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Faults & earthquakes

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At least 739 records · Page 41Linked to original sources

USGS2014

Using regional moment tensors to constrain the kinematics and stress evolution of the 2010–2013 Canterbury earthquake sequence, South Island, New Zealand

On September 3, 2010, a M W 7.0 (U.S. Geological Survey moment magnitude) earthquake ruptured across the Canterbury Plains in South Island, New Zealand. Since then, New Zealand GNS Science has recorded over 10,000 aftershocks M L 2.0 and larger, including three destructive ~ M W 6.0 earthquakes near Christchurch. We treat the Canterbury earthquake sequence as an intraplate earthquake sequence, and compare its kinematics to an Andersonian model for fault slip in a uniform stress field. We determined moment magnitudes and double couple solutions for 150 earthquakes having M W 3.7 and larger through the use of a waveform inversion technique using data from broadband seismic stations on South Island, New Zealand. The majority (126) of these double couple solutions have strike-slip focal mechanisms, with right-lateral slip on ENE fault planes or equivalently left-lateral slip on SSE fault planes. The remaining focal mechanisms indicate reverse faulting, except for two normal faulting events. The strike-slip segments have compatible orientations for slip in a stress field with a horizontal σ 1 oriented ~ N115°E, and horizontal σ 3 . The preference for right lateral strike-slip earthquakes suggests that these structures are inherited from previous stages of deformation. Reverse slip is interpreted to have occurred on previously existing structures in regions with an absence of existing structures optimally oriented for strike-slip deformation. Despite the variations in slip direction and faulting style, most aftershocks had nearly the same P-axis orientation, consistent with the regional σ 1 . There is no evidence for significant changes in these stress orientations throughout the Canterbury earthquake sequence.

South Island↗

USGS2013

Finite-fault source inversion using teleseismic P waves: Simple parameterization and rapid analysis

We examine the ability of teleseismic P waves to provide a timely image of the rupture history for large earthquakes using a simple, 2D finite‐fault source parameterization. We analyze the broadband displacement waveforms recorded for the 2010 M w &sim;7 Darfield (New Zealand) and El Mayor‐Cucapah (Baja California) earthquakes using a single planar fault with a fixed rake. Both of these earthquakes were observed to have complicated fault geometries following detailed source studies conducted by other investigators using various data types. Our kinematic, finite‐fault analysis of the events yields rupture models that similarly identify the principal areas of large coseismic slip along the fault. The results also indicate that the amount of stabilization required to spatially smooth the slip across the fault and minimize the seismic moment is related to the amplitudes of the observed P waveforms and can be estimated from the absolute values of the elements of the coefficient matrix. This empirical relationship persists for earthquakes of different magnitudes and is consistent with the stabilization constraint obtained from the L‐curve in Tikhonov regularization. We use the relation to estimate the smoothing parameters for the 2011 M w 7.1 East Turkey, 2012 M w 8.6 Northern Sumatra, and 2011 M w 9.0 Tohoku, Japan, earthquakes and invert the teleseismic P waves in a single step to recover timely, preliminary slip models that identify the principal source features observed in finite‐fault solutions obtained by the U.S. Geological Survey National Earthquake Information Center (USGS/NEIC) from the analysis of body‐ and surface‐wave data. These results indicate that smoothing constraints can be estimated a priori to derive a preliminary, first‐order image of the coseismic slip using teleseismic records.

