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Sample records for active seismic zone

  1. Structure and seismic activity of the Lesser Antilles subduction zone

    NASA Astrophysics Data System (ADS)

    Evain, M.; Galve, A.; Charvis, P.; Laigle, M.; Ruiz Fernandez, M.; Kopp, H.; Hirn, A.; Flueh, E. R.; Thales Scientific Party

    2011-12-01

    Several active and passive seismic experiments conducted in 2007 in the framework of the European program "Thales Was Right" and of the French ANR program "Subsismanti" provided a unique set of geophysical data highlighting the deep structure of the central part of the Lesser Antilles subduction zone, offshore Dominica and Martinique, and its seismic activity during a period of 8 months. The region is characterized by a relatively low rate of seismicity that is often attributed to the slow (2 cm/yr) subduction of the old, 90 My, Atlantic lithosphere beneath the Caribbean Plate. Based on tomographic inversion of wide-angle seismic data, the forearc can clearly be divided into an inner forearc, characterised by a high vertical velocity gradient in the igneous crust, and an outer forearc with lower crustal velocity gradient. The thick, high velocity, inner forearc is possibly the extension at depth of the Mesozoic Caribbean crust outcropping in La Désirade Island. The outer forearc, up to 70 km wide in the northern part of the study area, is getting narrower to the south and disappears offshore Martinique. Based on its seismic velocity structure with velocities higher than 6 km/s the backstop consists, at least partly, of magmatic rocks. The outer forearc is also highly deformed and faulted within the subducting trend of the Tiburon Ridge. With respect to the inner forearc velocity structure the outer forearc basement could either correspond to an accreted oceanic terrane or made of highly fractured rocks. The inner forearc is a dense, poorly deformable crustal block, tilted southward as a whole. It acts as a rigid buttress increasing the strain within both the overriding and subducting plates. This appears clearly in the current local seismicity affecting the subducting and the overriding plates that is located beneath the inner forearc. We detected earthquakes beneath the Caribbean forearc and in the Atlantic oceanic plate as well. The main seismic activity is

  2. Fault Activity, Seismicity and GPS Deformation of the Seismic Gap along the Red River Fault Zone (RRFZ) in Yunnan, China

    NASA Astrophysics Data System (ADS)

    Xue-Ze, Wen; Shengli, Ma; Fang, Du; Feng, Long

    2016-04-01

    Along the middle segment of the NW-trending and dextral-slip Red River fault zone (RRFZ), also the Honghe fault zone, Yunnan, China, there has been little of modern seismicity since the 1970's. Some Chinese researchers believed that this fault segment is inactive in the late Quaternary. However, more and more evidence shows that the middle segment of RRFZ is geologically-active in the late Quaternary, even is a Holocene-active one with evidence of paleo-earthquakes occurring. Our study suggests that along the fault segment there has been no any major earthquake occurring for over 500 years at least, and a large-scale seismic gap, the Honghe seismic gap, have formed there. On the modern seismicity, the middle segment of RRFZ has presented as a fault portion without or with very few small earthquakes occurring since the 1980's, but surrounded by several areas with low b-values, suggesting relatively high stress having built-up there. Also, GPS deformation analysis suggests that this fault segment has tightly locked already. Such tight locking would be associated with the fault geometry: A large-scale restraining bend of about 30°over a distance of ~100 km exists along the main fault trace along RRFZ between Yuanjiang and Yuanyang. However, how such a restraining bend makes the middle segment of RRFZ have tightly locked? How much strain has built up there? Moreover, how about the long-term seismic potential of major earthquake on the middle segment of RRFZ, and on some secondary active faults of the two sides of the segment, especially on the parallel faults Chuxiong, Qujiang and Shiping. All these are issues we want to study further. Keywords: Red River Fault Zone, Seismic Gap, Fault Activity, Seismicity, GPS Deformation

  3. Deep Seismic Researches Of Seismic-Active Zones With Use Of High-Power Vibrators - Technique, Outcomes, Outlooks

    NASA Astrophysics Data System (ADS)

    Soloviev, V.; Seleznev, V.; Emanov, A.; Sal`Nikov, A.; Kashun, V.; Glinsky, B.; Kovalevsky, V.; Zhemchugova, I.; Danilov, I.; Liseikin, A.

    2004-12-01

    There are presented the materials of deep vibroseism researches, carried out in seismic active regions of Siberia with use of stationary (100-tos power) and moveable vibration sources (40-60tons power) and mobile digital recording equipment. There are given some examples of unique, have no world analogues, correlograms from high-power vibrators on distances to 400km and more. Using new vibroseismic technology of deep seismic researches, there were got detail deep sections of the Earth's crust and upper mantle, including time-sections of CDP-DSS up to depth of 80km. Materials of vibroseismic investigations on 2500km of seismic profiles in hard-to-reach regions of the Altay-Sayan region, the Baikal rift zone and Okhotsko-Chukotski regions are evidence of high cost efficiency, ecological safety, possibility to be realized in hard-to-reach region and finally of availability of deep seismic investigations with use of high-power vibration sources.

  4. Seismic evidence for active underplating below the megathrust earthquake zone in Japan.

    PubMed

    Kimura, Hisanori; Takeda, Tetsuya; Obara, Kazushige; Kasahara, Keiji

    2010-07-01

    Determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes. The peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate (underplating) at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging, but direct seismic evidence of this process is lacking. By comparing seismic reflection profiles with microearthquake distributions in central Japan, we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting Philippine Sea plate. This region coincides with the location of slow-slip events that may serve as signals for monitoring active underplating. PMID:20616277

  5. Amplitude analysis of active source seismic data from the grounding zone of Whillans Ice Stream

    NASA Astrophysics Data System (ADS)

    Horgan, Huw; Anandakrishnan, Sridhar; Alley, Richard; Christianson, Knut

    2015-04-01

    Amplitude analysis of active source seismic data is often used to estimate acoustic properties and thereby infer the lithology of the substrate beneath glaciers and ice streams. The substrate beneath the ice streams of West Antarctica is of particular interest as here subglacial sediment deformation results in the rapid flow of the overriding ice. At the grounding zone, where the grounded ice sheet transitions to the floating ice shelf, this substrate is thought to stiffen due to tidal compaction resulting in a zone of higher basal shear stress which is manifest in the buckling of the internal layering in the overriding ice. Here we investigate these processes by estimating subglacial properties using active source seismic data acquired across the grounding zone of Whillans Ice Stream. Perhaps uniquely, we are able to test our methodology due to the survey crossing from an ice overlying sediment interface into a known ice overlying water interface. Our analysis indicates that lithological variations within the grounding zone are below the resolution of our methodology with the exception of a body of water trapped by a hydropotential reversal upstream of the grounding zone.

  6. Structural and Lithologic Characteristics of the Wenchuan Earthquake Fault Zone and its Relationship with Seismic Activity

    NASA Astrophysics Data System (ADS)

    Wang, H.; Li, H.; Pei, J.; Li, T.; Huang, Y.; Zhao, Z.

    2010-12-01

    the older earthquake, but rather along the edge of the gouge. According to the gouge statistics of the whole fault zone, seismic events have the obvious tendency towards the foot wall, and the thickness of gouge is proportional to the activity of the fault, indicating that the width of fault zone is directly related to the number and evolution history of earthquakes . Repeated earthquakes maybe the main cause for the formation of the Longmenshan Moutains

  7. Fault mirrors in seismically active fault zones: A fossil of small earthquakes at shallow depths

    NASA Astrophysics Data System (ADS)

    Kuo, Li-Wei; Song, Sheng-Rong; Suppe, John; Yeh, En-Chao

    2016-03-01

    Fault mirrors (FMs) are naturally polished and glossy fault slip surfaces that can record seismic deformation at shallow depths. They are important for investigating the processes controlling dynamic fault slip. We characterize FMs in borehole samples from the hanging wall damage zone of the active Hsiaotungshi reverse fault, Taiwan. Here we report the first documented occurrence of the combination of silica gel and melt patches coating FMs, with the silica gel resembling those observed on experimentally formed FMs that were cataclastically generated. In addition, the melt patches, which are unambiguous indicators of coseismic slip, suggest that the natural FMs were produced at seismic rates, presumably resulting from flash heating at asperities on the slip surfaces. Since flash heating is efficient at small slip, we propose that these natural FMs represent fossils of small earthquakes, formed in either coseismic faulting and folding or aftershock deformation in the active Taiwan fold-and-thrust belt.

  8. Active faults in the deformation zone off Noto Peninsula, Japan, revealed by high- resolution seismic profiles

    NASA Astrophysics Data System (ADS)

    Inoue, T.; Okamura, Y.; Murakami, F.; Kimura, H.; Ikehara, K.

    2008-12-01

    Recently, a lot of earthquakes occur in Japan. The deformation zone which many faults and folds have concentrated exists on the Japan Sea side of Japan. The 2007 Noto Hanto Earthquake (MJMA 6.9) and 2007 Chuetsu-oki Earthquake (MJMA 6.8) were caused by activity of parts of faults in this deformation zone. The Noto Hanto Earthquake occurred on 25 March, 2007 under the northwestern coast of Noto Peninsula, Ishikawa Prefecture, Japan. This earthquake is located in Quaternary deformation zone that is continued from northern margin of Noto Peninsula to southeast direction (Okamura, 2007a). National Institute of Advanced Industrial Science and Technology (AIST) carried out high-resolution seismic survey using Boomer and 12 channels short streamer cable in the northern part off Noto Peninsula, in order to clarify distribution and activities of active faults in the deformation zone. A twelve channels short streamer cable with 2.5 meter channel spacing developed by AIST and private corporation is designed to get high resolution seismic profiles in shallow sea area. The multi-channel system is possible to equip on a small fishing boat, because the data acquisition system is based on PC and the length of the cable is short and easy to handle. Moreover, because the channel spacing is short, this cable is very effective for a high- resolution seismic profiling survey in the shallow sea, and seismic data obtained by multi-channel cable can be improved by velocity analysis and CDP stack. In the northern part off Noto Peninsula, seismic profiles depicting geologic structure up to 100 meters deep under sea floor were obtained. The most remarkable reflection surface recognized in the seismic profiles is erosion surface at the Last Glacial Maximum (LGM). In the western part, sediments about 30 meters (40 msec) thick cover the erosional surface that is distributed under the shelf shallower than 100m in depth and the sediments thin toward offshore and east. Flexures like deformation in

  9. Peculiarities of ULF electromagnetic disturbances before strong earthquakes in seismic active zone of Kamchatka peninsula

    NASA Astrophysics Data System (ADS)

    Kopytenko, Y. A.; Ismagilov, V. S.; Schekotov, A.; Molchanov, O.; Chebrov, V.; Raspopov, O. M.

    2006-12-01

    Regular observations of ULF electromagnetic disturbances and acoustic emissions at st. Karymshino in seismic active zone of Kamchatka peninsula were carried out during 2001-2003 years. Five seismic active periods with strong earthquakes (M>5) were displayed during this period. These EQs occurred at the Pacific at 20-60 km depth at 100-140 km distances to the East from the st. Karymshino. Analysis of normalized dynamic power spectra of data of high-sensitive (0.2 pT/sqrt(Hz)) three-component induction magnetometer achieved a significant disorder of daily variation and increasing of the magnetic disturbance intensities (from 0.2 to ~1 pT) in the whole investigated frequency range (0.2-5 Hz). The anomaly intensity increasing was observed during the 12-18 hours before main seismic shocks. Maximum of the increasing occurred during 4-6 hours before the EQs. An increasing of acoustic emissions (F=30 Hz) was observed during the same period. A sharp decreasing of the magnetic disturbance intensities was observed 2-4 hours before the EQs. We suppose that physical processes in a hearth of forthcoming EQ lead to an irreversible avalanche-like formation of cracks and stimulation of the acoustic and ULF electromagnetic disturbances.

  10. Seismic sequences in the Sombrero Seismic Zone

    NASA Astrophysics Data System (ADS)

    Pulliam, J.; Huerfano, V. A.; ten Brink, U.; von Hillebrandt, C.

    2007-05-01

    The northeastern Caribbean, in the vicinity of Puerto Rico and the Virgin Islands, has a long and well-documented history of devastating earthquakes and tsunamis, including major events in 1670, 1787, 1867, 1916, 1918, and 1943. Recently, seismicity has been concentrated to the north and west of the British Virgin Islands, in the region referred to as the Sombrero Seismic Zone by the Puerto Rico Seismic Network (PRSN). In the combined seismicity catalog maintained by the PRSN, several hundred small to moderate magnitude events can be found in this region prior to 2006. However, beginning in 2006 and continuing to the present, the rate of seismicity in the Sombrero suddenly increased, and a new locus of activity developed to the east of the previous location. Accurate estimates of seismic hazard, and the tsunamigenic potential of seismic events, depend on an accurate and comprehensive understanding of how strain is being accommodated in this corner region. Are faults locked and accumulating strain for release in a major event? Or is strain being released via slip over a diffuse system of faults? A careful analysis of seismicity patterns in the Sombrero region has the potential to both identify faults and modes of failure, provided the aggregation scheme is tuned to properly identify related events. To this end, we experimented with a scheme to identify seismic sequences based on physical and temporal proximity, under the assumptions that (a) events occur on related fault systems as stress is refocused by immediately previous events and (b) such 'stress waves' die out with time, so that two events that occur on the same system within a relatively short time window can be said to have a similar 'trigger' in ways that two nearby events that occurred years apart cannot. Patterns that emerge from the identification, temporal sequence, and refined locations of such sequences of events carry information about stress accommodation that is obscured by large clouds of

  11. Quaternary grabens in southernmost Illinois: Deformation near an active intraplate seismic zone

    USGS Publications Warehouse

    Nelson, W.J.; Denny, F.B.; Follmer, L.R.; Masters, J.M.

    1999-01-01

    Narrow grabens displace Quaternary sediments near the northern edge of the Mississippi Embayment in extreme southern Illinois, east-central United States. Grabens are part of the Fluorspar Area Fault Complex (FAFC), which has been recurrently active throughout Phanerozoic time. The FAFC strikes directly toward the New Madrid Seismic Zone (NMSZ), scene of some of the largest intra-plate earthquakes in history. The NMSZ and FAFC share origin in a failed Cambrian rift (Reelfoot Rift). Every major fault zone of the FAFC in Illinois exhibits Quaternary displacement. The structures appear to be strike-slip pull-apart grabens, but the magnitude and direction of horizontal slip and their relationship to the current stress field are unknown. Upper Tertiary strata are vertically displaced more than 100 m, Illinoian and older Pleistocene strata 10 to 30 m, and Wisconsinan deposits 1 m or less. No Holocene deformation has been observed. Average vertical slip rates are estimated at 0.01 to 0.03 mm/year, and recurrence intervals for earthquakes of magnitude 6 to 7 are on the order of 10,000s of years for any given fault. Previous authors remarked that the small amount of surface deformation in the New Madrid area implies that the NMSZ is a young feature. Our findings show that tectonic activity has shifted around throughout the Quaternary in the central Mississippi Valley. In addition to the NMSZ and southern Illinois, the Wabash Valley (Illinois-Indiana), Benton Hills (Missouri), Crowley's Ridge (Arkansas-Missouri), and possibly other sites have experienced Quaternary tectonism. The NMSZ may be only the latest manifestation of seismicity in an intensely fractured intra-plate region.

  12. Seismically Articulating Kilauea Volcano's Active Conduits, Rift Zones, and Faults through HVO's Second Fifty Years

    NASA Astrophysics Data System (ADS)

    Okubo, P.; Nakata, J.; Klein, F.; Koyanagi, R.; Thelen, W.

    2011-12-01

    While seismic monitoring of active Hawaiian volcanoes began 100 years ago, the build-up of the U. S. Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) seismographic network to its current configuration began in 1955, when Jerry Eaton established remote stations that telemetered data via landline to recorders at HVO. With network expansion through the 1960's, earthquake location and cataloging capabilities have evolved to afford a computer processed seismic catalog now spanning fifty years. Location accuracy and catalog completeness to smaller magnitudes have increased. Research and insights developed using HVO's seismic record have exploited the ability to seismically monitor volcanic activity at depth, to identify active regions within the volcanoes on the basis of computed hypocentral locations, to infer regions of magma storage by recognizing different families of volcanic earthquakes, and to forecast volcanic activity in both short and longer term from seismicity patterns. HVO's seismicity catalog was central to calculations of probabilistic seismic hazards. The ability to develop and implement additional analytical and interpretive capabilities has kept pace with improvements in both field and laboratory hardware and software. While the basic capabilities continue as part of HVO's core monitoring, additional interpretive capabilities now include adding details of volcanic and earthquake source regions, and viewing seismic data in juxtaposition with other observatory data streams. As HVO looks to its next century of volcano studies, research and development continue to shape the future. Broadband seismic recording at HVO has enabled extensive study by Chouet, Dawson, and co-workers of the relationship of very-long-period seismic sources beneath Kilauea's summit caldera to magma supply and transport. Recent upgrades have improved the ability to use these data in seismic cataloging and research. Data processing upgrades have bolstered the ability to

  13. Active deformation along the Andaman-Nicobar subduction zone from seismic reflection studies

    NASA Astrophysics Data System (ADS)

    Moeremans, R. E.; Singh, S. C.

    2013-12-01

    The Andaman-Sumatra subduction zone is one of the most seismically active regions on Earth and is a prime example of oblique subduction. It is the result of the oblique convergence between the downgoing Indo-Australian and the overriding Eurasian plates, leading to slip partitioning into a trench-normal thrust component along the plate interface and a trench-subparallel strike-slip component along a sliver fault. The direction of convergence is 90° with respect to the trench near Java, reduces to 45° off of northern Sumatra, and becomes almost parallel to the trench along the Andaman-Nicobar portion of the subduction. Rates of subduction vary from 63 mm/yr off of Java, 50 mm/yr near Nias Island, 45 mm/yr northwest of Sumatra, and 39 mm/yr near the Andaman Islands. After the great December 2004 earthquake, the Sumatran section of the subduction zone was heavily investigated using marine geophysical studies, but the deformation processes in the Andaman-Nicobar region remain poorly understood due to the lack of data. Here, we present seismic reflection profiles from the Andaman-Nicobar region that cover the deformation front, the forearc high, and the forearc basin. We find that the presence of thick (> 3 s TWT) sediments lead to slip taking place predominantly along landward vergent frontal faults. The frontal fault vergence changes to seaward due to the thinning (< 2 s TWT) of the sediments in the region where the Ninetyeast ridge subducts. The presence of a thick (> 3 s TWT) 20 km-long unit of undeformed sediments, possibly resulting from the landward vergence of the frontal thrusts, suggests that ~40 km of the Ninetyeast ridge has subducted beneath the Andaman forearc. The forearc is widest between the Andaman and Nicobar Islands, likely due to the subduction of thick sediments. The forearc basin is bounded in the west by a series of backthrusts and is underlain by a continental crust, which was once a part of the Malay Peninsula. The forearc basin is crescent

  14. Structure of the active rift zone and margins of the northern Imperial Valley from Salton Seismic Imaging Project (SSIP) data

    NASA Astrophysics Data System (ADS)

    Livers, A.; Han, L.; Delph, J. R.; White-Gaynor, A. L.; Petit, R.; Hole, J. A.; Stock, J. M.; Fuis, G. S.

    2012-12-01

    First-arrival refraction data were used to create a seismic velocity model of the upper crust across the actively rifting northern Imperial Valley and its margins. The densely sampled seismic refraction data were acquired by the Salton Seismic Imaging Project (SSIP) , which is investigating rift processes in the northern-most rift segment of the Gulf of California extensional province and earthquake hazards at the southern end of the San Andreas Fault system. A 95-km long seismic line was acquired across the northern Imperial Valley, through the Salton Sea geothermal field, parallel to the five Salton Butte volcanoes and perpendicular to the Brawley Seismic Zone and major strike-slip faults. Nineteen explosive shots were recorded with 100 m seismometer spacing across the valley and with 300-500 m spacing into the adjacent ranges. First-arrival travel times were picked from shot gathers along this line and a seismic velocity model was produced using tomographic inversion. Sedimentary basement and seismic basement in the valley are interpreted to be sediment metamorphosed by the very high heat flow. The velocity model shows that this basement to the west of the Brawley Seismic Zone is at ~4-km depth. The basement shallows to ~2-km depth in the active geothermal field and Salton Buttes volcanic field which locally coincide with the Brawley Seismic Zone. At the eastern edge of the geothermal field, the basement drops off again to ~3.5-km depth. The eastern edge of the valley appears to be fault bounded by the along-strike extension of the Sand Hills Fault, an inactive strike-slip fault. The seismic velocities to the east of the fault correspond to metamorphic rock of the Chocolate Mountains, different from the metamorphosed basement in the valley. The western edge of the valley appears to be fault bounded by the active Superstition Hills Fault. To the west of the valley, >4-km deep valley basement extends to the active Superstition Hills Fault. Basement then shallows

  15. Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data

    SciTech Connect

    SGP-TR-150-16

    1995-01-26

    Active fault systems usually provide high-permeability channels for hydrothermal outflow in geothermal fields. Locating such fault systems is of a vital importance to plan geothermal production and injection drilling, since an active fault zone often acts as a fracture-extensive low-velocity wave guide to seismic waves. We have located an active fault zone in the Coso geothermal field, California, by identifying and analyzing a fault-zone trapped Rayleigh-type guided wave from microearthquake data. The wavelet transform is employed to characterize guided-wave's velocity-frequency dispersion, and numerical methods are used to simulate the guided-wave propagation. The modeling calculation suggests that the fault zone is {approx} 200m wide, and has a P wave velocity of 4.80 km/s and a S wave velocity of 3.00 km/s, which is sandwiched between two half spaces with relatively higher velocities (P wave velocity 5.60 km/s, and S wave velocity 3.20 km/s). zones having vertical or nearly vertical dipping fault planes.

  16. Physical modeling of the formation and evolution of seismically active fault zones

    USGS Publications Warehouse

    Ponomarev, A.V.; Zavyalov, A.D.; Smirnov, V.B.; Lockner, D.A.

    1997-01-01

    Acoustic emission (AE) in rocks is studied as a model of natural seismicity. A special technique for rock loading has been used to help study the processes that control the development of AE during brittle deformation. This technique allows us to extend to hours fault growth which would normally occur very rapidly. In this way, the period of most intense interaction of acoustic events can be studied in detail. Characteristics of the acoustic regime (AR) include the Gutenberg-Richter b-value, spatial distribution of hypocenters with characteristic fractal (correlation) dimension d, Hurst exponent H, and crack concentration parameter Pc. The fractal structure of AR changes with the onset of the drop in differential stress during sample deformation. The change results from the active interaction of microcracks. This transition of the spatial distribution of AE hypocenters is accompanied by a corresponding change in the temporal correlation of events and in the distribution of event amplitudes as signified by a decrease of b-value. The characteristic structure that develops in the low-energy background AE is similar to the sequence of the strongest microfracture events. When the AR fractal structure develops, the variations of d and b are synchronous and d = 3b. This relation which occurs once the fractal structure is formed only holds for average values of d and b. Time variations of d and b are anticorrelated. The degree of temporal correlation of AR has time variations that are similar to d and b variations. The observed variations in laboratory AE experiments are compared with natural seismicity parameters. The close correspondence between laboratory-scale observations and naturally occurring seismicity suggests a possible new approach for understanding the evolution of complex seismicity patterns in nature. ?? 1997 Elsevier Science B.V. All rights reserved.

  17. Onshore-offshore seismic networks: an inescapable approach to reveal the crustal structure and the seismic activity of large subduction zones

    NASA Astrophysics Data System (ADS)

    Charvis, P.; Galve, A.; Laigle, M.; Hirn, A.; Hello, Y. M.; Oge, A.; Yates, B. A.

    2013-12-01

    Ninety percent of the seismic energy released worldwide and ninety percent of the largest earthquakes and tsunamis occur in subduction zones. Several recent catastrophic subduction earthquakes surprised us on many aspects, either because we had been unable to anticipate their extremely large magnitude (2011 Tohoku Earthquake, Mw 9.0), or because we had considered the subduction as partly aseismic (2004 Sumatra Andaman earthquake, Mw 9.1). One of the reasons for our present ignorance of the behavior of large subduction earthquakes is the lack of marine data to image and monitor the structure and evolution of megathrust faults offshore. Over the last 15 years, our group has conducted several passive and active seismic experiments* in the forearc regions of the Ecuador-Colombia, Lesser Antilles and Hellenic subduction zones. The objectives of these experiments were to image the subduction interplate fault at depth and accurately locate the current earthquake activity of the megathrusts using arrays of combined ocean-bottom and land-based seismometers. In the case of very large events and in the absence of geodetic data in the offshore part of the faults, the precise knowledge of current seismicity is mandatory to estimate the seismogenic behavior and potential of the fault interface. 2D dense active seismic lines, shot jointly with multichannel acquisitions, provide invaluable images of the deep structure of the Lesser Antilles arc and forearc, which allow locating the updip and downdip limits of the expected seismogenic zone. Assuming that the Moho is the downdip limit of the seismogenic zone, the 26 km-thick crust of the arc makes the seismogenic zone 3 times wider than it is in ';standard' oceanic arcs (like Marianas). 3D active and passive experiments in the Lesser Antilles and Ecuador forearcs provide an unprecedented way to image the structure in 3D down to the lower plate. The tomography documents the spatial variability of the interplate fault structure and of

  18. Structure and evolution of the seismically active Ostler Fault Zone (New Zealand) based on interpretations of multiple high resolution seismic reflection profiles

    NASA Astrophysics Data System (ADS)

    Campbell, Fiona M.; Ghisetti, Francesca; Kaiser, Anna E.; Green, Alan G.; Horstmeyer, Heinrich; Gorman, Andrew R.

    2010-12-01

    To improve our understanding of active faulting away from the main plate boundary on New Zealand's South Island, we have acquired high resolution seismic data across the Ostler Fault Zone Twelve 1.2 km long lines perpendicular to fault strike and a 1.6 km long crossline were collected in a region of the MacKenzie Basin where surface mapping delineates significant complexity in the form of two non-overlapping reverse fault strands separated by a transfer zone characterised by multiple smaller strands and increased folding. Interpretation of the resultant images includes a 45-55° west-dipping principal fault and two 25-30° west-dipping subsidiary faults, one in the hanging wall and one in the footwall of the principal fault. The geologically mapped complexities are shown to be caused by shallow variations in the structure of the principal fault, which breaks the surface in the north and south but not within the transfer zone, where it forms a triangle zone with associated backthrusting and minor faulting. These complexities only extend to ~ 300 m depth. Structures below this level are markedly simpler and much more 2D in nature, with the principal fault strand extending over a much longer distance than the individual strands observed at the surface. Since longer faults are susceptible to larger earthquakes than shorter ones, seismic hazard at the study site may be higher than previously thought. Multiple surface fault strands that give way to a single more major stand at relatively shallow depths may be a common feature of segmented fault systems. The deepest layered reflections at our site are consistent with the presence of a Late Cretaceous (?)-Tertiary basin underlying the present-day MacKenzie Basin. Structural restoration of the seismic images back to the base of Quaternary fluvioglacial terraces and back to the top of a Late Pliocene-Pleistocene fluviolacustrine unit indicate that compression was initiated prior to the Late Pliocene and that it has continued

  19. On the seismic activity of the Malibu Coast Fault Zone, and other ethical problems in engineering geoscience

    SciTech Connect

    Cronin, V.S. . Geosciences Dept.)

    1992-01-01

    The Malibu Coast Fault Zone (MCFZ) merges eastward with the active Santa Monica, Hollywood, Raymond Hill, Sierra Madre, and Cucamonga Faults of the central Transverse Ranges. West of Point Dume, the MCFZ extends offshore to join the active Santa Cruz Island Fault. Active microearthquake seismicity along the MCFZ trend indicates that it is seismogenic. Focal mechanism solutions for several of these earthquakes indicate thrusting along faults with the same orientation as the MCFZ. The geomorphology of the MCFZ is consistent with the interpretation that the MCFZ is active. Scarps in unconsolidated sands along the continental shelf just south of Malibu indicate recent offset. In the Santa Monica Mountains, late Tertiary and Quaternary marine sedimentary strata are exposed on the hanging-wall side of the MCFZ, indicating active uplift of the Santa Monica Mountains. Given the other indicators of fault activity, the trench studies that must still be undertaken across the MCFZ are more likely to establish the chronology of recent displacement along the MCFZ than to indicate that the fault is not active. It has been suggested that the MCFZ has not yet been formally recognized as an active, seismogenic fault zone because of the expected loss of property value should the MCFZ be designated an active fault. Geoscientists fear being held liable for loss of property value, even though their assessment of fault activity may be scientifically valid. What are the ethical responsibilities of geoscientists involved in seismic risk assessment along the MCFZ Are political or financial considerations valid criteria to use in assessing the activity of a fault These are not abstract questions of geoethics, because the lives and properties of countless people are potentially at risk.

  20. New Madrid seismic zone recurrence intervals

    SciTech Connect

    Schweig, E.S. Center for Earthquake Research and Information, Memphis, TN ); Ellis, M.A. )

    1993-03-01

    Frequency-magnitude relations in the New Madrid seismic zone suggest that great earthquakes should occur every 700--1,200 yrs, implying relatively high strain rates. These estimates are supported by some geological and GPS results. Recurrence intervals of this order should have produced about 50 km of strike-slip offset since Miocene time. No subsurface evidence for such large displacements is known within the seismic zone. Moreover, the irregular fault pattern forming a compressive step that one sees today is not compatible with large displacements. There are at least three possible interpretations of the observations of short recurrence intervals and high strain rates, but apparently youthful fault geometry and lack of major post-Miocene deformation. One is that the seismological and geodetic evidence are misleading. A second possibility is that activity in the region is cyclic. That is, the geological and geodetic observations that suggest relatively short recurrence intervals reflect a time of high, but geologically temporary, pore-fluid pressure. Zoback and Zoback have suggested such a model for intraplate seismicity in general. Alternatively, the New Madrid seismic zone is geologically young feature that has been active for only the last few tens of thousands of years. In support of this, observe an irregular fault geometry associated with a unstable compressive step, a series of en echelon and discontinuous lineaments that may define the position of a youthful linking fault, and the general absence of significant post-Eocene faulting or topography.

  1. Offshore active faults of the Mikata fault zone in Fukui, Japan, revealed by high-resolution seismic profiles

    NASA Astrophysics Data System (ADS)

    Inoue, T.; Sugiyama, Y.; Sakamoto, I.; Takino, Y.; Murakami, F.; Hosoya, T.; Usami, T.

    2014-12-01

    The Mikata fault zone are located in coastal and shallow sea area off Fukui Prefecture, West Japan. National Institute of Advanced Industrial Science and Technology (AIST) and Tokai University conducted, as part of MEXT 2013 nearshore active fault survey project, a high-resolution multi-channel seismic survey using Boomer and a 12-channel streamer cable, acoustic profiling survey using parametric sub-bottom profiler and shallow-sea offshore drilling, in order to clarify distribution and activity of the Mikata fault zone. The seismic reflection surveys identified four reflection surfaces as vertical displacement markers in the post-glacial deposits at a depth ranging from ca. 4.5m to ca. 17m below the sea bottom on the downthrown side. We estimated the age of each marker reflection surface by using the C14 age and others from 4m-long core obtained on the downthrown side of fault and the sea level change in the latest Pleistocene and early Holocene around Japan. The results of these surveys have revealed that the fault system was reactivated three times since the latest Pleistocene. The vertical slip rate and average recurrence interval of the fault system are estimated at ca. 0.8-1.0 m/ky and 2,000-3,800 years, respectively.

  2. Real time electromagnetic monitoring system used for short-term earthquakes forecast related to the seismic-active Vrancea zone

    NASA Astrophysics Data System (ADS)

    Stanica, Dumitru; Armand Stanica, Dragos

    2016-04-01

    The existence of the pre-seismic electromagnetic signals related to the earthquakes is still under scientific debate and requires new reliable information about their possible inter-relationship. In this paper, to obtain new insights into the seismic active Vrancea zone (Romania), a 3-D magnetotelluric imaging has been used to strengthen the connection between the geodynamic model and a possible generation mechanism of the intermediate depth earthquakes. Consequently, it is considered that before an earthquake initiation, due to the torsion effect, a high stress reached inside the seismogenic volume that may generates dehydration and rupture processes of the rocks, associated with the fluid migration through the lithospheric faults system, what leads to the resistivity changes. These changes have been investigated by using ULF electromagnetic data recorded in real time at the Geodynamic Observatory Provita de Sus (GOPS), placed on the Carpathian Electrical Conductivity Anomaly (CECA) at about 100km far from the seismic active Vrancea zone. The daily mean distribution of the normalized function Bzn(f) = Bz(f)/Bperp(f) (where: Bz is vertical component of the geomagnetic field; Bperp is geomagnetic component perpendicular to strike; f is frequency in Hz) and its standard deviation are performed by using a FFT band-pass filter analysis in the ULF range 0.001Hz to 0.0083Hz, for which a 2-D geoelectrical structure under GOPS has been identified. To provide reliable information in anticipating the likelihood occurrence of an earthquake of Mw higher than 4, a statistical analysis based on standardized random variable equation has been used to identify the anomalous intervals on the new time series (Bzn*) carried out in a span of three years (2013-2015). The final conclusion is that the Bzn* shows a significant anomalous effect some days (weeks) before an impending earthquake and it should be used for short-term earthquakes forecast.

  3. Seismicity of the eastern Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Bruestle, A.; Kueperkoch, L.; Rische, M.; Meier, T.; Friederich, W.; Egelados Working Group

    2012-04-01

    The Hellenic Subduction Zone (HSZ) is the seismically most active region of Europe. The African plate is subducting beneath the Aegean lithosphere with a relative velocity of 4 cm per year. A detailed picture of the microseismicity of the eastern HSZ was obtained by the recordings of the temporary networks CYCNET (September 2002 - September 2005) and EGELADOS (October 2005 - March 2007). In total, nearly 7000 earthquakes were located with a location uncertainty of less than 20 km. The SE Aegean is dominated by (1) shallow intraplate seismicity within the Aegean plate, by (2) interplate seismicity at the plate contact and by (3) intermediate deep seismicity along the subducting African slab. Strong shallow seismicity in the upper plate is observed along the Ptolemy graben south of Crete extending towards the Karpathos Basin, indicating intense recent deformation of the forearc. In contrary, low shallow seismicity around Rhodes indicates only minor seismic crustal deformation of the upper plate. An almost NS-striking zone of microseismicity has been located, running from the Karpathos basin via the Nisyros volcanic complex towards the EW striking Gökova graben. In the SE Aegean the geometry of the Wadati-Benioff-Zone (WBZ) within the subducting African plate is revealed in detail by the observed microseismicity. Between about 50 to 100 km depth a continuous band of intermediate deep seismicity describes the strongly curved geometry of the slab. From the central to the eastern margin of the HSZ, the dip direction of the WBZ changes from N to NW with a strong increase of the dip angle beneath the eastern Cretan Sea. The margin of the dipping African slab is marked by an abrupt end of the observed WBZ beneath SW Anatolia. Below 100 km depth, the WBZ of the eastern HSZ is dominated by an isolated cluster of intense intermediate deep seismicity (at 100-180 km depth) beneath the Nisyros volcanic complex. It has an extension of about 100x80 km and is build up of 3 parallel

  4. Study on Seismic Zoning of Sino-Mongolia Arc Areas

    NASA Astrophysics Data System (ADS)

    Xu, G.

    2015-12-01

    According to the agreement of Cooperation on seismic zoning between Institute of Geophysics, China Earthquake Administration and Research Center of Astronomy and Geophysics, Mongolian Academy of Science, the data of geotectonics, active faults, seismicity and geophysical field were collected and analyzed, then field investigation proceeded for Bolnay Faults, Ar Hutul Faults and Gobi Altay Faults, and a uniform earthquake catalogue of Mongolia and North China were established for the seismic hazard study in Sino-Mongolia arc areas. Furthermore the active faults and epicenters were mapped and 2 seismic belts and their 54 potential seismic sources are determined. Based on the data and results above mentioned the seismicity parameters for the two seismic belts and their potential sources were studied. Finally, the seismic zoning with different probability in Sino-Mongolia arc areas was carried out using China probabilistic hazard analysis method. By analyzing the data and results, we draw the following main conclusions. Firstly, the origin of tectonic stress field in the study areas is the collision and pressure of the India Plate to Eurasian Plate, passing from the Qinghai-Tibet Plateau. This is the reason why the seismicity is higher in the west than in the east, and all of earthquakes with magnitude 8 or greater occurred in the west. Secondly, the determination of the 2 arc seismic belts, Altay seismic belt and Bolnay-Baikal seismic belt, are reasonable in terms of their geotectonic location, geodynamic origin and seismicity characteristics. Finally, there are some differences between our results and the Mongolia Intensity Zoning map published in 1985 in terms of shape of seismic zoning map, especially in the areas near Ulaanbaatar. We argue that our relsults are reasonable if we take into account the data use of recent study of active faults and their parameters, so it can be used as a reference for seismic design.

  5. Seismic fault zone trapped noise

    NASA Astrophysics Data System (ADS)

    Hillers, G.; Campillo, M.; Ben-Zion, Y.; Roux, P.

    2014-07-01

    Systematic velocity contrasts across and within fault zones can lead to head and trapped waves that provide direct information on structural units that are important for many aspects of earthquake and fault mechanics. Here we construct trapped waves from the scattered seismic wavefield recorded by a fault zone array. The frequency-dependent interaction between the ambient wavefield and the fault zone environment is studied using properties of the noise correlation field. A critical frequency fc ≈ 0.5 Hz defines a threshold above which the in-fault scattered wavefield has increased isotropy and coherency compared to the ambient noise. The increased randomization of in-fault propagation directions produces a wavefield that is trapped in a waveguide/cavity-like structure associated with the low-velocity damage zone. Dense spatial sampling allows the resolution of a near-field focal spot, which emerges from the superposition of a collapsing, time reversed wavefront. The shape of the focal spot depends on local medium properties, and a focal spot-based fault normal distribution of wave speeds indicates a ˜50% velocity reduction consistent with estimates from a far-field travel time inversion. The arrival time pattern of a synthetic correlation field can be tuned to match properties of an observed pattern, providing a noise-based imaging tool that can complement analyses of trapped ballistic waves. The results can have wide applicability for investigating the internal properties of fault damage zones, because mechanisms controlling the emergence of trapped noise have less limitations compared to trapped ballistic waves.

  6. Seismicity and seismotectonic study of the Kermit Seismic Zone, Texas

    SciTech Connect

    Keller, G.R.; Rogers, A.M.; Lund, R.J.; Orr, C.D.

    1981-01-01

    This work presents and summarizes all data that were collected during the operation of a 12-station seismograph network in the vicinity of Kermit, Texas, during the period December 1975 to September 1979. The results of this study have several implications relating to the seismic hazard at the Waste Isolation Pilot Plant (WIPP) facility. First, the widespread occurrence of small earthquakes within the seismic network area suggests that additional microearthquake studies in regions closer to the WIPP site are warranted. Second, the apparent relation between hydrocarbon production and earthquake occurrence indicates that the WIPP site probably should not be included in the same seismic source region as the central basin platform because the WIPP site is isolated from the major hydrocarbon producing zones. Third, the maximum magnitude of earthquakes in the active zone may be limited, if these earthquakes are in fact induced by secondary hydrocarbon recovery operations, because the spatial extent of artificial pressure increases is limited, and increase of more than 70 bars have resulted in earthquakes with magnitudes consistently less than 4.0. 14 references.

  7. Reassessing the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Atkinson, Gail; Bakun, Bill; Bodin, Paul; Boore, David; Camer, Chris; Frankel, Art; Gasperini, Paulo; Gomberg, Joan; Hanks, Tom; Hermann, Bob; Hough, Susan; Johnston, Arch; Kenner, Shelley; Langston, Chuck; Linker, Mark; Mayne, Paul; Petersen, Mark; Powell, Christine; Prescott, Will; Schweig, Eugene; Segall, Paul; Stein, Seth; Stuart, Bill; Tuttle, Martitia; VanArsdale, Roy

    The central enigma of the mid-continent region in the United States known as the New Madrid seismic zone (NMSZ; Figure 1) involves the mechanisms that give rise to recurrent great earthquakes far from plate boundaries. Given the lack of significant topographic relief that is the hallmark of tectonic activity in most actively deforming regions, most of us feel a need to “pinch ourselves to see if we're dreaming” when confronted with evidence that, at some probability levels, the earthquake hazard throughout the NMSZ is comparable to that estimated for the San Francisco Bay region.Although assessing the hazard in the NMSZ is in many ways more challenging than in the western United States, and the uncertainties are much greater, careful scientific study has led to a consensus on the issues most critical to seismic hazard assessment.

  8. An integrated approach to the seismic activity and structure of the central Lesser Antilles subduction megathrust seismogenic zone

    NASA Astrophysics Data System (ADS)

    Hirn, Alfred; Laigle, Mireille; Charvis, Philippe; Flueh, Ernst; Gallart, Josep; Kissling, Edi; Lebrun, Jean-Frederic; Nicolich, Rinaldo; Sachpazi, Maria

    2010-05-01

    In order to increase the understanding of plate boundaries that show currently low seismic activity, as was the Sumatra-Andaman subduction before the major earthquake in 2004, a cluster of surveys and cruises has been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project on the Lesser Antilles subduction zone of the Carribean-America plate boundary. A segment of the corresponding transform boundary just tragically ruptured in the 2010 January 12, Haïti earthquake. This cluster is composed by the German cruise TRAIL with the vessel F/S M. A. MERIAN, the French cruise SISMANTILLES II with the IFREMER vessel N/O ATALANTE), and French cruise OBSANTILLES with the IRD vessel N/O ANTEA. During these cruises and surveys, 80 OBS, Ocean Bottom Seismometers, 64 of which with 3-components seismometers and hydrophones, and 20 OBH with hydrophones have been brought together from several pools (Geoazur Nice, INSU/IPGP Paris, IfM-GEOMAR Kiel, AWI Bremerhaven), with up to 30 land stations (CSIC Barcelona, IPG Paris, INSU-RLBM and -Lithoscope, ETH Zurich). The deployment of all these instruments has been supported principally in addition by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT. The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the subduction mega-thrust fault plane, and which is here the

  9. Active seismic experiment

    NASA Technical Reports Server (NTRS)

    Kovach, R. L.; Watkins, J. S.; Talwani, P.

    1972-01-01

    The Apollo 16 active seismic experiment (ASE) was designed to generate and monitor seismic waves for the study of the lunar near-surface structure. Several seismic energy sources are used: an astronaut-activated thumper device, a mortar package that contains rocket-launched grenades, and the impulse produced by the lunar module ascent. Analysis of some seismic signals recorded by the ASE has provided data concerning the near-surface structure at the Descartes landing site. Two compressional seismic velocities have so far been recognized in the seismic data. The deployment of the ASE is described, and the significant results obtained are discussed.

  10. Crustal Structure Across the Okavango Rift Zone, Botswana: Initial Results From the PRIDE-SEISORZ Active-Source Seismic Profile

    NASA Astrophysics Data System (ADS)

    Canales, J. P.; Moffat, L.; Lizarralde, D.; Laletsang, K.; Harder, S. H.; Kaip, G.; Modisi, M.

    2015-12-01

    The PRIDE project aims to understand the processes of continental rift initiation and evolution by analyzing along-axis trends in the southern portion of the East Africa Rift System, from Botswana through Zambia and Malawi. The SEISORZ active-source seismic component of PRIDE focused on the Okavango Rift Zone (ORZ) in northwestern Botswana, with the main goal of imaging the crustal structure across the ORZ. This will allow us to estimate total crustal extension, determine the pattern and amount of thinning, assess the possible presence of melt within the rift zone, and assess the contrasts in crustal blocks across the rift, which closely follows the trend of a fold belt. In November 2014 we conducted a crustal-scale, 450-km-long seismic refraction/wide-angle reflection profile consisting of 19 sources (shots in 30-m-deep boreholes) spaced ~25 km apart from each other, and 900 receivers (IRIS/PASSCAL "Texan" dataloggers and 4.5Hz geophones) with ~500 m spacing. From NW to SE, the profile crosses several tectonic domains: the Congo craton, the Damara metamorphic belt and the Ghanzi-Chobe fold belt where the axis of the ORZ is located, and continues into the Kalahari craton. The record sections display clear crustal refraction (Pg) and wide-angle Moho reflection (PmP) phases for all 17 of the good-quality shots, and a mantle refraction arrival (Pn), with the Pg-PmP-Pn triplication appearing at 175 km offset. There are distinct changes in the traveltime and amplitude of these phases along the transect, and on either side of the axis, that seem to correlate with sharp transitions across tectonic terrains. Initial modeling suggests: (1) the presence of a sedimentary half-graben structure at the rift axis beneath the Okavango delta, bounded to the SE by the Kunyere-Thamalakane fault system; (2) faster crustal Vp in the domains to the NW of the ORZ; and (3) thicker crust (45-50 km) at both ends of the profile within the Congo and Kalahari craton domains than at the ORZ and

  11. The New Madrid Seismic Zone: not dead yet.

    PubMed

    Page, Morgan T; Hough, Susan E

    2014-02-14

    The extent to which ongoing seismicity in intraplate regions represents long-lived aftershock activity is unclear. We examined historical and instrumental seismicity in the New Madrid central U.S. region to determine whether present-day seismicity is composed predominantly of aftershocks of the 1811-1812 earthquake sequence. High aftershock productivity is required both to match the observation of multiple mainshocks and to explain the modern level of activity as aftershocks; synthetic sequences consistent with these observations substantially overpredict the number of events of magnitude ≥ 6 that were observed in the past 200 years. Our results imply that ongoing background seismicity in the New Madrid region is driven by ongoing strain accrual processes and that, despite low deformation rates, seismic activity in the zone is not decaying with time. PMID:24457216

  12. Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc

    NASA Astrophysics Data System (ADS)

    Laigle, M.; Hirn, A.; Sapin, M.; Bécel, A.; Charvis, P.; Flueh, E.; Diaz, J.; Lebrun, J.-F.; Gesret, A.; Raffaele, R.; Galvé, A.; Evain, M.; Ruiz, M.; Kopp, H.; Bayrakci, G.; Weinzierl, W.; Hello, Y.; Lépine, J.-C.; Viodé, J.-P.; Sachpazi, M.; Gallart, J.; Kissling, E.; Nicolich, R.

    2013-09-01

    The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70 km depth. At 145 km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45 km depth on the interplate

  13. Multiscale seismic imaging of active fault zones for hazard assessment: A case study of the Santa Monica fault zone, Los Angeles, California

    USGS Publications Warehouse

    Pratt, T.L.; Dolan, J.F.; Odum, J.K.; Stephenson, W.J.; Williams, R.A.; Templeton, M.E.

    1998-01-01

    High-resolution seismic reflection profiles at two different scales were acquired across the transpressional Santa Monica Fault of north Los Angeles as part of an integrated hazard assessment of the fault. The seismic data confirm the location of the fault and related shallow faulting seen in a trench to deeper structures known from regional studies. The trench shows a series of near-vertical strike-slip faults beneath a topographic scarp inferred to be caused by thrusting on the Santa Monica fault. Analysis of the disruption of soil horizons in the trench indicates multiple earthquakes have occurred on these strike-slip faults within the past 50 000 years, with the latest being 1000 to 3000 years ago. A 3.8-km-long, high-resolution seismic reflection profile shows reflector truncations that constrain the shallow portion of the Santa Monica Fault (upper 300 m) to dip northward between 30?? and 55??, most likely 30?? to 35??, in contrast to the 60?? to 70?? dip interpreted for the deeper portion of the fault. Prominent, nearly continuous reflectors on the profile are interpreted to be the erosional unconformity between the 1.2 Ma and older Pico Formation and the base of alluvial fan deposits. The unconformity lies at depths of 30-60 m north of the fault and 110-130 m south of the fault, with about 100 m of vertical displacement (180 m of dip-slip motion on a 30??-35?? dipping fault) across the fault since deposition of the upper Pico Formation. The continuity of the unconformity on the seismic profile constrains the fault to lie in a relatively narrow (50 m) zone, and to project to the surface beneath Ohio Avenue immediately south of the trench. A very high-resolution seismic profile adjacent to the trench images reflectors in the 15 to 60 m depth range that are arched slightly by folding just north of the fault. A disrupted zone on the profile beneath the south end of the trench is interpreted as being caused by the deeper portions of the trenched strike

  14. Gravity of the New Madrid seismic zone; a preliminary study

    USGS Publications Warehouse

    Langenheim, V.E.

    1995-01-01

    In the winter of 1811-12, three of the largest historic earthquakes in the United States occurred near New Madrid, Mo. Seismicity continues to the present day throughout a tightly clustered pattern of epicenters centered on the bootheel of Missouri, including parts of northeastern Arkansas, northwestern Tennessee, western Kentucky, and southern Illinois. In 1990, the New Madrid seismic zone/Central United States became the first seismically active region east of the Rocky Mountains to be designated a priority research area within the National Earthquake Hazards Reduction Program (NEHRP). This Professional Paper is a collection of papers, some published separately, presenting results of the newly intensified research program in this area. Major components of this research program include tectonic framework studies, seismicity and deformation monitoring and modeling, improved seismic hazard and risk assessments, and cooperative hazard mitigation studies.

  15. A seismotectonic model for the 300-kilometer-long eastern Tennessee seismic zone

    USGS Publications Warehouse

    Powell, C.A.; Bollinger, G.A.; Chapman, M.C.; Sibol, M.S.; Johnston, A.C.; Wheeler, R.L.

    1994-01-01

    Ten years of monitoring microearthquakes with a regional seismic network has revealed the presence of a well-defined, linear zone of seismic activity in eastern Tennessee. This zone produced the second highest release of seismic strain energy in the United States east of the Rocky Mountains during the last decade, when normalized by crustal area. The data indicate that seismicity produced by regional, intraplate stresses is now concentrating near the boundary between relatively strong and weak basement crustal blocks.

  16. Structural and thermal control of seismic activity and megathrust rupture dynamics in subduction zones: Lessons from the Mw 9.0, 2011 Tohoku earthquake

    NASA Astrophysics Data System (ADS)

    Satriano, Claudio; Dionicio, Viviana; Miyake, Hiroe; Uchida, Naoki; Vilotte, Jean-Pierre; Bernard, Pascal

    2014-10-01

    The 2011 Tohoku megathrust earthquake ruptured a vast region of the northeast Japan Trench subduction zone in a way that had not been enough anticipated by earthquake and tsunami risk scenarios. We analyzed the Tohoku rupture combining high-frequency back-projection analysis with low frequency kinematic inversion of the co-seismic slip. Results support the to-day well-accepted broadband characteristics of this earthquake. Most of the seismic moment is released during the first 100 s, with large co-seismic slip (up to 55 m) offshore Miyagi in a compact region on the landward side of the trench. Coherent high-frequency radiation areas and relatively low co-seismic slip are a distinctive signature of the slab-mantle interface. The broadband characteristics of the Tohoku rupture are interpreted, integrating the seismic activity and structure information on the NE Japan forearc region, as a signature of along-dip segmentation and segment interactions, that result from thermal structure, plate geometry, material composition and fracture heterogeneities along the plate boundary interface. Deep mantle corner flow and low dehydration rates along the cold subduction slab interface lead to an extended seismogenic slab-mantle interface, with strong bi-material contrast controlling larger propagation distance in the downdip preferred rupture direction. Off Miyagi, plate bending below the mantle wedge, ∼142.3°E at ∼25 km depth, is associated with the eastern limit of the deep M7-8-class thrust-earthquakes, and of the strongest coherent high-frequency generation areas. The region of the slab-crust interface between the mantle wedge limit, ∼142.7°E at ∼20 km depth, and a trenchward plate bending, ∼143.2°E at ∼15 km, acted as an effective barrier resisting for many centuries to stress-loading gradient induced by deep stable sliding and large earthquakes along the slab-mantle interface. The 2011 Tohoku earthquake, whose hypocenter is located on the east side of the

  17. A seismic hazard uncertainty analysis for the New Madrid seismic zone

    USGS Publications Warehouse

    Cramer, C.H.

    2001-01-01

    A review of the scientific issues relevant to characterizing earthquake sources in the New Madrid seismic zone has led to the development of a logic tree of possible alternative parameters. A variability analysis, using Monte Carlo sampling of this consensus logic tree, is presented and discussed. The analysis shows that for 2%-exceedence-in-50-year hazard, the best-estimate seismic hazard map is similar to previously published seismic hazard maps for the area. For peak ground acceleration (PGA) and spectral acceleration at 0.2 and 1.0 s (0.2 and 1.0 s Sa), the coefficient of variation (COV) representing the knowledge-based uncertainty in seismic hazard can exceed 0.6 over the New Madrid seismic zone and diminishes to about 0.1 away from areas of seismic activity. Sensitivity analyses show that the largest contributor to PGA, 0.2 and 1.0 s Sa seismic hazard variability is the uncertainty in the location of future 1811-1812 New Madrid sized earthquakes. This is followed by the variability due to the choice of ground motion attenuation relation, the magnitude for the 1811-1812 New Madrid earthquakes, and the recurrence interval for M>6.5 events. Seismic hazard is not very sensitive to the variability in seismogenic width and length. Published by Elsevier Science B.V.

  18. Fault-zone attenuation of high-frequency seismic waves

    SciTech Connect

    Blakeslee, S.; Malin, P.; Alvarez, M. )

    1989-11-01

    The authors have developed a technique to measure seismic attenuation within an active fault-zone at seismogenic depths. Utilizing a pair of stations and pairs of earthquakes, spectral ratios are performed to isolate attenuation produced by wave-propagation within the fault-zone. The empirical approach eliminates common source, propagation, instrument and near-surface site effects. The technique was applied to a cluster of 19 earthquakes recorded by a pair of downhole instruments located within the San Andreas fault-zone, at instruments located within the San Andreas fault-zone, at Parkfield, California. Over the 1-40 Hz bandwidth used in this analysis, amplitudes are found to decrease exponentially with frequency. Furthermore, the fault-zone propagation distance correlates with severity of attenuation. Assuming a constant Q attenuation operator, the S-wave quality factor within the fault-zone at a depth of 5-6 kilometers is 31 (+7,{minus}5). If fault-zones are low-Q environments, then near-source attenuation of high-frequency seismic waves may help to explain phenomenon such as f{sub max}. Fault-zone Q may prove to be a valuable indicator of the mechanical behavior and rheology of fault-zones. Specific asperities can be monitored for precursory changes associated with the evolving stress-field within the fault-zone. The spatial and temporal resolution of the technique is fundamentally limited by the uncertainty in earthquake location and the interval time between earthquakes.

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

    USGS Publications Warehouse

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

    2005-01-01

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

  20. Teleseismic Tomography of the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Olasanmi, Olorunfemi Temitope

    This research investigates the properties of the crust and the upper mantle beneath the eastern Tennessee seismic zone (ETSZ). The ETSZ is a major seismic feature that is located in the southeastern United States. The zone spans portions of eastern Tennessee, North Carolina, Virginia, Georgia and Alabama and is, after the New Madrid seismic zone, the second most active seismic region of the North America east of the Rocky Mountains. This NE trending zone of intraplate seismicity is about 300km long and 100km wide. A striking geophysical anomaly crossing this region is called the New York-Alabama magnetic lineament. The most seismically active part of this zone is along and to the SW of this aeromagnetic anomaly. In this thesis 3-D velocity images of the earth beneath the ETSZ were obtained by using Fast Marching Teleseismic Tomography package. The starting data was adopted from the previous study by Agbaje (2012) and consisted of 2855 residuals from 217 teleseismic events that were recorded by 28 stations within the ETSZ. The tomographic images show significant velocity anomalies, confirming complex tectonic evolution and revealing basement features that can be correlated with regional gravity and magnetic anomalies. The results of the tomographic inversion in the crust agree with the previous tomographic studies that used local earthquake data (Powell et al., 2014). However, the most significant anomaly resolved persists through most of the upper mantle and suggests the presence of a major, southeast dipping, high velocity anomaly located beneath the Blue Ridge province. The anomaly is interpreted to possibly be a fossil slab dating back to the accretion of Carolina terrane during Devonian.

  1. Cockade-textured cataclasite and silica gel from damage zone in carbonated ultramafics: markers of cycles of seismic activity?

    NASA Astrophysics Data System (ADS)

    Scarsi, Marco; Crispini, Laura; Garofalo, Paolo; Capponi, Giovanni

    2016-04-01

    Shallow crustal processes occurring during seismic slips and generating fracture networks are of great interest due to their complex interplay with a spectrum of other geological processes . Our study focuses on faults with peculiar core textures, similar to those of "cockade breccia" (Genna et al., 1996) and "clast cortex grains" (Rempe et al., 2014), and on their relation with syntectonic hydrothermal alteration linked with Au bearing-quartz and chalcedony veins. Our work aims to study the enviromental conditions for the formation of such peculiar texture, their relation with the hydrothermal vein system and their potential as shallow seismic indicators. We present field, microstructural and petrochemical data of a peculiar damage zone of fault rocks located in carbonated peridotites and serpentinites of the Ligurian Alps (Voltri Massif, Italy). These are mainly reverse faults, which are coeval with syntectonic Au-bearing quartz veins and chalcedony veins (Giorza et al., 2010), in which lherzolites occupy the hangingwall of the faults and serpentinites the footwall. The fault rocks show evidence for carbonation, as olivine and serpentine are clearly transformed into an assemblage made of magnesite, dolomite and minor ankerite. The damage zones of the faults are serpentinite-rich and about 10 m in thickness, while the cataclasite cores are carbonate-rich and ca. 1 m thick. The top of the fault core shows the occurrence of a chalcedony shear veins with chatter marks and slikenlines on the surface. The "cockade breccia" is made of spherical aggregates of Fe-Mg carbonates and are 1 mm to 3 cm in size. These aggregates show cores of microcrystalline Fe-Mg carbonates, and concentric outer layers of relatively coarser Fe-Mg carbonates with radial or laminated texture. In some cases, these aggregates show evidence for rotation along secondary slip zones. We interpret all these features as the products of chemical interaction between the olivine and serpentine initially

  2. Assessing the Seismic Potential Hazard of the Makran Subduction Zone

    NASA Astrophysics Data System (ADS)

    Frohling, E.; Szeliga, W. M.; Melbourne, T. I.; Abolghasem, A.; Lodi, S. H.

    2013-12-01

    Long quiescent subduction zones like the Makran, Sunda, and Cascadia, which have long recurrence intervals for large (> Mw 8) earthquakes, often have poorly known seismic histories and are particularly vulnerable and often ill-prepared. The Makran subduction zone has not been studied extensively, but the 1945 Mw 8.1 earthquake and subsequent tsunami, as well as more recent mid magnitude, intermediate depth (50-100 km) seismicity, demonstrates the active seismic nature of the region. Recent increases in regional GPS and seismic monitoring now permit the modeling of strain accumulations and seismic potential of the Makran subduction zone. Subduction zone seismicity indicates that the eastern half of the Makran is presently more active than the western half. It has been hypothesized that the relative quiescence of the western half is due to aseismic behavior. However, based on GPS evidence, the entire subduction zone generally appears to be coupled and has been accumulating stress that could be released in another > 8.0 Mw earthquake. To assess the degree of coupling, we utilize existing GPS data to create a fault coupling model for the Makran using a preliminary 2-D fault geometry derived from ISC hypocenters. Our 2-D modeling is done using the backslip approach and defines the parameters in our coupling model; we forego the generation of a 3-D model due to the low spatial density of available GPS data. We compare the use of both NUVEL-1A plate motions and modern Arabian plate motions derived from GPS station velocities in Oman to drive subduction for our fault coupling model. To avoid non-physical inversion results, we impose second order smoothing to eliminate steep strain gradients. The fit of the modeled inter-seismic deformation vectors are assessed against the observed strain from the GPS data. Initial observations indicate that the entire subduction zone is currently locked and accumulating strain, with no identifiable gaps in the interseismic locking

  3. Seismic coupling and uncoupling at subduction zones

    NASA Technical Reports Server (NTRS)

    Ruff, L.; Kanamori, H.

    1983-01-01

    Some of the correlations concerning the properties of subduction zones are reviewed. A quantitative global comparison of many subduction zones reveals that the largest earthquakes occur in zones with young lithosphere and fast convergence rates. Maximum earthquake size is directly related to the asperity distribution on the fault plane. This observation can be translated into a simple model of seismic coupling where the horizontal compressive stress between two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. Plate age and rate can control asperity distribution directly through the horizontal compressive stress associated with the vertical and horizontal velocities of subducting slabs. The basalt to eclogite phase change in the down-going oceanic crust may be largely responsible for the uncoupling of subduction zones below a depth of about 40 km.

  4. Improving Seismic Constraints on Subduction Zone Geometry

    NASA Astrophysics Data System (ADS)

    Syracuse, E. M.; Abers, G. A.; Fischer, K. M.; van Keken, P. E.; Kneller, E. A.; Rychert, C. A.

    2007-12-01

    Accurate slab geometries are necessary for 3D flow modeling, and for understanding the variations in temperature and melting geometry between different subduction zones. Recent studies have shown that the depth to slab beneath arc volcanoes varies by as much as a factor of two between subduction zones, but these results are based on teleseismic earthquake catalogs with potentially large errors. When available, local seismic arrays provide better constraints. The TUCAN array (Tomography Under Costa Rica and Nicaragua) deployed 48 three component broadband PASSCAL instruments for 18 months with station spacing of 10-50 km across the Central America arc. This dataset provides some of the best control anywhere for ground-truth comparison of teleseismic catalogs in steeply dipping subduction zones. Joint inversion of TUCAN arrival times for velocity and hypocenters illuminate a 10-15 km thick Wadati-Benioff zone (WBZ), with absolute hypocenter uncertainties of 1-5 km. Besides providing accurate hypocenters, the tomographic images provide independent constraints on melting and temperature, through the imaging of low Vp (7.5-7.8 km/s) and highly attenuating (40Seismic Centre (ISC) and EHB (Engdahl et al., 1998) hypocenters show a teleseismic Wadati- Benioff zone (TWBZ) that lies 15 km below that of the TUCAN catalog on average at 80-200 km depth, with similar results for local catalogs based on a 1D velocity model. However, the width of the TWBZ is 30-80 km or 3-5 times that indicated by TUCAN hypocenters; this additional width suggests errors of +/- 10-33 km. Commonly, the top of the subducting slab is assumed to lie at the top of the WBZ seismicity, for example if double seismic zones are expected. Because of the large scatter, the TWBZ is biased too shallow compared to the TUCAN data, vertically by as much as 50 km for the steeply-dipping Nicaragua slab. Relative

  5. The mechanics of the South Iceland Seismic Zone

    NASA Technical Reports Server (NTRS)

    Hackman, M. Christine; Bilham, Roger; King, Geoffrey C. P.

    1990-01-01

    The mechanics of the South Iceland Seismic Zone is examined by means of boundary element modeling. The differences between the observed geometry of the major tectonically active features and that of a geometrically orthogonal ridge-transform system (which assumes that the upper part of the earth's crust is an elastic plate containing vertical cuts) are examined. It is suggested that north-south faults can accommodate transform deformation only if the faults are longer or more numerous than those observed so far. This is considered reasonable because earthquake surface rupture lengths are commonly less than the inferred fault length at depth. The South Iceland Seismic Zone is subject to sequences of large earthquake every 45-112 years. In comparing the seismic moment release derived from earthquake magnitudes with that predicted by the models, it is confirmed that the system of north-south faults can act as a transform fault.

  6. Teleseismic Tomography of the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Olasanmi, O. T.; Arroucau, P.; Vlahovic, G.

    2014-12-01

    In this work we perform a tomographic inversion of teleseismic data to investigate the properties of the crust and the uppermost mantle beneath the eastern Tennessee seismic zone (ETSZ). The ETSZ is a major seismic feature located in the southeastern United States. The zone spans portions of eastern Tennessee, North Carolina, Virginia, Georgia and Alabama and is, after the New Madrid seismic zone, the second most active seismic region of the North America east of the Rocky Mountains. Earthquakes in the ETSZ appear to align along a sharp, linear magnetic feature, called the New York-Alabama Lineament (NYAL), which acts as the northwest edge of the seismic zone and is attributed to a strike-slip fault affecting the Precambrian basement. A total of 2652 relative P-wave arrival time residuals from 201 teleseismic events recorded at 28 regional seismic station have been extracted from the continuous records using the adaptive stacking code. The three-dimensional model was computed down to 300km. The tomographic images show significant velocity anomalies, confirming complex tectonic evolution and revealing basement features that can be correlated with regional gravity and magnetic anomalies. One of the main features of the three-dimensional model is a significant velocity contrast across the NYAL that extends through the crust and the uppermost mantle, with high velocity anomalies northwest of the NYAL and lower velocities southwest of the NYAL. Our results support the hypothesis that the lineament is a major basement fault associated with a tectonic boundary produced by merging of the southern Appalachian basement with the Granite-Rhyolite basement during the Grenville orogeny.

  7. Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

    SciTech Connect

    Rutqvist, Jonny; Rinaldi, Antonio P.; Cappa, Frédéric; Moridis, George J.

    2015-03-01

    We conducted three-dimensional coupled fluid-flow and geomechanical modeling of fault activation and seismicity associated with hydraulic fracturing stimulation of a shale-gas reservoir. We simulated a case in which a horizontal injection well intersects a steeply dip- ping fault, with hydraulic fracturing channeled within the fault, during a 3-hour hydraulic fracturing stage. Consistent with field observations, the simulation results show that shale-gas hydraulic fracturing along faults does not likely induce seismic events that could be felt on the ground surface, but rather results in numerous small microseismic events, as well as aseismic deformations along with the fracture propagation. The calculated seismic moment magnitudes ranged from about -2.0 to 0.5, except for one case assuming a very brittle fault with low residual shear strength, for which the magnitude was 2.3, an event that would likely go unnoticed or might be barely felt by humans at its epicenter. The calculated moment magnitudes showed a dependency on injection depth and fault dip. We attribute such dependency to variation in shear stress on the fault plane and associated variation in stress drop upon reactivation. Our simulations showed that at the end of the 3-hour injection, the rupture zone associated with tensile and shear failure extended to a maximum radius of about 200 m from the injection well. The results of this modeling study for steeply dipping faults at 1000 to 2500 m depth is in agreement with earlier studies and field observations showing that it is very unlikely that activation of a fault by shale-gas hydraulic fracturing at great depth (thousands of meters) could cause felt seismicity or create a new flow path (through fault rupture) that could reach shallow groundwater resources.

  8. Cockade-textured cataclasite and silica gel from damage zone in carbonated ultramafics: markers of cycles of seismic activity?

    NASA Astrophysics Data System (ADS)

    Scarsi, Marco; Crispini, Laura; Garofalo, Paolo; Capponi, Giovanni

    2016-04-01

    Shallow crustal processes occurring during seismic slips and generating fracture networks are of great interest due to their complex interplay with a spectrum of other geological processes . Our study focuses on faults with peculiar core textures, similar to those of "cockade breccia" (Genna et al., 1996) and "clast cortex grains" (Rempe et al., 2014), and on their relation with syntectonic hydrothermal alteration linked with Au bearing-quartz and chalcedony veins. Our work aims to study the enviromental conditions for the formation of such peculiar texture, their relation with the hydrothermal vein system and their potential as shallow seismic indicators. We present field, microstructural and petrochemical data of a peculiar damage zone of fault rocks located in carbonated peridotites and serpentinites of the Ligurian Alps (Voltri Massif, Italy). These are mainly reverse faults, which are coeval with syntectonic Au-bearing quartz veins and chalcedony veins (Giorza et al., 2010), in which lherzolites occupy the hangingwall of the faults and serpentinites the footwall. The fault rocks show evidence for carbonation, as olivine and serpentine are clearly transformed into an assemblage made of magnesite, dolomite and minor ankerite. The damage zones of the faults are serpentinite-rich and about 10 m in thickness, while the cataclasite cores are carbonate-rich and ca. 1 m thick. The top of the fault core shows the occurrence of a chalcedony shear veins with chatter marks and slikenlines on the surface. The "cockade breccia" is made of spherical aggregates of Fe-Mg carbonates and are 1 mm to 3 cm in size. These aggregates show cores of microcrystalline Fe-Mg carbonates, and concentric outer layers of relatively coarser Fe-Mg carbonates with radial or laminated texture. In some cases, these aggregates show evidence for rotation along secondary slip zones. We interpret all these features as the products of chemical interaction between the olivine and serpentine initially

  9. Method for processing seismic data to identify anomalous absorption zones

    DOEpatents

    Taner, M. Turhan

    2006-01-03

    A method is disclosed for identifying zones anomalously absorptive of seismic energy. The method includes jointly time-frequency decomposing seismic traces, low frequency bandpass filtering the decomposed traces to determine a general trend of mean frequency and bandwidth of the seismic traces, and high frequency bandpass filtering the decomposed traces to determine local variations in the mean frequency and bandwidth of the seismic traces. Anomalous zones are determined where there is difference between the general trend and the local variations.

  10. Geochemical Monitoring of Geothermal Waters (2002 2004) along the North Anatolian Fault Zone, Turkey: Spatial and Temporal Variations and Relationship to Seismic Activity

    NASA Astrophysics Data System (ADS)

    Süer, Selin; Güleç, Nilgün; Mutlu, Halim; Hilton, David R.; Çifter, Candan; Sayin, Mesut

    2008-01-01

    A total of nine geothermal fields located along an 800-km long E-W transect of the North Anatolian Fault Zone (NAFZ), Turkey were monitored for three years (2002 2004 inclusive; 3-sampling periods per year) to investigate any possible relationship between seismic activity and temporal variations in the chemistry and isotope characteristics of waters in the fields. The geothermal fields monitored in the study were, from west to east, Yalova, Efteni, Bolu, Mudurnu, Seben, Kurşunlu-Çankırı, Hamamözü, Gözlek and Reşadiye. The chemical (major anion-cation contents) and isotopic (18O/16O, D/H, 3H) compositions of hot and cold waters of the geothermal sites were determined in order to both characterize the chemical nature of the individual fields and identify possible temporal variations associated with localized seismic activity. The geothermal waters associated with the NAFZ are dominantly Na-HCO3, whereas the cold waters are of the Ca-HCO3 type. The oxygen- and hydrogen-isotope compositions reveal that the hot waters are meteoric in origin as are their cold water counterparts. However, the lower δ18O, δD and 3H contents of the hot waters point to the fact that they are older than the cold waters, and that their host aquifers are recharged from higher altitudes with virtually no input from recent (post-bomb) precipitation. Although no major earthquakes (e.g., with M ≥ 5) were recorded along the NAFZ during the course of the monitoring period, variations in the chemical and isotopic compositions of some waters were observed. Indeed, the timing of the chemical/isotopic changes seems to correlate with the occurrence of seismic activity of moderate magnitude (3 < M < 5) close to the sampling sites. In this respect, Cl, 3H and Ca seem to be the most sensitive tracers of seismically-induced crustal perturbations, and the Yalova and Efteni fields appear to be the key localities where the effects of seismic activity on the geothermal fluids are most pronounced over

  11. Seismicity Of The Charlie-Gibbs Fracture Zone

    NASA Astrophysics Data System (ADS)

    Novitsky, C. G.; Páll, E.

    2014-12-01

    Due to the low seismicity of the Charlie-Gibbs Fracture Zone it has not been possible to examine in the past. Now there is over 50 years of seismic data to reexamine the fracture zone and past-hypothesized relationships, such as an inactive southern trough of the fracture zone. The results indicate the southern trough of the fracture zone still appears to be inactive and the fracture zone has reached a full seismic time cycle. It is postulated that the southern troughs inactive behavior is from a low velocity, low viscosity zone from unusually thin crust (3.0-4.5 km).

  12. Modeling of fault activation and seismicity by injection directly into a fault zone associated with hydraulic fracturing of shale-gas reservoirs

    DOE PAGESBeta

    Rutqvist, Jonny; Rinaldi, Antonio P.; Cappa, Frédéric; Moridis, George J.

    2015-03-01

    We conducted three-dimensional coupled fluid-flow and geomechanical modeling of fault activation and seismicity associated with hydraulic fracturing stimulation of a shale-gas reservoir. We simulated a case in which a horizontal injection well intersects a steeply dip- ping fault, with hydraulic fracturing channeled within the fault, during a 3-hour hydraulic fracturing stage. Consistent with field observations, the simulation results show that shale-gas hydraulic fracturing along faults does not likely induce seismic events that could be felt on the ground surface, but rather results in numerous small microseismic events, as well as aseismic deformations along with the fracture propagation. The calculated seismicmore » moment magnitudes ranged from about -2.0 to 0.5, except for one case assuming a very brittle fault with low residual shear strength, for which the magnitude was 2.3, an event that would likely go unnoticed or might be barely felt by humans at its epicenter. The calculated moment magnitudes showed a dependency on injection depth and fault dip. We attribute such dependency to variation in shear stress on the fault plane and associated variation in stress drop upon reactivation. Our simulations showed that at the end of the 3-hour injection, the rupture zone associated with tensile and shear failure extended to a maximum radius of about 200 m from the injection well. The results of this modeling study for steeply dipping faults at 1000 to 2500 m depth is in agreement with earlier studies and field observations showing that it is very unlikely that activation of a fault by shale-gas hydraulic fracturing at great depth (thousands of meters) could cause felt seismicity or create a new flow path (through fault rupture) that could reach shallow groundwater resources.« less

  13. Double Seismic Zones and Dehydration Embrittlement

    NASA Astrophysics Data System (ADS)

    Seno, T.; Yamasaki, T.

