sequence filling the Po Plain is characterized by an uneven thickness, ranging from several thousands of meters to a few tens of meters. This particular context results especially in a resonance basin below 1 Hz and strong surface waves, which makes it particularly difficult to model wave propagation and hence to obtain robust images of the rupture propagation. This study proposes to take advantage of the large set of recorded aftershocks, considered as point sources, to model wave propagation. Due to the heterogeneous distribution of the aftershocks on the fault plane, an interpolation technique is proposed to compute an approximation of the Green's function between each fault point and each strong motion station in the frequency range [0.2-1Hz]. We then use a Bayesian inversion technique (Monte Carlo Markov Chain algorithm) to obtain images of the rupture propagation from the strong motion data. We propose to retrieve the slip distribution by inverting the final slip value at some control points, which are allowed to move on the fault plane, and by interpolating the slip value between these points. We show that the use of 5 control points to describe the slip, coupled with the hypothesis of spatially constant rupture velocity and rise-time (that is 18 free source parameters), results in a good level of fit with the data. This indicates that despite their complexity, the strong motion data can be properly modeled up to 1 Hz using a relatively simple rupture. The inversion results also reveal that the rupture propagated slowly, at a speed of about 45% of the shear wave velocity.

Seismic images and fault relations of the Santa Monica thrust fault, West Los Angeles, California

USGS Publications Warehouse

Catchings, R.D.; Gandhok, G.; Goldman, M.R.; Okaya, D.

2001-01-01

In May 1997, the US Geological Survey (USGS) and the University of Southern California (USC) acquired high-resolution seismic reflection and refraction images on the grounds of the Wadsworth Veterans Administration Hospital (WVAH) in the city of Los Angeles (Fig. 1a,b). The objective of the seismic survey was to better understand the near-surface geometry and faulting characteristics of the Santa Monica fault zone. In this report, we present seismic images, an interpretation of those images, and a comparison of our results with results from studies by Dolan and Pratt (1997), Pratt et al. (1998) and Gibbs et al. (2000). The Santa Monica fault is one of the several northeast-southwest-trending, north-dipping, reverse faults that extend through the Los Angeles metropolitan area (Fig. 1a). Through much of area, the Santa Monica fault trends subparallel to the Hollywood fault, but the two faults apparently join into a single fault zone to the southwest and to the northeast (Dolan et al., 1995). The Santa Monica and Hollywood faults may be part of a larger fault system that extends from the Pacific Ocean to the Transverse Ranges. Crook et al. (1983) refer to this fault system as the Malibu Coast-Santa Monica-Raymond-Cucamonga fault system. They suggest that these faults have not formed a contiguous zone since the Pleistocene and conclude that each of the faults should be treated as a separate fault with respect to seismic hazards. However, Dolan et al. (1995) suggest that the Hollywood and Santa Monica faults are capable of generating Mw 6.8 and Mw 7.0 earthquakes, respectively. Thus, regardless of whether the overall fault system is connected and capable of rupturing in one event, individually, each of the faults present a sizable earthquake hazard to the Los Angeles metropolitan area. If, however, these faults are connected, and they were to rupture along a continuous fault rupture, the resulting hazard would be even greater. Although the Santa Monica fault represents a hazard to millions of people, its lateral extent and rupture history are not well known, due largely to limited knowledge of the fault location, geometry, and relationship to other faults. The Santa Monica fault has been obscured at the surface by alluvium and urbanization. For example, Dolan et al. (1995) could find only one 200-m-long stretch of the Santa Monica fault that was not covered by either streets or buildings. Of the 19-km length onshore section of the Santa Monica fault, its apparent location has been delineated largely on the basis of geomorphic features and oil-well drilling. Seismic imaging efforts, in combination with other investigative methods, may be the best approach in locating and understanding the Santa Monica fault in the Los Angeles region. This investigation and another recent seismic imaging investigation (Pratt et al., 1998) were undertaken to resolve the near-surface location, fault geometry, and faulting relations associated with the Santa Monica fault.

Co-seismic and cumulative slip along the Kokoxili Mw 7.9 earthquake rupture (Kunlun Fault, northeastern Tibet)

NASA Astrophysics Data System (ADS)

van der Woerd, J.; Klinger, Y.; Xu, X.; Ledortz, K.; Tapponnier, P.; Li, H.; King, G.; Ma, W.; Chen, W.

2009-04-01

Co-seismic slip values along a strike-slip rupture are found to be very irregular with variations up to one order of magnitude. Data usually scattered and sparse, are more dense and continuous with slip functions derived from InSAR or image correlations. Whether the fast variations in slip along strike reveals long-lived structures of the fault plane at depth, only incomplete slip at the surface or inelastic accommodation of slip remains debated. In addition, how these slip disparities are accommodated with time is unclear. The surface breaks of the Kokoxili Mw 7.9 event sytematically follow the geomorphic trace of the fault, which bears evidence for cumulative displacements. In the epicentral area, the rupture steps along the highest ice-capped summit of the region, the Buka Daban Feng. Evidence for normal fault breaks, left-lateral ruptures and steep triangular facets indicate that the Buka Daban Feng, a 40 km-long range reaching about 6800 m a.s.l. formed as a result of continuous oblique left-normal faulting. Normal faulting is attested by hanging glaciers, a steep southeastern flank and hot springs along coseismic and cumulative surface ruptures. Left-lateral movement along the main oblique normal fault has displaced the distal frontal moraines of almost all glacial valleys. West of the Buka Daban Feng, the western most ruptured strand is continuous between Kushiwan and Tayang lakes (about 60 km long) with coseismic left-lateral offsets reaching 4-5 m. To the east, at one site, the rupture splays into 4 main, N90°E-striking strands across a 1200x300m pull-apart. Three strands show right-stepping scarps with maximum throws of 1 m. The northern strand shows 3 m of purely normal throw. On NS profiles, the coseismic subsidence of the pull-apart floor was about 2 m, 1/10th of its 20 m depth, consistent with the repetition of 10 comparable earthquakes. East of the pull-apart 5 +/- 0.2 m sinistral slip are measured on the single stranded rupture. A similar coseismic sinistral slip value (4.8 +/- 0.4 m), with a thrust throw of 0.75 m, is found across two lake shorelines (1 and 2.3 meters above lake level), about 1.5 km farther east. At one river channel site, 15 km farther east, we measured a larger coseismic sinistral offset. The lowest terrace riser, between the active river bed and the lowest terrace is offset by about 10 m. Two older terrace risers show cumulative offsets twice as large (about 20 m). The largest cumulative offset measured reaches 50 m. Thirty kilometers yet farther east, near a glacial outlet at the foot of the Yuxi Shan, 4 distinct rills are offset by 3.5 m along the main surface break while smaller offsets (1-1.5 m) characterize a secondary break 250 m southwards. We have investigated in more detail the co-seismic and paleo-seismic slip near the central part of the rupture, where a set of about 10 inset terraces is used to infer the slip history of the fault. The particularly low co-seismic slip value at the site, about 2.4 +/- 0.6 m, lower than the value derived from Spot image correlation or from Insar, follows a maximum slip value of about 8 m only a few kilometres to the west. Site interpretation from field data, total station measurements and high resolution Ikonos images allow to reconstruct the different riser offsets across the damaged rupture zone. 10Be cosmogenic nuclide exposure ages of cobbles from the surface of the terraces allow to constrain the age the terraces and to infer an average slip-rate of 10-15 mm/yr in good agreement with previous results along the fault. When a characteristic slip model is considered, and depending of the measurement method or location of coseismic slip, the recurrence time for events comparable to the Mw 7.9 Kokoxili event may vary from 200 to 800 years.

An insight on correlations between kinematic rupture parameters from dynamic ruptures on rough faults

NASA Astrophysics Data System (ADS)

Thingbijam, Kiran Kumar; Galis, Martin; Vyas, Jagdish; Mai, P. Martin

2017-04-01

We examine the spatial interdependence between kinematic parameters of earthquake rupture, which include slip, rise-time (total duration of slip), acceleration time (time-to-peak slip velocity), peak slip velocity, and rupture velocity. These parameters were inferred from dynamic rupture models obtained by simulating spontaneous rupture on faults with varying degree of surface-roughness. We observe that the correlations between these parameters are better described by non-linear correlations (that is, on logarithm-logarithm scale) than by linear correlations. Slip and rise-time are positively correlated while these two parameters do not correlate with acceleration time, peak slip velocity, and rupture velocity. On the other hand, peak slip velocity correlates positively with rupture velocity but negatively with acceleration time. Acceleration time correlates negatively with rupture velocity. However, the observed correlations could be due to weak heterogeneity of the slip distributions given by the dynamic models. Therefore, the observed correlations may apply only to those parts of rupture plane with weak slip heterogeneity if earthquake-rupture associate highly heterogeneous slip distributions. Our findings will help to improve pseudo-dynamic rupture generators for efficient broadband ground-motion simulations for seismic hazard studies.

Dynamic Rupture and Energy Partition in Models of Earthquake Faults

NASA Astrophysics Data System (ADS)

Shi, Z.; Needleman, A.; Ben-Zion, Y.