Darfield, El Mayor-Cucapah↗

USGS2016

Earthquake source properties from pseudotachylite

The motions radiated from an earthquake contain information that can be interpreted as displacements within the source and therefore related to stress drop. Except in a few notable cases, the source displacements can neither be easily related to the absolute stress level or fault strength, nor attributed to a particular physical mechanism. In contrast paleo-earthquakes recorded by exhumed pseudotachylite have a known dynamic mechanism whose properties constrain the co-seismic fault strength. Pseudotachylite can also be used to directly address a longstanding discrepancy between seismologically measured static stress drops, which are typically a few MPa, and much larger dynamic stress drops expected from thermal weakening during localized slip at seismic speeds in crystalline rock [Sibson, 1973; McKenzie and Brune, 1969; Lachenbruch, 1980; Mase and Smith, 1986; Rice, 2006] as have been observed recently in laboratory experiments at high slip rates [Di Toro et al., 2006a]. This note places pseudotachylite-derived estimates of fault strength and inferred stress levels within the context and broader bounds of naturally observed earthquake source parameters: apparent stress, stress drop, and overshoot, including consideration of roughness of the fault surface, off-fault damage, fracture energy, and the 'strength excess'. The analysis, which assumes stress drop is related to corner frequency by the Madariaga [1976] source model, is restricted to the intermediate sized earthquakes of the Gole Larghe fault zone in the Italian Alps where the dynamic shear strength is well-constrained by field and laboratory measurements. We find that radiated energy exceeds the shear-generated heat and that the maximum strength excess is ~16 MPa. More generally these events have inferred earthquake source parameters that are rate, for instance a few percent of the global earthquake population has stress drops as large, unless: fracture energy is routinely greater than existing models allow, pseudotachylite is not representative of the shear strength during the earthquake that generated it, or unless the strength excess is larger than we have allowed.

Gole Larghe fault zone↗

USGS2010

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

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

Scientific Investigations Map↗

USGS1999

Trench investigation along the Merida section of the Bocono fault (central Venezuelan Andes), Venezuela

The Bocono fault is a major NE-SW-trending, dextral fault that extends for about 500 km along the backbone of the Venezuelan Andes. Several large historical earthquakes in this region have been attributed to the Bocono fault, and some of these have been recently associated with specific parts through paleoseismologic investigations. A new trench study has been performed, 60 km to the northeast of Merida in the central Venezuelan Andes, where the fault forms a releasing bend, comprising two conspicuous late Holocene fault strands that are about 1 km apart. The southern and northern strands carry about 70% and 30% (respectively) of the 7-10 mm/yr net slip rate measured in this sector, which is based on a 40 vs. 85-100 m right-lateral offset of the Late Pleistocene Los Zerpa moraines. A trench excavated on the northern strand of the fault (near Morros de los Hoyos, slightly northeast of Apartaderos) across a twin shutter ridge and related sag pond exposed two main fault zones cutting Late Pleistocene alluvial and Holocene peat deposits. Each zone forms a shutter ridge with peat deposits ponded against the uplifted block. The paleoearthquake reconstruction derived from this trench allow us to propose the occurrence of at least 6-8 earthquakes in the past 9000 yr, yielding a maximum average recurrence interval of about 1100-1500 yr. Based on the northern strands average slip rate (2.6 mm/yr), such as earthquake sequence should have accommodated about 23 m of slip since 9 ka, suggesting that the maximum slip per event ranges between 3 and 4 m. No direct evidence for the large 1812 earthquake has been found in the trench, although this earthquake may have ruptured this section of the fault. Further paleoseismic studies will investigate the possibility that this event occurred in the Bocono fault, but ruptured mainly its southern strand in this region.

Tectonophysics↗

USGS2012

Rupture process of a multiple main shock sequence: analysis of teleseismic, local and field observations of the Tennant Creek, Australia, earthquakes of January 22, 1988