    2001-12-01

    Dehydration embrittlement is still an attractive mechanism for intermediate-depth earthquake occurrence. We explore the possibility whether this hypothesis can explain the observed geometry of the double seismic zones. We calculate transient temperature structures of slabs using a finite element method, based on geologically estimated subduction histories for NE Japan, SW Japan, E. Aleutian, N. Chile, Taiwan, and Cape Mendocino, where double zones have been observed. We then delineate the dehydration loci utilizing the phase diagrams of serpentine by Wunder and Schreyer (1997), Bose and Navrotsky (1998) and Ulmer and Trommsdorff (1995), and of meta-basalt by Kerrick and Connolly (2001). For slabs whose crust passes from blueschist to lawsonite-eclogite facies, we take the glaucophane-out boundary as the upper limit of the significant dehydration of meta-basalt. The dehydration loci of serpentine produces a double-layered structure for NE Japan, E. Aleutian, N. Chile, and Taiwan, whose slabs are older than 40 Ma at the trench. In these regions, the lower zone of the double zones can be explained by the lower dehydration locus of serpentine, and the upper zone by that of meta-basalt. The deepest portion of the upper zone might contain the upper dehydration locus of serpentine; however, this occurs only in NE Japan and E. Aleutian. The dehydration loci of serpentine degenerate into a single one in SW Japan and Cape Mendocino whose slabs are younger than 20 Ma at the trench. For these regions, the lower zone of the double zone is explained by the degenerated dehydration locus of serpentine and the upper zone by successive dehydration of meta-basalt from greenschist to dry eclogite facies in the shallow depth. Provided that the oceanic crust is metamorphosed into hydrated minerals at the mid-ocean ridge, the condition for the existence of a double zone is the serpentinization of the subducting oceanic plate at mid-plate depth for a cold slab and at shallow depth for a

  14. Identifying active interplate and intraplate fault zones in the western Caribbean plate from seismic reflection data and the significance of the Pedro Bank fault zone in the tectonic history of the Nicaraguan Rise

    NASA Astrophysics Data System (ADS)

    Ott, B.; Mann, P.

    2015-12-01

    The offshore Nicaraguan Rise in the western Caribbean Sea is an approximately 500,000 km2 area of Precambrian to Late Cretaceous tectonic terranes that have been assembled during the Late Cretaceous formation of the Caribbean plate and include: 1) the Chortis block, a continental fragment; 2) the Great Arc of the Caribbean, a deformed Cretaceous arc, and 3) the Caribbean large igneous province formed in late Cretaceous time. Middle Eocene to Recent eastward motion of the Caribbean plate has been largely controlled by strike-slip faulting along the northern Caribbean plate boundary zone that bounds the northern margin of the Nicaraguan Rise. These faults reactivate older rift structures near the island of Jamaica and form the transtensional basins of the Honduran Borderlands near Honduras. Recent GPS studies suggest that small amount of intraplate motion within the current margin of error of GPS measurements (1-3 mm/yr) may occur within the center of the western Caribbean plate at the Pedro Bank fault zone and Hess Escarpment. This study uses a database of over 54,000 km of modern and vintage 2D seismic data, combined with earthquake data and results from previous GPS studies to define the active areas of inter- and intraplate fault zones in the western Caribbean. Intraplate deformation occurs along the 700-km-long Pedro Bank fault zone that traverses the center of the Nicaraguan Rise and reactivates the paleo suture zone between the Great Arc of the Caribbean and the Caribbean large igneous province. The Pedro Bank fault zone also drives active extension at the 200-km-long San Andres rift along the southwest margin of the Nicaraguan Rise. Influence of the Cocos Ridge indentor may be contributing to reactivation of faulting along the southwesternmost, active segment of the Hess Escarpment.

  15. Building a GIS database in the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Akinpelu, M. O.; Vlahovic, G.; Arroucau, P.; Malhotra, R.; Powell, C. A.

    2010-12-01

    Eastern Tennessee contains one of the most seismically active regions in the eastern North America. The Eastern Tennessee Seismic Zone (ETSZ) is about 300 kilometers long and extends from northwestern Georgia through eastern Tennessee [Study Area: 34°N to 37°N; 86°W to 82.5°W]. It is the second most active earthquake zone of the United States east of the Rocky Mountains. Only the New Madrid Seismic Zone is releasing more seismic strain energy. Unlike the New Madrid Seismic Zone, the ETSZ did not experience any destructive earthquake in historical time; however, its seismogenic potential is not well understood. The spatial dimensions of the ETSZ and its association with potential field anomalies suggest that collecting and organizing all the relevant data into a GIS geodatabase could increase our understanding of that region. Geographic Information System (GIS) software can be used to acquire, share, maintain and modify geospatial data sets. In this work, ArcGIS 9.3.2 is used to build a geodatabase which includes topography, earthquake information such as locations, magnitudes and focal mechanisms, potential field data, P and S wave velocity anomalies inferred from local tomographic inversions of local events, seismic transects, digital geological maps and others relevant datasets. Raw datasets were downloaded from several earth science institutions and were edited before being imported to ArcGIS. Various geoprocessing techniques, such as geo-referencing, digitizing, and surface interpolation were used to manipulate and analyze these data. We show how this compilation can be used to analyze the spatial relationships between earthquake locations and other data layers. The long-term idea behind this project is to build an information resource that will be continuously updated and will eventually encompass data related to intraplate seismicity in the entire central and eastern United States. It will be made available to researchers, students, the general public and

  16. Imaging fault zones using 3D seismic image processing techniques

    NASA Astrophysics Data System (ADS)

    Iacopini, David; Butler, Rob; Purves, Steve

    2013-04-01

    and collecting these into "disturbance geobodies". These seismic image processing methods represents a first efficient step toward a construction of a robust technique to investigate sub-seismic strain, mapping noisy deformed zones and displacement within subsurface geology (Dutzer et al.,2011; Iacopini et al.,2012). In all these cases, accurate fault interpretation is critical in applied geology to building a robust and reliable reservoir model, and is essential for further study of fault seal behavior, and reservoir compartmentalization. They are also fundamental for understanding how deformation localizes within sedimentary basins, including the processes associated with active seismogenetic faults and mega-thrust systems in subduction zones. Dutzer, JF, Basford., H., Purves., S. 2009, Investigating fault sealing potential through fault relative seismic volume analysis. Petroleum Geology Conference series 2010, 7:509-515; doi:10.1144/0070509 Marfurt, K.J., Chopra, S., 2007, Seismic attributes for prospect identification and reservoir characterization. SEG Geophysical development Iacopini, D., Butler, RWH. & Purves, S. (2012). 'Seismic imaging of thrust faults and structural damage: a visualization workflow for deepwater thrust belts'. First Break, vol 5, no. 30, pp. 39-46.

  17. Active Seismic Imaging Experiment

    NASA Astrophysics Data System (ADS)

    Berge, Patricia A.; Dawson, Phillip B.; Evans, John R.

    In September 1985 the U.S. Geological Survey (USGS) and Lawrence Livermore National Laboratory (LLNL) will conduct an active seismic experiment in the Medicine Lake area of northern California. The work is supported by the Geothermal Research Program of USGS and by the Geothermal and Hydropower Technologies Division of the U.S. Department of Energy. We invite interested organizations or individuals to record our explosions from Medicine Lake volcano and surrounding areas not covered by the USGS-LLNL array.

  18. Seismic-wave attenuation associated with crustal faults in the New Madrid seismic zone

    USGS Publications Warehouse

    Hamilton, R.M.; Mooney, W.D.

    1990-01-01

    The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/ or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.

  19. 40 CFR 258.14 - Seismic impact zones.

    Code of Federal Regulations, 2012 CFR

    2012-07-01

    ... 40 Protection of Environment 26 2012-07-01 2011-07-01 true Seismic impact zones. 258.14 Section 258.14 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES CRITERIA FOR MUNICIPAL SOLID WASTE LANDFILLS Location Restrictions § 258.14 Seismic impact zones. (a) New MSWLF units and lateral expansions shall not be...

  20. 40 CFR 258.14 - Seismic impact zones.

    Code of Federal Regulations, 2014 CFR

    2014-07-01

    ... 40 Protection of Environment 25 2014-07-01 2014-07-01 false Seismic impact zones. 258.14 Section 258.14 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES CRITERIA FOR MUNICIPAL SOLID WASTE LANDFILLS Location Restrictions § 258.14 Seismic impact zones. (a) New MSWLF units and lateral expansions shall not be...

  1. Global distribution of carbon dioxide discharges, and major zones of seismicity

    USGS Publications Warehouse

    Barnes, Ivan; Irwin, William P.; White, Donald E.

    1978-01-01

    Carbon dioxide discharges of the circum-Pacific belt are in a seismically active zone and in part stem from contact metamorphism. Carbon dioxide discharges in Europe and Asia Minor, also in an area of high seismic activity, are in part from regional metamorphism and are in areas of very high heat flow.

  2. Seismic structure of the Costa Rican subduction system from active-source onshore-offshore seismic data and imaging plate boundary processes at the Cascadia subduction zone offshore Washington

    NASA Astrophysics Data System (ADS)

    Everson, Erik D.

    of ~6.5 -7.2 km/s under the active arc. Our modeled lower crustal velocities and densities fit approximately at or within the error bounds for bulk continental crust. Using the crustal structure from our velocity model, we were able to determine a magmatic production rate of ~80 km3/km/Ma for the Costa Rican volcanic arc. The third chapter uses iterative pre-stack velocity analysis to create pre-stack depth migrated seismic images and velocity models. The PSDM reveal: (1) landward vergence of faults; (2) extensive BSR's; (3) a zone of low acoustic impedance underneath the Pleistocene accretionary prism; (4) a lack of a strong decollement reflection throughout the section; (5) discontinuous reflectivity from the subducting oceanic crust; (6) and a shallow dip of the top of the subducting oceanic crust ~1.5 - 4° beneath the Pleistocene accretionary prism. From the inferred porosity variations from our velocity model we are able to estimate the volume of expelled fluid from the Pleistocene accretionary prism. We estimate that over the ~32 km along the deformation front covered by our seismic lines that ~ 750 +150/-110 km3 of expelled fluid has been released at a rate of ~ 1.1 mm/yr.

  3. Definition of Exclusion Zones Using Seismic Data

    NASA Astrophysics Data System (ADS)

    Bartal, Y.; Villagran, M.; Ben Horin, Y.; Leonard, G.; Joswig, M.

    - In verifying compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT), there is a motivation to be effective, efficient and economical and to prevent abuse of the right to conduct an On-site Inspection (OSI) in the territory of a challenged State Party. In particular, it is in the interest of a State Party to avoid irrelevant search in specific areas. In this study we propose several techniques to determine `exclusion zones', which are defined as areas where an event could not have possibly occurred. All techniques are based on simple ideas of arrival time differences between seismic stations and thus are less prone to modeling errors compared to standard event location methods. The techniques proposed are: angular sector exclusion based on a tripartite micro array, half-space exclusion based on a station pair, and closed area exclusion based on circumferential networks.

  4. A seismic source zone model for the seismic hazard assessment of the Italian territory

    NASA Astrophysics Data System (ADS)

    Meletti, Carlo; Galadini, Fabrizio; Valensise, Gianluca; Stucchi, Massimiliano; Basili, Roberto; Barba, Salvatore; Vannucci, Gianfranco; Boschi, Enzo

    2008-04-01

    We designed a new seismic source model for Italy to be used as an input for country-wide probabilistic seismic hazard assessment (PSHA) in the frame of the compilation of a new national reference map. We started off by reviewing existing models available for Italy and for other European countries, then discussed the main open issues in the current practice of seismogenic zoning. The new model, termed ZS9, is largely based on data collected in the past 10 years, including historical earthquakes and instrumental seismicity, active faults and their seismogenic potential, and seismotectonic evidence from recent earthquakes. This information allowed us to propose new interpretations for poorly understood areas where the new data are in conflict with assumptions made in designing the previous and widely used model ZS4. ZS9 is made out of 36 zones where earthquakes with Mw > = 5 are expected. It also assumes that earthquakes with Mw up to 5 may occur anywhere outside the seismogenic zones, although the associated probability is rather low. Special care was taken to ensure that each zone sampled a large enough number of earthquakes so that we could compute reliable earthquake production rates. Although it was drawn following criteria that are standard practice in PSHA, ZS9 is also innovative in that every zone is characterised also by its mean seismogenic depth (the depth of the crustal volume that will presumably release future earthquakes) and predominant focal mechanism (their most likely rupture mechanism). These properties were determined using instrumental data, and only in a limited number of cases we resorted to geologic constraints and expert judgment to cope with lack of data or conflicting indications. These attributes allow ZS9 to be used with more accurate regionalized depth-dependent attenuation relations, and are ultimately expected to increase significantly the reliability of seismic hazard estimates.

  5. Seismic Zone of Craiova and its Surroundings

    NASA Astrophysics Data System (ADS)

    Nita, Laura-Simona

    2013-04-01

    The mapping of the Romanian area, based of the seismic range, put Craiova and the metropolitan zone, in seismic class C of level 8. This increased level is due to crustal fissure, oriented N-S on the Craiova meridian, which resonates with seismic waves from Vrancea epicenter. Craiova and the metropolitan zone is placed on the separated lithologic substrates, so, the effects of seism are not uniform between urban and peri-urban areas. The highest piedmont area and the terraces (V, IV, III), with a predominant substrate of conglomerate and sandstone, decrease effect of the earthquake. The first and the second terraces and the meadow, with a predominant content of marls and clays, are very elastic and increase the effect of earthquake. The most exposed are the villages placed on the right side of the Jiu River, where these landforms are very common and in case of earthquake, can be reactivated. In Romania there are just shallow depth and intermediate depth earthquakes. The frequency of earthquakes with a greater than 7.2 magnitude on the Richter scale is three times in a century. The most magnitude for a Romanian earthquake took place on 10 of November 1940 and it has a magnitude of 7.6 on the Richter scale. The 1977 Vrancea Earthquake occurred on March 4, 21:20 local time. It had a magnitude of 7.2 with an epicenter in Vrancea. The fracture occurred at a depth of 94 kilometers but it was a multishock earthquake with several outbreaks in the south-west part of Vrancea County. In 55 seconds the earthquake killed about 1,500 people in Romania (1,400 in Bucharest), wounded more than 11,300 and about 33,000 buildings and structures collapsed. The shock wave was felt in almost all countries in the Balkan Peninsula, as well as Ukraine and Moldova, but with a lower intensity. When the moving is vertically, in a time of 7-8 seconds, the next move is horizontally. The horizontally waves, shear waves, carry 75% of total energy. The poorly constructed buildings will fall even

  6. Active Seismic Monitoring for Earthquake Forecasting

    NASA Astrophysics Data System (ADS)

    Artamonova, M.; Korneev, V.

    2005-12-01

    Earthquake prediction remains high priority issue for disaster prevention. Study of the M6.0 2004 Parkfield and M7.0 1989 Loma Prieta strike-slip earthquakes on the San Andreas Fault (SAF) reveal seismicity peaks in the surrounding crust several months prior to the main events. Earthquakes directly within the SAF zone were intentionally excluded from the analysis because they manifest stress-release processes rather than stress accumulation. The observed increase in seismicity is interpreted as a signature of the increasing stress level in the surrounding crust, while the peak that occurs several months prior to the main event and the subsequent decrease in seismicity are attributed to damage-induced softening processes. Furthermore, in both cases there is a distinctive zone of low seismic activity that surrounds the epicentral region in the pre-event period. The increase of seismicity in the crust surrounding a potential future event and the development of a low-seismicity epicentral zone can be regarded as promising precursory information that could help signal the arrival of large earthquakes. We modeled the seismicity precursor phenomena using finite-element 2D model capable to replicate non-linear breaking of elastic rock. The distinctive seismicity peak was observed for a model simulating SAF properties at Park field. Such peaks are likely to be a good mid-term precursors allowing to declare alerts several months before earthquakes and pointing on their epicenter regions. The short tern alerts require use of active sources and their proper placement in order to monitor the developments of rock softening processes.

  7. Exploring the seismic expression of fault zones in 3D seismic volumes

    NASA Astrophysics Data System (ADS)

    Iacopini, D.; Butler, R. W. H.; Purves, S.; McArdle, N.; De Freslon, N.

    2016-08-01

    Mapping and understanding distributed deformation is a major challenge for the structural interpretation of seismic data. However, volumes of seismic signal disturbance with low signal/noise ratio are systematically observed within 3D seismic datasets around fault systems. These seismic disturbance zones (SDZ) are commonly characterized by complex perturbations of the signal and occur at the sub-seismic (10 s m) to seismic scale (100 s m). They may store important information on deformation distributed around those larger scale structures that may be readily interpreted in conventional amplitude displays of seismic data. We introduce a method to detect fault-related disturbance zones and to discriminate between this and other noise sources such as those associated with the seismic acquisition (footprint noise). Two case studies from the Taranaki basin and deep-water Niger delta are presented. These resolve SDZs using tensor and semblance attributes along with conventional seismic mapping. The tensor attribute is more efficient in tracking volumes containing structural displacements while structurally-oriented semblance coherency is commonly disturbed by small waveform variations around the fault throw. We propose a workflow to map and cross-plot seismic waveform signal properties extracted from the seismic disturbance zone as a tool to investigate the seismic signature and explore seismic facies of a SDZ.

  8. Exploring the seismic expression of fault zones in 3D seismic volumes

    NASA Astrophysics Data System (ADS)

    Iacopini, David; Butler, Rob; Purves, Steve

    2016-04-01

    Mapping and understanding distributed deformation is a major challenge for the structural interpretation of seismic data. However, volumes of seismic signal disturbance with low signal/noise ratio are systematically observed within 3D seismic datasets around fault systems. These seismic disturbance zones (SDZ) are commonly characterized by complex perturbations of the signal and occur at the sub-seismic to seismic scale. They may store important information on deformation distributed around those larger scale structures that may be readily interpreted in conventional amplitude displays of seismic data scale. We introduce a method to detect fault-related disturbance zones and to discriminate between this and other noise sources such as those associated with the seismic acquisition (footprint noise). Two case studies, from the Taranaki basin and deep-water Niger delta are presented. These resolve structure within SDZs using tensor and semblance attributes along with conventional seismic mapping. The tensor attribute is more efficient in tracking volumes containing structural displacements while structurally-oriented semblance coherency is commonly disturbed by small waveform variations around the fault throw. We propose a workflow to map and cross-plot seismic waveform signal properties extracted from the seismic disturbance zone as a tool to investigate the seismic signature and explore seismic facies of a SDZ.

  9. Seismicity around the Cimandiri fault zone, West Java, Indonesia

    NASA Astrophysics Data System (ADS)

    Febriani, Febty

    2016-02-01

    We analyzed the seismicity activity around the Cimandiri fault zone, West Java, Indonesia by using the earthquake catalogs listed by Indonesian Meteorological Climatological and Geophysical (BMKG) and International Seismological Centre (ISC) from 1973 to 2013 (M>=1 and depth ≤ 0-50 km), along with the focal mechanism data from National Research Institute of Earth Science and Disaster Prevention (NIED) from 2007 to 2014 (M>4; depth ≤ 50 km) and Global CMT catalog from 1976 to 2014 (M=0-10 and depth ≤ 50 km). The result from earthquake catalogs suggest that there are earthquake activities around the Cimandiri fault zone in the recent years, which is also supported by the results of focal mechanism data analysis from NIED data and Global CMT catalog.

  10. Receiver Function Analysis of the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Graw, J. H.; Powell, C. A.; Langston, C. A.

    2011-12-01

    We present receiver/transfer functions determined for a seismic network associated with an active, intraplate seismic zone. Basement studies within eastern Tennessee are sparse despite the fact that these rocks host the eastern Tennessee seismic zone (ETSZ) and are associated with an extensive aeromagnetic lineament called the New York-Alabama (NY-AL) lineament. The NY-AL lineament is prominent in eastern Tennessee, with a SW-NE trend, and is characterized by a lateral change in magnetic and gravity anomalies in a NW to SE direction; high magnetic and low gravity anomalies lie west of the lineament, while low magnetic and high gravity anomalies are located east of the lineament. The NY-AL lineament is thought to be an ancient strike-slip fault that is reactivating in the present day stress field. A better understanding of the basement structure within the ETSZ will aid in the assessment of its seismic hazard potential. A network maintained by the Center for Earthquake Research and Information (CERI) at the University of Memphis is located within the study area and consists of 23 short-period and three broadband seismometers. An additional station (TZTN) is maintained by IRIS and is included in our dataset. Receiver functions are computed using teleseismic earthquakes within a 30°-90° epicentral distance, at hypocentral depths greater than 30 km, and with magnitudes greater than Mw 6.0. A vertical component stack is used to obtain the best source function. A spectral waterlevel deconvolution is then used to calculate the receiver functions. Results indicate a thickening of the crust west of the NY-AL lineament and show vertical variation within the crust and upper mantle with abrupt polarity changes and strong positive and negative amplitude values. Crustal structure west of the NY-AL lineament appears to be much more complex than that east of the NY-AL lineament.

  11. Observations of seismic activity in Southern Lebanon

    NASA Astrophysics Data System (ADS)

    Meirova, T.; Hofstetter, R.

    2013-04-01

    Recent seismic activity in southern Lebanon is of particular interest since the tectonic framework of this region is poorly understood. In addition, seismicity in this region is very infrequent compared with the Roum fault to the east, which is seismically active. Between early 2008 and the end of 2010, intense seismic activity occurred in the area. This was manifested by several swarm-like sequences and continuous trickling seismicity over many days, amounting in total to more than 900 earthquakes in the magnitude range of 0.5 ≤ M d ≤ 5.2. The region of activity extended in a 40-km long zone mainly in a N-S direction and was located about 10 km west of the Roum fault. The largest earthquake, with a duration magnitude of M d = 5.2, occurred on February 15, 2008, and was located at 33.327° N, 35.406° E at a depth of 3 km. The mean-horizontal peak ground acceleration observed at two nearby accelerometers exceeded 0.05 g, where the strongest peak horizontal acceleration was 55 cm/s2 at about 20 km SE of the epicenter. Application of the HypoDD algorithm yielded a pronounced N-S zone, parallel to the Roum fault, which was not known to be seismically active. Focal mechanism, based on full waveform inversion and the directivity effect of the strongest earthquake, suggests left-lateral strike-slip NNW-SSE faulting that crosses the NE-SW traverse faults in southern Lebanon.

  12. Western Rainier Seismic Zone Airborne Laser Swath Mapping

    NASA Technical Reports Server (NTRS)

    Harding, David J.; Haugerud, Ralph A.; Johnson, Samuel Y.; Scott, Kevin M.; Weaver, Craig S.; Martinez, Diana M.; Zeigler, John C.; Latypov, Damir

    2003-01-01

    Airborne laser swath mapping (ALSM) of the Puget Lowland conducted by TerraPoint LLC for the Purget Sound Lidar Concortium (PSLC), has been successful in revealing Holocene fault scarps and lendsliders hidden beneath the dense, temperate rain forest cover and in quantifying shoreline terrace uplift. Expanding the PSLC efforts, NASA-USGS collaboration is now focusing on topographic mapping of seismogenic zones adjacent to volcanois in the western Cascades range in order to assess the presence of active faulting and tectonic deformation, better define the extend of lahars and understand their flow processes, and characterize landslide occurrence. Mapping of the western Rainier zone (WRZ) was conducted by TerraPoint in late 2002, after leaf fall and before snow accumulation. The WRZ is a NNW-trending, approx. 30 km-long zone of seismicity west of Mount Rainier National Park. The Puget Lowland ALSM methods were modified to accommodate challenges posed by the steep, high relief terrian. The laser data, acquired with a density of approx. 2 pulses /sq m, was filtered to identify returns from the ground from which a bare Earth digital elevation model (DEM) was produced with a grid size of 1.8 m. The RMS elevation accuracy of the DEM in flat, unvegetated areas is approx. 10cm based on consistency between overlapping flight swaths and comparisons to ground control points. The resulting DEM substantially improves upon Shuttle Radar Topography Mission and USGS photogrammetric mapping. For example, the DEM defines the size and spatial distribution of flood erratics left by the Electron lahar and of megaclasts within the Round Pass lahar, important for characterizing the lahar hydraulics. A previously unknown lateral levee on the Round Pass lahar is also revealed. In addition, to illustrating geomorfic feature within the WRZ, future plans for laser mapping of the Saint Helens and Darrington seismic zones will be described.

  13. An analysis of seismic hazard in the Upper Rhine Graben enlightened by the example of the New Madrid seismic zone.

    NASA Astrophysics Data System (ADS)

    Doubre, Cécile; Masson, Frédéric; Mazzotti, Stéphane; Meghraoui, Mustapha

    2014-05-01

    Seismic hazard in the "stable" continental regions and low-level deformation zones is one of the most difficult issues to address in Earth sciences. In these zones, instrumental and historical seismicity are not well known (sparse seismic networks, seismic cycle too long to be covered by the human history, episodic seismic activity) and many active structures remain poorly characterized or unknown. This is the case of the Upper Rhine Graben, the central segment of the European Cenozoic rift system (ECRIS) of Oligocene age, which extends from the North Sea through Germany and France to the Mediterranean coast over a distance of some 1100 km. Even if this region has already experienced some destructive earthquakes, its present-day seismicity is moderate and the deformation observed by geodesy is very small (below the current measurement accuracy). The strain rate does not exceed 10-10 and paleoseismic studies indicate an average return period of 2.5 to 3 103 ka for large earthquakes. The largest earthquake known for this zone is the 1356 Basel earthquake, with a magnitude generally estimated about 6.5 (Meghraoui et al., 2001) but recently re-evaluated between 6.7 and 7.1 (Fäh et al et al., 2009). A comparison of the Upper Rhine Graben with equivalent regions around the world could help improve our evaluation of seismic hazard of this region. This is the case of the New Madrid seismic zone, one of the best studied intraplate system in central USA, which experienced an M 7.0 - 7.5 earthquake in 1811-1812 and shares several characteristics with the Upper Rhine Graben, i.e. the general framework of inherited geological structures (reactivation of a failed rift / graben), seismicity patterns (spatial variability of small and large earthquakes), the null or low rate of deformation, and the location in a "stable" continental interior. Looking at the Upper Rhine Graben as an analogue of the New Madrid seismic zone, we can re-evaluate its seismic hazard and consider the

  14. Imaging the West Bohemia Seismic Zone

    NASA Astrophysics Data System (ADS)

    Alexandrakis, C.; Calo, M.; Bouchaala, F.; Vavrycuk, V.

    2013-12-01

    West Bohemia is located at the suture of three mantle lithosphere plates, the Eger Rift, the Cheb basin and is the site of Quaternary volcanism. This complex tectonic setting results in localized, periodic earthquake swarms throughout the region and many CO2 springs and gas exhalation sites. Nový Kostel, the most active swarm area, experiences frequent swarms of several hundreds to thousands of earthquakes over a period of weeks to several months. It is a unique study area, since the swarm region is surrounded by the West Bohemia Seismic Network (WEBNET), providing observations in all directions. Larger swarms, such as those in 1985/1986, 1997, 2000, 2007 and 2008, have been studied in terms of source mechanisms and swarm characteristics (Fischer and Michálek, 2003; Fischer et al., 2010; Vavryčuk, 2011). The seismicity is always located in the same area and depth range (6-15 km), however the active fault planes differ. This indicates changes to the local stress field, and may relate to the complicated tectonic situation and/or migrating fluids. Many studies have examined individual swarms and compared the earthquake episodes, however the mechanisms behind the phenomenon are still not understood. This has motivated many studies, including recent proposals for a reflection seismic profile directly over the swarm area and multidisciplinary monitoring through ICDP. In this study, we image the velocity structure within and around the swarm area using double-difference tomography (Zhang and Thurber, 2003) and Weighted Average Model (WAM) post-processing analysis (Calò et al., 2011). The WAM analysis averages together velocity models calculated with a variety of reasonable starting parameters. The velocities are weighted by the raypath proximity and density at an inversion node. This reduces starting model bias and artifacts, and yields a weighted standard deviation at each grid point. Earthquake locations and WEBNET P and S arrival times for the two most recent large

  15. Serpentine in active subduction zones

    NASA Astrophysics Data System (ADS)

    Reynard, Bruno

    2013-09-01

    Serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10-200 km depth range. Serpentines are a major water carrier, and their rheological properties have a strong influence on deformation partitioning and seismicity at depths. I review experimental investigations that have been conducted on serpentines, with emphasis on the large body of data acquired over the past decade. Determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology, and to link the presence of serpentinites with deformation and fluid circulation. The fluid budget can be partially constrained from geophysical data. Elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography. Anisotropy suggests the existence of thin serpentinite channels above the plate interface, that account for mechanical decoupling inferred from down-dip limit of the seismogenic zone and heat flow. Strain-rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high-pressure rocks, and with the geological record of subduction-related deformation. Circulation of serpentinizing fluids depends on the permeability structure, and is imaged by electrical conductivity tomography. It could be controlled by fracturing in the undeformed cold nose of the mantle wedge, and by plastic deformation along the plate interface. Fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites. Estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up

  16. Crustal imaging across the North Anatolian Fault Zone from the autocorrelation of ambient seismic noise

    NASA Astrophysics Data System (ADS)

    Taylor, George; Rost, Sebastian; Houseman, Gregory

    2016-03-01

    Seismic images of active fault zones can be used to examine the structure of faults throughout the crust and upper mantle and give clues as to whether the associated deformation occurs within a narrow shear zone or is broadly distributed through the lower crust. Limitations on seismic resolution within the crust and difficulties imaging shallow structures such as the crust-mantle boundary (Moho) place constraints on the interpretation of seismic images. In this study we retrieve body wave reflections from autocorrelations of ambient seismic noise. The instantaneous phase coherence autocorrelations allow unprecedented ambient noise images of the North Anatolian Fault Zone (NAFZ). Our reflection profiles show a Moho reflected P wave and additional structure within the crust and upper mantle. We image a distinct vertical offset of the Moho associated with the northern branch of the NAFZ indicating that deformation related to the fault remains narrow in the upper mantle.

  17. Development of GIS Database for New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Birhanemeskel, Y. T.; Vlahovic, G.; Arroucau, P.; Malhotra, R.; Powell, C. A.

    2010-12-01

    The New Madrid Seismic Zone (NMSZ) of the central Mississippi river valley is currently the most seismically active region in the central and eastern United States. A number of earthquakes occurred in NMSZ between 1811 and 1812, of which three major earthquakes with magnitudes greater than 7 destroyed the town of New Madrid, Missouri. Intraplate seismicity like the New Madrid seismicity is difficult to explain in the framework of plate tectonics and requires analyzing various geological, geophysical and seismological data to better understand its causes. ArcGIS® 9.3.1 software with license type ArcEditor was used to build a geodatabase containing multiple layers that are useful for the study of intraplate seismicity. These layers include earthquake locations, gravity and magnetic anomalies, lithology, topography, velocity anomalies as resolved by arrival time tomography and geological structures like intrusions and faults. The data for these layers were obtained from the U.S Geological Survey, from the Center for Earthquake Research and Information at the University of Memphis, TN, and from paper maps. Zipped files of various formats (.xls, .shp, .txt, .tar, etc) were downloaded and converted to a format compatible with ArcGIS. To keep compatibility of the data, editing of the attribute table of the raw data was completed before importing the data to Arc Catalog. Geo-referencing and digitizing processes were also done to import layers of contour lines and geological structures with correct vector information from papers maps. Layers were clipped in order to make sure that they fit the spatial extent of the study area (from 34°S to 40°N in latitude and from 93°W to 86°W in longitude). The New Madrid seismicity will be analyzed by looking for possible relationships that exist between the data layers using various spatial and geostatistical tools. For example the distribution of earthquakes will be analyzed with respect to the potential field and velocity

  18. Prediction of subsurface fracture in mining zone of Papua using passive seismic tomography based on Fresnel zone

    NASA Astrophysics Data System (ADS)

    Setiadi, Herlan; Nurhandoko, Bagus Endar B.; Wely, Woen; Riyanto, Erwin

    2015-04-01

    Fracture prediction in a block cave of underground mine is very important to monitor the structure of the fracture that can be harmful to the mining activities. Many methods can be used to obtain such information, such as TDR (Time Domain Relectometry) and open hole. Both of them have limitations in range measurement. Passive seismic tomography is one of the subsurface imaging method. It has advantage in terms of measurements, cost, and rich of rock physical information. This passive seismic tomography studies using Fresnel zone to model the wavepath by using frequency parameter. Fresnel zone was developed by Nurhandoko in 2000. The result of this study is tomography of P and S wave velocity which can predict position of fracture. The study also attempted to use sum of the wavefronts to obtain position and time of seismic event occurence. Fresnel zone tomography and the summation wavefront can predict location of geological structure of mine area as well.

  19. Prediction of subsurface fracture in mining zone of Papua using passive seismic tomography based on Fresnel zone

    SciTech Connect

    Setiadi, Herlan; Nurhandoko, Bagus Endar B.; Wely, Woen; Riyanto, Erwin

    2015-04-16

    Fracture prediction in a block cave of underground mine is very important to monitor the structure of the fracture that can be harmful to the mining activities. Many methods can be used to obtain such information, such as TDR (Time Domain Relectometry) and open hole. Both of them have limitations in range measurement. Passive seismic tomography is one of the subsurface imaging method. It has advantage in terms of measurements, cost, and rich of rock physical information. This passive seismic tomography studies using Fresnel zone to model the wavepath by using frequency parameter. Fresnel zone was developed by Nurhandoko in 2000. The result of this study is tomography of P and S wave velocity which can predict position of fracture. The study also attempted to use sum of the wavefronts to obtain position and time of seismic event occurence. Fresnel zone tomography and the summation wavefront can predict location of geological structure of mine area as well.

  20. Three Dimensional velocity Structure in the New Madrid and Other SCR Seismic Zones

    NASA Astrophysics Data System (ADS)

    Powell, C. A.

    2002-12-01

    Recent tomographic inversions of travel time data accumulated for active SCR seismic zones have revealed strong velocity contrasts that appear to control the distribution of seismicity. Velocity images have been obtained for the New Madrid seismic zone (NMSZ), the eastern Tennessee seismic zone (ETSZ), and the Charlevoix seismic zone (CSZ). We have also obtained a preliminary velocity model for the aftershock region associated with the Mw=7.7 January 26th Bhuj, India earthquake. Both P and S waves were inverted for velocity structure in the NMSZ. High velocity bodies were imaged and are interpreted to be intrusions associated with the axis and edge of the Reelfoot graben. Low velocities were imaged near the intersection of the long NE arm of seismicity and the NW trending arm; the low velocities are attributed to highly fractured, fluid saturated crust and are associated with shallow earthquake swarms. In general, earthquakes tend to avoid regions with higher than average velocities and concentrate in areas of low velocity or along the edges of high velocity zones. Similar results were obtained for both the ETSZ and the CSZ. A prominent low-velocity zone was detected in the ETSZ; most earthquakes occur in rocks that surround the lowest-velocity regions. An elongated, high velocity region is present at mid-crustal depths in the CSZ; earthquakes avoid the high velocity body and separate into two bands, one on either side of the feature. Larger earthquakes (exceeding magnitude 4) have occurred along the northern edge of the high velocity region. Our results suggest that earthquakes in SCR seismic zones tend to occur in rocks where strain energy is concentrating. This observation is consistent with results from high resolution tomographic images of fault zones in California.

  1. Lunar seismic profiling experiment natural activity study

    NASA Technical Reports Server (NTRS)

    Duennebier, F. K.

    1976-01-01

    The Lunar Seismic Experiment Natural Activity Study has provided a unique opportunity to study the high frequency (4-20 Hz) portion to the seismic spectrum on the moon. The data obtained from the LSPE was studied to evaluate the origin and importance of the process that generates thermal moonquakes and the characteristics of the seismic scattering zone at the lunar surface. The detection of thermal moonquakes by the LSPE array made it possible to locate the sources of many events and determine that they are definitely not generated by astronaut activities but are the result of a natural process on the moon. The propagation of seismic waves in the near-surface layers was studied in a qualitative manner. In the absence of an adequate theoretical model for the propagation of seismic waves in the moon, it is not possible to assign a depth for the scattering layer. The LSPE data does define several parameters which must be satisfied by any model developed in the future.

  2. A non extensive statistical physics analysis of the Hellenic subduction zone seismicity

    NASA Astrophysics Data System (ADS)

    Vallianatos, F.; Papadakis, G.; Michas, G.; Sammonds, P.

    2012-04-01

    The Hellenic subduction zone is the most seismically active region in Europe [Becker & Meier, 2010]. The spatial and temporal distribution of seismicity as well as the analysis of the magnitude distribution of earthquakes concerning the Hellenic subduction zone, has been studied using the concept of Non-Extensive Statistical Physics (NESP) [Tsallis, 1988 ; Tsallis, 2009]. Non-Extensive Statistical Physics, which is a generalization of Boltzmann-Gibbs statistical physics, seems a suitable framework for studying complex systems (Vallianatos, 2011). Using this concept, Abe & Suzuki (2003;2005) investigated the spatial and temporal properties of the seismicity in California and Japan and recently Darooneh & Dadashinia (2008) in Iran. Furthermore, Telesca (2011) calculated the thermodynamic parameter q of the magnitude distribution of earthquakes of the southern California earthquake catalogue. Using the external seismic zones of 36 seismic sources of shallow earthquakes in the Aegean and the surrounding area [Papazachos, 1990], we formed a dataset concerning the seismicity of shallow earthquakes (focal depth ≤ 60km) of the subduction zone, which is based on the instrumental data of the Geodynamic Institute of the National Observatory of Athens (http://www.gein.noa.gr/, period 1990-2011). The catalogue consists of 12800 seismic events which correspond to 15 polygons of the aforementioned external seismic zones. These polygons define the subduction zone, as they are associated with the compressional stress field which characterizes a subducting regime. For each event, moment magnitude was calculated from ML according to the suggestions of Papazachos et al. (1997). The cumulative distribution functions of the inter-event times and the inter-event distances as well as the magnitude distribution for each seismic zone have been estimated, presenting a variation in the q-triplet along the Hellenic subduction zone. The models used, fit rather well to the observed

  3. Rapid intraplate strain accumulation in the New Madrid seismic zone

    USGS Publications Warehouse

    Liu, L.; Zoback, M.D.; Segall, P.

    1992-01-01

    Remeasurement of a triangulation network in the southern part of the New Madrid seismic zone with the Global Positioning System has revealed rapid crustal strain accumulation since the 1950s. This area experienced three large (moment magnitudes >8) earthquakes in 1811 to 1812. The orientation and sense of shear is consistent with right-lateral strike slip motion along a northeast-trending fault zone (as indicated by current seismicity). Detection of crustal strain accumulation may be a useful discriminant for identifying areas where potentially damaging intraplate earthquakes may occur despite the absence of large earthquakes during historic time.

  4. Towards a bayesian seismotectonic zoning for use in Probabilistic Seismic Hazard Assessment (PSHA)

    SciTech Connect

    Le Goff, Boris; Fitzenz, Delphine; Beauval, Celine

    2011-03-14

    The mathematical representation of seismic sources is an important part of probabilistic seismic hazard assessment. It reflects the association of the seismicity with the tectonically-active geological structures evidenced by seismotectonic studies. Given that most active faults are not characterized well enough, seismic sources are generally defined as areal zones, delimited with finite boundary polygons, within which the geological features of active tectonics and the seismicity are deemed homogeneous (e.g., focal depth, seismicity rate, and maximum magnitude). Besides the lack of data (e.g., narrow range of recorded magnitudes), the application of this representation generates different problems: 1) a large sensitivity of resulting hazard maps on the location of zone boundaries, while these boundaries are set by expert decision; 2) the zoning can not represent any variation in faulting mechanism; 3) the seismicity rates are distributed throughout the zones and we lose the location of the determinant information used for their calculation. We propose an exploratory study for an alternative procedure in area source modeling. First, different data (e.g., geomorphology, geology, fault orientations) will be combined by using automated spatial partitioning (investigation of both supervised and unsupervised methods) in order to obtain several information classes, which may be defined as areal source zones. Then, a given hypocenter belonging to a given ''zone'', from now on called seismicity model, will be expressed by a probability computed from the 2D (spatial) probability density function (pdf) for the active tectonic model used as an a priori and updated with specific data from seismicity catalogs (e.g., focal mechanism) or other new data sources (e.g., geomorphology, subsurface exploration). This hypocenter will thus be allowed to contribute to several models, with weights given by the value of the pdf for each model. The annual rate of occurrence, for a given

  5. Teleseismic Tomography in the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Agbaje, T.; Arroucau, P.; Vlahovic, G.; Powell, C. A.

    2011-12-01

    The Eastern Tennessee Seismic Zone (ETSZ) is the second most active seismic region in the eastern United States and is located in the southern Appalachian fold-and-thrust belt. The earthquakes mostly occur between 5 and 25 km depth, below the decollement surface, and tend to align along the New York Alabama magnetic lineament, a linear feature attributed to a strike-slip fault affecting the Precambrian basement but having no signature in surface geology. Recent results from local tomography also show some relationship between the body-wave velocity field and earthquake distribution down to about 20 km depth. In this work, we investigate the deep 3D P-wave velocity structure of the lithosphere in the ETSZ by means of teleseismic tomography We use seismograms recorded in the last 10 years at a local array of 30 short-period stations operated by the Center for Earthquake Research and Information (CERI) in Memphis, TN. Events with magnitude greater than 5.5 and epicentral distance greater than 2500 km were selected. Relative P-wave arrival time residuals were obtained from an adaptive stacking procedure and were subsequently used in a tomographic inversion to map the 3D P-wave velocity variations beneath the array.

  6. Seismic Holography of Solar Activity

    NASA Technical Reports Server (NTRS)

    Lindsey, Charles

    2000-01-01

    The basic goal of the project was to extend holographic seismic imaging techniques developed under a previous NASA contract, and to incorporate phase diagnostics. Phase-sensitive imaging gives us a powerful probe of local thermal and Doppler perturbations in active region subphotospheres, allowing us to map thermal structure and flows associated with "acoustic moats" and "acoustic glories". These remarkable features were discovered during our work, by applying simple acoustic power holography to active regions. Included in the original project statement was an effort to obtain the first seismic images of active regions on the Sun's far surface.

  7. Seismic gaps and source zones of recent large earthquakes in coastal Peru

    USGS Publications Warehouse

    Dewey, J.W.; Spence, W.

    1979-01-01

    The earthquakes of central coastal Peru occur principally in two distinct zones of shallow earthquake activity that are inland of and parallel to the axis of the Peru Trench. The interface-thrust (IT) zone includes the great thrust-fault earthquakes of 17 October 1966 and 3 October 1974. The coastal-plate interior (CPI) zone includes the great earthquake of 31 May 1970, and is located about 50 km inland of and 30 km deeper than the interface thrust zone. The occurrence of a large earthquake in one zone may not relieve elastic strain in the adjoining zone, thus complicating the application of the seismic gap concept to central coastal Peru. However, recognition of two seismic zones may facilitate detection of seismicity precursory to a large earthquake in a given zone; removal of probable CPI-zone earthquakes from plots of seismicity prior to the 1974 main shock dramatically emphasizes the high seismic activity near the rupture zone of that earthquake in the five years preceding the main shock. Other conclusions on the seismicity of coastal Peru that affect the application of the seismic gap concept to this region are: (1) Aftershocks of the great earthquakes of 1966, 1970, and 1974 occurred in spatially separated clusters. Some clusters may represent distinct small source regions triggered by the main shock rather than delimiting the total extent of main-shock rupture. The uncertainty in the interpretation of aftershock clusters results in corresponding uncertainties in estimates of stress drop and estimates of the dimensions of the seismic gap that has been filled by a major earthquake. (2) Aftershocks of the great thrust-fault earthquakes of 1966 and 1974 generally did not extend seaward as far as the Peru Trench. (3) None of the three great earthquakes produced significant teleseismic activity in the following month in the source regions of the other two earthquakes. The earthquake hypocenters that form the basis of this study were relocated using station

  8. The Salton Seismic Imaging Project: Seismic velocity structure of the Brawley Seismic Zone, Salton Buttes and Geothermal Field, Salton Trough, California

    NASA Astrophysics Data System (ADS)

    Delph, J.; Hole, J. A.; Fuis, G. S.; Stock, J. M.; Rymer, M. J.

    2011-12-01

    The Salton Trough is an active rift in southern California in a step-over between the plate-bounding Imperial and San Andreas Faults. In March 2011, the Salton Seismic Imaging Project (SSIP) investigated the rift's crustal structure by acquiring several seismic refraction and reflection lines. One of the densely sampled refraction lines crosses the northern-most Imperial Valley, perpendicular to the strike-slip faults and parallel to a line of small Quaternary rhyolitic volcanoes. The line crosses the obliquely extensional Brawley Seismic Zone and goes through one of the most geothermally productive areas in the United States. Well logs indicate the valley is filled by several kilometers of late Pliocene-recent lacustrine, fluvial, and shallow marine sediment. The 42-km long seismic line was comprised of eleven 110-460 kg explosive shots and receivers at a 100 m spacing. First arrival travel times were used to build a tomographic seismic velocity image of the upper crust. Velocity in the valley increases smoothly from <2 km/s to >5 km/s, indicating diagenesis and gradational metamorphism of rift sediments at very shallow depth due to an elevated geotherm. The velocity gradient is much smaller in the relatively low velocity (<6 km/s) crystalline basement comprised of recently metamorphosed sediment reaching greenschist to lower amphibolite facies. The depth of this basement is about 4-km below the aseismic region of the valley west of the Brawley Seismic Zone, but rises sharply to ~2 km depth beneath the seismically, geothermally, and volcanically active area of the Brawley Seismic Zone. The basement deepens to the northeast of the active tectonic zone and then is abruptly offset to shallower depth on the northeast side of the valley. This offset may be the subsurficial expression of a paleofault, most likely an extension of the Sand Hills Fault, which bounds the basin to the east. Basement velocity east of the fault is ~5.7 km/s, consistent with the granitic rocks

  9. On the mechanism of seismic decoupling and back are spreading at subduction zones

    SciTech Connect

    Scholz, C.H.; Campos, J.

    1995-11-10

    This report discusses a force model for the mechanics of seismic decoupling and back arc spreading at subduction zones. This model predicts three regimes: seismically coupled compressional arcs; seismically decoupled extensional arcs; and strongly extensional arcs with back arc spreading.

  10. Monitoring of stressed state in seismic-prone zones using vibroseismic interferometry method

    NASA Astrophysics Data System (ADS)

    Kovalevsky, V.

    2003-04-01

    Experiments with powerful seismic vibrators carried out in the Siberian Branch of RAS have shown the possibility to investigate the small changes of the tensely-deformed state of a seismic-prone zone of 300 - 500 km size. The method of vibroseismic interferometry was used in these experiments. It is based on the seismic sounding of the region by powerful seismic vibrators with the long-time narrow-band harmonic signal radiation. Changes of the tensely-deformed state are determined through variations of the amplitude- phase characteristics of the stationary wave fields, which are excited in a medium due to the long-time radiation of harmonic signals of constant frequency from the vibrator. The method of vibroseismic interferometry has high sensitivity to the time changes of parameters of the medium in the case of the long-distance observations. The influence of the lunar-solar tides deformations of the Earth's crust on the seismic waves velocities was investigated in the experiments with a 100-ton force seismic vibrator and recording systems of vibroseismic signals, located at distances of 356 - 430 km from a source. It was determined that the variations of the seismic waves velocities are about 10-5 - 10-6 and have 12- and 24-hour periodicity well correlated with the lunar-solar tides periodicity. This method can be efficiently used to define the first changes of the stress in the medium and location of the areas of such changes in the seismic-prone zone. Now an experimental system of active vibroseismic monitoring of the seismic-prone zones, which includes powerful 100- ton force vibrators, mobile seismic arrays for vibrosignals recording and computer systems for the vibromonitoring data processing is created.

  11. Spatial distribution of seismic energy rate of tectonic tremors in subduction zones

    NASA Astrophysics Data System (ADS)

    Yabe, Suguru; Ide, Satoshi

    2014-11-01

    The sizes of deep tectonic tremors have never been accurately evaluated as a physical quantity. Here we estimate tremor size as the band-limited seismic energy rate at 2-8 Hz, with accurate evaluation of the path attenuation and site amplification of seismic waves in four subduction zones: Nankai, Cascadia, Jalisco, and South Chile. The size-frequency statistics of seismic energy rate, which are characterized by the median measure for each subregion, are spatially variable. The spatial variations are categorized into three types, with each type corresponding to a different tremor migration behavior. In type A regions where tremor zone is wider, seismic energy rates are highly variable in the dip direction, and tremor activities are usually initiated in the less energetic tremor zone. Some of them further penetrate into the energetic tremor zone and subsequently migrate for long distances in the strike direction. Type B regions are characterized by relatively narrow tremor zones, minor variations in energy rates in the dip direction, and long-distance migration in the strike direction. Type C regions are characterized by isolated clusters of tremor activities without migration and by independent failure of each small tremor cluster. Given that the spatial distributions of tremor energy rates reflect heterogeneities in the strength of the plate interface, such distributions, which would be controlled by the width of tremor zone, may determine the regional style of slow-earthquake behavior. Some energetic tremor regions act as switches that trigger large slow slip events, especially in type A regions.

  12. Seismic measurements of the internal properties of fault zones

    USGS Publications Warehouse

    Mooney, W.D.; Ginzburg, A.

    1986-01-01

    The internal properties within and adjacent to fault zones are reviewed, principally on the basis of laboratory, borehole, and seismic refraction and reflection data. The deformation of rocks by faulting ranges from intragrain microcracking to severe alteration. Saturated microcracked and mildly fractured rocks do not exhibit a significant reduction in velocity, but, from borehole measurements, densely fractured rocks do show significantly reduced velocities, the amount of reduction generally proportional to the fracture density. Highly fractured rock and thick fault gouge along the creeping portion of the San Andreas fault are evidenced by a pronounced seismic low-velocity zone (LVZ), which is either very thin or absent along locked portions of the fault. Thus there is a correlation between fault slip behavior and seismic velocity structure within the fault zone; high pore pressure within the pronounced LVZ may be conductive to fault creep. Deep seismic reflection data indicate that crustal faults sometimes extend through the entire crust. Models of these data and geologic evidence are consistent with a composition of deep faults consisting of highly foliated, seismically anisotropic mylonites. ?? 1986 Birkha??user Verlag, Basel.

  13. Recurrent intraplate tectonism in the New Madrid seismic zone

    USGS Publications Warehouse

    Zoback, M.D.; Hamilton, R.M.; Crone, A.J.; Russ, D.P.; McKeown, F.A.; Brockman, S.R.

    1980-01-01

    For the first time, New Madrid seismicity can be linked to specific structural features that have been reactivated through geologic time. Extensive seismic reflection profiling reveals major faults coincident with the main earthquake trends in the area and with structural deformation apparently caused by repeated episodes of igneous activity. Copyright ?? 1980 AAAS.

  14. Tsunami potential assessment based on rupture zones, focal mechanisms and repeat times of strong earthquakes in the major Atlantic-Mediterranean seismic fracture zone

    NASA Astrophysics Data System (ADS)

    Agalos, Apostolos; Papadopoulos, Gerassimos A.; Kijko, Andrzej; Papageorgiou, Antonia; Smit, Ansie; Triantafyllou, Ioanna

    2016-04-01

    In the major Atlantic-Mediterranean seismic fracture zone, extended from Azores islands in the west to the easternmost Mediterranean Sea in the east, including the Marmara and Black Seas, a number of 22 tsunamigenic zones have been determined from historical and instrumental tsunami documentation. Although some tsunamis were produced by volcanic activity or landslides, the majority of them was generated by strong earthquakes. Since the generation of seismic tsunamis depends on several factors, like the earthquake size, focal depth and focal mechanism, the study of such parameters is of particular importance for the assessment of the potential for the generation of future tsunamis. However, one may not rule out the possibility for tsunami generation in areas outside of the 22 zones determined so far. For the Atlantic-Mediterranean seismic fracture zone we have compiled a catalogue of strong, potentially tsunamigenic (focal depth less than 100 km) historical earthquakes from various data bases and other sources. The lateral areas of rupture zones of these earthquakes were determined. Rupture zone is the area where the strain after the earthquake has dropped substantially with respect the strain before the earthquake. Aftershock areas were assumed to determine areas of rupture zones for instrumental earthquakes. For historical earthquakes macroseismic criteria were used such as spots of higher-degree seismic intensity and of important ground failures. For the period of instrumental seismicity, focal mechanism solutions from CMT, EMMA and other data bases were selected for strong earthquakes. From the geographical distribution of seismic rupture zones and the corresponding focal mechanisms in the entire Atlantic-Mediterranean seismic fracture zone we determined potentially tsunamigenic zones regardless they are known to have produced seismic tsunamis in the past or not. An attempt has been made to calculate in each one of such zones the repeat times of strong

  15. Is there a zone of weakness beneath the New Madrid and Wabash Valley Seismic Zones?

    NASA Astrophysics Data System (ADS)

    Chen, C.; Gilbert, H. J.; Pavlis, G. L.; Hamburger, M. W.; Yang, X.; Marshak, S.; Larson, T. H.

    2014-12-01

    The US midcontinent contains several intraplate seismic zones, including the New Madrid Seismic Zone (NMSZ) sitting above the Reelfoot Rift, and the Wabash Valley Seismic Zone (WVSZ), associated with a smaller Grayville Graben, stretching along the border between Illinois and Indiana. This study provides a new estimate of the velocity structure beneath the area based on observations from the Transportable Array and the OIINK (Ozarks-Illinois-INdiana-Kentucky) FlexArray experiment in the region. Numerous models, involving a zone of weakness either in the crust or the mantle, have been used to explain the seismicity in the NMSZ. Here we present a shear velocity model of the lithosphere beneath the midcontinent by inverting dispersion curves of fundamental mode Rayleigh waves, which are primarily sensitive to the shear wave speeds. We find no spatial correlation between the crustal velocity variations and the two seismic zones. But we do observe that low velocities (~ 4% lower than the measured average of the area) exist in the mantle beneath the NMSZ at depths between 90 and 125 km. The low upper mantle velocities extend to the north and reach the WVSZ, where they are about 3% lower than the average. Velocity variations can result from thermal or compositional heterogeneities, although a thermal perturbation is less likely in this area because no clear surface heat flow anomaly is observed. Compositional heterogeneities, such as the presence of hydrous minerals or contamination by enriched mantle from a plume can reduce seismic velocities as well as the mechanical strength of a region, which would produce a weak zone. The lithosphere beneath a failed rift which has already undergone an earlier phase of deformation is more susceptible to compositional modification and weakening compared to an intact part of a craton. Thus, the two seismic zones may mark locations where deformation has been localized in the crust above a weak mantle due to their lower integrated

  16. Velocity Structure in the West Bohemia Seismic Zone: Velocity Models Retrieved from different Earthquake Swarms

    NASA Astrophysics Data System (ADS)

    Alexandrakis, C.; Löberich, E.; Kieslich, A.; Calo, M.; Vavrycuk, V.; Buske, S.

    2015-12-01

    Earthquake swarms, fluid migration and gas springs are indications of the ongoing geodynamic processes within the West Bohemia seismic zone located at the Czech-German border. The possible relationship between the fluids, gas and seismicity is of particular interest and has motivated numerous past, ongoing and future studies, including a multidisciplinary monitoring proposal through the International Continental Scientific Drilling Program (ICDP). The most seismically active area within the West Bohemia seismic zone is located at the Czech town Nový Kostel. The Nový Kostel zone experiences frequent swarms of several hundreds to thousands of earthquakes over a period of weeks to several months. The seismicity is always located in the same area and depth range (~5-15 km), however the activated fault segments and planes differ. For example, the 2008 swarm activated faults along the southern end of the seismic zone, the 2011 swarm activated the northern segment, and the recent 2014 swarm activated the middle of the seismic zone. This indicates changes to the local stress field, and may relate to fluid migration and/or the complicated tectonic situation. The West Bohemia Seismic Network (WEBNET) is ideally located for studying the Nový Kostel swarm area and provides good azimuthal coverage. Here, we use the high quality P- and S-wave arrival picks recorded by WEBNET to calculate swarm-dependent velocity models for the 2008 and 2011 swarms, and an averaged (swarm independent) model using earthquakes recorded between 1991 and 2011. To this end, we use double-difference tomography to calculate P- and S-wave velocity models. The models are compared and examined in terms of swarm-dependent velocities and structures. Since the P-to-S velocity ratio is particularly sensitive to the presence of pore fluids, we derive ratio models directly from the inverted P- and S-wave models in order to investigate the potential influence of fluids on the seismicity. Finally, clustering

  17. Fault zones and seismicity in western Connecticut and southeastern New York

    NASA Astrophysics Data System (ADS)

    Tillman, J. E.

    1981-10-01

    Two previously unmapped, major fault zones were identified and investigated in southeastern New York and western Connecticut. These fault zones were clearly active in the Paleozoic and Mesozoic and probably also in the Cenozoic. Although surface rupture has not been associated with historic events along these zones, their locations do coincide with alignments of historic and instrumentally measured seismicity. Multiple episodes of epigenetic mineralization that occur on these faults indicate that their faulting and circulation histories are indeed complex and that a detailed record of their stress release history from the Paleozoic to the present can be obtained for each fault zone.

  18. Seismic signature of a hydrous mantle transition zone

    NASA Astrophysics Data System (ADS)

    Thio, Vincent; Cobden, Laura; Trampert, Jeannot

    2016-01-01

    Although water has a major influence on tectonic and other geodynamic processes, little is known about its quantity and distribution within the deep Earth. In the last few decades, laboratory experiments on nominally anhydrous minerals (NAMs) of the transition zone have shown that these minerals can contain significant amounts of water, up to 3.3 wt%. In this study, we investigate if it is possible to use seismic observations to distinguish between a hydrous and anhydrous transition zone. We perform an extensive literature search of mineral experimental data, to generate a compilation of the water storage capacities, elastic parameters and phase boundary data for potentially hydrous minerals in the transition zone, and use thermodynamic modelling to compute synthetic seismic profiles of density, VP and VS at transition zone temperatures and pressures. We find that large uncertainties on the mineral phase equilibria (ca. 2 GPa) and elastic properties produce a wide range of seismic profiles. In particular, there is a lack of data at temperatures corresponding to those along a 1300 °C adiabat or hotter, which may be expected at transition zone pressures. Comparing our hydrous transition zone models with equivalent profiles at anhydrous conditions, we see that the depths of the 410 and 660 discontinuities cannot at present be used to map the water content of the transition zone due to these uncertainties. Further, while average velocities and densities inside the transition zone clearly decrease with increasing water content, there is a near-perfect trade-off with increases in temperature. It is therefore difficult to distinguish thermal from water effects, and the conventional view of a slow and thick transition zone for water and slow and thin transition zone for high temperature should be regarded with caution. A better diagnostic for water may be given by the average velocity gradients of the transition zone, which increase with increasing water content (but

  19. Double seismic zone and dehydration embrittlement of the subducting slab

    NASA Astrophysics Data System (ADS)

    Yamasaki, Tadashi; Seno, Tetsuzo

    2003-04-01

    Dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate-depth earthquakes. We explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast Japan, southwest Japan, northeast Taiwan, northern Chile, Cape Mendocino, and eastern Aleutians. We calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions. We then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams. The depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age. The dehydration loci of serpentine produce a double-layered structure. Because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab, we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust, and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab. We find that the lower plane seismicity is located at the lower dehydration loci of serpentine, which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement.

  20. Seismic investigation of an ocean-continent transition zone in the northern South China Sea

    NASA Astrophysics Data System (ADS)

    Zhu, J.; Qiu, X.; Xu, H.; Zhan, W.; Sun, Z.

    2011-12-01

    Rifted continental margins and basins are mainly formed by the lithospheric extension. Thined lithosphere of passive continental margins results in decompression melt of magma and created oceanic crust and thined ocean-continent transition (OCT) zone. Two refraction profiles used ocean bottom seismometers deployed in the broad continental shelf and three multi-channel seismic reflection lines in the northern South China Sea, acquired by the ship "Shiyan 2" of the South China Sea Institute of Oceanology, Chinese Academy of Sciences in 2010, are processed and interpreted in this study. Seismic reflection lines cut through the Dongsha rise, Zhu-1 and Zhu-2 depression within a Tertiary basin, Pear River Mouth basin (called as Zhujiangkou basin). These tectonic features are clear imaged in the seismic reflection records. Numerous normal faults, cutted through the basement and related to the stretch of the northern South China Sea margin, are imaged and interpreted. Reflection characteristics of the ocean-continent transition (OCT) zone are summaried and outlined. The COT zone is mainly divided into the northern syn-rift subsidence zone, central volcano or buried volcano uplift zone and tilt faulted block near the South Chia Sea basin. Compared to the previous seismic reflection data and refraction velocity models, the segmentation range of the OCT zone is outlined, from width of about 225 km in the northeastern South China Sea , of 160 km in the central to of 110 km in the north-central South China Sea. Based on the epicenter distribution of sporadic and large than 6 magnitude earthquakes, it suggests the OCT zone in the northern South China Sea at present is still an active seismic zone.

  1. Major Existence of Very Low Frequency Earthquakes in Background Seismicity Along Subduction Zone of South-western Japan

    NASA Astrophysics Data System (ADS)

    Ishihara, Y.

    2003-12-01

    The condense, high quality and equalized broadband seismic network provided us to recognize the variety of seismic sources. The active volcanoes excite seismic waves with various frequency characteristics. Some cases show the long period seismic waves greater than 10 sec associates with volcanic activities. The tectonic seismic events originated at the close to trench zone are frequently lack of high frequency, greater than 1 Hz, seismic wave component. Meanwhile, the many low frequency earthquakes and tremors whose sources are not explicated are occurred in lower crust and subcrustal region. The subduction zone of Philippine Sea plate in south-western Japan is actively genetic area of low frequency earthquake group. The broadband seismic array of Japan region observed unknown long period ground motions. The seismograms are higher amplitude between 10 and 30 sec period than ground noise level. The earthquake JMA and USGS catalogues don_ft list about these long period seismograms. The arrival order of wave packet means that these events locate subduction zone around Japan. The hypocenters of unknown events are estimated by arrival times of vertical peak amplitude using the assumption that the ground motion dominates Rayleigh wave. The more detailed determination of major events is performed by combined technique for moment tensor inversion and grid search. The moment magnitude of uncatalogued event is greater than 3.5 because of the detection limitation. The largest event is distributed to about 4.5 Mw level and special event is greater than 5.0. The frequency characteristics show that source time is 7 to 20 sec by comparison with synthetic seismograms. We call these uncatalogued events _gvery low frequency earthquake_h. The hypocenters are located to two kinds of zones along the Philippine Sea subducting plate in south-western Japan. The one zone is very close to the trough. The seismicity listed by earthquake catalogues is low level in the zone and hypocenters are

  2. Storage of fluids and melts at subduction zones detectable by seismic tomography

    NASA Astrophysics Data System (ADS)

    Luehr, B. G.; Koulakov, I.; Rabbel, W.; Brotopuspito, K. S.; Surono, S.

    2015-12-01

    During the last decades investigations at active continental margins discovered the link between the subduction of fluid saturated oceanic plates and the process of ascent of these fluids and partial melts forming a magmatic system that leads to volcanism at the earth surface. For this purpose the geophysical structure of the mantle and crustal range above the down going slap has been imaged. Information is required about the slap, the ascent paths, as well as the reservoires of fluids and partial melts in the mantle and the crust up to the volcanoes at the surface. Statistically the distance between the volcanoes of volcanic arcs down to their Wadati Benioff zone results of approximately 100 kilometers in mean value. Surprisingly, this depth range shows pronounced seismicity at most of all subduction zones. Additionally, mineralogical laboratory investigations have shown that dehydration of the diving plate has a maximum at temperature and pressure conditions we find at around 100 km depth. The ascent of the fluids and the appearance of partial melts as well as the distribution of these materials in the crust can be resolved by seismic tomographic methods using records of local natural seismicity. With these methods these areas are corresponding to lowered seismic velocities, high Vp/Vs ratios, as well as increased attenuation of seismic shear waves. The anomalies and their time dependence are controlled by the fluids. The seismic velocity anomalies detected so far are within a range of a few per cent to more than 30% reduction. But, to explore plate boundaries large and complex amphibious experiments are required, in which active and passive seismic investigations should be combined to achieve best results. The seismic station distribution should cover an area from before the trench up to far behind the volcanic chain, to provide under favorable conditions information down to 150 km depth. Findings of different subduction zones will be compared and discussed.

  3. Pre-eruption deformation and seismic anomalies in 2012 in Tolbachik volcanic zone, Kamchatka

    NASA Astrophysics Data System (ADS)

    Kugaenko, Yulia; Saltykov, Vadim; Titkov, Nikolay

    2014-05-01

    Tolbachik volcanic zone (active volcano Plosky Tolbachik, dormant volcano Ostry Tolbachik and Tolbachik zone of cinder cones) is situated in the south part of Klyuchevskaya group of volcanoes in Kamchatka. All historical fissure eruptions of Tolbachik volcanic zone (1740, 1941, 1975-76 and 2012-13) were connected with one or another activity of Plosky Tolbachik volcano. In 1941 the fissure vent was occurred during the completion of 1939-41 terminal eruption of Plosky Tolbachik. In 1975 the Large Tolbachik Fissure Eruption (LTFE) was forestalled by Plosky Tolbachik terminal activity of the Hawaiian type and then was accompanied by the catastrophic collapse in the crater of Plosky Tolbachik. What events took place in the vicinity of Plosky Tolbachik in 2012 before the 2012-13 fissure eruption? In contrast of the 1975-76 LTFE the eruption 2012-13 was not preceded by intensive seismic preparation. Nowadays Klyuchevskaya group of volcanoes is under monitoring by 12 seismic stations, so we can investigate seismicity in details on the lower energy level then forty years ago. We analyzed seismicity of Plosky Tolbachik using regional catalogue 1999-2012. Anomalies of low-energy (M≥1.5) seismicity parameters (increase of seismicity rate and seismic energy) were discovered. This is evidence of seismic activization covered the whole Plosky Tolbachik volcano. The significance of this anomaly was estimated by distribution function of emitted seismic energy. Statistically significant transition of seismicity from background level to high and extremely high levels was revealed. It corresponds to multiple growth of earthquake number and seismic energy in 2012, July-November (five months before the eruption). The seismicity transition from background level to high level was happen in August 2012. During last three weeks before fissure eruption seismicity of analyzed seismoactive volume was on extremely high level. Earthquakes from fissure site directly appeared only on November 27

  4. Seismic no-data zone, offshore Mississippi delta: depositional controls on geotechnical properties, velocity structure, and seismic attenuation

    SciTech Connect

    May, J.A.; Meeder, C.A.; Tinkle, A.R.; Wener, K.R.

    1986-09-01

    Seismic acquisition problems plague exploration and production offshore the Mississippi delta. Geologic and geotechnical analyses of 300-ft borings and 20-ft piston cores, combined with subbottom acoustic measurements, help identify and predict the locations, types, and magnitudes of anomalous seismic zones. This knowledge is used to design acquisition and processing techniques to circumvent the seismic problems.

  5. Multiscale seismic imaging of the Western-Pacific subduction zone

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2011-12-01

    We used multiscale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific subduction zone. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). High-resolution local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by locating suboceanic events precisely with sP depth phases. Strong structural heterogeneity is revealed in the megathrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes including the great 2011 Tohoku-oki earthquake (Mw 9.0). A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. A big mantle wedge (BMW) has formed in the upper mantle above the stagnant slab. Convective circulations in the BMW and deep dehydration of the stagnant slab may have caused the intraplate volcanoes in NE Asia, such as the Changbai and Wudalianchi volcanoes. The active Tengchong volcanism in SW China is caused by a similar process in the BMW above the subducting Burma (or Indian) slab. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a

  6. On subduction zone earthquakes and the Pacific Northwest seismicity

    SciTech Connect

    Chung, Dae H.

    1991-12-01

    A short review of subduction zone earthquakes and the seismicity of the Pacific Northwest region of the United States is provided for the purpose of a basis for assessing issues related to earthquake hazard evaluations for the region. This review of seismotectonics regarding historical subduction zone earthquakes and more recent seismological studies pertaining to rupture processes of subduction zone earthquakes, with specific references to the Pacific Northwest, is made in this brief study. Subduction zone earthquakes tend to rupture updip and laterally from the hypocenter. Thus, the rupture surface tends to become more elongated as one considers larger earthquakes (there is limited updip distance that is strongly coupled, whereas rupture length can be quite large). The great Aleutian-Alaska earthquakes of 1957, 1964, and 1965 had rupture lengths of greater than 650 km. The largest earthquake observed instrumentally, the M{sub W} 9.5, 1960 Chile Earthquake, had a rupture length over 1000 km. However, earthquakes of this magnitude are very unlikely on Cascadia. The degree of surface shaking has a very strong dependency on the depth and style of rupture. The rupture surface during a great earthquake shows heterogeneous stress drop, displacement, energy release, etc. The high strength zones are traditionally termed asperities and these asperities control when and how large an earthquake is generated. Mapping of these asperities in specific subduction zones is very difficult before an earthquake. They show up more easily in inversions of dynamic source studies of earthquake ruptures, after an earthquake. Because seismic moment is based on the total radiated-energy from an earthquake, the moment-based magnitude M{sub W} is superior to all other magnitude estimates, such as M{sub L}, m{sub b}, M{sub bLg}, M{sub S}, etc Probably, just to have a common language, non-moment magnitudes should be converted to M{sub W} in any discussions of subduction zone earthquakes.

  7. Fault zone regulation, seismic hazard, and social vulnerability in Los Angeles, California: Hazard or urban amenity?

    NASA Astrophysics Data System (ADS)

    Toké, Nathan A.; Boone, Christopher G.; Arrowsmith, J. Ramón

    2014-09-01

    Public perception and regulation of environmental hazards are important factors in the development and configuration of cities. Throughout California, probabilistic seismic hazard mapping and geologic investigations of active faults have spatially quantified earthquake hazard. In Los Angeles, these analyses have informed earthquake engineering, public awareness, the insurance industry, and the government regulation of developments near faults. Understanding the impact of natural hazards regulation on the social and built geography of cities is vital for informing future science and policy directions. We constructed a relative social vulnerability index classification for Los Angeles to examine the social condition within regions of significant seismic hazard, including areas regulated as Alquist-Priolo (AP) Act earthquake fault zones. Despite hazard disclosures, social vulnerability is lowest within AP regulatory zones and vulnerability increases with distance from them. Because the AP Act requires building setbacks from active faults, newer developments in these zones are bisected by parks. Parcel-level analysis demonstrates that homes adjacent to these fault zone parks are the most valuable in their neighborhoods. At a broad scale, a Landsat-based normalized difference vegetation index shows that greenness near AP zones is greater than the rest of the metropolitan area. In the parks-poor city of Los Angeles, fault zone regulation has contributed to the construction of park space within areas of earthquake hazard, thus transforming zones of natural hazard into amenities, attracting populations of relatively high social status, and demonstrating that the distribution of social vulnerability is sometimes more strongly tied to amenities than hazards.

  8. Velocity Inhomogeneity of The Seismic Focal Zone In Kamchatka

    NASA Astrophysics Data System (ADS)

    Sanina, I.; Gontovaya, L.; Levina, V.; Stepanova, M.

    Kamchatka is situated in the junction zone of Kuril-Kamchatka and Aleutian island arcs. According to P.R.Vogt et.al the geometry of this conjunction is determined by the Hawaiian-Emperor Volcanic Seamount Chain. The Kluchevsky volcanic group be- longs to intersection of these structures on the Kamchatka Peninsula. Due to high yield of magmatic material this group can be considered as one of the so-called hot spots on Earth. We studied the velocity structure of the Earth's crust and upper mantle in the transition zone continent-Pacific Ocean up to the depth about 120 km. 3-D veloc- ity structure is reconstructed by seismic tomography method (SSA modification). We used regional catalogue of Kamchatka earthquakes for the period 1975-1999 recorded by 35 seismological stations within the territory of Kamchatka (Kuril-Kamchatka is- land arc) and the Bering Island. Data are carefully selected according to special re- quirements on depth, magnitude, accuracy in coordinates and origin time. The amount of data provides high resolution of observation pattern and accuracy of obtained veloc- ity residuals. Distributions of Vp and Vs velocities are presented for the depth ranges 0-20, 20-35, 35-60, 60-90 and 90-120 km, and for vertical crossections along and across the Kuril-Kamchatka island arc. Significant lateral and vertical inhomogeneity of the seismic focal zone is revealed. We identify narrow steep dipping low velocity zones, related to deep faults with continuation on the Peninsula. These zones sepa- rate crust and mantle blocks with high contrast within the focal layer. Usually strong earthquakes are related to these boundaries. The conjunction zone of the island arc and the Emperor fault (its continuation under Kamchatka) coincides with the low velocity zone in the mantle at depth 60-90 km, continued in the continental block. A chain of volcanoes oriented across Eastern-Kamchatka volcanic belt is related to this zone. Volcanic belt on the whole is shifted westwards in

  9. Landslides in the New Madrid seismic zone

    SciTech Connect

    Jibson, R.W.; Keefer, D.K.