2006-12-01

We study properties of dynamic rupture and the partition of energy between radiation and dissipative mechanisms using 2D finite element calculations. The goal is to improve the understanding of these processes on faults at different evolutionary stages associated with different levels of geometrical complexity and possible presence of contrasting elastic properties across the fault. The initial calculations employ homogeneous media and a planar internal interface governed by a general rate- and state-dependent friction law that accounts for the gradual response of shear stress to abrupt changes of normal stress. Ruptures are initiated by gradually increasing the shear traction in a limited nucleation zone near the origin. By changing the rate dependency of the friction law and the size of the nucleation zone, we obtain four rupture modes: (i) supershear crack-like rupture; (ii) subshear crack-like rupture; (iii) subshear single pulse; and (iv) supershear train of pulses. Increasing the initial shear stress produces a transition from a subshear crack to a supershear crack, while increasing the rate dependency of the friction produces self-healing and the transition from a crack-like to a pulse mode of rupture. Properties of the nucleation process can strongly affect the rupture mode. In the cases examined, the total release of strain energy (over the same propagation distance) decreases following the order: supershear crack, subshear crack, train of pulses and single pulse. The ratio of the radiated kinetic energy to the energy dissipated in friction is about 5% for the supershear crack case and about 2% for the other three cases. Future work will involve similar calculations accounting for the generation of plastic strain in the bulk, the material contrast across the fault, and the addition of cohesive surfaces in the bulk to allow for the generation of new surfaces. The study may provide fundamental information on rupture processes in geologically-relevant circumstances and improve the understanding of physical limits on extreme ground motion. The results may be used to check assumptions made in observational works and may help to guide new observational research.

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).

Afterslip-dominated surface rupture in the M6.0 South Napa Earthquake as constrained by structure-from-motion analysis and terrestrial laser scanning

NASA Astrophysics Data System (ADS)

DeLong, S. B.; Pickering, A.; Scharer, K. M.; Hudnut, K. W.; Lienkaemper, J. J.

2014-12-01

Near-fault surface deformation associated with the August 24, 2014 M6.0 South Napa earthquake included both coseismic and post-seismic slip. Initial synthesis of field observations and initial measurement and modeling of afterslip from traditional survey methods indicate that coseismic slip was minimal (<10 cm) within 8 km northward from the epicenter but post-seismic slip, in places, approached 40 cm. We collected reconnaissance photographs using professional-grade SLR cameras from a helicopter within 12 hours after the earthquake, and a more systematic collection of air photos the day after the earthquake. We also collected terrestrial laser scanner (TLS) data two days (on August 26) and twenty-two days (on September 22) after the earthquake along a 0.5 km length of the main fault trace, just south of State Highway 12. This study site is 6 km north of the southern end of the 15 km long surface rupture and 5 km south of the highest measured co-seismic slip. We used structure-from-motion (SfM) methods to mosaic, orthorectify, and generate dense point clouds from the photos. SfM data corroborates survey-based ground observations of limited (~5 cm or less) coseismic slip along the fault trace between CA State Highway 12 and Withers Road, on discontinuous left-stepping en echelon ruptures. By August 26, the surface rupture became nearly continuous, and cultural features extracted from the TLS point clouds indicate horizontal slip magnitudes between 15 and 27 cm, increasing northward. By September 22, slip magnitudes had increased to between 26 and 46 cm. The lower slip magnitudes are to the south at Withers Road, and the general trend is increased slip to the north, but there is more slip variability along the fault trace in the September 15 data. From August 26 to September 15, the west side of the fault trace uplifted between 0.5 and 5 cm relative to east side. Increased relief on the surface rupture itself indicated a slight compressional component of the deformation. These results confirm that post-event air photos can be useful for rapid 3D mapping, and that the unparalleled accuracy of TLS data can be used to quantify even very subtle deformation patterns in three dimensions and document changes through time.

Active Tectonics of Himalayan Faults/Thrusts System in Northern India on the basis of recent & Paleo earthquake Studies

NASA Astrophysics Data System (ADS)

Kumar, S.; Biswal, S.; Parija, M. P.

2016-12-01

The Himalaya overrides the Indian plate along a decollement fault, referred as the Main Himalayan Thrust (MHT). The 2400 km long Himalayan mountain arc in the northern boundary of the Indian sub-continent is one of the most seismically active regions of the world. The Himalayan Frontal Thrust (HFT) is characterized by an abrupt physiographic and tectonic break between the Himalayan front and the Indo-Gangetic plain. The HFT represents the southern surface expression of the MHT on the Himalayan front. The tectonic zone between the Main Boundary Thrust (MBT) and the HFT encompasses the Himalayan Frontal Fault System (HFFS). The zone indicates late Quaternary-Holocene active deformation. Late Quaternary intramontane basin of Dehradun flanked to the south by the Mohand anticline lies between the MBT and the HFT in Garhwal Sub Himalaya. Slip rate 13-15 mm/yr has been estimated on the HFT based on uplifted strath terrace on the Himalyan front (Wesnousky et al. 2006). An out of sequence active fault, Bhauwala Thrust (BT), is observed between the HFT and the MBT. The Himalayan Frontal Fault System includes MBT, BT, HFT and PF active fault structures (Thakur, 2013). The HFFS structures were developed analogous to proto-thrusts in subduction zone, suggesting that the plate boundary is not a single structure, but series of structures across strike. Seismicity recorded by WIHG shows a concentrated belt of seismic events located in the Main Central Thrust Zone and the physiographic transition zone between the Higher and Lesser Himalaya. However, there is quiescence in the Himalayan frontal zone where surface rupture and active faults are reported. GPS measurements indicate the segment between the southern extent of microseismicity zone and the HFT is locked. The great earthquake originating in the locked segment rupture the plate boundary fault and propagate to the Himalaya front and are registered as surface rupture reactivating the fault in the HFFS.

Systematic Underestimation of Earthquake Magnitudes from Large Intracontinental Reverse Faults: Historical Ruptures Break Across Segment Boundaries

NASA Technical Reports Server (NTRS)

Rubin, C. M.

1996-01-01

Because most large-magnitude earthquakes along reverse faults have such irregular and complicated rupture patterns, reverse-fault segments defined on the basis of geometry alone may not be very useful for estimating sizes of future seismic sources. Most modern large ruptures of historical earthquakes generated by intracontinental reverse faults have involved geometrically complex rupture patterns. Ruptures across surficial discontinuities and complexities such as stepovers and cross-faults are common. Specifically, segment boundaries defined on the basis of discontinuities in surficial fault traces, pronounced changes in the geomorphology along strike, or the intersection of active faults commonly have not proven to be major impediments to rupture. Assuming that the seismic rupture will initiate and terminate at adjacent major geometric irregularities will commonly lead to underestimation of magnitudes of future large earthquakes.

Using Paleoseismic Trenching and LiDAR Analysis to Evaluate Rupture Propagation Through Segment Boundaries of the Central Wasatch Fault Zone, Utah

NASA Astrophysics Data System (ADS)

Bennett, S. E. K.; DuRoss, C. B.; Reitman, N. G.; Devore, J. R.; Hiscock, A.; Gold, R. D.; Briggs, R. W.; Personius, S. F.

2014-12-01

Paleoseismic data near fault segment boundaries constrain the extent of past surface ruptures and the persistence of rupture termination at segment boundaries. Paleoseismic evidence for large (M≥7.0) earthquakes on the central Holocene-active fault segments of the 350-km-long Wasatch fault zone (WFZ) generally supports single-segment ruptures but also permits multi-segment rupture scenarios. The extent and frequency of ruptures that span segment boundaries remains poorly known, adding uncertainty to seismic hazard models for this populated region of Utah. To address these uncertainties we conducted four paleoseismic investigations near the Salt Lake City-Provo and Provo-Nephi segment boundaries of the WFZ. We examined an exposure of the WFZ at Maple Canyon (Woodland Hills, UT) and excavated the Flat Canyon trench (Salem, UT), 7 and 11 km, respectively, from the southern tip of the Provo segment. We document evidence for at least five earthquakes at Maple Canyon and four to seven earthquakes that post-date mid-Holocene fan deposits at Flat Canyon. These earthquake chronologies will be compared to seven earthquakes observed in previous trenches on the northern Nephi segment to assess rupture correlation across the Provo-Nephi segment boundary. To assess rupture correlation across the Salt Lake City-Provo segment boundary we excavated the Alpine trench (Alpine, UT), 1 km from the northern tip of the Provo segment, and the Corner Canyon trench (Draper, UT) 1 km from the southern tip of the Salt Lake City segment. We document evidence for six earthquakes at both sites. Ongoing geochronologic analysis (14C, optically stimulated luminescence) will constrain earthquake chronologies and help identify through-going ruptures across these segment boundaries. Analysis of new high-resolution (0.5m) airborne LiDAR along the entire WFZ will quantify latest Quaternary displacements and slip rates and document spatial and temporal slip patterns near fault segment boundaries.

Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry

USGS Publications Warehouse

Mayer, Larry; Lu, Zhong

2001-01-01

A basic model incorporating satellite synthetic aperture radar (SAR) interferometry of the fault rupture zone that formed during the Kocaeli earthquake of August 17, 1999, documents the elastic rebound that resulted from the concomitant elastic strain release along the North Anatolian fault. For pure strike-slip faults, the elastic rebound function derived from SAR interferometry is directly invertible from the distribution of elastic strain on the fault at criticality, just before the critical shear stress was exceeded and the fault ruptured. The Kocaeli earthquake, which was accompanied by as much as ∼5 m of surface displacement, distributed strain ∼110 km around the fault prior to faulting, although most of it was concentrated in a narrower and asymmetric 10-km-wide zone on either side of the fault. The use of SAR interferometry to document the distribution of elastic strain at the critical condition for faulting is clearly a valuable tool, both for scientific investigation and for the effective management of earthquake hazard.

The 2016 central Italy earthquake sequence: surface effects, fault model and triggering scenarios

NASA Astrophysics Data System (ADS)

Chatzipetros, Alexandros; Pavlides, Spyros; Papathanassiou, George; Sboras, Sotiris; Valkaniotis, Sotiris; Georgiadis, George

2017-04-01

The results of fieldwork performed during the 2016 earthquake sequence around the karstic basins of Norcia and La Piana di Castelluccio, at an altitude of 1400 m, on the Monte Vettore (altitude 2476 m) and Vettoretto, as well as the three mapped seismogenic faults, striking NNW-SSW, are presented in this paper. Surface co-seismic ruptures were observed in the Vettore and Vettoretto segment of the fault for several kilometres ( 7 km) in the August earthquakes at high altitudes, and were re-activated and expanded northwards during the October earthquakes. Coseismic ruptures and the neotectonic Mt. Vettore fault zone were modelled in detail using images acquired from specifically planned UAV (drone) flights. Ruptures, typically with displacement of up to 20 cm, were observed after the August event both in the scree and weathered mantle (elluvium), as well as the bedrock, consisting mainly of fragmented carbonate rocks with small tectonic surfaces. These fractures expanded and new ones formed during the October events, typically of displacements of up to 50 cm, although locally higher displacements of up to almost 2 m were observed. Hundreds of rock falls and landslides were mapped through satellite imagery, using pre- and post- earthquake Sentinel 2A images. Several of them were also verified in the field. Based on field mapping results and seismological information, the causative faults were modelled. The model consists of five seismogenic sources, each one associated with a strong event in the sequence. The visualisation of the seismogenic sources follows INGV's DISS standards for the Individual Seismogenic Sources (ISS) layer, while strike, dip and rake of the seismic sources are obtained from selected focal mechanisms. Based on this model, the ground deformation pattern was inferred, using Okada's dislocation solution formulae, which shows that the maximum calculated vertical displacement is 0.53 m. This is in good agreement with the statistical analysis of the observed surface rupture displacement. Stress transfer analysis was also performed in the five modelled seismogenic sources, using seismologically defined parameters. The resulting stress transfer pattern, based on the sequence of events, shows that the causative fault of each event was influenced by loading from the previous ones.

Micro-geomorphology Surveying and Analysis of Xiadian Fault Scarp, China

NASA Astrophysics Data System (ADS)

Ding, R.

2014-12-01

Historic records and field investigations reveal that the Mw 8.0 Sanhe-Pinggu (China) earthquake of 1679 produced a 10 to 18 km-long surface rupture zone, with dominantly dip-slip accompanied by a right-lateral component along the Xiadian fault, resulting in extensive damage throughout north China. The fault scarp that was coursed by the co-seismic ruptures from Dongliuhetun to Pangezhang is about 1 to 3 meters high, and the biggest vertical displacement locates in Pangezhuang, it is easily to be seen in the flat alluvial plain. But the 10 to 18 km-long surface rupture couldn't match the Mw 8.0 earthquake scale. After more than 300 years land leveling, the fault scarps in the meizoseismal zone which is farmland are retreat at different degree, some small scarps are becoming disappeared, so it is hard to identify by visual observation in the field investigations. The meizoseismal zone is located in the alluvial plain of the Chaobai river and Jiyun river, and the fault is perpendicular to the river. It is easy to distinguish fault scarps from erosion scarps. Land leveling just changes the slope of the fault scarp, but it can't eliminate the height difference between two side of the fault. So it is possible to recover the location and height of the fault scarp by using Digital Elevation Model (DEM) analysis and landform surveying which is constrained by 3D centimeter-precision RTK GPS surveying method in large scale crossing the fault zone. On the base of the high-precision DEM landform analysis, we carried out 15 GPS surveying lines which extends at least 10km for each crossing the meizoseismal zone. Our findings demonstrate that 1) we recover the complete rupture zone of the Sanhe-Pinggu earthquake in 1679, and survey the co-seismic displacement at 15 sites; 2) we conform that the Xiadian fault scarp is consist of three branches with left stepping. Height of the scarp is from 0.5 to 4.0 meters, and the total length of the scarp is at least 50km; 3) Combined with the analysis of offset strata of the trench, we conform that the middle segment of the fault scarp is made by 1679 earthquake; 4) The fault scarp strikes along with the Ju river at the northeast segment of the Xiadian fault which course the asymmetrical valley geomorphology.

The role of large strike-slip faults in a convergent continental setting - first results from the Dzhungarian Fault in Eastern Kazakhstan

NASA Astrophysics Data System (ADS)

Grützner, Christoph; Campbell, Grace; Elliott, Austin; Walker, Richard; Abdrakhmatov, Kanatbek

2016-04-01

The Tien Shan and the Dzhungarian Ala-tau mountain ranges in Eastern Kazakhstan and China take up a significant portion of the total convergence between India and Eurasia, despite the fact that they are more than 1000 km away from the actual plate boundary. Shortening is accommodated by large thrust faults that strike more or less perpendicular to the convergence vector, and by a set of conjugate strike-slip faults. Some of these strike-slip faults are major features of several hundred kilometres length and have produced great historical earthquakes. In most cases, little is known about their slip-rates and earthquake history, and thus, about their role in the regional tectonic setting. This study deals with the NW-SE trending Dzhungarian Fault, a more than 350 km-long, right-lateral strike slip feature. It borders the Dzhungarian Ala-tau range and forms one edge of the so-called Dzhungarian Gate. The fault curves from a ~305° strike at its NW tip in Kazakhstan to a ~328° strike in China. No historical ruptures are known from the Kazakh part of the fault. A possible rupture in 1944 in the Chinese part remains discussed. We used remote sensing, Structure-from-Motion (SfM), differential GPS, field mapping, and Quaternary dating of offset geological markers in order to map the fault-related morphology and to measure the slip rate of the fault at several locations along strike. We also aimed to find out the age of the last surface rupturing earthquake and to determine earthquake recurrence intervals and magnitudes. We were further interested in the relation between horizontal and vertical motion along the fault and possible fault segmentation. Here we present first results from our 2015 survey. High-resolution digital elevation models of offset river terraces allowed us to determine the slip vector of the most recent earthquake. Preliminary dating results from abandoned fluvial terraces allow us to speculate on a late Holocene surface rupturing event. Morphological data indicate that more than one fault strand was activated in the Holocene. Folded river terraces testify to the amplitude of long-term deformation associated with the Dzhungarian Fault, but no dating results are available yet.

Dynamic rupture simulations on complex fault zone structures with off-fault plasticity using the ADER-DG method

NASA Astrophysics Data System (ADS)

Wollherr, Stephanie; Gabriel, Alice-Agnes; Igel, Heiner

2015-04-01

In dynamic rupture models, high stress concentrations at rupture fronts have to to be accommodated by off-fault inelastic processes such as plastic deformation. As presented in (Roten et al., 2014), incorporating plastic yielding can significantly reduce earlier predictions of ground motions in the Los Angeles Basin. Further, an inelastic response of materials surrounding a fault potentially has a strong impact on surface displacement and is therefore a key aspect in understanding the triggering of tsunamis through floor uplifting. We present an implementation of off-fault-plasticity and its verification for the software package SeisSol, an arbitrary high-order derivative discontinuous Galerkin (ADER-DG) method. The software recently reached multi-petaflop/s performance on some of the largest supercomputers worldwide and was a Gordon Bell prize finalist application in 2014 (Heinecke et al., 2014). For the nonelastic calculations we impose a Drucker-Prager yield criterion in shear stress with a viscous regularization following (Andrews, 2005). It permits the smooth relaxation of high stress concentrations induced in the dynamic rupture process. We verify the implementation by comparison to the SCEC/USGS Spontaneous Rupture Code Verification Benchmarks. The results of test problem TPV13 with a 60-degree dipping normal fault show that SeisSol is in good accordance with other codes. Additionally we aim to explore the numerical characteristics of the off-fault plasticity implementation by performing convergence tests for the 2D code. The ADER-DG method is especially suited for complex geometries by using unstructured tetrahedral meshes. Local adaptation of the mesh resolution enables a fine sampling of the cohesive zone on the fault while simultaneously satisfying the dispersion requirements of wave propagation away from the fault. In this context we will investigate the influence of off-fault-plasticity on geometrically complex fault zone structures like subduction zones or branched faults. Studying the interplay of stress conditions and angle dependence of neighbouring branches including inelastic material behaviour and its effects on rupture jumps and seismic activation helps to advance our understanding of earthquake source processes. An application is the simulation of a real large-scale subduction zone scenario including plasticity to validate the coupling of our dynamic rupture calculations to a tsunami model in the framework of the ASCETE project (http://www.ascete.de/). Andrews, D. J. (2005): Rupture dynamics with energy loss outside the slip zone, J. Geophys. Res., 110, B01307. Heinecke, A. (2014), A. Breuer, S. Rettenberger, M. Bader, A.-A. Gabriel, C. Pelties, A. Bode, W. Barth, K. Vaidyanathan, M. Smelyanskiy and P. Dubey: Petascale High Order Dynamic Rupture Earthquake Simulations on Heterogeneous Supercomputers. In Supercomputing 2014, The International Conference for High Performance Computing, Networking, Storage and Analysis. IEEE, New Orleans, LA, USA, November 2014. Roten, D. (2014), K. B. Olsen, S.M. Day, Y. Cui, and D. Fäh: Expected seismic shaking in Los Angeles reduced by San Andreas fault zone plasticity, Geophys. Res. Lett., 41, 2769-2777.