On January 22, 1988, three large intraplate earthquakes (with M S 6.3, 6.4, and 6.7) occurred within a 12-hour period near Tennant Creek, Australia. These earthquakes, which occurred over a small interval of time and within a small volume of space, present a unique opportunity to study the rupture process of the class of intraplate earthquakes that occur as multiple main shocks. Broadband displacement and velocity records of body waves from teleseismically recorded data are analyzed to determine source mechanisms, depths, and complexity of rupture of each of the three main shocks. Hypocenters of an additional 150 foreshocks and aftershocks constrained by local arrival time data and field observations of surface rupture are used to complement the source characteristics of the main shocks in order to derive as complete a description of the rupture process as possible. The interpretation of the combined data sets suggests that the overall rupture process involved unusually complicated stress release. As locations of the main shock hypocenters progressively moved from west to east, we infer that the first and third main shocks, denoted as MS 1 and MS 3 , produced the southeast-northwest trending scarps observed at the western end (the Kunayungku fault) and at the eastern end (the east end of the Lake Surprise fault), respectively, of the rupture zone. The epicenter of the only immediate foreshock was located in the gap between these two fault scarps. MS 1 nucleated near this epicenter and ruptured upward and to the northwest from a depth of 6.5 km. MS 3 ruptured predominantly to the SE at a depth of 4.5 km. The second main shock, MS 2 , is inferred to have produced the deformation of the southwest trending central scarp segment (the western end of the Lake Surprise fault). From the sense of thrusting seen at the surface and from the distribution of aftershock hypocenters, the south dipping nodal planes derived from waveform modeling are identified as the fault planes for earthquakes MS1 and MS 3 . In contrast, the dip of the central fault scarp is reversed relative to the dips of the western and eastern fault scarps. The rupture process Of MS 2 turns out to be commensurately complex and sufficiently explains the geological complexity. MS 2 consisted of three subevents. The southeast dipping nodal plane of the first two subevents is coplanar with a southeast dipping plane implied by locations of aftershocks which did not break the surface. Choice of the north dipping plane as the rupture plane of the third subevent, consistent with the surface deformation and coplanar with a second plane delineated by aftershocks, would imply conjugate faulting. The majority of the aftershocks are concentrated near the edges of the fault planes, and there is an absence of activity in the center of the planes. The areas of absent activity may represent the failed asperities of the main shocks in which substantial stress relief occurred. The rupture process of each main shock is characterized by the rapid release of energy followed by a much slower release of moment and by aftershock zones whose dimensions exceed the inferred dimensions of the rupture. These characteristics suggest that substantial slow slip occurred on each of the three fault interfaces that was not accompanied by major energy release. The first main shock nucleated at the deepest part of its aftershock zone and ruptured upward. In contrast, MS 2 and MS 3 nucleated at depths that were one third to one half of the maximum depth of their aftershock zones. This variation of focal depth and the strong increase of moment and radiated energy with each main shock imply that lateral variations of strength were more important than vertical gradients of shear stress in controlling the progression of rupture.

Journal of Geophysical Research Solid Earth↗

USGS2002

Triggered surface slips in the Salton Trough associated with the 1999 Hector Mine, California, earthquake

Surface fracturing occurred along the southern San Andreas, Superstition Hills, and Imperial faults in association with the 16 October 1999 (Mw 7.1) Hector Mine earthquake, making this at least the eighth time in the past 31 years that a regional earthquake has triggered slip along faults in the Salton Trough. Fractures associated with the event formed discontinuous breaks over a 39-km-long stretch of the San Andreas fault, from the Mecca Hills southeastward to Salt Creek and Durmid Hill, a distance from the epicenter of 107 to 139 km. Sense of slip was right lateral; only locally was there a minor (~1 mm) vertical component of slip. Dextral slip ranged from 1 to 13 mm. Maximum slip values in 1999 and earlier triggered slips are most common in the central Mecca Hills. Field evidence indicates a transient opening as the Hector Mine seismic waves passed the southern San Andreas fault. Comparison of nearby strong-motion records indicates several periods of relative opening with passage of the Hector Mine seismic wave-a similar process may have contributed to the field evidence of a transient opening. Slip on the Superstition Hills fault extended at least 9 km, at a distance from the Hector Mine epicenter of about 188 to 196 km. This length of slip is a minimum value, because we saw fresh surface breakage extending farther northwest than our measurement sites. Sense of slip was right lateral; locally there was a minor (~1 mm) vertical component of slip. Dextral slip ranged from 1 to 18 mm, with the largest amounts found distributed (or skewed) away from the Hector Mine earthquake source. Slip triggered on the Superstition Hills fault commonly is skewed away from the earthquake source, most notably in 1968, 1979, and 1999. Surface slip on the Imperial fault and within the Imperial Valley extended about 22 km, representing a distance from the Hector Mine epicenter of about 204 to 226 km. Sense of slip dominantly was right lateral; the right-lateral component of slip ranged from 1 to 19 mm. Locally there was a minor (~1-2 mm) vertical component of slip; larger proportions of vertical slip (up to 10 mm) occurred in Mesquite basin, where scarps indicate long-term oblique-slip motion for this part of the Imperial fault. Slip triggered on the Imperial fault appears randomly distributed relative to location along the fault and source direction. Multiple surface slips, both primary and triggered slip, indicate that slip repeatedly is small at locations of structural complexity.