    1985-01-01

    During the New Madrid earthquakes of 1811-12, bluffs bordering the Mississippi alluvial plain in the epicentral region underwent large-scale landsliding. Between Cairo, Illinois and Memphis, Tennessee, the authors mapped 221 large landslides of three types: (1) old, eroded, coherent block slides and slumps; (2) old earth flows; and (3) young, fresh slumps that occur only along near-river bluffs and are the only landslides present along such bluffs. Historical accounts and field evidence indicate that most or all old coherent slides and earth flows date to the 1811-12 earthquakes and that the only currently active, large-scale landsliding in the area occurs along bluffs bordering the river. Analysis of old coherent slides and earth flows indicates that landslide distribution is most strongly affected by slope height, but that proximity to the hypocenters of the 1811-12 earthquakes also has a significant effect. Slope-stability analyses of an old coherent slide and an earth flow selected as representative of the principal kinds of landslides present indicate that both were stable in aseismic conditions even when water tables were at highest possible levels. However, a dynamic Newmark displacement analysis shows that ground shaking such as that in 1811-12 would cause large displacements leading to catastrophic failure in both slides. These results indicate that in large earthquakes landsliding in much of the study are is likely. Moderate earthquakes may also trigger landslides at some locations.

  10. Neotectonic structure in the central new madrid seismic zone: Evidence from multimode seismic-reflection data

    USGS Publications Warehouse

    Woolery, E.W.; Street, R.L.; Wang, Z.; Harris, J.B.; McIntyre, J.

    1999-01-01

    Approximately 14.5 km of conventional P-wave and 2.2 km of horizontally polarized shear-wave seismic-reflection data acquired in the Kentucky Bend area of the central New Madrid Seismic Zone provide evidence of extensive neotectonic near-surface structure. The style and geometry of the deformation are consistent with documented historical geomorphic features, contemporary geomorphic features, and contemporary seismicity. The data image high-angle transpressional faults that strike between N30??W and N50??W. The fault planes exhibit apparent northeast and southwest dips. The opposing high-angle planes represent secondary splay or imbricate faults that responded to torsional bending of a lower-angle master fault.

  11. CPT site characterization for seismic hazards in the New Madrid seismic zone

    USGS Publications Warehouse

    Liao, T.; Mayne, P.W.; Tuttle, M.P.; Schweig, E.S.; Van Arsdale, R.B.

    2002-01-01

    A series of cone penetration tests (CPTs) were conducted in the vicinity of the New Madrid seismic zone in central USA for quantifying seismic hazards, obtaining geotechnical soil properties, and conducting studies at liquefaction sites related to the 1811-1812 and prehistoric New Madrid earthquakes. The seismic piezocone provides four independent measurements for delineating the stratigraphy, liquefaction potential, and site amplification parameters. At the same location, two independent assessments of soil liquefaction susceptibility can be made using both the normalized tip resistance (qc1N) and shear wave velocity (Vs1). In lieu of traditional deterministic approaches, the CPT data can be processed using probability curves to assess the level and likelihood of future liquefaction occurrence. ?? 2002 Elsevier Science Ltd. All rights reserved.

  12. Seismic imaging constraints on megathrust fault zone properties

    NASA Astrophysics Data System (ADS)

    Abers, G. A.; Janiszewski, H. A.; Keranen, K. M.; Saffer, D. M.; Shillington, D. J.

    2014-12-01

    Several lines of evidence suggest that subduction zone thrusts lie within overpressured channels. Seismic reflection data often shows a relatively thin, high-reflectivity surface with occasional bright spots, indicative of rapidly varying impedance contrasts over length scales of tens of meters. Scattered coda of teleseismic P waves, such as in receiver functions, often show a thin low-velocity layer corresponding to the top of the subducting plate. The latter have been best documented in Cascadia, where a 2-4 km thick very low velocity channel is seen above a moderately slow subducting crust, and in Alaska where similar structure has been seen. High-reflectivity bright spots occur in the same region, although perhaps over more limited areas. The low velocity zones are characterized by elevated Vp/Vs ratios (>2.0), and extend both throughout the locked, seismogenic fault zone and downdip into the region where episodic tremor and slip occur. Commonly, this combination of low velocities and high Vp/Vs is taken to indicate high pore pressures, and hence a fault zone that can withstand only very low shear stresses. However, models of the low wavespeeds suggest static porosities of 2-5% throughout a 2-4 km thick layer, extending to depths of 40 km, a situation that seems difficult to sustain. At both the Alaska and Cascadia margins, low Vp, high Poisson's ratios, and high anisotropies should result in part from the subduction of sediments well into and beyond the seismogenic zone. The presence of a significant thickness of subducted and underplated sediment is consistent with observations of preserved subduction "channels" in exhumed examples from tens of km depth. Although some elevation of pore pressure may be still needed to explain observations, if the subduction of 2-4 km of sediment is a significant factor in generating the seismic signatures, then the geophysical observations could reflect a much stronger thrust zone than one sustained by high pore pressure alone.

  13. Subduction zone locking, strain partitioning, intraplate deformation and their implications to Seismic Hazards in South America

    NASA Astrophysics Data System (ADS)

    Galgana, G. A.; Mahdyiar, M.; Shen-Tu, B.; Pontbriand, C. W.; Klein, E.; Wang, F.; Shabestari, K.; Yang, W.

    2014-12-01

    We analyze active crustal deformation in South America (SA) using published GPS observations and historic seismicity along the Nazca Trench and the active Ecuador-Colombia-Venezuela Plate boundary Zone. GPS-constrained kinematisc models that incorporate block and continuum techniques are used to assess patterns of regional tectonic deformation and its implications to seismic potential. We determine interplate coupling distributions, fault slip-rates, and intraplate crustal strain rates in combination with historic earthquakes within 40 seismic zones crust to provide moment rate constraints. Along the Nazca subduction zone, we resolve a series of highly coupled patches, interpreted as high-friction producing "asperities" beneath the coasts of Ecuador, Peru and Chile. These include areas responsible for the 2010 Mw 8.8 Maule Earthquake and the 2014 Mw 8.2 Iquique Earthquake. Predicted tectonic block motions and fault slip rates reveal that the northern part of South America deforms rapidly, with crustal fault slip rates as much as ~20 mm/a. Fault slip and locking patterns reveal that the Oca Ancón-Pilar-Boconó fault system plays a key role in absorbing most of the complex eastward and southward convergence patterns in northeastern Colombia and Venezuela, while the near-parallel system of faults in eastern Colombia and Ecuador absorb part of the transpressional motion due to the ~55 mm/a Nazca-SA plate convergence. These kinematic models, in combination with historic seismicity rates, provide moment deficit rates that reveal regions with high seismic potential, such as coastal Ecuador, Bucaramanga, Arica and Antofagasta. We eventually use the combined information from moment rates and fault coupling patterns to further constrain stochastic seismic hazard models of the region by implementing realistic trench rupture scenarios (see Mahdyiar et al., this volume).

  14. New Madrid Seismic Zone: a test case for naturally induced seismicity

    SciTech Connect

    Nava, S.J.

    1983-09-01

    Induced seismicity caused by man-made events, such as the filling of reservoirs has been well documented. In contrast, naturally induced seismicity has received little attention. It has been shown that a fluctuation of as little as several bars can trigger reservoir induced earthquakes. Naturally occurring phenomena generate similar fluctuations and could trigger earthquakes where the faults in ambient stress field are suitably oriented and close to failure. The New Madrid Seismic Zone (NMSZ) presents an ideal test case for the study of naturally induced seismicity. The ideal data set for a study of triggering effects must contain a statistically significant number of events, a constant accumulated strain, and a limited focal region. New Madrid earthquakes are well documented from 1974 to the present, down to a magnitude approx. 1.8. They lie in a distinct fault pattern and occur as a reaction to the regional stress regime. A statistical correlation was made between the earthquakes and a variety of different types of loads, to see if New Madrid seismicity could be triggered by natural fluctuations. The types of triggers investigated ranged from solid earth tides to variations in barometric pressure, rainfall, and stages of the Mississippi River. This analysis becomes complex because each factor investigated creates individual stresses, as well as having imbedded in it a reaction to other factors.

  15. A classification of morphoseismic features in the New Madrid seismic zone

    SciTech Connect

    Knox, R.; Stewart, D. . Dept. of Geosciences)

    1993-03-01

    The New Madrid Seismic Zone (NMSZ) contains thousands of surface features distributed over 5,000 square miles in four states. These are attributable to some combination of (1) seismically-induced liquefaction (SIL), (2) secondary deformation, and (3) seismically-induced slope failures. Most of these features were produced by the 1811--12 series of great earthquakes, but some predate and some postdate 1811--12. Subsequent non-seismic factors, such as hydrologically-induced liquefaction (HIL), mechanically-induced liquefaction (MIL), human activities, mass wasting, eolian and fluvial processes have modified all of these features. Morphoseismic features are new landforms produced by earthquakes, or are pre-existing landforms modified by them. Involved are complex interrelationships among several variables, including: (1) intensity and duration of seismic ground motion, (2) surface wave harmonics, (3) depth to water table, (4) depth to basement, (5) particle size, composition, and sorting of sediment making up the liquefied (LZ) and non-liquefied zones (NLZ), (6) topographic parameters, and (7) attitudes of beds and lenses susceptible to liquefaction. Morphoseismic features are depicted as results of a time-flow sequence initiated by primary basement disturbances which produce three major categories of surface response: secondary deformation, liquefaction and slope failure. Nine subcategories incorporate features produced by or resulting in: extruded sand, intruded sand, lateral spreading, faulting, subsidence of large areas, uplift of large areas, altered streams, coherent landslides, and incoherent landslides. The total morphoseismic features identified by this classification are 34 in number.

  16. Deep Structure and Earthquake Generating Properties in the Yamasaki Fault Zone Estimated from Dense Seismic Observation

    NASA Astrophysics Data System (ADS)

    Nishigami, K.; Shibutani, T.; Katao, H.; Yamaguchi, S.; Mamada, Y.

    2010-12-01

    We have been estimating crustal heterogeneous structure and earthquake generating properties in and around the Yamasaki fault zone, which is a left-lateral strike-slip active fault with a total length of about 80 km in southwest Japan. We deployed dense seismic observation network, composed of 32 stations with average spacing of 5-10 km around the Yamasaki fault zone. We estimate detailed fault structure such as fault dip and shape, segmentation, and possible location of asperities and rupture initiation point, as well as generating properties of earthquakes in the fault zone, through analyses of accurate hypocenter distribution, focal mechanism, 3-D velocity tomography, coda wave inversion, and other waveform analyses. We also deployed a linear seismic array across the fault, composed of 20 stations with about 20 m spacing, in order to delineate the fault-zone structure in more detail using the seismic waves trapped inside the low velocity zone. We also estimate detailed resistivity structure at shallow depth of the fault zone by AMT (audio-frequency magnetotelluric) and MT surveys. In the scattering analysis of coda waves, we used 2,391 wave traces from 121 earthquakes that occurred in 2002, 2003, 2008 and 2009, recorded at 60 stations, including dense temporary and routine stations. We estimated 3-D distribution of relative scattering coefficients along the Yamasaki fault zone. Microseismicity is high and scattering coefficient is relatively larger in the upper crust along the entire fault zone. The distribution of strong scatterers suggests that the Ohara and Hijima faults, which are the segments in the northwestern part of the Yamasaki fault zone, have almost vertical fault plane from surface to a depth of about 15 km. We used seismic network data operated by Universities, NIED, AIST, and JMA. This study has been carried out as a part of the project "Study on evaluation of earthquake source faults based on surveys of inland active faults" by Japan Nuclear

  17. Seismicity along Subduction Zones: Visualization with a Physical Basis

    NASA Astrophysics Data System (ADS)

    Chen, W.; Brudzinski, M. R.

    2002-12-01

    It is a common practice to plot epicenters and hypocenters as symbols of equal sizes on maps and cross-sections. While this representation is effective in relating seismicity to high-angle faults, it lacks a physical basis because equal weights are assigned to events that span several orders of magnitude in seismic moment. Furthermore, for cases such as subducted lithosphere where seismicity does not occur along major, through-going faults, "connecting the dots" leads to erroneous impressions of true seismogenic structures. Another common practice is to plot each event according to its magnitude. In principle, this preserves the size of earthquakes by using the logarithmic magnitude scale. The results, however, are not intuitive and the physical meaning of the magnitude scale is unclear. In contrast, we use simple scaling laws between fault area and seismic moment to plot seismicity in a way that conforms to the true scale of maps and cross-sections. The results are intuitive and often quite distinct from plots produced from common practices. We will illustrate the utility of our approach with examples from several different tectonic settings, including the large (Mw 7.6 and 7.7) August 19, 2002 Tonga deep earthquakes, configurations of sub-horizontal outboard earthquakes and complex Wadati-Benioff zones [Chen and Brudzinski, Science, v. 292, p. 2475, 2001], aftershock productivity of deep earthquakes [Wu and Chen, GRL, v. 26, p. 1977, 1999], and dual, out-of-sequence thrusts at mid- to lower-crustal depths [Kao and Chen, Science, v. 288, p. 2346, 2000].

  18. Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone.

    PubMed

    Calvert, Andrew J

    2004-03-11

    At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there. PMID:15014496

  19. Seismic exploration noise reduction in the Marginal Ice Zone.

    PubMed

    Tollefsen, Dag; Sagen, Hanne

    2014-07-01

    A sonobuoy field was deployed in the Marginal Ice Zone of the Fram Strait in June 2011 to study the spatial variability of ambient noise. High noise levels observed at 10-200 Hz are attributed to distant (1400 km range) seismic exploration. The noise levels decreased with range into the ice cover; the reduction is fitted by a spreading loss model with a frequency-dependent attenuation factor less than for under-ice interior Arctic propagation. Numerical modeling predicts transmission loss of the same order as the observed noise level reduction and indicates a significant loss contribution from under-ice interaction. PMID:24993237

  20. Seismic Evidence of A Widely Distributed West Napa Fault Zone, Hendry Winery, Napa, California

    NASA Astrophysics Data System (ADS)

    Goldman, M.; Catchings, R.; Chan, J. H.; Criley, C.

    2015-12-01

    Following the 24 August 2014 Mw 6.0 South Napa earthquake, surface rupture was mapped along the West Napa Fault Zone (WNFZ) for a distance of ~ 14 km and locally within zones up to ~ 2 km wide. Near the northern end of the surface rupture, however, several strands coalesced to form a narrow, ~100-m-wide zone of surface rupture. To determine the location, width, and shallow (upper few hundred meters) geometry of the fault zone, we acquired an active-source seismic survey across the northern surface rupture in February 2015. We acquired both P- and S-wave data, from which we developed reflection images and tomographic images of Vp, Vs, Vp/Vs, and Poisson's ratio of the upper 100 m. We also used small explosive charges within surface ruptures located ~600 m north of our seismic array to record fault-zone guided waves. Our data indicate that at the latitude of the Hendry Winery, the WNFZ is characterized by at least five fault traces that are spaced 60 to 200 m apart. Zones of low-Vs, low-Vp/Vs, and disrupted reflectors highlight the fault traces on the tomography and reflection images. On peak-ground-velocity (PGV) plots, the most pronounced high-amplitude guided-wave seismic energy coincides precisely with the mapped surface ruptures, and the guided waves also show discrete high PGV zones associated with unmapped fault traces east of the surface ruptures. Although the surface ruptures of the WNFZ were observed only over a 100-m-wide zone at the Hendry Winery, our data indicate that the fault zone is at least 400 m wide, which is probably a minimum width given the 400-m length of our seismic profile. Slip on the WNFZ is generally considered to be low relative to most other Bay Area faults, but we suggest that the West Napa Fault is a zone of widely distributed shear, and to fully account for the total slip on the WNFZ, slip on all traces of this wide fault zone must be considered.

  1. Spatio-temporal changes of seismic anisotropy in seismogenic zones

    NASA Astrophysics Data System (ADS)

    Saade, M.; Montagner, J.; Roux, P.; Paul, C.; Brenguier, F.; Enescu, B.; Shiomi, K.

    2013-12-01

    Seismic anisotropy plays a key role in the study of stress and strain fields in the earth. Potential temporal change of seismic anisotropy can be interpreted as change of the orientation of cracks in seismogenic zones and thus change of the stress field. Such temporal changes have been observed in seismogenic zones before and after earthquakes (Durand et al. , 2011) but are still not well understood. In this study, from a numerical point of view, we investigate the variations of the polarization of surface waves in anisotropic media. These variations are related to the elastic properties of the medium, in particular to anisotropy. The technique used is based on the calculation of the whole cross-correlation tensor (CCT) of ambient seismic noise. If the sources are randomly distributed in homogeneous medium, it allows us to reconstruct the Green's tensor between two stations continuously and to monitor the region through the use of its fluctuations. Therefore, the temporal change of the Green's cross-correlation tensor enables the monitoring of stress and strain fields. This technique is applied to synthetic seismograms computed in a transversally isotropic medium with horizontal symmetry axis (hereafter referred to an HTI medium) using a code RegSEM (Cupillard et al. , 2012) based on the spectral element method. We designed an experiment in order to investigate the influence of anisotropy on the CCT. In homogeneous, isotropic medium the off-diagonal terms of the Green's tensor are null. The CCT is computed between each pair of stations and then rotated in order to approximate the Green's tensor by minimizing the off-diagonal components. This procedure permits the calculation of the polarization angle of quasi-Rayleigh and quasi-Love waves, and to observe the azimuthal variation of their polarization. The results show that even a small variation of the azimuth of seismic anisotropy with respect to a certain pair of stations can induce, in some cases, a large

  2. Methods of seismic zone localization in the highly stressed geological environment in mining natural-engineering system

    NASA Astrophysics Data System (ADS)

    Kozyrev, A.; Fedotova, Iu.; Zhuravleva, O.

    2012-04-01

    was carried out of changing parameters of seismicity in the Kukisvumchorrsky and Uksporsky sides of the United Kirovsky mine owned by "Apatit" JSC. As a result there were determined an average value of a complex assessment before large seismic events (106-108J), as well as minimum and maximum values. Based on investigations carried out there was made a classification of seismic active zones according to types of their possible development. Four types of seismic active zones were determined: increasing, pulsatory, stable and attenuated ones. The most dangerous rock mass parts are those with increasing and pulsatory seismic active zones revealed. Seismic active zone can transfer from one type to another, i.e., there can be selected stages of preparation of a large seismic event or series of events, what is an important prediction stage. Thus, based on analysis of dynamics of complex assessment and configuration of seismic active zones there can be determined periods of rock mass parts transition into the hazardous or safe state. Diminution of complex assessment values indicates usually beginning of rock mass parts transition into the safe state. It is a basis for development of preventive measures to prevent manifestations of mining-induced seismicity.

  3. Seismic velocity structure in the western part of Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Yamamoto, Y.; Obana, K.; Takahashi, T.; Nakanishi, A.; Kodaira, S.; Kaneda, Y.

    2011-12-01

    In the Nankai Trough, three major seismogenic zones of megathrust earthquake exist (Tokai, Tonankai and Nankai earthquake regions). The Hyuga-nada region was distinguished from these seismogenic zones because of the lack of megathrust earthquake. However, recent studies show the possibility of simultaneous rupture of the Nankai and Hyuga-nada segments was also pointed out [e.g., Furumura et al, 2010 JGR]. Because seismic velocity structure is one of the useful and basic information for understanding the possibility of seismic linkage of Nankai and Hyuga-nada segments, Japan Agency for Marine-Earth Science and Technology has been carried out a series of wide-angle active source surveys and local seismic observations among the three major seismogenic zones and Hyuga-nada segment from 2008, as a part of "Research concerning Interaction Between the Tokai, Tonankai and Nankai Earthquakes' funded by Ministry of Education, Culture, Sports, Science and Technology, Japan". We are performing two set of three-dimensional seismic velocity tomographic inversions, one is in the Hyuga-nada region and the other is western part of the coseismic rupture area of 1946 Nankai earthquake, to discuss the relationship between the structural heterogeneities and the location of segment boundary between Hyuga-nada and Nankai segment. For the analysis of Hyuga-nada segment, we used both active and passive source data. The obtained velocity model clearly showed the subducted Kyushu-Palau ridge as thick low velocity Philippine Sea slab in the southwestern part. Our velocity image also indicates that "the thin oceanic crust zone" located between Nankai segment and Kyushu-Palau Ridge segment, founded by Nakanishi et al [2010, AGU] by analyzing of the active source survey, continuously exists from trough axis to near the coastline of Kyushu Island. The overriding plate just above the coseismic slip area of 1968 Hyuga-nada earthquake shows relatively high velocity. Although the tomographic study in

  4. Seismicity and structure of Nazca Plate subduction zone in southern Peru

    NASA Astrophysics Data System (ADS)

    Lim, H.; Kim, Y.; Clayton, R. W.

    2015-12-01

    We image the Nazca plate subduction zone system by detecting and (re)locating intra-slab earthquakes in southern Peru. Dense seismic arrays (PeruSE, 2013) were deployed along four lines to target geophysical characterization of the subduction system in the transition zone between flat and normal dipping segments of the Nazca plate (2-15°S). The arc volcanism is absent near the flat slab segment, and currently, the correlation between the location of the active volcanic front and corresponding slab depth is neither clear nor consistent between previously published models from seismicity. We detect 620 local earthquakes from August 2008 to February 2013 by manually picking 6559 and 4145 arrival times for P- and S-phases, respectively. We observe that the S-phase data is helpful to reduce the trade-off between origin time and depth of deeper earthquakes (>100 km). Earthquake locations are relocated to constrain the Nazca slab-mantle interface in the slab-dip transition zone using 7322 measurements of differential times of nearby earthquake pairs by waveform cross-correlation. We also employ the double-difference tomography (Zhang and Thurber, 2003) to further improve earthquake source locations and the spatial resolution of the velocity structure simultaneously. The relocated hypocenters clearly delineate the dipping Wadati-Benioff zone in the slab-dip transition zone between the shallow- (25°) to-flat dipping slab segment in the north and the normal (40°) dipping segment in the south. The intermediate-depth seismicity in the flat slab region stops at a depth of ~100 km and a horizontal distance of ~400 km from the trench. We find a significant slab-dip difference (up to 10°) between our relocated seismicity and previously published slab models along the profile region sampling the normal-dip slab at depth (>100 km).

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

    USGS Publications Warehouse

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

    2001-01-01

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

  6. Waveform cross-correlation and relocations for seismic events in the San Jacinto Fault Zone

    NASA Astrophysics Data System (ADS)

    Galipchak, E.; Kurzon, I.; Vernon, F.; Pavlis, G. L.; Ben-Zion, Y.

    2012-12-01

    We introduce a new approach for the relocation of local seismic events using waveform cross-correlation and automatic detection algorithm. This approach is developed and implemented for the San Jacinto Fault Zone (SJFZ), where recent cross-correlation and double-difference relocation methods (e.g., Hauksson et al. 2011) account up to ~75% of the seismic events, due to the complex nature of the SJFZ. The fault zone complexity features include a mismatch between the fault traces and seismicity clusters and strong heterogeneity of focal mechanisms. Our goal is to develop an efficient relocation method in which ~90% of the seismic events would be considered. The 'dbxcor' tool of the Antelope software package (e.g., Pavlis & Vernon 2010) is a graphic cross-correlation method involving an active reviewing of the cross-correlation process by a seismic analyst. The method is adjusted here for the analysis of local events from the original algorithm developed mainly for the processing of teleseismic events. The advantage of this approach is that the analyst may keep many of the waveforms that would have been dropped out due to the cross-correlation threshold, thereby increasing the percentage of events considered in the process. Moreover, the method allows an interactive demonstration and identification of different nearby source mechanisms, thus helping to examine the heterogeneity of the fault zone. A pre-request of the cross-correlation algorithm is the existence of arrivals for each waveform in the process. This required tuning a set of efficient automated detectors for grasping the specific nature of seismicity in the SJFZ. Applying such detectors we manage to increase the catalogue by up to 40% of additional events not reviewed previously by analysts. This improvement allows incorporating not only more events into the relocation process, but also additional stations, which were missed by previous automatic or manual picking of P and S arrivals. The relocated events

  7. Shallow seismicity patterns in the northwestern section of the Mexico Subduction Zone

    NASA Astrophysics Data System (ADS)

    Abbott, Elizabeth R.; Brudzinski, Michael R.

    2015-11-01

    This study characterizes subduction related seismicity with local deployments along the northwestern section of the Mexico Subduction Zone where 4 portions of the plate interface have ruptured in 1973, 1985, 1995, and 2003. It has been proposed that the subducted boundary between the Cocos and Rivera plates occurs beneath this region, as indicated by inland volcanic activity, a gap in tectonic tremor, and the Manzanillo Trough and Colima Graben, which are depressions thought to be associated with the splitting of the two plates after subduction. Data from 50 broadband stations that comprised the MARS seismic array, deployed from January 2006 to June 2007, were processed with the software program Antelope and its generalized source location algorithm, genloc, to detect and locate earthquakes within the network. Slab surface depth contours from the resulting catalog indicate a change in subduction trajectory between the Rivera and Cocos plates. The earthquake locations are spatially anti-correlated with tectonic tremor, supporting the idea that they represent different types of fault slip. Hypocentral patterns also reveal areas of more intense seismic activity (clusters) that appear to be associated with the 2003 and 1973 megathrust rupture regions. Seismicity concentrated inland of the 2003 rupture is consistent with slip on a shallowly dipping trajectory for the Rivera plate interface as opposed to crustal faulting in the overriding North American plate. A prominent cluster of seismicity within the suspected 1973 rupture zone appears to be a commonly active portion of the megathrust as it has been active during three previous deployments. We support these interpretations by determining focal mechanisms and detailed relocations of the largest events within the 1973 and inland 2003 clusters, which indicate primarily thrust mechanisms near the plate interface.

  8. Probing the Detailed Seismic Velocity Structure of Subduction Zones Using Advanced Seismic Tomography Methods

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Thurber, C. H.

    2005-12-01

    Subduction zones are one of the most important components of the Earth's plate tectonic system. Knowing the detailed seismic velocity structure within and around subducting slabs is vital to understand the constitution of the slab, the cause of intermediate depth earthquakes inside the slab, the fluid distribution and recycling, and tremor occurrence [Hacker et al., 2001; Obara, 2002].Thanks to the ability of double-difference tomography [Zhang and Thurber, 2003] to resolve the fine-scale structure near the source region and the favorable seismicity distribution inside many subducting slabs, it is now possible to characterize the fine details of the velocity structure and earthquake locations inside the slab, as shown in the study of the Japan subduction zone [Zhang et al., 2004]. We further develop the double-difference tomography method in two aspects: the first improvement is to use an adaptive inversion mesh rather than a regular inversion grid and the second improvement is to determine a reliable Vp/Vs structure using various strategies rather than directly from Vp and Vs [see our abstract ``Strategies to solve for a better Vp/Vs model using P and S arrival time'' at Session T29]. The adaptive mesh seismic tomography method is based on tetrahedral diagrams and can automatically adjust the inversion mesh according to the ray distribution so that the inversion mesh nodes are denser where there are more rays and vice versa [Zhang and Thurber, 2005]. As a result, the number of inversion mesh nodes is greatly reduced compared to a regular inversion grid with comparable spatial resolution, and the tomographic system is more stable and better conditioned. This improvement is quite valuable for characterizing the fine structure of the subduction zone considering the highly uneven distribution of earthquakes within and around the subducting slab. The second improvement, to determine a reliable Vp/Vs model, lies in jointly inverting Vp, Vs, and Vp/Vs using P, S, and S

  9. Stress development in heterogenetic lithosphere: Insights into earthquake processes in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Zhan, Yan; Hou, Guiting; Kusky, Timothy; Gregg, Patricia M.

    2016-03-01

    The New Madrid Seismic Zone (NMSZ) in the Midwestern United States was the site of several major M 6.8-8 earthquakes in 1811-1812, and remains seismically active. Although this region has been investigated extensively, the ultimate controls on earthquake initiation and the duration of the seismicity remain unclear. In this study, we develop a finite element model for the Central United States to conduct a series of numerical experiments with the goal of determining the impact of heterogeneity in the upper crust, the lower crust, and the mantle on earthquake nucleation and rupture processes. Regional seismic tomography data (CITE) are utilized to infer the viscosity structure of the lithosphere which provide an important input to the numerical models. Results indicate that when differential stresses build in the Central United States, the stresses accumulating beneath the Reelfoot Rift in the NMSZ are highly concentrated, whereas the stresses below the geologically similar Midcontinent Rift System are comparatively low. The numerical observations coincide with the observed distribution of seismicity throughout the region. By comparing the numerical results with three reference models, we argue that an extensive mantle low velocity zone beneath the NMSZ produces differential stress localization in the layers above. Furthermore, the relatively strong crust in this region, exhibited by high seismic velocities, enables the elevated stress to extend to the base of the ancient rift system, reactivating fossil rifting faults and therefore triggering earthquakes. These results show that, if boundary displacements are significant, the NMSZ is able to localize tectonic stresses, which may be released when faults close to failure are triggered by external processes such as melting of the Laurentide ice sheet or rapid river incision.

  10. Deep Structure of the Northern Cascadia Subduction Zone From Reflection, Tomography and Seismicity Studies

    NASA Astrophysics Data System (ADS)

    Nedimović, M. R.; Ramachandran, K.; Hyndman, R. D.

    2002-12-01

    To study the structure of southwestern British Columbia and northwestern Washington State, a multidisciplinary seismic survey named SHIPS (Seismic Hazards Investigation in Puget Sound) was carried out in 1998. The main objective was to map active crustal faults in the high seismicity region of Strait of Juan de Fuca, Georgia Strait and Puget Sound, and to gather information about other earthquake controlling structures such as are the position and nature of the subducted Juan de Fuca oceanic plate. We carried out a comparative analysis of the reflection, tomography and seismicity results for the Strait of Juan de Fuca region. Shallow forearc sedimentary basins of glacial and tectonic origin are well outlined on reflection sections. Leech River Fault and southern Whidbey Island Fault are imaged directly. Devils Mountain Fault is indirectly imaged on several profiles by an offset in the basement structure. At greater depth, a thick group of gently landward dipping events is present in the reflection images: The "E" reflection zone previously detected on Lithoprobe data. We believe that this reflection band, earlier interpreted as a shear zone, is situated just above the subducted slab. We use it to map the topography of the subducted oceanic crust. Because oceanic Moho is visible on reflection profiles only within the western edge of the survey area, we also use tomography and seismicity results to delineate it. The reflection, tomography and seismicity results are in good agreement and confirm the existence of an upward bulge in the subducting oceanic crust beneath northwestern Washington. By integrating our results with previous reflection profiles across the accreted wedge and Vancouver Island, we study the structure of the subducted Juan de Fuca oceanic slab and the nature of its contact with the overriding North America plate, from the deformation front to the forearc Moho.

  11. Seismo-thermo-mechanical modeling of subduction zone seismicity

    NASA Astrophysics Data System (ADS)

    van Dinther, Ylona; Gerya, Taras; Dalguer, Luis; Mai, Martin

    2014-05-01

    Recent megathrust earthquakes, e.g., the 2011 M9.0 Tohoku and the 2004 M9.2 Sumatra events, illustrated both their disastrous human and economic impact and our limited physical understanding of their spatial occurrence. To improve long-term seismic hazard assessment by overcoming the restricted direct observations in time and space, we developed a new numerical seismo-thermo-mechanical (STM) modeling approach. This approach may help to shed light onto the interaction between long-term subduction dynamics and deformation and associated short-term seismicity. Additional advantages of this STM approach include the physically consistent emergence of rupture paths, both on- and off-megathrust, and the inclusion of three key ingredients for seismic cycling --rate-dependent friction, slow tectonic loading, and visco-elastic relaxation--. The validation of this approach was accomplished through a comparison with a laboratory seismic cycle model (van Dinther et al., JGR, 2013a). A more realistic geometry and physical setup of the Southern Chilean margin showed that results also agree with a range of seismological, geodetic, and geological observations, albeit at lower coseismic speeds (van Dinther et al., JGR, 2013b). This setup also suggests that a) ~5% of cyclic deformation is being stored on the long-term, b) a self-consistent downdip transition zone between 350°C and 450°C arises from temperature-dependent viscosity, and c) megathrusts are weak (i.e., pore fluid pressures of ˜75% to 99% of that of solid pressures). After introducing the main features of this innovative approach, this study focuses on analyzing the spontaneous unstable rupturing of off-megathrust events. Shallow off-megathrust subduction events are important in terms of hazard assessment and coseismic energy budget. The characteristics of simulated normal events within the outer-rise and splay and normal antithetic events within the wedge resemble seismic and seismological observations in terms of

  12. Fault zone structure and seismic reflection characteristics in zones of slow slip and tsunami earthquakes

    NASA Astrophysics Data System (ADS)

    Bell, Rebecca; Henrys, Stuart; Sutherland, Rupert; Barker, Daniel; Wallace, Laura; Holden, Caroline; Power, William; Wang, Xiaoming; Morgan, Joanna; Warner, Michael; Downes, Gaye

    2015-04-01

    Over the last couple of decades we have learned that a whole spectrum of different fault slip behaviour takes place on subduction megathrust faults from stick-slip earthquakes to slow slip and stable sliding. Geophysical data, including seismic reflection data, can be used to characterise margins and fault zones that undergo different modes of slip. In this presentation we will focus on the Hikurangi margin, New Zealand, which exhibits marked along-strike changes in seismic behaviour and margin characteristics. Campaign and continuous GPS measurements reveal deep interseismic coupling and deep slow slip events (~30-60 km) at the southern Hikurangi margin. The northern margin, in contrast, experiences aseismic slip and shallow (<10-15 km) slow slip events (SSE) every 18-24 months with equivalent moment magnitudes of Mw 6.5-6.8. Updip of the SSE region two unusual megathrust earthquakes occurred in March and May 1947 with characteristics typical of tsunami earthquakes. The Hikurangi margin is therefore an excellent natural laboratory to study differential fault slip behaviour. Using 2D seismic reflection, magnetic anomaly and geodetic data we observe in the source areas of the 1947 tsunami earthquakes i) low amplitude interface reflectivity, ii) shallower interface relief, iii) bathymetric ridges, iv) magnetic anomaly highs and in the case of the March 1947 earthquake v) stronger geodetic coupling. We suggest that this is due to the subduction of seamounts, similar in dimensions to seamounts observed on the incoming Pacific plate, to depths of <10 km. We propose a source model for the 1947 tsunami earthquakes based on geophysical data and find that extremely low rupture velocities (c. 300 m/s) are required to model the observed large tsunami run-up heights (Bell et al. 2014, EPSL). Our study suggests that subducted topography can cause the nucleation of moderate earthquakes with complex, low velocity rupture scenarios that enhance tsunami waves, and the role of

  13. Late Pleistocene and Holocene paleoseismology of an intraplate seismic zone in a large alluvial valley, the New Madrid seismic zone, Central USA

    NASA Astrophysics Data System (ADS)

    Guccione, Margaret J.

    2005-10-01

    . Seven fault segments are recognized by microseismicity and geomorphology. Surface faulting has been recognized at three of these segments, Reelfoot fault, New Madrid North fault, and Bootheel fault. The Reelfoot fault is a compressive stepover along the strike-slip fault and has up to 11 m of surface relief ([Carlson, S.D., 2000. Formation and geomorphic history of Reelfoot Lake: insight into the New Madrid seismic zone. M.S. Thesis, University of Arkansas, Fayetteville, Arkansas, U.S.A]) deforming abandoned and active Mississippi River channels ([Guccione, M.J., Mueller, K., Champion, J., Shepherd, S., Odhiambo, B., 2002b. Stream response to repeated co-seismic folding, Tiptonville dome, western Tennessee. Geomorphology 43 (2002), 313 349]). The New Madrid North fault apparently has only strike-slip motion and is recognized by modern microseismicity, geomorphic anomalies, and sand cataclasis ([Baldwin, J.N., Barron A.D., Kelson, K.I., Harris, J.B., Cashman, S., 2002. Preliminary paleoseismic and geophysical investigation of the North Farrenburg lineament: primary tectonic deformation associated with the New Madrid North Fault?. Seismological Research Letters 73, 393 413]). The Bootheel fault, which is not identified by the modern microseismicity, is associated with extensive liquefaction and offset channels ([Guccione, M.J., Marple, R., Autin, W.J., 2005, Evidence for Holocene displacements on the Bootheel fault (lineament) in southeastern Missouri: Seismotectonic implications for the New Madrid region. Geological Society of America Bulletin 117, 319 333]). The fault has dominantly strike-slip motion but also has a vertical component of slip. Other recognized surface deformation includes relatively low-relief folding at Big Lake/Manila high ([Guccione, M.J., VanArdale, R.B., Hehr, L.H., 2000. Origin and age of the Manila high and associated Big Lake “Sunklands”, New Madrid seismic zone, northeastern Arkansas. Geological Society of America Bulletin 112, 579 590

  14. Transient Deformation, Stress and Strain in the South Iceland Seismic Zone

    NASA Astrophysics Data System (ADS)

    Arnadottir, T.; Decriem, J.; Hreinsdottir, S.; Michalczewska, K.; Hensch, M.; Lund, B.; Geirsson, H.; Ofeigsson, B. G.

    2012-12-01

    The South Iceland Seismic Zone (SISZ) is an ~80 km long E-W transform zone in SW Iceland that accommodates E-W left-lateral shear at depth by faulting on a number of 10-20 km long N-S, right lateral strike slip faults in the brittle crust. Earthquake sequences in the SISZ vary in duration, location, number of events, and magnitudes. For example, the 1896 sequence included five Ms6-6.9 main shocks in the SISZ during a period of 2 weeks, followed by a sixteen-year repose until an Ms7 earthquake struck the eastern-most part of the zone in 1912. We present a simple conceptual model of the seismic cycle in the SISZ based on estimated size of historical earthquakes. The model compares the accumulated geometric moment due to plate motion to the seismic moment release in M>6 events estimated back to the 1600's. The model indicates that accumulated moment is completely released in each earthquake sequence. Two series of main shocks have occurred in the SISZ in recent years. The first in June 2000 when two Mw6.5 earthquakes ruptured two faults in the central part of the zone, located about 17 km apart within a span of three days, and the second on 29 May 2008 when two Mw6 earthquakes ruptured faults spaced 4 km apart within 3 seconds, in the western part of the zone. Geodetic source models and aftershock locations indicate that the June 2000 and May 2008 main shocks ruptured 10-15 km long N-S right-lateral strike slip faults, with the slip occurring above 10 km and 5 km depths, respectively. Since the June 2000 main shock, annual GPS measurements have been conducted in the SISZ. The deformation transient following the May 2008 main shocks extends over a fairly large area compared to the June 2000 post-seismic signal, with changes continuing in 2012. The GPS data also indicate anomalously high strain rates in the May 2008 epicentral area during 2004-2007. Analysis of micro-seismicity and focal mechanisms are used to estimate stress tensor orientation and search for possible

  15. Seismicity, shear failure and modes of deformation in deep subduction zones

    NASA Technical Reports Server (NTRS)

    Lundgren, Paul R.; Giardini, Domenico

    1992-01-01

    The joint hypocentral determination method is used to relocate deep seismicity reported in the International Seismological Center catalog for earthquakes deeper than 400 km in the Honshu, Bonin, Mariannas, Java, Banda, and South America subduction zones. Each deep seismic zone is found to display planar features of seismicity parallel to the Harvard centroid-moment tensor nodal planes, which are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration.

  16. Gravity anomalies, forearc morphology and seismicity in subduction zones

    NASA Astrophysics Data System (ADS)

    Bassett, D.; Watts, A. B.; Das, S.

    2012-12-01

    We apply spectral averaging techniques to isolate and remove the long-wavelength large-amplitude trench-normal topographic and free-air gravity anomaly "high" and "low" associated with subduction zones. The residual grids generated illuminate the short-wavelength structure of the forearc. Systematic analysis of all subduction boundaries on Earth has enabled a classification of these grids with particular emphasis placed on topography and gravity anomalies observed in the region above the shallow seismogenic portion of the plate interface. The isostatic compensation of these anomalies is investigated using 3D calculations of the gravitational admittance and coherence. In the shallow region of the megathrust, typically within 100 km from the trench, isolated residual anomalies with amplitudes of up to 2.5 km and 125 mGal are generally interpreted as accreted/subducting relief in the form of seamounts and other bathymetric features. While most of these anomalies, which have radii < 50km, are correlated with areas of reduced seismicity, several in regions such as Japan and Java appear to have influenced the nucleation and/or propagation of large magnitude earthquakes. Long-wavelength (500 - >1000 km) trench-parallel forearc ridges with residual anomalies of up to 1.5 km and 150 mGal are identified in approximately one-third of the subduction zones analyzed. Despite great length along strike, these ridges are less than 100 km wide and several appear uncompensated. A high proportion of arc-normal structure and the truncation/morphological transition of trench-parallel forearc ridges is explained through the identification and tracking of pre-existing structure on the over-riding and subducting plates into the seismogenic portion of the plate boundary. Spatial correlations between regions with well-defined trench-parallel forearc ridges and the occurrence of large magnitude interplate earthquakes, in addition to the uncompensated state of these ridges, suggest links

  17. Subduction zone deformation and geometry: Influence on megathrust seismicity

    NASA Astrophysics Data System (ADS)

    Kopp, H.

    2011-12-01

    The fundamental concept of plate tectonics has been established decades ago, along with the observation that the largest earthquakes on the planet occur along the megathrust fault of subduction zones. In that timespan, however, our understanding of what governs the magnitude, source region and recurrence interval of megathrust events has not advanced sufficiently to provide robust answers to open problems. This must be attributed to the fact that large parts of the seismogenic zone and forearc are commonly submerged in deep water and difficult to access at the majority of margins. Marine geophysical techniques, which are able to image the complex structures in these settings with sufficient coherency and depth penetration, have only evolved in recent years. And while satellite altimetry provides images of the seafloor and its large-scale structures on a global scale, local tectonic features may only be identified in ship-based high- resolution bathymetric maps. It is thus crucial for the advancement of our scientific knowledge to expand the marine observational basis of convergent margins and to literally overcome the shoreline- an effort that is certainly not completed. The 2004 Sumatra earthquake sparked many of the questions addressed here: why, along a single convergent margin, do some segments produce large megathrust events whereas other portions of the very same margin only nucleate earthquakes of moderate magnitude? How and why are devastating tsunamis generated in both segments? These observations implicate the notion that individual subduction zones or segments thereof differ in their structure and geometry to induce such diverse behavior. Marine geophysical techniques help to unravel the structural diversity of convergent margins and between individual subduction zone segments. Seismic images and multibeam bathymetry offer a detailed view into the shallow and deeper portions of subduction zones. Field data from Indonesia's Sunda Margin have provided an

  18. Seismic Readings from the Deepest Borehole in the New Madrid Seismic Zone

    SciTech Connect

    Woolery, Edward W; Wang, Zhenming; Sturchio, Neil C

    2006-03-01

    Since the 1980s, the research associated with the UK network has been primarily strong-motion seismology of engineering interest. Currently the University of Kentucky operates a strong-motion network of nine stations in the New Madrid Seismic Zone. A unique feature of the network is the inclusions of vertical strong-motion arrays, each with one or two downhole accelerometers. The deepest borehole array is 260 m below the surfaces at station VASA in Fulton County, Kentucky. A preliminary surface seismic refraction survey was conducted at the site before drilling the hole at VSAS (Woolery and Wang, 2002). The depth to the Paleozoic bedrock at the site was estimated to be approximately 595 m, and the depth to the first very stiff layer (i.e. Porters Creek Clay) was found to be about 260 m. These depths and stratigraphic interpretation correlated well with a proprietary seismic reflection line and the Ken-Ten Oil Exploration No. 1 Sanger hole (Schwalb, 1969), as well as our experience in the area (Street et al., 1995; Woolery et al., 1999).

  19. Summary of Pre-2011 Seismic Monitoring Results for the Central Virginia Seismic Zone

    NASA Astrophysics Data System (ADS)

    Chapman, M. C.

    2011-12-01

    The M 5.7 earthquake affecting Louisa county, Virginia on September 23, 2011 occurred in the central Virginia seismic zone, in the Piedmont physiographic province of Virginia. The largest previous shock occurred on December 22, 1875 with magnitude 5.0 estimated from intensity reports. Other events causing minor damage and widespread alarm occurred in 1774, 1833, 1852, 1907, 1929, 1984 (mbLg 4.2) and 2003 (M 4.3). Previous studies have documented that the seismic zone overlies a major change in crustal thickness and that the seismogenic upper crust is comprised of Appalachian thrust sheets. However, the area was affected by early Mesozoic extensional tectonics and includes several Triassic fault-bounded basins (Culpepper, Scottsville, Farmville, and Richmond). Local network stations have operated in central Virginia since the mid-1970's, although hypocenter location capability has declined since the mid-1990's. The monitoring indicates a mean focal depth of 8 km and maximum depth of approximately 13 km: approximately 75% of the well-located shocks have focal depths above 10 km, and most events are in allochthonous rocks above the Blue Ridge thrust (resolved on seismic reflection profiles). This is in contrast to the situation west of the Blue Ridge province in the Southern Appalachians, where earthquakes generally occur in Grenville basement. The central Virginia seismicity occurs on multiple seismogenic structures. Focal mechanisms of the pre-2011 events exhibit a mixture of strike-slip and reverse faulting on steeply dipping planes showing variable strike. The seismicity has exhibited both temporal and spatial clustering. The M 4.3 December 9, 2003 compound earthquake occurred at a depth of 10 km, and probably represented reverse faulting on a steeply dipping (69 deg) N190E-striking nodal plane, although no aftershocks were recorded to confirm actual fault orientation. The epicenter of the August 23, 2011 shock was only 20 km to the north-northeast of the 2003

  20. Influence of the continental margin on the stress field and seismicity in the intraplate Acaraú Seismic Zone, NE Brazil

    NASA Astrophysics Data System (ADS)

    Oliveira, Paulo H. S.; Ferreira, Joaquim M.; Bezerra, Francisco H. R.; Assumpção, Marcelo; do Nascimento, Aderson F.; Sousa, Maria O. L.; Menezes, Eduardo A. S.

    2015-09-01

    The Borborema province in NE Brazil is characterized by seismic sequences with small earthquakes that can last 10 yr or more. The seismicity in this region is concentrated in three main seismic zones. In this work, we investigate the stress field in one of these zones, the Acaraú Seismic Zone, which is located in the NW part of the Borborema province. This seismic zone exhibits earthquake sequences that contain repeated earthquakes with similar waveforms and a shallow depth. Using a local network, we investigated a seismic sequence close to the town of Santana do Acaraú from December 2009 to December 2010, and we present detailed results (velocity model, hypocentres and focal mechanism) from this network. In addition, we inverted seven focal mechanisms, including six that were used in previous studies, and determined the directions of the three main axes of the regional stress field. Selecting a very precise set of 12 earthquakes, we found an active seismic zone with a depth between 3.5 and 4.8 km and with a horizontal dimension of approximately 2.5 km in the NW-SE direction (azimuth of 118°) and a strike-slip focal mechanism. The new seismic fault and some of the previous seismic faults determined in previous studies occur near the continental-scale Transbrasiliano lineament, but they exhibit no direct relationship with that ancient structure. The stress field is characterized by NW-SE trending compression and NE-SW trending extension. This result suggests that the rheological contrast between the continental-oceanic crusts created flexural stresses with maximum horizontal compression parallel to the continental margin. This stress pattern occurs along the Potiguar basin and continues west as far as the Amazon fan along the Equatorial margin of Brazil. This stress field and related seismicity may be a characteristic of this type of passive margin that is generated during the transform shearing between the South America and Africa plates and that exhibits an

  1. Geophysical setting of the Reelfoot Rift and relations between rift structures and the New Madrid seismic zone

    USGS Publications Warehouse

    Hildenbrand, T.G.; Hendricks, J.D.

    1995-01-01

    In the winter of 1811-12, three of the largest historic earthquakes in the United States occurred near New Madrid, Missouri. Seismicity continues to the present day throughout a tightly clustered pattern of epicenters centered on the bootheel of Missouri, including parts of northeastern Arkansas, northwestern Tennessee, western Kentucky, and southern Illinois. In 1990, the New Madrid seismic zone/Central United States became the first seismically active region east of the Rocky Mountains to be designated a priority research area within the National Earthquake Hazards Reduction Program (NEHRP). This Professional Paper is a collection of papers, some published separately, presenting results of the newly intensified research program in this area. Major components of this research program include tectonic framework studies, seismicity and deformation monitoring and modeling, improved seismic hazard and risk assessments, and cooperative hazard mitigation studies.

  2. Investigating the time dynamics of seismicity by using the visibility graph approach: Application to seismicity of Mexican subduction zone

    NASA Astrophysics Data System (ADS)

    Telesca, Luciano; Lovallo, Michele; Ramirez-Rojas, Alejandro; Flores-Marquez, Leticia

    2013-12-01

    By using the method of the visibility graph (VG), five magnitude time series extracted from the seismic catalog of the Mexican subduction zone were investigated. The five seismic sequences represent the seismicity which occurred between 2005 and 2012 in five seismic areas: Guerrero, Chiapas, Oaxaca, Jalisco and Michoacán. Among the five seismic sequences, the Jalisco sequence shows VG properties significantly different from those shown by the other four. Such a difference could be inherent in the different tectonic settings of Jalisco with respect to those characterizing the other four areas. The VG properties of the seismic sequences have been put in relationship with the more typical seismological characteristics (b-value and a-value of the Gutenberg-Richter law).

  3. Seismogenic structures in the central Virginia seismic zone

    SciTech Connect

    Coruh, C.; Bollinger, G.A.; Costain, J.K.

    1988-08-01

    A correlation between earthquake hypocenters and seismic reflection data in central Virginia has become apparent on an automatic line drawing (ALD) display of seismic reflection data. With the reprocessed Virginia I-64 reflection Vibroseis data extended to 14 s, reflectors are imaged from the lower crust as well as from the upper crust. Specifically, the improved resolution and data quality of ALDs have produced an image of an antiformal structure bounded by mid-crustal reflections on the bottom and by major thrusts at the top. The reflections that define the roof of the antiform are most prominent from about 6 s (18 km) on the east near Richmond under the Coastal Plain sediments, to 1-1.3 s (3-4 km) between Richmond and Charlottesville, and to 3.5 s (10.5 km) on the west. Seismic signatures that can be followed downward from the surface between Charlottesville and Richmond appear to be truncated at the roof of the antiform. The dominant reflections that define the roof correlate with the seismic signature of the transported Taconic suture on the west flank and mylonites on the east flank. The distribution of hypocenters in the area shows an excellent correlation with the westward-dipping reflections that form the roof of the antiform on its western flank. Earthquake activity in this locale may be related to reactivation of the thrusts defining the roof and/or faults above the antiformal structure; however, distribution of the easternmost and deepest set of hypocenters appears to be related to an extensive near-vertical diabase dike swarm of Mesozoic age.

  4. The Nature of Co-seismic Rupture Zone of the 2010 Mentawai Tsunami Earthquake from Full Waveform Inversion of Long Offset Seismic Reflection Data

    NASA Astrophysics Data System (ADS)

    Singh, S. C.; Huot, G.

    2015-12-01

    The Sumatra subduction zone is one of the most seismically active zone on Earth. In the last one decade alone, it has hosted three Mw>8.4 great earthquakes (2004, 2005, 2007) along with 2010 tsunami earthquake. Although the 2007 Mentawai earthquake had Mw=8.4, it did not produce tsunami whereas the 2010 earthquake had Mw=7.8 only in the same region, it produced a large tsunami with a run up height of up to 8 m on Pagai Island, taking 800 lives. Therefore, understanding why an earthquake produce tsunami is fundamental for risk assessment as well for subduction zone processes. Prior to the 2010 earthquake we had acquired ultra-long offsets seismic reflection data in 2009 in the co-seismic slip zone using a 15 km long streamer, the longest streamer ever used, and found that the earth ruptured the frontal section of the subduction zone, which is normally believed to be aseismic, and possibly produced the tsunami. In order to quantify the nature of the co-seismic rupture zone and its link with the tsunami generation, we performed full waveform inversion of seismic reflection data. In order to obtain the high-resolution velocity model for the full waveform inversion, we first downward continue the data to the seafloor, picked first arrivals, and performed tomography. We used the tomographic velocity model as an input to the full waveform inversion. This process also reduced the computation cost significantly as the water depth in this area is 5.5 km. The resulting models shows the presence of thrust faults extending up to the subducting oceanic plate, suggesting that the frontal section of the subduction in this region was indeed locked, capable of hosting great earthquakes. Our inverted model provides the resolution of tens of meters, allowing to characterize the nature of the megathrust and other faults, and hence estimate the effective porosity, permeability and stress along these faults, subsequently the pore pressure.

  5. Stress changes induced at neighbouring faults by the June 2000 earthquakes, South Iceland Seismic Zone

    NASA Astrophysics Data System (ADS)

    Plateaux, Romain; Angelier, Jacques; Bergerat, Françoise; Cappa, Frédéric; Stefansson, Ragnar

    2010-05-01

    The Icelandic rift system belongs to the Mid-Atlantic Ridge and is connected to the offshore Reykjanes and Kolbeinsey ridges by two active transform zones. Plate separation occurs at a rate of nearly 2 cm/yr along the N105°E direction. With respect to the Icelandic Hotspot, westward plate velocities in Iceland are 1.8-2.2 cm/yr for North America and 0-0.4 cm/yr for Eurasia, resulting in a westward displacement of the Icelandic Rift relative to the hotspot. Rift jumps occur when the plate boundary has migrated to a critical point to the west, and a new rift develops above the hotspot apex while the old rift is dying out. The two active transform zones, the Tjörnes Fracture Zone (TFZ) and the South Iceland Seismic Zone (SISZ), resulted from such eastward rift jumps. Our study focuses on the SISZ which is an onland, E-W trending transform zone where N-S trending right-lateral strike-slip faults accommodate left-lateral transform motion as revealed by historical seismicity. During the most recent seismic crisis, in June 2000, two major earthquakes of magnitude (Mw) 6.4 occurred along N-S right-lateral faults in the central segment of the SISZ. The high sensitivity SIL (South Iceland Lowlands) seismic network run by the Icelandic Meteorological Office (IMO) provided a complete record of earthquakes down to magnitude Mw = -1. Here, we present an analysis of this earthquakes sequence in term of stress regimes in order to examine the response of two faults that did not experience significant motion during the earthquakes, and hence to determine how far such fault zones provide information about stress changes in space and time when large earthquakes occur at distance of some tens of kilometres. The faults considered are the Skard and Leirubakki faults, along which large earthquakes and significant displacement occurred in the past Using seismological data recorded from 1991 to 2007, we carried out stress inversion of focal mechanisms of 1,340 earthquakes that affected

  6. Seismicity and shallow slab geometry in the central Vanuatu subduction zone

    NASA Astrophysics Data System (ADS)

    Baillard, Christian; Crawford, Wayne C.; Ballu, Valérie; Régnier, Marc; Pelletier, Bernard; Garaebiti, Esline

    2015-08-01

    The Vanuatu arc in the southwest Pacific Ocean is one of the world's most seismically active regions, with almost 39 magnitude 7+ earthquakes in the past 43 years. Convergence rates are around 90-120 mm/yr along most of the arc, but drop to 25-43 mm/yr in the central section, probably due to the subduction of the d'Entrecasteaux ridge. We characterize the slab geometry and tectonic state in this central section by analyzing data from a 10 month deployment of 30 seismometers over this section. We located more than 30,000 events (all less than magnitude 5.5), constructed an improved 1-D velocity model, calculated focal mechanisms and cluster geometries, and determined the 3-D geometry of the interplate seismogenic zone. The seismogenic zone has a shallow bulge in front of the d'Entrecasteaux ridge, which could be explained by the ridge's buoyancy contributing to the uplift of the fore-arc islands. The seismogenic zone extends to ~45 km depth, significantly below the 26-27 km depth of the fore-arc Moho, indicating that the upper mantle wedge is not significantly serpentinized, which is consistent with the relatively high thermal parameter of the subducting plate. The maximum width of the seismogenic zone is 80 km, indicating an upper earthquake magnitude limit of Mw 7.85 ± 0.4, assuming standard rupture zone aspect ratios. The data also reveal a double seismic zone, 20 to 30 km below the seismogenic zone, which is presumably caused by flexure of the downgoing plate.

  7. Crustal Structure across The Southwest Longmenshan Fault Zone from Seismic Controlled Source Seismic Data

    NASA Astrophysics Data System (ADS)

    Tian, X.; Wang, F.; Liu, B.

    2014-12-01

    The Lushan eathquake, which epicenter and focal depth were at 30.308° N, 102.888° E, and 14.0 km, is the latest intense earthquake occurring in the southwest section of the Longmenshan fault zone after the Ms 8.0 Wenchuan earthquake in 2008. According to the emergency field observations, the slip distribution of the Lushan earthquake was concentrated at the hypocenter, and did not rupture to the surface(Chen et al, 2013). The rupture history constrained by inverting waveforms showed that the causative fault plane of the Lushan event is apparently not a simple extension of either the Pengguan fault or the Beichuan fault that ruptured during the 2008 Mw 8.0 Wenchuan earthquake. The focal mechanism using the Cut and paste algorithm showed this event occurred on a high dip-angle fault, but its dip angle is not steep enough to rupture the surface. All these research is not independent on the heterogeneous crust structure of the Longmenshan fault zone. A 450 km-long wide-angle reflection/refraction profile executed during September and October 2013. This experiment have provided the best opportunities to obtain better knowledge of seismic structure and properties of crust and uppermost mantle beneath the Southwest Longmenshan fault zone. This seismic profile extends from the west Sichuan Plain, through the Longmenshan Fault zone, and into the west Sichuan Plateau. We observed clear Pg, refraction Phase from the upper crust, Pi1/Pi2/Pi3, reflection/refraction Phase from intra-crust, PmP, reflection from the Moho boundary, and the Pn phase, refraction Phase from uppermost mantle. We present a hybrid tomographic and layered velocity model of the crust and uppermost mantle along the profile. The final velocity model reveals large variations both in structure and velocity, and is demonstrated that a particular model has minimum structure. The model shows the crustal thickness of the region is very variable. The Moho topography varies more than 10km in the southwest

  8. Lattice-Preferred orientations of olivine in subducting oceanic lithosphere derived from the observed seismic anisotropies in double seismic zones

    NASA Astrophysics Data System (ADS)

    Han, Peng; Wei, Dongping; Zhang, Keliang; Sun, Zhentian; Zhou, Xiaoya

    2016-08-01

    Subduction zones can generally be classified into Mariana type and Chilean type depending on plate ages, plate thicknesses, subduction angles, back-arc deformation patterns, etc. The double seismic zones (DSZs) in subduction zones are mainly divided into type I and type II which, respectively, correspond to the Mariana type and Chilean type in most cases. Seismic anisotropy is an important parameter characterizing the geophysical features of the lithosphere, including the subduction zones, and can be described by the two parameters of delay time δt and fast wave polarization direction ϕ. We totally collected 524 seismic anisotropy data records from 24 DSZs and analyzed the statistical correlations between seismic anisotropy and the related physical parameters of DSZs. Our statistical analysis demonstrated that the fast wave polarization directions are parallel to the trench strike with no more than 30° for most type I DSZs, while being nearly perpendicular to the trench strike for type II DSZs. We also calculated roughly linear correlations that the delay time δt increases with dip angles but decreases with subduction rates. A linear equation was summarized to describe the strong correlation between DSZ's subduction angle α DSZ and seismic anisotropy in subduction zones. These results suggest that the anisotropic structure of the subducting lithosphere can be described as a possible equivalent crystal similar to the olivine crystal with three mutually orthogonal polarization axes, of which the longest and the second axes are nearly along the trench-perpendicular and trench-parallel directions, respectively.

  9. Analysis of the seismic origin of landslides: examples from the New Madrid seismic zone

    USGS Publications Warehouse

    Jibson, R.W.; Keefer, D.K.

    1993-01-01

    By analyzing two landslides in the New Madrid seismic zone, we develop an approach for judging if a landslide or group of landslides of unknown origin was more likely to have formed as a result of earthquake shaking or in aseismic conditions. The two landslides analyzed are representative of two groups of land-slides that previous research on the geomorphology and regional distribution of landslides in this region indicates may have been triggered by the 1811-1812 New Madrid earthquakes. Slope-stability models of aseismic conditions show that neither landslide is likely to have formed aseismically even in unrealistically high ground-water conditions. Our analysis yields a general relationship between Newmark landslide displacement, earthquake shaking intensity, and the critical acceleration of a landslide. -from Authors

  10. Strain Accumulation and Release in the South Iceland Seismic Zone (Invited)

    NASA Astrophysics Data System (ADS)

    Arnadottir, T.; Hreinsdottir, S.; Geirsson, H.; Ofeigsson, B.

    2013-12-01

    Iceland is located on the Mid-Atlantic ridge, straddling the plate boundary of the North-American and Eurasian plates. Several active volcanic zones and two main transforms accommodate the plate spreading across the island. In the South, the South Iceland Seismic Zone (SISZ) forms the active plate boundary between the Hengill triple junction in the west, and the Eastern Volcanic Zone. The SISZ translates the E-W left lateral shear at depth by faulting on numerous N-S oriented faults in the brittle crust forming the southern margin of the proposed Hreppar micro-plate in South Iceland. In June 2000 and May 2008, two sets of magnitude 6.5 and 6.0 main shocks struck the SISZ. Both earthquake episodes consisted of a pair of main shocks of similar size rupturing closely spaced faults, where static and dynamic stress changes generated by the first event triggered the second main shock further west. The June 2000 earthquakes occurred in the central part of the SISZ, and the May 2008 events in the western part, close to the Hengill triple junction. Since June 2000 annual GPS measurements have been conducted in a geodetic network in South Iceland and a number of continuous GPS stations have been installed. We report strain rate variations in South Iceland derived from GPS observations during 2000 to 2013. In addition to plate motion, and post-seismic signals, the surface deformation is complicated by magma accumulation under active volcanoes at the eastern border of the SISZ - Hekla, and Eyjafjallajökull - as well as subsidence and contraction in the Hengill area caused by fluid withdrawal for geothermal energy production. We also note an increase in strain rates in the epicentral area of the May 2008 main shocks during 2004 to 2007. Previous studies have indicated that the seismic moment released in the June 2000 and May 2008 earthquakes is only half of the accumulated stress since the last major earthquake sequence in 1896-1912. Thus, magnitude 6-7 events may be expected

  11. Patterns of seismic activity preceding large earthquakes

    NASA Technical Reports Server (NTRS)

    Shaw, Bruce E.; Carlson, J. M.; Langer, J. S.

    1992-01-01

    A mechanical model of seismic faults is employed to investigate the seismic activities that occur prior to major events. The block-and-spring model dynamically generates a statistical distribution of smaller slipping events that precede large events, and the results satisfy the Gutenberg-Richter law. The scaling behavior during a loading cycle suggests small but systematic variations in space and time with maximum activity acceleration near the future epicenter. Activity patterns inferred from data on seismicity in California demonstrate a regional aspect; increased activity in certain areas are found to precede major earthquake events. One example is given regarding the Loma Prieta earthquake of 1989 which is located near a fault section associated with increased activity levels.

  12. The Eastern Tennessee Seismic Zone: Reactivation of an Ancient Continent-Continent Suture Zone

    NASA Astrophysics Data System (ADS)

    Powell, C. A.

    2014-12-01

    The eastern Tennessee seismic zone (ETSZ) may represent reactivation of an ancient shear zone that accommodated left-lateral, transpressive motion of the Amazon craton during the Grenville orogeny. Several different lines of evidence support this concept including velocity models for the crust, earthquake hypocenter alignments, focal mechanism solutions, potential field anomalies, paleomagnetic pole positions, and isotopic geochemical studies. The ETSZ trends NE-SW for about 300 km and displays remarkable correlation with the prominent New York - Alabama (NY-AL) aeromagnetic lineament. Vp and Vs models for the crust derived from a local ETSZ earthquake tomography study reveal the presence of a narrow, NE-SW trending, steeply dipping zone of low velocities that extends to a depth of at least 24 km and is associated with the vertical projection of the NY-AL aeromagnetic lineament. The low velocity zone is interpreted as a major basement fault. The recent Mw 4.2 Perry County eastern Kentucky earthquake occurred north of the ETSZ but has a focal depth and mechanism that are similar to those for ETSZ earthquakes. We investigate the possibility that the proposed ancient shear zone extends into eastern Kentucky using Bouguer and aeromagnetic maps. The southern end of the ETSZ is characterized by hypocenters that align along planes dipping at roughly 45 degrees and focal mechanisms that contain large normal faulting components. The NY-AL aeromagnetic lineament also changes trend in the southern end of the ETSZ and the exact location of the lineament is ambiguous. We suggest that the southern portion of the ETSZ involves reactivation of reverse faults (now as normal faults) that mark the ancient transition between a collisional to a more transpressive boundary between Amazonia and Laurentia during the formation of the super continent Rodinia.

  13. Stress development in heterogenetic lithosphere: Insights into earthquake processes in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Zhan, Y.; Gregg, P. M.; Hou, G.

    2015-12-01

    The New Madrid Seismic Zone (NMSZ) in the Central and Eastern United States (CEUS) is the site of several major M 6.8-8 earthquakes in 1811-1812, and remains seismically active. Although extensive investigations have been carried out, the ultimate controls on earthquake initiation and the duration of the seismicity remains unclear. Especially ambiguous is the role of a heterogenetic lithosphere in the development and propagation of stress throughout the crust in intraplate settings. In this study, we develop a finite element model to conduct a series of numerical experiments, the goal of which is to determine the impact of heterogeneity in the Upper Crust, the Lower Crust, and the Mantle on earthquake nucleation and rupture processes. Results indicate that when the differential stresses are built up from boundary displacements, similar to tectonic loading, the stresses below the Reelfoot Rift in the NMSZ are highly concentrated, whereas the stresses below the geologically similar Midcontinent Rift System are low, corresponding with the earthquakes distribution. By comparing the results with three reference models, we argue that the extensive Mantle Low Velocity Zone (MLVZ) beneath the NMSZ produces differential stress localization in the layers above. Furthermore, the relatively strong crust in this region, exhibited by high seismic velocity, enables the elevated stress to extend to the bottom of the ancient rift system, reactivating fossil rifting faults and therefore triggering earthquakes. Although our numerical models focus on loading by a far-field stress source, they explain why the New Madrid inevitably became the most earthquake susceptible region in the CEUS since a heterogeneous lithosphere. Specifically, the presence of the MLVZ will further concentrate stresses resulting from other unloading process, such as melting of the ice sheets or sudden river incision.

  14. Principal Slip Zones in Carbonate: Microstructural Characterization and Implications for the Seismic Cycle

    NASA Astrophysics Data System (ADS)

    Smith, Steven; Billi, Andrea; di Toro, Giulio

    2010-05-01

    Main shocks in central Italy, such as the L'Aquila Mw 6.3 earthquake on 6 April 2009, and associated foreshock and aftershock sequences, often nucleate within, and rupture through, carbonate-bearing rocks within the upper-crust. One way to understand the mechanical behaviour of such rocks during the passage of earthquake ruptures is to study the Principal Slip Zones (PSZs) of exhumed faults. The PSZs are thought to accommodate a majority of displacement during individual earthquake slip events, and potentially contain a rich variety of information about earthquake-related processes and, more generally, deformation mechanisms throughout the seismic cycle. At present, however, there are no reliable microstructural or geochemical indicators of seismic slip in carbonate rocks. We present detailed field and microstructural observations of the PSZs of large-displacement, seismically active normal fault zones in the central Apennines of Italy. The fault zones are exhumed from <3km depth and cut 3-5km thick sequences of platform limestone. Samples were collected from individual PSZs containing polished slip surfaces with both small (centimetres to metres) and large (metres to hundreds of metres) displacements, including the main PSZ that defines the active Quaternary fault scarp. Small displacement slipping zones are characterized by typical cataclastic fabrics, including angular grains cross-cut by brittle fractures, and a gradual decrease in grain-size towards the polished slip surface. In contrast, large-displacement slipping zones always contain a continuous, texturally-distinct layer up to 2-3mm in thickness that lies immediately beneath the polished slip surface. This layer is itself internally zoned; up to 6 distinct zones can be present, each between 200-300µm in thickness, and recognized on the basis of grain-size, colour, and textural variations. In some cases, the zones developed at the expense of one another. 100-200µm-thick, syn-tectonic calcite veins

  15. Two-dimensional seismic image of the San Andreas Fault in the Northern Gabilan Range, central California: Evidence for fluids in the fault zone

    USGS Publications Warehouse

    Thurber, C.; Roecker, S.; Ellsworth, W.; Chen, Y.; Lutter, W.; Sessions, R.

    1997-01-01

    A joint inversion for two-dimensional P-wave velocity (Vp), P-to-S velocity ratio (Vp/Vs), and earthquake locations along the San Andreas fault (SAF) in central California reveals a complex relationship among seismicity, fault zone structure, and the surface fault trace. A zone of low Vp and high Vp/Vs lies beneath the SAF surface trace (SAFST), extending to a depth of about 6 km. Most of the seismic activity along the SAF occurs at depths of 3 to 7 km in a southwest-dipping zone that roughly intersects the SAFST, and lies near the southwest edge of the low Vp and high Vp/Vs zones. Tests indicate that models in which this seismic zone is significantly closer to vertical can be confidently rejected. A second high Vp/Vs zone extends to the northeast, apparently dipping beneath the Diablo Range. Another zone of seismicity underlies the northeast portion of this Vp/Vs high. The high Vp/Vs zones cut across areas of very different Vp values, indicating that the high Vp/Vs values are due to the presence of fluids, not just lithology. The close association between the zones of high Vp/Vs and seismicity suggests a direct involvement of fluids in the faulting process. Copyright 1997 by the American Geophysical Union.

  16. 3-D Crustal Velocity Structure Across the Vrancea Zone in Romania, Derived From Seismic Data

    NASA Astrophysics Data System (ADS)

    Landes, M.; Hauser, F.; Popa, M.

    2002-12-01

    The Vrancea zone in the south-eastern Carpathians is one of the most active seismic zones in Europe. In order to study the crustal and upper-mantle structure in this region, two seismic refraction/wide-angle reflection experiments were carried out in 1999 and 2001. The 1999 campaign comprised a 320 km long N-S profile and a 80 km long transverse profile (E-W). All shots were recorded simultaneously on both profiles. The profile conducted in 2001 extended in E-W direction from the Hungarian border across the Vrancea zone to the Black Sea. We present an application of a 3-D refraction and reflection tomography algorithm (Hole 1992, 1995), elaborating the crustal velocity and interface structure within a 115 x 235 km wide region around the Vrancea zone. In order to enhance the model resolution, first arrival data from local earthquakes of the CALIXTO-99 teleseismic project were also included. The results indicate a high-velocity structure beneath the northern part of the Vrancea zone extending from shallow levels to depths of about 11 km. This structure may be related to the Trotus and Capidava-Ovidiu faults, which converge to the north of it. The high-velocity region is surrounded by the lower velocity Focsani and Brasov basins. The sedimentary succession beneath the southern part of the model extends to 18 km depth, while in the north sediment thickness varies between 10 and 15 km. Further results of the interface modelling of prominent reflections show that the mid-crustal and Moho interfaces shallow northwards from 30 km to 22 km and from 42 km to 38 km, respectively. This correlates well with previous results of Hauser et al. (2001).

