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Egill Hauksson - One of the best experts on this subject based on the ideXlab platform.
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Aftershocks driven by afterslip and fluid pressure sweeping through a fault fracture mesh
AGU Fall Meeting Abstracts, 2017Co-Authors: Zachary E Ross, Egill Hauksson, Chris Rollins, Elizabeth S Cochran, Jean Philippe Avouac, Yehuda BenzionAbstract:A variety of physical mechanisms are thought to be responsible for the triggering and spatiotemporal evolution of Aftershocks. Here we analyze a vigorous Aftershock sequence and postseismic geodetic strain that occurred in the Yuha Desert following the 2010 M_w 7.2 El Mayor-Cucapah earthquake. About 155,000 detected Aftershocks occurred in a network of orthogonal faults and exhibit features of two distinct mechanisms for Aftershock triggering. The earliest Aftershocks were likely driven by afterslip that spread away from the main shock with the logarithm of time. A later pulse of Aftershocks swept again across the Yuha Desert with square root time dependence and swarm-like behavior; together with local geological evidence for hydrothermalism, these features suggest that the events were driven by fluid diffusion. The observations illustrate how multiple driving mechanisms and the underlying fault structure jointly control the evolution of an Aftershock sequence.
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The 2010 M _w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin
Pure and Applied Geophysics, 2011Co-Authors: Egill Hauksson, Kate Hutton, Joann Stock, Wenzheng Yang, J. Antonio Vidal-villegas, Hiroo KanamoriAbstract:The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M > 2 (up to M4.4) that occurred during the 24 h preceding the mainshock. The foreshocks occurred along a north–south trend near the mainshock epicenter. The M _w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15 s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The Aftershock zone extends for 120 km from the south end of the Elsinore fault zone north of the US–Mexico border almost to the northern tip of the Gulf of California. The waveform-relocated Aftershocks form two abutting clusters, each about 50 km long, as well as a 10 km north–south Aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large Aftershocks is asymmetric with five M5+ Aftershocks located to the south of the mainshock, and only one M5.7 Aftershock, but numerous smaller Aftershocks to the north. Further, the northwest Aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the Aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the Aftershocks is similar to the rate of decay of a generic California Aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of Aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific–North America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.
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a structural interpretation of the Aftershock cloud of the 1992 mw 7 3 landers earthquake
Bulletin of the Seismological Society of America, 2003Co-Authors: Jing Liu, Kerry Sieh, Egill HaukssonAbstract:We analyze the spatial relationship of relocated Aftershocks to the principal rupture planes of the M w 7.3 1992 Landers mainshock from a structural point of view. We find that the Aftershocks constitute primarily a several-kilometer-wide damage zone centered on the mainshock rupture planes. The intensity of damage decreases away from the principal faults. Less than half of the Aftershocks occurred within 1 km of the mainshock planes, and perhaps only 5% of the Aftershocks are candidates for rerupture of the mainshock faults. Moreover, it seems that Aftershocks along the Landers rupture have b -values that correlate well with the complexity of the mainshock rupture. Low b -values occur along segments that are simple, whereas higher b -values correlate with sections that are more complex. Thus, structural complexity appears to correlate with a greater relative abundance of small earthquakes. These observations imply that Aftershock populations reflect fault populations in the medium surrounding the principal faults rather than the behavior of the mainshock planes themselves. Online material : Arcview information about the surface rupture of the Landers mainshock.
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the static stress change triggering model constraints from two southern california Aftershock sequences
Journal of Geophysical Research, 1998Co-Authors: Jeanne L Hardebeck, Julie J Nazareth, Egill HaukssonAbstract:Static stress change has been proposed as a mechanism of earthquake triggering. We quantitatively evaluate this model for the apparent triggering of Aftershocks by the 1992 M_W 7.3 Landers and 1994 M_W 6.7 Northridge earthquakes. Specifically, we test whether the fraction of Aftershocks consistent with static stress change triggering is greater than the fraction of random events which would appear consistent by chance. Although static stress changes appear useful in explaining the triggering of some Aftershocks, the model's capability to explain Aftershock occurrence varies significantly between sequences. The model works well for Landers Aftershocks. Approximately 85% of events between 5 and 75 km distance from the mainshock fault plane are consistent with static stress change triggering, compared to ∼50% of random events. The minimum distance is probably controlled by limitations of the modeling, while the maximum distance may be because static stress changes of <0.01 MPa trigger too few events to be detected. The static stress change triggering model, however, can not explain the first month of the Northridge Aftershock sequence significantly better than it explains a set of random events. The difference between the Landers and Northridge sequences may result from differences in fault strength, with static stress changes being a more significant fraction of the failure stress of weak Landers-area faults. Tectonic regime, regional stress levels, and fault strength may need to be incorporated into the static stress change triggering model before it can be used reliably for seismic hazard assessment.