Dynamic rupture simulation of the 2017 Mw 7.8 Kaikoura (New Zealand) earthquake: Is spontaneous multi-fault rupture expected?

NASA Astrophysics Data System (ADS)

Ando, R.; Kaneko, Y.

2017-12-01

The coseismic rupture of the 2016 Kaikoura earthquake propagated over the distance of 150 km along the NE-SW striking fault system in the northern South Island of New Zealand. The analysis of In-SAR, GPS and field observations (Hamling et al., 2017) revealed that the most of the rupture occurred along the previously mapped active faults, involving more than seven major fault segments. These fault segments, mostly dipping to northwest, are distributed in a quite complex manner, manifested by fault branching and step-over structures. Back-projection rupture imaging shows that the rupture appears to jump between three sub-parallel fault segments in sequence from the south to north (Kaiser et al., 2017). The rupture seems to be terminated on the Needles fault in Cook Strait. One of the main questions is whether this multi-fault rupture can be naturally explained with the physical basis. In order to understand the conditions responsible for the complex rupture process, we conduct fully dynamic rupture simulations that account for 3-D non-planar fault geometry embedded in an elastic half-space. The fault geometry is constrained by previous In-SAR observations and geological inferences. The regional stress field is constrained by the result of stress tensor inversion based on focal mechanisms (Balfour et al., 2005). The fault is governed by a relatively simple, slip-weakening friction law. For simplicity, the frictional parameters are uniformly distributed as there is no direct estimate of them except for a shallow portion of the Kekerengu fault (Kaneko et al., 2017). Our simulations show that the rupture can indeed propagate through the complex fault system once it is nucleated at the southernmost segment. The simulated slip distribution is quite heterogeneous, reflecting the nature of non-planar fault geometry, fault branching and step-over structures. We find that optimally oriented faults exhibit larger slip, which is consistent with the slip model of Hamling et al. (2017). We conclude that the first order characteristics of this event may be interpreted by the effect of irregularity in the fault geometry.

Geodetic Insights into the Earthquake Cycle in a Fold and Thrust Belt

NASA Astrophysics Data System (ADS)

Ingleby, T. F.; Wright, T. J.; Butterworth, V.; Weiss, J. R.; Elliott, J.

2017-12-01

Geodetic measurements are often sparse in time (e.g. individual interferograms) and/or space (e.g. GNSS stations), adversely affecting our ability to capture the spatiotemporal detail required to study the earthquake cycle in complex tectonic systems such as subaerial fold and thrust belts. In an effort to overcome these limitations we combine 3 generations of SAR satellite data (ERS 1/2, Envisat & Sentinel-1a/b) to obtain a 25 year, high-resolution surface displacement time series over the frontal portion of an active fold and thrust belt near Quetta, Pakistan where a Mw 7.1 earthquake doublet occurred in 1997. With these data we capture a significant portion of the seismic cycle including the interseismic, coseismic and postseismic phases. Each satellite time series has been referenced to the first ERS-1 SAR epoch by fitting a ground deformation model to the data. This allows us to separate deformation associated with each phase and to examine their relative roles in accommodating strain and creating topography, and to explore the relationship between the earthquake cycle and critical taper wedge mechanics. Modeling of the coseismic deformation suggests a long, thin rupture with rupture length 7 times greater than rupture width. Rupture was confined to a 20-30 degree north-northeast dipping reverse fault or ramp at depth, which may be connecting two weak decollements at approximately 8 km and 13 km depth. Alternatively, intersections between the coseismic fault plane and pre-existing steeper splay faults underlying folds may have played a significant role in inhibiting rupture, as evidenced by intersection points bordering the rupture. These fault intersections effectively partition the fault system down-dip and enable long, thin ruptures. Postseismic deformation is manifest as uplift across short-wavelength folds at the thrust front, with displacement rates decreasing with time since the earthquake. Broader patterns of postseismic uplift are also observed surrounding the coseismic rupture in both the down- and up-dip directions. We examine how coseismic stress changes may be driving the postseismic deformation by jointly inverting the InSAR-derived displacements for the rupture and fault friction parameters using a rate-strengthening friction model.

The 2015 April 25 Gorkha (Nepal) earthquake and its aftershocks: implications for lateral heterogeneity on the Main Himalayan Thrust

NASA Astrophysics Data System (ADS)

Kumar, Ajay; Singh, Shashwat K.; Mitra, S.; Priestley, K. F.; Dayal, Shankar

2017-02-01

The 2015 Gorkha earthquake (Mw 7.8) occurred by thrust faulting on a ˜150 km long and ˜70 km wide, locked downdip segment of the Main Himalayan Thrust (MHT), causing the Himalaya to slip SSW over the Indian Plate, and was followed by major-to-moderate aftershocks. Back projection of teleseismic P-wave and inversion of teleseismic body waves provide constraints on the geometry and kinematics of the main-shock rupture and source mechanism of aftershocks. The main-shock initiated ˜80 km west of Katmandu, close to the locking line on the MHT and propagated eastwards along ˜117° azimuth for a duration of ˜70 s, with varying rupture velocity on a heterogeneous fault surface. The main-shock has been modelled using four subevents, propagating from west-to-east. The first subevent (0-20 s) ruptured at a velocity of ˜3.5 km s- 1 on a ˜6°N dipping flat segment of the MHT with thrust motion. The second subevent (20-35 s) ruptured a ˜18° W dipping lateral ramp on the MHT in oblique thrust motion. The rupture velocity dropped from 3.5 km s- 1 to 2.5 km s- 1, as a result of updip propagation of the rupture. The third subevent (35-50 s) ruptured a ˜7°N dipping, eastward flat segment of the MHT with thrust motion and resulted in the largest amplitude arrivals at teleseismic distances. The fourth subevent (50-70 s) occurred by left-lateral strike-slip motion on a steeply dipping transverse fault, at high angle to the MHT and arrested the eastward propagation of the main-shock rupture. Eastward stress build-up following the main-shock resulted in the largest aftershock (Mw 7.3), which occurred on the MHT, immediately east of the main-shock rupture. Source mechanisms of moderate aftershocks reveal stress adjustment at the edges of the main-shock fault, flexural faulting on top of the downgoing Indian Plate and extensional faulting in the hanging wall of the MHT.

Rupture complexity and the supershear transition on rough faults

NASA Astrophysics Data System (ADS)

Bruhat, Lucile; Fang, Zijun; Dunham, Eric M.

2016-01-01

Field investigations suggest that supershear earthquakes occur on geometrically simple, smooth fault segments. In contrast, dynamic rupture simulations show how heterogeneity of stress, strength, and fault geometry can trigger supershear transitions, as well as other complex rupture styles. Here we examine the Fang and Dunham (2013) ensemble of 2-D plane strain dynamic ruptures on fractally rough faults subject to strongly rate weakening friction laws to document the effect of fault roughness and prestress on rupture behavior. Roughness gives rise to extremely diverse rupture styles, such as rupture arrests, secondary slip pulses that rerupture previously slipped fault sections, and supershear transitions. Even when the prestress is below the Burridge-Andrews threshold for supershear on planar faults with uniform stress and strength conditions, supershear transitions are observed. A statistical analysis of the rupture velocity distribution reveals that supershear transients become increasingly likely at higher stress levels and on rougher faults. We examine individual ruptures and identify recurrent patterns for the supershear transition. While some transitions occur on fault segments that are favorably oriented in the background stress field, other transitions happen at the initiation of or after propagation through an unfavorable bend. We conclude that supershear transients are indeed favored by geometric complexity. In contrast, sustained supershear propagation is most common on segments that are locally smoother than average. Because rupture style is so sensitive to both background stress and small-scale details of the fault geometry, it seems unlikely that field maps of fault traces will provide reliable deterministic predictions of supershear propagation on specific fault segments.

The 2016 Mw7.0 Kumamoto, Japan earthquake: the rupture propagation under extensional stress

NASA Astrophysics Data System (ADS)

Zhang, Y.; Shan, X.; Zhang, G.; Gong, W.