  17. Seismic constraints on a large dyking event and initiation of a transform fault zone in Western Gulf of Aden

    NASA Astrophysics Data System (ADS)

    Ahmed, AbdulHakim; Doubre, Cecile; Leroy, Sylvie; Perrot, Julie; Audin, Laurence; Rolandone, Frederique; Keir, Derek; Al-Ganad, Ismael; Sholan, Jamal; Khanbari, Khaled; Mohamed, Kassim; Vergne, Jerome; Jacques, Eric; Nercessian, Alex

    2013-04-01

    In November 2010, a large number of events were recorded by the world seismic networks showing important activity occurring along the western part of the Aden Ridge. West of the Shulka El Sheik fracture zone, events in this large seismic swarm (magnitudes above 5) occurred in a complex area, where the change of both the ridge direction and the bathymetry suggest the propagation of the ridge into a continental lithosphere and the influence of the Afar plume. We combine several sets of data from permanent networks and temporary 3C broad stations installed after the beginning of the event along the southern and eastern coasts of Yemen and Djibouti respectively, we located more than 600 earthquakes with magnitudes ranging from 2.5 to 5.6 that occurred during the first months following the first event. The spatial distribution of the main seismicity reveals a very clear N115° -trending alignment, parallel to the mean direction of the en-echelon spreading segments that form the ridge at this longitude. Half of the events, which represent half of the total seismic energy released during the first months, are located in the central third section of the segment. Here several volcanic cones and recent lava flows observed from bathymetric and acoustic reflectivity data during the Tadjouraden cruise (Audin, 1999, Dauteuil et al., 2001) constitute the sea floor. In addition to this main activity, two small groups of events suggest the activiation of landslides into a large fan and the activity in a volcanic area 50 km due east from the main active zone. The time evolution of the seismicity shows several bursts of activity. Some of them are clearly related to sudden activities within the volcanic areas, when others exhibit horizontal migration of the events, with velocity around ˜ 1 km/h. The time-space evolution of the seismicity clearly reveals the intrusion of dykes associated with magma propagation from the crustal magmatic centres into the rift zone. Taking into account

  18. Quaternary layer anomalies around the Carlsberg Fault zone mapped with high-resolution shear-wave seismics south of Copenhagen

    NASA Astrophysics Data System (ADS)

    Kammann, Janina; Hübscher, Christian; Nielsen, Lars; Boldreel, Lars Ole

    2015-04-01

    The Carlsberg Fault zone is located in the N-S striking Höllviken Graben and traverses the city of Copenhagen. The fault zone is a NNW-SSE striking structure in direct vicinity to the transition zone of the Danish Basin and the Baltic Shield. Recent small earthquakes indicate activity in the area, although none of the mapped earthquakes appear to have occurred on the Carlsberg Fault. We examined the fault evolution by a combination of very high resolution onshore shear-wave seismic data, one conventional onshore seismic profile and marine reflection seismic profiles. The chalk stratigraphy and the localization of the fault zone at depth was inferred from previous studies by other authors. We extrapolated the Jurassic and Triassic stratigraphy from the Pomeranian Bay to the area of investigation. The fault zone shows a flower structure in the Triassic as well as in Cretaceous sediments. The faulting geometry indicates strong influence of Triassic processes when subsidence and rifting prevailed in the Central European Basin System. Growth strata within the surrounding Höllviken Graben reveal syntectonic sedimentation in the lower Triassic, indicating the opening to be a result of Triassic rifting. In the Upper Cretaceous growth faulting documents continued rifting. This finding contrasts the Late Cretaceous to Paleogene inversion tectonics in neighbouring structures, as the Tornquist Zone. The high-resolution shear-wave seismic method was used to image structures in Quaternary layers in the Carlsberg Fault zone. The portable compact vibrator source ElViS III S8 was used to acquire a 1150 m long seismic section on the island Amager, south of Copenhagen. The shallow subsurface in the investigation area is dominated by Quaternary glacial till deposits in the upper 5-11 m and Danian limestone below. In the shear-wave profile, we imaged the 30 m of the upward continuation of the Carlsberg Fault zone. In our area of investigation, the fault zone appears to comprise

  19. Seismic-reflection profiles of the New Madrid seismic zone-data along the Mississippi River near Caruthersville, Missouri

    USGS Publications Warehouse

    Crone, A.J.; Harding, S.T.; Russ, D.P.; Shedlock, K.M.

    1986-01-01

    Three major seismic-reflection programs have been conducted by the USGS in the New Madrid seismic zone. The first program consisted of 32 km of conventional Vibroseis profiling designed to investigate the subsurface structure associated with scarps and lineaments in northwestern Tennessee (Zoback, 1979). A second, more extensive Vibroseis program collected about 250 km of data from all parts of the New Madrid seismic zone in Missouri, Arkansas, and Tennessee (Hamilton and Zoback, 1979, 1982; Zoback and others, 1980). The profiles presented here are part of the third program that collected about 240 km of high-resolution seismic-reflection data from a boat along the Mississippi River between Osceola, Ark., and Wickliffe, Ky. (fig. 1). The data for profiles A, B, C, and D were collected between river miles 839-1/2 and 850-1/2 from near the Interstate-155 bridge to upstream of Caruthersville, Mo. (fig. 2). Profiles on this part of the river are important for three reasons: (1) they connect many of the land-based profiles on either side of the river, (2) they are near the northeast termination of a linear, 120km-long, northeast-southwest zone of seismicity that extends from northeast Arkansas to Caruthersville, Mo. (Stauder, 1982; fig. 1), and (3) they cross the southwesterly projection of the Cottonwood Grove fault (fig. 1), a fault having a substantial amount of vertical Cenozoic offset (Zoback and others, 1980).

  20. The Antillean-South American Collision Zone as Imaged by BOLIVAR Seismic Data

    NASA Astrophysics Data System (ADS)

    Levander, A.; Bezada, M. J.; Niu, F.; Miller, M. S.; Masy, J.; Clark, S. A.; Magnani, M.; Ave Lallemant, H. G.; Pindell, J. L.

    2009-12-01

    Most models for the evolution of the southeastern Caribbean plate boundary invoke a time transgressive collision of the Antilles arc against northern South America (SA) as the Antillean subduction zone rolled back from Colombia to its present location east of Trindad during the Cenozoic. We present seismic images from the BOLIVAR experiment that detail crustal, lithospheric, and upper mantle structures across northern Venezuela resulting from this collision between the Caribbean and South American plates. A combination of seismicity, active seismic profiles, receiver functions, shear wave splits, and surface wave inversions image crustal, lithospheric and upper most mantle structures, while receiver functions and finite-frequency body-wave tomography provide control on deeper structures through the transition zone. We observe a number of first order structures that relate to Caribbean-South American collision: 1) The right-lateral San Sebastian-El Pilar strike-slip system originates at the southeastern corner of the Antillean subduction zone at about (10.75oN, 62.25oW) offshore eastern Venezuela as a subduction-transform edge propagator fault system. As the Atlantic plate descends into the mantle, tearing from the South American plate, it also depresses the edge of the SA continental margin, providing room for coastal mountain building and massive sedimentary basins. 2) After tearing from South America, the Atlantic slab descends rapidly to the transition zone as seen in body wave tomography and receiver function images. 3) The Caribbean plate has been underthrust beneath Colombia and western Venezuela from the west, and now extends as far east as the foreland of the Merida Andes, its western edge roughly paralleling the Bocono fault. 4) The mountains of central and western Venezuela owe much of their elevation to the Caribbean slab lying beneath them. In particular east-directed flat-slab Caribbean subduction beneath westernmost Venezuela has produced the Merida

  1. Permafrost Active Layer Seismic Interferometry Experiment (PALSIE).

    SciTech Connect

    Abbott, Robert; Knox, Hunter Anne; James, Stephanie; Lee, Rebekah; Cole, Chris

    2016-01-01

    We present findings from a novel field experiment conducted at Poker Flat Research Range in Fairbanks, Alaska that was designed to monitor changes in active layer thickness in real time. Results are derived primarily from seismic data streaming from seven Nanometric Trillium Posthole seismometers directly buried in the upper section of the permafrost. The data were evaluated using two analysis methods: Horizontal to Vertical Spectral Ratio (HVSR) and ambient noise seismic interferometry. Results from the HVSR conclusively illustrated the method's effectiveness at determining the active layer's thickness with a single station. Investigations with the multi-station method (ambient noise seismic interferometry) are continuing at the University of Florida and have not yet conclusively determined active layer thickness changes. Further work continues with the Bureau of Land Management (BLM) to determine if the ground based measurements can constrain satellite imagery, which provide measurements on a much larger spatial scale.

  2. Multiharmonic model of seismic activity in Kamchatka

    NASA Astrophysics Data System (ADS)

    Sobolev, G. A.; Valeev, S. G.; Faskhutdinova, V. A.

    2010-12-01

    Based on the uniform catalogue of earthquakes of the minimum energy class 8.5 for 1962-2008, multiharmonic models of seismic activity in Kamchatka are developed. The main harmonic components with periods from a few days to 12 years are identified. Both the entire catalogue and its modified versions obtained by the elimination of aftershocks and clusters, as well as nonoverlapping time series were used to study the stability of the models. The forward-prediction testing showed that in the models with weekly averaged initial data, periods of increased and reduced seismic activity lasting for several weeks are predicted with high confidence on an interval of up to 1.8% of the education period. This testifies for the presence of deterministic components in the seismic activity.

  3. Apollo 14 active seismic experiment.

    PubMed

    Watkins, J S; Kovach, R L

    1972-03-17

    Explosion seismic refraction data indicate that the lunar near-surface rocks at the Apollo 14 site consist of a regolith 8.5 meters thick and characterized by a compressional wave velocity of 104 meters per second. The regolith is underlain by a layer with a compressional wave velocity of 299 meters per second. The thickness of this layer, which we interpret to be the Fra Mauro Formation, is between 16 and 76 meters. The layer immediately beneath this has a velocity greater than 370 meters per second. We found no evidence of permafrost. PMID:17794200

  4. Apollo 14 active seismic experiment.

    NASA Technical Reports Server (NTRS)

    Watkins, J. S.; Kovach, R. L.

    1972-01-01

    Explosion seismic refraction data indicate that the lunar near-surface rocks at the Apollo 14 site consist of a regolith 8.5 meters thick and characterized by a compressional wave velocity of 104 meters per second. The regolith is underlain by a layer with a compressional wave velocity of 299 meters per second. The thickness of this layer, which we interpret to be the Fra Mauro Formation, is between 16 and 76 meters. The layer immediately beneath this has a velocity greater than 370 meters per second. We found no evidence of permafrost.

  5. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°-18°S)

    NASA Astrophysics Data System (ADS)

    Métois, M.; Vigny, C.; Socquet, A.

    2016-05-01

    The recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity, but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear. Here, we investigate the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw 8.1 in 2014 and Illapel Mw 8.4 in 2015). We combine recent GPS data acquired over the margin (38°-18°S) with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface. Here, we show that at least six low coupling zones (LCZ), areas where coupling is low relatively to the neighboring highly coupled segments can be identified. We also find that for the three most recent Mw > 8 events, co-seismic asperities correlate well with highly coupled segments, while LCZs behaved as barriers and stopped the ruptures. The relation between coupling and background seismicity in the interseismic period before the events is less clear. However, we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between LCZ and neighboring segments, and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone. Thus, highly coupled segments usually exhibit low background seismicity. In this overall context, the Metropolitan segment that partly ruptured during the 2015 Illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture, raising the issue of the remaining seismic hazard in this very densely populated area.

  6. Interseismic Coupling, Megathrust Earthquakes and Seismic Swarms Along the Chilean Subduction Zone (38°-18°S)

    NASA Astrophysics Data System (ADS)

    Métois, M.; Vigny, C.; Socquet, A.

    2016-04-01

    The recent expansion of dense GPS networks over plate boundaries allows for remarkably precise mapping of interseismic coupling along active faults. The interseismic coupling coefficient is related to the ratio between slipping velocity on the fault during the interseismic period and the long-term plates velocity, but the interpretation of coupling in terms of mechanical behavior of the fault is still unclear. Here, we investigate the link between coupling and seismicity over the Chilean subduction zone that ruptured three times in the last 5 years with major earthquakes (Maule Mw 8.8 in 2010, Iquique Mw 8.1 in 2014 and Illapel Mw 8.4 in 2015). We combine recent GPS data acquired over the margin (38°-18°S) with older data to get the first nearly continuous picture of the interseismic coupling variations on the subduction interface. Here, we show that at least six low coupling zones (LCZ), areas where coupling is low relatively to the neighboring highly coupled segments can be identified. We also find that for the three most recent Mw > 8 events, co-seismic asperities correlate well with highly coupled segments, while LCZs behaved as barriers and stopped the ruptures. The relation between coupling and background seismicity in the interseismic period before the events is less clear. However, we note that swarm sequences are prone to occur in intermediate coupling areas at the transition between LCZ and neighboring segments, and that the background seismicity tends to concentrate on the downdip part of the seismogenic locked zone. Thus, highly coupled segments usually exhibit low background seismicity. In this overall context, the Metropolitan segment that partly ruptured during the 2015 Illapel earthquake appears as an outlier since both coupling and background seismicity were high before the rupture, raising the issue of the remaining seismic hazard in this very densely populated area.

  7. 3D absolute hypocentral determination - 13 years of seismicity in Ecuadorian subduction zone

    NASA Astrophysics Data System (ADS)

    Font, Yvonne; Segovia, Monica; Theunissen, Thomas

    2010-05-01

    In Ecuador, the Nazca plate is subducting beneath the North Andean Block. This subduction triggered, during the last century, 4 major earthquakes of magnitude greater than 7.7. Between 1994 and 2007, the Geophysical Institute (Escuela National Politecnica, Quito) recorded about 40 000 events in whole Ecuador ranging from Mb 1.5 to 6.9. Unfortunately, the local network shows great density discrepancy between the Coastal and Andean regions where numerous stations were installed to survey volcanic activity. Consequently, seismicity in and around the interplate seismogenic zone - producer of the most destructive earthquakes and tsunamis - is not well constrained. This study aims to improve the location of 13 years seismicity occurred during an interseismic period in order to better localize the seismic deformation and gaps. The first step consists in the construction of a 3D "georealistic" velocity model. Because local tomography cannot provide satisfactory model, we combined all local crustal/lithospheric information on the geometry and velocity properties of different geological units. Those information cover the oceanic Nazca plate and sedimentary coverture the subducting plate dip angle; the North Andean Block margin composed of accreted oceanic plateaus (the Moho depth is approximated using gravity modeling); the metamorphic volcanic chain (oceanic nature for the occidental cordillera and inter-andean valley, continental one for the oriental cordillera); The continental Guyana shield and sedimentary basins. The resulting 3D velocity model extends from 2°N to 6.5°S and 277°E to 283°E and reaches a depth of 300 km. It is discretized in constant velocity blocks of 12 x 12 x 3 km in x, y and z, respectively. The second step consists in selecting an adequate sub-set of seismic stations in order to correct the effect of station density disequilibrium between coastal and volcanic regions. Consequently, we only keep the most representative volcanic stations in terms

  8. Regional Observation of Seismic Activity in Baekdu Mountain

    NASA Astrophysics Data System (ADS)

    Kim, Geunyoung; Che, Il-Young; Shin, Jin-Soo; Chi, Heon-Cheol

    2015-04-01

    Seismic unrest in Baekdu Mountain area between North Korea and Northeast China region has called attention to geological research community in Northeast Asia due to her historical and cultural importance. Seismic bulletin shows level of seismic activity in the area is higher than that of Jilin Province of Northeast China. Local volcanic observation shows a symptom of magmatic unrest in period between 2002 and 2006. Regional seismic data have been used to analyze seismic activity of the area. The seismic activity could be differentiated from other seismic phenomena in the region by the analysis.

  9. A Trial of the Delineation of Gas Hydrate Bearing Zones using Seismic Methods Offshore Tokai Japan

    NASA Astrophysics Data System (ADS)

    Inamori, T.; Hato, M.

    2002-12-01

    MITI Research Well 'Nankai Trough' was drilled at offshore Tokai Japan in 1999/2000 and the existence of gas hydrate was confirmed by various proofs through borehole measurement or coring. It gave so big impact to the view of Japan_fs future energy resources and other scientific interests.The METI, Ministry of Economy, Trade and Industry, has started the national project "Methane Hydrate Exploration study" in Japan since the fall 2001. Bottom Simulating Reflectors (BSRs) were widely found on the marine seismic data acquired offshore Japan especially in the shelf-slope near Nankai Trough. BSRs are thought to be the bottom of gas hydrate stability zones, we cannot, however, get the information of gas hydrate bearing zones, such as the height of those, the porosity, the gas hydrate saturation etc, only from BSRs. In order to estimate the amount of gas hydrate accurately, we have to get those reservoir parameters of gas hydrate bearing zones from marine seismic data. The velocity of these zones is greater than that of the surrounding sediment, because pure gas hydrate has high velocity that is more than 3,000 m/s. This means the interval velocity is the key for exploration of gas hydrate. First, we have tried to image the gas hydrate bearing zones from seismic stacking velocity analysis. After the conversion to interval velocity from NMO velocity by Dix's equation, we imaged the P-wave velocity section through 2D seismic line. We successfully imaged high velocity zones above BSRs and low velocity zones beneath BSRs on P-wave velocity section. But the resolution of the section from the velocity analysis is not so high. Although we have only two adjacent well log data on the seismic line, in order to make more detailed map, we tried to execute the seismic impedance inversion with MITI Nankai Trough Well data. We made a simple initial model and inverted to seismic impedance value. We got the good impedance section and delineated the gas hydrate bearing zones through it

  10. Seismic hazard and risk assessment in the intraplate environment: The New Madrid seismic zone of the central United States

    USGS Publications Warehouse

    Wang, Z.

    2007-01-01

    Although the causes of large intraplate earthquakes are still not fully understood, they pose certain hazard and risk to societies. Estimating hazard and risk in these regions is difficult because of lack of earthquake records. The New Madrid seismic zone is one such region where large and rare intraplate earthquakes (M = 7.0 or greater) pose significant hazard and risk. Many different definitions of hazard and risk have been used, and the resulting estimates differ dramatically. In this paper, seismic hazard is defined as the natural phenomenon generated by earthquakes, such as ground motion, and is quantified by two parameters: a level of hazard and its occurrence frequency or mean recurrence interval; seismic risk is defined as the probability of occurrence of a specific level of seismic hazard over a certain time and is quantified by three parameters: probability, a level of hazard, and exposure time. Probabilistic seismic hazard analysis (PSHA), a commonly used method for estimating seismic hazard and risk, derives a relationship between a ground motion parameter and its return period (hazard curve). The return period is not an independent temporal parameter but a mathematical extrapolation of the recurrence interval of earthquakes and the uncertainty of ground motion. Therefore, it is difficult to understand and use PSHA. A new method is proposed and applied here for estimating seismic hazard in the New Madrid seismic zone. This method provides hazard estimates that are consistent with the state of our knowledge and can be easily applied to other intraplate regions. ?? 2007 The Geological Society of America.

  11. Imaging the Carboneras fault zone at depth: preliminary results from reflection/refraction seismic tomography

    NASA Astrophysics Data System (ADS)

    Nippress, S.; Rietbrock, A.; Faulkner, D. R.; Rutter, E.; Haberland, C. A.; Teixido, T.

    2009-12-01

    Understanding and characterizing fault zone structure at depth is vital to predicting the slip behaviour of faults in the brittle crust. We aim to combine detailed field mapping and laboratory velocity/physical property determinations with seismic measurements on the Carboneras fault zone (S.E. Spain) to improve our knowledge of how fault zone structure affects seismic signals. The CFZ is a large offset (10s of km) strike-slip fault that constitutes part of the diffuse plate boundary between Africa and Iberia. It has been largely passively exhumed from ca. 4 to 6 km depth. The friable fault zone components are excellently preserved in the region’s semi-arid climate, and consist of multiple strands of phyllosilicate-rich fault gouge ranging from 1 to 20 m in thickness. In May 2009 we conducted 4 high-resolution seismic reflection and refraction/first break tomography lines. Two of these lines (~1km long) crossed the entire fault zone while the remaining lines (~150 and ~300m long) concentrated on individual fault strands and associated damage zones. For each of the lines a 2 m-geophone spacing was used with a combination of accelerated drop weight, sledgehammer and 100g explosives as seismic sources. Initial seismic reflection processing has been carried out on each of the 4 lines. First breaks have been picked for each of the shot gathers and inputted into a 2D traveltime inversion and amplitude-modeling package (Zelt & Smith, 1992) to obtain first break tomography images. During this field campaign we also carried out numerous fault zone guided wave experiments on two of the dense seismic lines. At the larger offsets (~600-700m) we observe low frequency guided waves. These experiments will capture the various length scales involved in a mature fault zone and will enable the surface mapping and petrophysical studies to be linked to the seismic field observations.

  12. The May 29 2008 earthquake aftershock sequence within the South Iceland Seismic Zone: Fault locations and source parameters of aftershocks

    NASA Astrophysics Data System (ADS)

    Brandsdottir, B.; Parsons, M.; White, R. S.; Gudmundsson, O.; Drew, J.

    2010-12-01

    The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. The South Iceland Seismic Zone (SISZ) is a complex transform zone where left-lateral E-W shear between the Reykjanes Peninsula Rift Zone and the Eastern Volcanic Zone is accommodated by bookshelf faulting along N-S lateral strike-slip faults. The SISZ is also a transient feature, migrating sideways in response to the southward propagation of the Eastern Volcanic Zone. Sequences of large earthquakes (M > 6) lasting from days to years and affecting most of the seismic zone have occurred repeatedly in historical time (last 1100 years), separated by intervals of relative quiescence lasting decades to more than a century. On May 29 2008, a Mw 6.1 earthquake struck the western part of the South Iceland Seismic Zone, followed within seconds by a slightly smaller event on a second fault ~5 km further west. Aftershocks, detected by a temporal array of 11 seismometers and three permanent Icelandic Meteorological Office stations were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two major and several smaller N-S faults as well as an E-W zone of activity stretching further west into the Reykjanes Peninsula Rift Zone. Fault plane solutions show both right lateral and oblique strike slip mechanisms along the two major N-S faults. The aftershocks deepen from 3-5 km in the north to 8-9 km in the south, suggesting that the main faults dip southwards. The faulting is interpreted to be driven by the local stress due to transform motion between two parallel segments of the divergent plate boundary crossing Iceland.

  13. Monitoring deep geodynamic processes within Vrancea intermediate-depth seismic zone by geodetic means

    NASA Astrophysics Data System (ADS)

    Besutiu, Lucian; Zlagnean, Luminita

    2015-04-01

    Background Located in the bending zone of East Carpathians, the so-called Vrancea zone is one of the most active seismic regions in Europe. Despite many years of international research, its intermediate-depth seismicity within full intra-continental environment still represents a challenge of the 21st century. Infrastructure In the attempt to join the above-mentioned efforts, the Solid Earth Dynamics Department (SEDD) in the Institute of Geodynamics of the Romanian Academy has developed a special research infrastructure, mainly devoted to gravity and space geodesy observations. A geodetic network covering the epicentre area of the intermediate-depth earthquakes has been designed and implemented for monitoring deep geodynamic processes and their surface echoes. Within each base-station of the above-mentioned network, a still-reinforced concrete pillar allows for high accuracy repeated gravity and GPS determinations. Results Starting from some results of the previously run CERGOP and UNIGRACE European programmes, to which additional SEDD repeated field campaigns were added, an unusual geodynamic behaviour has been revealed in the area. 1) Crust deformation: unlike the overall uprising of East Carpathians, as a result of denudation followed by erosion, their SE bending zone, with Vrancea epicentre area exhibits a slight subsidence. 2) Gravity change: more than 200 microgals non-tidal gravity decrease over a 20 years time-span has been noticed within the subsiding area. Extended observations showed the gravity lowering as a nowadays continuing process. Interpretation This strange combination of topography subsidence and gravity lowering has been interpreted in terms of crust stretching in the Vrancea epicentre zone due to the gravity pull created by densification of the lower crust as a result of phase-transform processes taking place in the lithospheric compartment sunken into the upper mantle. The occurrence of crust earthquakes with vertical-extension focal

  14. Seismic imaging of the stagnant Pacific slab in the mantle transition zone under East Asia

    NASA Astrophysics Data System (ADS)

    Zhao, D.

    2008-12-01

    We used regional and global seismic tomography to determine high-resolution 3-D P-wave velocity structure of the crust and mantle down to 1200 km depth under Western Pacific to East Asia (Zhao, 2004, 2007; Huang and Zhao, 2006). A large number of arrival times of P, pP, PP and PcP waves recorded by many seismic stations in East Asia are used in the tomographic inversions. The subducting Pacific slab is imaged clearly as a high-velocity zone from the oceanic trenches down to 670-km depth, and intermediate-depth and deep earthquakes are located within the slab. The Pacific slab becomes stagnant in the mantle transition zone under eastern China. The western edge of the stagnant slab is generally parallel with the Japan trench and the Ryukyu trench and roughly coincides with a prominent surface topographic boundary in East China. Although there are some discrepancies between the topographic boundary and the western edge of the stagnant slab, both of them are located approximately 1800 km west of the trenches. The entire Pacific slab is stagnant in the mantle transition zone under Northeast China (53-37 degree north latitude). Under 37-28 degree north latitude, however, some of the slab materials are visible below the 670-km discontinuity, though most of the slab materials are still in the transition zone, suggesting that part of the slab materials have started to drop down to the lower mantle. Under the Mariana arc, the Pacific slab penetrates directly down to the lower mantle. It is also visible that the Philippine Sea slab has subducted down to the mantle transition zone depth under western Japan and the Ryukyu back-arc region (Abdelwahed and Zhao, 2007). There are three active intraplate volcanoes in China. The Changbai and Wudalianchi volcanoes in Northeast China are underlain by significant slow anomalies in the upper mantle, above the stagnant Pacific slab, suggesting that the two active volcanoes are not hot spots but a kind of back-arc volcanoes associated with

  15. Seismic trapped modes in the oroville and san andreas fault zones.

    PubMed

    Li, Y G; Leary, P; Aki, K; Malin, P

    1990-08-17

    Three-component borehole seismic profiling of the recently active Oroville, California, normal fault and microearthquake event recording with a near-fault three-component borehole seismometer on the San Andreas fault at Parkfield, California, have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals. These wave trains are interpreted as fault zone-trapped seismic modes. Parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers, as shallow as 4 kilometers, and extend 12 kilometers along the fault on either side of the recording station. Selected Oroville and Parkfield wave forms are modeled as the fundamental and first higher trapped SH modes of a narrow low-velocity layer at the fault. Modeling results suggest that the Oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second, whereas at Parkfield, the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1.1 to 1.8 kilometers per second. These low-velocity layers are probably the rupture planes on which earthquakes occur. PMID:17756789

  16. Seismic imaging of mantle transition zone discontinuities beneath the northern Red Sea and adjacent areas

    NASA Astrophysics Data System (ADS)

    Mohamed, A. A.; Gao, S. S.; Elsheikh, A. A.; Liu, K. H.; Yu, Y.; Fat-Helbary, R. E.

    2014-11-01

    The dramatic asymmetry in terms of surface elevation, Cenozoic volcanisms and earthquake activity across the Red Sea is an enigmatic issue in global tectonics, partially due to the unavailability of broad-band seismic data on the African Plate adjacent to the Red Sea. Here, we report the first comprehensive image of the mantle transition zone (MTZ) discontinuities using data from the Egyptian National Seismic Network, and compare the resulting depths of the 410 and 660-km discontinuities with those observed on the Arabian side. Our results show that when a standard earth model is used for time-to-depth conversion, the resulting depth of the discontinuities increases systematically towards the axis of the Afro-Arabian Dome (AAD) from both the west and east. Relative to the westernmost area, the maximum depression of the 410-km discontinuity is about 30 km, and that of the 660-km discontinuity is about 45 km. The observed systematic variations can best be explained by a model involving a hydrated MTZ and an upper-mantle low-velocity zone beneath the AAD. Models invoking one or more mantle plumes originated from the MTZ or the lower-mantle beneath the study area are not consistent with the observations.

  17. A joint local and teleseismic tomography study of the Mississippi Embayment and New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Nyamwandha, Cecilia A.; Powell, Christine A.; Langston, Charles A.

    2016-05-01

    Detailed, upper mantle P and S wave velocity (Vp and Vs) models are developed for the northern Mississippi Embayment (ME), a major physiographic feature in the Central United States (U.S.) and the location of the active New Madrid Seismic Zone (NMSZ). This study incorporates local earthquake and teleseismic data from the New Madrid Seismic Network, the Earthscope Transportable Array, and the FlexArray Northern Embayment Lithospheric Experiment stations. The Vp and Vs solutions contain anomalies with similar magnitudes and spatial distributions. High velocities are present in the lower crust beneath the NMSZ. A pronounced low-velocity anomaly of ~ -3%--5% is imaged at depths of 100-250 km. High-velocity anomalies of ~ +3%-+4% are observed at depths of 80-160 km and are located along the sides and top of the low-velocity anomaly. The low-velocity anomaly is attributed to the presence of hot fluids upwelling from a flat slab segment stalled in the transition zone below the Central U.S.; the thinned and weakened ME lithosphere, still at slightly higher temperatures from the passage of the Bermuda hotspot in mid-Cretaceous, provides an optimal pathway for the ascent of the fluids. The observed high-velocity anomalies are attributed to the presence of mafic rocks emplaced beneath the ME during initial rifting in the early Paleozoic and to remnants of the depleted, lower portion of the lithosphere.

  18. Seismic trapped modes in the Oroville and San Andreas Fault zones

    SciTech Connect

    Li, Yong-Gang; Leary, P.; Aki, K. ); Malin, P. )

    1990-08-17

    Three-component borehole seismic profiling of the recently active Oroville, California, normal fault and microearthquake event recording with a near-fault three-component borehole seismometer on the San Andreas fault at Parkfield, California, have shown numerous instances of pronounced dispersive wave trains following the shear wave arrivals. These wave trains are interpreted as fault zone-trapped seismic modes. Parkfield earthquakes exciting trapped modes have been located as deep as 10 kilometers, as shallow as 4 kilometers, and extend 12 kilometers along the fault on either side of the recording station. Selected Oroville and Parkfield wave forms are modeled as the fundamental and first higher trapped SH modes of a narrow low-velocity layer at the fault. Modeling results suggest that the Oroville fault zone is 18 meters wide at depth and has a shear wave velocity of 1 kilometer per second, whereas at Parkfield, the fault gouge is 100 to 150 meters wide and has a shear wave velocity of 1.1 to 1.8 kilometers per second. These low-velocity layers are probably the rupture planes on which earthquakes occur. 15 refs., 5 figs., 1 tab.

  19. Seismicity and Geometry Properties of the Hellenic Subduction Zone

    NASA Astrophysics Data System (ADS)

    Papadimitriou, E. E.; Karakostas, V. G.; Vallianatos, F.; Makropoulos, C.; Drakatos, G.

    2014-12-01

    Recent seismicity and fault plane solutions of earthquakes that occurred along the Hellenic Arc-Trench system are engaged for approximating the geometry of the subducted oceanic plate. Seismicity and focal mechanisms confirm the gentle subduction (~15o-20o) of the oceanic crust reaching a depth of 20 km at a distance of 100 km from the trench. The slab is then bending at larger angles, and in particular at ~45o up to the depth of 80 km and at ~65o up to the depth of 180 km, when seismicity ceased. This geometry of the slab is shown in a bunch of cross sections normal to the convergence strike, up to ~25o (east Crete Island). To the east the sparse inslab seismicity reveals an almost vertical dipping of the lower part (from 80 km downdip) of the descending slab. The slab interface that accommodates hazardous earthquakes is clearly nonplanar with the main seismic moment release taking place on its up-dip side. The fore-arc, upper plate seismicity, is remarkably low in comparison with both subduction and back arc seismicity, and confined inside a seismogenic layer having a width not exceeding the 20km. Offshore seismicity is spatially variable forming distinctive streaks thus revealing that parts of the oceanic crust are probably slipped aseismically. This observation along with the fact that coupling in the Hellenic arc is only about a tenth of the plate motion, imply the presence of areas of lower and higher coupling across the subduction interface. Areas of high coupling imply areas of the slab interface subjected to high normal forces and correlate with earthquake asperities. Although asperity distributions vary substantially through time, identification of such characteristics in the seismogenesis can have a significant impact in the seismic hazard assessment. This research has been co-funded by the European Union (European Social Fund) and Greek national resources under the framework of the "THALES Program: SEISMO FEAR HELLARC" project.

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

    USGS Publications Warehouse

    Trehu, Anne M.; Solomon, Sean C.

    1983-01-01

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

  1. Imaging 3D seismic velocity along the seismogenic zone of Algarve region (southern Portugal)

    NASA Astrophysics Data System (ADS)

    Rocha, João.; Bezzeghoud, Mourad; Caldeira, Bento; Dias, Nuno; Borges, José; Matias, Luís.; Dorbath, Catherine; Carrilho, Fernando

    2010-05-01

    seismotectonic picture of the region. This work is expected to produce a more detailed knowledge of the structure of the crust over the region of Algarve, being able to identify seismogenic zones, potentially generators of significant seismic events and also the identification of zones of active faults.

  2. Seismic reflection imaging over the South Portuguese Zone fold-and-thrust belt, SW Iberia

    NASA Astrophysics Data System (ADS)

    Schmelzbach, C.; Simancas, J. F.; Juhlin, C.; Carbonell, R.

    2008-08-01

    We reprocessed an ˜35-km-long part of the IBESREIS seismic reflection profile which runs over the Iberian Pyrite Belt section of the South Portuguese Zone, SW Iberia, with the goal to image the upper crust (<15-km depth). The applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5-s travel time relative to high-amplitude source-generated noise. A complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked-line prestack migration scheme for coherent imaging. To interpret sources of diffracted energy, we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections.The final seismic images show south-vergent imbricate fold-and-thrust tectonics, documenting the contractive deformation that the South Portuguese Zone experienced during the Variscan Orogeny. Based on surface geological information, we correlate a low reflective unit with the shallow Upper Carboniferous Flysch Group, a highly reflective unit ranging in depth from 2 to 4 km with the Middle Carboniferous Volcano-Sedimentary Complex Group, which hosts massive sulfide deposits, and a moderately reflective unit with the Upper Devonian Phyllite-Quartzite Group. Below these units, another low-reflective facies is present, which may represent older Paleozoic metasediments. In addition, the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions. These proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in Early Carbonifereous times.

  3. Imaging the Middle America subduction zone with body waves extracted from ambient noise by seismic interferometry

    NASA Astrophysics Data System (ADS)

    Vargas, W.; Brown, L. D.; Cabolova, A.; Quiros, D. A.; Chen, C.

    2011-12-01

    Subduction zones have long been a prime target for seismic imaging with a variety of active and passive methodologies. Here we report an attempt to use seismic interferometry to extract body waves (P and S) from ambient noise recorded during a broadband experiment in southwestern Mexico for reflection imaging of the crust and subducting Cocos plate. The Middle America Subduction Experiment (MASE; Kim et al., 2010) included a quasi linear array of 100 broadband seismic instruments deployed at a nominal spacing of 6 km which continuously recorded for up to 30 months. Our focus was on using cross-correlation and autocorrelation of ambient noise along this array to 1) determine if useful body waves could be extracted, 2) assess which conditions were most favorable for such extraction, and 3) evaluate whether these waves could be used to image deep lithospheric structure, with particular interest in the seismogenic zone. While surface wave tomography using cross-correlation techniques have found widespread success in mapping crustal structure, examples of body wave imaging of crustal targets using this approach are still very few. In our analysis, we have found it necessary to suppress the surface wave energy to enhance body waves from virtual sources. Our pre-processing sequence includes bias removal, bandpass filtering, deconvolution (spectral whitening), and sign- bit conversion. The resulting data windows are cross-correlated and stacked until useful signals are apparent. The virtual shot gathers thus far produced show clear Rayleigh and Pg waves, with weaker but distinct Sg phases. We have also found arrivals with hyperbolic travel times that match those expected for deep reflections. Crustal imaging is limited by the large station spacing, which results in relatively few stations at sub-critical offsets. However several apparent reflections from sub-Moho depths suggest that key elements of the subduction process can be imaged using reflections derived from ambient

  4. Developing Advanced Seismic Imaging Methods For Characterizing the Fault Zone Structure

    NASA Astrophysics Data System (ADS)

    Zhang, Haijiang

    2015-04-01

    Here I present a series of recent developments on seismic imaging of fault zone structure. The goals of these advanced methods are to better determine the physical properties (including seismic velocity, attenuation, and anisotropy) around the fault zone and its boundaries. In order to accurately determine the seismic velocity structure of the fault zone, we have recently developed a wavelet-based double-difference seismic tomography method, in which the wavelet coefficients of the velocity model, rather than the model itself, are solved using both the absolute and differential arrival times. This method takes advantage of the multiscale nature of the velocity model and the multiscale wavelet representation property. Because of the velocity model is sparse in the wavelet domain, a sparsity constraint is applied to tomographic inversion. Compared to conventional tomography methods, the new method is both data- and model-adaptive, and thus can better resolve the fault zone structure. In addition to seismic velocity property of the fault zone, seismic anisotropy and attenuation properties are also important to characterize the fault zone structure. For this reason, we developed the seismic anisotropy tomography method to image the three-dimensional anisotropy strength model of the fault zone using shear wave splitting delay times between fast and slow shear waves. The applications to the San Andreas fault around Parkfield, California and north Anatolian fault in Turkey will be shown. To better constrain the seismic attenuation structure, we developed a new seismic attenuation tomography method using measured t* values for first arrival body waves, in which the structures of attenuation and velocity models are similar through the cross-gradient constraint. Seismic tomography can, however, only resolve the smooth variations in elastic properties in Earth's interior. To image structure at length scales smaller than what can be resolved tomographically, including

  5. Seismic velocity models for the Denali fault zone along the Richardson Highway, Alaska

    USGS Publications Warehouse

    Brocher, T.M.; Fuis, G.S.; Lutter, W.J.; Christensen, N.I.; Ratchkovski, N.A.

    2004-01-01

    Crustal-scale seismic-velocity models across the Denali fault zone along the Richardson Highway show a 50-km-thick crust, a near vertical fault trace, and a 5-km-wide damage zone associated with the fault near Trans-Alaska Pipeline Pump Station 10, which provided the closest strong ground motion recordings of the 2002 Denali fault earthquake. We compare models, derived from seismic reflection and refraction surveys acquired in 1986 and 1987, to laboratory measurements of seismic velocities for typical metamorphic rocks exposed along the profiles. Our model for the 1986 seismic reflection profile indicates a 5-km-wide low-velocity zone in the upper 1 km of the Denali fault zone, which we interpret as fault gouge. Deeper refractions from our 1987 line image a 40-km wide, 5-km-deep low-velocity zone along the Denali fault and nearby associated fault strands, which we attribute to a composite damage zone along several strands of the Denali fault zone and to the obliquity of the seismic line to the fault zone. Our velocity model and other geophysical data indicate a nearly vertical Denali fault zone to a depth of 30 km. After-shocks of the 2002 Denali fault earthquake and our velocity model provide evidence for a flower structure along the fault zone consisting of faults dipping toward and truncated by the Denali fault. Wide-angle reflections indicate that the crustal thickness beneath the Denali fault is transitional between the 60-km-thick crust beneath the Alaska Range to the south, and the extended, 30-km-thick crust of the Yukon-Tanana terrane to the north.

  6. Seismic blanking zones in the deep-water Ullung Basin, East Sea of Korea.

    NASA Astrophysics Data System (ADS)

    Ryu, Byong-Jae; Riedel, Michael; Yoo, Dong-Geun

    2015-04-01

    A total 12366.395 L.km of 2D multichannel seismic data were acquired by the Korea Institute of Geoscience and Mineral Resources (KIGAM) for detecting and mapping seismic indicators for the presence of gas hydrate in the deep-water Ulleung Basin, East Sea of Korea. The seismic data were acquired using Trilogy System of Geco-Prakla, Bolt Air-gun System onboard the R/V TAMHAE II of KIGAM during the years of 2000 to 2004. The seismic faices of shallow sediments were also analyzed to understand the sedimentary strata developed in the basin. Seismic data were processed to define gas hydrate indicators such as bottom simulating reflectors (BSRs) and seismic blank zones. The BSR was identified by (a) its polarity opposite to the seafloor, (b) its seafloor-parallel reflection behavior, and (c) its occurrence at a sub-bottom depth corresponding to the expected base of gas hydrate stability zone, on heat flow and other thermal data for the region and on seismic velocity data. The seismic velocity analysis was also conducted for determining the velocity deviation effect of high-velocity gas hydrate and underlying low-velocity free gas. The BSRs occur mainly in the southern part of the basin where mass transport deposits are widely occurring. A number of vertical to sub-vertical seismic blanking zones were identified in the basin. The blanking zones are near-vertical broad chimney-like structures of reduced seismic reflectivity. They may be formed by gas and/or fluid upwelling through fractures and faults. Many of the blanking zones show apparent velocity pull-up effects of sediment layering structures that are interpreted to be a result of higher velocity gas hydrate. The presence of substantial amounts of gas hydrate in the blank zones were first found by piston coring in 2007, and subsequently confirmed by two deep-drilling expeditions in 2007 and 2010. Most of the blanking zones occur in well-bedded turbidite/hemi-pelagic sediments in the northern deep basin. The

  7. Stress drop at the Kephalonia Transform Zone estimated from the 2014 seismic sequence

    NASA Astrophysics Data System (ADS)

    Caporali, Alessandro; Bruyninx, Carine; Fernandes, Rui; Ganas, Athanassios; Kenyeres, Ambrus; Lidberg, Martin; Stangl, Guenter; Steffen, Holger; Zurutuza, Joaquin

    2016-01-01

    The Kephalonia Transform Zone (KTZ) is a seismically active dextral transform fault decoupling the extensional deformed area of the Ionian Abyssal Plain and the compressional deformed area of the Mediterranean Ridge. We estimate a prominent steady state strain rate of 225 ± 20 nstrain/year across the KTZ from the mean velocities of permanent Global Navigation Satellite System (GNSS) stations in East Italy and West Greece, which confirms previous estimates. Based on the regional statistical seismicity and using the local Gutenberg-Richter relation we investigate the energetic balance between stress accumulated as a consequence of the continuous dextral shear deformation, and the average stress released by shallow seismicity. If the maximum expected magnitude is Mw = 7.4, the estimated a and b parameters of the local Gutenberg-Richter relation and the geodetically determined shear-strain rate set an upper limit to the regional stress drop ∆σ ≤ 0.4 MPa. We verify these values by analyzing a seismic sequence on the KTZ between January and February 2014, which culminated in the Mw= 6.09 event of Jan. 26, and the Mw= 6.14 event of Feb. 3. The estimated epicenters lie within a few kilometers from each other, on the western side of the Kephalonia Island, at a hypocentral depth ≤ 10 km. The measured coseismic displacements of GNSS stations in this area fit the expected surface dislocation, which can be predicted for an elastic half-space using the measured fault-plane solutions as input. If the hypocenters of the aftershocks are taken as indicator of optimal dextral shear-stress orientation, then a low regional deviatoric stress of 0.4 MPa, that is comparable with the maximum regional stress drop estimated above, is required for the Coulomb stress to match the pattern of the aftershocks of the 2014 sequence. As a consequence, we conclude that the regional deviatoric stress and the seismically released shear stress are in close balance in the KTZ seismic province.

  8. Crustal structure of the Izu Collision zone, central Japan, revealed by dense seismic array observations

    NASA Astrophysics Data System (ADS)

    Kurashimo, E.; Sato, H.; Abe, S.; Kato, N.; Ishikawa, M.; Obara, K.

    2009-12-01

    In central Japan, the Philippine Sea Plate (PSP) subducts beneath the Tokyo Metropolitan area, the Kanto region. In western Kanto region, the Izu-Bonin arc (IBA) within the PSP has been colliding from the south with the Honshu arc, forming a complex structure called the Izu-Collision zone (ICZ). Several active faults were formed in and around the ICZ. The geometry of the subducting PSP and the overlying crustal structure of the ICZ are important to constrain the process of earthquake occurrence and the crustal evolution process associated with arc-arc collision. Recent seismic experiments reveal the geometry of the subducting PSP beneath the Kanto region (Sato et al., 2005). The Japanese islands, including the ICZ, are covered with dense arrays of permanent seismic stations, which provide good constraints on velocity structures by a tomographic method. Such studies reveal a general picture of the lithospheric structure such as a descending plate configuration (e.g. Matsubara et al., 2008). However, since an average spacing of the permanent station is typically 20 km, a detailed structure in the upper crust, which is imperative for an understanding of the active tectonics, cannot be well constrained by permanent array alone. Two dense seismic array observations were conducted to obtain a structural image beneath the ICZ. One is a 40-km-long line (EW-line) located in the northern part of the ICZ and the other is a 55-km-long line (NS-line) located in the central part of the ICZ. Seventy-five 3-component portable seismographs were deployed on EW-line with 500 to 700 m interval and waveforms were recorded during a four-month period from October, 2008. Forty 3-component portable seismographs were deployed on NS-line with about 1 km spacing and waveforms were recorded during the three month period from January, 2006. In order to obtain a high-resolution velocity model, a well-controlled hypocenter is essential. Due to this, we combined the seismic array data with

  9. Seismic imaging of the 2001 Bhuj Mw7.7 earthquake source zone: b-value, fractal dimension and seismic velocity tomography studies

    NASA Astrophysics Data System (ADS)

    Mandal, Prantik; Rodkin, Mikhail V.

    2011-11-01

    We use precisely located aftershocks of the 2001 Mw7.7 Bhuj earthquake (2001-2009) to explore the structure of the Kachchh seismic zone by mapping the 3-D distributions of b-value, fractal dimension (D) and seismic velocities. From frequency-magnitude analysis, we find that the catalog is complete above Mw = 3.0. Thus, we analyze 2159 aftershocks with Mw ≥ 3.0 to estimate the 3-D distribution of b-value and fractal dimensions using maximum-likelihood and spatial correlation dimension approaches, respectively. Our results show an area of high b-, D- and Vp/Vs ratio values at 15-35 km depth in the main rupture zone (MRZ), while relatively low b- and D values characterize the surrounding rigid regions and Gedi fault (GF) zone. We propose that higher material heterogeneities in the vicinity of the MRZ and/or circulation of deep aqueous fluid/volatile CO 2 is the main cause of the increased b-, D- and Vp/Vs ratio values at 15-35 km depth. Seismic velocity images also show some low velocity zones continuing in to the deep lower crust, supporting the existence of circulation of deep aqueous fluid / volatile CO 2 in the region (probably released from the eclogitasation of olivine rich lower crustal rocks). The presence of number of high and low velocity patches further reveals the heterogeneous and fractured nature of the MRZ. Interestingly, we observe that Aki (1981)'s relation (D = 2b) is not valid for the spatial b-D correlation of the events in the GF (D 2 = 1.2b) zone. However, the events in the MRZ (D 2 = 1.7b) show a fair agreement with the D = 2b relationship while the earthquakes associated with the remaining parts of the aftershock zone (D 2 = 1.95b) show a strong correlation with the Aki (1981)'s relationship. Thus, we infer that the remaining parts of the aftershock zone are probably behaving like locked un-ruptured zones, where larger stresses accumulate. We also propose that deep fluid involvement may play a key role in generating seismic activity in the

  10. Variation of seismic coupling with slab detachment and upper plate structure along the western Hellenic subduction zone

    NASA Astrophysics Data System (ADS)

    Laigle, Mireille; Sachpazi, Maria; Hirn, Alfred

    2004-10-01

    The western Hellenic subduction zone is characterized by a trenchward velocity of the upper plate. In the Ionian islands segment, complete seismic coupling is achieved, as is predicted by standard plate-tectonic models in which there is no slab pull force because the slab has broken off. The moderate local seismic moment rate relates to a shallow downdip limit for the seismogenic interface. This characteristic may be attributed to the ductility of the lower crust of the upper plate, which allows a décollement between the upper crust of the overriding plate and the subducting plate. Farther south, a deeper downdip limit of the seismogenic interface is indicated by thrust-faulting earthquakes, which persist much deeper in western Crete. A correspondingly larger downdip width of this seismogenic zone is consistent with the suggested larger maximum magnitude of earthquakes here. However, since the seismic moment release rate seems to be moderate in the Peloponnese and western Crete, like in in the Ionian islands, this seismically active interface cannot maintain complete seismic coupling across its larger downdip width. A cause may be the lateral addition of overweight to the part of the slab still attached in Crete, by the free fall of its part that has broken off from the surface further north. This increased slab pull reduces the compressive normal stress across the seismogenic interface and thus causes partial seismic coupling in its shallower part. However, the width of this part may provide an additional area contributing to slip in large earthquakes, which may nucleate deeper on stick-slip parts of the interface. Hints at anomalies in structure and seismicity, which need to be resolved, may relate to the present location of the edge of the tear in the slab.

  11. Evidences of Attenuation Zones Under Vesuvius Volcano By Local and Regional Seismicity

    NASA Astrophysics Data System (ADS)

    Cubellis, E.; Marturano, A.

    The seismicity at Vesuvius is characterised by events of moderate-energy concentrated in the caldera area. The foci of events are shallow, with depths less than 6 km under sea level. Periods of greater actvity were recorded in 1989, 1990, and, more recently, in 1995 and 1996. On October, 9, 1999 an earthquake (Ml=3.6) felt outside vesuvian area took place at Vesuvius-crater. It was not only the most energetic one since the last eruption of 1944 but also one of the most energetic among those occurring in the Vesuvian area since Roman times, as shown by an analysis of historical seismicity. Following the 9 october 1999 event, questionnaires were sent to all middle schools in the Vesuvian area and surrounding towns in order to define the extent to which the earthquake had been felt. The felt index was thus obtained, which represent the per- centage response to the question: Did you feel the earthquake? and used in later data processing. The felt index is a continuous parameter and this feature makes it possible, among other things, to relate it to ground motion parameters and overcome the prob- lem of the limits involved in using integer values of intensity. In particular, Q quality factor was determined by assuming direct proportionality between energy and felt in- dex. The values obtained were Q=60-90 and, Qa=100-150, in reasonable agreement with the P-wave quality factor of 70 to 100 reported below active volcanoes, consis- tent with high temperatures and generally associated with the presence of magmatic bodies. The near Southern Apennine seismogenetic zone, 50-100 km from Vesuvius, is characterised by prevalent normal faulting and large historical earthquakes. The last, the Irpinia earthquake of November 23, 1980 (Ms=6.9), developed on three fault sources at least, with apenninic trend (NW-SE), was characterised by elevated atten- uation zones in epicentral and external areas too. In particular, the macroseismic field showed a 25 km wide circular attenuation zone

  12. Effects of Shear Zone Development on Seismic Anisotropy in the Lower Grenvillian Crust

    NASA Astrophysics Data System (ADS)

    Song, W. J.; Gerbi, C. C.; Johnson, S. E.; Vel, S. S.

    2014-12-01

    Deep crustal structure, particularly the geometry of shear zones, affects the degree of crust-mantle coupling and the kinematics of crustal deformation. In principle, shear zones in the deep crust can be visible using seismic imaging due to the change in the orientations and modes of anisotropic minerals. However, matching the seismic signals to structures present remains a challenge. This work seeks to bridge some of that gap. We utilize the Parry Sound domain in the western Central Gneiss Belt of the Grenville orogen, Ontario, Canada, to develop quantitative relationships between geologic structures and seismic anisotropy. This region provides excellent examples of granulite and amphibolite facies shear zones up to several km wide. We investigated three rock types: (1) regionally deformed mafic and felsic granulite facies orthogneiss, (2) granulite facies shear zones, and (3) amphibolite facies shear zones. Both of the latter two derived from (1). Using the numerical architecture of asymptotic expansion homogenization (which considers grain-scale elastic interactions), we computed much higher precision seismic velocities than is possible with conventional Voigt-Reuss-Hill algorithms. In all sheared felsic rocks, the dominant quartz slip system was prism + rhomb indicating slowest Vp direction paralleled lineation because in quartz a-axis is near the slowest direction. In contrast, in all sheared mafic rocks, the fastest amphibole direction is strongly parallel to the lineation. As a consequence of combining the quartz and amphibole deformation, rocks comprising felsic and mafic layers have a weak seismic anisotropy. In monolithological shear zones, anisotropy can exceed 10%. Despite the promise this work illustrates, we must continue to consider the influence of inherited fabrics in the host rock. In a second line of investigation, we explored how shear zone volume fraction affects seismic anisotropy.

  13. Comparing the New Madrid Seismic Zone with the Osning Thrust: implications for GIA-induced intraplate tectonics in northern Germany

    NASA Astrophysics Data System (ADS)

    Brandes, Christian; Steffen, Holger; Wu, Patrick; Tanner, David; Winsemann, Jutta

    2013-04-01

    Continental intraplate tectonics is a widespread phenomenon that causes significant earthquakes. These earthquakes even occur in areas that are characterized by low strain rates and there are often long intervals between the individual seismic events (Gangopadhyay & Talwani, 2003) that result in a hazard potential. To better understand the controlling factors of intraplate plate earthquakes in northern Germany, we compare the Osning Thrust with the intensively-studied New Madrid Seismic Zone in the Midwest USA. Both areas share major similarities such as a failed rift-basin setting, the presence of intrusive magmatic bodies in the subsurface, tectonic reactivation during the Late Cretaceous, paleo- and historic seismicity and comparable fault parameters. In addition, both areas have a very similar Late Pleistocene deglaciation history. New Madrid was c. 340 km south of the Laurentide ice sheet and ice retreat started around 21 ka and was completed by 8.5 ka (Grollimund & Zoback, 2001). The Osning Thrust was c. 310 km south of the Scandinavian ice sheet and deglaciation began at 24 ka. Both areas show historic seismicity in a similar time frame (New Madrid Seismic Zone: 1811-1812, Johnston & Schweig, 1996); Osning Thrust: 1612 and 1767, Grünthal & Bosse, 1997). We use numerical simulations to identify the timing of potentially GIA-induced fault activity, which are based on the fault stability margin concept of Wu & Hasegawa (1996). From our modelling results it is evident that the fault stability margin changed to negative between 16 and 13 ka for the Osning Thrust, which matches the OSL data of fault-related growth strata (Brandes et al., 2012). For the New Madrid Seismic Zone, the fault stability margin becomes zero between 2.5 ka BP (before 1812) to about 2 ka after the 1812 event, depending on the parameters of the model. This indicates that for both seismic zones, seismicity due to deglaciation was and still is very likely. From this study it can be derived

  14. Seismicity of the St. Lawrence paleorift faults overprinted by a meteorite impact crater: Implications for crustal strength based on new earthquake relocations in the Charlevoix Seismic Zone, Eastern Canada

    NASA Astrophysics Data System (ADS)

    Yu, H.; Harrington, R. M.; Liu, Y.; Lamontagne, M.; Pang, M.

    2015-12-01

    The Charlevoix Seismic Zone (CSZ), located along the St. Lawrence River (SLR) ~100 km downstream from Quebec City, is the most active seismic zone in eastern Canada with five historic earthquakes of M 6-7 and ~ 200 events/year reported by the Canadian National Seismograph Network. Cataloged earthquake epicenters outline two broad linear zones along the SLR with little shallow seismicity in between. Earthquakes form diffuse clusters between major dipping faults rather than concentrating on fault planes. Detailed fault geometry in the CSZ is uncertain and the effect on local seismicity of a meteorite impact structure that overprints the paleorift faults remains ambiguous. Here we relocate 1639 earthquakes occurring in the CSZ between 01/1988 - 10/2010 using the double-difference relocation method HypoDD and waveforms primarily from 7 local permanent stations. We use the layered SLR north shore velocity model from Lamontagne (1999), and travel time differences based on both catalog and cross-correlated P and S-phase picks. Of the 1639 relocated earthquakes, 1236 (75.4%) satisfied selection criteria of horizontal and vertical errors less than 2 km and 1 km respectively. Cross-sections of relocated seismicity show hypocenters along distinct active fault segments. Earthquakes located beneath the north shore of the SLR are likely correlated with the NW Gouffre fault, forming a ~10 km wide seismic zone parallel to the river, with dip angle changing to near vertical at the northern edge of the impact zone. In contrast, seismicity beneath the SLR forms a diffuse cloud within the impact structure, likely representing a highly fractured volume. It further implies that faults could be locally weak and subject to high pore-fluid pressures. Seismicity outside the impact structure defines linear structures aligning with the Charlevoix fault. Relocated events of M > 4 all locate outside the impact structure, indicating they nucleated on the NE-SW-oriented paleorift faults.

  15. Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy)

    NASA Astrophysics Data System (ADS)

    Smith, Steven A. F.; Billi, Andrea; Toro, Giulio Di; Spiess, Richard

    2011-12-01

    Earthquakes in central Italy, and in other areas worldwide, often nucleate within and rupture through carbonates in the upper crust. During individual earthquake ruptures, most fault displacement is thought to be accommodated by thin principal slip zones. This study presents detailed microstructural observations of the slip zones of the seismically active Tre Monti normal fault zone. All of the slip zones cut limestone, and geological constraints indicate exhumation from <2 km depth, where ambient temperatures are ≪100°C. Scanning electron microscope observations suggest that the slip zones are composed of 100% calcite. The slip zones of secondary faults in the damage zone contain protocataclastic and cataclastic fabrics that are cross-cut by systematic fracture networks and stylolite dissolution surfaces. The slip zone of the principal fault has much more microstructural complexity, and contains a 2-10 mm thick ultracataclasite that lies immediately beneath the principal slip surface. The ultracataclasite itself is internally zoned; 200-300 μm-thick ultracataclastic sub-layers record extreme localization of slip. Syn-tectonic calcite vein networks spatially associated with the sub-layers suggest fluid involvement in faulting. The ultracataclastic sub-layers preserve compelling microstructural evidence of fluidization, and also contain peculiar rounded grains consisting of a central (often angular) clast wrapped by a laminated outer cortex of ultra-fine-grained calcite. These "clast-cortex grains" closely resemble those produced during layer fluidization in other settings, including the basal detachments of catastrophic landslides and saturated high-velocity friction experiments on clay-bearing gouges. An overprinting foliation is present in the slip zone of the principal fault, and electron backscatter diffraction analyses indicate the presence of a weak calcite crystallographic preferred orientation (CPO) in the fine-grained matrix. The calcite c-axes are

  16. Enriquillo–Plantain Garden fault zone in Jamaica: paleoseismology and seismic hazard

    USGS Publications Warehouse

    Koehler, R.D.; Mann, P.; Prentice, Carol S.; Brown, L.; Benford, B.; Grandison-Wiggins, M.

    2013-01-01

    The countries of Jamaica, Haiti, and the Dominican Republic all straddle the Enriquillo–Plantain Garden fault zone ( EPGFZ), a major left-lateral, strike-slip fault system bounding the Caribbean and North American plates. Past large earthquakes that destroyed the capital cities of Kingston, Jamaica (1692, 1907), and Port-au-Prince, Haiti (1751, 1770), as well as the 2010 Haiti earthquake that killed more than 50,000 people, have heightened awareness of seismic hazards in the northern Caribbean. We present here new geomorphic and paleoseismic information bearing on the location and relative activity of the EPGFZ, which marks the plate boundary in Jamaica. Documentation of a river bank exposure and several trenches indicate that this fault is active and has the potential to cause major destructive earthquakes in Jamaica. The results suggest that the fault has not ruptured the surface in at least 500 yr and possibly as long as 28 ka. The long period of quiescence and subdued geomorphic expression of the EPGFZ indicates that it may only accommodate part of the ∼7–9 mm=yr plate deformation rate measured geodetically and that slip may be partitioned on other undocumented faults. Large uncertainties related to the neotectonic framework of Jamaica remain and more detailed fault characterization studies are necessary to accurately assess seismic hazards.

  17. Subduction Zone Science - Examples of Seismic Images of the Central Andes and Subducting Nazca Slab

    NASA Astrophysics Data System (ADS)

    Beck, S. L.; Zandt, G.; Scire, A. C.; Ward, K. M.; Portner, D. E.; Bishop, B.; Ryan, J. C.; Wagner, L. S.; Long, M. D.

    2015-12-01

    Subduction has shaped large regions of the Earth and constitute over 55,000 km of convergent plate margin today. The subducting slabs descend from the surface into the lower mantle and impacts earthquake occurrence, surface uplift, arc volcanism and mantle convection as well as many other processes. The subduction of the Nazca plate beneath the South America plate is one example and constitutes the largest present day ocean-continent convergent margin system and has built the Andes, one of the largest actively growing mountain ranges on Earth. This active margin is characterized by along-strike variations in arc magmatism, upper crustal shortening, crustal thickness, and slab geometry that make it an ideal region to study the relationship between the subducting slab, the mantle wedge, and the overriding plate. After 20 years of portable seismic deployments in the Central Andes seismologists have combined data sets and used multiple techniques to generate seismic images spanning ~3000 km of the South American subduction zone to ~800 km depth with unprecedented resolution. For example, using teleseismic P- waves we have imaged the Nazca slab penetrating through the mantle transition zone (MTZ) and into the uppermost lower mantle. Our tomographic images show that there is significant along-strike variation in the morphology of the Nazca slab in the upper mantle, MTZ, and the lower mantle, including possible tears, folding, and internal deformation. Receiver function studies and surface wave tomography have revealed major changes in lithospheric properties in the Andes. Improved seismic images allow us to more completely evaluate tectonic processes in the formation and uplift of the Andes including: (1) overthickened continental crust driven by crustal shortening, (2) changes in slab dip and coupling with the overlying plate (3) localized lithospheric foundering, and (4) large-scale mantle and crustal melting leading to magmatic addition and/or crustal flow. Although

  18. Space Geodesy and the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Smalley, Robert; Ellis, Michael A.

    2008-07-01

    One of the most contentious issues related to earthquake hazards in the United States centers on the midcontinent and the origin, magnitudes, and likely recurrence intervals of the 1811-1812 New Madrid earthquakes that occurred there. The stakeholder groups in the debate (local and state governments, reinsurance companies, American businesses, and the scientific community) are similar to the stakeholder groups in regions more famous for large earthquakes. However, debate about New Madrid seismic hazard has been fiercer because of the lack of two fundamental components of seismic hazard estimation: an explanatory model for large, midplate earthquakes; and sufficient or sufficiently precise data about the causes, effects, and histories of such earthquakes.

  19. Seismic activity of Erebus volcano, antarctica

    NASA Astrophysics Data System (ADS)

    Kaminuma, Katsutada

    1987-11-01

    Mount Erebus is presently the only Antarctic volcano with sustained eruptive activity in the past few years. It is located on Ross Island and a convecting anorthoclase phonolite lava lake has occupied the summit crater of Mount Erebus from January 1973 to September 1984. A program to monitor the seismic activity of Mount Erebus named IMESS was started in December 1980 as an international cooperative program among Japan, the United States and New Zealand. A new volcanic episode began on 13 September, 1984 and continued until December. Our main observations from the seismic activity from 1982 1985 are as follows: (1) The average numbers of earthquakes which occurred around Mount Erebus in 1982, 1983 and January August 1984 were 64, 134 and 146 events per day, respectively. Several earthquake swarms occurred each year. (2) The averag number of earthquakes in 1985 is 23 events per day, with only one earthquake swarm. (3) A remarkable decrease of the background seismicity is recognized before and after the September 1984 activity. (4) Only a few earthquakes were located in the area surrounding Erebus mountain after the September 1984 activity. A magma reservoir is estimated to be located in the southwest area beneath the Erebus summit, based on the hypocenter distributions of earthquakes.

  20. Fault zone reverberations from cross-correlations of earthquake waveforms and seismic noise

    NASA Astrophysics Data System (ADS)

    Hillers, Gregor; Campillo, Michel

    2016-03-01

    Seismic wavefields interact with low-velocity fault damage zones. Waveforms of ballistic fault zone head waves, trapped waves, reflected waves and signatures of trapped noise can provide important information on structural and mechanical fault zone properties. Here we extend the class of observable fault zone waves and reconstruct in-fault reverberations or multiples in a strike-slip faulting environment. Manifestations of the reverberations are significant, consistent wave fronts in the coda of cross-correlation functions that are obtained from scattered earthquake waveforms and seismic noise recorded by a linear fault zone array. The physical reconstruction of Green's functions is evident from the high similarity between the signals obtained from the two different scattered wavefields. Modal partitioning of the reverberation wavefield can be tuned using different data normalization techniques. The results imply that fault zones create their own ambiance, and that the here reconstructed reverberations are a key seismic signature of wear zones. Using synthetic waveform modelling we show that reverberations can be used for the imaging of structural units by estimating the location, extend and magnitude of lateral velocity contrasts. The robust reconstruction of the reverberations from noise records suggests the possibility to resolve the response of the damage zone material to various external and internal loading mechanisms.

  1. Reflection seismic and petrophysical investigations over a series of major deformation zones in eastern Sweden

    NASA Astrophysics Data System (ADS)

    Malehmir, Alireza; Ahmadi, Pouya; Lundberg, Emil; Dahlin, Peter; Juhlin, Christopher; Sjöström, Håkan; Högdahl, Karin

    2013-04-01

    The Bergslagen region is one of the most ore prospective districts in south-central Sweden. We will present an overview of results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora mining area. Seismic interpretations are constrained by ultrasonic velocity measurements on a series of rock samples, cross-dip analysis, prestack time migration, and swath 3D imaging, as well as by other available geophysical and geological observations. A series of major fault zones is imaged by the seismic data, as is a large mafic intrusion. However, the most prominent feature is a package of east-dipping reflectors found east of the Dannemora area that extend down to at least 3 km depth. This package is associated with a poly-phase, ductile-brittle deformation zone with the latest ductile movement showing east-side-up or reverse kinematics. Its total vertical displacement is estimated to be in the order of 2.5 km. In order to better understand the nature of reflectivity from the deformation zone, we extended our petrophysical studies to include Laser Doppler Interferometer (LDI) measurements, which are capable of providing information about anisotropy and potentially its system in available rock samples from the deformation zone. Up to 10% velocity-anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone. To link the lab measurements with the real seismic data, we recently processed the seismic data over the deformation zone using anisotropy parameters obtained from the LDI measurements. A partial improvement of the deformation zone image is obtained. This suggests that rock anisotropy may also contribute to generation of reflections from the deformation zones in the study area.

  2. Location, structure, and seismicity of the Seattle fault zone, Washington: Evidence from aeromagnetic anomalies, geologic mapping, and seismic-reflection data

    USGS Publications Warehouse

    Blakely, R.J.; Wells, R.E.; Weaver, C.S.; Johnson, S.Y.

    2002-01-01

    A high-resolution aeromagnetic survey of the Puget Lowland shows details of the Seattle fault zone, an active but largely concealed east-trending zone of reverse faulting at the southern margin of the Seattle basin. Three elongate, east-trending magnetic anomalies are associated with north-dipping Tertiary strata exposed in the hanging wall; the magnetic anomalies indicate where these strata continue beneath glacial deposits. The northernmost anomaly, a narrow, elongate magnetic high, precisely correlates with magnetic Miocene volcanic conglomerate. The middle anomaly, a broad magnetic low, correlates with thick, nonmagnetic Eocene and Oligocene marine and fluvial strata. The southern anomaly, a broad, complex magnetic high, correlates with Eocene volcanic and sedimentary rocks. This tripartite package of anomalies is especially clear over Bainbridge Island west of Seattle and over the region east of Lake Washington. Although attenuated in the intervening region, the pattern can be correlated with the mapped strike of beds following a northwest-striking anticline beneath Seattle. The aeromagnetic and geologic data define three main strands of the Seattle fault zone identified in marine seismic-reflection profiles to be subparallel to mapped bedrock trends over a distance of >50 km. The locus of faulting coincides with a diffuse zone of shallow crustal seismicity and the region of uplift produced by the M 7 Seattle earthquake of A.D. 900-930.

  3. Triggered tremors beneath the seismogenic zone of an active fault zone, Kyushu, Japan

    NASA Astrophysics Data System (ADS)

    Miyazaki, Masahiro; Matsumoto, Satoshi; Shimizu, Hiroshi

    2015-11-01

    Non-volcanic tremors were induced by the surface waves of the 2012 Sumatra earthquake around the Hinagu fault zone in Kyushu, Japan. We inferred from dense seismic observation data that the hypocenters of these tremors were located beneath the seismogenic zone of the Hinagu fault. Focal mechanisms of the tremors were estimated using S-wave polarization angles. The estimated focal mechanisms show similarities to those of shallow earthquakes in this region. In addition, one of the nodal planes of the focal mechanisms is almost parallel to the strike direction of the Hinagu fault. These observations suggest that the tremors were triggered at the deeper extension of the active fault zone under stress conditions similar to those in the shallower seismogenic region. A low-velocity anomaly beneath the hypocentral area of the tremors might be related to the tremor activity.

  4. An Examination of Seismicity Linking the Solomon Islands and Vanuatu Subduction Zones

    NASA Astrophysics Data System (ADS)

    Neely, J. S.; Furlong, K. P.

    2015-12-01

    The Solomon Islands-Vanuatu composite subduction zone represents a tectonically complex region along the Pacific-Australia plate boundary in the southwest Pacific Ocean. Here the Australia plate subducts under the Pacific plate in two segments: the South Solomon Trench and the Vanuatu Trench. The two subducting sections are offset by a 200 km long, transform fault - the San Cristobal Trough (SCT) - which acts as a Subduction-Transform Edge Propagator (STEP) fault. The subducting segments have experienced much more frequent and larger seismic events than the STEP fault. The northern Vanuatu trench hosted a M8.0 earthquake in 2013. In 2014, at the juncture of the western terminus of the SCT and the southern South Solomon Trench, two earthquakes (M7.4 and M7.6) occurred with disparate mechanisms (dominantly thrust and strike-slip respectively), which we interpret to indicate the tearing of the Australia plate as its northern section subducts and southern section translates along the SCT. During the 2013-2014 timeframe, little seismic activity occurred along the STEP fault. However, in May 2015, three M6.8-6.9 strike-slip events occurred in rapid succession as the STEP fault ruptured east to west. These recent events share similarities with a 1993 strike-slip STEP sequence on the SCT. Analysis of the 1993 and 2015 STEP earthquake sequences provides constraints on the plate boundary geometry of this major transform fault. Preliminary research suggests that plate motion along the STEP fault is partitioned between larger east-west oriented strike-slip events and smaller north-south thrust earthquakes. Additionally, the differences in seismic activity between the subducting slabs and the STEP fault can provide insights into how stress is transferred along the plate boundary and the mechanisms by which that stress is released.

  5. Use of archaeology to date liquefaction features and seismic events in the new madrid seismic zone, Central United States

    USGS Publications Warehouse

    Tuttle, M.P.; Lafferty, R. H., III; Guccione, M.J.; Schweig, E. S., III; Lopinot, N.; Cande, R.F.; Dyer-Williams, K.; Haynes, M.

    1996-01-01

    Prehistoric earthquake-induced liquefaction features occur in association with Native American occupation horizons in the New Madrid seismic zone. Age control of these liquefaction features, including sand-blow deposits, sand-blow craters, and sand dikes, can be accomplished by extensive sampling and flotation processing of datable materials as well as archaeobotanical analysis of associated archaeological horizons and pits. This approach increases both the amount of carbon for radiocarbon dating and the precision dating of artifact assemblages. Using this approach, we dated liquefaction features at four sites northwest of Blytheville, Arkansas, and found that at least one significant earthquake occurred in the New Madrid seismic zone between A.D. 1180 and 1400, probably about A.D. 1300 ?? 100 yr. In addition, we found three buried sand blows that formed between 3340 B.C. and A.D. 780. In this region where very large to great earthquakes appear to be closely timed, archaeology is helping to develop a paleoearthquake chronology for the New Madrid seismic zone. ?? 1996 John Wiley & Sons, Inc.

  6. The Carboneras Fault Zone (SE Spain): Constraints on Fault Zone Properties and Geometry from Controlled-Source-Generated Guided Seismic Waves

    NASA Astrophysics Data System (ADS)

    Rietbrock, A.; Haberland, C. A.; Faulkner, D. R.; Nippress, S.; Rutter, E. H.; Kelly, C. M.; Teixido, T.

    2014-12-01

    We combine geophysical data, field-geological mapping and lab measurements to study the Carboneras fault zone (CFZ) in SE Spain. The CFZ is part of the Trans-Alborán Shear Zone which constitutes part of the diffuse plate boundary between Africa and Iberia. The CFZ is inferred to behave as a stretching transform fault with˜40 km left-lateral offset. It was active principally between 12 - 6 Ma BP, and has been exhumed from ca. 1 - 2 km depth. The relatively recent movement history and the semi-arid terrain lead to excellent exposure.The phyllosilicate-rich fault gougeis excellently preserved. In 2010 we conducted a controlled source seismic experiment at the CFZ in which explosive sources in boreholes (two groups of 3 explosions) were placed in two strands of the fault zone. The signals were observed with dense linear seismic arrays crossing the CFZ at 3.5km and 8.3km distance, respectively. The recordings show clear high-energy P-phases at receivers and from sources located at or near the fault zone. We interpret these phases as P-waves trapped in the low-velocity layer (waveguide) formed by the damage zone of the fault(s). With waveform modeling (using an analytical solution assuming a straight waveguide embedded in two quarter spaces and a line source at depth) we derive basic models well-explaining the observations. Lab-measurements of the different rocks constrain the possible models. Additionally, we employed extensive three-dimensional finite-difference (3D-FD) modeling with more realistic (curved and anastomosing) waveguide geometries. It seems that the studied segments of the CFZ form effective waveguides for seismic waves with connectivity over several kilometers. The derived seismic models together with lab measurements of the seismic velocities indicate that the average fault zone core widths are in the order of 15 to 25m which is in good agreement with surface geological mapping.

  7. Critical zone weathering in the southern Sierra Nevada and Laramie Mountains imaged by seismic tomography

    NASA Astrophysics Data System (ADS)

    Hayes, J. L.; Holbrook, W. S.; Riebe, C. S.

    2011-12-01

    Near-surface variations in seismic velocity reflect differences in physical properties such as density and porosity, which in turn reflect differences in alteration of parent material by exposure to water and biologic activity. Here we present tomographic analysis of the extent of weathering from seismic refraction experiments at two areas underlain by granite: the Southern Sierra Critical Zone Observatory (SSCZO) in the fall of 2010 & 2011 and the Laramie Mountains in the spring of 2010. A 48-channel geophone array and hammer source was deployed in both surveys. In both areas seismic velocities suggest that weathering has progressed to depths of 10 m or more. When coupled with geochemical measurements of the degree of regolith weathering, these depths imply that the potential for subsurface water storage in regolith may be a larger component of the water budget than previously thought at the SSCZO. The velocity of granite bedrock was determined independently in both studies to be ~4 km/s by seismic experiments directly on outcropping granite. Two other ranges of seismic velocities seem consistent between the studies: a saprolite layer of chemically altered but still intact rock (2-4 km/s) and a regolith layer more altered than the underlying saprolite layer (<2 km/s). Using these parameters we tested hypotheses in two different granite-weathering environments. In the SSCZO a velocity-depth profile that crosses a water-saturated meadow and an adjoining forest reveal relatively low gradients in the first ten meters beneath the forest (0.75 km/s per 10 m) and higher gradients beneath the meadow (3 km/s per 10 m). From these observations, we hypothesize that the saturated meadow may provide a reducing environment that inhibits chemical weathering relative to better-drained, more oxidizing conditions in the forest. In the 1.4 Ga Sherman batholith of the Laramie Mountains we observe isolated outcrops of Lincoln Granite within the Sherman Granite. Two 0.5 km profiles

  8. Microstructural study of the Mertz shear zone, East Antarctica. Implications for deformation processes and seismic anisotropy.