Bulletin of the Seismological Society of America↗

USGS2006

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

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

California↗

USGS2007

Preliminary Earthquake Hazard Map of Afghanistan

Introduction Earthquakes represent a serious threat to the people and institutions of Afghanistan. As part of a United States Agency for International Development (USAID) effort to assess the resource potential and seismic hazards of Afghanistan, the Seismic Hazard Mapping group of the United States Geological Survey (USGS) has prepared a series of probabilistic seismic hazard maps that help quantify the expected frequency and strength of ground shaking nationwide. To construct the maps, we do a complete hazard analysis for each of ~35,000 sites in the study area. We use a probabilistic methodology that accounts for all potential seismic sources and their rates of earthquake activity, and we incorporate modeling uncertainty by using logic trees for source and ground-motion parameters. See the Appendix for an explanation of probabilistic seismic hazard analysis and discussion of seismic risk. Afghanistan occupies a southward-projecting, relatively stable promontory of the Eurasian tectonic plate (Ambraseys and Bilham, 2003; Wheeler and others, 2005). Active plate boundaries, however, surround Afghanistan on the west, south, and east. To the west, the Arabian plate moves northward relative to Eurasia at about 3 cm/yr. The active plate boundary trends northwestward through the Zagros region of southwestern Iran. Deformation is accommodated throughout the territory of Iran; major structures include several north-south-trending, right-lateral strike-slip fault systems in the east and, farther to the north, a series of east-west-trending reverse- and strike-slip faults. This deformation apparently does not cross the border into relatively stable western Afghanistan. In the east, the Indian plate moves northward relative to Eurasia at a rate of about 4 cm/yr. A broad, transpressional plate-boundary zone extends into eastern Afghanistan, trending southwestward from the Hindu Kush in northeast Afghanistan, through Kabul, and along the Afghanistan-Pakistan border. Deformation here is expressed as a belt of major, north-northeast-trending, left-lateral strike-slip faults and abundant seismicity. The seismicity intensifies farther to the northeast and includes a prominent zone of deep earthquakes associated with northward subduction of the Indian plate beneath Eurasia that extends beneath the Hindu Kush and Pamirs Mountains. Production of the seismic hazard maps is challenging because the geological and seismological data required to produce a seismic hazard model are limited. The data that are available for this project include historical seismicity and poorly constrained slip rates on only a few of the many active faults in the country. Much of the hazard is derived from a new catalog of historical earthquakes: from 1964 to the present, with magnitude equal to or greater than about 4.5, and with depth between 0 and 250 kilometers. We also include four specific faults in the model: the Chaman fault with an assigned slip rate of 10 mm/yr, the Central Badakhshan fault with an assigned slip rate of 12 mm/yr, the Darvaz fault with an assigned slip rate of 7 mm/yr, and the Hari Rud fault with an assigned slip rate of 2 mm/yr. For these faults and for shallow seismicity less than 50 km deep, we incorporate published ground-motion estimates from tectonically active regions of western North America, Europe, and the Middle East. Ground-motion estimates for deeper seismicity are derived from data in subduction environments. We apply estimates derived for tectonic regions where subduction is the main tectonic process for intermediate-depth seismicity between 50- and 250-km depth. Within the framework of these limitations, we have developed a preliminary probabilistic seismic-hazard assessment of Afghanistan, the type of analysis that underpins the seismic components of modern building codes in the United States. The assessment includes maps of estimated peak ground-acceleration (PGA), 0.2-second spectral acceleration (SA), and 1.0-secon