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kinematics of postseismic relaxation from Aftershock focal mechanisms of the 1994 northridge california earthquake
Journal of Geophysical Research, 1997Co-Authors: Jeffrey R Unruh, Robert J Twiss, Egill HaukssonAbstract:Geodetic observations of surface deformation associated with the 1994 Northridge, southern California, earthquake generally are reproduced by simple models of a large-scale elastic dislocation on a blind or buried thrust fault. The smaller-scale Aftershocks of the Northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock. These Aftershocks, averaged over volumes that are large relative to their rupture radii, reflect a distributed, permanent deformation that is accommodated by local brittle fracture. We use a micropolar continuum model to invert the Aftershocks in such volumes for the average incremental strain, and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain. Aftershock deformation that occurred at depths below about 6 km, and which is associated with the primary rupture zone, is consistent with slow continuation of the southwest-side-up reverse slip on the blind Northridge thrust fault. In contrast, Aftershock deformation from the upper 5-7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal NE-SW shortening and horizontal NW-SE (i.e., fault-parallel) extension. This pattern of deformation is similar to that associated with the mainshock, as observed geodetically and as calculated from the elastic dislocation model. We interpret that the Aftershock activity in the hanging wall represents the quasi-ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block. The Aftershocks along the mainshock rupture zone are interpreted as resulting from either (1) the time-dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or (2) the continued slip in the weakened fault zone driven by the deformation of a ductile-elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust. In either case, the Aftershock activity suggests that the crust undergoes quasi-ductile flow as a brittle-elastic material, and is not a strictly elastic material.
Toshihiko Kanazawa - One of the best experts on this subject based on the ideXlab platform.
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imaging heterogeneous velocity structures and complex Aftershock distributions in the source region of the 2007 niigataken chuetsu oki earthquake by a dense seismic observation
Earth Planets and Space, 2008Co-Authors: Aitaro Kato, Naoshi Hirata, Shinichi Sakai, Eiji Kurashimo, Toshihiro Igarashi, Takashi Iidaka, Takaya Iwasaki, Toshihiko KanazawaAbstract:The velocity structure and accurate Aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 Aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate Aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, Aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-V p body beneath this Aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the Aftershocks are aligned along SE-dipping planes. The high-V p body beneath this Aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex Aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.
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precise Aftershock distribution of the 2007 chuetsu oki earthquake obtained by using an ocean bottom seismometer network
Earth Planets and Space, 2008Co-Authors: Toshihiko Kanazawa, Tomoaki Yamada, Yoshio Murai, K Nakahigashi, Shinichi Sakai, Akira Yamazaki, Koichiro Obana, Yoshihiro ItoAbstract:The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise Aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed Aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four Aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the Aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the Aftershocks occurred along a plane dipping to the southeast in the whole Aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few Aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.
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Aftershock observation of the noto hanto earthquake in 2007 using ocean bottom seismometers
Earth Planets and Space, 2008Co-Authors: Tomoaki Yamada, Toshihiko Kanazawa, K Nakahigashi, Masanao Shinohara, Kimihiro Mochizuki, Ryota Hino, Asako Kuwano, Kenji Uehira, T Yagi, Naoto TakedaAbstract:The Noto Hanto earthquake in 2007 (Mj 6.9) occurred on March 25, 2007 near the west coast of the Noto peninsula, Honshu, Japan. To study the Aftershock activity under the sea, we deployed pop-up type ocean bottom seismometers (OBSs) from April 5 to May 8, 2007. We combined data from ten ocean bottom and four onshore seismic stations located around the rupture area of the earthquake and determined the preliminary distribution of the Aftershocks. Most of the offshore Aftershocks are located in a depth range between 2 and 10 km, and no earthquakes are observed in the lower crust. Hypocenters of deep events occurring at depths greater than 5 km are confined to an area northeastward from the largest Aftershock in offshore region. Most of the Aftershocks aligned along a high angle and southeast dipping plane, which is consistent with the geometry of the active faults revealed by previous seismic reflection surveys.