    NASA Astrophysics Data System (ADS)

    Lamarque, Gaëlle; Bascou, Jérôme; Maurice, Claire; Cottin, Jean-Yves; Ménot, René-Pierre

    2015-04-01

    The Mertz Shear Zone (MSZ; 146°E 67°S; East Antarctica) is one major lithospheric-scale structure which outcrops on the eastern edge of the Terre Adélie Craton (Ménot et al., 2007) and that could connected with shear zones of South Australia (e.g., Kalinjala or Coorong shear zone (Kleinschmidt and Talarico, 2000; Gibson et al., 2013)) before the Cretaceous opening of the Southern Ocean. Geochronological and metamorphic studies indicated an MSZ activity at 1.7 and 1.5 Ga respectively in amphibolite and greenschists facies conditions. The deformation affects both the intermediate and lower crust levels, without associated voluminous magma injection. Granulite crop out in the area of the MSZ. They were dated at 2.4 Ga (Ménot et al., 2005) and could represent some preserved Neoarchean tectonites. These rocks show various degrees of deformation including penetrative structures that may display comparable features with that observed in amphibolite and greenschists facies rocks, i.e. NS-striking and steeply dipping foliation with weekly plunging lineation. In the field, cinematic indicators for the MSZ argue for a dominant dextral shear sense. We proceed to optical analysis and crystallographic preferred orientation (CPO) measurements using EBSD technique in order to better constrain the deformation processes. Our results highlight (1) a microstructural gradient from highly deformed rocks (mylonites), forming plurimetric large shear bands and showing evidences of plastic deformation, to slightly deformed rocks in preserved cores with no evidences of plastic deformation or with a clear strong static recrystallization; (2) CPO of minerals related with variations on deformation conditions. Feldspar and quartz CPO argue for plastic deformation at high temperature in the most deformed domains and for the absence of deformation or an important stage of static recrystallization in preserved cores; (3) uncommon CPO in orthopyroxene which are characterized by [010]-axes

  9. Seismological investigation of earthquakes in the New Madrid seismic zone and the northeastern extent of the New Madrid seismic zone: Final report, September 1981-December 1986

    SciTech Connect

    Herrmann, R.B.; Taylor, K.; Nguyen, B.

    1988-07-01

    Earthquake activity in the Central Mississippi Valley has been monitored by an eight station seismograph network in the Wabash River Valley of southeastern Illinois and by a six station seismograph network in the New Madrid seismic zone. This network is a major component of a larger network in the region, jointly sponsored by the NRC, USGS, universities and states. During the time period of the contract, October 1981 through December 1986, 1206 earthquakes were located in the Central Mississippi Valley, of which 808 were in the New Madrid, Missouri area. Significant earthquakes studied in detail occurred in northeastern Ohio on January 31, 1986 and in southeastern Illinois on June 10, 1987. Focal mechanisms have been calculated for the 10 June 1987 southern Illinois earthquake using both P-wave first motions and long-period surface-wave spectral amplitude data. The long-period surface-wave and strong ground motion accelerogram recordings of the January 3, 1986, northeastern Ohio earthquake were used to estimate the focal mechanism and source time function of the source.reverse arrow

  10. A New Set of Focal Mechanisms and a Geodynamic Model for the Eastern Tennessee Seismic Zone

    NASA Astrophysics Data System (ADS)

    Cooley, M. T.; Powell, C. A.; Choi, E.

    2014-12-01

    We present a new set of 26 focal mechanisms for the eastern Tennessee seismic zone (ETSZ) and discuss the implications for regional uplift. The mechanisms are for earthquakes with magnitudes 2.5 and greater occurring after 1999. The ETSZ is the second largest seismic zone in the central and eastern US and the seismicity is attributed to reactivation of a major Grenville-age shear zone. P- and S- wave velocity models, the distribution of hypocenters, focal mechanisms, and potential field anomalies suggest the presence of a basement shear zone. The new focal mechanism solutions supplement and are consistent with a previously calculated set of 26 focal mechanisms for the period 1983-1993. Focal mechanisms fall into two groups. The first group shows strike-slip motion on steeply dipping nodal planes striking N-S/E-W and NE-SW/NW-SE. Mechanisms in the second group display primarily dip-slip motion and are constrained geographically to the southern portion of the seismic zone. Events in the second group are among the shallowest in the dataset (8-12 km). We are developing a geodynamic model of the regional structure to examine the stress regime, which may be changing with depth. This model will be used to determine a possible relationship between the localized normal faulting and previously established recent regional uplift.

  11. Seismic imaging of deformation zones associated with normal fault-related folding

    NASA Astrophysics Data System (ADS)

    Lapadat, Alexandru; Imber, Jonathan; Iacopini, David; Hobbs, Richard

    2016-04-01

    Folds associated with normal faulting, which are mainly the result of fault propagation and linkage of normal fault segments, can exhibit complex deformation patterns, with multiple synthetic splay faults, reverse faults and small antithetic Riedel structures accommodating flexure of the beds. Their identification is critical in evaluating connectivity of potential hydrocarbon reservoirs and sealing capacity of faults. Previous research showed that seismic attributes can be successfully used to image complex structures and deformation distribution in submarine thrust folds. We use seismic trace and coherency attributes, a combination of instantaneous phase, tensor discontinuity and semblance attributes to identify deformation structures at the limit of seismic resolution, which accommodate seismic scale folding associated with normal faulting from Inner Moray Firth Basin, offshore Scotland. We identify synthetic splay faults and reverse faults adjacent to the master normal faults, which are localized in areas with highest fold amplitudes. This zone of small scale faulting is the widest in areas with highest fault throw / fold amplitude, or where a bend is present in the main fault surface. We also explore the possibility that changes in elastic properties of the rocks due to deformation can contribute to amplitude reductions in the fault damage zones. We analyse a pre-stack time-migrated 3D seismic data-set, where seismic reflections corresponding to a regionally-continuous and homogeneous carbonate layer display a positive correlation between strain distribution and amplitude variations adjacent to the faults. Seismic amplitude values are homogeneously distributed within the undeformed area of the footwall, with a minimum deviation from a mean amplitude value calculated for each seismic line. Meanwhile, the amplitude dimming zone is more pronounced (negative deviation increases) and widens within the relay zone, where sub-seismic scale faults, which accommodate

  12. Monitoring in situ deformation induced by a fluid injection in a fault zone in shale using seismic velocity changes

    NASA Astrophysics Data System (ADS)

    Rivet, D.; De Barros, L.; Guglielmi, Y.; Castilla, R.

    2015-12-01

    We monitor seismic velocity changes during an experiment at decametric scale aimed at artificially reactivate a fault zone by a high-pressure hydraulic injection in a shale formation of the underground site of Tournemire, South of France. A dense and a multidisciplinary instrumentation, with measures of pressure, fluid flow, strain, seismicity, seismic properties and resistivity allow for the monitoring of this experiment. We couple hydromechanical and seismic observations of the fault and its adjacent areas to better understand the deformation process preceding ruptures, and the role played by fluids. 9 accelerometers recorded repeated hammers shots on the tunnel walls. For each hammer shot we measured small travel time delays on direct P and S waves. We then located the seismic velocity perturbations using a tomography method. At low injection pressure, i.e. P< 15 Bars, we observe an increase of P-waves velocity around the injection, while we measure no change in S waves velocity. When the pressure overcomes 15 Bars, velocity perturbations dramatically increase with both P and S waves affected. A decrease of velocity is observed close to the injection point and is surrounded by regions of increasing velocity. Our observations are consistent with hydromechanical measures. Below 15 Bars, we interpret the P-wave velocity increase to be related to the compression of the fault zone around the injection chamber. Above 15 Bars, we measure a shear and dilatant fault movement, and a rapid increase in the injected fluid flow. At this step, our measures are coherent with a poroelastic opening of the fault with velocities decrease at the injection source and velocities increase related to stress transfer in the far field. Velocity changes prove to be efficient to monitor stress/strain variation in an activated fault, even if these observations might produce complex signals due to the highly contrasted hydromechanical responses in a heterogeneous media such as a fault zone.

  13. High Resolution Seismic Imaging of the Trench Canyon Fault Zone, Mono Lake, Northeastern California

    NASA Astrophysics Data System (ADS)

    Novick, M. W.; Jayko, A. S.; Roeske, S.; McClain, J. S.; Hart, P. E.; Boyle, M.

    2009-12-01

    High resolution seismic imaging of Mono Lake, located in northeastern California, has revealed an approximately northwest striking fault in the area to the west of aerially exposed Negit Volcano. This fault, henceforth referred to as the Trench Canyon Fault (TCF), has also been mapped onshore along a correlating strike as far north as Cedar Hill Volcano, located to the northeast of the lake on the California/Nevada border. Onshore, the TCF was mapped for approximately 10 kilometers using air photos, DEM images, and standard geologic pace and compass mapping techniques. The TCF post- dates the last glacial maximum, evidenced by the cutting of wave cut benches along Cedar Hill Volcano. Relict, non-historic shorelines, left by the steady evaporation of Mono Lake beginning approximately 13k, are also repeatedly cut by the fault. Additional evidence of fault presence includes sag ponds, pressure ridges, tectonically fractured rocks, and normal fault scarps found along strike. Offshore, DEM images show a northeast striking structure to the northwest of Negit Volcano, which is co-linear with the onshore TCF. High resolution seismic imaging of the structure, using an applied acoustic/SIG mini-sparker system, reveals steeply dipping Holocene sediments, as well as volcanic deposits from active vents which have erupted in the last 1000 years, offset by the fault. Detailed structural analysis of the previously unstudied Trench Canyon Fault (TFC) and faults in the Cedar Hill region of northern California, along with seismic studies of sediments beneath Mono Lake not only allow for a better comprehension of this minor fault system, but provide greater understanding of the larger and more complex Walker Lane Shear Zone. Fault analyses, combined and correlated with those from CHV, give a better understanding of how slip is transferred into the complicated Mina defection to the east, from the dextral and normal faults along the Sierra Nevada Range front.

  14. Microseismicity and b-values of the Wabash Valley Intraplate Seismic Zone from short-period phased arrays

    NASA Astrophysics Data System (ADS)

    Conder, J. A.; Milliron, K.; Zhu, L.

    2014-12-01

    Two phased arrays of 9 short-period stations each are currently recording in the Wabash Valley Seismic Zone (WVSZ) as part of the EarthScope Wabash FlexArray project. The phased arrays aim to address the level of microseismicity produced by the intraplate seismic zone. Although seismic hazard maps of the U.S. Midwest are dominated by the New Madrid Seismic Zone (NMSZ), the WVSZ has released 40% more seismic energy than the NMSZ over the last half century with four events larger than M5 and only one in the NMSZ reaching that threshold. A comparison of event frequency statistics suggests two markedly different systems. The NMSZ exhibits b-values near unity, but the WVSZ exhibits much smaller b-values in the 0.6-0.7 range. Deviations less than unity may be controlled through crack geometry and/or greater shear stresses possibly indicating a time-dependent, or migrating, behavior in mid-continent. Alternatively, it may be the case that the low b-values are simply a reflection of less complete catalog than the NMSZ. A previous short-term microseismicity study of the WVSZ shows a dearth of non-anthropogenic sources in the Wabash. The phased array near the central portion of the WVSZ largely confirms the previously noted lack of substantial natural seismicity along the central portion of the fault system and the associated low b-values. However, the phased array near the southern termination of the fault system shows significantly more activity. Importantly, the largest events from the Wabash, including the 2008 M5.4 Mt. Carmel and the 1968 M5.5 Harrisburg events occurred near the northern and southern ends of the fault system. The phased arrays seem to indicate different portions of the fault system yielding different levels of activity. As the catalogs become more complete, there is a preliminary suggestion that the anomalously low b-values for the Wabash do not denote a system under significantly larger stresses, but rather a conflation of regions along-strike of the

  15. Seismic Activity in the Gulf of Mexico: a Preliminary Analysis

    NASA Astrophysics Data System (ADS)

    Franco, S. I.; Canet, C.; Iglesias, A.; Valdes-Gonzales, C. M.

    2013-05-01

    The southwestern corner of Gulf of Mexico (around the northern Isthmus of Tehuantepec) is exposed to an intense deep (> 100 km) seismic activity caused by the subduction of the Cocos plate. Aside from this, the gulf has been considered as a zone of low or no-seismicity. However, a sparse shallow seismic activity is observed across the Gulf of Mexico; some of these earthquakes have been strongly felt (e.g. 23/05/2007 and 10/09/2006), and the Jaltipan, 1959 earthquake caused fatalities and severe destruction in central and southern Veracruz. In this work we analyze 5 relevant earthquakes that occurred since 2001. At the central Gulf of Mexico focal mechanisms show inverse faults oriented approximately NW-SE with dip near 45 degrees, suggesting a link to sediment loading and/or to salt tectonics. On the other hand, in the southwestern corner of the gulf we analyzed some clear examples of strike-slip faults and activity probably related to the Veracruz Fault. One anomalous earthquake, recorded in 2007 in the western margin of the gulf, shows a strike-slip mechanism indicating a transform regime probably related with the East Mexican Fault. The recent improvement of the Mexican Seismological broadband network have allowed to record small earthquakes distributed in and around the Gulf of Mexico. Although the intermediate and large earthquakes in the region are infrequent, the historic evidence indicates that the magnitudes could reach Mw~6.4. This fact could be taken in consideration to reassess the seismic hazard for oil and industrial infrastructure in the region.

  16. Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China.

    PubMed

    Zhou, Xiaocheng; Du, Jianguo; Chen, Zhi; Cheng, Jianwu; Tang, Yi; Yang, Liming; Xie, Chao; Cui, Yueju; Liu, Lei; Yi, Li; Yang, Panxin; Li, Ying

    2010-01-01

    The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity. PMID:21134257

  17. Geochemistry of soil gas in the seismic fault zone produced by the Wenchuan Ms 8.0 earthquake, southwestern China

    PubMed Central

    2010-01-01

    The spatio-temporal variations of soil gas in the seismic fault zone produced by the 12 May 2008 Wenchuan Ms 8.0 earthquake were investigated based on the field measurements of soil gas concentrations after the main shock. Concentrations of He, H2, CO2, CH4, O2, N2, Rn, and Hg in soil gas were measured in the field at eight short profiles across the seismic rupture zone in June and December 2008 and July 2009. Soil-gas concentrations of more than 800 sampling sites were obtained. The data showed that the magnitudes of the He and H2 anomalies of three surveys declined significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H2 (40 and 279.4 ppm, respectively) were found in three replicates at the south part of the rupture zone close to the epicenter. The spatio-temporal variations of CO2, Rn, and Hg concentrations differed obviously between the north and south parts of the fault zone. The maximum He and H2 concentrations in Jun 2008 occurred near the parts of the rupture zone where vertical displacements were larger. The anomalies of He, H2, CO2, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activity. PMID:21134257

  18. Ionospheric Response Due to Seismic Activity

    NASA Astrophysics Data System (ADS)

    Sharma, Dinesh Kumar

    2016-07-01

    Signatures of the seismic activity in the ionospheric F2 region have been studied by analyzing the measurement of electron and ion temperatures during the occurrence of earthquake. The ionospheric electron and ion temperatures data recorded by the RPA payload aboard the Indian SROSS-C2 satellite during the period from January 1995 to December 2000 were used for the altitude range 430-630 km over Indian region. The normal day's electron and ion temperatures have been compared to the temperatures recorded during the seismic activity. The details of seismic events were obtained from USGS earthquake data information website. It has been found that the average electron temperature is enhanced during the occurrence of earthquakes by 1.2 to 1.5 times and this enhancement was for ion temperature ranging from 1.1to 1.3 times over the normal day's average temperatures. The above careful quantitative analysis of ionospheric electron and ion temperatures data shows the consistent enhancement in the ionospheric electron and ion temperatures. It is expected that the seismogenic vertical electrical field propagates up to the ionospheric heights and induces Joule heating that may cause the enhancement in ionospheric temperatures.

  19. Issues Related to Seismic Activity Induced by the Injection of CO2 in Deep Saline Aquifers

    SciTech Connect

    Sminchak, Joel; Gupta, Neeraj; Byrer, Charles; Bergman, Perry

    2001-05-31

    technology provides effective tools for investigating and preventing induced seismic activity. More research is recommended on developing site selection criteria and operational constraints for CO2 storage sites near zones of seismic concerns.

  20. The Afar triple junction accommodation zone from InSAR derived strain and seismicity

    NASA Astrophysics Data System (ADS)

    Pagli, C.; Ebinger, C. J.; Keir, D.; Wang, H.

    2015-12-01

    Strain and seismicity show us the mode by which deformation is accommodated in rifting continents. Here we present a combined analysis of InSAR derived strain maps and seismicity of the Afar triple junction from 2006 to 2010. Our analysis shows that that the plate spreading motion is accommodated in different modes. A dogbone-shaped seismicity and strain distribution dominates the northern Red Sea branch of the triple junction, likely as a result of repeated dike intrusions 2005-2010. East of the triple junction, in the Gulf of Aden branch the strain and seismicity distribution appears decoupled. The strain focuses across the central part of several overlapping rifts, while the seismicity mainly occurs at the rift tips. Conversely, the Main Ethiopian Rift branch shows a narrow and elongated zone of both high strain and seismicity. The pattern suggests that the recent history of magmatic intrusions in the northern branch and mainly tectonic extension in the other branches creates a diverse triple junction accommodation zone.

  1. Thrust-type subduction-zone earthquakes and seamount asperites: A physical model for seismic rupture

    SciTech Connect

    Cloos, M. )

    1992-07-01

    A thrust-type subduction-zone earthquake of M{sub W} 7.6 ruptures an area of {approximately}6,000 km{sup 2}, has a seismic slip of {approximately}1 m, and is nucleated by the rupture of an asperity {approximately}25km across. A model for thrust-type subduction-zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick-slip faulting. A M{sub W} 7.6 event would correspond to the near-basal rupture of a {approximately}2-km-tall seamount. The base of the seamount is surrounded by a low shear-strength layer composed of subducting sediment that also deforms between seismic events by distributed strain (viscous flow). Planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second. The faults in the shear zone are disrupted after the event by aseismic, slow viscous flow of the subducting sediment layer. Consequently, the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture.

  2. Deep critical zone weathering at the southern Sierra Nevada Critical Zone Observatory imaged by seismic waveform tomography

    NASA Astrophysics Data System (ADS)

    Hayes, J. L.; Holbrook, W.; Riebe, C. S.

    2012-12-01

    We present seismic velocity profiles that constrain the extent of weathering and frequency of velocity heterogeneities at depths less than 40 m in the southern Sierra Nevada Critical Zone Observatory (SSCZO) from waveform tomography modeling of a seismic refraction experiment. Near-surface variations in seismic velocity reflect differences in alteration of parent material by chemical, hydrological and biological processes. Previous traveltime tomography models from these data suggest that the depth to bedrock in the SSCZO is typically ~25 m; thus the potential for subsurface water storage in regolith may be a larger component of water storage than previously thought. Traveltime tomography is unable to resolve heterogeneities with horizontal wavelengths less than 10 m, such as those observed along a surveyed road cut beneath our seismic profile. For a higher resolution seismic image, we apply waveform tomography, which is more robust than traveltime tomography at approximating the wave equation and thus should provide images of subsurface heterogeneities such as corestones and fracture networks. This technique uses a weak scattering approximation to account for the amplitude and phase of the recorded waveforms, rather than just the traveltimes. A 48-channel vertical geophone array and hammer source was deployed over a 7 m high road cut with receiver and shot spacing of 2 m and 4 m respectively. The road cut displays lateral variation in weathering from a friable saprolite to coherent granodiorite which are compared to velocity variations modeled using waveform tomography.

  3. Geyser's Eruptive Activity in Broadband Seismic Records

    NASA Astrophysics Data System (ADS)

    Kugaenko, Yulia; Saltykov, Vadim

    2010-05-01

    A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapor phase (steam). The formation of geysers is due to particular hydrogeological conditions, which exist in only a few places on Earth, so they are a fairly rare phenomenon. The reasons of geyser periodicity and specifics of the activity for every particular geyser are not completely clear yet. So almost for all known geysers it is necessary to develop the personal model. In given study we first use seismic method for detection of possible hidden feature of geyser's eruptive activity in Kamchatkan Valley of the Geysers. Broadband seismic records of geyser generated signals were obtained in hydrothermal field. The Valley of the Geysers belongs to Kronotskiy State Natural Biosphere Reserve and the UNESCO World Natural Heritage Site "Volcanoes of Kamchatka". Neither seismological nor geophysical investigations were carried out here earlier. In September, 2009 seismic observation was organized in geyser's field by 24-bit digital output broadband seismometers (GURALP CMG-6TD flat velocity response 0.033-50 Hz). Four geysers were surveyed: the fountain type Big and Giant geysers; the cone type Pearl geyser and the short-period Gap geyser. Seismometers were set as possible close to the geyser's surface vent (usually at the distance near 3-5 m). Main parameters of the eruptions for the investigated geysers: - The Giant geyser is the most powerful among the regular active geysers in Kamchatkan Valley of the Geysers. The height of the fountain reaches 30 meters, the mass of water erupted is about 40-60 tons. The main cycle of activity varies significantly: in 1945 the intervals between eruptions was near 3 hours, nowadays it is 5-6 hours. As a geyser of fountain type, the Giant geyser erupts from the 2*3 m2 pool of water. - The Big geyser was flooded by the lake after the natural catastrophe (giant mud-stone avalanche, formed by landslide, bed into Geiyzernaya

  4. Boundary separating the seismically active reelfoot rift from the sparsely seismic Rough Creek graben, Kentucky and Illinois

    USGS Publications Warehouse

    Wheeler, R.L.

    1997-01-01

    The Reelfoot rift is the most active of six Iapetan rifts and grabens in central and eastern North America. In contrast, the Rough Creek graben is one of the least active, being seismically indistinguishable from the central craton of North America. Yet the rift and graben adjoin. Hazard assessment in the rift and graben would be aided by identification of a boundary between them. Changes in the strikes of single large faults, the location of a Cambrian transfer zone, and the geographic extent of alkaline igneous rocks provide three independent estimates of the location of a structural boundary between the rift and the graben. The boundary trends north-northwest through the northeastern part of the Fluorspar Area Fault Complex of Kentucky and Illinois, and has no obvious surface expression. The boundary involves the largest faults, which are the most likely to penetrate to hypocentral depths, and the boundary coincides with the geographic change from abundant seismicity in the rift to sparse seismicity in the graben. Because the structural boundary was defined by geologic variables that are expected to be causally associated with seismicity, it may continue to bound the Reelfoot rift seismicity in the future.

  5. Reconciling short recurrence intervals with minor deformation in the New Madrid seismic zone

    USGS Publications Warehouse

    Schweig, E.S.; Ellis, M.A.

    1994-01-01

    At least three great earthquakes occurred in the New Madrid seismic zone in 1811 and 1812. Estimates of present-day strain rates suggest that such events may have a repeat time of 1000 years or less. Paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years. However, pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed. This suggests that the seismic zone is a young feature, possibly as young as several tens of thousands of years old and no more than a few million years old.At least three great earthquakes occurred in the New Madrid seismic zone in 1811 and 1812. Estimates of present-day strain rates suggest that such events may have a repeat time of 1000 years or less. Paleoseismological data also indicate that earthquakes large enough to cause soil liquefaction have occurred several times in the past 5000 years. However, pervasive crustal deformation expected from such a high frequency of large earthquakes is not observed. This suggests that the seismic zone is a young feature, possibly as young as several tens of thousands of years old and no more than a few million years old.

  6. Evidence of contemporary and ancient excess fluid pressure in the New Madrid seismic zone of the Reelfoot Rift, central United States

    USGS Publications Warehouse

    McKeown, F.A.; Diehl, S.

    1994-01-01

    In the winter of 1811-12, three of the largest historic earthquakes in the United States occurred near New Madrid, Missouri. Seismicity continues to the present day throughout a tightly clustered pattern of epicenters centered on the bootheel of Missouri, including parts of northeastern Arkansas, northwestern Tennessee, western Kentucky, and southern Illinois. In 1990, the New Madrid seismic zone/central United States became the first seismically active region east of the Rocky Mountains to be designated a priority research area within the National Earthquake Hazards Reduction Program (NEHRP). This professional paper is a collection of papers, some published separately, presenting results of the newly intensified research program in this area. Major components of this research program include tectonic framework studies, seismicity and deformation monitoring and modeling, improved seismic hazard and risk assessments, and cooperative hazard mitigation studies.

  7. 2010 Maule earthquake slip correlates with pre-seismic locking of Andean subduction zone.

    PubMed

    Moreno, Marcos; Rosenau, Matthias; Oncken, Onno

    2010-09-01

    The magnitude-8.8 Maule (Chile) earthquake of 27 February 2010 ruptured a segment of the Andean subduction zone megathrust that has been suspected to be of high seismic potential. It is the largest earthquake to rupture a mature seismic gap in a subduction zone that has been monitored with a dense space-geodetic network before the event. This provides an image of the pre-seismically locked state of the plate interface of unprecedentedly high resolution, allowing for an assessment of the spatial correlation of interseismic locking with coseismic slip. Pre-seismic locking might be used to anticipate future ruptures in many seismic gaps, given the fundamental assumption that locking and slip are similar. This hypothesis, however, could not be tested without the occurrence of the first gap-filling earthquake. Here we show evidence that the 2010 Maule earthquake slip distribution correlates closely with the patchwork of interseismic locking distribution as derived by inversion of global positioning system (GPS) observations during the previous decade. The earthquake nucleated in a region of high locking gradient and released most of the stresses accumulated in the area since the last major event in 1835. Two regions of high seismic slip (asperities) appeared to be nearly fully locked before the earthquake. Between these asperities, the rupture bridged a zone that was creeping interseismically with consistently low coseismic slip. The rupture stopped in areas that were highly locked before the earthquake but where pre-stress had been significantly reduced by overlapping twentieth-century earthquakes. Our work suggests that coseismic slip heterogeneity at the scale of single asperities should indicate the seismic potential of future great earthquakes, which thus might be anticipated by geodetic observations. PMID:20829792

  8. Seismic Wave Attenuation Estimated from Tectonic Tremor and Radiated Energy in Tremor for Various Subduction Zones

    NASA Astrophysics Data System (ADS)

    Yabe, S.; Baltay, A.; Ide, S.; Beroza, G. C.

    2013-12-01

    Ground motion prediction is an essential component of earthquake hazard assessment. Seismic wave attenuation with distance is an important, yet difficult to constrain, factor for such estimation. Using the empirical method of ground motion prediction equations (GMPEs), seismic wave attenuation with distance, which includes both the effect of anelastic attenuation and scattering, can be estimated from the distance decay of peak ground velocity (PGV) or peak ground acceleration (PGA) of ordinary earthquakes; however, in some regions where plate-boundary earthquakes are infrequent, such as Cascadia and Nankai, there are fewer data with which to constrain the empirical parameters. In both of those subduction zones, tectonic tremor occurs often. In this study, we use tectonic tremor to estimate the seismic wave attenuation with distance, and in turn use the attenuation results to estimate the radiated seismic energy of tremor. Our primary interest is in the variations among subduction zones. Ground motion attenuation and the distribution of released seismic energy from tremors are two important subduction zone characteristics. Therefore, it is very interesting to see whether there are variations of these parameters in different subduction zones, or regionally within the same subduction zone. It is also useful to estimate how much energy is released by tectonic tremor from accumulated energy to help understand subduction dynamics and the difference between ordinary earthquakes and tremor. We use the tectonic tremor catalog of Ide (2012) in Nankai, Cascadia, Mexico and southern Chile. We measured PGV and PGA of individual tremor bursts at each station. We assume a simple GMPE relationship and estimate seismic attenuation and relative site amplification factors from the data. In the Nankai subduction zone, there are almost no earthquakes on the plate interface, but intra-slab earthquakes occur frequently. Both the seismic wave attenuation with distance and the site

  9. The Crustal Structure of the Eastern Tennessee Seismic Zone Imaged by means of Seismic Noise Tomography and Potential Fields Inversion Methods

    NASA Astrophysics Data System (ADS)

    Brandmayr, E.; Arroucau, P.; Kuponiyi, A.; Vlahovic, G.

    2015-12-01

    We investigate the crustal structure of the Eastern Tennessee Seismic Zone (ETSZ) by means of group velocity tomography maps from seismic noise data analysis and potential fields inversion with the located Euler deconvolution method. Preliminary tomography results show that, in the uppermost crust, the New York-Alabama (NY-AL) magnetic lineament surface projection represents the boundary between a low velocity anomaly to the NW of the lineament and a high velocity anomaly to the SE of it. The low velocity anomaly migrates towards SE with increasing depth, suggesting a possible SE dipping weak structure in which most of the seismic activity takes place. Inversion of magnetic field data shows that the top of the magnetic basement ranges between 5 and 10 km of depth in the Valley and the Ridge physiographic province while it is shallower (less than 2 km of depth) and locally outcropping in the Blue Ridge province and in the Cumberland Plateau province. The estimated depth of the top of the magnetic basement is in general agreement with existing sedimentary cover map of the broad study area, although the local features of the ETSZ presented in this work are not resolved by previous studies due to poor resolution. The correlation between the magnetic signature and the position of the seismic velocity anomalies support the interpretation of the low velocity zone as a major basement fault, trending NE-SW and juxtaposing Granite-Rhyolite basement to the NW from Grenville southern Appalachian basement to the SE, of which the NY-AL magnetic lineament is the projection on the surface. In order to better constrain our interpretation, inversion of tomography results to obtain absolute shear waves velocity models will be performed as a next step.

  10. Seismic evidence for fluids in fault zones on top of the subducting Cocos Plate beneath Costa Rica

    NASA Astrophysics Data System (ADS)

    Van Avendonk, Harm J. A.; Holbrook, W. Steven; Lizarralde, Daniel; Mora, Mauricio M.; Harder, Steven; Bullock, Andrew D.; Alvarado, Guillermo E.; Ramírez, Carlos J.

    2010-05-01

    In the 2005 TICOCAVA explosion seismology study in Costa Rica, we observed crustal turning waves with a dominant frequency of ~10 Hz on a linear array of short-period seismometers from the Pacific Ocean to the Caribbean Sea. On one of the shot records, from Shot 21 in the backarc of the Cordillera Central, we also observed two seismic phases with an unusually high dominant frequency (~20 Hz). These two phases were recorded in the forearc region of central Costa Rica and arrived ~7 s apart and 30-40 s after the detonation of Shot 21. We considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active-source seismic experiment. Such high-frequency phases following Shot 21 were not recorded after Shots 22, 23 and 24, all in the backarc of Costa Rica, which might suggest that they were produced by some other source. However, earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude. In addition, we would have expected to see more arrivals from such an earthquake on other seismic stations in central Costa Rica. We therefore investigate whether the high-frequency arrivals may be the result of a deep seismic reflection from the subducting Cocos Plate. The timing of these phases is consistent with a shear wave from Shot 21 that was reflected as a compressional (S×P) and a shear (S×S) wave at the top of the subducting Cocos slab between 35 and 55 km depth. The shift in dominant frequency from ~10 Hz in the downgoing seismic wave to ~20 Hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency. The spectral amplitude characteristics of the S×P and S×S phases from Shot 21 are consistent with a very high Vp/Vs ratio of 6 in ~5 m thick, slab-parallel layers. This result suggests that a system of thin shear

  11. Earthquake Hazard in the New Madrid Seismic Zone Remains a Concern

    USGS Publications Warehouse

    Frankel, A.D.; Applegate, D.; Tuttle, M.P.; Williams, R.A.

    2009-01-01

    There is broad agreement in the scientific community that a continuing concern exists for a major destructive earthquake in the New Madrid seismic zone. Many structures in Memphis, Tenn., St. Louis, Mo., and other communities in the central Mississippi River Valley region are vulnerable and at risk from severe ground shaking. This assessment is based on decades of research on New Madrid earthquakes and related phenomena by dozens of Federal, university, State, and consulting earth scientists. Considerable interest has developed recently from media reports that the New Madrid seismic zone may be shutting down. These reports stem from published research using global positioning system (GPS) instruments with results of geodetic measurements of strain in the Earth's crust. Because of a lack of measurable strain at the surface in some areas of the seismic zone over the past 14 years, arguments have been advanced that there is no buildup of stress at depth within the New Madrid seismic zone and that the zone may no longer pose a significant hazard. As part of the consensus-building process used to develop the national seismic hazard maps, the U.S. Geological Survey (USGS) convened a workshop of experts in 2006 to evaluate the latest findings in earthquake hazards in the Eastern United States. These experts considered the GPS data from New Madrid available at that time that also showed little to no ground movement at the surface. The experts did not find the GPS data to be a convincing reason to lower the assessment of earthquake hazard in the New Madrid region, especially in light of the many other types of data that are used to construct the hazard assessment, several of which are described here.

  12. Variable post-Paleozoic deformation detected by seismic reflection profiling across the northwestern "prong" of New Madrid seismic zone

    USGS Publications Warehouse

    McBride, J.H.; Pugin, Andre J.M.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.; Devera, J.A.; Denny, F.B.; Woolery, E.W.

    2003-01-01

    High-resolution shallow seismic reflection profiles across the northwesternmost part of the New Madrid seismic zone (NMSZ) and northwestern margin of the Reelfoot rift, near the confluence of the Ohio and Mississippi Rivers in the northern Mississippi embayment, reveal intense structural deformation that apparently took place during the late Paleozoic and/or Mesozoic up to near the end of the Cretaceous Period. The seismic profiles were sited on both sides of the northeast-trending Olmsted fault, defined by varying elevations of the top of Mississippian (locally base of Cretaceous) bedrock. The trend of this fault is close to and parallel with an unusually straight segment of the Ohio River and is approximately on trend with the westernmost of two groups of northeast-aligned epicenters ("prongs") in the NMSZ. Initially suspected on the basis of pre-existing borehole data, the deformation along the fault has been confirmed by four seismic reflection profiles, combined with some new information from drilling. The new data reveal (1) many high-angle normal and reverse faults expressed as narrow grabens and anticlines (suggesting both extensional and compressional regimes) that involved the largest displacements during the late Cretaceous (McNairy); (2) a different style of deformation involving probably more horizontal displacements (i.e., thrusting) that occurred at the end of this phase near the end of McNairy deposition, with some fault offsets of Paleocene and younger units; (3) zones of steeply dipping faults that bound chaotic blocks similar to that observed previously from the nearby Commerce geophysical lineament (CGL); and (4) complex internal deformation stratigraphically restricted to the McNairy, suggestive of major sediment liquefaction or landsliding. Our results thus confirm the prevalence of complex Cretaceous deformations continuing up into Tertiary strata near the northern terminus of the NMSZ. ?? 2003 Elsevier Science B.V. All rights reserved.

  13. Structural Analysis of Active North Bozgush Fault Zone (NW Iran)

    NASA Astrophysics Data System (ADS)

    Saber, R.; Isik, V.; Caglayan, A.

    2013-12-01

    NW Iran is one of the seismically active regions between Zagros Thrust Belt at the south and Caucasus at the north. Not only large magnitude historical earthquakes (Ms>7), but also 1987 Bozgush, 1997 Ardebil (Mw 6.1) and 2012 Ahar-Varzagan (Mw 6.4) earthquakes reveal that the region is seismically active. The North Bozgush Fault Zone (NBFZ) in this region has tens of kilometers in length and hundreds of meters in width. The zone has produced some large and destructive earthquakes (1593 M:6.1 and 1883 M:6.2). The NBFZ affects the Cenozoic units and along this zone Eocene units thrusted over Miocene and/or Plio-Quaternary sedimentary units. Together with morphologic features (stream offsets and alluvial fan movements) affecting the young unites reveal that the zone is active. The zone is mainly characterized by strike-slip faults with reverse component and reverse faults. Reverse faults striking N55°-85°E and dip of 40°-50° to the SW while strike-slip faults show right lateral slip with N60°-85°W and N60°-80°E directions. Our structural data analysis in NBFZ indicates that the axis direction of σ2 principal stress is vertical and the stress ratio (R) is 0.12. These results suggest that the tectonic regime along the North Bozgush Fault Zone is transpressive. Obtained other principal stresses (σ1, σ3) results are compatible with stress directions and GPS velocity suggested for NW Iran.

  14. Detailed structures of the subducted Philippine Sea Plate beneath northeast Taiwan: A new type of double seismic zone

    NASA Astrophysics Data System (ADS)

    Kao, Honn; Rau, Ruey-Juin

    1999-01-01

    We studied the detailed structure of the subducted Philippine Sea plate beneath northeast Taiwan where oblique subduction, regional collision, and back arc opening are all actively occurring. Simultaneous inversion for velocity structure and earthquake hypocenters are performed using the vast, high-quality data recorded by the Taiwan Seismic Network. We further supplement the inversion results with earthquake source parameters determined from inversion of teleseismic P and SH waveforms, a critical step to define the position of plate interface and the state of strain within the subducted slab. The most interesting feature is that relocated hypocenters tend to occur along a two-layered structure. The upper layer is located immediately below the plate interface and extends down to 70-80 km at a dip of 40°-50°. Below approximately 100 km, the dip increases dramatically to 70°-80°. The lower layer commences at 45-50 km and stays approximately parallel to the upper layer with a separation of 15±5 km in between down to 70-80 km. Below that the separation decreases and the two layers seem to gradually merge into one Wadati-Benioff Zone. We propose to term the classic double seismic zones observed beneath Japan and Kuril as "type I" and that we observed as "type II," respectively. A global survey indicates that type II double seismic zones are also observed in New Zealand near the southernmost North Island, Cascadia, just north of the Mendocino triple junction, and the Cook Inlet area of Alaska. All of them are located near the termini of subducted slabs in a tectonic setting of oblique subduction. We interpret the seismogenesis of type II double seismic zones as reflecting the lateral compressive stress between the subducted plate and the adjacent lithosphere (originating from oblique subduction) and the downdip extension (from slab pulling force). The upper seismic layer represents seismicity occurring in the upper crust of a subducted plate and/or along the plate

  15. Seismic active control by neural networks.

    SciTech Connect

    Tang, Y.

    1998-01-01

    A study on the application of artificial neural networks (ANNs) to activate structural control under seismic loads is carried out. The structure considered is a single-degree-of-freedom (SDF) system with an active bracing device. The control force is computed by a trained neural network. The feed-forward neural network architecture and an adaptive back-propagation training algorithm is used in the study. The neural net is trained to reproduce the function that represents the response-excitation relationship of the SDF system under seismic loads. The input-output training patterns are generated randomly. In the back-propagation training algorithm, the learning rate is determined by ensuring the decrease of the error function at each epoch. The computer program implemented is validated by solving the classification of the XOR problem. Then, the trained ANN is used to compute the control force according to the control strategy. If the control force exceeds the actuator's capacity limit, it is set equal to that limit. The concept of the control strategy employed herein is to apply the control force at every time step to cancel the system velocity induced at the preceding time step so that the gradual rhythmic buildup of the response is destroyed. The ground motions considered in the numerical example are the 1940 El Centro earthquake and the 1979 Imperial Valley earthquake in California. The system responses with and without the control are calculated and compared. The feasibility and potential of applying ANNs to seismic active control is asserted by the promising results obtained from the numerical examples studied.

  16. Seismic active control by neutral networks

    SciTech Connect

    Tang, Yu

    1995-12-31

    A study on the application of artificial neural networks (ANNs) to active structural control under seismic loads is carried out. The structure considered is a single-degree-of-freedom (SDF) system with an active bracing device. The control force is computed by a trained neural network. The feedforward neural network architecture and an adaptive backpropagation training algorithm is used in the study. The neural net is trained to reproduce the function that represents the response-excitation relationship of the SDF system under seismic loads. The input-output training patterns are generated randomly. In the backpropagation training algorithm, the learning rate is determined by ensuring the decrease of the error function at each epoch. The computer program implemented is validated by solving the classification of the XOR problem. Then, the trained ANN is used to compute the control force according to the control strategy. If the control force exceeds the actuator`s capacity limit, it is set equal to that limit. The concept of the control strategy employed herein is to apply the control force at every time step to cancel the system velocity induced at the preceding time step so that the gradual rhythmic buildup of the response is destroyed. The ground motions considered in the numerical example are the 1940 El Centro earthquake and the 1979 Imperial Valley earthquake in California. The system responses with and without the control are calculated and compared. The feasibility and potential of applying ANNs to seismic active control is asserted by the promising results obtained from the numerical examples studied.

  17. The seismicity of Ethiopia; active plate tectonics

    USGS Publications Warehouse

    Mohr, P.

    1981-01-01

    Ethiopia, descended from the semimythical Kingdom of Punt, lies at the strategic intersection of Schmidt's jigsaw puzzle where the Red Sea, Gulf of Aden, and the African Rift System meet. Because of geologically recent uplift combined with rapid downcutting erosion by rivers, notably the Blue Nile (Abbay), Ethiopia is the most mountainous country in Africa. It is also the most volcanically active, while its historical seismicity matches that of the midocean ridges. And, in a sense, Ethiopia is host to an evoloving ocean ridge system. 

  18. Methods and systems for low frequency seismic and infrasound detection of geo-pressure transition zones

    DOEpatents

    Shook, G. Michael; LeRoy, Samuel D.; Benzing, William M.

    2006-07-18

    Methods for determining the existence and characteristics of a gradational pressurized zone within a subterranean formation are disclosed. One embodiment involves employing an attenuation relationship between a seismic response signal and increasing wavelet wavelength, which relationship may be used to detect a gradational pressurized zone and/or determine characteristics thereof. In another embodiment, a method for analyzing data contained within a response signal for signal characteristics that may change in relation to the distance between an input signal source and the gradational pressurized zone is disclosed. In a further embodiment, the relationship between response signal wavelet frequency and comparative amplitude may be used to estimate an optimal wavelet wavelength or range of wavelengths used for data processing or input signal selection. Systems for seismic exploration and data analysis for practicing the above-mentioned method embodiments are also disclosed.

  19. Systems for low frequency seismic and infrasound detection of geo-pressure transition zones

    DOEpatents

    Shook, G. Michael; LeRoy, Samuel D.; Benzing, William M.

    2007-10-16

    Methods for determining the existence and characteristics of a gradational pressurized zone within a subterranean formation are disclosed. One embodiment involves employing an attenuation relationship between a seismic response signal and increasing wavelet wavelength, which relationship may be used to detect a gradational pressurized zone and/or determine characteristics thereof. In another embodiment, a method for analyzing data contained within a response signal for signal characteristics that may change in relation to the distance between an input signal source and the gradational pressurized zone is disclosed. In a further embodiment, the relationship between response signal wavelet frequency and comparative amplitude may be used to estimate an optimal wavelet wavelength or range of wavelengths used for data processing or input signal selection. Systems for seismic exploration and data analysis for practicing the above-mentioned method embodiments are also disclosed.

  20. CRUSTAL STRUCTURE OF THE SOUTHERN CALAVERAS FAULT ZONE, CENTRAL CALIFORNIA, FROM SEISMIC REFRACTION INVESTIGATIONS.

    USGS Publications Warehouse

    Blumling, Peter; Mooney, Walter D.; Lee, W.H.K.

    1985-01-01

    A magnitude 5. 7 earthquake on August 6, 1979, within the Calaveras fault zone, near Coyote Lake of west-central California, motivated a seismic-refraction investigation in this area. A northwest-southeast profile along the fault, as well as two fan profiles across the fault were recorded to examine the velocity structure of this region. The analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions. Velocities within the fault zone were determined from the fan profiles. Near Anderson Lake, a pronounced delay of first arrivals on the fan records indicates a vertical 1- to 2-km-wide near-surface, low-velocity zone along the fault. Near Coyote Lake, the delays observed in the fan records correlate with two subsurface en-echelon fault planes which have been previously identified from lineations in the seismicity pattern. Refs.

  1. A missing-link in the tectonic configuration of the Almacık Block along the North Anatolian Fault Zone (NW Turkey): Active faulting in the Bolu plain based on seismic reflection studies

    NASA Astrophysics Data System (ADS)

    Seyitoğlu, Gürol; Ecevitoğlu, Berkan; Kaypak, Bülent; Esat, Korhan; Çağlayan, Ayşe; Gündoğdu, Oğuz; Güney, Yücel; Işık, Veysel; Pekkan, Emrah; Tün, Muammer; Avdan, Uğur

    2015-06-01

    The North Anatolian Fault Zone (NAFZ) starts to branch off in the western Bolu plain. The branches of the NAFZ in this location create the Almacık block which is surrounded by the latest surface ruptures of significant earthquakes that occurred between 1944 and 1999, but its northeastern part remains unruptured. The most recently formed rupture, that was a result of the 1999 November 12 Düzce earthquake, ended to the northwest of the Bakacak Fault. The connection between the Bakacak Fault and the main branch of the NAFZ via the Bolu plain has until now remained unknown. This paper establishes that the route of the missing link runs through the Dağkent, Kasaplar and Bürnük faults, a finding achieved with the help of seismic reflection studies. The paper also argues that the cross cutting nature of these newly determined faults and a stress analysis based on focal mechanism solutions of recent earthquakes demonstrate the termination of the suggested pull-apart nature of the Bolu plain.

  2. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity

    USGS Publications Warehouse

    McCrory, Patricia A.; Blair, J. Luke; Waldhause, Felix; Oppenheimer, David H.

    2012-01-01

    A new model of the subducted Juan de Fuca plate beneath western North America allows first-order correlations between the occurrence of Wadati-Benioff zone earthquakes and slab geometry, temperature, and hydration state. The geo-referenced 3D model, constructed from weighted control points, integrates depth information from earthquake locations and regional seismic velocity studies. We use the model to separate earthquakes that occur in the Cascadia forearc from those that occur within the underlying Juan de Fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes. Seismicity within the slab is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic dehydration processes, promotes earthquake occurrence within the subducted oceanic plate. Earthquake patterns beneath western Vancouver Island are consistent with slab dehydration processes. Conversely, the lack of slab earthquakes beneath western Oregon is consistent with an anhydrous slab. Double-differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 M7.1 Mendocino earthquake. We infer that this earthquake ruptured a surface within the Cascadia accretionary margin above the Juan de Fuca plate. We further speculate that this earthquake is associated with a detached fragment of former Farallon plate. Other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes.

  3. Kinematics of the New Madrid seismic zone, central United States, based on stepover models

    USGS Publications Warehouse

    Pratt, Thomas L.

    2012-01-01

    Seismicity in the New Madrid seismic zone (NMSZ) of the central United States is generally attributed to a stepover structure in which the Reelfoot thrust fault transfers slip between parallel strike-slip faults. However, some arms of the seismic zone do not fit this simple model. Comparison of the NMSZ with an analog sandbox model of a restraining stepover structure explains all of the arms of seismicity as only part of the extensive pattern of faults that characterizes stepover structures. Computer models show that the stepover structure may form because differences in the trends of lower crustal shearing and inherited upper crustal faults make a step between en echelon fault segments the easiest path for slip in the upper crust. The models predict that the modern seismicity occurs only on a subset of the faults in the New Madrid stepover structure, that only the southern part of the stepover structure ruptured in the A.D. 1811–1812 earthquakes, and that the stepover formed because the trends of older faults are not the same as the current direction of shearing.

  4. Juan de Fuca slab geometry and its relation to Wadati-Benioff zone seismicity

    NASA Astrophysics Data System (ADS)

    McCrory, Patricia A.; Blair, J. Luke; Waldhauser, Felix; Oppenheimer, David H.

    2012-09-01

    A new model of the subducted Juan de Fuca plate beneath western North America allows first-order correlations between the occurrence of Wadati-Benioff zone earthquakes and slab geometry, temperature, and hydration state. The geo-referenced 3D model, constructed from weighted control points, integrates depth information from earthquake locations and regional seismic velocity studies. We use the model to separate earthquakes that occur in the Cascadia forearc from those that occur within the underlying Juan de Fuca plate and thereby reveal previously obscured details regarding the spatial distribution of earthquakes. Seismicity within the slab is most prevalent where the slab is warped beneath northwestern California and western Washington suggesting that slab flexure, in addition to expected metamorphic dehydration processes, promotes earthquake occurrence within the subducted oceanic plate. Earthquake patterns beneath western Vancouver Island are consistent with slab dehydration processes. Conversely, the lack of slab earthquakes beneath western Oregon is consistent with an anhydrous slab. Double-differenced relocated seismicity resolves a double seismic zone within the slab beneath northwestern California that strongly constrains the location of the plate interface and delineates a cluster of seismicity 10 km above the surface that includes the 1992 M7.1 Mendocino earthquake. We infer that this earthquake ruptured a surface within the Cascadia accretionary margin above the Juan de Fuca plate. We further speculate that this earthquake is associated with a detached fragment of former Farallon plate. Other subsurface tectonic elements within the forearc may have the potential to generate similar damaging earthquakes.

  5. Correlations between streamflow and intraplate seismicity in the central Virginia, U.S.A., seismic zone: evidence for possible climatic controls

    NASA Astrophysics Data System (ADS)

    Costain, John K.; Bollinger, G. A.

    1991-02-01

    There is no widely accepted explanation for the origin of intraplate earthquakes. The central Virginia seismic zone, like other seismically active intraplate areas, is a spatially isolated area of persistent, diffuse earthquake activity. We suggested earlier that rainfall plays a key role in the generation of intraplate seismicity ("hydroseismicity"). Observed long-period (10-30 years) changes in streamflow (rainfall) are hypothesized to generate intraplate seismicity by diffusion of pore pressure transients from recharge areas of groundwater basins to depth as deep as the brittle-ductile transition. Streamflow and earthquake strain for a 62-year sample from 1925 to 1987 in the central Virginia seismic zone were cumulated, and a least-squares straight-line fit was subtracted to obtain residuals of streamflow and strain. Residual streamflow was differentiated to obtain the rate of change of residual streamflow. We observed common cyclicities with periods of 10-30 years for residual streamflow and strain. From the one-dimensional diffusion equation, we determined the time response of fluid pressure at depths, ψ, in a hydraulically diffusive crust to an impulsive change in fluid pressure at the surface of a groundwater basin. These responses were convolved with the surface streamflow residuals, or, because of results from reservoir-induced seismicity, with the derivatives of these residuals. Root-mean-square values (rms) of the convolutions were computed for ψ = 5, 10, 15 and 20 km and various values of D. For central Virginia, the number of earthquakes, N, within a crustal slice centered on a depth, ψ, was found to be proportional to the rms value of the convolution, suggesting that the number of intraplate earthquakes generated is directly proportional to the magnitude of the rms changes in fluid pore pressure within the crustal slice. These fluctuations in pore pressure, in concert with stress corrosion and hydrolytic weakening, are hypothesized to trigger

  6. Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise.

    PubMed

    Chaves, Esteban J; Schwartz, Susan Y

    2016-01-01

    In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise-based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [M w (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations. PMID:26824075

  7. Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise

    PubMed Central

    Chaves, Esteban J.; Schwartz, Susan Y.

    2016-01-01

    In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw (moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations. PMID:26824075

  8. Cyclic stressing and seismicity at strongly coupled subduction zones

    USGS Publications Warehouse

    Taylor, M.A.J.; Zheng, G.; Rice, J.R.; Stuart, W.D.; Dmowska, R.

    1996-01-01

    We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long-term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench-normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate

  9. Nucleation, propagation and arrest of seismic swarms in the Tjörnes Fracture Zone (North Iceland)

    NASA Astrophysics Data System (ADS)

    Rivalta, E.; Passarelli, L.; Maccaferri, F.; Hensch, M.; Metzger, S.; Jakobsdottir, S. S.; Corbi, F.; Jonsson, S.; Dahm, T.

    2015-12-01

    The Tjörnes Fracture Zone (TFZ) connects the Northern Volcanic Zone to the Mid-Atlantic ridge north of Iceland. It primarily consists of two transform structures, the Húsavík-Flatey Fault (HFF) and the Grímsey Oblique Rift (GOR), which together have experienced about ten M>6 earthquakes since 1750. There is growing concern that a large earthquake may be due in the TFZ. The GOR and the northwestern part of the HFF have been seismically very active during the past two decades, often in the form of seismic swarms of various intensity and duration. The most energetic swarms during this period took place in October 2012 and March 2013, with several M>5 earthquakes. These and previous swarms occurred offshore and are poorly understood, in particular regarding the physical mechanism behind their generation and to what extent the swarm-like activity might temporally or permanently modify the hazard in the entire TFZ. Here we study the spatio-temporal pattern of earthquake swarms occurring in the TFZ since 1996. We find that the swarms show spatial complementarity, i.e., in general they do not overlap spatially with earlier swarms. Moreover, together they have progressively filled up the entire GOR and the western half of the HFF. Each swarm shows a clear migration of hypocenters on their respective fault planes. This is particularly visible in the September-October 2012 and March 2013 sequences, where the earthquakes expanded concentrically from a central, focused patch on the fault to the entire fault area excited by seismicity. The swarms typically start as very localized microseismicity, lasting for a few hours to a few days. Then, the hypocenter area starts to expand, with earthquakes migrating at velocities ranging from 1 km/day up to 1 km/h. The migration sometimes accelerates or decelerates, depending on the case, and then comes to a sudden stop, with microseismicity continuing on the fault plane for a few days or weeks. The estimated rupture duration and

  10. Lesser Antilles Subduction Zone Investigation by a Cluster of Large Seismic Experiments in the Forearc Region

    NASA Astrophysics Data System (ADS)

    Last, T.

    2007-12-01

    Thales LAST stands for Lesser Antilles Subduction zone Team which gathers the scientific teams of a cluster of surveys and cruises that have been carried out in 2007 and coordinated under the European Union THALES WAS RIGHT project (Coord. A. Hirn). This cluster is composed by the German cruise TRAIL with the vessel F/S Merian (PI E. Flueh and H. Kopp, IFM-GEOMAR), the French cruise SISMANTILLES 2 with the IFREMER vessel N/O Atalante (PI M. Laigle, IPG Paris and JF. Lebrun, Univ. Antilles Guyane), and French cruise OBSANTILLES with the IRD vessel N/O Antea (PI P. Charvis, Geoazur, Nice, France). During these cruises and surveys, 84 Ocean Bottom 3-components Seismometers (OBS) and 20 Hydrophones (OBHs) have been brought together from several pools (Geoazur, INSU, IPGP, IFM-GEOMAR, AWI,), with up to 30 land stations (CSIC Barcelone, IPG Paris, INSU-RLBM and -LITHOSCOPE) in addition to the permanent onshore arrays of IPGP and SRU. The deployment of all these instruments has been supported principally by ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI), by the EU SALVADOR Programme of IFM-GEOMAR, as well as by the EU project THALES WAS RIGHT on the Antilles and Hellenic active subductions to which contribute IPGP, Geoazur, IFM-GEOMAR (Germany), ETH Zurich (Switzerland), CSIC Barcelona (Spain), Univ. Trieste (Italy) and NOA Athens (Greece). The main goal of this large seismic investigation effort is the understanding of the behaviour of the seismogenic zone and location of potential source regions of mega-thrust earthquakes. Specific goals are the mapping of the subduction interplate in the range where it may be seismogenic along the Lesser Antilles Arc from Antigua to southern Martinique Islands, as a contribution to identification and localisation in advance of main rupture zones of possible future major earthquakes, and to the search for transient signals of the activity. The forearc region, commonly considered as a proxy to the seismogenic portion of the

  11. Evidences of a Lithospheric Fault Zone in the Sicily Channel Continental Rift (Southern Italy) from Instrumental Seismicity Data

    NASA Astrophysics Data System (ADS)

    Parisi, L.; Calo, M.

    2013-12-01

    The Sicily Channel continental rift is located in the African Plate and is submerged by a shallow sea extending from the northern coast of Africa to the southern coast of Sicily (southern Italy). The area is affected by an extensional regime since early Pliocene, which thins the continental crust and produces NW-SE oriented Pantelleria, Linosa and Malta grabens. The rift-related volcanic activity is represented by Pantelleria and Linosa Islands and a series of magmatic manifestations roughly NNE-SSW aligned, from Linosa Island to the Nameless Bank, in proximity of the Sicilian coast. Recent rapid magmatic ascents occurred along the strip near to the Sicilian coast in a region named Graham Bank. The NNE-SSW strip has already been recognised as a separation belt between the western sector of the rift (Pantelleria graben) and the eastern one (Linosa and Malta grabens). Seismic profiles suggest the presence of near vertical structures associated with strike slip fault zones. Bathymetric data show a 15-20 km wide zone characterised by several shallow basins irregularly alternated by topographic highs. However, evidences of a N-S or NNE-SSW orientated faults have not been found. In this work we re-localised the instrumental seismicity recorded between 1981 and 2012 in the Sicily Channel and western Sicily using the Double Difference method (Waldhauser, 2001, 2012) and 3D Vp and Vs models (Calò et al., 2013). The statistical analysis of the relocated seismicity together with the study of seismic energy release distribution allows us to describe the main patterns associated with the active faults in the western Sicily Straits. Here we find that most of the events in the Sicily Channel are highly clustered between 12.5°- 13.5°E and 35.5°-37°N with hypocentral depth between 5-40 km, reaching in some cases 70 km of depth. Seismic events seem to be aligned along a sub-vertical shear zone that is long at least 250 km and oriented approximately NNE-SSW. The spatial

  12. Resonant seismic and microseismic ground motion of the Cascadia subduction zone accretionary prism and implications for seismic velocity

    NASA Astrophysics Data System (ADS)

    Davis, Earl E.; Heesemann, Martin

    2015-02-01

    Seafloor pressure and seismic observations have been made along a transect of sites off southwestern Canada using connections to the NEPTUNE Canada cabled network beginning in the fall of 2009. A comparison of the vertical ground motion response to oceanographic and seismic loading at a site on the outer Cascadia subduction zone accretionary prism to that at a site on the adjacent Juan de Fuca Plate shows generally stronger ground motion at the prism site across the full bandwidths of infragravity waves and microseisms and a strong sharp peak in the relative response at a period of 9 s. This peak is seen in the response to loading by local storm waves and dispersive swell sequences, as well as in the average response to storm- and swell-generated pressure fluctuations averaged over long periods of time. Tuned response to teleseismic surface waves is also seen at the same frequency. We infer that this behavior results from quarter-wavelength harmonic resonance of the prism, with the two-way travel time of compressional waves between the seafloor and underlying igneous crust being one half the resonance period. The consistency of the anomalous spectral peak from year to year at this particular site suggests that the behavior might be used to track small (≈1%) changes in the vertical seismic velocity of the prism if variations related to strain or pore fluid pressure changes through a subduction thrust earthquake cycle were to occur.

  13. Induced seismicity and CO2 leakage through fault zones during large-scale underground injection in a multilayered sedimentary system

    NASA Astrophysics Data System (ADS)

    Rinaldi, A.; Rutqvist, J.; Jeanne, P.; Cappa, F.

    2013-12-01

    The importance of geomechanics including the potential for reactivating faults associated with large-scale geologic carbon sequestration operations has recently become more widely recognized. However, not withstanding the potential for triggering notable (felt) seismic events, the potential for buoyancy-driven CO2 to reach potable groundwater and the ground surface is more important from safety and storage-efficiency perspectives. In this context, this work extends previous studies on the geomechanical modeling of fault responses during underground carbon dioxide injection, focusing on short-term integrity of the sealing caprock, and hence of potential leakage of either brine or CO2 to shallow groundwater aquifers during active injection. We account for a stress/strain-dependent permeability and study the leakage through a fault zone as its permeability changes during a reactivation, also causing seismicity. We analyze several scenarios related to the injected amount of CO2 (and hence as a function of the overpressure) both involving minor and major faults, and analyze the profile risks of leakage for different stress/strain permeability coupling functions, as well as increasing the complexity of the fault zone in terms of hydromechanical heterogeneities. We conclude that whereas it is very difficult to predict how much fault permeability could change upon reactivation, this process can have a significant impact on the leakage rate. The presence of hydromechanical heterogeneity influences the pressure diffusion, as well as the effective normal and shear stress evolution. Hydromechanical heterogeneities (i) strengthen the fault zone resulting in earthquake of small magnitude, and (ii) prevent a good fluid migration upward along the fault. We also study the effects of the caprock and aquifer thickness on the resulting induced seismicity and CO2 leakage, both in heterogeneous and homogeneous fault zone. Results show that a thin caprock or aquifer allows smaller events

  14. Seismic evidence for deep fluid circulation in the overriding plate of subduction zones

    NASA Astrophysics Data System (ADS)

    Tauzin, B.; Reynard, B.; Bodin, T.; Perrillat, J. P.; Debayle, E.

    2015-12-01

    In subduction zones, non-volcanic tremors are associated with fluid circulations (Obara, 2002). Their sources are often located on the interplate boundary (Rogers and Dragert, 2003; Shelly et al, 2006; La Rocca, 2009), consistent with fluids released by the dehydration of subducted plates (Hacker et al., 2003). Reports of tremors in the overriding continental crust of several subduction zones in the world (Kao et al., 2005; Payero et al., 2008; Ide, 2012) suggest fluid circulation at shallower depths but potential fluid paths are poorly documented. Here we obtained seismic observations from receiver functions that evidence the close association between the shallow tremor zone, electrical conductivity, and tectonic features of the Cascadia overriding plate. A seismic discontinuity near 15 km depth in the crust of the overriding North American plate is attributed to the Conrad discontinuity. This interface is segmented, and its interruption is spatially correlated with conductive regions and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that shallow fluid circulation, tremors and seismicity are controlled by fault zones limiting blocks of accreted terranes in the overriding plate (Brudzinski and Allen, 2007). These zones constitute fluid "escape" routes that may contribute unloading fluid pressure on the megathrust. Obara, K. (2002). Science, 296, 1679-1681. Rogers, G., & Dragert, H. (2003). Science, 300, 1942-1943. Shelly, D. R., et al. (2006). Nature, 442, 188-191. La Rocca, M., et al. (2009). Science, 323, 620-623. Kao, H., et al. (2005). Nature, 436, 841-844. Payero, J. S., et al. (2008). Geophysical Research Letters, 35. Ide, S. (2012). Journal of Geophysical Research: Solid Earth, 117. Brudzinski, M. R., & Allen, R. M. (2007). Geology, 35, 907-910.

  15. Seismic properties of lawsonite eclogites from the southern Motagua fault zone, Guatemala

    NASA Astrophysics Data System (ADS)

    Kim, Daeyeong; Wallis, Simon; Endo, Shunsuke; Ree, Jin-Han

    2016-05-01

    We present new data on the crystal preferred orientation (CPO) and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern Motagua fault zone, Guatemala, to discuss the seismic anisotropy of subducting oceanic crust. The CPO of omphacite is characterized by (010)[001], and it shows P-wave seismic anisotropies (AVP) of 1.4%-3.2% and S-wave seismic anisotropies (AVS) of 1.4%-2.7%. Lawsonite exhibits (001) planes parallel to the foliation and [010] axes parallel to the lineation, and seismic anisotropies of 1.7%-6.6% AVP and 3.4%-14.7% AVS. The seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1.2%-4.1% AVP and 1.8%-6.8% AVS. For events that propagate more or less parallel to the maximum extension direction, X, the fast S-wave velocity (VS) polarization is parallel to the Z in the Y-Z section (rotated from the X-Z section), causing trench-normal seismic anisotropy for orthogonal subduction. Based on the high modal abundance and strong fabric of lawsonite, the AVS of eclogites is estimated as ~ 11.7% in the case that lawsonite makes up ~ 75% of the rock mass. On this basis, we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in NE Japan: weak trench-parallel anisotropy in the forearc basin domains and trench-normal anisotropy in the backarc region.

  16. Seismic structure of the North Pacific oceanic crust prior plate bending at the Alaska subduction zone

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    Seismic reflection profiles across North Pacific oceanic Plate reveal the internal structure of a mature oceanic crust (42-56Ma) formed at fast (70mm/yr, half rate) to intermediate (28mm/yr, half rate) spreading rates. Data used in this study were collected with the R/V Langseth in summer 2011 as part of the ALEUT (Alaska Langseth Experiment to Understand the megathrust) program. MCS data were acquired with two 8-km streamers and a 6600 cu. in. air gun array. We collected a series of profiles across the subduction zone system but also across the preexisting structures of the oceanic crust before being affected by subduction zone processes. Additionally, two 400-km OBS refraction lines were shot coincident with MCS profiles. The multi-channel seismic (MCS) data across oceanic crust formed at fast spreading rates contain abundant bright reflectors mostly confined in the lower crust above the Moho discontinuity and dipping predominantly toward the paleo-ridge. Along these profiles, the Moho discontinuity is observed as a bright event with remarkable lateral continuity. The lengths of the dipping reflectors are on the order of 5-km, with apparent dips between 10 and 30°. These reflectors represent discrete events, with spacing between 0.3 to 5 km without any obvious regularity. These dipping events appear to sole out within the middle crust (1 to 1.5 s beneath basement) and most of them terminate at the Moho. The Moho is much weaker or absent on the northern profiles acquired across the North Pacific oceanic crust formed at intermediate spreading rates. Basement topography is rougher and no clear dipping events have been imaged suggesting that the spreading rate may be an important factor that controls the strength and abundance of such dipping reflectors and the lateral change in the Moho reflection characters. Lower crustal dipping reflections (LCDR) have been only imaged at very few places across the Pacific oceanic crust: (Eittreim et al., 1988, Reston et al. 1999

  17. Transient Deformation at the Seismic-Aseismic Transition in a Mature Plate Boundary Fault Zone - New Zealand's Alpine Fault

    NASA Astrophysics Data System (ADS)

    Toy, V. G.; Norris, R. J.; Prior, D. J.

    2008-12-01

    During the seismic cycle, stresses and strain rates fluctuate in the viscously-deforming zones down-dip of large faults. These transient events produce geological records that can be preserved in exhumed fault zones that have experienced single ruptures (e.g. Sesia Zone, European Western Alps). On the other hand, in major faults that have not experienced a simple, single rupture history, coseismic structures are likely to be destroyed during subsequent cycles of postseismic creep. New Zealand's active Alpine Fault has likely experienced upwards of 20,000 Mw~8 earthquakes, on average one every 200-300 years, over the last ≥5 million years of dextral-reverse slip. Fault rocks generated during these events are exhumed in the hangingwall, exposing materials deformed throughout the seismogenic zone at the surface. We have recognised a structural record of transient events in these rocks that differs from that previously reported elsewhere. Mylonites were formed by viscous shearing of a metasedimentary protolith downdip of the seismogenic structure. Rheological models predict these mylonites should have passed through a crustal strength peak (τ ≥100 MPa) around the brittle-viscous transition. Immediately prior to passing through this transition, they should have developed a small recrystallised grainsize (~10-15μm) and a crystallographic preferred orientation (CPO) indicating slip on the basal system during quartz dislocation creep, as well as a retrograde greenschist-facies mineralogy. However, the high-strain mylonites preserve a large recrystallised grainsize (>~30μm), amphibolite-facies mineralogy and CPO characteristic of prism slip. This suggests they were not significantly deformed at temperatures below ~450°C, significantly above the lower temperature limit for quartz crystal-plasticity at steady-state strain rates in the fault zone Microstructural observations and textural data indicate variable deformation style through the seismic cycle. Large fault

  18. Estimation of recurrence interval of large earthquakes on the central Longmen Shan fault zone based on seismic moment accumulation/release model.

    PubMed

    Ren, Junjie; Zhang, Shimin

    2013-01-01

    Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 10¹⁷ N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region. PMID:23878524

  19. Estimation of Recurrence Interval of Large Earthquakes on the Central Longmen Shan Fault Zone Based on Seismic Moment Accumulation/Release Model

    PubMed Central

    Zhang, Shimin

    2013-01-01

    Recurrence interval of large earthquake on an active fault zone is an important parameter in assessing seismic hazard. The 2008 Wenchuan earthquake (Mw 7.9) occurred on the central Longmen Shan fault zone and ruptured the Yingxiu-Beichuan fault (YBF) and the Guanxian-Jiangyou fault (GJF). However, there is a considerable discrepancy among recurrence intervals of large earthquake in preseismic and postseismic estimates based on slip rate and paleoseismologic results. Post-seismic trenches showed that the central Longmen Shan fault zone probably undertakes an event similar to the 2008 quake, suggesting a characteristic earthquake model. In this paper, we use the published seismogenic model of the 2008 earthquake based on Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data and construct a characteristic seismic moment accumulation/release model to estimate recurrence interval of large earthquakes on the central Longmen Shan fault zone. Our results show that the seismogenic zone accommodates a moment rate of (2.7 ± 0.3) × 1017 N m/yr, and a recurrence interval of 3900 ± 400 yrs is necessary for accumulation of strain energy equivalent to the 2008 earthquake. This study provides a preferred interval estimation of large earthquakes for seismic hazard analysis in the Longmen Shan region. PMID:23878524

  20. Periodic variation of stress field in the Koyna-Warna reservoir triggered seismic zone inferred from focal mechanism studies

    NASA Astrophysics Data System (ADS)

    Rao, N. Purnachandra; Shashidhar, D.

    2016-06-01

    The Koyna-Warna region in western India is globally recognized as the premier site of reservoir triggered seismicity (RTS) associated with the Koyna and Warna reservoirs. The region is characterized by continuous seismic activity observed since several decades, including the world's largest triggered earthquake of M6.3 which occurred in Koyna in 1967. While the role of reservoirs in triggering earthquakes has been widely discussed, the actual tectonic mechanism controlling earthquake genesis in this region is hardly understood. The Koyna-Warna region is exclusively governed by earthquakes of strike-slip and normal fault mechanism distinct from the thrust faulting seen in other active zones in the Indian region. In the present study, a comprehensive catalog of 50 focal mechanism solutions of earthquakes that occurred during the last 45 years in the Koyna-Warna region is developed, both from previous literature and from moment tensor inversion studies by the authors using broadband data from a local seismic network operating since 2005. The seismicity and fault plane data have enabled precise delineation of trends of the major causative faults, which are further accentuated using the double-difference technique. Stress inversion of the focal mechanism data has provided the best fitting principal compressive and tensile stress field of the region, which in conjunction with the deciphered fault zones provides a feasible model of seismogenesis in this region. Based on the observed temporal variation of faulting mechanism a model of alternating cycles of predominantly strike-slip and normal faulting is proposed, which is attributed to a periodic peaking and relaxation respectively of the horizontal compressive stress field in this region due to the Indian plate collision with Eurasia.

  1. Possible Biases in Characteristic Earthquake and Seismic Hazard Studies: Illustrations for the Wasatch, New Madrid, North Africa, and Eastern Canadian Seismic Zones

    NASA Astrophysics Data System (ADS)

    Swafford, L.; Stein, S.; Newman, A.; Friedrich, A.

    2005-12-01

    Attempts to study earthquake recurrence in space and time are limited in simple but frustrating ways by the short history of instrumental seismology compared to the long and variable recurrence time of large earthquakes. If major seismicity is spread uniformly with a seismic zone but the recurrence interval is long compared to the earthquake record, apparent differences in seismic hazard within the seismic zone inferred from the earthquake history are likely to simply reflect the short earthquake record. Simple numerical simulations suggest that this may be the case for the concentrated areas of high predicted hazard with seismic zones in North Africa, along the St. Lawrence valley, and the eastern coast of Canada. If so, with more time and a longer earthquake record, large earthquakes would likely occur over the entire seismic zone and show that the real hazard is uniform. Similarly, large earthquakes are likely to appear "characteristic", more frequent than would be inferred from the rates of smaller ones, for two reasons. First, a short history is likely to overestimate the rate of large earthquakes because fractions of earthquakes cannot be observed. Second, because the rates of small earthquakes are typically determined from the seismological record whereas the rates of large earthquakes are inferred from paleoseismology, biases in estimating paleomagnitudes can produce apparent characteristic earthquakes, as appears to have occurred for New Madrid. A further complexity is illustrated by results for the Wasatch seismic zone, where some studies find characteristic earthquake behavior whereas others do not. The discrepancy arises primarily because some studies consider the entire Wasatch front area whereas others focus on the Wasatch fault, on which only some of the smaller earthquakes but all of the large paleoearthquakes occur. Similar situations may arise in other seismic zones containing a major fault and a number of smaller ones.