Open-File Report↗

USGS2013

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

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

Conference Paper↗

USGS1998

Preliminary results from the investigation of the Pymatuning earthquake of September 25, 1998

The Pymatuning earthquake occurred on Friday, September 25, 1998, at 19:52:52 Universal Coordinated Time (UTC), or 3:52:52 p.m. EDT, near Jamestown, Pa., at the southern end of the Pymatuning Reservoir, which straddles the Ohio-Pennsylvania border. The National Earthquake Information Center (NEIC) determined that the event had a magnitude of 5.2 mbLg (a magnitude scale used to measure the size of earthquakes that are regional distances away [100 to 1,000 km, or 60 to 600 mi]), an epicenter of 41.5°N latitude, 80.4°W longitude, and an estimated depth of 5 km (3 mi). One person was reported injured as a result of being thrown to the ground by the earthquake, and it caused minor damage to buildings and seriously disrupted many water wells in the GreenvilleJamestown, Pa., area. The earthquake was generally felt over an area of approximately 200,000 km 2 (77,230 mi 2 ) throughout northern Ohio, western Pennsylvania and New York, and much of southern Ontario, Canada (see map on back cover). It was also felt as far west as Illinois and Wisconsin, as far east as New Jersey, Connecticut, and the District of Columbia, and as far south as Kentucky and Virginia. During the aftershock field investigation that commenced within 12 hours of the main shock, a World Wide Web site, < http://groundmotion.cr.usgs.gov/pym/pym.htm >, was established from the field headquarters. The web site was used not only to transmit investigation results to the world in near real time but also to receive information from the local community as new earthquake effects were reported. As of March 1999, at least 11 aftershocks have occurred, the largest being a magnitude 2.3. The largest recent previous earthquake in the region was the northeastern Ohio (Leroy) earthquake of magnitude 5.0 that occurred on January 31, 1986, about 65 km (40 mi) west-northwest of the Pymatuning shock. This event was also felt by many of those who felt the Pymatuning earthquake. Similar to most of the seismicity east of the Rocky Mountains, earthquakes in the region are probably shallow (5 to 10 km, or 3 to 6 mi), and Seeber and Armbruster (1993) hypothesized that the earthquakes occurred along preexisting zones of weakness in Precambrian rocks. Wegweiser and others (1998) suggested that seismicity in northwestern Pennsylvania may be associated with the northwest-trending “cross-strike discontinuities” that are recognized in Paleozoic rocks and may represent reactivation of faults in the Precambrian basement. Using structure-contour maps constructed on the tops of lower Paleozoic strata, Alexandrowicz and Cole (1999) found evidence of preexisting northwest-striking faults in the epicentral region of the Pymatuning shock. The Harvard focal mechanism for the Pymatuning earthquake (a method used to infer the slip and orientation of the fault that generated an earthquake) indicates thrust faulting on a northwest striking plane, which is consistent with the regional northeast-southwest compressive stress regime observed in the area. Seeber and Armbruster (1993) plotted three prior earthquakes in the epicentral area having magnitudes greater than 3; two were instrumentally located near the Pymatuning earthquake, and the third event occurred 20 to 30 km (12 to 19 mi) to the northeast in 1852 (Figure 1).