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imaging the seismic structure and stress field in the source region of the 2004 mid niigata prefecture earthquake structural zones of weakness and seismogenic stress concentration by ductile flow
Journal of Geophysical Research, 2006Co-Authors: Aitaro Kato, Naoshi Hirata, Shinichi Sakai, Eiji Kurashimo, Takashi Iidaka, Takaya Iwasaki, Toshihiko KanazawaAbstract:[1] We deployed a dense temporal seismic network in the source region of the 2004 mid-Niigata prefecture earthquake (thrust fault), Japan. A detailed velocity structure and accurate Aftershock distributions were elucidated by inverting Aftershock arrival times using double-difference tomography. A stress tensor inversion using the first-motion data was also conducted in order to investigate the stress field. The seismic velocities in the hanging wall above the main shock fault are lower than those in the footwall, with the velocity contrast extending to a depth of approximately 10 km. The Aftershocks along the main shock rupture zone are distributed around the sharp boundary between the low- and high-velocity bodies. Furthermore, Aftershocks associated with the largest Aftershock appear to be aligned on a boundary between low- and high-velocity zones, in the footwall. The orientation of maximum principal stress (σ1) is consistent with the regional compressional strain rate axis inferred from GPS data, except in the southwestern side of the main shock hypocenter where the azimuth of σ1 rotates approximately 20° counterclockwise. The main shock hypocenter was located roughly at the transition zone where the structure of the hanging wall changes laterally and the azimuth of σ1 rotates. Heterogeneous structures of the seismic velocity and the stress field, combined with the ductile deformation of the upper crust, may have concentrated seismogenic stress around the hypocenter area to cause the complex distributions of Aftershock sequence on structural boundaries.
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Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network
Earth Planets and Space, 2004Co-Authors: Tomoaki Yamada, Toshihiko Kanazawa, Naoshi Hirata, Kiyoshi Suyehiro, Yoshiyuki Kaneda, Tetsuo Takanami, Hitoshi Mikada, Shin’ichi Sakai, Tomoki WatanabeAbstract:The Tokachi-Oki earthquake occurred on September 26, 2003. Precise Aftershock distribution is important to understand the mechanism of this earthquake generation. To study the Aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of Aftershocks near the mainshock. From the data overall OBS, seventy-four Aftershocks were located with high spatial resolution. Most of the Aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No Aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the Aftershock distribution.
Karen R Felzer - One of the best experts on this subject based on the ideXlab platform.
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decay of Aftershock density with distance indicates triggering by dynamic stress
Nature, 2006Co-Authors: Karen R Felzer, Emily E BrodskyAbstract:Aftershocks, the most common type of earthquake, were thought to be triggered by static stresses induced by an earlier ‘mainshock’. Recent work suggested that dynamic stresses, or shaking, may also be a factor, and a study based on analysis of the earthquake locations in the 1984–2002 Southern California catalogue confirms that view. Precise measurements of the decay of Aftershock density with distance show that the probability of an Aftershock is consistent with a maximum amplitude of seismic shaking at distances of 0.2 to 50 km from a mainshock. The majority of earthquakes are Aftershocks1, yet Aftershock physics is not well understood. Many studies suggest that static stress changes2,3 trigger Aftershocks, but recent work suggests that shaking (dynamic stresses) may also play a role4,5. Here we measure the decay of Aftershocks as a function of distance from magnitude 2–6 mainshocks in order to clarify the Aftershock triggering process. We find that for short times after the mainshock, when low background seismicity rates allow for good Aftershock detection, the decay is well fitted by a single inverse power law over distances of 0.2–50 km. The consistency of the trend indicates that the same triggering mechanism is working over the entire range. As static stress changes at the more distant Aftershocks are negligible, this suggests that dynamic stresses may be triggering all of these Aftershocks. We infer that the observed Aftershock density is consistent with the probability of triggering Aftershocks being nearly proportional to seismic wave amplitude. The data are not fitted well by models that combine static stress change with the evolution of frictionally locked faults3.