  2. The Lusi seismic experiment: An initial study to understand the effect of seismic activity to Lusi

    NASA Astrophysics Data System (ADS)

    Karyono, Mazzini, Adriano; Lupi, Matteo; Syafri, Ildrem; Masturyono, Rudiyanto, Ariska; Pranata, Bayu; Muzli, Widodo, Handi Sulistyo; Sudrajat, Ajat; Sugiharto, Anton

    2015-04-01

    The spectacular Lumpur Sidoarjo (Lusi) eruption started in northeast Java on the 29 of May 2006 following a M6.3 earthquake striking the island [1,2]. Initially, several gas and mud eruption sites appeared along the reactivated strike-slip Watukosek fault system [3] and within weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. The Lusi seismic experiment is a project aims to begin a detailed study of seismicity around the Lusi area. In this initial phase we deploy 30 seismometers strategically distributed in the area around Lusi and along the Watukosek fault zone that stretches between Lusi and the Arjuno Welirang (AW) complex. The purpose of the initial monitoring is to conduct a preliminary seismic campaign aiming to identify the occurrence and the location of local seismic events in east Java particularly beneath Lusi.This network will locate small event that may not be captured by the existing BMKG network. It will be crucial to design the second phase of the seismic experiment that will consist of a local earthquake tomography of the Lusi-AW region and spatial and temporal variations of vp/vs ratios. The goal of this study is to understand how the seismicity occurring along the Sunda subduction zone affects to the behavior of the Lusi eruption. Our study will also provide a large dataset for a qualitative analysis of earthquake triggering studies, earthquake-volcano and earthquake-earthquake interactions. In this study, we will extract Green's functions from ambient seismic noise data in order to image the shallow subsurface structure beneath LUSI area. The waveform cross-correlation technique will be apply to all of recordings of ambient seismic noise at 30 seismographic stations around the LUSI area. We use the dispersive behaviour of the retrieved Rayleigh waves to infer velocity structures in the shallow subsurface.

  3. The Lusi seismic experiment: An initial study to understand the effect of seismic activity to Lusi

    SciTech Connect

    Karyono; Mazzini, Adriano; Sugiharto, Anton; Lupi, Matteo; Syafri, Ildrem; Masturyono,; Rudiyanto, Ariska; Pranata, Bayu; Muzli,; Widodo, Handi Sulistyo; Sudrajat, Ajat

    2015-04-24

    The spectacular Lumpur Sidoarjo (Lusi) eruption started in northeast Java on the 29 of May 2006 following a M6.3 earthquake striking the island [1,2]. Initially, several gas and mud eruption sites appeared along the reactivated strike-slip Watukosek fault system [3] and within weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. The Lusi seismic experiment is a project aims to begin a detailed study of seismicity around the Lusi area. In this initial phase we deploy 30 seismometers strategically distributed in the area around Lusi and along the Watukosek fault zone that stretches between Lusi and the Arjuno Welirang (AW) complex. The purpose of the initial monitoring is to conduct a preliminary seismic campaign aiming to identify the occurrence and the location of local seismic events in east Java particularly beneath Lusi.This network will locate small event that may not be captured by the existing BMKG network. It will be crucial to design the second phase of the seismic experiment that will consist of a local earthquake tomography of the Lusi-AW region and spatial and temporal variations of vp/vs ratios. The goal of this study is to understand how the seismicity occurring along the Sunda subduction zone affects to the behavior of the Lusi eruption. Our study will also provide a large dataset for a qualitative analysis of earthquake triggering studies, earthquake-volcano and earthquake-earthquake interactions. In this study, we will extract Green’s functions from ambient seismic noise data in order to image the shallow subsurface structure beneath LUSI area. The waveform cross-correlation technique will be apply to all of recordings of ambient seismic noise at 30 seismographic stations around the LUSI area. We use the dispersive behaviour of the retrieved Rayleigh waves to infer velocity structures in the shallow subsurface.

  4. Microstructures, deformation mechanisms and seismic properties of a Palaeoproterozoic shear zone: The Mertz shear zone, East-Antarctica

    NASA Astrophysics Data System (ADS)

    Lamarque, Gaëlle; Bascou, Jérôme; Maurice, Claire; Cottin, Jean-Yves; Riel, Nicolas; Ménot, René-Pierre

    2016-06-01

    The Mertz shear zone (MSZ) is a lithospheric scale structure that recorded mid-crustal deformation during the 1.7 Ga orogeny. We performed a microstructural and crystallographic preferred orientation (CPO) study of samples from both mylonites and tectonic boudins that constitute relics of the Terre Adélie Craton (TAC). The deformation is highly accommodated in the MSZ by anastomosed shear bands, which become more scattered elsewhere in the TAC. Most of the MSZ amphibolite-facies mylonites display similar CPO, thermal conditions, intensity of deformation and dominant shear strain. Preserved granulite-facies boudins show both coaxial and non-coaxial strains related to the previous 2.45 Ga event. This former deformation is more penetrative and less localized and shows a deformation gradient, later affected by a major phase of recrystallization during retrogression at 2.42 Ga. Both MSZ samples and granulite-facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted CPO of minerals (quartz, feldspar, biotite, amphibole and orthopyroxene). In particular, we highlight the development of an "uncommon" CPO in orthopyroxene from weakly deformed samples characterized by (010)-planes oriented parallel to the foliation plane, [001]-axes parallel to the stretching lineation and clustering of [100]-axes near the Y structural direction. Lastly, we computed the seismic properties of the amphibolite and granulite facies rocks in the MSZ area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the MSZ. Our results reveal that (i) the low content of amphibole and biotite in the rock formations of the TAC, and (ii) the interactions between the CPO of the different mineralogical phases, generate a seismically isotropic crust. Thus, the seismic anisotropy recorded by the seismic stations of the TAC, including the

  5. Investigating the Brittle Ductile Transition Zone Using Southern California Seismicity and the SCEC Community Fault Model

    NASA Astrophysics Data System (ADS)

    Hauksson, E.; Shearer, P.; Lin, G.

    2008-12-01

    The depth to the brittle-ductile transition zone depends on crustal parameters such as rock type, temperature, fluid pressure, and strain rate. We analyze the relocated background seismicity (1981-2005) in southern California to identify features that may be associated with rupture patterns near the brittle-ductile transition zone in past major earthquakes. The bulk of this seismicity is aftershocks that decay with time since the occurrence of the mainshock. We focus our analysis on the seismicity associated with the 1987 Mw6.6 Superstition Hills, 1992 Mw6.1 Joshua Tree, 1992 Mw7.3 Landers, 1994 Mw6.7 Northridge, and 1999 Mw7.1 Hector Mine earthquakes. Both observational and modeling studies suggest that the coseismic stress changes from major earthquakes should cause a sudden increase in the strain rate in the vicinity of the brittle-ductile transition zone at the time of the mainshock. For instance, the average depth of seismicity after the 1992 Mw7.3 Landers earthquake became ~2 km deeper as compared to before the mainshock. Apparently, the stress redistribution due to the mainshock faulting caused a strain-rate dependent downward displacement of the brittle-ductile transition. We have also determined the changes in the mean and maximum depths of seismicity for these southern California mainshocks. At the time of the mainshocks, the mean depth of seismicity exhibits a transient change of about 1 to 2 km by decaying to a new deeper depth, except for the Superstition Hills earthquake, where the seismicity depth remained unchanged. Similarly, the D95% (the depth above which 95% of the earthquakes occur) and D5% (the average depth of the deepest 5% of the earthquakes) behave like the mean depth, with a rapid transient and subsequent returning to the same depth or a slightly shallower depth. Only the thrust-faulting Northridge sequence exhibits a deep cluster that is separate from the main aftershock zone, which is predicted by geodynamic the thrust-fault models. We

  6. Geyser's Eruptive Activity in Broadband Seismic Records

    NASA Astrophysics Data System (ADS)

    Kugaenko, Yulia; Saltykov, Vadim

    2010-05-01

    A geyser is a spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapor phase (steam). The formation of geysers is due to particular hydrogeological conditions, which exist in only a few places on Earth, so they are a fairly rare phenomenon. The reasons of geyser periodicity and specifics of the activity for every particular geyser are not completely clear yet. So almost for all known geysers it is necessary to develop the personal model. In given study we first use seismic method for detection of possible hidden feature of geyser's eruptive activity in Kamchatkan Valley of the Geysers. Broadband seismic records of geyser generated signals were obtained in hydrothermal field. The Valley of the Geysers belongs to Kronotskiy State Natural Biosphere Reserve and the UNESCO World Natural Heritage Site "Volcanoes of Kamchatka". Neither seismological nor geophysical investigations were carried out here earlier. In September, 2009 seismic observation was organized in geyser's field by 24-bit digital output broadband seismometers (GURALP CMG-6TD flat velocity response 0.033-50 Hz). Four geysers were surveyed: the fountain type Big and Giant geysers; the cone type Pearl geyser and the short-period Gap geyser. Seismometers were set as possible close to the geyser's surface vent (usually at the distance near 3-5 m). Main parameters of the eruptions for the investigated geysers: - The Giant geyser is the most powerful among the regular active geysers in Kamchatkan Valley of the Geysers. The height of the fountain reaches 30 meters, the mass of water erupted is about 40-60 tons. The main cycle of activity varies significantly: in 1945 the intervals between eruptions was near 3 hours, nowadays it is 5-6 hours. As a geyser of fountain type, the Giant geyser erupts from the 2*3 m2 pool of water. - The Big geyser was flooded by the lake after the natural catastrophe (giant mud-stone avalanche, formed by landslide, bed into Geiyzernaya

  7. Continuous, Large-Scale Processing of Seismic Archives for High-Resolution Monitoring of Seismic Activity and Seismogenic Properties

    NASA Astrophysics Data System (ADS)

    Waldhauser, F.; Schaff, D. P.

    2012-12-01

    the computational framework for double-difference processing the combined parametric and waveform archives of the ISC, NEIC, and IRIS with over three million recorded earthquakes worldwide. Since our methods are scalable and run on inexpensive Beowulf clusters, periodic re-analysis of such archives may thus become a routine procedure to continuously improve resolution in existing global earthquake catalogs. Results from subduction zones and aftershock sequences of recent great earthquakes demonstrate the considerable social and economic impact that high-resolution images of active faults, when available in real-time, will have in the prompt evaluation and mitigation of seismic hazards. These results also highlight the need for consistent long-term seismic monitoring and archiving of records.

  8. Monitoring seismic and silent faulting along the Atacama Fault System and its relation to the subduction zone seismic cycle: A Creepmeter Study in N-CHile

    NASA Astrophysics Data System (ADS)

    Victor, Pia; Ziegenhagen, Thomas; Bach, Christoph; Walter, Thomas; Oncken, Onno

    2010-05-01

    The relationship between crustal forearc faults and subduction zone processes is little understood and therefore the modern seismogenic capacity of these faults cannot be determined. The Atacama Fault System (AFS) is the dominant trench parallel fault in N-Chile with an along strike extent of 1000km. In order to characterize the mode of deformation accumulation and its spatio-temporal distribution, we are continuously monitoring displacement accumulation along active fault branches with a recently installed Creepmeter array. All the installed Creepmeters use 12 mm thick Invar-rod as length standard buried up to 0.7 m depth to reduce the signal to noise ratio, and measure the length standard change across a fault on outcrop scale. The currently deployed 9 sites are designed for displacement detection in the range of 0.001 - 50 mm/yr with a sampling rate of 1/min. The monitored fault branches have been chosen such that 3 Creepmeter sites are located in the Iquique seismic gap of the subduction zone, 5 instruments are located in the segment that recently ruptured in the 2007 Tocopilla earthquake, whereof 2 are located on the Mejillones Peninsula and one is located in the Antofagasta segment that last ruptured in the 1995 Antofagasta Earthquake. This enables us to compare the mode of strain accumulation in different stages of the subduction zone seismic cycle. The first datasets (> 1 yr) show that the instruments both in the Antofagasta and Tocopilla segments display a continuous creep signal equivalent to extensional displacement across the fault zone superimposed by sudden displacement events related to subduction zone earthquakes. The sum of both amounts to 0.02 mm/y - 0.1 mm/y of displacement which is less than predicted by the geological long-term observation. The data from the Chomache Fault located in the Iquique segment shows only a creep signal for the first year after installation with an average extensional displacement rate of 0.05 mm/y. No sudden

  9. An inverted double seismic zone in chile: evidence of phase transformation in the subducted slab.

    PubMed

    Comte, D; Suárez, G

    1994-01-14

    Data from two microseismic field experiments in northern Chile revealed an elongated cluster of earthquakes in the subducted Nazca plate at a depth of about 100 kilometers in which down-dip tensional events were consistently shallower than a family of compressional earthquakes. This double seismic zone shows a distribution of stresses of opposite polarity relative to that observed in other double seismic zones in the world. The distribution of stresses in northern Chile supports the notion that at depths of between 90 to 150 kilometers, the basalt to eclogite transformation of the subducting oceanic crust induces tensional deformation in the upper part of the subducted slab and compressional deformation in the underlying mantle. PMID:17839180

  10. Seismic imaging of transition zone discontinuities suggests hot mantle west of Hawaii.

    PubMed

    Cao, Q; van der Hilst, R D; de Hoop, M V; Shim, S-H

    2011-05-27

    The Hawaiian hotspot is often attributed to hot material rising from depth in the mantle, but efforts to detect a thermal plume seismically have been inconclusive. To investigate pertinent thermal anomalies, we imaged with inverse scattering of SS waves the depths to seismic discontinuities below the Central Pacific, which we explain with olivine and garnet transitions in a pyrolitic mantle. The presence of an 800- to 2000-kilometer-wide thermal anomaly (ΔT(max) ~300 to 400 kelvin) deep in the transition zone west of Hawaii suggests that hot material does not rise from the lower mantle through a narrow vertical plume but accumulates near the base of the transition zone before being entrained in flow toward Hawaii and, perhaps, other islands. This implies that geochemical trends in Hawaiian lavas cannot constrain lower mantle domains directly. PMID:21617072

  11. A slow slip event in the south central Alaska Subduction Zone and related seismicity anomaly

    NASA Astrophysics Data System (ADS)

    Wei, Meng; McGuire, Jeffrey J.; Richardson, Eliza

    2012-08-01

    We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down-dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude of Mw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.

  12. New seismic images of the cascadia subduction zone from cruise SO 108-ORWELL

    USGS Publications Warehouse

    Flueh, E.R.; Fisher, M.A.; Bialas, J.; Childs, J. R.; Klaeschen, D.; Kukowski, Nina; Parsons, T.; Scholl, D. W.; ten Brink, U.; Trehu, A.M.; Vidal, N.

    1998-01-01

    In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was conducted aboard the German R/V Sonne. This cooperative experiment by GEOMAR and the USGS acquired wide-angle reflection and refraction seismic data, using ocean-bottom seismometers (OBS) and hydrophones (OBH), and multichannel seismic reflection (MCS) data. The main goal of this experiment was to investigate the internal structure and associated earthquake hazard of the Cascadia subduction zone and to image the downgoing plate. Coincident MCS and wide-angle profiles along two tracks are presented here. The plate boundary has been imaged precisely beneath the wide accretionary wedge close to shore at c13km depth. Thus, the downgoing plate dips more shallowly than previously assumed. The dip of the plate changes from 2?? to 4?? at the eastern boundary of the wedge on the northern profile, whereas approximately 3km of sediment is entering the subduction zone. On the southern profile, where the incoming sedimentary section is about 2.2km thick, the plate dips about 0.5?? to 1.5?? near the deformation front and increases to 3.5?? further landwards. On both profiles, the deformation of the accretionary wedge has produced six ridges on the seafloor, three of which represent active faulting, as indicated by growth folding. The ridges are bordered by landward verging faults which reach as deep as the top of the oceanic basement. Thus, the entire incoming sediment package is being accreted. At least two phases of accretion are evident, and the rocks of the older accretionary phase(s) forms the backstop for the younger phase, which started around 1.5 Ma ago. This documents that the 30 to 50km wide frontal part of the accretionary wedge, which is characterized by landward vergent thrusts, is a Pleistocene feature which was formed in response to the high input of sediment building the fans during glacial periods. Velocities increase quite rapidly within the wedge, both

  13. Quiet zone within a seismic gap near western Nicaragua: Possible location of a future large earthquake

    USGS Publications Warehouse

    Harlow, D.H.; White, R.A.; Cifuentes, I.L.; Aburto, Q.A.

    1981-01-01

    A 5700-square-kilometer quiet zone occurs in the midst of the locations of more than 4000 earthquakes off the Pacific coast of Nicaragua. The region is indicated by the seismic gap technique to be a likely location for an earthquake of magnitude larger than 7. The quiet zone has existed since at least 1950; the last large earthquake originating from this area occurred in 1898 and was of magnitude 7.5. A rough estimate indicates that the magnitude of an earthquake rupturing the entire quiet zone could be as large as that of the 1898 event. It is not yet possible to forecast a time frame for the occurrence of such an earthquake in the quiet zone. Copyright ?? 1981 AAAS.

  14. Rigidity of the fault zones in the Earth's crust estimated from seismic data

    NASA Astrophysics Data System (ADS)

    Spivak, A. A.

    2011-07-01

    Nonlinear effects in seismic wave propagation are analyzed to determine the mechanical rigidity of different-order faults that thread the tectonic structures in the central part of the East European platform (Moscow syneclise and Voronezh Crystalline Massif) and the fault zones of the Balapan and Degelen mountain regions in Kazakhstan (the Degelen magmatic node in the Central Chingiz zone). The dependency of the rigidity of the fault zone on the fault's length is obtained. The rigidity of the tectonic structures is found to experience well-expressed temporal variations with periods of 13-15 days, 27-32 days, and about one year. In the different-order fault zones, the amplitudes of both normal k n and the shear k s rigidity for semimonthly, monthly, and annual variations can span a factor of 1.3, 1.5, and 2.5, respectively.

  15. Angola Seismicity MAP

    NASA Astrophysics Data System (ADS)

    Neto, F. A. P.; Franca, G.

    2014-12-01

    The purpose of this job was to study and document the Angola natural seismicity, establishment of the first database seismic data to facilitate consultation and search for information on seismic activity in the country. The study was conducted based on query reports produced by National Institute of Meteorology and Geophysics (INAMET) 1968 to 2014 with emphasis to the work presented by Moreira (1968), that defined six seismogenic zones from macro seismic data, with highlighting is Zone of Sá da Bandeira (Lubango)-Chibemba-Oncócua-Iona. This is the most important of Angola seismic zone, covering the epicentral Quihita and Iona regions, geologically characterized by transcontinental structure tectono-magmatic activation of the Mesozoic with the installation of a wide variety of intrusive rocks of ultrabasic-alkaline composition, basic and alkaline, kimberlites and carbonatites, strongly marked by intense tectonism, presenting with several faults and fractures (locally called corredor de Lucapa). The earthquake of May 9, 1948 reached intensity VI on the Mercalli-Sieberg scale (MCS) in the locality of Quihita, and seismic active of Iona January 15, 1964, the main shock hit the grade VI-VII. Although not having significant seismicity rate can not be neglected, the other five zone are: Cassongue-Ganda-Massano de Amorim; Lola-Quilengues-Caluquembe; Gago Coutinho-zone; Cuima-Cachingues-Cambândua; The Upper Zambezi zone. We also analyzed technical reports on the seismicity of the middle Kwanza produced by Hidroproekt (GAMEK) region as well as international seismic bulletins of the International Seismological Centre (ISC), United States Geological Survey (USGS), and these data served for instrumental location of the epicenters. All compiled information made possible the creation of the First datbase of seismic data for Angola, preparing the map of seismicity with the reconfirmation of the main seismic zones defined by Moreira (1968) and the identification of a new seismic

  16. Seismic activity noted at Medicine Lake Highlands

    SciTech Connect

    Blum, D.

    1988-12-01

    The sudden rumble of earthquakes beneath Medicine Lake Highlands this fall gave geologists an early warning that one of Northern California's volcanoes may be stirring back to life. Researchers stressed that an eruption of the volcano is not expected soon. But the flurry of underground shocks in late September, combined with new evidence of a pool of molten rock beneath the big volcano, has led them to monitor Medicine Lake with new wariness. The volcano has been dormant since 1910, when it ejected a brief flurry of ash - worrying no one. A federal team plans to take measurements of Medicine Lake, testing for changes in its shape caused by underground pressures. The work is scheduled for spring because snows have made the volcano inaccessible. But the new seismic network is an effective lookout, sensitive to very small increases in activity.

  17. Variations of seismic velocities in the Kachchh rift zone, Gujarat, India, during 2001-2013

    NASA Astrophysics Data System (ADS)

    Mandal, Prantik

    2016-03-01

    We herein study variations of seismic velocities in the main rupture zone (MRZ) of the Mw 7.7 2001 Bhuj earthquake for the time periods [2001-05, 2006-08, 2009-10 and 2011-13], by constructing dVp(%), dVs(%) and d(Vp/Vs)(%) tomograms using high-quality arrival times of 28,902 P- and 28,696 S-waves from 4644 precise JHD (joint hypocentral determination) relocations of local events. Differential tomograms for 2001-05 reveal a marked decrease in seismic velocities (low dVp, low dVs and high d(Vp/Vs)) in the MRZ (at 5-35 km depths) during 2001-10, which is attributed to an increase in crack/fracture density (higher pore fluid pressure) resulted from the intense fracturing that occurred during the mainshock and post-seismic periods. While we observe a slight recovery or increase in seismic velocities 2011-13, this could be related to the healing process (lower pore fluid pressure due to sealing of cracks) of the causative fault zone of the 2001 Bhuj mainshock. The temporal reduction in seismic velocities is observed to be higher at deeper levels (more fluid enrichment under near-lithostatic pressure) than that at shallower levels. Fluid source for low velocity zone (LVZ) at 0-10 km depths (with high d(Vp/Vs)) could be attributed to the presence of meteoric water or soft alluvium sediments with higher water content, while fluid source for LVZ at 10-35 km depths could be due to the presence of brine fluids (released from the metamorphic dewatering) and volatile CO2 (emanating from the crystallization of carbonatite melts in the asthenosphere), in fractures and pores. We also imaged two prominent LVZs associated with the Katrol Hill fault zone and Island Belt fault zone, extending from shallow upper-crust to sub-crustal depth, which might be facilitating the deeper circulation of metamorphic fluids/volatile CO2, thereby, the generation of lower crustal earthquakes occurring in the Kachchh rift zone.

  18. Finite-frequency sensitivity kernels of seismic waves to fault zone structures

    NASA Astrophysics Data System (ADS)

    Allam, A. A.; Tape, C.; Ben-Zion, Y.

    2015-12-01

    We analyse the volumetric sensitivity of fault zone seismic head and trapped waves by constructing finite-frequency sensitivity (Fréchet) kernels for these phases using a suite of idealized and tomographically derived velocity models of fault zones. We first validate numerical calculations by waveform comparisons with analytical results for two simple fault zone models: a vertical bimaterial interface separating two solids of differing elastic properties, and a `vertical sandwich' with a vertical low velocity zone surrounded on both sides by higher velocity media. Establishing numerical accuracy up to 12 Hz, we compute sensitivity kernels for various phases that arise in these and more realistic models. In contrast to direct P body waves, which have little or no sensitivity to the internal fault zone structure, the sensitivity kernels for head waves have sharp peaks with high values near the fault in the faster medium. Surface wave kernels show the broadest spatial distribution of sensitivity, while trapped wave kernels are extremely narrow with sensitivity focused entirely inside the low-velocity fault zone layer. Trapped waves are shown to exhibit sensitivity patterns similar to Love waves, with decreasing width as a function of frequency and multiple Fresnel zones of alternating polarity. In models that include smoothing of the boundaries of the low velocity zone, there is little effect on the trapped wave kernels, which are focused in the central core of the low velocity zone. When the source is located outside a shallow fault zone layer, trapped waves propagate through the surrounding medium with body wave sensitivity before becoming confined. The results provide building blocks for full waveform tomography of fault zone regions combining high-frequency head, trapped, body, and surface waves. Such an imaging approach can constrain fault zone structure across a larger range of scales than has previously been possible.

  19. Shear Wave Splitting from Local Earthquakes in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Martin, P.; Arroucau, P.; Vlahovic, G.

    2012-12-01

    In this study we investigate crustal anisotropy in the New Madrid seismic zone (NMSZ), by analyzing shear wave splitting from local earthquake data. The NMSZ is centrally located in the United States, spanning portions of western Tennessee, northeastern Arkansas, and southeastern Missouri. The NMSZ is also the location in which three of the largest known earthquakes took place in North America, occurring in 1811-1812. Although many seismic studies have been performed in this region, there is no consensus about which driving mechanism could satisfy both the current observations, as well as the historically observed seismicity. Therefore, it is important to continue investigating the NMSZ, to gain a better understanding of its seismicity, and the possible mechanisms that drive it. The automated technique developed by Savage et al. (2010) is used to perform the shear wave splitting measurements at 120 seismic stations within the NMSZ. The Center for Earthquake Research and Information (CERI) at the University of Memphis provided data for 1151 earthquakes spanning the years 2003-2011. The initial event selection was reduced to 245 earthquakes ranging in magnitude from 2.0 to 4.6, which fell within the shear wave window of one or more of the stations. The results of this study provide information about orientation of microcracks in the upper portion of the crust; future work will include analysis for temporal and spatial variations in order to assess the state of stress in the region.

  20. Erosion influences the seismicity of active thrust faults.

    PubMed

    Steer, Philippe; Simoes, Martine; Cattin, Rodolphe; Shyu, J Bruce H

    2014-01-01

    Assessing seismic hazards remains one of the most challenging scientific issues in Earth sciences. Deep tectonic processes are classically considered as the only persistent mechanism driving the stress loading of active faults over a seismic cycle. Here we show via a mechanical model that erosion also significantly influences the stress loading of thrust faults at the timescale of a seismic cycle. Indeed, erosion rates of about ~0.1-20 mm yr(-1), as documented in Taiwan and in other active compressional orogens, can raise the Coulomb stress by ~0.1-10 bar on the nearby thrust faults over the inter-seismic phase. Mass transfers induced by surface processes in general, during continuous or short-lived and intense events, represent a prominent mechanism for inter-seismic stress loading of faults near the surface. Such stresses are probably sufficient to trigger shallow seismicity or promote the rupture of deep continental earthquakes up to the surface. PMID:25412707

  1. Precursory seismic quiescence along the Sumatra-Andaman subduction zone: past and present

    NASA Astrophysics Data System (ADS)

    Sukrungsri, Santawat; Pailoplee, Santi

    2016-07-01

    In this study, the seismic quiescence prior to hazardous earthquakes was analyzed along the Sumatra-Andaman subduction zone (SASZ). The seismicity data were screened statistically with mainshock earthquakes of M w ≥ 4.4 reported during 1980-2015 being defined as the completeness database. In order to examine the possibility of using the seismic quiescence stage as a marker of subsequent earthquakes, the seismicity data reported prior to the eight major earthquakes along the SASZ were analyzed for changes in their seismicity rate using the statistical Z test. Iterative tests revealed that Z factors of N = 50 events and T = 2 years were optimal for detecting sudden rate changes such as quiescence and to map these spatially. The observed quiescence periods conformed to the subsequent major earthquake occurrences both spatially and temporally. Using suitable conditions obtained from successive retrospective tests, the seismicity rate changes were then mapped from the most up-to-date seismicity data available. This revealed three areas along the SASZ that might generate a major earthquake in the future: (i) Nicobar Islands (Z = 6.7), (ii) the western offshore side of Sumatra Island (Z = 7.1), and (iii) western Myanmar (Z = 6.7). The performance of a stochastic test using a number of synthetic randomized catalogues indicated these levels of anomalous Z value showed the above anomaly is unlikely due to chance or random fluctuations of the earthquake. Thus, these three areas have a high possibility of generating a strong-to-major earthquake in the future.

  2. San Andreas fault zone velocity structure at SAFOD at core, log, and seismic scales

    NASA Astrophysics Data System (ADS)

    Jeppson, Tamara N.; Tobin, Harold J.

    2015-07-01

    The San Andreas Fault (SAF), like other mature brittle faults, exhibits a zone of low seismic velocity hypothesized to result from fluid pressure effects and/or development of a damage zone. To address the relative contributions of these mechanisms in developing low-velocity zones, we measured P and S wave velocities ultrasonically at elevated confining and pore pressures on core samples from the San Andreas Fault Observatory at Depth (SAFOD). We compared those data to wireline and seismic-scale velocities to examine the scale dependence of acoustic properties of the fault core and damage zone. Average laboratory P and S wave velocities of the fault gouge at estimated in situ conditions are 3.1 and 1.5 km/s, respectively, consistent with the sonic log from the same intervals. These data show that fault core has intrinsically low velocity, even if no anomalous pore pressure is assumed, due to alteration and mechanical damage. In contrast, laboratory average P and S wave velocities for the damage zone are 4.7 and 2.5 km/s, up to 41% greater than the sonic log in the damage zone. This scale dependence indicates that stress conditions or macroscale features dominate the damage zone's acoustic properties, although velocity dispersion could play a role. Because no pressure anomaly was detected while drilling the SAFOD borehole, we infer that damage at a scale larger than core samples controls the elastic properties of the broader damage zone. This result bolsters other independent lines of evidence that the SAF does not contain major pore fluid overpressure at SAFOD.

  3. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

    PubMed Central

    Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J. -Michael

    2016-01-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

  4. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone.

    PubMed

    Nagaya, Takayoshi; Walker, Andrew M; Wookey, James; Wallis, Simon R; Ishii, Kazuhiko; Kendall, J-Michael

    2016-01-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed. PMID:27436676

  5. Seismic evidence for flow in the hydrated mantle wedge of the Ryukyu subduction zone

    NASA Astrophysics Data System (ADS)

    Nagaya, Takayoshi; Walker, Andrew M.; Wookey, James; Wallis, Simon R.; Ishii, Kazuhiko; Kendall, J.-Michael

    2016-07-01

    It is widely accepted that water-rich serpentinite domains are commonly present in the mantle above shallow subducting slabs and play key roles in controlling the geochemical cycling and physical properties of subduction zones. Thermal and petrological models show the dominant serpentine mineral is antigorite. However, there is no good consensus on the amount, distribution and alignment of this mineral. Seismic velocities are commonly used to identify antigorite-rich domains, but antigorite is highly-anisotropic and depending on the seismic ray path, its properties can be very difficult to distinguish from non-hydrated olivine-rich mantle. Here, we utilize this anisotropy and show how an analysis of seismic anisotropy that incorporates measured ray path geometries in the Ryukyu arc can constrain the distribution, orientation and amount of antigorite. We find more than 54% of the wedge must consist of antigorite and the alignment must change from vertically aligned to parallel to the slab. This orientation change suggests convective flow in the hydrated forearc mantle. Shear wave splitting analysis in other subduction zones indicates large-scale serpentinization and forearc mantle convection are likely to be more widespread than generally recognized. The view that the forearc mantle of cold subduction zones is dry needs to be reassessed.

  6. Seismic hazard assessment for oil-and-gas-bearing shelf zones: A case study of the North Caspian region

    NASA Astrophysics Data System (ADS)

    Krylov, A. A.; Ivashchenko, A. I.; Kovachev, S. A.

    2015-11-01

    Seismic hazard assessment is done for oil field areas in the North Caspian region by a method earlier successfully applied to other areas. The method involves general seismic zoning data, available regional databases on recurrence of seismic shaking, known models of ground motion attenuation in seismoactive regions, and data on geological and geophysical surveys in the studied area. The assigned level of seismic hazard of the region is refined using probabilistic analysis; additionally, disaggregation is made and accelerograms are synthesized (the latter are necessary for seismic microzoning of particular sites using the numerical nonlinear analysis of ground seismic response). The amplitude and spectral characteristics of ground motions are obtained which are necessary for seismic resistance design and construction of petroleum industry objects.

  7. A source generation model for near-field seismic impact of coal fractures in stress concentration zones

    NASA Astrophysics Data System (ADS)

    Feng, Junjun; Wang, Enyuan; Shen, Rongxi; Chen, Liang; Li, Xuelong; Li, Nan

    2016-08-01

    To study the near-field seismic impact of coal fractures in stress concentration zones, we established a source generation model based on finite dislocation source theory and dynamic fracture mechanics, derived an analytical expression for near-field seismic displacements caused by coal fractures in the zone and numerically computed the resultant near-field seismic displacements within the coal mass. The results show that (1) the larger difference between the vertical and horizontal normal stresses in the stress concentration zone leads to a greater fracture speed, which thereby causes a stronger seismic impact; (2) the P-wave component in the near-field seismic displacements mainly impacts on the middle of the roadway, while the SH- and SV wave components mainly affect the junctions between the roadway and both the roof and the floor, and the damage caused by the SH- and SV waves within the coal mass is more significant than that caused by the P-waves; and (3) the effective way to mitigate the seismic impact induced by coal fractures in stress concentration zones is to reduce the difference between the vertical and horizontal normal stresses as far as possible. It is hoped that this study will provide a better understanding of the seismic impacts induced by coal fractures in stress concentration zones and thus help engineers to discover ways to prevent roadway failure.

  8. Deep Mantle Dynamics under the North American Continent Drives Localised Flow and Stress Below the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Forte, A. M.; Mitrovica, J. X.; Moucha, R.; Simmons, N. A.; Grand, S. P.

    2007-12-01

    The origin of intraplate earthquakes represents one of the outstanding problems in modern geophysical research, and the major earthquake sequence that struck the central Mississippi River Valley in 1811-1812, the so-called New Madrid seismic sequence, has become a principal target of this research. As Johnston and Schweig (1996) have noted, the occurrence of such large magnitude earthquakes in "stable" North American crust, far from any plate boundaries, remains an enigma. To understand the possible origin of this enigmatic seismic activity we have developed a new high resolution model of mantle flow below North America. The model is constrained by simultaneously inverting global seismic and mantle-convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere. Moreover, it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment (GIA) and convection data. The flow model successfully reproduces plate velocities and observations of surface gravity and topography, including the continent-scale quasi-linear depression (after corrections for GIA and crustal heterogeneity) extending from northern Alaska to Venezuela. The predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation. We demonstrate that these signals are largely driven by viscous flow coupled to density anomalies within the lower mantle associated with the descent of the ancient Kula-Farallon plate system. More importantly, the flow calculations elucidate how these large-scale heterogeneities give rise to flow and stress patterns below the New Madrid Seismic Zone which are favourably oriented with respect the local fault geometry in this portion of the Mississippi valley.

  9. Multicomponent seismic loss estimation on the North Anatolian Fault Zone (Turkey)

    NASA Astrophysics Data System (ADS)

    karimzadeh Naghshineh, S.; Askan, A.; Erberik, M. A.; Yakut, A.

    2015-12-01

    Seismic loss estimation is essential to incorporate seismic risk of structures into an efficient decision-making framework. Evaluation of seismic damage of structures requires a multidisciplinary approach including earthquake source characterization, seismological prediction of earthquake-induced ground motions, prediction of structural responses exposed to ground shaking, and finally estimation of induced damage to structures. As the study region, Erzincan, a city on the eastern part of Turkey is selected which is located in the conjunction of three active strike-slip faults as North Anatolian Fault, North East Anatolian Fault and Ovacik fault. Erzincan city center is in a pull-apart basin underlain by soft sediments that has experienced devastating earthquakes such as the 27 December 1939 (Ms=8.0) and the 13 March 1992 (Mw=6.6) events, resulting in extensive amount of physical as well as economical losses. These losses are attributed to not only the high seismicity of the area but also as a result of the seismic vulnerability of the constructed environment. This study focuses on the seismic damage estimation of Erzincan using both regional seismicity and local building information. For this purpose, first, ground motion records are selected from a set of scenario events simulated with the stochastic finite fault methodology using regional seismicity parameters. Then, existing building stock are classified into specified groups represented with equivalent single-degree-of-freedom systems. Through these models, the inelastic dynamic structural responses are investigated with non-linear time history analysis. To assess the potential seismic damage in the study area, fragility curves for the classified structural types are derived. Finally, the estimated damage is compared with the observed damage during the 1992 Erzincan earthquake. The results are observed to have a reasonable match indicating the efficiency of the ground motion simulations and building analyses.

  10. Recognizing and dating prehistoric liquefaction features: Lessons learned in the New Madrid seismic zone, central United States

    USGS Publications Warehouse

    Tuttle, M.P.; Schweig, E.S.

    1996-01-01

    The New Madrid seismic zone (NMSZ), which experienced severe liquefaction during the great New Madrid, Missouri, earthquakes of 1811 and 1812 as well as during several prehistoric earthquakes, is a superb laboratory for the study of world-class, arthquake-induced liquefaction features and their use in paleoseismology. In seismically active regions like the NMSZ, frequent large earthquakes can produce a complex record of liquefaction events that is difficult to interpret. Lessons learned studying liquefaction features in the NMSZ may help to unravel the paleoseismic record in other seismically active regions. Soil characteristics of liquefaction features, as well as their structural and sratigraphic relations to Native American occupation horizons and other cultural features, an help to distinguish prehistoric liquefaction features from historic features. In addition, analyses of artifact assemblages and botanical content of cultural horizons can help to narrow the age ranges of liquefaction features. Future research should focus on methods for defining source areas and estimating magnitudes of prehistoric earthquakes from liquefaction features. Also, new methods for dating liquefaction features are needed.

  11. The Calabrian subduction zone (Ionian Sea): Historical seismicity and a new review of the system from multi-channel seismic data

    NASA Astrophysics Data System (ADS)

    Gallais, Flora; Gutscher, Marc-Andre; Torelli, Luigi; Polonia, Alina; Graindorge, David

    2010-05-01

    The Calabrian subduction zone is included in the long W-E elongated compressive South Mediterranean belt. This subduction is located in the complex Central Mediterranean area and accommodates the African/Eurasian convergence at very slow rates (<5 mm/y reported by a recent GPS study). The presence of shallow to deep earthquakes (down to 500 km depth) under Calabria and the South East Tyrrhenian Sea images a 70° NW dipping slab, associated with an active volcanic arc: the Aeolian Islands in the Tyrrhenian Sea. But no thrusts events characteristic of active subduction have been recorded during the instrumental era. However, the South Calabrian/East Sicilian region is well-known to have been affected by strong historical seismicity with Mercalli intensities reaching XI. The sources of these events is often linked to the activity of crustal, normal faults in the Calabrian region: 1638, 1783, 1905. Furthermore, important details of the Messina 1908 earthquake (72000 killed) and tsunami remain unresolved, in particular the origin of the tsunami (fault induced or submarine landslide). Moreover, the origin of two of the most destructive earthquakes (1169 and 1693) remains enigmatic. For the 1169 and 1693 (60000 killed and 5 to 10 m tsunami wave) Catania earthquakes, the source faults are the subject of debate and linked alternatively to the activity of the Malta escarpment or of the subduction fault plane (because the isoseismals are open to the sea). In this case, the 1169 earthquake which had similar intensities and a comparable isoseismal pattern, is suggested to have the same source and so the fault plane may have be locked between these two events. To better understand the origins of the 1169 and 1693 major events and seek evidence of activity of Calabrian system, we present new results from reprocessed 96-channels seismic reflection profiles (French Archimede cruise, 1997) offshore Sicily. Interpretation of the seismic dataset is based on correlations with published

  12. The Pollino Seismic Sequence: Activated Graben Structures in a Seismic Gap

    NASA Astrophysics Data System (ADS)

    Rößler, Dirk; Passarelli, Luigi; Govoni, Aladino; Bindi, Dino; Cesca, Simone; Hainzl, Sebatian; Maccaferri, Francesco; Rivalta, Eleonora; Woith, Heiko; Dahm, Torsten

    2015-04-01

    The Mercure Basin (MB) and the Castrovillari Fault (CF) in the Pollino range (Southern Apennines, Italy) represent one of the most prominent seismic gaps in the Italian seismic catalogue, with no M>5.5 earthquakes during the last centuries. In historical times several swarm-like seismic sequences occurred in the area including two intense swarms within the past two decades. The most energetic one started in 2010 and has been still active in 2014. The seismicity culminated in autumn 2012 with a M=5 event on 25 October. The range hosts a number of opposing normal faults forming a graben-like structure. Their rheology and their interactions are unclear. Current debates include the potential of the MB and the CF to host large earthquakes and the style of deformation. Understanding the seismicity and the behaviour of the faults is necessary to assess the tectonics and the seismic hazard. The GFZ German Research Centre for Geosciences and INGV, Italy, have jointly monitored the ongoing seismicity using a small-aperture seismic array, integrated in a temporary seismic network. Based on this installation, we located more than 16,000 local earthquakes that occurred between November 2012 and September 2014. Here we investigate quantitatively all the phases of the seismic sequence starting from January 2010. Event locations along with moment tensor inversion constrain spatially the structures activated by the swarm and the migration pattern of the seismicity. The seismicity forms clusters concentrated within the southern part of the MB and along the Pollino Fault linking MB and CF. Most earthquakes are confined to the upper 10 km of the crust in an area of ~15x15 km2. However, sparse seismicity at depths between 15 and 20 km and moderate seismicity further north with deepening hypocenters also exist. In contrast, the CF appears aseismic; only the northern part has experienced micro-seismicity. The spatial distribution is however more complex than the major tectonic structures

  13. Post-Seismic Crustal Deformation Following The 1999 Izmit Earthquake, Western Part Of North Anatolian Fault Zone, Turkey

    NASA Astrophysics Data System (ADS)

    Gurkan, O.; Ozener, H.

    2004-12-01

    The North Anatolian Fault is an about 1500 km long, extending from the Karliova to the North Aegean. Turkey is a natural laboratory with high tectonic activity caused by the relative motion of the Eurasian, Arabian and Anatolian plates. Western part of Turkey and its vicinity is a seismically active area. Since 1972 crustal deformation has been observed by various kinds of geodetic measurements in the area. Three GPS networks were installed in this region by Geodesy Department of Kandilli Observatory and Earthquake Research Institute( KOERI ) of Bogazici University: (1) Iznik Network, installed on the Iznik-Mekece fault zone, seismically low active part, (2) Sapanca Network, installed on the Izmit-Sapanca fault zone, seismically active part, (3) Akyazi Network, installed on their intersection area, the Mudurnu fault zone. First period observations were performed by using terrestrial methods in 1990 and these observations were repeated annually until 1993. Since 1994, GPS measurements have been carried out at the temporary and permanent points in the area and the crustal movements are being monitored. Horizontal deformations, which have not been detected by terrestrial methods, were determined from the results of GPS measurements. A M=7.4 earthquake hit Izmit, northern Turkey, on August 17, 1999. After this earthquake many investigations have been started in the region. An international project has been performed with the collaboration of Massachussets Institute of Technology, Turkish General Command of Mapping, Istanbul Technical University, TUBITAK-Marmara Research Center and Geodesy Department of KOERI. Postseismic movements have been observed by the region-wide network. A GPS network including 49 well spread points in Marmara region was observed twice a year between 1999 and 2003 years. During these surveys, another network with 6 points has been formed by using 2 points from each 3 microgeodetic networks on NAFZ with appropriate coverage and geometry. These

  14. Subsurface structure along the eastern marginal fault zone of Yokote Basin by Seismic reflection profiling studies, Northeast Japan

    NASA Astrophysics Data System (ADS)

    Kagohara, K.; Imaizumi, T.; Echigo, T.; Miyauchi, T.; Sato, H.

    2005-12-01

    Typical reverse faults, which are known as Senya earthquake faults appeared along the western foot of the Mahiru Mountains, associated with The Rikuu Earthquake (Mj7.2) of 1896 in Northeast Japan. Eastern marginal fault zone of the Yokote Basin consist of four main surface ruptures, about 35 km long, Obonai fault, Shiraiwa fault, Ota fault and Senya fault, depending on their continuity and strike (Matsuda et al., 1980). We carried out the seismic reflection profiling across these faults (Kawaguchi03 Seismic line, Unjono04 Seismic line and Kotaki05 Seismic line) to clarify the subsurface structure of these reverse fault system based on the data of tectonic geomorphology and structural geology and furthermore, to discuss the timing of migration of the thrusting from the range front to the basin margin. The seismic source was mini-vibrator trucks, with 20seconds of 10-100Hz signals at 10m or 5m intervals. The sweep signals were recorded by a digital telemetry system (GDAPS-4a) with 10 Hz geophones. The obtained seismic reflection data were processed by conventional Common mid-point (CMP) methods, including post-stack migration and depth conversion. The resulting seismic reflection profile reveals a thrust structure beneath these areas. At the Center of Senya hills there are two thrusts and one high angle reverse fault (1997 Seismic Line). Senya fault is an active frontal emergent thrust with flat and ramp structure. Although, the high angle reverse fault, located along the foot of the range is a short-cut branching fault from the Senya fault in the central part of the Senya hill (Sato et al., 1998), in the Unjono04 seismic line, the depth of the flat and ramp structure gradually shallow in the north part of the Senya hill, where the flexure scarp accompanied with antithetic faults formed on the fluvial terraces. In the Kawaguchi03 seismic line, the concealed fault, 0.5km below the surface, branched from the master Ota fault, form a flexure scarplet on the alluvial fan

  15. The shallow velocity structure of the Carboneras fault zone from high-resolution seismic investigations

    NASA Astrophysics Data System (ADS)

    Jones, C.; Nippress, S.; Rietbrock, A.; Faulkner, D. R.; Rutter, E. H.; Haberland, C. A.; Teixido, T.

    2010-12-01

    Understanding and characterizing fault zone structure at depth is vital to predicting the slip behaviour of faults in the brittle crust. The CFZ is a large offset (10s of km) strike-slip fault that constitutes part of the diffuse plate boundary between Africa and Iberia. It has been largely passively exhumed from ca. 4 to 6 km depth. The friable fault zone components are excellently preserved in the region’s semi-arid climate, and consist of multiple strands of phyllosilicate-rich fault gouge ranging from 1 to 20 m in thickness. We conducted four high-resolution seismic refraction tomography lines. Two of these lines crossed the entire width of the fault zone (~1km long) while the remaining lines concentrated on individual fault strands and associated damage zones (~100m long). For each line a combination of seismic sources (accelerated drop weight, sledgehammer and 100g explosives) was used, with 2m-geophone spacing. First breaks have been picked for each of the shot gathers and inputted into a 2D travel time inversion and amplitude-modeling package (Zelt & Smith, 1992) to obtain first break tomography images down to a depth 100m for the longer lines. The fault zone is imaged as a series of low velocity zones associated with the gouge strands, with Vp=1.5-1.75 km/s a velocity reduction of 40-60% compared to the wall-rock velocities (Vp=2.8-3.2km/s). These velocities are consistent with first break tomographic observations across the Dead Sea Transform fault (Haberland et al., 2007), but lower than the velocities imaged along the Punchbowl fault zone (part of the San Andreas system). Along the longer profiles we image multiple fault strands that exhibit a variety of thicknesses (~20-80m).

  16. Mapping brittle fracture zones in three dimensions: high resolution traveltime seismic tomography in a granitic pluton

    NASA Astrophysics Data System (ADS)

    Martí, D.; Carbonell, R.; Tryggvason, A.; Escuder, J.; Pérez-Estaún, A.

    2002-04-01

    Fractured and altered zones within a granitic pluton are mapped in three dimensions by means of high resolution seismic traveltime tomography. The input traveltimes were picked from offset and azimuth variable vertical seismic profiles (OVSP) acquired in three boreholes and from seismic shot gathers of four CDP high resolution seismic reflection profiles recorded on the surface. For the OVSP data a hydrophone streamer placed in the boreholes recorded the acoustic energy generated (a signal with a frequency content between 15 to 150 Hz) by a Vibroseis truck at source points distributed every 30 m in a rectangular grid of 620 m by 150 m. The combination of borehole and surface seismic data resulted in an increase in the ray density of the shallow subsurface. The tomographic algorithm uses a variable model grid, with a finer grid spacing close to the surface were ray density is highest and the velocity variations are strongest. Therefore the resulting velocity models feature more detail at shallow levels. A simple and smooth starting velocity model was derived from P -wave velocity logs. Careful surface geological mapping, and borehole geophysical data, P - and S -wave velocity logs and Poisson's ratio depth functions, provided key constraints for a physically reasonable 3-D interpretation of the tomograms. The low velocity anomalies constrained by the tomographic images were interpreted as unconsolidated rock, fractures and altered zones which correlate with structures mapped at the surface or velocity anomalies identified in the logs. Subsequent resolution analysis revealed that the derived velocity model is well constrained to depths of 60 m.

  17. Peru Subduction Zone Seismic Experiment (PeruSZE): Preliminary Results From a Seismic Network Between Mollendo and Lake Titicaca, Peru.

    NASA Astrophysics Data System (ADS)

    Guy, R.; Stubailo, I.; Skinner, S.; Phillips, K.; Foote, E.; Lukac, M.; Aguilar, V.; Tavera, H.; Audin, L.; Husker, A.; Clayton, R.; Davis, P. M.

    2008-12-01

    This work describes preliminary results from a 50 station broadband seismic network recently installed from the coast to the high Andes in Peru. UCLA's Center for Embedded Network Sensing (CENS) and Caltech's Tectonic Observatory are collaborating with the IRD (French L'Institut de Recherche pour le Developpement) and the Institute of Geophysics, in Lima Peru in a broadband seismic experiment that will study the transition from steep to shallow slab subduction. The currently installed line has stations located above the steep subduction zone at a spacing of about 6 km. In 2009 we plan to install a line of 50 stations north from this line along the crest of the Andes, crossing the transition from steep to shallow subduction. A further line from the end of that line back to the coast, completing a U shaped array, is in the planning phase. The network is wirelessly linked using multi-hop network software designed by computer scientists in CENS in which data is transmitted from station to station, and collected at Internet drops, from where it is transmitted over the Internet to CENS each night. The instrument installation in Peru is almost finished and we have been receiving data daily from 10 stations (out of total 50) since June 2008. The rest are recording on-site while the RF network is being completed. The software system provides dynamic link quality based routing, reliable data delivery, and a disruption tolerant shell interface for managing the system from UCLA without the need to travel to Peru. The near real-time data delivery also allows immediate detection of any problems at the sites. We are building a seismic data and GPS quality control toolset that would greatly minimize the station's downtime by alerting the users of any possible problems.

  18. Fracturing and Seismicity at the Prestahnúkur Fissure Swarm in the Ultra-Slowly Spreading Western Volcanic Zone, Iceland

    NASA Astrophysics Data System (ADS)

    Hjartardottir, A. R.; Hjaltadottir, S.; Einarsson, P.; Vogfjord, K. S.

    2014-12-01

    Extension across the mid-Atlantic plate boundary in south Iceland is taken up by two parallel zones, the Western and the Eastern Volcanic Zones. Spreading across the western zone is of the order of 1-7 mm/year, qualifying it as an ultra-slow rift. The Prestahnúkur fissure swarm in the Western Volcanic Zone offers an opportunity to study magma-tectonic interaction in an ultra-slow spreading setting. In this study, fractures and faults were mapped in detail from aerial photographs to determine the extent of the fissure swarm. The fissure swarm is about 60 km long and 10 km wide, although its extent to the south is uncertain due to its linkage with the adjacent Hengill fissure swarm. The fissure swarm has both open fractures, indicating postglacial activation, and normal faults with up to 45 m vertical offset. Hyaloclastite ridges („tindars"), produced by subglacial fissure eruptions during the Pleistocene, are found within the fissure swarm, as well as postglacial lava flows. Intermittent seismicity occurs in the area. Earthquakes during the last ~24 years were relocated to study their relation with the surface fractures. Relocation of these events indicate that some of the fractures seen on surface have been activated during these years. The small cumulative seismic moment and slow spreading measured by geodetic methods nevertheless show that no dike intrusions have occurred during this period. Pleistocene tindars and Holocene lavas are clear indications of magmatism, however. We therefore suggest that the Prestahnúkur fissure swarm is mainly formed by diking and rifting during rifting episodes, whereas small scale fracture movements can occur during inter-rifting periods. The present small-scale fracture movements may indicate lack of magma to drive dike intrusions.

  19. Modelling of Seismic Ground Motion in Santiago de Cuba City from Earthquakes in Oriente Fault Seismic Zone

    NASA Astrophysics Data System (ADS)

    Alvarez, L.; Panza, G. F.; Vaccari, F.; González, B. E.

    We present the results of complete P-SV and SH waves modelling, up to a maximum frequency of 1Hz, along two profiles in Santiago de Cuba city. The seismic sources are located in the depth range from 10 to 40km on the Oriente fault zone at distances of several tens of kilometres from the city. The calculation has been made by a hybrid method: Modal summation in the regional anelastic model (one-dimensional) where the source is buried, and finite differences in the local sedimentary anelastic models (two-dimensional). The analysis of the influence of the depth and of the distance of the source on the site effects shows that standard traditional methods, based on the deconvolution analysis of the rock outcrop motion, can lead to erroneous results.

  20. Seismicity rate changes in the Salton Sea Geothermal Field and the San Jacinto Fault Zone after the 2010 Mw 7.2 El Mayor-Cucapah earthquake

    NASA Astrophysics Data System (ADS)

    Meng, Xiaofeng; Peng, Zhigang

    2014-06-01

    Whether static or dynamic stress changes play the most important role in triggering earthquakes in the near field is still in debate. Here, we examine the seismicity rate changes in southern California following the 2010 Mw 7.2 El Mayor-Cucapah earthquake. We focus on the Salton Sea Geothermal Field (SSGF) and the San Jacinto Fault Zone (SJFZ) because of high-sensitivity continuous borehole recordings and ample background seismicity. A significant increase in seismic activity is found in both study regions immediately following the main shock. However, near the SSGF where the static Coulomb stress decreased, the seismicity rate dropped below the pre-main-shock rate after ˜1 month. In comparison, along the SJFZ with an increase in the static Coulomb stress, the seismicity rate remained higher than the background rate with several moderate-size earthquakes occurring in the subsequent months. While we cannot completely rule out other mechanisms, these observations are best consistent with a widespread increase in seismicity from dynamic stress changes immediately after the main shock, and longer term seismicity rate changes from static stress changes. Our observation, together with other recent studies, suggests that both static and dynamic stress changes are important in triggering near-field earthquakes, but their affected regions and timescales are different.

  1. The double seismic zone in downgoing slabs and the viscosity of the mesosphere

    NASA Technical Reports Server (NTRS)

    Sleep, N. H.

    1979-01-01

    The seismic zone beneath several island arcs between about 100 and 200 km depth consists of an upper zone having down-dip compression and a lower zone having down-dip tension. Several numerical models of the Aleutian arc were computed to test the hypothesis that these double seismic zones are due to sagging of the slab under its own weight. This sagging occurs because the asthenosphere (between about 100 and 200 km) provides little support or resistance to the slab, which is supported from below by the more viscous mesosphere and from above by the lithosphere. The viscosity of the mesosphere was constrained to the interval between 0.25 x 10 to the 22nd and 0.5 x 10 to the 22nd P by noting that the slab would have mainly down-dip compression at higher viscosities and mainly down-dip tension at lower viscosities. The deviatoric stress in the slab and the fault plane between the slab and the island arc is about 200-300 bars (expressed as shear stress). The models were calibrated to the observed depth and gravity anomalies in the trench.

  2. Directly imaging steeply-dipping fault zones in geothermal fields with multicomponent seismic data

    DOE PAGESBeta

    Chen, Ting; Huang, Lianjie

    2015-07-30

    For characterizing geothermal systems, it is important to have clear images of steeply-dipping fault zones because they may confine the boundaries of geothermal reservoirs and influence hydrothermal flow. Elastic reverse-time migration (ERTM) is the most promising tool for subsurface imaging with multicomponent seismic data. However, conventional ERTM usually generates significant artifacts caused by the cross correlation of undesired wavefields and the polarity reversal of shear waves. In addition, it is difficult for conventional ERTM to directly image steeply-dipping fault zones. We develop a new ERTM imaging method in this paper to reduce these artifacts and directly image steeply-dipping fault zones.more » In our new ERTM method, forward-propagated source wavefields and backward-propagated receiver wavefields are decomposed into compressional (P) and shear (S) components. Furthermore, each component of these wavefields is separated into left- and right-going, or downgoing and upgoing waves. The cross correlation imaging condition is applied to the separated wavefields along opposite propagation directions. For converted waves (P-to-S or S-to-P), the polarity correction is applied to the separated wavefields based on the analysis of Poynting vectors. Numerical imaging examples of synthetic seismic data demonstrate that our new ERTM method produces high-resolution images of steeply-dipping fault zones.« less

  3. Directly imaging steeply-dipping fault zones in geothermal fields with multicomponent seismic data

    SciTech Connect

    Chen, Ting; Huang, Lianjie

    2015-07-30

    For characterizing geothermal systems, it is important to have clear images of steeply-dipping fault zones because they may confine the boundaries of geothermal reservoirs and influence hydrothermal flow. Elastic reverse-time migration (ERTM) is the most promising tool for subsurface imaging with multicomponent seismic data. However, conventional ERTM usually generates significant artifacts caused by the cross correlation of undesired wavefields and the polarity reversal of shear waves. In addition, it is difficult for conventional ERTM to directly image steeply-dipping fault zones. We develop a new ERTM imaging method in this paper to reduce these artifacts and directly image steeply-dipping fault zones. In our new ERTM method, forward-propagated source wavefields and backward-propagated receiver wavefields are decomposed into compressional (P) and shear (S) components. Furthermore, each component of these wavefields is separated into left- and right-going, or downgoing and upgoing waves. The cross correlation imaging condition is applied to the separated wavefields along opposite propagation directions. For converted waves (P-to-S or S-to-P), the polarity correction is applied to the separated wavefields based on the analysis of Poynting vectors. Numerical imaging examples of synthetic seismic data demonstrate that our new ERTM method produces high-resolution images of steeply-dipping fault zones.

  4. Seismic Probing of the Base of a Tectonic Plate from Subduction Zone to Trench Outer Rise: Hikurangi Margin, New Zealand

    NASA Astrophysics Data System (ADS)

    Louie, J. N.; Kent, G. M.; Lamb, S. H.; Savage, M. K.; Stern, T. A.; Stratford, W. R.

    2015-12-01

    The nature of the base of a tectonic plate (LAB) is the 3rd dimension of plate tectonics. Recent seismic studies of the LAB have revealed planar features that show very different characteristics. In the oceans, the top of the S-wave low velocity zone shows a systematic deepening with plate age that fits plate-cooling models. However, the change in radial anisotropy has a markedly constant depth of ~70 km, regardless of plate age. A recent land-based study (SAHKE 2) of the subducted Pacific Plate in the southern Hikurangi margin has imaged a pair of distinct reflectors defining a ~10 km thick channel parallel to and ~70 km below the top of the ~15° dipping plate. Low velocities indicate that the channel is a zone of partial melt or high volatile content, acting as a weak base to the plate. Receiver function studies along the Japan margin have also imaged layers at these depths, parallel to the top of the plate and dipping up to 45°. We propose probing the base of the tectonic plate by tracing potential LAB seismic reflectors from their dipping portions in the subduction zone to where they bend in the trench outer rise. If the seismically identified boundary represents a 'frozen-in' feature, created at the mid ocean ridge, then it will remain parallel to the top of the plate, and its nature will remain unchanged as it is tracked over the outer rise. Alternatively, if the base of the plate is a thin channel of partial melt, then one would expect thickening of the channel beneath the outer rise due to melt ponding in the core of the flexure; this melt ponding may be the source of volcanic activity. A 500-km survey will trace the Pacific plate LAB from the subduction zone into the trench outer rise. The deeper part of the line coincides with the part of the plate where the ~10 km thick 'melt' channel was clearly imaged with the SAHKE 2 experiment. We show with synthetic experiments that given seismic energy scatter and attenuation comparable to that observed in the

  5. Mapping Deep Low Velocity Zones in Alaskan Arctic Coastal Permafrost using Seismic Surface Waves

    NASA Astrophysics Data System (ADS)

    Dou, S.; Ajo Franklin, J. B.; Dreger, D. S.

    2012-12-01

    Permafrost degradation may be an important amplifier of climate change; Thawing of near-surface sediments holds the potential of increasing greenhouse gas emissions due to microbial decomposition of preserved organic carbon. Recently, the characterization of "deep" carbon pools (several meters below the surface) in circumpolar frozen ground has increased the estimated amount of soil carbon to three times higher than what was previously thought. It is therefore potentially important to include the characteristics and processes of deeper permafrost strata (on the orders of a few to tens of meters below surface) in climate models for improving future predictions of accessible carbon and climate feedbacks. This extension is particularly relevant if deeper formations are not completely frozen and may harbor on-going microbial activity despite sub-zero temperatures. Unfortunately, the characterization of deep permafrost systems is non-trivial; logistics and drilling constraints often limit direct characterization to relatively shallow units. Geophysical measurements, either surface or airborne, are often the most effective tools for evaluating these regions. Of the available geophysical techniques, the analysis of seismic surface waves (e.g. MASW) has several unique advantages, mainly the ability to provide field-scale information with good depth resolution as well as penetration (10s to 100s of m with small portable sources). Surface wave methods are also able to resolve low velocity regions, a class of features that is difficult to characterize using traditional P-wave refraction methods. As part of the Department of Energy (DOE) Next-Generation Ecosystem Experiments (NGEE-Arctic) project, we conducted a three-day seismic field survey (May 12 - 14, 2012) at the Barrow Environmental Observatory, which is located within the Alaskan Arctic Coastal Plain. Even though permafrost at the study site is continuous, ice-rich and thick (>= 350m), our Multichannel Analysis of

  6. Imaging the Seismic Cycle in the Central Andean Subduction Zone from Geodetic Observations

    NASA Astrophysics Data System (ADS)

    Ortega-Culaciati, F.; Becerra-Carreño, V. C.; Socquet, A.; Jara, J.; Carrizo, D.; Norabuena, E. O.; Simons, M.; Vigny, C.; Bataille, K. D.; Moreno, M.; Baez, J. C.; Comte, D.; Contreras-Reyes, E.; Delorme, A.; Genrich, J. F.; Klein, E.; Ortega, I.; Valderas, M. C.

    2015-12-01

    We aim to quantify spatial and temporal evolution of fault slip behavior during all stages of the seismic cycle in subduction megathrusts, with the eventual goal of improving our understanding of the mechanical behavior of the subduction system and its implications for earthquake and tsunami hazards. In this work, we analyze the portion of the Nazca-SouthAmerican plates subduction zone affected by the 1868 southern Peru and 1877 northern Chile mega-earthquakes. The 1868 and 1878 events defined a seismic gap that did not experience a large earthquake for over 124 years. Only recently, the 1995 Mw 8.1 Antofagasta, 2001 Mw 8.4 Arequipa, 2007 Mw 7.7 Tocopilla, and 2014 Mw 8.2 Pisagua earthquakes released only a small fraction of the potential slip budget, thereby raising concerns about continued seismic and tsunami hazard. We use over a decade of observations from continuous and campaign GPS networks to analyze inter-seismic strain accumulation, as well as co-seimic deformation associated to the more recent earthquakes in the in the Central Andean region. We obtain inferences of slip (and back-slip) behavior using a consistent and robust inversion framework that accounts for the spatial variability of the constraint provided by the observations on slip across the subduction megathrust. We present an updated inter-seismic coupling model and estimates of pre-, co- and post- seismic slip behavior associated with the most recent 2014 Mw 8.2 Pisagua earthquake. We analyze our results, along with published information on the recent and historical large earthquakes, to characterize the regions of the megathrust that tend to behave aseismically, and those that are capable to accumulate a slip budget (ultimately leading to the generation of large earthquakes), to what extent such regions may overlap, and discuss the potential for large earthquakes in the region.

  7. Geomorphological features of active tectonics and ongoing seismicity of northeastern Kumaun Himalaya, Uttarakhand, India

    NASA Astrophysics Data System (ADS)

    Pathak, Vivekanand; Pant, Charu C.; Darmwal, Gopal Singh

    2015-08-01

    The northeastern part of Kumaun Lesser Himalaya, Uttarakhand, India, lying between the rupture zones of 1905, Kangra and 1934, Bihar-Nepal earthquakes and known as `central seismic gap' is a segment of an active fault known to produce significant earthquakes and has not slipped in an unusually long time when compared to other segments. The studied section forms a part of this seismic gap and is seismically an active segment of the Himalayan arc, as compared to the remaining part of the Kumaun Lesser Himalaya and it is evident by active geomorphological features and seismicity data. The geomorphological features of various river valley transects suggest that the region had a history of tectonic rejuvenation which is testified by the deposition of various levels of terraces and their relative uplift, shifting and ponding of river channels, uplifted potholes, triangular facets on fault planes, fault scarps, etc. Further, the seismic data of five-station digital telemetered seismic network along with two stand alone systems show the distribution of earthquakes in or along the analyzed fault transects. It is observed that the microseismic earthquakes (magnitude 1.0-3.0) frequently occur in the region and hypocenters of these earthquakes are confined to shallow depths (10-20 km), with low stress drop values (1.0-10 bar) and higher peak ground velocity (PGV). The cluster of events is observed in the region, sandwiched between the Berinag Thrust (BT) in south and Main Central Thrust (MCT) in north. The occurrences of shallow focus earthquakes and the surface deformational features in the different river valley transect indicates that the region is undergoing neotectonic rejuvenation. In absence of chronology of the deposits it is difficult to relate it with extant seismicity, but from the geomorphic and seismic observations it may be concluded that the region is still tectonically active. The information would be very important in identifying the areas of hazard prone and

  8. Imaging the Seattle Fault Zone with high-resolution seismic tomography

    USGS Publications Warehouse

    Calvert, A.J.; Fisher, M.A.

    2001-01-01

    The Seattle fault, which trends east-west through the greater Seattle metropolitan area, is a thrust fault that, around 1100 years ago, produced a major earthquake believed to have had a magnitude greater than 7. We present the first high resolution image of the shallow P wave velocity variation across the fault zone obtained by tomographic inversion of first arrivals recorded on a seismic reflection profile shot through Puget Sound adjacent to Seattle. The velocity image shows that above 500 m depth the fault zone extending beneath Seattle comprises three distinct fault splays, the northernmost of which dips to the south at around 60??. The degree of uplift of Tertiary rocks within the fault zone suggests that the slip-rate along the northernmost splay during the Quaternary is 0.5 mm a-1, which is twice the average slip-rate of the Seattle fault over the last 40 Ma.

  9. Fault zone amplified waves as a possible seismic hazard along the Calaveras fault in central California

    USGS Publications Warehouse

    Spudich, P.; Olsen, K.B.

    2001-01-01

    The Calaveras fault lies within a low velocity zone (LVZ) 1-2 km wide near Gilroy, California. Accelerographs G06, located in the LVZ 1.2 km from the Calaveras fault, and G07, 4 km from G06, recorded both the M 6.2 1984 Morgan Hill and the M 6.9 1989 Loma Prieta earthquakes. Comparison of the ground motions shows that a large 0.6-1.0 Hz velocity pulse observed at G06 during the Morgan Hill event may be amplified by focussing caused by the LVZ. Such amplified waves might be a mappable seismic hazard, and the zone of increased hazard can extend as much as 1.2 km from the surface trace of the fault. Finite-difference simulations of ground motions in a simplified LVZ model show a zone of amplified motion similar to the observations.

  10. Seismic detection of rigid zones at the top of the core.

    PubMed

    Rost, S; Revenaugh, J

    2001-11-30

    Data from earthquakes in the Tonga-Fiji region recorded at a seismic array in northern Australia show evidence for rigid zones at the top of the outer core. The ScP waveforms can be modeled by thin (0.12 to 0.18 kilometer) zones of molten iron mixed with solid material with a small, but positive, S-wave velocity (0.6 to 0.8 kilometer per second) that enables the propagation of S-waves in the outermost core. The zones may be topographic highs of the core-mantle boundary filled by light core sediments and might be important for variation of Earth's nutation and for convection of the outer core. PMID:11729313

  11. Searching for unconventional seismic signals on a subduction zone with a submerged forearc: OBS offshore the Lesser Antilles

    NASA Astrophysics Data System (ADS)

    Bécel, Anne; Diaz, Jordi; Laigle, Mireille; Hirn, Alfred

    2013-09-01

    Detecting unconventional seismic signals related to subduction zone processes at depth in continuous ocean bottom seismometer (OBS) records requires the analysis and identification of noise due to instrumental problems, deployment sites or sea state conditions. The temporary OBS deployment at the Lesser Antilles subduction zone provides new insights into the feasibility of detecting unconventional signals such as non volcanic tremor (NVT), long-period (LP) or ultra-long period (ULP) events. Analysis of noise at an array comprising several sites and types of instruments and comparison with recordings on land shows transients in the noise. Episodes can be identified considering the diversity of sites and instrument types and comparing the seismic signals with meteorological and oceanographic data. In order to reliably detect NVT (1-10 Hz) originating from inside the solid Earth, one must first characterize noise induced by the activity of the atmosphere and hydrosphere at the sea-bottom as well as on land. The semidiurnal modulation of noise amplitude can be shown here not to be due to that of the NVT from a seismic source at depth which is related to the subduction interplate and whose activity is modulated by the tidal stresses as inferred for other megathrusts on emerged forearcs. Here, the semidiurnal modulation is rather due to the effect of the tides themselves, such as tidal currents, since they do not affect all types and all components of the unique multi-station array of OBS that could be deployed on this submerged forearc. The short period cut-off of the strong noise due to ocean surface infragravity waves increases to longer periods with OBS depth, thereby increasing the observational window with low noise to lower frequencies, and deep OBS sites may be advantageous for detecting LP events.

  12. Seismic Intensity Maps for North Anatolian Fault Zone (Turkey) using Local Felt Intensity and Strong Motion Datasets

    NASA Astrophysics Data System (ADS)

    Askan, A.

    2014-12-01

    Seismic intensity maps indicate the spatial distribution of ground shaking levels in the meizoseismal area affected from an earthquake. Intensity maps provide guidance for the rapid assessment of shaking intensity and consequently the physical damage involved with an earthquake. Local correlations between the instrumental ground motion parameters and shaking intensity values are used to prepare these maps. There are several correlations derived using data from different regions in the world. However, since local damage characteristics of the built environment affect the felt-intensity values directly, different felt-intensity values may be reported in two different regions subjected to ground motions with similar amplitude and frequency contents. Thus such relationships should be derived based on regional strong motion and intensity datasets. Despite the intense seismic activity, as of now there are no such local correlations for the North Anatolian Fault Zone. In this study, we use the recently-compiled Turkish strong motion dataset along with the corresponding felt intensity data from past earthquakes to derive local relationships between MMI and a selected ground motion parameter (PGA, PGV, and SA at selected periods). We provide two sets of predictive equations: first group expresses the intensity values as a function of a selected ground motion parameter while the second set is more refined involving the event magnitude, distance and site class terms as independent variables. We present intensity maps of selected past events against the observed maps. We conclude that regional data from seismic networks is crucial for preparing realistic maps for use disaster management purposes.

  13. Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle

    NASA Astrophysics Data System (ADS)

    Garnero, Edward J.; McNamara, Allen K.; Shim, Sang-Heon

    2016-07-01

    Seismic images of Earth's interior reveal two massive anomalous zones at the base of the mantle, above the core, where seismic waves travel slowly. The mantle materials that surround these anomalous regions are thought to be composed of cooler rocks associated with downward advection of former oceanic tectonic plates. However, the origin and composition of the anomalous provinces is uncertain. These zones have long been depicted as warmer-than-average mantle materials related to convective upwelling. Yet, they may also be chemically distinct from the surrounding mantle, and potentially partly composed of subducted or primordial material, and have therefore been termed thermochemical piles. From seismic, geochemical and mineral physics data, the emerging view is that these thermochemical piles appear denser than the surrounding mantle materials, are dynamically stable and long-lived, and are shaped by larger-scale mantle flow. Whether remnants of a primordial layer or later accumulations of more-dense materials, the composition of the piles is modified over time by stirring and by chemical reactions with material from the surrounding mantle, underlying core and potentially from volatile elements transported into the deep Earth by subducted plates. Upwelling mantle plumes may originate from the thermochemical piles, so the unusual chemical composition of the piles could be the source of distinct trace-element signatures observed in hotspot lavas.

  14. Scaling of the critical slip distance for seismic faulting with shear strain in fault zones

    USGS Publications Warehouse

    Marone, C.; Kilgore, B.

    1993-01-01

    THEORETICAL and experimentally based laws for seismic faulting contain a critical slip distance1-5, Dc, which is the slip over which strength breaks down during earthquake nucleation. On an earthquake-generating fault, this distance plays a key role in determining the rupture nucleation dimension6, the amount of premonitory and post-seismic slip7-10, and the maximum seismic ground acceleration1,11. In laboratory friction experiments, Dc has been related to the size of surface contact junctions2,5,12; thus, the discrepancy between laboratory measurements of Dc (??? 10-5 m) and values obtained from modelling earthquakes (??? 10-2 m) has been attributed to differences in roughness between laboratory surfaces and natural faults5. This interpretation predicts a dependence of Dc on the particle size of fault gouge 2 (breccia and wear material) but not on shear strain. Here we present experimental results showing that Dc scales with shear strain in simulated fault gouge. Our data suggest a new physical interpretation for the critical slip distance, in which Dc is controlled by the thickness of the zone of localized shear strain. As gouge zones of mature faults are commonly 102-103 m thick13-17, whereas laboratory gouge layers are 1-10 mm thick, our data offer an alternative interpretation of the discrepancy between laboratory and field-based estimates of Dc.

  15. Revised magnitude-bound relation for the Wabash Valley seismic zone of the central United States

    USGS Publications Warehouse

    Olson, S.M.; Green, R.A.; Obermeier, S.F.