2016-12-01

On April 16, 2016, the Kumamoto city was hit by an Mw7.0 earthquake, the largest earthquake since 1900 in the central part of Kyushu Island in Japan. It is an event with two foreshocks and rather complex source faults and surface rupture scarps. The Mw7.0 Kumamoto earthquake and its foreshocks and aftershocks occurred on the Futagawa and Hinagu faults, which are previously mapped and formed the southwest portion of the median tectonic line on Kyushu Island. These faults are mainly controlled by extensional and right-lateral shear stress. In this study, we obtained the deformation filed of the Kumamoto earthquake using both of descending and ascending Sentinel-1A data. We then invert the fault slip distribution based on the displacements obtained by InSAR. A three-segment fault model is established by trial and error. We analyze the rupture propagation and the conclusions are listed as following: The Mw 7.0 earthquake is a right-lateral striking event with a slight normal component. Most of the slip distributed on the Futagawa fault segment, with a maximum slip of 4.9 m at 5 km depth below the surface. The energy released on this Futagawa fault segment is equivalent to an Mw6.9 event. The slip distribution on the Hinagu fault segment is also right-lateral, but with a maximum slip of 2 m. Compared to the southern two segments, the northern source fault segment has the steepest dipping segment, which is almost vertical, with a dip as high as 80°; The normal component of the Kumamoto event is controlled by extensional stress due to the tectonic background. The Beppu-Shimabara half graben is the largest extensional structure on Kyushu Island and its formation could strongly be affected by Philippine Sea slab (PHS) convergence and Okinawa Trough extension, so we argue the Kumamoto event maybe exhibits the concrete manifestation of Okinawa Trough extension to Kyushu Island; Continuous surface rupture trace is observed from InSAR coseismic deformation and field investigation, based on which we confirm that the Kumamoto event jumped a 1 km wide step over of the Kiyama fault and two 0.6km wide gaps. However, the mainshock do not jump a 1.7 km wide step over of the Futagawa fault, so its magnitude moment is constrained. In addition, both the Mw6.4 and Mw6.5 events could not go through a 2 km wide at the northeast termination of the Hinagu faults.

Timing of large earthquakes since A.D. 800 on the Mission Creek strand of the San Andreas fault zone at Thousand Palms Oasis, near Palm Springs, California

USGS Publications Warehouse

Fumal, T.E.; Rymer, M.J.; Seitz, G.G.

2002-01-01

Paleoseismic investigations across the Mission Creek strand of the San Andreas fault at Thousand Palms Oasis indicate that four and probably five surface-rupturing earthquakes occurred during the past 1200 years. Calendar age estimates for these earthquakes are based on a chronological model that incorporates radio-carbon dates from 18 in situ burn layers and stratigraphic ordering constraints. These five earthquakes occurred in about A.D. 825 (770-890) (mean, 95% range), A.D. 982 (840-1150), A.D. 1231 (1170-1290), A.D. 1502 (1450-1555), and after a date in the range of A.D. 1520-1680. The most recent surface-rupturing earthquake at Thousand Palms is likely the same as the A.D. 1676 ?? 35 event at Indio reported by Sieh and Williams (1990). Each of the past five earthquakes recorded on the San Andreas fault in the Coachella Valley strongly overlaps in time with an event at the Wrightwood paleoseismic site, about 120 km northwest of Thousand Palms Oasis. Correlation of events between these two sites suggests that at least the southernmost 200 km of the San Andreas fault zone may have ruptured in each earthquake. The average repeat time for surface-rupturing earthquakes on the San Andreas fault in the Coachella Valley is 215 ?? 25 years, whereas the elapsed time since the most recent event is 326 ?? 35 years. This suggests the southernmost San Andreas fault zone likely is very near failure. The Thousand Palms Oasis site is underlain by a series of six channels cut and filled since about A.D. 800 that cross the fault at high angles. A channel margin about 900 years old is offset right laterally 2.0 ?? 0.5 m, indicating a slip rate of 4 ?? 2 mm/yr. This slip rate is low relative to geodetic and other geologic slip rate estimates (26 ?? 2 mm/yr and about 23-35 mm/yr, respectively) on the southernmost San Andreas fault zone, possibly because (1) the site is located in a small step-over in the fault trace and so the rate is not be representative of the Mission Creek fault, (2) slip is partitioned northward from the San Andreas fault and into the eastern California shear zone, and/or (3) slip is partitioned onto the Banning strand of the San Andreas fault zone.

Source model and Coulomb stress change of 2017 Mw 6.5 Philippine (Ormoc) Earthquake revealed by SAR interferometry

NASA Astrophysics Data System (ADS)

Tsai, M. C.; Hu, J. C.; Yang, Y. H.; Hashimoto, M.; Aurelio, M.; Su, Z.; Escudero, J. A.

2017-12-01

Multi-sight and high spatial resolution interferometric SAR data enhances our ability for mapping detailed coseismic deformation to estimate fault rupture model and to infer the Coulomb stress change associated with a big earthquake. Here, we use multi-sight coseismic interferograms acquired by ALOS-2 and Sentinel-1A satellites to estimate the fault geometry and slip distribution on the fault plane of the 2017 Mw 6.5 Ormoc Earthquake in Leyte island of Philippine. The best fitting model predicts that the coseismic rupture occurs along a fault plane with strike of 325.8º and dip of 78.5ºE. This model infers that the rupture of 2017 Ormoc earthquake is dominated by left-lateral slip with minor dip-slip motion, consistent with the left-lateral strike-slip Philippine fault system. The fault tip has propagated to the ground surface, and the predicted coseismic slip on the surface is about 1 m located at 6.5 km Northeast of Kananga city. Significant slip is concentrated on the fault patches at depth of 0-8 km and an along-strike distance of 20 km with varying slip magnitude from 0.3 m to 2.3 m along the southwest segment of this seismogenic fault. Two minor coseismic fault patches are predicted underneath of the Tononan geothermal field and the creeping segment of the northwest portion of this seismogenic fault. This implies that the high geothermal gradient underneath of the Tongonan geothermal filed could prevent heated rock mass from the coseismic failure. The seismic moment release of our preferred fault model is 7.78×1018 Nm, equivalent to Mw 6.6 event. The Coulomb failure stress (CFS) calculated by the preferred fault model predicts significant positive CFS change on the northwest segment of the Philippine fault in Leyte Island which has coseismic slip deficit and is absent from aftershocks. Consequently, this segment should be considered to have increasing of risk for future seismic hazard.

Continuous borehole strain in the San Andreas fault zone before, during, and after the 28 June 1992, MW 7.3 Landers, California, earthquake

USGS Publications Warehouse

Johnston, M.J.S.; Linde, A.T.; Agnew, D.C.

1994-01-01

High-precision strain was observed with a borehole dilational strainmeter in the Devil's Punchbowl during the 11:58 UT 28 June 1992 MW 7.3 Landers earthquake and the large Big Bear aftershock (MW 6.3). The strainmeter is installed at a depth of 176 m in the fault zone approximately midway between the surface traces of the San Andreas and Punchbowl faults and is about 100 km from the 85-km-long Landers rupture. We have questioned whether unusual amplified strains indicating precursive slip or high fault compliance occurred on the faults ruptured by the Landers earthquake, or in the San Andreas fault zone before and during the earthquake, whether static offsets for both the Landers and Big Bear earthquakes agree with expectation from geodetic and seismologic models of the ruptures and with observations from a nearby two-color geodimeter network, and whether postseismic behavior indicated continued slip on the Landers rupture or local triggered slip on the San Andreas. We show that the strain observed during the earthquake at this instrument shows no apparent amplification effects. There are no indications of precursive strain in these strain data due to either local slip on the San Andreas or precursive slip on the eventual Landers rupture. The observations are generally consistent with models of the earthquake in which fault geometry and slip have the same form as that determined by either inversion of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. Finally, there are some indications of minor postseismic behavior, particularly during the month following the earthquake.

Heterogeneous rupture on homogenous faults: Three-dimensional spontaneous rupture simulations with thermal pressurization

NASA Astrophysics Data System (ADS)

Urata, Yumi; Kuge, Keiko; Kase, Yuko

2008-11-01

To understand role of fluid on earthquake rupture processes, we investigated effects of thermal pressurization on spatial variation of dynamic rupture by computing spontaneous rupture propagation on a rectangular fault. We found thermal pressurization can cause heterogeneity of rupture even on a fault of uniform properties. On drained faults, tractions drop linearly with increasing slip in the same way everywhere. However, by changing the drained condition to an undrained one, the slip-weakening curves become non-linear and depend on locations on faults with small shear zone thickness w, and the dynamic frictional stresses vary spatially and temporally. Consequently, the super-shear transition fault length decreases for small w, and the final slip distribution can have some peaks regardless of w, especially on undrained faults. These effects should be taken into account of determining dynamic rupture parameters and modeling earthquake cycles when the presence of fluid is suggested in the source regions.

Quaternary Slip History for the Agua Blanca Fault, northern Baja California, Mexico

NASA Astrophysics Data System (ADS)

Gold, P. O.; Behr, W. M.; Rockwell, T. K.; Fletcher, J. M.