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triggering of the 1999 mw 7 1 hector mine earthquake by Aftershocks of the 1992 mw 7 3 landers earthquake
Journal of Geophysical Research, 2002Co-Authors: Karen R Felzer, Thorsten W Becker, Rachel E Abercrombie, Goran Ekstrom, James R RiceAbstract:[1] There is strong observational evidence that the 1999 MW 7.1 Hector Mine earthquake in the Mojave Desert, California, was triggered by the nearby 1992 MW 7.3 Landers earthquake. Many authors have proposed that the Landers earthquake directly stressed the Hector Mine fault. Our model of the Landers Aftershock sequence, however, suggests there is an 85% chance that the Hector Mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the Landers main shock. We perform our model simulations using the Monte Carlo method based on the Gutenberg-Richter relationship, Omori's law, Bath's law, and assumptions that all earthquakes, including Aftershocks, are capable of producing Aftershocks and that Aftershocks produce their own Aftershocks at the same rate that other earthquakes do. In general, our simulations show that if it has been more than several days since an M ≥ 7 main shock, most new Aftershocks will be the result of secondary triggering. These secondary Aftershocks are not physically constrained to occur where the original main shock increased stress. This may explain the significant fraction of Aftershocks that have been found to occur in main shock stress shadows in static Coulomb stress triggering studies.
Kate Hutton - One of the best experts on this subject based on the ideXlab platform.
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The 2010 M _w 7.2 El Mayor-Cucapah Earthquake Sequence, Baja California, Mexico and Southernmost California, USA: Active Seismotectonics along the Mexican Pacific Margin
Pure and Applied Geophysics, 2011Co-Authors: Egill Hauksson, Kate Hutton, Joann Stock, Wenzheng Yang, J. Antonio Vidal-villegas, Hiroo KanamoriAbstract:The El Mayor-Cucapah earthquake sequence started with a few foreshocks in March 2010, and a second sequence of 15 foreshocks of M > 2 (up to M4.4) that occurred during the 24 h preceding the mainshock. The foreshocks occurred along a north–south trend near the mainshock epicenter. The M _w 7.2 mainshock on April 4 exhibited complex faulting, possibly starting with a ~M6 normal faulting event, followed ~15 s later by the main event, which included simultaneous normal and right-lateral strike-slip faulting. The Aftershock zone extends for 120 km from the south end of the Elsinore fault zone north of the US–Mexico border almost to the northern tip of the Gulf of California. The waveform-relocated Aftershocks form two abutting clusters, each about 50 km long, as well as a 10 km north–south Aftershock zone just north of the epicenter of the mainshock. Even though the Baja California data are included, the magnitude of completeness and the hypocentral errors increase gradually with distance south of the international border. The spatial distribution of large Aftershocks is asymmetric with five M5+ Aftershocks located to the south of the mainshock, and only one M5.7 Aftershock, but numerous smaller Aftershocks to the north. Further, the northwest Aftershock cluster exhibits complex faulting on both northwest and northeast planes. Thus, the Aftershocks also express a complex pattern of stress release along strike. The overall rate of decay of the Aftershocks is similar to the rate of decay of a generic California Aftershock sequence. In addition, some triggered seismicity was recorded along the Elsinore and San Jacinto faults to the north, but significant northward migration of Aftershocks has not occurred. The synthesis of the El Mayor-Cucapah sequence reveals transtensional regional tectonics, including the westward growth of the Mexicali Valley and the transfer of Pacific–North America plate motion from the Gulf of California in the south into the southernmost San Andreas fault system to the north. We propose that the location of the 2010 El Mayor-Cucapah, as well as the 1992 Landers and 1999 Hector Mine earthquakes, may have been controlled by the bends in the plate boundary.
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the 1994 northridge earthquake sequence in california seismological and tectonic aspects
Journal of Geophysical Research, 1995Co-Authors: Egill Hauksson, Lucile M Jones, Kate HuttonAbstract:The M_w 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north-northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fault plane dipping 35° to the south-southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic Aftershock sequence. Eight Aftershocks of M ≥ 5.0 and 48 Aftershocks of 4 ≤ M ≤ 5 occurred between January 17 and September 30, 1994. The Aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illuminating a previously unmapped thrust ramp, extending from 7–10 km depth into the lower crust to a depth of 23 km. No flattening of the Aftershock distribution is observed near its bottom. At shallow depths, above 7–10 km, the thrust ramp is topped by a dense distribution of Aftershock hypocenters bounded by some of the surficial faults. The dip of the ramp increases from east to west. The west side of the Aftershock zoae is characterized by a dense, steeply dipping, and north-northeast striking planar cluster of Aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Canyon lateral ramp. Along the east side of the Aftershock zone the Aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the Aftershocks were dominated by thrust faulting in the large Aftershocks, with some strike-slip and normal faulting in the smaller Aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north-south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 north-northeast trend of left-lateral faulting (Chatsworth trend) was not activated in 1994.