    2005-01-01

    Seismic hazard assessment in the central United States, and in particular the Wabash Valley seismic zone of Indiana-Illinois, frequently relies on empirical estimates of paleoearthquake magnitudes (M). In large part these estimates have been made using the magnitude-bound method. Existing region-specific magnitude-bound relations rely heavily on only a few historical earthquakes in the central United States and eastern Canada that induced reported liquefaction features. Recent seismological studies have suggested smaller magnitudes than previously presumed for some of these earthquakes, however, and other studies have reinterpreted site-to-source distances to liquefaction features associated with some of these earthquakes. In this paper, we re-examine historical earthquakes (M > ???5) that occurred in the central and eastern United States and eastern Canada; some of these earthquakes triggered liquefaction and others did not. Based on our findings, we reinterpret the region-specific magnitude-bound relation for the Wabash Valley. Using this revised magnitude-bound relation, we present magnitude estimates for four prehistoric earthquakes that occurred in the Wabash Valley seismic zone during Holocene time.

  16. A mixed seismic-aseismic stress release episode in the Andean subduction zone

    NASA Astrophysics Data System (ADS)

    Villegas-Lanza, J. C.; Nocquet, J.-M.; Rolandone, F.; Vallée, M.; Tavera, H.; Bondoux, F.; Tran, T.; Martin, X.; Chlieh, M.

    2016-02-01

    In subduction zones, stress is released by earthquakes and transient aseismic slip. The latter falls into two categories: slow slip and afterslip. Slow-slip events emerge spontaneously during the interseismic phase, and show a progressive acceleration of slip with a negligible contribution of synchronous tremors or microseismicity to the energy, or moment release. In contrast, afterslip occurs immediately after large and moderate earthquakes, decelerates over time, and releases between 20 and 400% of the moment released by the preceding earthquake. Here we use seismic and GPS data to identify transient aseismic slip that does not fit into either of these categories. We document a seismic-aseismic slip sequence which occurred at shallow depths along a weakly coupled part of the Andean subduction zone in northern Peru and lasted seven months. The sequence generated several moderate earthquakes that together account for about 25% of the total moment released during the full sequence, equivalent to magnitude 6.7. Transient slip immediately followed two of the earthquakes, with slip slowing at a logarithmic rate. Considered separately, the moment released by transient slip following the second earthquake was more than 1,000% of the moment released during the earthquake itself, a value incompatible with classical models of afterslip. Synchronous seismic swarms and aseismic slip may therefore define a stress-release process that is distinct from slow-slip events and afterslip.

  17. Global seismic data reveal little water in the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Houser, C.

    2016-08-01

    Knowledge of the Earth's present water content is necessary to constrain the amount of water and other volatiles the Earth acquired during its formation and the amount that is cycled back into the interior from the surface. This study compares 410 and 660 km discontinuity depth with shear wave tomography within the mantle transition zone to identify regions with seismic signals consistent with water. The depth of the 410 and 660 km discontinuities is determined from a large updated dataset of SS-S410S and SS-S660S differential travel times, known as SS precursors. The discontinuity depths measured from binning and stacking the SS precursor data are then compared to the shear velocity model HMSL-S06 in the transition zone. Mapping all the possible combinations, very few locations match the predictions from mineral physics for the effects of water on discontinuity depth and shear velocity. The predictions, although not yet measured at actual transition zone temperatures and pressures, are a shallow 410 km discontinuity, a deep 660 km discontinuity, and a slow shear velocity. Only 8% of the bins with high-quality data are consistent with these predictions, and the calculated average water content within these bins is around 0.6 wt.%. A few isolated locations have patterns of velocity/topography that are consistent with water, while there are large regional-scale patterns consistent with cold/hot temperature anomalies. Combining this global analysis of long period seismic data and the current mineral physics predictions for water in transition zone minerals, I find that the mantle transition zone is generally dry, containing less than one Earth ocean of water. Although subduction zones could be locally hydrated, the combined discontinuity and velocity data show no evidence that wadsleyite or ringwoodite have been globally hydrated by subduction or initial Earth conditions.

  18. Seismic anisotropy in mylonites: an example from the Mannin Thrust Zone, southwest Connemara, Ireland

    USGS Publications Warehouse

    Chroston, P.N.; Max, M.D.

    1988-01-01

    Mylonites associated with the Mannin Thrust zone of southwesternmost Connemara formed when the high-grade metamorphic rocks typical of most of the Connemara massif were thrust to the southeast over low metamorphic grade (low greenschist facies?) acid volcanics and volcaniclastic sediments, while being metamorphosed in the epidote-amphibolite facies. Triaxial and biaxial ultrasonic velocity measurements of mylonite specimens from a 240 m borehole have established that there is significant seismic anisotropy up to about 11% when comparing velocities perpendicular and parallel to the foliation. This would ultimately lead to a reflection coefficient of about 0.02 when comparing the mean "isotropic" seismic velocity with that perpendicular to the foliation. The finely striped, discontinunous mineral lithons that define mylonitic foliation, but which form no real and continuous surfaces, could interact with seismic energy to produce "reflections" that do not relate to lithological contacts within the rocks but to a tectonically induced, orientated acoustic impedance. However, the results support the work of others in suggesting that on its own the fabric would not produce the high amplitude reflections observed on deep seismic lines and other mechanisms need to be investigated. ?? 1988.

  19. Evidences for higher nocturnal seismic activity at the Mt. Vesuvius

    NASA Astrophysics Data System (ADS)

    Mazzarella, Adriano; Scafetta, Nicola

    2016-07-01

    We analyze hourly seismic data measured at the Osservatorio Vesuviano Ovest (OVO, 1972-2014) and at the Bunker Est (BKE, 1999-2014) stations on the Mt. Vesuvius. The OVO record is complete for seismic events with magnitude M ≥ 1.9. We demonstrate that before 1996 this record presents a daily oscillation that nearly vanishes afterwards. To determine whether a daily oscillation exists in the seismic activity of the Mt. Vesuvius, we use the higher quality BKE record that is complete for seismic events with magnitude M ≥ 0.2. We demonstrate that BKE confirms that the seismic activity at the Mt. Vesuvius is higher during nighttime than during daytime. The amplitude of the daily oscillation is enhanced during summer and damped during winter. We speculate possible links with the cooling/warming diurnal cycle of the volcanic edifice, with external geomagnetic field and with magnetostriction, which stress the rocks. We find that the amplitude of the seismic daily cycle changes in time and has been increasing since 2008. Finally, we propose a seismic activity index to monitor the 24-hour oscillation that could be used to complement other methodologies currently adopted to determine the seismic status of the volcano to prevent the relative hazard.

  20. Seismic Reflection Imaging of the Deep Seismogenic Zone: Lessons from Raytrace Modeling of Nankaido and Other Major Earthquake Rupture Zones

    NASA Astrophysics Data System (ADS)

    Kwiatkowski, A. M.; Brown, L. D.

    2001-12-01

    Seismic reflection techniques offer great promise for detailing, and perhaps monitoring, structure and physical properties associated with seismogenic faults at depths beyond direct sampling. However, success with these techniques depends upon a) effective imaging thru complex overlying structure, usually in rugged terrain and noisy environments when on land, b) sensitivity to physical properties relevant to seismogenesis (and their temporal variation?), and c) survey cost. We have used GXII ray-tracing software (GX Technology) and ProMaAX seismic processing software strategies for detecting structural details (asperties), fault offsets, and pore pressure variations at depths that correspond to the lower portions of the seismogenic zone associated with major lithospheric thrusts. Our models include geometries representing a) the Nankaido rupture zone of SE Japan (a prime target of the SEIZE initiative), b) the ramp and flat geometry proposed for the Himalayan megathrust beneath Nepal (an intracontinental seismogenic zone), and c) the Chelungpu fault of central Taiwan, whose slip during the 1999 Chi-Chi earthquake is well-documented. Among the specific issues addressed by this modeling were a) source requirements for detecting spatial variations in slip at a depth comparable to those of the Chi-Chi event; b) surface source/receiver requirements for imaging the base of the Nankaido seismogenic zone beneath Shikoku; c) detectability of temporal variations in physical conditions (e.g. pore pressure) at lower seismogenic depths for realistic terrains and current acquisition hardware; d) low-fold vs. high-fold strategies for imaging the seismogenic zone in remote areas; e) the relative importance of 3-D acquisition in detecting possible seismogenic factors at depth; and f) the practicality of 4-D imaging at seismogenic depths. While modeling confirms that certian aspects of deep structure relevant to rupture may realistically be targeted by reflection surveys (e.g. fault

  1. Seismic Refraction Profiles Across Shear Zones: What Can We Expect To See ?

    NASA Astrophysics Data System (ADS)

    Rumpel, H.-M.; Levander, A. R.

    In recent years more high-resolution refraction / wide-angle reflection data sets have been recorded to elucidate our picture of the Earth. For the 1999 seismic refraction/ wide-angle reflection CDROM profile more than 1200 stations were deployed be- tween New Mexico and Wyoming (Southern Rocky Mountain Region) at an average receiver spacing of 800m. A ray tracing code was used to analyse the data set. Though we have a good velocity model for the crust with a vertical resolution of less than 1km for the sediments and circa 2km at Moho depths, explaing more than 90% of the travel time data, we still find features we cannot explain by ray tracing only. One of those is the influence of shear zones on seismic sections. It is well known that the CDROM profile crosses many shear zones which formed in Phanerozoic time or even earlier. Therefore we test this idea in the following study. A two-dimensional visco-elastic finite-difference code was used to calculate synthetic seismogramms. Several models were created. The shear zone was realized as an elongate tabular body with velocities reduced at a constant percentage relative to the positive linear velocity gradient back- ground model. Variable parameters were the thickness of the shear zone, the relative location to the source, the dip angle and the velocity reduction within the shear zone relative to the background model. Recorder spacing was chosen to be comparable to the CDROM wide-angle data set and the source was located along the profile. Beside the normal first arrivals the synthetic seismogramms show diffuse arrivals between the front and back shear zone reflections. The time difference between front and back reflections depends on the shear zone thickness and the amplitudes on the velocity contrast. This band of arrivals comes in steeply crossing other reflections, thus creat- ing ambiguties in the data. A small influence on the first arrivals is seen as well, but might not be resolvable in field data. Varying

  2. Crustal structure of a land-ocean transitional zone in the northern South China Sea, from an onshore-offshore seismic survey

    NASA Astrophysics Data System (ADS)

    Cao, J.; Xia, S.; Xu, H.; Sun, J.

    2012-12-01

    The littoral fault zone(LFZ), a east-northeast-trending rupture region, which is located at an land-ocean transition area in the northern margin of South China Sea(SCS). Previous work in the northern SCS identified the LFZ as the most hazardous fault in the coastal South China, in history distribution of destructive erathquakes with magnitudes greater than 7.0 occured almost all along the LFZ. But in the maritime space off the Pearl River Mouth(PRM), the LFZ is intersected with a northwest-trending fault zone, where the seismic activity level to be significantly lower in recent hundreds of years, therefore it is very important to obtain detailed deep seismogenic structure in this potential strong earthquake zone. To investigate the crustal structure of the LFZ and the potential strong earthquake zone off the PMR, a wide-angle onshore-offshore seismic experiment and a coincident multi-channel seismic(MCS) profile were carried out in the northern SCS during July and August, 2010. The seismic source was an array of four guns and the shots were recorded simultaneously by ocean bottom seismometers and portable and permanent land stations. The preliminary processing result demonstrated good quality data, clear shallow-crustal structure is obtained from the MCS data, and the permanent seismic stations received air-gun signals over a distance of 360 km. Observed and calculated P-wave traveltimes were matched by forwad modelling prior to the inversion. Finally we got the optimal two-dimensional P-wave velocity model, and the model cross the northern margin of SCS shows the Moho depth is gradually thinned from land to sea, and the LFZ is a 10 km wide low-velocity rupture zone.

  3. Anatomy of a Complex Fault Zone: Land Seismic Reflection Imaging of the Tacoma Fault Zone, Washington State

    NASA Astrophysics Data System (ADS)

    Pape, K.; Liberty, L. M.; Pratt, T. L.

    2005-12-01

    Preliminary interpretations of new land-based seismic reflection images across the Tacoma fault zone in western Washington State document a complex pattern of faulting and folding. The Tacoma fault zone bounds gravity and aeromagnetic anomalies for 50 km across the central Puget Lowland west of the city of Tacoma, and tomography data suggest there is as much as 6 km of post-Eocene uplift of the hanging wall relative to Tacoma basin sediments to the south. We acquired four north-south seismic reflection profiles to define the character and tectonic history of the Tacoma fault zone. The 6-km long Powerline Road profile, located west of Case Inlet, perpendicularly crosses the 4-km-long Catfish Lake scarp discerned from Lidar data and trenching. The profile shows flat-lying strata on the south, but the north part of the profile is dominated by south-dipping Tertiary and older strata that appear to form the limb of an anticline. There appears to be at least one, and likely two faults in the Tertiary and older strata, although it is not clear these faults penetrate the shallowest Pleistocene strata. The 8.5-km long Carney Lake profile is located east of Case Inlet and spans two scarps imaged on Lidar data. This profile shows a similar geometry to the Powerline Road profile, folded and faulted Tertiary and older strata adjacent to flat-lying marine sediments of the Tacoma Basin. The 9-km long Bethel-Burley profile across the east portion of the Tacoma fault near Gig Harbor shows a significantly different reflector geometry than the profiles to the west. The Bethel-Burley profile is dominated by a strong, south-dipping reflection that becomes a prominent arch near the north end of the section. The strength of the reflector suggests that it marks the top of the Eocene basement rocks. South-dipping strata on this profile match those imaged on marine profiles from Carr Inlet. The new seismic reflection data support an interpretation in which the north edge of the Tacoma basin

  4. The Obsidian Creep Project: Seismic Imaging in the Brawley Seismic Zone and Salton Sea Geothermal Field, Imperial County, California

    NASA Astrophysics Data System (ADS)

    Catchings, R. D.; Rymer, M. J.; Goldman, M.; Lohman, R. B.; McGuire, J. J.

    2010-12-01

    In March 2010, we acquired medium- and high-resolution P- and S-wave seismic reflection and refraction data across faults in the Brawley seismic zone (BSZ) and across part of the Salton Sea Geothermal Field (SSGF), Imperial Valley, California. Our objectives were to determine the dip, possible structural complexities, and seismic velocities associated with the BSZ and SSGF. We acquired multiple seismic data sets along a north-south profile and a high-resolution P-wave profile along an east-west profile. The north-south profile included: 1) a 6.4-km-long P-wave (main) profile that was recorded on 320 Texan seismographs spaced at 20-m intervals, 2) a 1.2-km-long cabled, high-resolution profile along the northern end of the main profile, and 3) an approximately 1.2-km-long S-wave profile along the cabled profile. P-wave sources along the main profile were generated by 0.15- to 0.45-kg buried explosions spaced every 40 m, and P-wave sources along the cabled profile were generated by Betsy-Seisgun ‘shots’ spaced every 10 m. S-waves sources were generated by hammer impacts on the ends of an aluminum block. The east-west profile consisted of a 3.4-km-long high-resolution P-wave seismic profile with shots (Betsy-Seisgun) and geophones spaced every 10 m. Preliminary interpretation of shot gathers from blasts in the north-south profile suggests that the BSZ and SSGF are structurally complex, with abundant faults extending to or near the ground surface. Also, we observe relatively high-velocity material, apparent velocities of about 4.0 km/s in one direction and about 2.8 km/s in another relative to about 1.6 km/s for shallower material, that shallows beneath the SSGF. This may be due to high temperatures and resultant metamorphism of buried materials in the SSGF. From preliminary interpretation of shot gathers along the east-west profile we interpret a prominent fault that extends to the ground surface. This fault is on projection of the Kalin fault, from about 40 m to

  5. Water, oceanic fracture zones and the lubrication of subducting plate boundaries—insights from seismicity

    NASA Astrophysics Data System (ADS)

    Schlaphorst, David; Kendall, J.-Michael; Collier, Jenny S.; Verdon, James P.; Blundy, Jon; Baptie, Brian; Latchman, Joan L.; Massin, Frederic; Bouin, Marie-Paule

    2016-03-01

    We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg-Richter law. This power law describes the earthquake-magnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its along-strike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov-Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north-south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value `bull's-eyes' along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the

  6. Predicting brittle zones in the Bakken Formation using well logs and seismic data

    NASA Astrophysics Data System (ADS)

    Beecher, Michael E.

    The oil-in-place estimate for the Bakken Formation has varied from 10 billion barrels in 1974 to 503 billion barrels in 1999. However, only a small fraction of this estimate is recoverable due to the formation having very low porosity and permeability. Implementation of hydraulic fracture stages along horizontal wells in the Bakken has been productive. Recently, identification of zones where the formation is brittle has been used to improve hydraulic fracture stimulation efficiency in an effort to improve production. The first goal for this thesis is to identify a correlation between brittleness and production data by using elastic moduli and normalized production values. The hypothesis for this study is that rock with a low Poisson's ratio and high Young's modulus will be more brittle and will ultimately produce a higher amount of oil than more ductile rock. The next goal was to create and test a method to identify brittle zones with high normalized production in a 3D seismic data set without well control using producing wells from outside the survey with dipole sonic logs from the Bakken Formation. Correlations between normalized production values and elastic moduli were subsequently identified. Cumulative first-four-months' production was found to have the best correlation to the elastic moduli. Correlations of normalized production values and Poisson's ratio showed that sections of the middle Bakken with low Poisson's ratio yield higher normalized production values. Correlations of Young's modulus and normalized production showed that middle Bakken zones with low Young's modulus have higher normalized production values. However, when using additional wells that were not used for well-to-3D seismic correlations, the correlation shows that higher Young's modulus yield higher normalized production. The correlation with additional wells best represented the data and agrees with the initial hypothesis. Brittle zones were mapped in a 3D seismic data set by

  7. Constraining physical properties of ultra-low velocity zones using multiple seismic phases

    NASA Astrophysics Data System (ADS)

    Jensen, K. J.; Thorne, M. S.; Rost, S.; Nissen-Meyer, T.

    2010-12-01

    Ultra-low velocity zones (ULVZs) are a prominent feature of the lower mantle and may be related to many lower mantle dynamic processes. Several studies have indicated a possible partial melt origin to ULVZs however chemical reactions between the mantle and outer core cannot be ruled out giving a compositional component to ULVZ makeup. In order to determine the true importance of ULVZs, it is first necessary to determine their physical properties and geographic location. ULVZ physical properties (P- and S-wave velocities, density, thickness, and lateral coverage) are not well constrained due to sparse coverage of seismic phases used to probe the CMB, and extensive modeling tradeoffs. In this study we focus on examining ULVZ structure in the western Pacific region. Although several studies have confirmed the presence of ULVZs in this region, these studies have primarily been conducted using a single seismic phase. Yet, these past studies indicate that multiple seismic phases may interact with the same ULVZ. We seek to further constrain ULVZ physical properties using the seismic phases SPdKS, ScP, and PcP. Our primary focus is on ULVZ structure where at least 2 of these 3 seismic phases pass through the same ULVZ. Our SPdKS data set consists of broadband seismic recordings from 150 deep earthquakes occurring between January 1990 to April 2010 along the west boundary of the Pacific plate, and the east and south boundaries of the Eurasian plate. Our ScP and PcP data sets consist of 785 and 819 events, respectively, occurring between the years 1995 and 2000 recorded at the short period arrays of the International Monitoring System. We constrain ULVZ properties by waveform modeling using two new techniques: (1) We use the axi-symmetric finite difference technique PSVaxi to model broadband SPdKS waveforms, and (2) we use the axi-symmetric spectral-element method (AXISEM) to model high frequency (1 Hz) ScP and PcP waveforms. We determine the acceptable model space for each

  8. Strong ground motion synthesis along the Sanyi-Tungshih-Puli seismic zone using empirical Green`s functions

    SciTech Connect

    Hutchings, L.; Foxall, W.; Kasameyer, P.; Wu, F.T.; Rau, R.-J.; Jarpe, S.

    1997-01-01

    We synthesize strong ground motion from a M=7.25 earthquake along the NW-trending Sanyi-Tungshih-Puli seismic zone. This trend extends from Houlong to Taichung and forms a nearly continuous 78 km long seismic zone identified by the occurrence of M<5 events. It extends from a shallow depth all the way down to about 40 km. The entire length of the fault, if activated at one time, can lead to an event comparable to that the 1995 Kobe earthquake. With the improved digital CWBSN data now provided routinely by CWBSN, it becomes possible to use these data as empirical Green`s functions to synthesize potential ground motion for future large earthquakes. We developed a suite of 100 rupture scenarios for the earthquake and computed the commensurate strong ground motion time histories. We synthesized strong ground motion with physics-based solutions of earthquake rupture and applied physical bounds on rupture parameters. the synthesized ground motions obtained for a fixed magnitude and identifying the hazard to a site from the statistical distribution of engineering parameters, we have introduced a probabilistic component to the deterministic hazard calculation, The time histories suggested for engineering design are the ones that most closely match either the average or one standard deviation absolute acceleration response values.

  9. Characterization and application of microearthquake clusters to problems of scaling, fault zone dynamics, and seismic monitoring at Parkfield, California

    SciTech Connect

    Nadeau, R.M.

    1995-10-01

    This document contains information about the characterization and application of microearthquake clusters and fault zone dynamics. Topics discussed include: Seismological studies; fault-zone dynamics; periodic recurrence; scaling of microearthquakes to large earthquakes; implications of fault mechanics and seismic hazards; and wave propagation and temporal changes.

  10. SEISMIC STRUCTURE AND STRATIGRAPHY OF NORTHERN EDGE OF BAHAMAN-CUBAN COLLISION ZONE.

    USGS Publications Warehouse

    Ball, M.M.; Martin, R.G.; Bock, W.D.; Sylwester, R.E.; Bowles, R.M.; Taylor, D.; Coward, E.L.; Dodd, J.E.; Gilbert, L.

    1985-01-01

    Common-depth-point (CDP) seismic reflection data in the southwestern Bahamas reveal the northern edge of the tectonized zone that resulted from the late Mesozoic-early Cenozoic collision of Cuba and the Bahamas. Two seismic facies are present. A 10-km broad anticline occurs at the south end of Santaren Channel. Platform carbonates in the core of this structure overlie Early Cretaceous and older basinal carbonate deposits and are onlapped by Late Cretaceous and Cenozoic basinal facies. The structure is inferred to be a hanging-wall anticline at the northern limit of the Cuban fold-thrust belt formed in the Late Cretaceous. A deeper water embayment extended northward into the Straits of Florida, around northern Cay Sal Bank, and back into Santaren Channel during the Early Cretaceous.

  11. Seismic slip and down-dip strain rates in wadati-benioff zones.

    PubMed

    Bevis, M

    1988-06-01

    The rate of accumulation of seismic moment in Wadati-Benioff zones is used to estimate strain rates in subducting slabs that are sinking through the asthenosphere. Between depths of 75 and 175 kilometers a typical down-dip strain rate is about 10(-15) per second, which implies that slabs in this depth range typically accumulate strain of order 10(-1). This result is in accord with geometrical arguments that subducted slabs must experience large membrane strains to deform to their observed shapes. Mantle seismicity (repeated catastrophic shear failure) is apparently a primary mechanism by which large membrane strains accumulate in the cold cores of subducting slabs. Slabs are penetratively deformed, and they have low flexural rigidity compared to oceanic plates at the earth's surface. PMID:17815851

  12. Preliminary results of characteristic seismic anisotropy beneath Sunda-Banda subduction-collision zone

    SciTech Connect

    Wiyono, Samsul H.; Nugraha, Andri Dian

    2015-04-24

    Determining of seismic anisotropy allowed us for understanding the deformation processes that occured in the past and present. In this study, we performed shear wave splitting to characterize seismic anisotropy beneath Sunda-Banda subduction-collision zone. For about 1,610 XKS waveforms from INATEWS-BMKG networks have been analyzed. From its measurements showed that fast polarization direction is consistent with trench-perpendicular orientation but several stations presented different orientation. We also compared between fast polarization direction with absolute plate motion in the no net rotation and hotspot frame. Its result showed that both absolute plate motion frame had strong correlation with fast polarization direction. Strong correlation between the fast polarization direction and the absolute plate motion can be interpreted as the possibility of dominant anisotropy is in the asthenosphere.

  13. Variation of the Earth tide-seismicity compliance parameter during the recent seismic activity of Fthiotida, Greece

    NASA Astrophysics Data System (ADS)

    Contadakis, Michael; Aarabelos, Dimitrios; Vergos, Georgios; Spatalas, Spyridon

    2014-05-01

    Applying the Hi(stogram)Cum(ulation) method, which was introduced recently by Cadicheanu, van Ruymbecke and Zhu (2007), we analyze the series of the earthquakes occurred in the last 50 years in seismic active areas of Greece, i.e. the areas (a) of the Mygdonian Basin(Contadakis et al. 2007), (b) of the Ionian Islands (Contadakis et al. 2012 ) and (c) of the Hellenic Arc (Vergos et al. 2012 ) . The result of the analysis for all the areas indicate that the monthly variation of the frequencies of earthquake occurrence is in accordance with the period of the tidal lunar monthly and semi-monthly (Mm and Mf) variations and the same happens with the corresponding daily variations of the frequencies of earthquake occurrence with the diurnal luni-solar (K1) and semidiurnal lunar (M2) tidal variations. In addition the confidence level for the identifiation of such period accordance between earthquakes occurrence frequency and tidal periods varies with seismic activity, i.e. the higher confidence level corresponds to periods with stronger seismic activity. These results are in favor of a tidal triggering process on earthquakes when the stress in the focal area is near the critical level. Based on these results, we consider the confidence level of earthquake occurrence - tidal period accordance, p, as an index of tectonic stress criticality for earthquake occurrence and we check on posterior if the variation of the confidence level index, p, indicate the fault matureness in the case of the recent seismic activity at Fthiotida, Greece. In this paper we present the results of this test. References Cadicheanu, N., van Ruymbeke, M andZhu P.,2007:Tidal triggering evidence of intermediate depth earthquakes in Vrancea zone(Romania), NHESS 7,733-740. Contadakis, M. E., Arabelos, D. N., Spatalas, S., 2009, Evidence for tidal triggering on the shallow earthquakes of the seismic area of Mygdonia basin, North Greece, in Terrestrial and Stellar Environment, eds.D. Arabelos, M

  14. Structure and Deformation of the Hikurangi-Kermadec Subduction Zone - Transitions Revealed by Seismic Wide-angle Data

    NASA Astrophysics Data System (ADS)

    Scherwath, M.; Kopp, H.; Flueh, E. R.; Henrys, S. A.; Sutherland, R.

    2008-12-01

    The Hikurangi-Kermadec subduction zone northeast of New Zealand represents an ideal target to study lateral variations of subduction zone processes. The incoming Pacific plate changes from being a large igneous province, called the Hikurangi Plateau, in the south to normal oceanic plate north of the Rapuhia Scarp. The overriding Australian plate of continental character in the south, forming the North Island of New Zealand, and changes to an island arc in the north. Further lateral variability exists in changes in volcanic and hydro-thermal activity, transitions from accretion to subduction erosion, backarc spreading and rifting, and is accompanied by northward increasing seismicity. As part of the MANGO project (Marine Geoscientific Investigations on the Input and Output of the Kermadec Subduction Zone), four marine geophysical transects of largely seismic reflection and refraction data provide constraints on the upper lithospheric structures across the Hikurangi-Kermadec Trench between 29-38 deg S. On MANGO profile 1 in the south, the initially shallow subduction of the incoming plateau coincides with crustal underplating beneath the East Cape ridge. To the west lies the 100 km wide and over 10 km deep Raukumara Basin. Seismic velocities of the upper mantle of both plates are around 8 km/s and are considered normal. In contrast, on MANGO profile 4, about 1000 km to the north around the volcanically active Raoul Island, the incoming oceanic crust appears to bend considerably steeper and thus causes a 50 km narrower forearc with a smaller forearc basin. Furthermore, the upper mantle velocities in both plates are relatively low (7.4-7.7 km/s), likely indicating strong bending related deformation of the incoming plate and thermal activity within the arc possibly due to spreading. The central two transects MANGO 2 and 3, though without data coverage of the structure of the incoming plate, are more similar to MANGO 4. The arc regions appear to be strongly affected by

  15. Seismic Attenuation in the Vrancea Zone and Surroundings: Delamination or Subduction?

    NASA Astrophysics Data System (ADS)

    Dardac, M.; Russo, R. M.; Mocanu, V.; Popa, M.; Radulian, M.; Bonjer, K.

    2005-12-01

    We present results of seismic attenuation measurements focused on the Vrancea bend zone of the Carpathians, in order to delimit the lithosphere and asthenosphere in this area. Attenuation observed at 27 stations of the German-Romanian K2 seismic network allows us to evaluate models of subduction or delamination proposed for this region. The seismic stations are homogeneously distributed above concentrated intermediate depth Vrancea seismicity, which occurs beneath the contact between the Moldavian East European Platform, the Scythian Platform, and the Moesian Platform, to the east and southeast, and terranes of the Transylvania Basin lying within the Carpathian arc. The delamination model implies strong upwelling and horizontal inflow of asthenosphere into the gap between the delaminating and remnant lithosphere. The other model implies downwelling and perhaps lateral-horizontal inflow along the slab detachment or tear. The models imply different distributions of density and rheological properties associated with the different lithosphere - asthenosphere structures. We use the ratio of spectral amplitudes of P and S waves from vertical and transverse seismograms to estimate the S to P ratio in the frequency domain, and then we calculate Qs, the relative shear wave attenuation via two complementary techniques: For 400 combinations of iteratively increasing-length windows of P and S, we estimate individual and composite Qs. This allows us to calculate uncertainties in Qs. We find that stations located near and above the Vrancea zone and in the Transylvanian Basin, attenuation is high (low Q). Stations situated on the East European, Moesian, and Scythian Platforms are characterized by higher Qs (low attenuation). We interpret the high attenuation in the Vrancea and Transylvanian Basin is the result of shallow hot asthenosphere present in this area.

  16. Structure of the North Anatolian Fault Zone from the Autocorrelation of Ambient Seismic Noise

    NASA Astrophysics Data System (ADS)

    Taylor, George; Rost, Sebastian; Houseman, Gregory

    2016-04-01

    In recent years the technique of cross-correlating the ambient seismic noise wavefield at two seismometers to reconstruct empirical Green's Functions for the determination of Earth structure has been a powerful tool to study the Earth's interior without earthquakes or man-made sources. However, far less attention has been paid to using auto-correlations of seismic noise to reveal body wave reflections from interfaces in the subsurface. In principle, the Green's functions thus derived should be comparable to the Earth's impulse response to a co-located source and receiver. We use data from a dense seismic array (Dense Array for Northern Anatolia - DANA) deployed across the northern branch of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip system that extends ~1200 km across northern Turkey and continues to pose a high level of seismic hazard, in particular to the mega-city of Istanbul. We construct body wave images for the entire crust and the shallow upper mantle over the ~35 km by 70 km footprint of the 70-station DANA array. Using autocorrelations of the vertical component of ground motion, P-wave reflections can be retrieved from the wavefield to constrain crustal structure. We show that clear P-wave reflections from the crust-mantle boundary (Moho) can be retrieved using the autocorrelation technique, indicating topography on the Moho on horizontal scales of less than 10 km. Offsets in crustal structure can be identified that seem to be correlated with the surface expression of the northern branch of the fault zone, indicating that the NAFZ reaches the upper mantle as a narrow structure. The southern branch has a less clear effect on crustal structure. We also see evidence of several discontinuities in the mid-crust in addition to an upper mantle reflector that we interpret to represent the Hales discontinuity.

  17. Encouraging the use of seismic methods for the hydrogeophysical characterization of the critical zone

    NASA Astrophysics Data System (ADS)

    Pasquet, S.; Bodet, L.; Chalikakis, K.; Flipo, N.; Longuevergne, L.; Guérin, R.

    2015-12-01

    The characterization, study and monitoring of hydrosystems mainly rely on piezometric and log data, e.g. on local information. Fortunately, hydrogeophysics provide appropriate tools to interpolate boreholes information and to image heterogeneities in the critical zone. When electrical and electromagnetic methods predominate in such context, we recently suggested the use of classical seismic methods not only to provide a characterization of the subsurface geometry, but also to estimate the mechanical properties of the critical zone influenced by its water content. We tested, on two critical zone observatories with distinct hydrogeological characteristics, the simultaneous estimation of pressure (P-) and shear (S-) wave seismic velocities (VP and VS, respectively) from P-wave travel-time tomography and surface-wave dispersion inversion respectively. On both sites, e.g. a fractured environment with strong discontinuities and a continuous multi-layered hydrosystem, we were able to image spatial and/or temporal variations of VP/VS ratio, whose evolution was strongly associated to the water content observed locally.

  18. Estimation of earthquake effects associated with a great earthquake in the New Madrid seismic zone

    USGS Publications Warehouse

    Hopper, Margaret G.; Algermissen, Sylvester Theodore; Dobrovolny, Ernest E.

    1983-01-01

    Estimates have been made of the effects of a large Ms = 8.6, Io = XI earthquake hypothesed to occur anywhere in the New Madrid seismic zone. The estimates are based on the distributions of intensities associated with the earthquakes of 1811-12, 1843 and 1895 although the effects of other historical shocks are also considered. The resulting composite type intensity map for a maximum intensity XI is believed to represent the upper level of shaking likely to occur. Specific intensity maps have been developed for six cities near the epicentral region taking into account the most likely distribution of site response in each city. Intensities found are: IX for Carbondale, IL; VIII and IX for Evansville, IN; VI and VIII for Little Rock, AR; IX and X for Memphis, TN; VIII, IX, and X for Paducah, KY; and VIII and X for Poplar Bluff, MO. On a regional scale, intensities are found to attenuate from the New Madrid seismic zone most rapidly to the west and southwest sides of the zone, most slowly to the northwest along the Mississippi River, on the northeast along the Ohio River, and on the southeast toward Georgia and South Carolina. Intensities attenuate toward the north, east, and south in a more normal fashion. Known liquefaction effects are documented but much more research is needed to define the liquefaction potential.

  19. Microstructural study of the partition between seismic and aseismic deformation along the North Anatolian Fault zone, Turkey

    NASA Astrophysics Data System (ADS)

    Kaduri, M.; Gratier, J. P.; Renard, F.; Cakir, Z.; Lasserre, C.

    2014-12-01

    Along the North Anatolian Fault (Turkey), fault sliding is accommodated both by earthquakes and by aseismic creep. The creep processes develop either as transient (post-seismic or interseismic) sliding or as permanent sliding along zones localized on specific segments of the fault. Creep processes relax the stress and contribute to stress redistribution within the seismogenic zone. They participate to the deformation budget during the seismic cycle, sometimes delaying or on the contrary helping triggering the occurrence of large earthquakes. Identifying the mechanisms controlling creep and their evolution with time and space represents a major challenge for predicting the mechanical evolution of active faults. Our goal is to answer three main questions: How to identify at the outcrop scale permanent creep from transient creep? Is aseismic creep controlled by lithology? How does creep evolve before and after earthquakes? The challenge is to understand which key parameters control the shift from seismic to aseismic deformation, such as the effect of fabric, rock lithology, fault roughness, strain-rate, fluid pressure or stress.We collected samples from a dozen of fresh and well-preserved fault outcrops along creeping and locked segments of the North Anatolian Fault. We used various methods such as microscopic and geological observations, SEM, XRD analysis, strain measurements from image processing approaches in order to quantitatively characterize the amount of deformation and the mechanisms involved. Results show different relationships between lithology and mechanisms of deformation: (i) Along the locked segments of the North Anatolian Fault, in massive limestone, we found evidence of large earthquakes followed by pre- or post-seismic (i.e. afterslip) creep. (ii) Along some creeping segments, we observed gouges with weak clay (saponite) that could accommodate (or have accommodated in the past) large permanent creep. (iii) Along other creeping segments, we observed

  20. What can satellite geodesy tell us about fault zone mechanics and seismic hazard in the continents?

    NASA Astrophysics Data System (ADS)

    Wright, Tim

    2015-04-01

    Reliable assessment of hazard from short-term geodetic observations requires physical models that can explain any time-dependent surface deformation. In this lectures, I will review the observations, show models that are consistent with all the data, and discuss the implications for the mechanics of fault zones and the strength of the continental lithosphere. The last twenty years has seen a dramatic growth in our ability to measure surface deformation in fault zones using satellite geodesy. Collectively, these observations require any successful model to be capable of producing rapid postseismic deformation transients that decay with a 1/t dependency, and steady strain focussed in relatively narrow regions around the fault later in the cycle. I will show that these observations require (i) the lower crust outside of fault zones to have a viscosity greater than ~1020 Pa s, (ii) a region beneath the seismogenic upper crust that can respond rapidly to a stress perturbation. Rapid postseismic relaxation can occur through afterslip on a downward continuation of the fault, or by viscoelastic relaxation in a weak zone beneath the fault. If the relaxation is occurring viscoelastically, explaining the 1/t dependency requires a non-linear power-law relationship between stress and strain, and/or a viscosity that varies spatially due to temperature. It has been shown that such rheologies concentrate lower-crustal shear into narrow zones, a few kilometres wide. A model with narrow shear in the lower crust beneath major faults is also consistent with geological observations and results from a recent seismic experiment on the North Anatolian Fault conducted by the University of Leeds with Turkish partners at Kandilli Observatory and Sakarya University. I will conclude by discussing the implications of this synthesis for the use of satellite geodesy for seismic hazard assessment, the mechanics of continental deformation, and the strength of the continental lithosphere, and by

  1. Incorporating fault zone head wave and direct wave secondary arrival times into seismic tomography: Application at Parkfield, California

    NASA Astrophysics Data System (ADS)

    Bennington, N. L.; Thurber, C. H.; Zhang, H.; Peng, Z.; Zhao, P.

    2011-12-01

    /relocation inversion. Through these efforts, we hope to refine the 3D tomographic image of seismic velocity structure and the complex geometry of the active fault strands near SAFOD and along the Parkfield rupture zone.

  2. Wide Angle Seismic Imaging of (serpentinite ?) Fault Zones that Pass Through the Moho in a - Foreland Basin Setting

    NASA Astrophysics Data System (ADS)

    Stern, T. A.; Henrys, S. A.; Okaya, D. A.; Dimech, J.; Sato, H.; Iwasaki, T.

    2015-12-01

    We report strong, wide-angle, seismic reflections from the mantle beneath the Taranaki Fault zone, southwest North Island, New Zealand. We attribute these reflections to a serpentinised fault zone that accommodated large amounts (~100km) of shortening during the Miocene. Other manifestations of the collision zone are a fold and thrust belt in the overriding Australian plate, a 3-4 km deep foreland basin and fragments of serpentine outcropping at the surface. The 2011 SAHKE experiment carried out multi-component, active-source seismic surveys along a length of about 300 km. The middle 100 km of the line is on land. Twelve large explosions (500 kg weight) were detonated onshore into an array of ~ 900 vertical component and 300 horizontal component seismographs in the on land section. Deep reflections are evident on both the land and marine seismic data. At 30 s two way travel time (twtt) are ~ 100 km deep reflections that have been interpreted to be due to arise from the base of the subducting Pacific plate*. But at 20 s twtt there are back-dipping reflections that when migrated are located in the mantle wedge of the overriding Australian plate. These reflections are in the depth range of 40-70 km and dip ~45 degrees to the southeast. Coincident S-S reflections are also seen on the horizontal geophones of the land array coming in at ~ 34 s twtt. This timing implies a Vp/Vs ratio = 34/20= 1.7 for the time averaged reflection path. Zoeprittz equation solutions show that if both P-P and S-S arrivals are generated from a common reflector a negative impedance contrast is required. We suggest serpentinite is the most likely cause of the reflectivity. Marine reflection data also show low-frequency reflections around the Moho depths that we also associate with both serpentinite, and recently documented slow-slip events on the subducted plate interface. * Stern T.A. et al (2015). A seismic reflection image for the base of a tectonic plate, Nature, 518, pp. 85-88, doi10

  3. Induced seismicity of a normal blind undetected reservoir-bounding fault influenced by dissymmetric fractured damage zones

    NASA Astrophysics Data System (ADS)

    Rohmer, J.

    2014-04-01

    Fluid injection in deep sedimentary porous formations might induce shear reactivation of reservoir bounding faults. Here, we focus on `blind' 1000-m-long normal faults (with shear displacement ≤10 m), which can hardly be detected using conventional seismic surveys, but might potentially induce seismicity felt on surface. The influence of the dissymmetry in the internal structure of the fractured damage zone DZ is numerically investigated by using 2-D plane-strain finite-element simulations of a 1500-m-deep fluid injection into a porous reservoir. The problem is solved within the framework of fully saturated isothermal elasto-plastic porous media by both accounting for fault slip weakening and shear-induced degradation of fault core permeability. The numerical results show that the presence of a thick fractured hanging wall's DZ (with Young's modulus decreasing with the distance to the fault core due to the presence of fractures) strongly controls the magnitude M of the seismic event induced by the rupture. In the case modelled, M changed by more than 1.0 unit when the DZ thickness is varied from 5 to 50 m (M ranges from ˜0.1 to ˜1.5, i.e. from a `low' to a `low-to-moderate' seismicity activity). However, further extending DZ up to 90 m has little effect and the relationship reaches a quasi-horizontal plateau. This tendency is confirmed considering other initial conditions and injection scenarios. Finally, the presence of a thicker footwall DZ appears to lower the influence of hanging wall's DZ, but with lesser impact than the degree of fracturing.

  4. New constraints on micro-seismicity and stress state in the western part of the North Anatolian Fault Zone: Observations from a dense seismic array

    NASA Astrophysics Data System (ADS)

    Altuncu Poyraz, Selda; Teoman, M. Uğur; Türkelli, Niyazi; Kahraman, Metin; Cambaz, Didem; Mutlu, Ahu; Rost, Sebastian; Houseman, Gregory A.; Thompson, David A.; Cornwell, David; Utkucu, Murat; Gülen, Levent

    2015-08-01

    With the aim of extensively investigating the crustal structure beneath the western segment of the North Anatolian Fault Zone where it splays into northern and southern branches, a temporary seismic network (dense array for North Anatolia-DANA) consisting of 70 stations was deployed in early May 2012 and operated for 18 months in the Sakarya region during the FaultLab experiment. Out of 2437 events contaminated by explosions, we extracted 1371 well located earthquakes. The enhanced station coverage having a nominal station spacing of 7 km, lead to a minimum magnitude calculation of 0.1. Horizontal and vertical location uncertainties within the array do not exceed 0.8 km and 0.9 km, respectively. We observe considerable seismic activity along both branches of the fault where the depth of the seismogenic zone was mostly confined to 15 km. Using our current earthquake catalog we obtained a b-value of 1. We also mapped the b-value variation with depth and observed a gradual decrease. Furthermore, we determined the source parameters of 41 earthquakes with magnitudes greater than 1.8 using P-wave first motion polarity method. Regional Moment Tensor Inversion method was also applied to earthquakes with magnitudes greater than 3.0. Focal mechanism solutions confirm that Sakarya and its vicinity is stressed by a compressional regime showing a primarily oblique-slip motion character. Stress tensor analysis indicates that the maximum principal stress is aligned in WNW-ESE direction and the tensional axis is aligned in NNE-SSW direction.

  5. Seismic-reflection technique used to verify shallow rebound fracture zones in the Pierre Shale of South Dakota ( USA).

    USGS Publications Warehouse

    Nichols, T.C., Jr.; King, K.W.; Collins, D.S.; Williams, R.A.

    1988-01-01

    Shallow seismic-reflection data are presented to demonstrate their usefulness for locating and showing the continuity and lateral extent of rebound fracture zones in the Pierre Shale. Rebound fracture zones, identified in boreholes near Hayes, South Dakota, have variable depth, thickness, and character, thus making questionable the correlation of these zones between holes. Thus, the subsequent determination of dip and of continuity of the zones is somewhat tenuous, especially if the fracture characteristics change significantly between holes. Once rebound fracture zones have been identified and located by borehole geotechnical and geologic data, seismic profiles can reveal the extent and geometry of fractures in these zones, thus providing valuable preconstruction information without the cost of additional drilling.-Authors

  6. Analysis of the seismic activity associated with the 2010 eruption of Merapi Volcano, Java

    NASA Astrophysics Data System (ADS)

    Budi-Santoso, Agus; Lesage, Philippe; Dwiyono, Sapari; Sumarti, Sri; Subandriyo; Surono; Jousset, Philippe; Metaxian, Jean-Philippe

    2013-07-01

    The 2010 eruption of Merapi is the first large explosive eruption of the volcano that has been instrumentally observed. The main characteristics of the seismic activity during the pre-eruptive period and the crisis are presented and interpreted in this paper. The first seismic precursors were a series of four shallow swarms during the period between 12 and 4 months before the eruption. These swarms are interpreted as the result of perturbations of the hydrothermal system by increasing heat flow. Shorter-term and more continuous precursory seismic activity started about 6 weeks before the initial explosion on 26 October 2010. During this period, the rate of seismicity increased almost constantly yielding a cumulative seismic energy release for volcano-tectonic (VT) and multiphase events (MP) of 7.5 × 1010 J. This value is 3 times the maximum energy release preceding previous effusive eruptions of Merapi. The high level reached and the accelerated behavior of both the deformation of the summit and the seismic activity are distinct features of the 2010 eruption. The hypocenters of VT events in 2010 occur in two clusters at of 2.5 to 5 km and less than 1.5 km depths below the summit. An aseismic zone was detected at 1.5-2.5 km depth, consistent with studies of previous eruptions, and indicating that this is a robust feature of Merapi's subsurface structure. Our analysis suggests that the aseismic zone is a poorly consolidated layer of altered material within the volcano. Deep VT events occurred mainly before 17 October 2010; subsequent to that time shallow activity strongly increased. The deep seismic activity is interpreted as associated with the enlargement of a narrow conduit by an unusually large volume of rapidly ascending magma. The shallow seismicity is interpreted as recording the final magma ascent and the rupture of a summit-dome plug, which triggered the eruption on 26 October 2010. Hindsight forecasting of the occurrence time of the eruption is performed

  7. Studying geodesy and earthquake hazard in and around the New Madrid Seismic Zone

    USGS Publications Warehouse

    Boyd, Oliver Salz; Magistrale, Harold

    2011-01-01

    Workshop on New Madrid Geodesy and the Challenges of Understanding Intraplate Earthquakes; Norwood, Massachusetts, 4 March 2011 Twenty-six researchers gathered for a workshop sponsored by the U.S. Geological Survey (USGS) and FM Global to discuss geodesy in and around the New Madrid seismic zone (NMSZ) and its relation to earthquake hazards. The group addressed the challenge of reconciling current geodetic measurements, which show low present-day surface strain rates, with paleoseismic evidence of recent, relatively frequent, major earthquakes in the region. The workshop presentations and conclusions will be available in a forthcoming USGS open-file report (http://pubs.usgs.gov).

  8. Studying geodesy and earthquake hazard in and around the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Boyd, Oliver Salz; Magistrale, Harold

    2011-09-01

    Workshop on New Madrid Geodesy and the Challenges of Understanding Intraplate Earthquakes; Norwood, Massachusetts, 4 March 2011Twenty-six researchers gathered for a workshop sponsored by the U.S. Geological Survey (USGS) and FM Global to discuss geodesy in and around the New Madrid seismic zone (NMSZ) and its relation to earthquake hazards. The group addressed the challenge of reconciling current geodetic measurements, which show low present-day surface strain rates, with paleoseismic evidence of recent, relatively frequent, major earthquakes in the region. The workshop presentations and conclusions will be available in a forthcoming USGS open-file report (http://pubs.usgs.gov).

  9. Upper-mantle seismic discontinuities and the thermal structure of subduction zones

    USGS Publications Warehouse

    Vidale, J.E.; Benz, H.M.

    1992-01-01

    The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410 km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.

  10. Seismic hazard assessment of Syria using seismicity, DEM, slope, active tectonic and GIS

    NASA Astrophysics Data System (ADS)

    Ahmad, Raed; Adris, Ahmad; Singh, Ramesh

    2016-07-01

    In the present work, we discuss the use of an integrated remote sensing and Geographical Information System (GIS) techniques for evaluation of seismic hazard areas in Syria. The present study is the first time effort to create seismic hazard map with the help of GIS. In the proposed approach, we have used Aster satellite data, digital elevation data (30 m resolution), earthquake data, and active tectonic maps. Many important factors for evaluation of seismic hazard were identified and corresponding thematic data layers (past earthquake epicenters, active faults, digital elevation model, and slope) were generated. A numerical rating scheme has been developed for spatial data analysis using GIS to identify ranking of parameters to be included in the evaluation of seismic hazard. The resulting earthquake potential map delineates the area into different relative susceptibility classes: high, moderate, low and very low. The potential earthquake map was validated by correlating the obtained different classes with the local probability that produced using conventional analysis of observed earthquakes. Using earthquake data of Syria and the peak ground acceleration (PGA) data is introduced to the model to develop final seismic hazard map based on Gutenberg-Richter (a and b values) parameters and using the concepts of local probability and recurrence time. The application of the proposed technique in Syrian region indicates that this method provides good estimate of seismic hazard map compared to those developed from traditional techniques (Deterministic (DSHA) and probabilistic seismic hazard (PSHA). For the first time we have used numerous parameters using remote sensing and GIS in preparation of seismic hazard map which is found to be very realistic.

  11. A Three-dimensional Seismic Tomographic Study of The Gas Hydrate Stability Zone Offshore Vancouver Island

    NASA Astrophysics Data System (ADS)

    Minshull, T. A.; Hobro, J. W. D.; Singh, S. C.

    We present results from a high-resolution three-dimensional tomographic seismic ex- periment conducted in an around Ocean Drilling Program Site 889 offshore Vancouver Island. During the experiment, five ocean bottom hydrophones (OBH) were deployed twice, with a 3.3 km spacing and then a 1 km spacing, and approximately 26,000 shots were fired from a single 120 cu. in. airgun. A dense grid of single-channel seismic re- flection data acquired in the same region constrains the depth and reflection coefficient of the gas hydrate bottom simulating reflector (BSR), which is present over much of the survey area. Seismic data were analysed using a traveltime inversion technique for refracted and reflected phases recorded by the OBHs, which incorporates also the normal incidence seismic reflection constraints, and which tightly constrains the seis- mic velocity structure between the seabed and the base of the hydrate stability zone over a 8-km2 region. The inversion is regularised to find the smoothest velocity model consistent with the data and an estimate of the velocity uncertainty, and the final model is independent of the starting model. The inferred velocity structure in- cludes a steady increase in velocity from 1.5 km/s near the seabed to 1.85 km/s at the BSR, which lies 250 m below the seabed, and is consistent with the results of vertical seismic profiling at Site 889. Lateral variations in seismic velocity close to the seabed are correlated with lithological changes (e.g. folded vs slope basin sediments). Deeper lateral variations are inferred to be controlled by variations in hydrate con- tent. Throughout the well-constrained region, the seismic velocity immediately above the BSR is significantly higher than that of normally compacting, hydrate-free sed- iments at the same depth, and the velocity is poorly correlated with BSR reflection coefficients derived by Fink and Spence (1999). These observations suggest hydrate is present whether or not a BSR is observed

  12. Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

    USGS Publications Warehouse

    Bexfield, C.E.; McBride, J.H.; Pugin, Andre J.M.; Ravat, D.; Biswas, S.; Nelson, W.J.; Larson, T.H.; Sargent, S.L.; Fillerup, M.A.; Tingey, B.E.; Wald, L.; Northcott, M.L.; South, J.V.; Okure, M.S.; Chandler, M.R.

    2006-01-01

    Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between "deeper" deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and "shallower" deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard. ?? 2006 Elsevier B.V. All rights reserved.

  13. Seismic velocity structure of the central Taupo Volcanic Zone, New Zealand, from local earthquake tomography

    NASA Astrophysics Data System (ADS)

    Sherburn, Steven; Bannister, Stephen; Bibby, Hugh

    2003-03-01

    The 3-D distribution of P-wave velocity (Vp) and the P-wave/S-wave velocity ratio (Vp/Vs) are derived for the crust in the central Taupo Volcanic Zone (TVZ), New Zealand, by tomographic inversion of P- and S-wave arrival time data from local earthquakes. Resolution in the seismogenic mid-crust (4-6 km) is good, but poorer above and below these depths. The 3-D velocity model has several Vp anomalies as large as ±5% in the mid-lower crust (4-10 km) and more than ±10% in the upper crust (0-4 km). The model achieves a 55% reduction in data variance from an initial 1-D model. Young caldera structures, Okataina, Rotorua, and Reporoa, are characterised by low Vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies. We attribute these anomalies to large volumes of low Vp, low-density, volcaniclastic sediments that have filled these caldera collapse structures. Although there are no Vp anomalies which suggest the presence of molten or semi-molten magma beneath the TVZ, a large, high Vp anomaly of more than +15% and a high Vp/Vs anomaly are observed coincident with a diorite pluton beneath the Ngatamariki geothermal field. However, Vp anomalies cannot be seen beneath the largest geothermal fields, Waimangu, Waiotapu, and Reporoa, and, consequently, if such anomalies exist, they must be below the resolution of our data. A prominent Vp contrast of 5-10% occurs at a depth of about 6 km beneath the boundary between the Taupo-Reporoa Depression and the Taupo Fault Belt (TFB), coincident with the eastern limit of the seismic activity beneath the TFB. We interpret this velocity contrast as being caused by the presence of extensive, non-molten, intrusives beneath the Taupo-Reporoa Depression. We suggest that the high-velocity material beneath the Taupo-Reporoa Depression is isolated from regional extension in the TVZ, and from the resulting faulting and seismicity, which occurs preferentially within the weaker material of the TFB. We

  14. Earthquake-induced liquefaction features in the coastal setting of South Carolina and in the fluvial setting of the New Madrid seismic zone

    USGS Publications Warehouse

    Obermeier, S.F.; Jacobson, R.B.; Smoot, J.P.; Weems, R.E.; Gohn, G.S.; Monroe, J.E.; Powars, D.S.

    1990-01-01

    Many types of liquefaction-related features (sand blows, fissures, lateral spreads, dikes, and sills) have been induced by earthquakes in coastal South Carolina and in the New Madrid seismic zone in the Central United States. In addition, abundant features of unknown and nonseismic origin are present. Geologic criteria for interpreting an earthquake origin in these areas are illustrated in practical applications; these criteria can be used to determine the origin of liquefaction features in many other geographic and geologic settings. In both coastal South Carolina and the New Madrid seismic zone, the earthquake-induced liquefaction features generally originated in clean sand deposits that contain no or few intercalated silt or clay-rich strata. The local geologic setting is a major influence on both development and surface expression of sand blows. Major factors controlling sand-blow formation include the thickness and physical properties of the deposits above the source sands, and these relationships are illustrated by comparing sand blows found in coastal South Carolina (in marine deposits) with sand blows found in the New Madrid seismic zone (in fluvial deposits). In coastal South Carolina, the surface stratum is typically a thin (about 1 m) soil that is weakly cemented with humate, and the sand blows are expressed as craters surrounded by a thin sheet of sand; in the New Madrid seismic zone the surface stratum generally is a clay-rich deposit ranging in thickness from 2 to 10 m, in which case sand blows characteristically are expressed as sand mounded above the original ground surface. Recognition of the various features described in this paper, and identification of the most probable origin for each, provides a set of important tools for understanding paleoseismicity in areas such as the Central and Eastern United States where faults are not exposed for study and strong seismic activity is infrequent.

  15. A 80 OBS and 30 Land 3-component seismometers array encompassing the 280 km segment of the Lesser Antilles subduction megathrust seismogenic zone: view of current seismicity

    NASA Astrophysics Data System (ADS)

    Laigle, Mireille; Sapin, Martine; Ruiz, Mario; Diaz, Jordi; Kissling, Edi; Charvis, Philippe; Flueh, Ernst; Hirn, Alfred

    2010-05-01

    An extensive onshore and offshore seismic station array in the Lesser Antilles subduction zone allows to monitor microearthquake activity for a period of 4 months in a region previously outside of reach for detailed observation. Such a network has been possible thanks to a cluster of 3 seismic surveys (TRAIL - F/S Merian, SISMANTILLESII - N/O Atalante, and OBSANTILLES - N/O Antea) for deploying and recovering the instruments from several pools (Geoazur, INSU-IPGP, IFM-GEOMAR, AWI ). It has been followed by an additional deployment of the 28 GeoAzur OBSs (OBSANTILLES - N/O Antea) during 5 months in the south-western half. These operations have been carried out for the seismic investigation of the Antilles megathrust seismogenic zone in the framework of the THALES WAS RIGHT european project, and with also the financial support of the french ANR Catastrophes Telluriques et Tsunamis (SUBSISMANTI) and by the EU SALVADOR Programme of IFM-GEOMAR. Onshore, 30 3-components land stations (CSIC Barcelone, IPG Paris, INSU-RLBM and -LITHOSCOPE) have been temporarily deployed. The deep seismic structure of the whole area has been investigated during these seismic surveys by wide-angle reflection and refraction seismics recorded by these instruments as well as multi-channel reflection seismic imaging (MCS) along a dense grid of crossing profiles at the OBS positions providing excellent velocity information for the upper plate. Both the location and the interpretation of the recorded earthquake activity require constraints on the deep seismic structure, which will be discussed with respect to the 3D geometry of the interplate boundary and oceanic Moho, as well as those of the forearc basement and Moho. Preliminary locations have been obtained within a simple 1D velocity model by taking into account corrections for the variable thickness of the mud- and sediments layers beneath each OBS. The latter are estimated for both P- and S-waves to compensate for the huge structural

  16. Deep seismic survey images crustal structure of Tornquist Zone beneath southern Baltic Sea

    SciTech Connect

    Not Available

    1991-06-01

    The Tornquist Zone is Europe's longest tectonic lineament and bisects the continent in a NW-SE direction from the North Sea (off NW Denmark) to the Black Sea. New deep seismic reflection and coincident refraction data have been collected across its 50 km wide, intensely faulted and inverted NW part. The marine reflection profile in the area north of Bornholm Island shows a tilted block structure in the rigid upper crust, whereas the lower crust seems to be more gently uplifted. A complex transition from the highly reflective lower crust to the mantle is indicated by mantle reflections and a curious wide-angle event recorded by a landstation on Bornholm Island. The authors suggest that deep-reaching inversion tectonics, induced by Alpine and Carpathian orogeny, were responsible for the development of the gross crust-mantle structure of the Tornquist Zone in the study area, which seems to be similar to that in Poland.

  17. Two-dimensional seismic attenuation images of Stromboli Island using active data

    NASA Astrophysics Data System (ADS)

    Prudencio, J.; Del Pezzo, E.; Ibáñez, J. M.; Giampiccolo, E.; Patané, D.

    2015-03-01

    In this work we present intrinsic and scattering seismic attenuation 2-D images of Stromboli Volcano. We used 21,953 waveforms from air gun shots fired by an oceanographic vessel and recorded at 33 inland and 10 ocean bottom seismometer seismic stations. Coda wave envelopes of the filtered seismic traces were fitted to the energy transport equation in the diffusion approximation, obtaining a couple of separate Qi and Qs in six frequency bands. Using numerically estimated sensitivity kernels for coda waves, separate images of each quality factor were produced. Results appear stable and robust. They show that scattering attenuation prevails over intrinsic attenuation. The scattering pattern shows a strong concordance with the tectonic lineaments in the area, while an area of high total attenuation coincides with the zone where most of the volcanic activity occurs. Our results provide evidence that the most important attenuation effects in volcanic areas are associated with the presence of geological heterogeneities.

  18. Seismic sequence near Zakynthos Island, Greece, April 2006: Identification of the activated fault plane

    NASA Astrophysics Data System (ADS)

    Serpetsidaki, A.; Sokos, E.; Tselentis, G.-A.; Zahradnik, J.

    2010-01-01

    The April 2006 earthquake sequence near Zakynthos (Western Greece) is analysed to identify the fault plane(-s). The sequence (33 events) was relocated to assess physical insight into the hypocenter uncertainty. Moment tensor solution of three major events was performed, simultaneously with the determination of the centroid position. Joint analysis of the hypocenter position, centroid position and nodal planes indicated sub-horizontal fault planes. Moment tensor solutions of 15 smaller events were performed under assumption that the source positions are those of the hypocenters (without seeking centroids). Their focal mechanisms are highly similar and agree with the analysis of the three major events. The preferable seismotectonic interpretation is that the whole sequence activated a single sub-horizontal fault zone at a depth of about 13 km, corresponding to the interplate subduction boundary. Considering that the Ionian Sea is a high-seismicity area, the identification of the seismic fault is significant for the seismic hazard investigation of the region.

  19. Multiscale seismic signature of a small fault zone in a carbonate reservoir: Relationships between VP imaging, fault zone architecture and cohesion

    NASA Astrophysics Data System (ADS)

    Jeanne, Pierre; Guglielmi, Yves; Cappa, Frédéric

    2012-07-01

    The seismic (P-waves velocity, VP) signature of a small fault zone intersecting carbonate reservoir layers with contrasted properties of the southeastern French sedimentary basin was studied from the micro-scale to the fault zone pluri-meter scale architecture. VP measurements were done both at the meter scale, at 250-m depth in a gallery within the LSBB-URL, and at the centimeter scale (laboratory scale), on samples collected through boreholes. Results were compared to a VP tomography at the reservoir scale and to the fault rock mechanical properties. Our investigations indicated that P-wave velocity variations across the fault zone are strongly correlated to some key parameters, such as the uniaxial compressive strength (σc) of the fault core, the intact rock porosity (ϕ) and the fracturation intensity (RQD) of the damage zone. The seismic visibility of the fault zone depends on the contrasts between the porosity and the fracturation density of the sedimentary layers. In porous layers the fault induced deformations are mainly accommodated at the micro-scale (grain scale) with few macroscopic fractures, and the damage zone is thin leading to a high VP contrast with the fault core. In the low-porosity layers where deformations are mainly accommodated through brittle fractures, the seismic visibility of the fault is moderate, characterized by a decrease in the VP value which remains within the magnitude of the VP variations within the layers outside the fault zone. Interestingly, the fault seismic signature in the highly fractured layers appears clearly in the frequency domain at 3 dominant frequencies (2000, 9000 and 28,000 Hz), each of which exhibiting different spectral amplitudes for each components of the fault zone. Finally, the seismic signature of a relatively small fault zone included in a layered sedimentary series appears discontinuous, characterized by more or less thick high velocity patches more or less extended within the stratigraphic layers. We

  20. Slow slip events and seismic tremor at circum-Pacific subduction zones

    NASA Astrophysics Data System (ADS)

    Schwartz, Susan Y.; Rokosky, Juliana M.

    2007-09-01

    It has been known for a long time that slip accompanying earthquakes accounts for only a fraction of plate tectonic displacements. However, only recently has a fuller spectrum of strain release processes, including normal, slow, and silent earthquakes (or slow slip events) and continuous and episodic slip, been observed and generated by numerical simulations of the earthquake cycle. Despite a profusion of observations and modeling studies the physical mechanism of slow slip events remains elusive. The concurrence of seismic tremor with slow slip episodes in Cascadia and southwestern Japan provides insight into the process of slow slip. A perceived similarity between subduction zone and volcanic tremor has led to suggestions that slow slip involves fluid migration on or near the plate interface. Alternatively, evidence is accumulating to support the notion that tremor results from shear failure during slow slip. Global observations of the location, spatial extent, magnitude, duration, slip rate, and periodicity of these aseismic slip transients indicate significant variation that may be exploited to better understand their generation. Most slow slip events occur just downdip of the seismogenic zone, consistent with rate- and state-dependent frictional modeling that requires unstable to stable transitional properties for slow slip generation. At a few convergent margins the occurrence of slow slip events within the seismogenic zone makes it highly likely that transitions in frictional properties exist there and are the loci of slow slip nucleation. Slow slip events perturb the surrounding stress field and may either increase or relieve stress on a fault, bringing it closer to or farther from earthquake failure, respectively. This paper presents a review of slow slip events and related seismic tremor observed at plate boundaries worldwide, with a focus on circum-Pacific subduction zones. Trends in global observations of slow slip events suggest that (1) slow slip is a

  1. Rheological control in subduction zones: slab dynamics, fluid flow and seismic anisotropy

    NASA Astrophysics Data System (ADS)

    Van Keken, P. E.; Spiegelman, M.; Wilson, C. R.

    2011-12-01

    The strong temperature, strain-rate and compositional gradients in subduction zones provide extreme challenges to the modeling of their dynamics. Major questions remain regarding the state of stress in the slab, the nature of the downdip transition of the seismogenic zone, the rheological control on the release of fluids and the role of fluids in magma generation, earthquake source processes and the formation of seismic anisotropy. We use high resolution finite element models of convergent margins to address some of these topics. First, we explore the role of fluids on wedge rheology and how the presence or absence of fluids changes the dynamics of the wedge. The presence of fluids in the cold fore-arc may be the cause of trench-parallel anisotropy due to B-type olivine fabric or due to shape preferred orientation by the alignment of serpentinite filled cracks. Second, The cold fore-arc generally extends to where the slab is at ~80 km depth. This depth has a strong sensitivity to the depth where the overriding wedge couples with the slab. Weak phases such as chlorite, serpentinite and talc may play a critical control on the region where the slab remains decoupled, but the strong non-linearities involved make it difficult to determine a single process for all subduction zones. Finally, the increasing metamorphic grade of rocks in the subducting slab cause progressive fluid production, which has been linked to intermediate depth seismicity. The fluids also are considered the primary cause for arc volcanism, but it is not yet clear how the fluids escape from the slab and travel to the zones of arc magmatism. We use a new set of coupled solid state and porous flow models to determine how fluids affect the shear and bulk viscosity of these rocks and how this in turn controls the flow of fluids from the slab.

  2. Spatial heterogeneities of deviatoric stress and pore-pressure in Kyushu, Japan, and their implication for seismic activity

    NASA Astrophysics Data System (ADS)

    Matsumoto, Satoshi; Chikura, Hiromi; Ohkura, Takahiro; Miyazaki, Masahiro; Shimizu, Hiroshi; Abe, Yuki; Inoue, Hiroyuki; Yoshikawa, Shin; Yamashita, Yusuke

    2013-04-01

    We investigated the spatial variation in stress fields and pore fluid pressures on Kyushu Island, southwestern Japan. High seismic activity is found not only along active faults in Kyushu Island (southwestern Japan) but also in the central area of the island where there are active volcanoes. We consider the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field and pore fluid factor. Generally, the stress field corresponds to a strike slip regime in this area. A decline in the maximum principal compressional stress is found in the western part of the high seismicity area, in the middle of Kyushu Island; this may be caused by a thickening of the seismogenic zone, as estimated from D90 analysis. At thin seismogenic layer, strike slip faulting dominates and strain rate from GPS study is high. In the active fault zone, seismic activity along the fault is high, and the pore pressure within the zone is higher than the values observed elsewhere, suggesting a mechanism explained by the fault valve model of Sibson [1992]. The pore pressure in the high seismic area with scattered hypocenter distribution in the middle part is lower than that in the active fault zones.

  3. 3D Modelling of Seismically Active Parts of Underground Faults via Seismic Data Mining

    NASA Astrophysics Data System (ADS)

    Frantzeskakis, Theofanis; Konstantaras, Anthony

    2015-04-01

    During the last few years rapid steps have been taken towards drilling for oil in the western Mediterranean sea. Since most of the countries in the region benefit mainly from tourism and considering that the Mediterranean is a closed sea only replenishing its water once every ninety years careful measures are being taken to ensure safe drilling. In that concept this research work attempts to derive a three dimensional model of the seismically active parts of the underlying underground faults in areas of petroleum interest. For that purpose seismic spatio-temporal clustering has been applied to seismic data to identify potential distinct seismic regions in the area of interest. Results have been coalesced with two dimensional maps of underground faults from past surveys and seismic epicentres, having followed careful reallocation processing, have been used to provide information regarding the vertical extent of multiple underground faults in the region of interest. The end product is a three dimensional map of the possible underground location and extent of the seismically active parts of underground faults. Indexing terms: underground faults modelling, seismic data mining, 3D visualisation, active seismic source mapping, seismic hazard evaluation, dangerous phenomena modelling Acknowledgment This research work is supported by the ESPA Operational Programme, Education and Life Long Learning, Students Practical Placement Initiative. References [1] Alves, T.M., Kokinou, E. and Zodiatis, G.: 'A three-step model to assess shoreline and offshore susceptibility to oil spills: The South Aegean (Crete) as an analogue for confined marine basins', Marine Pollution Bulletin, In Press, 2014 [2] Ciappa, A., Costabile, S.: 'Oil spill hazard assessment using a reverse trajectory method for the Egadi marine protected area (Central Mediterranean Sea)', Marine Pollution Bulletin, vol. 84 (1-2), pp. 44-55, 2014 [3] Ganas, A., Karastathis, V., Moshou, A., Valkaniotis, S., Mouzakiotis

  4. Erosion influence the seismicity of active thrust faults

    NASA Astrophysics Data System (ADS)

    Steer, Philippe; Simoes, Martine; Cattin, Rodolphe; Shyu, J. Bruce H.

    2015-04-01

    Assessing seismic hazards remains one of the most challenging scientific issue in Earth sciences. Deep tectonic processes are classically considered as the only persistent mechanism driving the stress loading of active faults over a seismic cycle. Here we show with a mechanical model that erosion also significantly influences the stress loading of thrust faults at the timescale of a seismic cycle. Indeed, erosion rates of about ~0.1 to 20 mm/yr, as documented in Taiwan and in other active compressional orogens, can raise the Coulomb stress by ~0.1 to ~10 bar on the nearby thrust faults over the inter-seismic phase. Mass transfers induced by surface processes in general, during continuous or short-lived and intense events, represent a prominent mechanism for inter-seismic stress loading of faults near the surface. Such stresses are probably sufficient to promote the rupture of deep continental earthquakes up to the surface or to trigger shallow seismicity. We illustrate this last point by identifying seismic events in Taiwan, by the mean of a coupled statistical and mechanical approach, that were induced by intense erosional events.

  5. Erosion influence the seismicity of active thrust faults

    NASA Astrophysics Data System (ADS)

    Steer, Philippe; Simoes, Martine; Cattin, Rodolphe; Shyu, J. Bruce H.

    2016-04-01

    Assessing seismic hazards remains one of the most challenging scientific issue in Earth sciences. Deep tectonic processes are classically considered as the only persistent mechanism driving the stress loading of active faults over a seismic cycle. Here we show with a mechanical model that erosion also significantly influences the stress loading of thrust faults at the timescale of a seismic cycle. Indeed, erosion rates of about ˜0.1 to 20 mm/yr, as documented in Taiwan and in other active compressional orogens, can raise the Coulomb stress by ˜0.1 to ˜10 bar on the nearby thrust faults over the inter-seismic phase. Mass transfers induced by surface processes in general, during continuous or short-lived and intense events, represent a prominent mechanism for inter-seismic stress loading of faults near the surface. Such stresses are probably sufficient to promote the rupture of deep continental earthquakes up to the surface or to trigger shallow seismicity. We illustrate this last point by identifying seismic events in Taiwan, by the mean of a coupled statistical and mechanical approach, that were induced by intense erosional events.

  6. Characterizing fractures and shear zones in crystalline rock using seismic and GPR methods

    NASA Astrophysics Data System (ADS)

    Doetsch, Joseph; Jordi, Claudio; Laaksonlaita, Niko; Gischig, Valentin; Schmelzbach, Cedric; Maurer, Hansruedi

    2016-04-01

    Understanding the natural or artificially created hydraulic conductivity of a rock mass is critical for the successful exploitation of enhanced geothermal systems (EGS). The hydraulic response of fractured crystalline rock is largely governed by the spatial organization of permeable fractures. Defining the 3D geometry of these fractures and their connectivity is extremely challenging, because fractures can only be observed directly at their intersections with tunnels or boreholes. Borehole-based and tunnel-based ground-penetrating radar (GPR) and seismic measurements have the potential to image fractures and other heterogeneities between and around boreholes and tunnels, and to monitor subtle time-lapse changes in great detail. We present the analysis of data acquired in the Grimsel rock laboratory as part of the In-situ Stimulation and Circulation (ISC) experiment, in which a series of stimulation experiments have been and will be performed. The experiments in the granitic rock range from hydraulic fracturing to controlled fault-slip experiments. The aim is to obtain a better understanding of coupled seismo-hydro-mechanical processes associated with high-pressure fluid injections in crystalline rocks and their impact on permeability creation and enhancement. GPR and seismic data have been recorded to improve the geological model and characterize permeable fractures and shear zones. The acquired and processed data include reflection GPR profiles measured from tunnel walls, single-borehole GPR images, and borehole-to-borehole and tunnel-to-tunnel seismic and GPR tomograms. The reflection GPR data reveal the geometry of shear zones up to a distance of 30 m from the tunnels and boreholes, but the interpretation is complicated by the geometrical ambiguity around tunnels and boreholes and by spurious reflections from man-made structures such as boreholes. The GPR and seismic traveltime tomography results reveal brittle fractured rock between two ductile shear zones. The

  7. Geomorphic evidence of deformation in the northern part of the New Madrid seismic zone

    SciTech Connect

    Fischer, K.J.; Schumm, S.A. )

    1993-03-01

    A geomorphic evaluation of the northern portion of the New Madrid seismic zone between Hickman, KY and Osceola, AR has identified several locations where anomalous geomorphic conditions indicate possible surface deformation. For example, the slope, course, sinuosity and dimensions of the Mississippi River have been affected by the Lake County uplift and Tertiary-age sediments are exposed in its channel. Also, anomalous channel behavior near Caruthersville, MO and Barfield, AR suggests that these two reaches of the Mississippi River are structurally controlled. The Black River northeast of Pocahontas follows a peculiar angular course that suggests fracture control, and course changes of the Black, St. Francis, and Little Rivers may be related to subsurface faulting, uplift, or downwarping, as well as to differential compaction or the effects of groundwater withdrawal. The topography of Crowley's Ridge suggests that, between Jonesboro and Castor River, it is composed of at least three structural blocks, that are bounded by northeast-southwest trending faults. Near Jonesboro, river patterns appear to be affected by the Jonesboro, AR pluton. The geomorphic evaluation has identified anomalous surface features in the New Madrid seismic zone. Some can be directly linked to mapped structures in the region, whereas others may result from previously unidentified areas of surface deformation. The identification of these anomalies should provide direction for scientists who are employing subsurface techniques in order to locate tectonic deformation in the area.