2017-12-01

The Agua Blanca Fault (ABF) is the primary structure accommodating San Andreas-related right-lateral slip across the Peninsular Ranges of northern Baja California. Activity on this fault influences offshore faults that parallel the Pacific coast from Ensenada to Los Angeles and is a potential threat to communities in northern Mexico and southern California. We present a detailed Quaternary slip history for the ABF, including new quantitative constraints on geologic slip rates, slip-per-event, the timing of most recent earthquake, and the earthquake recurrence interval. Cosmogenic 10Be exposure dating of clasts from offset fluvial geomorphic surfaces at 2 sites located along the western, and most active, section of the ABF yield preliminary slip rate estimates of 2-4 mm/yr and 3 mm/yr since 20 ka and 2 ka, respectively. Fault zone geomorphology preserved at the younger site provides evidence for right-lateral surface displacements measuring 2.5 m in the past two ruptures. Luminescence dating of an offset alluvial fan at a third site is in progress, but is expected to yield a slip rate relevant to the past 10 kyr. Adjacent to this third site, we excavated 2 paleoseismic trenches across a sag pond formed by a right step in the fault. Preliminary radiocarbon dates indicate that the 4 surface ruptures identified in the trenches occurred in the past 6 kyr, although additional dating should clarify earthquake timing and the mid-Holocene to present earthquake recurrence interval, as well as the likely date of the most recent earthquake. Our new slip rate estimates are somewhat lower than, but comparable within error to, previous geologic estimates based on soil morphology and geodetic estimates from GPS, but the new record of surface ruptures exposed in the trenches is the most complete and comprehensively dated earthquake history yet determined for this fault. Together with new and existing mapping of tectonically generated geomorphology along the ABF, our constraints show that contrary to some theories of fault interaction and activity for this section of the San Andreas system, the Agua Blanca Fault has been active over the late Holocene, and should be considered as a potential source of seismic hazard.

Faulting, damage, and intensity in the Canyondam earthquake of May 23, 2013

USGS Publications Warehouse

Chapman, K.; Gold, M.B.; Boatwright, John; Sipe, J.; Quitoriano, V.; Dreger, D.; Hardebeck, Jeanne

2016-09-23

On Thursday evening, May 23, 2013 (0347 May 24 UTC), a moment magnitude (Mw) = 5.7 earthquake occurred northeast of Canyondam, California. A two-person team of U.S. Geological Survey scientists went to the area to search for surface rupture and to canvass damage in the communities around Lake Almanor. While the causative fault had not been identified at the time of the field survey, surface rupture was expected to have occurred just south of Lake Almanor, approximately 2–4 kilometers south of the epicenter. No surface rupture was discovered. Felt intensity among the communities around Lake Almanor appeared to vary significantly. Lake Almanor West (LAW), Lake Almanor Country Club (LACC), and Hamilton Branch (HB) experienced Modified Mercalli Intensity (MMI) ≥7, whereas other communities around the lake experienced MMI ≤6; the maximum observed intensity was MMI 8, in LAW. Damage in the high intensity areas consisted of broken and collapsed chimneys, ruptured pipes, and some damage to foundations and to structural elements within houses. Although this shaking damage is not usually expected for an Mw 5.7 earthquake, the intensities at Lake Almanor Country Club correlate with the peak ground acceleration (38 percent g) and peak ground velocity (30 centimeters per second) recorded by the California Strong Motion Instrumentation Program accelerometer located at the nearby Lake Almanor Fire Station. The intensity distribution for the three hardest hit areas (LAW, LACC, and HB) appears to increase as the azimuth from epicenter to the intensity sites approaches the fault strike. The small communities of Almanor and Prattville on the southwestern shore of Lake Almanor experienced somewhat lower intensities. The town of Canyondam experienced a lower intensity as well, despite its location up-dip of the earthquake rupture. This report contains information on the earthquake itself, the search for surface rupture, and the damage we observed and compiled from other sources. 

A multiple fault rupture model of the November 13 2016, M 7.8 Kaikoura earthquake, New Zealand

NASA Astrophysics Data System (ADS)

Benites, R. A.; Francois-Holden, C.; Langridge, R. M.; Kaneko, Y.; Fry, B.; Kaiser, A. E.; Caldwell, T. G.

2017-12-01

The rupture-history of the November 13 2016 MW7.8 Kaikoura earthquake recorded by near- and intermediate-field strong-motion seismometers and 2 high-rate GPS stations reveals a complex cascade of multiple crustal fault rupture. In spite of such complexity, we show that the rupture history of each fault is well approximated by simple kinematic model with uniform slip and rupture velocity. Using 9 faults embedded in a crustal layer 19 km thick, each with a prescribed slip vector and rupture velocity, this model accurately reproduces the displacement waveforms recorded at the near-field strong-motion and GPS stations. This model includes the `Papatea Fault' with a mixed thrust and strike-slip mechanism based on in-situ geological observations with up to 8 m of uplift observed. Although the kinematic model fits the ground-motion at the nearest strong station, it doesn not reproduce the one sided nature of the static deformation field observed geodetically. This suggests a dislocation based approach does not completely capture the mechanical response of the Papatea Fault. The fault system as a whole extends for approximately 150 km along the eastern side of the Marlborough fault system in the South Island of New Zealand. The total duration of the rupture was 74 seconds. The timing and location of each fault's rupture suggests fault interaction and triggering resulting in a northward cascade crustal ruptures. Our model does not require rupture of the underlying subduction interface to explain the data.

The Wasatch fault zone, utah—segmentation and history of Holocene earthquakes

NASA Astrophysics Data System (ADS)

Machette, Michael N.; Personius, Stephen F.; Nelson, Alan R.; Schwartz, David P.; Lund, William R.

The Wasatch fault zone (WFZ) forms the eastern boundary of the Basin and Range province and is the longest continuous, active normal fault (343 km) in the United States. It underlies an urban corridor of 1.6 million people (80% of Utah's population) representing the largest earthquake risk in the interior of the western United States. We have used paleoseismological data to identify 10 discrete segments of the WFZ. Five are active, medial segments with Holocene slip rates of 1-2 mm a -1, recurrence intervals of 2000-4000 years and average lengths of about 50 km. Five are less active, distal segments with mostly pre-Holocene surface ruptures, late Quaternary slip rates of <0.5 mm a -1 recurrence intervals of ≥10,000 years and average lengths of about 20 km. Surface-faulting events on each of the medial segments of the WFZ formed 2-4-m-high scarps repeatedly during the Holocene; latest Pleistocene (14-15 ka) deposits commonly have scarps as much as 15-20 m in height. Segments identified from paleoseismological studies of other major late Quaternary normal faults in the northern Basin and Range province are 20-25 km long, or about half of that proposed for the medial segments of the WFZ. Paleoseismological records for the past 6000 years indicate that a major surface-rupturing earthquake has occurred along one of the medial segments about every 395 ± 60 years. However, between about 400 and 1500 years ago, the WFZ experienced six major surface-rupturing events, an average of one event every 220 years, or about twice as often as expected from the 6000-year record. This pattern of temporal clustering is similar to that of the central Nevada—eastern California Seismic Belt in the western part of the Basin and Range province, where 11 earthquakes of M > 6.5 have occurred since 1860. Although the time scale of the clustering is different—130 years vs 1100 years—we consider the central Nevada—eastern California Seismic Belt to be a historic analog for movement on the WFZ during the past 1500 years. We have found no evidence that surface-rupturing events occurred on the WFZ during the past 400 years, a time period which is twice the average intracluster recurrence interval and equal to the average Holocene recurrence interval. In particular, the Brigham City segment (the northernmost medial segment) has not ruptured in the past 3600 years—a period that is about three times longer than this segment's average recurrence interval during the early and middle Holocene. Although the WFZ's seismological record is one of relative quiescence, a comparison with other historic surface-rupturing earthquakes in the region suggests that earthquakes having moment magnitudes of 7.1-7.4 (or surface-wave magnitudes of 7.5-7.7)—each associated with tens of kilometers of surface rupture and several meters of normal dip slip—have occurred about every four centuries during the Holocene and should be expected in the future.

Poroelastic rebound along the Landers 1992 earthquake surface rupture

USGS Publications Warehouse

Peltzer, G.; Rosen, P.; Rogez, F.; Hudnut, K.