Connecticut;District of Columbia;Illinois;Kentucky↗

USGS1994

Ductile creep and compaction: A mechanism for transiently increasing fluid pressure in mostly sealed fault zones

A simple cyclic process is proposed to explain why major strike-slip fault zones, including the San Andreas, are weak. Field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock. Ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure. The increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic. The frictional slip associated with earthquakes creates porosity in the fault zone. The cycle adjusts so that no net porosity is created (if the fault zone remains constant width). The fluid pressure within the fault zone reaches long-term dynamic equilibrium with the (hydrostatic) pressure in the country rock. One-dimensional models of this process lead to repeatable and predictable earthquake cycles. However, even modest complexity, such as two parallel fault splays with different pressure histories, will lead to complicated earthquake cycles. Two-dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes. A possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip, stabilizing the fault, and then decreases with further slip, destabilizing it. ?? 1994 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS1985

Instability model for recurring large and great earthquakes in southern California

The locked section of the San Andreas fault in southern California has experienced a number of large and great earthquakes in the past, and thus is expected to have more in the future. To estimate the location, time, and slip of the next few earthquakes, an earthquake instability model is formulated. The model is similar to one recently developed for moderate earthquakes on the San Andreas fault near Parkfield, California. In both models, unstable faulting (the earthquake analog) is caused by failure of all or part of a patch of brittle, strain-softening fault zone. In the present model the patch extends downward from the ground surface to about 12 km depth, and extends 500 km along strike from Parkfield to the Salton Sea. The variation of patch strength along strike is adjusted by trial until the computed sequence of instabilities matches the sequence of large and great earthquakes since a.d. 1080 reported by Sieh and others. The last earthquake was the M=8.3 Ft. Tejon event in 1857. The resulting strength variation has five contiguous sections of alternately low and high strength. From north to south, the approximate locations of the sections are: (1) Parkfield to Bitterwater Valley, (2) Bitterwater Valley to Lake Hughes, (3) Lake Hughes to San Bernardino, (4) San Bernardino to Palm Springs, and (5) Palm Springs to the Salton Sea. Sections 1, 3, and 5 have strengths between 53 and 88 bars; sections 2 and 4 have strengths between 164 and 193 bars. Patch section ends and unstable rupture ends usually coincide, although one or more adjacent patch sections may fail unstably at once. The model predicts that the next sections of the fault to slip unstably will be 1, 3, and 5; the order and dates depend on the assumed length of an earthquake rupture in about 1700. ?? 1985 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗

USGS2022

Evidence of active Quaternary deformation on the Great Valley fault system near Winters, northern California

The Great Valley fault system defines the tectonic boundary between the Coast Ranges and the Central Valley in California, is active throughout the Quaternary, and has been the source of several significant ( M > 6) historic earthquakes, including the 1983 M 6.5 Coalinga earthquake and the 1892 Vacaville–Winters earthquake sequence. However, the locations and geometries of individual faults in the Great Valley fault system are poorly constrained, and fault slip rates and paleoearthquake chronology are largely unknown. Here, we report geomorphic and subsurface geophysical evidence of surface‐deforming displacement on a strand of the Great Valley fault system west of Winters, California. Detailed geomorphic mapping and a high‐resolution seismic reflection and tomography survey along an ∼800 m profile across the Bigelow Hills document a fault, which we call the West Winters strand of the Great Valley fault system, with apparent east side‐up displacement of surficial geologic units. These data together suggest that the West Winters strand is active in the latest Quaternary. Together with local reports from the time, this raises the possibility that the West Winters strand may have ruptured and deformed the surface during the 1892 M 6 Vacaville–Winters earthquake sequence. Future earthquakes with vertical displacement on this and Great Valley fault system structures could have significant hazard implications, given the region’s low relief and the presence of major water conveyance infrastructure.

California↗

USGS2023

Rapid Source Characterization of the 2023 Mw 6.8 Al Haouz, Morocco, Earthquake

The U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC) estimates source characteristics of significant damaging earthquakes, aiming to place events within their seismotectonic framework. Contextualizing the 8 September 2023, M w 6.8 Al Haouz, Morocco, earthquake is challenging, because it occurred in an enigmatic region of active surface faulting, and low seismicity yet produced significant damage and loss of life. Here, we present the rapid earthquake source products produced by the USGS NEIC, describing how the source model was derived using both seismic and geodetic observations. Our analysis indicates that the earthquake was the result of oblique‐reverse faulting in the lower crust on either a steeply north‐dipping fault or a moderately south‐dipping fault. Finite‐slip models using seismic and geodetic data reveal a compact source, with slip occurring at depths of 15–35 km. The causative fault is not apparent, because the rupture did not break the surface, and it is not possible to definitively attribute the earthquake to a known structure. The earthquake centroid depth of 25 km is noteworthy, because it shows slip extending beyond common estimates of seismogenic depth. This earthquake highlights that the seismogenic processes associated with mountain building in this wide plate boundary region are poorly understood.