  8. Compressive sensing of frequency-dependent seismic radiation from subduction zone megathrust ruptures

    PubMed Central

    Yao, Huajian; Shearer, Peter M.; Gerstoft, Peter

    2013-01-01

    Megathrust earthquakes rupture a broad zone of the subducting plate interface in both along-strike and along-dip directions. The along-dip rupture characteristics of megathrust events, e.g., their slip and energy radiation distribution, reflect depth-varying frictional properties of the slab interface. Here, we report high-resolution frequency-dependent seismic radiation of the four largest megathrust earthquakes in the past 10 y using a compressive-sensing (sparse source recovery) technique, resolving generally low-frequency radiation closer to the trench at shallower depths and high-frequency radiation farther from the trench at greater depths. Together with coseismic slip models and early aftershock locations, our results suggest depth-varying frictional properties at the subducting plate interfaces. The shallower portion of the slab interface (above ∼15 km) is frictionally stable or conditionally stable and is the source region for tsunami earthquakes with large coseismic slip, deficient high-frequency radiation, and few early aftershocks. The slab interface at intermediate depths (∼15–35 km) is the main unstable seismogenic zone for the nucleation of megathrust quakes, typically with large coseismic slip, abundant early aftershocks, and intermediate- to high-frequency radiation. The deeper portion of the slab interface (∼35–45 km) is seismically unstable, however with small coseismic slip, dominant high-frequency radiation, and relatively fewer aftershocks.

  9. Sinking mafic body in a reactivated lower crust: A mechanism for stress concentration at the New Madrid seismic zone

    USGS Publications Warehouse

    Pollitz, F.F.; Kellogg, L.; Burgmann, R.

    2001-01-01

    We propose a geodynamic model for stress concentration in the New Madrid seismic zone (NMSZ). The model postulates that a high-density (mafic) body situated in the deep crust directly beneath the most seismically active part of the NMSZ began sinking several thousands of years ago when the lower crust was suddenly weakened. Based on the fact that deformation rates in the NMSZ have accelerated over the past 9 k.y., we envision the source of this perturbation to be related to the last North American deglaciation. Excess mass of the mafic body exerts a downward pull on the elastic upper crust, leading to a cycle of primary thrust faulting with secondary strike-slip faulting, after which continued sinking of the mafic body reloads the upper crust and renews the process. This model is consistent with the youth of activity, the generation of a sequence of earthquakes, and the velocity evolution during interseismic periods, which depend upon the density contrast of the mafic body with respect to the surrounding crust, its volume, and the viscosity of the lower crust.

  10. Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone

    NASA Astrophysics Data System (ADS)

    ValléE, Martin; Nocquet, Jean-Mathieu; Battaglia, Jean; Font, Yvonne; Segovia, Monica; RéGnier, Marc; Mothes, Patricia; Jarrin, Paul; Cisneros, David; Vaca, Sandro; Yepes, Hugo; Martin, Xavier; BéThoux, Nicole; Chlieh, Mohamed

    2013-06-01

    document a 1 week long slow slip event (SSE) with an equivalent moment magnitude of 6.0-6.3 which occurred in August 2010 below La Plata Island (Ecuador), south of the rupture area of the 1906 Mw = 8.8 megathrust earthquake. GPS data reveal that the SSE occurred at a depth of about 10 km, within the downdip part of a shallow (<15 km), isolated, locked patch along the subduction interface. The availability of both broadband seismometer and continuous geodetic station located at the La Plata Island, 10 km above the SSE, enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface. During the slow slip sequence, the seismic data show a sharp increase of the local seismicity, with more than 650 earthquakes detected, among which 50 have a moment magnitude between 1.8 and 4.1. However, the cumulative moment released through earthquakes accounts, at most, for 0.2% of the total moment release estimated from GPS displacements. Most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion. While the earthquake sizes show a classical distribution (Gutenberg-Richter law with a b-value close to 1), the space-time occurrence presents a specific pattern. First, the largest earthquakes appear to occur randomly during the slow slip sequence, which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip. Moreover, the seismicity observed during the SSE consists in individual events and families of repeating earthquakes. These observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches. This study offers an a posteriori interpretation of the seismogenesis in the Central Ecuador subduction zone, where intense seismic swarms have been regularly observed (1977

  11. Joint seismic, hydrogeological, and geomechanical investigations of a fracture zone in the Grimsel Rock Laboratory, Switzerland

    SciTech Connect

    Majer, E.L.; Myer, L.R.; Peterson, J.E. Jr.; Karasaki, K.; Long, J.C.S.; Martel, S.J. ); Bluemling, P.; Vomvoris, S. )

    1990-06-01

    This report is one of a series documenting the results of the Nagra-DOE Cooperative (NDC-I) research program in which the cooperating scientists explore the geological, geophysical, hydrological, geochemical, and structural effects anticipated from the use of a rock mass as a geologic repository for nuclear waste. From 1987 to 1989 the United States Department of Energy (DOE) and the Swiss Cooperative for the Storage of Nuclear Waste (Nagra) participated in an agreement to carryout experiments for understanding the effect of fractures in the storage and disposal of nuclear waste. As part of this joint work field and laboratory experiments were conducted at a controlled site in the Nagra underground Grimsel test site in Switzerland. The primary goal of these experiments in this fractured granite was to determine the fundamental nature of the propagation of seismic waves in fractured media, and to relate the seismological parameters to the hydrological parameters. The work is ultimately aimed at the characterization and monitoring of subsurface sites for the storage of nuclear waste. The seismic experiments utilizes high frequency (1000 to 10,000 Hertz) signals in a cross-hole configuration at scales of several tens of meters. Two-, three-, and four-sided tomographic images of the fractures and geologic structure were produced from over 60,000 raypaths through a 10 by 21 meter region bounded by two nearly horizontal boreholes and two tunnels. Intersecting this region was a dominant fracture zone which was the target of the investigations. In addition to these controlled seismic imaging experiments, laboratory work using core from this region were studied for the relation between fracture content, saturation, and seismic velocity and attenuation. In-situ geomechanical and hydrologic tests were carried out to determine the mechanical stiffness and conductivity of the fractures. 20 refs., 90 figs., 6 tabs.

  12. Investigating Fault Zone Damage Processes Using Waveform Relocated Seismicity and High Precision Focal Mechanisms

    NASA Astrophysics Data System (ADS)

    Hauksson, E.; Yang, W.

    2011-12-01

    We analyze waveform relocated seismicity and focal mechanisms located near the principal slip zones (PSZs) of the late Quaternary fault zones in southern California to address questions such as: 1) Do mainshocks, aftershocks, and interseismic seismicity occur in the same place? 2) Is there evidence for fault normal compression along strike-slip faults? 3) Does seismicity in the near-field (±2km) of PSZs respond strongly to stress changes from nearby large earthquakes? 4) Is there other evidence for weak faults? We show that major earthquakes that can slip more than several meters, aftershocks, and near-fault relocated background seismicity mostly rupture different surfaces within late Quaternary fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults, called the PSZs. Aftershocks illuminate damage zones in the immediate vicinity of the PSZs, typically ±2 km wide. In contrast, the near-fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces, and forms spatially decaying distributions extending out to distances of ±10 km away from the PSZs. As part of our efforts to address these questions, we have completed waveform relocations of more than 500,000 earthquakes. We have also determined a new focal mechanism catalog of more than 150,000 events, applying the latest techniques that include using S/P amplitude ratios for additional constraints, and taking advantage of the new waveform based relocations. We use the data set of focal mechanisms to analyze the style of faulting and the stress field next to PSZs of major faults. For detailed analysis we select all the events located within ±10 km of major PSZs as defined in the SCEC Community Fault Model. There is a difference in the depth distribution as well as in the rake distribution on each side of the PSZs. The events may be deeper on one side of the PSZ as compared to the other. Further, the focal mechanisms often have a

  13. Elasticity of Hydrous Olivine Polymorphs: Implications for Seismic Structure of the Transition Zone

    NASA Astrophysics Data System (ADS)

    Duffy, T. S.; Mao, Z.; Jacobsen, S. D.; Jiang, F.; Smyth, J. R.; Holl, C. M.; Frost, D. J.

    2007-12-01

    The presence of water in the upper mantle and transition zone has the potential to explain various phenomena such as shear velocity anomalies or uplift and broadening of the 410-km discontinuity. The presence of H2O in the transition zone has also been frequently invoked to reconcile laboratory elasticity data on olivine polymorphs with seismic data for the amplitude of the 410-km discontinuity (Li et al., 2001; Chambers et al., 2005). Recently, we have measured the single-crystal elastic properties of hydrous olivine (Jacobsen et al., 2006) and a suite of hydrous wadsleyites (Mao et al., 2007a) at ambient conditions and one hydrous wadsleyite composition (0.84 wt% H2O) up to 12 GPa (Mao et al., 2007b). These data provide new constraints on elastic moduli and their pressure derivatives for hydrous olivine and wadsleyite. Using this data, we first examine the effect of H2O on bulk sound velocities under transition zone conditions because anelastic effects can be neglected in this case. At 410 km depth (~13.8 GPa, along a 1400°C adiabat), the bulk sound velocity of wadsleyite with 1 wt% H2O is 3.1% lower than for dry wadsleyite. Comparison of the seismic velocity jump across the 410-km discontinuity with the measured velocity contrast between wadsleyite and olivine provides a means to estimate the olivine abundance at 410-km depth. For mantle wadsleyite with 0.1-0.2 wt% H2O (Huang et al., 2005) and using experimentally determined olivine- wadsleyite H2O partition coefficients, the olivine abundance is found to be 40%, much lower than a pyrolite model. In order for a pyrolite composition to satisfy the seismic data, 1.2 wt. % H2O is needed in wadsleyite- a value greater than its maximum solubility under these conditions. The anomalously steep seismic gradient in the transition zone has been another feature of the region that has long defied explanation. We show that the seismic gradient can be matched if there is a gradient in H2O concentration across the transition

  14. Insights into the nature of the transition zone from physically constrained inversion of long-period seismic data

    PubMed Central

    Cammarano, Fabio; Romanowicz, Barbara

    2007-01-01

    Imposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it. PMID:17483461

  15. Insights into the nature of the transition zone from physically constrained inversion of long-period seismic data.

    PubMed

    Cammarano, Fabio; Romanowicz, Barbara

    2007-05-29

    Imposing a thermal and compositional significance to the outcome of the inversion of seismic data facilitates their interpretation. Using long-period seismic waveforms and an inversion approach that includes constraints from mineral physics, we find that lateral variations of temperature can explain a large part of the data in the upper mantle. The additional compositional signature of cratons emerges in the global model as well. Above 300 km, we obtain seismic geotherms that span the range of expected temperatures in various tectonic regions. Absolute velocities and gradients with depth are well constrained by the seismic data throughout the upper mantle, except near discontinuities. The seismic data are consistent with a slower transition zone and an overall faster shallow upper mantle, which is not compatible with a homogenous dry pyrolite composition. A gradual enrichment with depth in a garnet-rich component helps to reduce the observed discrepancies. A hydrated transition zone would help to lower the velocities in the transition zone, but it does not explain the seismic structure above it. PMID:17483461

  16. Precursory seismic activity before the 1944 Tonankai (Japan) earthquake: focusing on the downward migration of seismic activity

    NASA Astrophysics Data System (ADS)

    Mogi, Kiyoo

    1987-08-01

    Based on the latest JMA earthquake catalog, the author investigated seismic activity around the time of the 1944 Tonankai earthquake ( M 7.9, M w 8.1 ) and the 1946 Nankaido earthquake ( M 8.1, M w 8.1 ), which were both great thrust-type earthquakes along the Nankai Trough. For about 20 years before these earthquakes their focal regions had been quiescent (appearance of a seismic gap of the second kind) and the surrounding areas had become increasingly active, forming a doughnut pattern). Several years before these earthquakes occurred seismic activity increased at shallow depths of the area to the north. This activity gradually migrated downwards, and the Tonankai earthquake occurred when it reached its limit (a depth of approximately 70 km). The author has previously reported on several cases of increased activity in the deep seismic plane at a depth of 300-500 km prior to large shallow earthquakes along the Japan Trench (Mogi, 1973). This paper will demonstrate that a similar phenomenon occurs when the depth of the deep seismic plane is only about 70 km. For several years before the Tonankai earthquake there had been a slight increase in seismicity in the area along the trough, which is where the plate subducts. Two or three days before tne earthquake marked ground tilt also proceeded at the northeastern tip of the focal region. It is evident that the Tonankai earthquake was preceded by various long-term and short-term precursory phenomena.

  17. Results of trench perpendicular wide angle seismic transects across the Manila subduction zone offshore southern Taiwan

    NASA Astrophysics Data System (ADS)

    Eakin, D. H.; McIntosh, K. D.; Van Avendonk, H. J.

    2011-12-01

    Multi-channel seismic reflection and wide-angle seismic data collected in 2009 aboard the R/V Marcus Langseth as part of the TAIGER program delineate the crustal structure of the Manila subduction zone in the northern South China Sea. As part of that project, we recorded marine seismic data using a deployment of ocean-bottom-seismometers (OBS) from the U.S. instrument pool and National Taiwan Ocean University. The region between northern Luzon and southern Taiwan evolves from oceanic subduction to incipient arc-continent collision. This presentation focuses on results of 2 offshore transects across the Manila subduction zone offshore southern Taiwan. Our goal here is to document the transition from pure oceanic subuction in the south to incipient arc-continent collision in the north, an understanding of which is integral for future geodynamic modeling of the advanced arc-continent collision in the north. The northern transect, line T2 is located at 21.4° N and used 30 OBSs. Line T1 was located at 20.5° N and used 27 OBSs across the Manila subduction zone. Data quality is extremely variable due to the local geology and quality of seafloor coupling at each instrument. Preliminary travel-time tomography of transect T2 shows a 10-15 km thick Eurasian crust with crustal velocities of 5-7.5 km/sec entering the Manila trench suggesting thinned continental crust, serpentinized upper mantle, or both in this region. The model shows the accretionary prism to be cored by high velocity material (6-7 km/sec) that may be the result of accretion of crustal material from the subducting Eurasian slab. We also observe asymmetric crustal thickening beneath the Gagua Ridge that is potentially a result of failed subduction of the Philippine Sea Plate westward along the Gagua Ridge. The wide-angle data is complimented by MCS reflection data to constrain sediment thickness, top of the crystalline basement, and moho. Preliminary work is in progress with transect T1 which will be

  18. Asymmetric active seismicity along the ultra-slow spreading Gakkel Ridge

    NASA Astrophysics Data System (ADS)

    Hopper, John R.; Voss, Peter H.; Lavier, Luc L.

    2015-04-01

    Ultra-slow spreading ridges are frequently characterised by spreading segments that are largely magma starved. Spreading along such segments does not occur by crustal creation/accretion processes such as intrusions, diking and volcanism, but rather by mechanical extension of the lithosphere, exposing the mantle to seafloor where it interacts with seawater to form serpentinite. Such exhumation is thought to occur along detachment faults that form concave down surfaces and produce an extensional geometry that is highly asymmetric. A consequence of all models that have been developed to simulate this type of extension is that stress and strain is focused primarily on the footwall block of the spreading system. This would predict that at any given time, only one side of the system should show active seismicity. In 2001, the Gakkel Ridge was extensively sampled by dredging during the AMORE cruise. These samples showed that the ridge is divided into distinct segments that today are either magmatically robust (only basalts recovered) or magmatically starved (dominantly serpentinised peridotite and gabbros recovered). We extracted earthquake data along the Gakkel Ridge from the global catalogs to investigate if these distinct segments exhibit any differences in active seismicity. We show that the western volcanic zone shows symmetric active seismicity, with earthquakes occurring on both sides of the ridge axis along a relatively restricted region. In contrast, the sparsely magmatic zone shows active seismicity dominantly along along the southern half of the ridge, with comparatively little seismicity to the north. These results are consistent with the proposed models for the formation of amagmatic spreading centers.

  19. Coulomb stress evolution over the past 200 years and seismic hazard along the Xianshuihe fault zone of Sichuan, China

    NASA Astrophysics Data System (ADS)

    Shao, Zhigang; Xu, Jing; Ma, Hongsheng; Zhang, Langping

    2016-02-01

    This study focuses on the M ≥ 6.7 earthquakes that have occurred since 1816 on the Xianshuihe fault zone in southwest China. The interseismic Coulomb stress accumulation and the Coulomb stress changes caused by coseismic dislocation and postseismic viscoelastic relaxation of the previous shocks were computed for different periods on the relevant fault segments. Based on these results, we analyzed the relationship between time-adjacent strong shocks and the Coulomb stress evolution before every earthquake. The analysis suggests that strong earthquakes mostly occurred in the Coulomb stress enhancement region caused by coseismic dislocation and postseismic viscoelastic relaxation of the last earthquake. Considering the Coulomb stress evolution at the fault planes of the epicentral area before earthquakes, we found that the Coulomb stress accumulation caused by the interseismic tectonic loading was dominant for most strong earthquakes. For some other earthquakes the stress changes caused by coseismic dislocation and postseismic viscoelastic relaxation of surrounding earthquakes were very significant, which may be equivalent to the effect of interseismic tectonic loading lasting hundreds of years. Based on the time-dependent probabilistic risk model and the Dieterich (1994) model, we estimate the background seismic activity and the future earthquake probability for different fault segments, using long term seismic activity and strong earthquake recurrence cycles. It is shown that the Bamei, Selaha, and Kangding segments of the Xianshuihe fault zone have high earthquake probability, and are likely to have strong earthquakes. If energy is accumulated up to the year 2050, the magnitude of an event on these three segments could reach Mw 7.2, Mw 7.0, and Mw 7.1 respectively, while if the S7 and S8 cascades rupture, the event on these segments could reach a magnitude of up to Mw 7.2.

  20. Nonbarrier seismic process in the subduction zone and principles of monitoring

    NASA Astrophysics Data System (ADS)

    Gufel'd, I. L.; Novoselov, O. N.

    2015-10-01

    The physics of seismic process in subduction is discussed with the case study of Kamchatka region being considered. Seismic process is not attributed to either cracking or rupturing. Hydrogen degassing controls variations in voluminous-stress state of the geomedium and such parameters of boundary structures as amorphization, texturization, and destruction. Both rapid and slow dynamic processes are controlled by the medium structure and by the parameters of boundaries; these processes are induced by strain autowaves related to reversible structural transformations of the medium at ascending diffusion of hydrogen. Seismic processes are related to rapid or slow disturbance of accommodation of the medium elements relative to each other. Owing to the properties of boundaries, this process runs in a non-barrier manner and has superplasticity features. The monitoring methods for media with active inner energy sources are proposed. Difference equations of system state evolution are used; these equations are derived and corrected from the measurements of structurally sensitive parameters (saying in more precise, seismic wave velocities). Equation of system state evolution, being derived in these dimensions, reflects the effects of interaction between object and medium. As a result of the study, monitoring of phase state of the geomedium is proposed to predict small probability of the strongest earthquakes during the controlled period.

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

    USGS Publications Warehouse

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

    2003-01-01

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

  2. Seismic attenuation structure associated with episodic tremor and slip zone beneath Shikoku and the Kii peninsula, southwestern Japan, in the Nankai subduction zone

    NASA Astrophysics Data System (ADS)

    Kita, Saeko; Matsubara, Makoto

    2016-03-01

    The three-dimensional seismic attenuation structure (frequency-independent Q) beneath southwestern Japan was analyzed using t* estimated by applying the S coda wave spectral ratio method to the waveform data from a dense permanent seismic network. The seismic attenuation (Qp-1) structure is clearly imaged for the region beneath Shikoku, the Kii peninsula, and eastern Kyushu at depths down to approximately 50 km. At depths of 5 to 35 km, the seismic attenuation structure changes at the Median tectonic line and other geological boundaries beneath Shikoku and the southwestern Kii peninsula. High-Qp zones within the lower crust of the overlying plate are found just above the slip regions at the centers of the long-term slow-slip events (SSEs) beneath the Bungo and Kii channels and central Shikoku. Beneath central Shikoku, within the overlying plate, a high-Qp zone bounded by low-Qp zones is located from the land surface to the plate interface of the subducting plate. The high-Qp zone and low-Qp zones correspond to high-Vp and low-Vp zones of previous study, respectively. The boundaries of the high- and low-Qp zones are consistent with the segment boundaries of tremors (segment boundaries of short-term SSEs). These results indicated that the locations of the long- and short-term SSEs could be limited by the inhomogeneous distribution of the materials and/or condition of the overlying plate, which is formed due to geological and geographical process. The heterogeneity of materials and/or condition within the fore-arc crust possibly makes an effect on inhomogeneous rheological strength distribution on the interface.

  3. Deep structure and historical earthquakes in the Calabrian subduction zone (Southern Italy): preliminary results from multi-channel seismic reflection profiles

    NASA Astrophysics Data System (ADS)

    Gallais, F.; Gutscher, M.; Torelli, L.; Polonia, A.; Riminucci, F.

    2009-12-01

    The Calabrian subduction zone is located in the complex Central Mediterranean area. This subduction is characterized by the presence of deep earthquakes under the Tyrrhenian Sea down to 500 km depth. The Tethyan remnant Ionian slab descends towards the NW at a dip of about 70° and is associated with an active volcanic arc (the Aeolian Islands). Recently reported GPS and seismicity studies suggest that the subduction of the Ionian lithosphere beneath the Calabrian Arc may be locally still active, though at very slow rates (<5 mm/yr). Moreover the offshore Calabrian accretionary wedge is known to include compressional anticlines and ongoing hydrological activity (mud volcanoes), evidence of an active deformation in the wedge. In the Calabria - Eastern Sicily area, significant historical seismicity is documented, with the strongest event occurring in 1693. The 1693 earthquake (MCS intensities = XI) destroyed the coast of Eastern Sicily (60000 people killed) and generated a 5-10 m high tsunami. Because of the tsunami generated and because the isoseismals are open to the sea, the source region appears to be offshore. The subduction fault plane would then be a good candidate for the 1693 event. Historical reports indicates that the 1169 earthquake had similar intensities and a comparable isoseismal pattern, suggesting the events may have the same source. However, a lack of instrumentally recorded thrust earthquakes, characteristic of active subduction zone, suggests that if subduction is active, the fault plane may be locked since the instrumental period. To seek evidence of continuous tectonic activity of the Calabrian system, we present preliminary results from reprocessed 96-channels seismic reflection profiles (French Archimede cruise, 1997) offshore Sicily. This analysis permits to recognize a well-defined stratigraphy in the Ionian Abyssal Plain, this stratigraphy becomes difficult to follow under the deformed Calabrian Prism. But the joint interpretation with the

  4. Gas emissions and active tectonics within the submerged section of the North Anatolian Fault zone in the Sea of Marmara

    NASA Astrophysics Data System (ADS)

    Géli, L.; Henry, P.; Zitter, T.; Dupré, S.; Tryon, M.; Çağatay, M. N.; de Lépinay, B. Mercier; Le Pichon, X.; Şengör, A. M. C.; Görür, N.; Natalin, B.; Uçarkuş, G.; Özeren, S.; Volker, D.; Gasperini, L.; Burnard, P.; Bourlange, S.; Marnaut Scientific Party

    2008-09-01

    The submerged section of the North Anatolian fault within the Marmara Sea was investigated using acoustic techniques and submersible dives. Most gas emissions in the water column were found near the surface expression of known active faults. Gas emissions are unevenly distributed. The linear fault segment crossing the Central High and forming a seismic gap - as it has not ruptured since 1766, based on historical seismicity, exhibits relatively less gas emissions than the adjacent segments. In the eastern Sea of Marmara, active gas emissions are also found above a buried transtensional fault zone, which displayed micro-seismic activity after the 1999 events. Remarkably, this zone of gas emission extends westward all along the southern edge of Cinarcik basin, well beyond the zone where 1999 aftershocks were observed. The long term monitoring of gas seeps could hence be highly valuable for the understanding of the evolution of the fluid-fault coupling processes during the earthquake cycle within the Marmara Sea.

  5. CAFE: a seismic investigation of water percolation in the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Rondenay, S.; Abers, G. A.; Creager, K. C.; Malone, S. D.; MacKenzie, L.; Zhang, Z.; van Keken, P. E.; Wech, A. G.; Sweet, J. R.; Melbourne, T. I.; Hacker, B. R.

    2008-12-01

    Subduction zones transport water into the Earth's interior. The subsequent release of this water through dehydration reactions may trigger intraslab earthquakes and arc volcanism, regulate slip on the plate interface, control plate buoyancy, and regulate the long-term budget of water on the planet's surface. As part of Earthscope, we have undertaken an experiment named CAFE (Cascadia Arrays for Earthscope) seeking to better constrain these effects in the Cascadia subduction zone. The basic experiment has four components: (1) a 47-element broadband imaging array of Flexible Array instruments integrated with Bigfoot; (2) three small-aperture seismic arrays with 15 additional short-period instruments near known sources of Episodic Tremor and Slip (ETS) events; (3) analysis of the PBO and PANGA GPS data sets to define the details of episodic slip events; and (4) integrative modeling with complementary constraints from petrology and geodynamics. Here, we present a summary of the results that have been obtained to date by CAFE, with a focus on high-resolution seismic imaging. A 250 km-long by 120 km-deep seismic profile extending eastward from the Washington coast was generated by 2-D Generalized Radon Transform Inversion of the broadband data. It images the subducted crust as a shallow-dipping, low-velocity layer from 20km depth beneath the coast to 40km depth beneath the forearc. The termination of the low-velocity layer is consistent with the depth at which hydrated metabasalts of the subducted crust are expected to undergo eclogitization, a reaction that is accompanied by the release of water and an increase in seismic velocities. Slab earthquakes are located in both the oceanic crust and mantle at depths <40 km, and exclusively in the oceanic mantle at greater depth, as would be expected if they are related to slab dehydration. Two ETS events have occurred during the course of the deployment. They were precisely located and are confined to the region above which the

  6. Detecting seismic activity with a covariance matrix analysis of data recorded on seismic arrays

    NASA Astrophysics Data System (ADS)

    Seydoux, L.; Shapiro, N. M.; de Rosny, J.; Brenguier, F.; Landès, M.

    2016-03-01

    Modern seismic networks are recording the ground motion continuously at the Earth's surface, providing dense spatial samples of the seismic wavefield. The aim of our study is to analyse these records with statistical array-based approaches to identify coherent time-series as a function of time and frequency. Using ideas mainly brought from the random matrix theory, we analyse the spatial coherence of the seismic wavefield from the width of the covariance matrix eigenvalue distribution. We propose a robust detection method that could be used for the analysis of weak and emergent signals embedded in background noise, such as the volcanic or tectonic tremors and local microseismicity, without any prior knowledge about the studied wavefields. We apply our algorithm to the records of the seismic monitoring network of the Piton de la Fournaise volcano located at La Réunion Island and composed of 21 receivers with an aperture of ˜15 km. This array recorded many teleseismic earthquakes as well as seismovolcanic events during the year 2010. We show that the analysis of the wavefield at frequencies smaller than ˜0.1 Hz results in detection of the majority of teleseismic events from the Global Centroid Moment Tensor database. The seismic activity related to the Piton de la Fournaise volcano is well detected at frequencies above 1 Hz.

  7. Crustal Structure across The Southwest Longmenshan Fault Zone from Seismic Wide Angle Reflection/Refraction Profile

    NASA Astrophysics Data System (ADS)

    Tian, Xiaofeng; Wang, Fuyun; Wang, Shuaijun; Duan, Yonghong

    2014-05-01

    The Lushan eathquake, which epicenter and focal depth were at 30.308° N, 102.888° E, and 14.0 km, is the latest intense earthquake occurring in the southwest section of the Longmenshan fault zone after the Ms 8.0 Wenchuan earthquake in 2008. According to the emergency field observations, the slip distribution of the Lushan earthquake was concentrated at the hypocenter, and did not rupture to the surface(Chen et al, 2013). The rupture history constrained by inverting waveforms showed that the causative fault plane of the Lushan event is apparently not a simple extension of either the Pengguan fault or the Beichuan fault that ruptured during the 2008 Mw 8.0 Wenchuan earthquake. The focal mechanism using the Cut and Paste algorithm showed this event occurred on a high dip-angle fault, but its dip angle is not steep enough to rupture the surface. All these research is not independent on the heterogeneous crust structure of the Longmenshan fault zone. A 450 km-long wide-angle reflection/refraction profile executed during September and October 2013. This experiment have provided the best opportunities to obtain better knowledge of seismic structure and properties of crust and uppermost mantle beneath the Southwest Longmenshan fault zone. This seismic profile extends from the west Sichuan Plain, through the Longmenshan Fault zone, and into the west Sichuan Plateau. We observed clear Pg, refraction Phase from the upper crust, Pi1/Pi2/Pi3, reflection/refraction Phase from intra-crust, PmP, reflection from the Moho boundary, and the Pn phase, refraction Phase from uppermost mantle. We present a hybrid tomographic and layered velocity model of the crust and uppermost mantle along the profile. The final velocity model reveals large variations both in structure and velocity, and is demonstrated that a particular model has minimum structure. The model shows the crustal thickness of the region is very variable. The Moho topography varies more than 10km in the southwest

  8. Recent high-resolution seismic reflection studies of active faults in the Puget Lowland

    NASA Astrophysics Data System (ADS)

    Liberty, L. M.; Pratt, T. L.

    2005-12-01

    In the past four years, new high-resolution seismic surveys have filled in key gaps in our understanding of active structures beneath the Puget Lowland, western Washington State. Although extensive regional and high-resolution marine seismic surveys have been fundamental to understanding the tectonic framework of the area, these marine profiles lack coverage on land and in shallow or restricted waterways. The recent high-resolution seismic surveys have targeted key structures beneath water bodies that large ships cannot navigate, and beneath city streets underlain by late Pleistocene glacial deposits that are missing from the waterways. The surveys can therefore bridge the gap between paleoseismic and marine geophysical studies, and test key elements of models proposed by regional-scale geophysical studies. Results from these surveys have: 1) documented several meters of vertical displacement on at least two separate faults in the Olympia area; 2) clarified the relationship between the Catfish Lake scarp and the underlying kink band in the Tacoma fault zone; 3) provided a first look at the structures beneath the north portion of the western Tacoma fault zone, north of previous marine profiles; 4) documented that deformation along the Seattle fault extends well east of Lake Sammamish; 5) imaged the Seattle fault beneath the Vasa Park trench; and 6) documented multiple fault strands in and south of the Seattle fault zone south of Bellevue. The results better constrain interpretations of paleoseismic investigations of past earthquakes on these faults, and provide targets for future paleoseismic studies.

  9. Comparing the Gibraltar and Calabrian subduction zones (central western Mediterranean) based on seismic tomography

    NASA Astrophysics Data System (ADS)

    Argnani, Andrea; Battista Cimini, Giovanni; Frugoni, Francesco; Monna, Stephen; Montuori, Caterina

    2016-04-01

    The Central Western Mediterranean (CWM) was shaped by a complex tectonic and geodynamic evolution. Deep seismicity and tomographic studies point to the existence, under the Alboran and Tyrrhenian Seas, of lithospheric slabs extending down to the bottom of the mantle transition zone, at 660 km depth. Two narrow arcs correspond to the two slabs, the Gibraltar and Calabrian Arcs (e.g., Monna et al., 2013; Montuori et al., 2007). Similarities in the tectonic and mantle structure of the two areas have been explained by a common subduction and roll-back mechanism for the opening of the CWM, in which the two arcs are symmetrical end products. In spite of this unifying model, a wide amount of literature from different disciplines shows that many aspects of the two areas are still controversial. We present a new 3-D tomographic model at mantle scale for the Calabrian Arc and compare it with a recently published 3-D tomographic model for the Gibraltar Arc by Monna et al (2013). The two models are based on non-linear inversion of teleseismic phase arrivals, and have scale and parametrization that allow for a direct comparison. Unlike previous studies the tomographic models here presented include Ocean Bottom Seismometer broadband data, which improved the resolution of the mantle structures in the marine areas surrounding the arcs. We focus on key features of the two models that constrain reconstructions of the geodynamic evolution of the CWM (e.g., Monna et al., 2015). At Tortonian time the opening of the Tyrrhenian basin was in its initial stage, and the Calabrian arc formed subsequently; on the contrary, the Gibraltar arc was almost completely defined. We hypothesize that the complexity of the continental margin approaching the subduction zone played a key role during the final stages of the arc formation. References Monna, S., G. B. Cimini, C. Montuori, L. Matias, W. H. Geissler, and P. Favali (2013), New insights from seismic tomography on the complex geodynamic evolution

  10. Seismic Attenuation in the Rupture Zone of the 2010 Maule, Chile, Earthquake: Two Spectral Ratio Methods

    NASA Astrophysics Data System (ADS)

    Torpey, M.; Russo, R. M.; Beck, S. L.; Meltzer, A.; Roecker, S. W.

    2013-12-01

    We used data from the IRIS CHAMP temporary seismic network, deployed for 6 months following the February 2010 Mw 8.8 Maule earthquake, to estimate differential attenuation of P and S waves in the Maule rupture zone, 33°S - 38°S. We used two complementary spectral ratio methods both of which assume identical source-to-station travel paths which allowed us to neglect the source-time function and instrument response of each P-S phase pair. The first method iteratively determines 400 individual Qs values and uncertainties for each phase pair and the second method stacks the spectra of each of the 400 measurements to yield a composite spectrum from which we derive a single Qs. Measurements are deemed acceptable when the two methods agree. We examined 235 local events yielding a total of 1083 Qs measurements.The majority of ray paths evaluated show low Qs values (100-400) with an average Qs over the entire rupture zone of 350 and an average standard deviation of +/- 569. We are evaluating spatial and temporal variability in Qs; however, from our preliminary measurements we do not observe a temporal variability in Qs throughout the rupture zone nor do we recognize any consistent spatial pattern in the measurements. Tomographic inversion of the Qs measurements made along ray paths spanning the upper mantle wedge and South American crust above the Maule rupture region will allow us to interpret the observed Qs variability.

  11. LiDAR Data Reveal New Details on Seismically Triggered Landslides in the New Madrid Seismic Zone

    NASA Astrophysics Data System (ADS)

    Williams, R. A.; Gold, R. D.; Jibson, R. W.

    2013-12-01

    Recently acquired LiDAR data along the bluffs east of the Mississippi River in western Tennessee illuminate new details about earthquake-induced landslides caused by the 1811-1812 New Madrid earthquake sequence. Earthquake-induced landslides along the Mississippi River bluffs extend about 300 km, from Kentucky to northern Mississippi, and were first described in the late 1800s. The new data, however, provide the clearest images yet of the ground surface through this densely forested region and reveal previously undetected landslides. The data also confirm previous landslide localities interpreted from air photos in the 1980s and provide new details about the structure and extent of these features. In one case a newly identified landslide (about 400 m wide by 150 m long) near the southern end of the Reelfoot fault appears to have diverted and might have temporarily dammed a prominent drainage. In another case the internal structure of a low-angle, multiple-block rotational slump is clearly imaged for the first time. Field checks of these interpretations are planned. These data, which were collected at four pulses per square meter and will soon be publicly available, were collected under guidance of the U.S. Geological Survey and with funds from the American Recovery and Reinvestment Act (ARRA). The new data will provide opportunities to study the landscape for clues about (1) prehistoric earthquakes in about 1450 and 900 A.D., (2) the Reelfoot fault and, (3) the potential impacts of landslides on roads and infrastructure in future large New Madrid seismic zone earthquakes.

  12. Mapping of crustal scale tectonic boundaries in the Ossa-Morena Zone using reprocessed IBERSEIS reflection seismic data

    NASA Astrophysics Data System (ADS)

    Kashubin, A. S.; Juhlin, C.

    2010-06-01

    The IBERSEIS deep seismic reflection profile imaged crustal scale structures in the SW Iberian Variscan belt, crossing the South Portuguese Zone, the Ossa-Morena Zone and the Central Iberian Zone in Spain. Two subsets of the profile, corresponding to the South Portuguese Zone-Ossa-Morena Zone and the Ossa-Morena Zone-Central Iberian Zone tectonic contacts, have been reprocessed with the aim of investigating the influence of cross-dip and to better image steeply dipping features. Alternative strategies for binning midpoints into common depth point (CDP) bins using different azimuths were examined for synthetic data. We show that the choice of the CDP-processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip-moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile. Multi-azimuth binning, normal moveout/dip-moveout, and migration velocity analysis on synthetic and real data show the presence of clear sub-vertical upper crustal structures near the South Portuguese Zone-Ossa-Morena Zone suture, the Aroche fault. This sub-vertical reflectivity that was not imaged earlier, projects into a location in the lower crust with low reflectivity.

  13. Fault zones ruptured during the early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquakes (January 26 and February 3, Mw 6.0) based on the associated co-seismic surface ruptures

    NASA Astrophysics Data System (ADS)

    Lekkas, Efthymios L.; Mavroulis, Spyridon D.

    2016-01-01

    The early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquake sequence comprised two main shocks with almost the same magnitude (moment magnitude (Mw) 6.0) occurring successively within a short time (January 26 and February 3) and space (Paliki peninsula in Western Cephalonia) interval. Εach earthquake was induced by the rupture of a different pre-existing onshore active fault zone and produced different co-seismic surface rupture zones. Co-seismic surface rupture structures were predominantly strike-slip-related structures including V-shaped conjugate surface ruptures, dextral and sinistral strike-slip surface ruptures, restraining and releasing bends, Riedel structures ( R, R', P, T), small-scale bookshelf faulting, and flower structures. An extensional component was present across surface rupture zones resulting in ground openings (sinkholes), small-scale grabens, and co-seismic dip-slip (normal) displacements. A compressional component was also present across surface rupture zones resulting in co-seismic dip-slip (reverse) displacements. From the comparison of our field geological observations with already published surface deformation measurements by DInSAR Interferometry, it is concluded that there is a strong correlation among the surface rupture zones, the ruptured active fault zones, and the detected displacement discontinuities in Paliki peninsula.

  14. b values and ω−γ seismic source models: Implications for tectonic stress variations along active crustal fault zones and the estimation of high-frequency strong ground motion

    USGS Publications Warehouse

    Hanks, Thomas C.

    1979-01-01

    In this study the tectonic stress along active crustal fault zones is taken to be of the form , where  is the average tectonic stress at depth y and Δσp(x, y) is a seismologically observable, essentially random function of both fault plane coordinates; the stress differences arising in the course of crustal faulting are derived from Δσp(x, y). Empirically known frequency of occurrence statistics, moment-magnitude relationships, and the constancy of earthquake stress drops may be used to infer that the number of earthquakes N of dimension ≥r is of the form N ∼ 1/r2 and that the spectral composition of Δσp(x, y) is of the form , where  is the two-dimensional Fourier transform of Δσp(x, y) expressed in radial wave number k. The γ = 2 model of the far-field shear wave displacement spectrum is consistent with the spectral composition , provided that the number of contributions to the spectral representation of the radiated field at frequency ƒ goes as (k/k0)2, consistent with the quasi-static frequency of occurrence relation N ∼ 1/r2;k0 is a reference wave number associated with the reciprocal source dimension. Separately, a variety of seismologic observations suggests that the γ = 2 model is the one generally, although certainly not always, applicable to the high-frequency spectral decay of the far-field radiation of earthquakes. In this framework, then, b values near 1, the general validity of the γ = 2 model, and the constancy of earthquake stress drops independent of size are all related to the average spectral composition of. Should one of these change as a result of premonitory effects leading to failure, as has been specifically proposed for b values, it seems likely that one or all of the other characteristics will change as well from their normative values. Irrespective of these associations, the far-field, high-frequency shear radiation for the γ = 2 model in the presence of anelastic attenuation may be interpreted as

  15. Synaptic Vesicle Proteins and Active Zone Plasticity

    PubMed Central

    Kittel, Robert J.; Heckmann, Manfred

    2016-01-01

    Neurotransmitter is released from synaptic vesicles at the highly specialized presynaptic active zone (AZ). The complex molecular architecture of AZs mediates the speed, precision and plasticity of synaptic transmission. Importantly, structural and functional properties of AZs vary significantly, even for a given connection. Thus, there appear to be distinct AZ states, which fundamentally influence neuronal communication by controlling the positioning and release of synaptic vesicles. Vice versa, recent evidence has revealed that synaptic vesicle components also modulate organizational states of the AZ. The protein-rich cytomatrix at the active zone (CAZ) provides a structural platform for molecular interactions guiding vesicle exocytosis. Studies in Drosophila have now demonstrated that the vesicle proteins Synaptotagmin-1 (Syt1) and Rab3 also regulate glutamate release by shaping differentiation of the CAZ ultrastructure. We review these unexpected findings and discuss mechanistic interpretations of the reciprocal relationship between synaptic vesicles and AZ states, which has heretofore received little attention. PMID:27148040

  16. Seismic prediction of prolific oil zones in carbonate reservoirs with extremely low porosity and permeability under salt

    NASA Astrophysics Data System (ADS)

    Zheng, Xiaodong; Xu, Anna; Yang, Zhifang; Li, Yonggen; Liu, Ying; Zhang, Xin

    2005-06-01

    The Carboniferous reservoir in KJ oilfield is a carbonate reservoir with extremely low porosity and permeability and high-pressure. The reservoir has severe heterogeneity, is deeply buried, has complex master control factors, is covered with thick salt, all of which result in the serious distortion of reflection time and amplitudes under the salt, the poor seismic imaging, and the low S/N ratio and resolution. The key to developing this kind of reservoir is to correctly predict the distribution of highly profitable oil zones. In this paper we start by analyzing the master control factors, perform seismic-log calibration, optimize the seismic attributes indicating the lithofacies, karst, petrophysical properties, and fractures, and combine these results with the seismic, geology, log, oil reservoir engineering, and well data. We decompose the seismic prediction into six key areas: structural interpretation, prediction of lithofacies, karst, petrophysical properties, fractures, and then perform an integrated assessment. First, based on building the models of faults and fractures, sedimentary facies, and karst, we predict the distribution of the most favorable reservoir zones qualitatively. Then, using multi-parameter inversion and integrated multi-attribute analysis, we predict the favorable reservoir distribution quantitatively and setni-quantitatively to clarify the distribution of high-yield zones. We finally have a reliable basis for optimal selection of exploration and development targets.

  17. High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

    USGS Publications Warehouse

    Cooper, A. K.; Hart, P.E.

    2002-01-01

    We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data. We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km2) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as 'chimney' features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes. Published by Elsevier Science Ltd.

  18. Seismic activity in the Sunnyside mining district, Utah, during 1967

    USGS Publications Warehouse

    Barnes, Barton K.; Dunrud, C. Richard; Hernandez, Jerome

    1969-01-01

    A seismic monitoring network near Sunnyside, Utah, consisting of a triangular array of seismometer stations that encompasses most of the mine workings in the district, recorded over 50,000 local earth tremors during 1967. About 540 of the tremors were of sufficient magnitude to be accurately located. Most of these were located within 2-3 miles of mine workings and were also near known or suspected faults. The district-wide seismic activity generally consisted of two different patterns--a periodic increase in the daily number of tremors at weekly intervals, and also a less regular and longer term increase and decrease of seismic activity that occurred over a period of weeks or even months. The shorter and more regular pattern can be correlated with the mine work week and seems to result from mining. The longer term activity, however, does not correlate with known mining causes sad therefore seems to be .caused by natural stresses.

  19. Induced seismicity and CO2 leakage through fault zones during large-scale underground injection in a multilayered sedimentary system

    NASA Astrophysics Data System (ADS)

    Pio Rinaldi, Antonio; Rutqvist, Jonny; Jeanne, Pierre; Cappa, Frederic; Guglielmi, Yves

    2014-05-01

    Overpressure caused by the direct injection of CO2 into a deep sedimentary system may produce changes in the state of stress, as well as, have an impact on the sealing capabilities of the targeted system. The importance of geomechanics including the potential for reactivating faults associated with large-scale geologic carbon sequestration operations has recently become more widely recognized. However, not withstanding the potential for triggering notable (felt) seismic events, the potential for buoyancy-driven CO2 to reach potable groundwater and the ground surface is more important from safety and storage-efficiency perspectives. In this context, this work extends previous studies on the geomechanical modeling of fault responses during underground carbon dioxide injection, focusing on both short- and long-term integrity of the sealing caprock, and hence of potential leakage of either brine or CO2 to shallow groundwater aquifers during active injection. The first part of this work aims to study the fault responses during underground carbon dioxide injection, focusing on the short-term (5 years) integrity of the CO2 repository, and hence on the potential leakage of CO2 to shallow groundwater aquifers. Increased pore pressure can alter the stress distribution on a fault/fracture zone, which may produce changes in the permeability related to the elastic and/or plastic strain (or stress) during single (or multiple) shear ruptures. We account for stress/strain-dependent permeability and study the leakage through the fault zone as its permeability changes along with strain and stress variations. We analyze several scenarios related to the injected amount of CO2 (and hence related to potential overpressure) involving both involving minor and major faults, and analyze the profile risks of leakage for different stress/strain permeability coupling functions, as well as increasing the complexity of the system in terms of hydromechanical heterogeneities. We conclude that

  20. Subduction of the Tehuantepec oceanic fracture zone and the relationship with a seismic gap in southern Mexico

    NASA Astrophysics Data System (ADS)

    Constantin Manea, Vlad; Manea, Marina; Taras, Gerya; Valenzuela, Raul W.

    2016-04-01

    It is accepted that key constraints on the size and recurrence time of large subduction earthquakes originate from the degree of locking between the subducting and overriding plates. Since the interseismic locking degree is influenced by the rheological properties of crustal and mantle rocks, any variations along strike will result in significant changes in seismic behavior due to a change in frictional stability. Additionally, recent seismic studies show that the subduction of hydrothermally altered oceanic fracture zones induces strong pore-fluid pressure variations that control the degree of interseismic locking. The Mexico Subduction Zone (MSZ) is characterized by major along-strike changes in subduction geometry, as well as important structural variations of the incoming oceanic plate. One of the main tectonic features of the Cocos plate is the Tehuantepec fracture zone (FZ) that is currently subducting beneath southern Mexico. The analysis of seismicity revealed that the area around where Tehuantepec fracture zone is currently subducting is conspicuously quiet and considered a seismic gap. Here, no significant quake (Ms ≥ 7.0) has occurred in more than 100 years, and the origin of Tehuantepec Seismic Gap (TSG) has not been elucidated yet. Based on the dimensions of the Tehuantepec gap (125 km length and 80 km width), an earthquake of Mw = 8.0 may be possible. This study aims to shed some light on the relationship between the TSG with the subduction of Tehuantepec oceanic fracture zone. Previous studies show that the uppermost oceanic lithosphere beneath the Tehuantepec FZ is partially serpentinized due to seawater infiltrations along faults. Using high-resolution three-dimensional coupled petrological-thermomechanical numerical simulations specifically tailored for the subduction of the Tehuantepec FZ at MSZ we show that the weakened serpentinized fracture zone is partially scraped out in the forearc region because of its low strength and positive buoyancy

  1. Shear-wave splitting in Quaternary sediments: Neotectonic implications in the central New Madrid seismic zone

    USGS Publications Warehouse

    Harris, J.B.

    1996-01-01

    Determining the extent and location of surface/near-surface structural deformation in the New Madrid seismic zone (NMSZ) is very important for evaluating earthquake hazards. A shallow shear-wave splitting experiment, located near the crest of the Lake County uplift (LCU) in the central NMSZ, shows the presence of near-surface azimuthal anisotropy believed to be associated with neotectonic deformation. A shallow fourcomponent data set, recorded using a hammer and mass source, displayed abundant shallow reflection energy on records made with orthogonal source-receiver orientations, an indicator of shear-wave splitting. Following rotation of the data matrix by 40??, the S1 and S2 sections (principal components of the data matrix) were aligned with the natural coordinate system at orientations of N35??W and N55??E, respectively. A dynamic mis-tie of 8 ms at a two-way traveltime of 375 ms produced an average azimuthal anisotropy of ???2% between the target reflector (top of Quaternary gravel at a depth of 35 m) and the surface. Based on the shear-wave polarization data, two explanations for the azimuthal anisotropy in the study area are (1) fractures/cracks aligned in response to near-surface tensional stress produced by uplift of the LCU, and (2) faults/fractures oriented parallel to the Kentucky Bend scarp, a recently identified surface deformation feature believed to be associated with contemporary seismicity in the central NMSZ. In addition to increased seismic resolution by the use of shear-wave methods in unconsolidated, water-saturated sediments, measurement of near-surface directional polarizations, produced by shear-wave splitting, may provide valuable information for identifying neotectonic deformation and evaluating associated earthquake hazards.

  2. Identification of a major segment boundary between two megathrust subduction zone earthquakes from aftershock seismicity

    NASA Astrophysics Data System (ADS)

    Sobiesiak, M.; Victor, P.; Eggert, S.

    2009-04-01

    Aftershock seismicity is commonly used to characterize the extent of rupture planes of megathrust earthquakes. From unique datasets, covering the two adjacent fault planes of the Mw 8.0, 1995, Antofagasta and the Mw 7.7, 2007, Tocopilla earthquakes, we were able to identify a segment boundary (SB), located beneath Mejillones Peninsula. This segment boundary hosted the onset of the Antofagasta rupture and constituted the end of the Tocopilla rupture plane. The data recorded during the mission of the German Task Force for Earthquakes after the 2007 Tocopilla earthquake is supporting our observations regarding the northern part of the SB. 34 seismological stations registered the aftershocks from November 2007 until May 2008. First hypocenter determinations show that the aftershock sequences of both events meet along this E-W oriented segment boundary. The segment boundary is furthermore conformed by the historic record of megathrust events. Evidence for long term persistency of this SB comes from geological observations as differential uplift rates across the boundary and different fault patterns. Geomorpholocical analysis defines a topographic anomaly ~ 20 km wide and oriented along strike the SB..The main shock hypocenter determinations (NEIC, local network, ISC) which are related to the start of the rupture are all located in this zone. The SB is further characterized by intermediate b-values derived from a spatial b-value study of the Antofagasta fault plane and hosts several elongated clusters of aftershock seismicity. A detailed study of the focal mechanism solutions in one of these clusters showed a number of aligned strike slip events with one E-W striking nodal plane having a strike angle which is similar to the angle of subduction obliquity of the oceanic Nazca plate in this area. In further investigations we will search for detailed information on the nature and dynamics of processes along such a segment boundary, their meaning for the initiation of large

  3. Multiple transition zone seismic discontinuities and low velocity layers below western United States

    NASA Astrophysics Data System (ADS)

    Tauzin, B.; van der Hilst, R. D.; Wittlinger, G.; Ricard, Y.

    2013-05-01

    With P-to-S converted waves recorded at seismic stations of the U.S. Transportable Array, we image the fine structure of upper mantle and transition zone (TZ) beneath the western U.S. We map the topographies of seismic discontinuities by stacking data by common conversion points along profiles. Systematic depth and amplitude measurements are performed not only for the well-known "410" and "660" interfaces but also for minor seismic discontinuities identified around 350, 590, and 630 km depths. The amplitude of conversion suggests shear wave velocity (Vs) increase by 4% at the 410 and the 660. The observed 660 velocity contrast is smaller than expected from the 6% in IASP91 but consistent with a pyrolitic model of mantle composition. The Gorda plate, subducted under northern California, is tracked to the TZ where it seems to flatten and induce uplift of the 410 under northern Nevada. Maps of 410/660 amplitude/topography reveal that the TZ is anomalous beneath the geographical borders of Washington, Oregon, and Idaho, with (1) a thickened TZ, (2) a sharp change in depth of the 660, (3) a reduced 410 conversion amplitude in the North, and (4) a positive "630" discontinuity. Such anomalous structure might be inherited from the past history of plate subduction/accretion. A thinned TZ under the Yellowstone suggests higher-than-average temperatures, perhaps due to a deep thermal plume. Both the "350" and the "590" negative discontinuities extend over very large areas. They might be related either to an increased water content in the TZ, a significant amount of oceanic material accumulated through the past 100 Myr, or both.

  4. Geoarchaeological evidence of strong prehistoric earthquakes in the New Madrid (Missouri) seismic zone

    SciTech Connect

    Saucier, R.T. )

    1991-04-01

    Sand blows and fissures that cover >10,500 km{sup 2} in northeastern Arkansas and southeastern Missouri attest to the severity of the 1811-1812 earthquake series in the New Madrid seismic zone. However, except for one occurence near New Madrid, Missouri, the region has been devoid of any evidence of other major shocks for at least 1.3 ka prior to 1811 and possibly for >9 ka. Stratigraphic relations and radiocarbon dating at a recently excavated archaeological site near East Prairie, Missouri, have revealed liquifaction phenomena attributable to a shock dated to within about 100 yr prior to A.D. 539 and a probable second one dated between about A.D. 539 and 991.

  5. The subduction zone flow field from seismic anisotropy: a global view.

    PubMed

    Long, Maureen D; Silver, Paul G

    2008-01-18

    Although the morphologies of subducting slabs have been relatively well characterized, the character of the mantle flow field that accompanies subduction remains poorly understood. To analyze this pattern of flow, we compiled observations of seismic anisotropy, as manifested by shear wave splitting. Data from 13 subduction zones reveal systematic variations in both mantle-wedge and subslab anisotropy with the magnitude of trench migration velocity |V(t)|. These variations can be explained by flow along the strike of the trench induced by trench motion. This flow dominates beneath the slab, where its magnitude scales with |V(t)|. In the mantle wedge, this flow interacts with classical corner flow produced by the convergence velocity V(c); their relative influence is governed by the relative magnitude of |V(t)| and V(c). PMID:18202286

  6. Crustal deformation in the New Madrid seismic zone and the role of postseismic processes

    USGS Publications Warehouse

    Boyd, Oliver; Robert Smalley, Jr; Zeng, Yuehua

    2015-01-01

    Global Navigation Satellite System data across the New Madrid seismic zone (NMSZ) in the central United States over the period from 2000 through 2014 are analyzed and modeled with several deformation mechanisms including the following: (1) creep on subsurface dislocations, (2) postseismic frictional afterslip and viscoelastic relaxation from the 1811–1812 and 1450 earthquakes in the NMSZ, and (3) regional strain. In agreement with previous studies, a dislocation creeping at about 4 mm/yr between 12 and 20 km depth along the downdip extension of the Reelfoot fault reproduces the observations well. We find that a dynamic model of postseismic frictional afterslip from the 1450 and February 1812 Reelfoot fault events can explain this creep. Kinematic and dynamic models involving the Cottonwood Grove fault provide minimal predictive power. This is likely due to the smaller size of the December 1811 event on the Cottonwood Grove fault and a distribution of stations better suited to constrain localized strain across the Reelfoot fault. Regional compressive strain across the NMSZ is found to be less than 3 × 10−9/yr. If much of the present-day surface deformation results from afterslip, it is likely that many of the earthquakes we see today in the NMSZ are aftershocks from the 1811–1812 New Madrid earthquakes. Despite this conclusion, our results are consistent with observations and models of intraplate earthquake clustering. Given this and the recent paleoseismic history of the region, we suggest that seismic hazard is likely to remain significant.

  7. Recordings from the deepest borehole in the New Madrid Seismic Zone

    USGS Publications Warehouse

    Wang, Z.; Woolery, E.W.

    2006-01-01

    The recordings at the deepest vertical strong-motion array (VSAS) from three small events, the 21 October 2004 Tiptonville, Tennessee, earthquake; the 10 February 2005 Arkansas earthquake; and the 2 June 2005 Ridgely, Tennessee, earthquake show some interesting wave-propagation phenomena through the soils: the S-wave is attenuated from 260 m to 30 m depth and amplified from 30 m to the surface. The S-wave arrival times from the three events yielded different shear-wave velocity estimates for the soils. These different estimates may be the result of different incident angles of the S-waves due to different epicentral distances. The epicentral distances are about 22 km, 110 km, and 47 km for the Tiptonville, Arkansas, and Ridgely earthquakes, respectively. These recordings show the usefulness of the borehole strong-motion array. The vertical strong-motion arrays operated by the University of Kentucky have started to accumulate recordings that will provide a database for scientists and engineers to study the effects of the near-surface soils on the strong ground motion in the New Madrid Seismic Zone. More information about the Kentucky Seismic and Strong-Motion Network can be found at www.uky.edu/KGS/geologichazards. The digital recordings are available at ftp://kgsweb.uky.edu.

  8. High-resolution seismic imaging, Mono Lake fault zone, eastern Sierra region, Walker Lane, California

    NASA Astrophysics Data System (ADS)

    Jayko, A. S.; Childs, J. R.; Hart, P. E.; Bursik, M. I.; McClain, J. S.

    2012-12-01

    Multiple strands of the Mono Lake fault zone (MLfz), a segment of the Sierra Nevada frontal fault zone, have been imaged on several high-resolution seismic reflection profiles collected during 2009 and 2011 at Mono Lake. The profiles show coherent reflectors to about 30-40 ms depth below the lake bottom (~30 m thick section) in nearshore areas north of the Lee Vining delta. The MLfz is well imaged on 8 lines including 4 lines ~normal to the trend of the fault zone. The fault zone is ~ 0.75 km wide. Deep reflection horizons appear gently tilted and rotated into the fault zone with a prominent clastic wedge overlying the west-tilted horizons. Shallow reflectors above the clastic wedge are generally east-sloping, but noticeably less inclined above the fault zone. At least two ruptures offset Holocene deposits, with ~0.5-1.8 m dip-slip offset around 2.5 ka and ~3.6-6.13 m dip-slip offset around 4.7 to 6.25 ka. The ages of reflection horizons are estimated using published Holocene and late Pleistocene sedimentation rates, as well as correlation with a published nearby shallow core. The short term fault slip rate based on the timing of the most recent event and multiple events in the profile lines suggests fault slip rates of about 0.26 to 0.55 m/ka using ages based only on sedimentation rate and of about 0.31 to 0.34 m/ka using correlation ages from nearby shallow core. This offshore dip-slip rate is significantly lower than previous published 1.0-2.0 m/ka dip-slip rates estimated using cosmogenic dating of Tioga glacial moraines in Lundy Canyon and offset older moraines. The offset on the large scarp in Lundy Canyon (~20 m) decreases both north and south of the canyon and flanking lateral moraines where the scarp is on the order of only ~4-7 m high. A possible explanation for the apparent difference in MLfz slip rates onshore and offshore in this part of Mono Basin may be soft sediment deformation of saturated glacial-deltaic sediment within Lundy Canyon that causes

  9. Seismic constraints on the nature of lower crustal reflectors beneath the extending Southern Transition Zone of the Colorado Plateau, Arizona

    USGS Publications Warehouse

    Parsons, Thomas E.; Howie, John M.; Thompson, George A.

    1992-01-01

    We determine the reflection polarity and exploit variations in P and S wave reflectivity and P wave amplitude versus offset (AVO) to constrain the origin of lower crustal reflectivity observed on new three-component seismic data recorded across the structural transition of the Colorado Plateau. The near vertical incidence reflection data were collected by Stanford University in 1989 as part of the U.S. Geological Survey Pacific to Arizona Crustal Experiment that traversed the Arizona Transition Zone of the Colorado Plateau. The results of independent waveform modeling methods are consistent with much of the lower crustal reflectivity resulting from thin, high-impedance layers. The reflection polarity of the cleanest lower crustal events is positive, which implies that these reflections result from high-velocity contrasts, and the waveform character indicates that the reflectors are probably layers less than or approximately equal to 200 m thick. The lower crustal events are generally less reflective to incident S waves than to P waves, which agrees with the predicted behavior of high-velocity mafic layering. Analysis of the P wave AVO character of lower crustal reflections demonstrates that the events maintain a constant amplitude with offset, which is most consistent with a mafic-layering model. One exception is a high-amplitude (10 dB above background) event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets. The event has a pronounced S wave response, which along with its negative AVO trend is a possible indication of the presence of fluids in the lower crust. The Arizona Transition Zone is an active but weakly extended province, which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers. Instead, we favor horizontal basaltic intrusions as the primary origin of high-impedance reflectors based on (1) The fact that

  10. Evidence for activity of the Calabrian arc system and implications for historical seismicity in Eastern Sicily

    NASA Astrophysics Data System (ADS)

    Gallais, F.; Gutscher, M.-A.; Graindorge, D.; Polonia, A.

    2009-04-01

    The Wadati-Benioff zone under Calabria and the Tyrrhenian Sea is located in the centre of the Mediterranean Sea, a region characterized by complex tectonics. The presence of deep earthquakes under the Tyrrhenian Sea to a depth of 500 km, depicting an Ionian slab dipping about 70° towards the NW (Selvaggi and Chiarabba, 95), related with an active volcanic arc (the Aeolian Islands). The Calabrian peninsula is among the most seismically active regions in the Mediterranean area. Several historical seismic events, such as 1169 and 1693 earthquakes, reached MCS intensities of XI and are associated with destructive tsunami (Piatanesi and Tinti, 1998). The source of these two strongest earthquakes has still not been identified with certainty. The 1693 earthquake struck Eastern Sicily (60000 people killed) and generated a 5-10 m high tsunami (Piatanesi and Tinti, 1998). The 1169 earthquake had similar intensities and a comparable isoseismal pattern, suggesting an equivalent source. Because of the tsunami generated in 1693 and because the isoseismals are open to the sea, the source region appears to be offshore. The subduction fault plane would then be a good candidate for the 1693 event. However, a lack of instrumentally recorded thrust earthquakes, characteristic of active subduction zone, suggests that if subduction is active, the fault plane may be locked since the instrumental period. Reported recent GPS motions suggest that the subduction of the Ionian lithosphere beneath the Tyrrhenian basin plays an minor role in controlling the active deformation of the Eurasia-Nubia plate boundary, but may be locally still active in particular in the Calabrian arc (D'Agostino et al., 08). Moreover the offshore accretionary wedge is known to include compressional anticlines and ongoing hydrological activity (mud volcanoes). We present preliminary results from reprocessed 96-channels seismic reflection profiles acquired during the French "Archimede" cruise (1997) crossing the

  11. Modeling spatio-temporal variations of seismicity in the San Jacinto Fault Zone

    NASA Astrophysics Data System (ADS)

    Zöller, G.; Ben-Zion, Y.

    2012-04-01

    We investigate spatio-temporal properties of earthquake patterns in the San Jacinto fault zone (SJFZ), California, between Cajon Pass and the Superstition Hill Fault, using long records of simulated seismicity constrained by available data. The model provides an effective realization (e.g. Ben-Zion 1996; Zöller et al. 2007) of a large segmented strike-slip fault zone in 3D elastic half space, with heterogeneous distributions of static/kinetic friction and creep properties, and boundary conditions consisting of constant velocity motion around the fault. The computational section of the fault contains small brittle slip patches which fail during earthquakes and may undergo some creep deformation between events. The creep rates increase to the end points of the computational section and with depth. Two significant offsets of the SJFZ at San Jacinto Valley and Coyote Ridge are modeled by strength heterogeneities. The simulated catalogs are compared to the seismicity recorded at the SJFZ since 1932 and to recently reported results on paleoearthquakes at sites along the SJFZ at Hog Lake (HL) and Mystic Lake (ML) in the last 1500 years (e.g. Onderdonk et al., 2012; Rockwell et al., 2012). We address several questions including the following intriguing issue raised by the available paleoseismological data: are large earthquakes with signatures in ML and HL typically correlated? In particular: is a typical paleoevent in HL an incomplete rupture that is continued later in ML, and vice versa? The simulation results provide insights on the statistical significance of these and other patterns, and the ability of the SJFZ to produce large earthquakes which have not been observed in recent decades.

  12. Characteristic and Uncharacteristic Earthquakes as Possible Artifacts: Applied to the New Madrid and Wabash Seismic Zones

    NASA Astrophysics Data System (ADS)

    Stein, S.; Newman, A. V.

    2003-12-01

    Generally, the largest events, characteristic earthquakes (CE), appear more often than expected from the Gutenberg-Richter relation. Whether this effect is real or apparent is an interesting question since apparent differences can arise from several possible situations. If the known history is shorter than or comparable to the recurrence time of largest events, apparent CEs can occur because sampling bias allows anomalous short recurrence to be observed more frequently (fractions of earthquakes cannot be observed). Alternatively, apparent CEs can occur if paleoseismic data overstimate magnitudes or underestimate recurrence. In the New Madrid Zone (NMSZ), simulations suggest that because the 2000 year paleoseismic record is 4x the accepted recurrence for CEs, there is a small probability of observing apparent CEs. A more significant effect is that paleoliquifaction data regionally appear to overestimate magnitudes. It is commonly assumed that the distribution of liquifaction, used for paleomagnitudes, near the NMSZ is smaller that seen globally. Liquefaction features extending 250 km from an earthquake in the NMSZ are interpreted as evidence for an M 8.3 event, rather than an M 7.6 as would be inferred from the global curve. This practice arose because the curve was calibrated assuming an M 8.3 for the 1811-12 events whereas more recent analysis finds M ˜7.4. Hence either the largest earthquakes are CEs or the paleoevents were smaller than those in 1811-12. The opposite effect occurs in the Wabash Valley Seismic zone, where the paleoearthquakes appear ``uncharacteristic'', less frequent than would be inferred from instrumental seismicity. The paleoseismic record is long enough that the discrepancy is unlikely to be a sampling artifact. Hence either the uncharacteristic behavior is real, or the paleoseismic record captures only a small fraction of the large preinstrumental earthquakes.

  13. Characterizing Seismic Anisotropy across the Peruvian Flat-Slab Subduction Zone: Shear Wave Splitting from PULSE

    NASA Astrophysics Data System (ADS)

    Eakin, C. M.; Long, M. D.; Beck, S. L.; Wagner, L. S.; Tavera, H.

    2013-12-01

    Although 10% of subduction zones worldwide today exhibit shallow or flat subduction, we are yet to fully understand how and why these slabs go flat. An excellent study location for such a problem is in Peru, where the largest region of flat-subduction currently exists, extending ~1500 km in length (from 3 °S to 15 °S) and ~300 km in width. Across this region we investigate the pattern of seismic anisotropy, an indicator for past and/or ongoing deformation in the upper mantle. To achieve this we conduct shear wave splitting analyzes at 40 broadband stations from the PULSE project (PerU Lithosphere and Slab Experiment). These stations were deployed for 2+ years across the southern half of the Peruvian flat-slab region. We present detailed shear wave splitting results for deep and teleseismic events, making use of a wide variety of available phases that sample the upper mantle directly beneath the stations (such as SKS, SKKS, PKS, sSKS, SKiKS, ScS and local/direct S). We analyze the variability of our results with respect to initial polarizations and ray paths, as well as spatial variability between stations as the underlying slab morphology changes. Preliminary results show predominately NW-SE fast polarizations (trench oblique to sub-parallel) over the flat-slab region east of Lima. These results are consistent with observations of more complex multi-layered anisotropy beneath a nearby permanent station (NNA). Further south, towards the transition to steeper subduction, the splitting pattern becomes increasingly dominated by null measurements. Over to the east however, beyond Cuzco, where the mantle wedge might begin to play a role, we record fast polarizations quasi-parallel to the local slab contours. We carefully evaluate the different possible source locations within the subduction zone for this seismic anisotropy and observe increasing evidence for distinct anisotropy within the slab as well as the sub-slab mantle.

  14. Collision tectonics of the Central Indian Suture zone as inferred from a deep seismic sounding study

    USGS Publications Warehouse

    Mall, D.M.; Reddy, P.R.; Mooney, W.D.

    2008-01-01

    The Central Indian Suture (CIS) is a mega-shear zone extending for hundreds of kilometers across central India. Reprocessing of deep seismic reflection data acquired across the CIS was carried out using workstation-based commercial software. The data distinctly indicate different reflectivity characteristics northwest and southeast of the CIS. Reflections northwest of the CIS predominantly dip southward, while the reflection horizons southeast of the CIS dip northward. We interpret these two adjacent seismic fabric domains, dipping towards each other, to represent a suture between two crustal blocks. The CIS itself is not imaged as a sharp boundary, probably due to the disturbed character of the crust in a 20 to 30-km-wide zone. The time sections also show the presence of strong bands of reflectors covering the entire crustal column in the first 65??km of the northwestern portion of the profile. These reflections predominantly dip northward creating a domal structure with the apex around 30??km northwest of the CIS. There are a very few reflections in the upper 2-2.5??s two-way time (TWT), but the reflectivity is good below 2.5??s TWT. The reflection Moho, taken as the depth to the deepest set of reflections, varies in depth from 41 to 46??km and is imaged sporadically across the profile with the largest amplitude occurring in the northwest. We interpret these data as recording the presence of a mid-Proterozoic collision between two micro-continents, with the Satpura Mobile Belt being thrust over the Bastar craton. ?? 2008.

  15. Testing the recent Santorini seismic activity for possible tidal triggering effect

    NASA Astrophysics Data System (ADS)

    Contadakis, Michael E.; Arabelos, Dimitrios N.; Vergos, George

    2013-04-01

    Applying the Hi(stogram)Cum(ulation) method, which was introduced recently by Cadicheanu, van Ruymbecke and Zhu (2007), we analyze the series of the earthquakes occurred in the last 50 years in seismic active areas of Greece, i.e. the areas (a) of the Mygdonian Basin(Contadakis et al. 2007), (b) of the Ionian Islands (Contadakis et al. 2012 ) and (c) of the Hellenic Arc (Vergos et al. 2012 ). The result of the analysis for all the areas indicate that the monthly variation of the frequencies of earthquake occurrence is in accordance with the period of the tidal lunar monthly and semi-monthly (Mm and Mf) variations and the same happens with the corresponding daily variations of the frequencies of earthquake occurrence with the diurnal luni-solar (K1) and semidiurnal lunar (M2) tidal variations. In addition the confidence level for the identification of such period accordance between earthquakes occurrence frequency and tidal periods varies with seismic activity, i.e. the higher confidence level corresponds to periods with stronger seismic activity. These results are in favor of a tidal triggering process on earthquakes when the stress in the focal area is near the critical level. Based on these results, we consider the confidence level of earthquake occurrence - tidal period accordance as an index of tectonic stress criticality for earthquake occurrence and we check if the recent increase in the seismic activity at the Santorini island complex indicate that the faults Kameni and Columbo (to which the seismicity is clustered) (Chouliaras et al. 2013) are mature for a stronger earthquake. In this paper we present the results of this test. References Cadicheanu, N., van Ruymbeke, M andZhu P.,2007:Tidal triggering evidence of intermediate depth earthquakes in Vrancea zone(Romania), NHESS 7,733-740. Contadakis, M. E., Arabelos, D. N., Spatalas, S., 2009, Evidence for tidal triggering on the shallow earthquakes of the seismic area of Mygdonia basin, North Greece, in

  16. High-Resolution Seismic Reflection Studies of Active Faults: a Case Study from Washington State

    NASA Astrophysics Data System (ADS)

    Liberty, L. M.; Pratt, T. L.

    2007-12-01

    In the past five years, new high-resolution seismic surveys have filled in gaps in our understanding of active structures beneath the Puget Lowland region of Washington State. The extensive forests have made recognition of active faults difficult, but new Light Distance and Ranging (LIDAR) detailed topographic data have made a major breakthrough in mapping active faults. Extensive regional and high-resolution marine seismic surveys have been fundamental to understanding the tectonic framework of the area. These marine profiles, however, lack coverage beneath water bodies that large ships cannot navigate and beneath city streets underlain by late Pleistocene glacial deposits that are missing from the waterways. Recent land surveys and profiles in restricted waterways can therefore bridge the gap between paleoseismic and marine geophysical studies, and test elements of models proposed by regional-scale geophysical studies. We have also been venturing into more congested areas to seismically image faults in key urban locations. Results from recent surveys have: 1) documented new faults that had long been suspected in the Olympia area; 2) clarified the relationship between the LIDAR scarps and observed structures across the Tacoma fault zone; 3) provided a window into structures beneath the north and eastern portions of the western Tacoma fault zone; 4) documented deformation along the Seattle fault near a paleoseismic trench; 5) mapped the eastern part the Seattle fault zone beyond its previously mapped limits; and 6) documented multiple fault strands in the Seattle fault zone in the cities of Bellevue and Seattle. The results better constrain interpretations of paleoseismic data collected on these faults, and provide targets for future paleoseismic studies.

  17. The energy release in earthquakes, and subduction <