1998-01-01

Maps of surface displacement following the 1992 Landers, California, earthquake, generated by interferometric processing of ERS-1 synthetic aperture radar (SAR) images, reveal effects of various postseismic deformation processes along the 1992 surface rupture. The large-scale pattern of the postseismic displacement field includes large lobes, mostly visible on the west side of the fault, comparable in shape with the lobes observed in the coseismic displacement field. This pattern and the steep displacement gradient observed near the Emerson-Camp Rock fault cannot be simply explained by afterslip on deep sections of the 1992 rupture. Models show that horizontal slip occurring on a buried dislocation in a Poisson's material produces a characteristic quadripole pattern in the surface displacement field with several centimeters of vertical motion at distances of 10-20 km from the fault, yet this pattern is not observed in the postseismic interferograms. As previously proposed to explain local strain in the fault step overs [Peltzer et al., 1996b], we argue that poroelastic rebound caused by pore fluid flow may also occur over greater distances from the fault, compensating the vertical ground shift produced by fault afterslip. Such a rebound is explained by the gradual change of the crustal rocks' Poisson's ratio value from undrained (coseismic) to drained (postseismic) conditions as pore pressure gradients produced by the earthquake dissipate. Using the Poisson's ratio values of 0.27 and 0.31 for the drained and undrained crustal rocks, respectively, elastic dislocation models show that the combined contributions of afterslip on deep sections of the fault and poroelastic rebound can account for the range change observed in the SAR data and the horizontal displacement measured at Global Positioning System (GPS) sites along a 60-km-long transect across the Emerson fault [Savage and Svarc, 1997]. Using a detailed surface slip distribution on the Homestead Valley, Kickapoo, and Johnson Valley faults, we modeled the poroelastic rebound in the Homestead Valley pull apart. A Poisson's ratio value of 0.35 for the undrained gouge rocks in the fault zone is required to account for the observed surface uplift in the 3.5 years following the earthquake. This large value implies a seismic velocity ratio Vp/Vs of 2.1, consistent with the observed low Vs values of fault zone guided waves at shallow depth [Li et al., 1997]. The SAR data also reveal postseismic creep along shallow patches of the Eureka Peak and Burnt Mountain faults with a characteristic decay time of 0.8 years. Coseismic, dilatant hardening (locking process) followed by post-seismic, pore pressure controlled fault creep provide a plausible mechanism to account for the decay time of the observed slip rate along this section of the fault. Copyright 1998 by the American Geophysical Union.

Seismic constraints on the architecture of the Newport-Inglewood/Rose Canyon fault: Implications for the length and magnitude of future earthquake ruptures

NASA Astrophysics Data System (ADS)

Sahakian, Valerie; Bormann, Jayne; Driscoll, Neal; Harding, Alistair; Kent, Graham; Wesnousky, Steve

2017-03-01

The Newport-Inglewood/Rose Canyon (NIRC) fault zone is an active strike-slip fault system within the Pacific-North American plate boundary in Southern California, located in close proximity to populated regions of San Diego, Orange, and Los Angeles counties. Prior to this study, the NIRC fault zone's continuity and geometry were not well constrained. Nested marine seismic reflection data with different vertical resolutions are employed to characterize the offshore fault architecture. Four main fault strands are identified offshore, separated by three main stepovers along strike, all of which are 2 km or less in width. Empirical studies of historical ruptures worldwide show that earthquakes have ruptured through stepovers with this offset. Models of Coulomb stress change along the fault zone are presented to examine the potential extent of future earthquake ruptures on the fault zone, which appear to be dependent on the location of rupture initiation and fault geometry at the stepovers. These modeling results show that the southernmost stepover between the La Jolla and Torrey Pines fault strands may act as an inhibitor to throughgoing rupture due to the stepover width and change in fault geometry across the stepover; however, these results still suggest that rupture along the entire fault zone is possible.

[X-ray diffraction and infrared spectrum analysis of fault gouge in Wenchuan seismic belt].

PubMed

Wang, Zheng-Yang; Cao, Jian-Jin; Luo, Song-Ying; Liao, Yi-Peng

2014-05-01

Wenchuan earthquake produced a series of co-seismic surface ruptures in Leigu and Zhaojiagou, and we collected samples of co-seismic fault gouge in the surface ruptures as well as the old gouge in the fault of Nanba. Testing The new and old fault gouge was tested with X-ray diffraction and infrared absorption spectra, and its characteristics such as mineral compositions, clay mineral contents and combinations were comprehensively analyzed. The results display obvious differences between the new and old fault gouge, showing that the old fault gouge is mainly composed of wall rock debris or milled powders, while the main components of new fault gouge are clay minerals. The assemblage of clay minerals composition shows that the environment of the fault activity was mainly warm and humid, and the clay minerals were mainly transformed by low temperature and low pressure dynamic metamorphism. And this also partly indicates that the latest way of the fault activity in this area may be a creeping. However the previous researches on the fault gouge of Wenchuan earthquake fault zone are mainly focused on its mechanical properties as well as its texture and structure, the research in this paper is to determine the physical and chemical environment of fault activity through the mineral compositions and clay mineral contents in the fault gouge characteristics, and this research has important scientific significance to the researches on the evolution of the fault environment and the activity mechanism of the earthquake.

Structure and mechanics of the Hayward-Rodgers Creek Fault step-over, San Francisco Bay, California

USGS Publications Warehouse

Parsons, T.; Sliter, R.; Geist, E.L.; Jachens, R.C.; Jaffe, B.E.; Foxgrover, A.; Hart, P.E.; McCarthy, J.

2003-01-01

A dilatational step-over between the right-lateral Hayward and Rodgers Creek faults lies beneath San Pablo Bay in the San Francisco Bay area. A key seismic hazard issue is whether an earthquake on one of the faults could rupture through the step-over, enhancing its maximum possible magnitude. If ruptures are terminated at the step-over, then another important issue is how strain transfers through the step. We developed a combined seismic reflection and refraction cross section across south San Pablo Bay and found that the Hayward and Rodgers Creek faults converge to within 4 km of one another near the surface, about 2 km closer than previously thought. Interpretation of potential field data from San Pablo Bay indicated a low likelihood of strike-slip transfer faults connecting the Hayward and Rodgers Creek faults. Numerical simulations suggest that it is possible for a rupture to jump across a 4-km fault gap, although special stressing conditions are probably required (e.g., Harris and Day, 1993, 1999). Slip on the Hayward and Rodgers Creek faults is building an extensional pull-apart basin that could contain hazardous normal faults. We investigated strain in the pull-apart using a finite-element model and calculated a ???0.02-MPa/yr differential stressing rate in the step-over on a least-principal-stress orientation nearly parallel to the strike-slip faults where they overlap. A 1- to 10-MPa stress-drop extensional earthquake is expected on normal faults oriented perpendicular to the strike-slip faults every 50-500 years. The last such earthquake might have been the 1898 M 6.0-6.5 shock in San Pablo Bay that apparently produced a small tsunami. Historical hydrographic surveys gathered before and after 1898 indicate abnormal subsidence of the bay floor within the step-over, possibly related to the earthquake. We used a hydrodynamic model to show that a dip-slip mechanism in north San Pablo Bay is the most likely 1898 rupture scenario to have caused the tsunami. While we find no strike-slip transfer fault between the Hayward and Rodgers Creek faults, a normal-fault link could enable through-going segmented rupture of both strike-slip faults and may pose an independent hazard of M ???6 earthquakes like the 1898 event.

Stress triggering of the 1999 Hector Mine earthquake by transient deformation following the 1992 Landers earthquake

USGS Publications Warehouse

Pollitz, F.F.; Sacks, I.S.

2002-01-01

The M 7.3 June 28, 1992 Landers and M 7.1 October 16, 1999 Hector Mine earthquakes, California, both right lateral strike-slip events on NNW-trending subvertical faults, occurred in close proximity in space and time in a region where recurrence times for surface-rupturing earthquakes are thousands of years. This suggests a causal role for the Landers earthquake in triggering the Hector Mine earthquake. Previous modeling of the static stress change associated with the Landers earthquake shows that the area of peak Hector Mine slip lies where the Coulomb failure stress promoting right-lateral strike-slip failure was high, but the nucleation point of the Hector Mine rupture was neutrally to weakly promoted, depending on the assumed coefficient of friction. Possible explanations that could account for the 7-year delay between the two ruptures include background tectonic stressing, dissipation of fluid pressure gradients, rate- and state-dependent friction effects, and post-Landers viscoelastic relaxation of the lower crust and upper mantle. By employing a viscoelastic model calibrated by geodetic data collected during the time period between the Landers and Hector Mine events, we calculate that postseismic relaxation produced a transient increase in Coulomb failure stress of about 0.7 bars on the impending Hector Mine rupture surface. The increase is greatest over the broad surface that includes the 1999 nucleation point and the site of peak slip further north. Since stress changes of magnitude greater than or equal to 0.1 bar are associated with documented causal fault interactions elsewhere, viscoelastic relaxation likely contributed to the triggering of the Hector Mine earthquake. This interpretation relies on the assumption that the faults occupying the central Mojave Desert (i.e., both the Landers and Hector Mine rupturing faults) were critically stressed just prior to the Landers earthquake.

Identification of Geomorphic Conditions Favoring Preservation of Multiple Individual Displacements Across Transform Faults

NASA Astrophysics Data System (ADS)

Williams, P. L.; Phillips, D. A.; Bowles-Martinez, E.; Masana, E.; Stepancikova, P.