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the 1992 landers earthquake sequence seismological observations
Journal of Geophysical Research, 1993Co-Authors: Egill Hauksson, Lucile M Jones, Kate Hutton, Donna EberhartphillipsAbstract:The (M_W 6.1, 7.3, 6.2) 1992 Landers earthquakes began on April 23 with the M_W6.1 1992 Joshua Tree preshock and form the most substantial earthquake sequence to occur in California in the last 40 years. This sequence ruptured almost 100 km of both surficial and concealed faults and caused Aftershocks over an area 100 km wide by 180 km long. The faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the San Andreas fault. The M_W6.1 Joshua Tree preshock at 33°N58′ and 116°W19′ on 0451 UT April 23 was preceded by a tightly clustered foreshock sequence (M≤4.6) beginning 2 hours before the mainshock and followed by a large Aftershock sequence with more than 6000 Aftershocks. The Aftershocks extended along a northerly trend from about 10 km north of the San Andreas fault, northwest of Indio, to the east-striking Pinto Mountain fault. The M_w7.3 Landers mainshock occurred at 34°N13′ and 116°W26′ at 1158 UT, June 28, 1992, and was preceded for 12 hours by 25 small M≤3 earthquakes at the mainshock epicenter. The distribution of more than 20,000 Aftershocks, analyzed in this study, and short-period focal mechanisms illuminate a complex sequence of faulting. The Aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults, and 40 km to the south, crossing the Pinto Mountain fault through the Joshua Tree Aftershock zone towards the San Andreas fault near Indio. The rupture initiated in the depth range of 3–6 km, similar to previous M∼5 earthquakes in the region, although the maximum depth of Aftershocks is about 15 km. The mainshock focal mechanism showed right-lateral strike-slip faulting with a strike of N10°W on an almost vertical fault. The rupture formed an arclike zone well defined by both surficial faulting and Aftershocks, with more westerly faulting to the north. This change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west. A 20-km-long linear cluster of Aftershocks occurred 10–20 km north of Barstow, or 30–40 km north of the end of the mainshock rupture. The most prominent off-fault Aftershock cluster occurred 30 km to the west of the Landers mainshock. The largest Aftershock was within this cluster, the M_w6.2 Big Bear Aftershock occurring at 34°N10′ and 116°W49′ at 1505 UT June 28. It exhibited left-lateral strike-slip faulting on a northeast striking and steeply dipping plane. The Big Bear Aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north. The Landers mainshock occurred near the southernmost extent of the Eastern California Shear Zone, an 80-km-wide, more than 400-km-long zone of deformation. This zone extends into the Death Valley region and accommodates about 10 to 20% of the plate motion between the Pacific and North American plates. The Joshua Tree preshock, its Aftershocks, and Landers Aftershocks form a previously missing link that connects the Eastern California Shear Zone to the southern San Andreas fault.
James R Rice - One of the best experts on this subject based on the ideXlab platform.
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triggering of the 1999 mw 7 1 hector mine earthquake by Aftershocks of the 1992 mw 7 3 landers earthquake
Journal of Geophysical Research, 2002Co-Authors: Karen R Felzer, Thorsten W Becker, Rachel E Abercrombie, Goran Ekstrom, James R RiceAbstract:[1] There is strong observational evidence that the 1999 MW 7.1 Hector Mine earthquake in the Mojave Desert, California, was triggered by the nearby 1992 MW 7.3 Landers earthquake. Many authors have proposed that the Landers earthquake directly stressed the Hector Mine fault. Our model of the Landers Aftershock sequence, however, suggests there is an 85% chance that the Hector Mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the Landers main shock. We perform our model simulations using the Monte Carlo method based on the Gutenberg-Richter relationship, Omori's law, Bath's law, and assumptions that all earthquakes, including Aftershocks, are capable of producing Aftershocks and that Aftershocks produce their own Aftershocks at the same rate that other earthquakes do. In general, our simulations show that if it has been more than several days since an M ≥ 7 main shock, most new Aftershocks will be the result of secondary triggering. These secondary Aftershocks are not physically constrained to occur where the original main shock increased stress. This may explain the significant fraction of Aftershocks that have been found to occur in main shock stress shadows in static Coulomb stress triggering studies.