The Seismological Record↗

USGS1979

Preliminary analysis of clay gouge from a well in the San Andreas fault zone in central California

Drilling into the San Andreas fault zone has begun in central California as part of the U.S.G.S. program of fault zone studies. The purpose of drilling into the fault zone is threefold. First, it will allow recovery of material from depth in order to determine the composition of both solid and fluid phases. Second, it will allow determination of the physical state of the fault zone, that is, the state of stress, pore pressure, and temperature. Finally, it will allow emplacement of instruments at depth in the fault zone for monitoring experiments related to short-term earthquake prediction. In an attempt to drill to the depth of earthquake focii, the initial drill site was chosen at a locale known as Dry Lake Valley (Fig. 1). Although the entire central section of the San Andreas fault is characterized by moderate seismicity and aseismic creep, Dry Lake Valley has particularly shallow earthquakes as well as a fairly high creep rate (18-20 mm/year). Figure 2 is a longitudinal section along the San Andreas fault. The high precision earthquake locations shown are magnitude one or greater events that occurred in 1973-1975 (from W. Ellsworth, pers. comm.). The depth estimates are accurate to about + 0.5 km. It is obvious from the figure that earthquakes in the section of the fault near the well are quite shallow.

California↗

USGS2017

Pulsed strain release on the Altyn Tagh fault, northwest China

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

Altyn Tagh fault↗

USGS2012

The 1979 Homestead Valley earthquake sequence, California: Control of aftershocks and postseismic deformation

The coseismic slip and geometry of the March 15, 1979, Homestead Valley, California, earthquake sequence are well constrained by precise horizontal and vertical geodetic observations and by data from a dense local seismic network. These observations indicate 0.52±0.10 m of right-lateral slip and 0.17±0.04 m of reverse slip on a buried vertical 6-km-long and 5-km-deep fault and yield a mean static stress drop of 7.2±1.3 MPa. The largest shock had M S = 5.6. Observations of the ground rupture revealed up to 0.1 m of right-lateral slip on two mapped faults that are subparallel to the modeled seismic slip plane. In the 1.9 years since the earthquakes, geodetic network displacements indicate that an additional 60±10 mm of postseismic creep took place. The rate of postseismic shear strain (0.53±0.13 μrad/yr) measured within a 30×30-km network centered on the principal events was anomalously high compared to its preearthquake value and the postseismic rate in the adjacent network. This transient cannot be explained by postseismic slip on the seismic fault but rather indicates that broadscale release of strain followed the earthquake sequence. We have calculated the postearthquake stress field caused by the modeled coseismic slip. We assume that failure is promoted when the sum of the shear stress plus 0.75 times the faultopening stress increases. Most aftershocks concentrate at points where the stresses are enhanced by 0.3 MPa (3 bars) or more; aftershocks are nearly absent where postearthquake stresses decrease by 0.3–0.5 MPa. Isolated off-fault clusters of aftershocks that locate at one fault length from the rupture plane are explainable by this hypothesis. We find that ground rupture and postseismic creep take place where near-surface stresses are calculated to increase within the preexisting fault zones. Two patches that extend 4 km from both ends of the seismic fault exhibited neither aftershocks nor measurable postseismic creep. The sensitivity of aftershocks and ground rupture to changes in stress that are less than 5% of the earthquake stress drop demonstrates that the region around the earthquakes was within a few percent of its failure threshold before the main shocks. The preearthquake stress field and the stress required for failure must also have been nearly uniform.

Journal of Geophysical Research Solid Earth↗