2010-12-01

Terrestrial and airborne LiDAR data, and low altitude aerial photography have been utilized in conjunction with field work to identify and map single and multiple-event stream-offsets along all strands of the San Andreas fault in the Coachella Valley. Goals of the work are characterizing the range of displacements associated with the fault’s prehistoric surface ruptures, evaluating patterns of along-fault displacement, and disclosing processes associated with the prominent Banning-Mission Creek fault junction. Preservation offsets is associated with landscape conditions including: (1) well-confined and widely spaced source streams up-slope of the fault; (2) persistent geomorphic surfaces below the fault; (3) slope directions oriented approximately perpendicular to the fault. Notably, a pair of multiple-event offset sites have been recognized in coarse fan deposits below the Mission Creek fault near 1000 Palms oasis. Each of these sites is associated with a single source drainage oriented approximately perpendicular to the fault, and preserves a record of individual fault displacements affecting the southern portion of the Mission Creek branch of the San Andreas fault. The two sites individually record long (>10 event) slip-per-event histories. Documentation of the sites indicates a prevalence of moderate displacements and a small number of large offsets. This is consistent with evidence developed in systematic mapping of individual and multiple event stream offsets in the area extending 70 km south to Durmid Hill. Challenges to site interpretation include the presence of closely spaced en echelon fault branches and indications of stream avulsion in the area of the modern fault crossing. Conversely, strong bar and swale topography produce high quality offset indicators that can be identified across en echelon branches in most cases. To accomplish the detailed mapping needed to fully recover the complex yet well-preserved geomorphic features under investigation, a program of terrestrial laser scanning (TLS) was conducted at the 1000 Palms oasis stream offset sites. Data products and map interpretations will be presented along with initial applications of the study to characterizing San Andreas fault rupture hazard. Continuing work will seek to more fully populate the dataset of larger offsets, evaluate means to objectively date the larger offsets, and, as completely as possible, to characterize magnitudes of past surface ruptures of the San Andreas fault in the Coachella Valley.

Fault Identification by Unsupervised Learning Algorithm

NASA Astrophysics Data System (ADS)

Nandan, S.; Mannu, U.

2012-12-01

Contemporary fault identification techniques predominantly rely on the surface expression of the fault. This biased observation is inadequate to yield detailed fault structures in areas with surface cover like cities deserts vegetation etc and the changes in fault patterns with depth. Furthermore it is difficult to estimate faults structure which do not generate any surface rupture. Many disastrous events have been attributed to these blind faults. Faults and earthquakes are very closely related as earthquakes occur on faults and faults grow by accumulation of coseismic rupture. For a better seismic risk evaluation it is imperative to recognize and map these faults. We implement a novel approach to identify seismically active fault planes from three dimensional hypocenter distribution by making use of unsupervised learning algorithms. We employ K-means clustering algorithm and Expectation Maximization (EM) algorithm modified to identify planar structures in spatial distribution of hypocenter after filtering out isolated events. We examine difference in the faults reconstructed by deterministic assignment in K- means and probabilistic assignment in EM algorithm. The method is conceptually identical to methodologies developed by Ouillion et al (2008, 2010) and has been extensively tested on synthetic data. We determined the sensitivity of the methodology to uncertainties in hypocenter location, density of clustering and cross cutting fault structures. The method has been applied to datasets from two contrasting regions. While Kumaon Himalaya is a convergent plate boundary, Koyna-Warna lies in middle of the Indian Plate but has a history of triggered seismicity. The reconstructed faults were validated by examining the fault orientation of mapped faults and the focal mechanism of these events determined through waveform inversion. The reconstructed faults could be used to solve the fault plane ambiguity in focal mechanism determination and constrain the fault orientations for finite source inversions. The faults produced by the method exhibited good correlation with the fault planes obtained by focal mechanism solutions and previously mapped faults.

Earthquake geology of the Bulnay Fault (Mongolia)

USGS Publications Warehouse

Rizza, Magali; Ritz, Jean-Franciois; Prentice, Carol S.; Vassallo, Ricardo; Braucher, Regis; Larroque, Christophe; Arzhannikova, A.; Arzhanikov, S.; Mahan, Shannon; Massault, M.; Michelot, J-L.; Todbileg, M.

2015-01-01

The Bulnay earthquake of July 23, 1905 (Mw 8.3-8.5), in north-central Mongolia, is one of the world's largest recorded intracontinental earthquakes and one of four great earthquakes that occurred in the region during the 20th century. The 375-km-long surface rupture of the left-lateral, strike-slip, N095°E trending Bulnay Fault associated with this earthquake is remarkable for its pronounced expression across the landscape and for the size of features produced by previous earthquakes. Our field observations suggest that in many areas the width and geometry of the rupture zone is the result of repeated earthquakes; however, in those areas where it is possible to determine that the geomorphic features are the result of the 1905 surface rupture alone, the size of the features produced by this single earthquake are singular in comparison to most other historical strike-slip surface ruptures worldwide. Along the 80 km stretch, between 97.18°E and 98.33°E, the fault zone is characterized by several meters width and the mean left-lateral 1905 offset is 8.9 ± 0.6 m with two measured cumulative offsets that are twice the 1905 slip. These observations suggest that the displacement produced during the penultimate event was similar to the 1905 slip. Morphotectonic analyses carried out at three sites along the eastern part of the Bulnay fault, allow us to estimate a mean horizontal slip rate of 3.1 ± 1.7 mm/yr over the Late Pleistocene-Holocene period. In parallel, paleoseismological investigations show evidence for two earthquakes prior to the 1905 event with recurrence intervals of ~2700-4000 years.

Complex rupture mechanism and topography control symmetry of mass-wasting pattern, 2010 Haiti earthquake

NASA Astrophysics Data System (ADS)

Gorum, Tolga; van Westen, Cees J.; Korup, Oliver; van der Meijde, Mark; Fan, Xuanmei; van der Meer, Freek D.

2013-02-01

The 12 January 2010 Mw 7.0 Haiti earthquake occurred in a complex deformation zone at the boundary between the North American and Caribbean plates. Combined geodetic, geological and seismological data posited that surface deformation was driven by rupture on the Léogâne blind thrust fault, while part of the rupture occurred as deep lateral slip on the Enriquillo-Plantain Garden Fault (EPGF). The earthquake triggered > 4490 landslides, mainly shallow, disrupted rock falls, debris-soil falls and slides, and a few lateral spreads, over an area of ~ 2150 km2. The regional distribution of these slope failures defies those of most similar earthquake-triggered landslide episodes reported previously. Most of the coseismic landslides did not proliferate in the hanging wall of the main rupture, but clustered instead at the junction of the blind Léogâne and EPGF ruptures, where topographic relief and hillslope steepness are above average. Also, low-relief areas subjected to high coseismic uplift were prone to lesser hanging wall slope instability than previous studies would suggest. We argue that a combined effect of complex rupture dynamics and topography primarily control this previously rarely documented landslide pattern. Compared to recent thrust fault-earthquakes of similar magnitudes elsewhere, we conclude that lower static stress drop, mean fault displacement, and blind ruptures of the 2010 Haiti earthquake resulted in fewer, smaller, and more symmetrically distributed landslides than previous studies would suggest. Our findings caution against overly relying on across-the-board models of slope stability response to seismic ground shaking.

Heterogeneity of direct aftershock productivity of the main shock rupture

NASA Astrophysics Data System (ADS)

Guo, Yicun; Zhuang, Jiancang; Hirata, Naoshi; Zhou, Shiyong

2017-07-01

The epidemic type aftershock sequence (ETAS) model is widely used to describe and analyze the clustering behavior of seismicity. Instead of regarding large earthquakes as point sources, the finite-source ETAS model treats them as ruptures that extend in space. Each earthquake rupture consists of many patches, and each patch triggers its own aftershocks isotropically. We design an iterative algorithm to invert the unobserved fault geometry based on the stochastic reconstruction method. This model is applied to analyze the Japan Meteorological Agency (JMA) catalog during 1964-2014. We take six great earthquakes with magnitudes >7.5 after 1980 as finite sources and reconstruct the aftershock productivity patterns on each rupture surface. Comparing results from the point-source ETAS model, we find the following: (1) the finite-source model improves the data fitting; (2) direct aftershock productivity is heterogeneous on the rupture plane; (3) the triggering abilities of M5.4+ events are enhanced; (4) the background rate is higher in the off-fault region and lower in the on-fault region for the Tohoku earthquake, while high probabilities of direct aftershocks distribute all over the source region in the modified model; (5) the triggering abilities of five main shocks become 2-6 times higher after taking the rupture geometries into consideration; and (6) the trends of the cumulative background rate are similar in both models, indicating the same levels of detection ability for seismicity anomalies. Moreover, correlations between aftershock productivity and slip distributions imply that aftershocks within rupture faults are adjustments to coseismic stress changes due to slip heterogeneity.

Holocene slip rates along the San Andreas Fault System in the San Gorgonio Pass and implications for large earthquakes in southern California

NASA Astrophysics Data System (ADS)

Heermance, Richard V.; Yule, Doug

2017-06-01

The San Gorgonio Pass (SGP) in southern California contains a 40 km long region of structural complexity where the San Andreas Fault (SAF) bifurcates into a series of oblique-slip faults with unknown slip history. We combine new 10Be exposure ages (Qt4: 8600 (+2100, -2200) and Qt3: 5700 (+1400, -1900) years B.P.) and a radiocarbon age (1260 ± 60 years B.P.) from late Holocene terraces with scarp displacement of these surfaces to document a Holocene slip rate of 5.7 (+2.7, -1.5) mm/yr combined across two faults. Our preferred slip rate is 37-49% of the average slip rates along the SAF outside the SGP (i.e., Coachella Valley and San Bernardino sections) and implies that strain is transferred off the SAF in this area. Earthquakes here most likely occur in very large, throughgoing SAF events at a lower recurrence than elsewhere on the SAF, so that only approximately one third of SAF ruptures penetrate or originate in the pass.