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Faults & earthquakes

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USGS2026

The potential impact of three-dimensional distributed slip models derived from real-time GNSS data on the performance of the ShakeAlert earthquake early warning system for slab interface earthquakes

The ShakeAlert® earthquake early warning (EEW) system is designed to warn users of imminent strong ground motion with sufficient time to take protective actions. ShakeAlert currently uses three algorithms to characterize the earthquake source. One estimates the location and magnitude using the first few seconds of the P wave and, while fast, tends to underestimate magnitude for M w 7.0+ earthquakes. A second estimates the location, orientation, length, and corresponding magnitude of a line source using observed peak ground acceleration and contributes primarily to M w 5.5+ earthquakes. The third infers earthquake magnitude from peak ground displacement measured using Global Navigation Satellite System (GNSS) data and offers nonsaturating magnitudes for M w 7.0+ earthquakes. Other EEW algorithms exist that infer temporally evolving spatially variable slip on a 3D fault surface from real‐time GNSS data, information that might enable more accurate and timely alerts in the event of large‐magnitude subduction interface earthquakes. Here, we evaluate the potential contribution of one such algorithm, BEFORES ( Minson et al. , 2014 ), to improve ShakeAlert performance through a simulated real‐time implementation of Bayesian evidence‐based fault orientation and real‐time earthquake slip (BEFORES) and other ShakeAlert algorithms using data for eight M w 7.6+ earthquakes. The test results demonstrate that BEFORES can produce well‐constrained and accurate magnitude estimates as soon as or sooner than other EEW algorithms, in turn enabling it to increase the amount of warning time users receive in many cases. However, with a modified Mercalli intensity (MMI) threshold of 3.5, which is commonly used for issuing alerts, BEFORES would tend to alert large geographic regions that did not feel strong shaking (MMI 6+). This effect can be mitigated using a higher alert threshold of MMI 4.5 without negative impact on the amount of warning time obtainable with BEFORES.

Bulletin of the Seismological Society of America↗

USGS2000

Preliminary report on the 16 October 1999 M 7.1 Hector mine, California, earthquake

The M w 7.1 Hector Mine, California, earthquake occurred at 9:46 GMT on 16 October 1999. The event caused minimal damage because it was located in a remote, sparsely populated part of the Mojave Desert, approximately 47 miles east-southeast of Barstow, with epicentral coordinates 34.59°N 116.27°W and a hypocentral depth of 5 ± 3 km. Twelve foreshocks, M 1.9-3.8, preceded the mainshock during the previous twelve hours. All of these events were located close to the hypocenter of the mainshock. The Hector Mine earthquake occurred within the Eastern California Shear Zone (ECSZ). By virtue of its remote location, the societal impact of the Hector Mine earthquake was, fortunately, minimal in spite of the event's appreciable size. The ECSZ is characterized by high seismicity, a high tectonic strain rate, and a broad, distributed zone of north-northwest-trending faults (ECSZ; Figure 1 ; Dokka and Travis, 1990 ; Sauber et al., 1986 ; Sauber et al., 1994 ; Sieh et al., 1993 ). Data regarding the slip rates of faults within the ECSZ suggest that on the order of 15% of the Pacific-North American plate motion occurs along this zone ( Sauber et al., 1986 ; Wesnousky, 1986 ). Most of the faults in the ECSZ have low slip rates and long repeat times for major earthquakes, on the order of several thousands to tens of thousands of years. The occurrence of the Hector Mine earthquake within seven years and only about 30 km east of the 1992 M w 7.3 Landers earthquake suggests that the closely spaced surface faults in the ECSZ are mechanically related. The Hector Mine event involved rupture on two previously mapped fault zones—the Bullion Fault and an unnamed, more northerly-trending fault that is informally referred to in this paper as the Lavic Lake Fault (Dibblee, 1966 , 1967a , b ). Traces of the Bullion Fault exhibit evidence of Holocene displacement and were zoned as active in 1988 under California's Mquist-Priolo Earthquake Fault Zoning Act ( Hart and Bryant, 1997 ). The pattern of rupture along more than one named fault was also observed from the 1992 Landers earthquake ( Hauksson et al., 1993 ; Sieh et al., 1994). Much of the fault zone that produced the Hector Mine earthquake had been buried by relatively young stream deposits, and the fault scarps in bedrock have a subdued morphology. It appears that these faults have not experienced significant offset for perhaps 10,000 years or more ( Hart, 1987 ). Planned future investigations will refine the age of the last event on these faults. The portion of the Lavic Lake Fault that ruptured between the northern end of the Bullion Mountains and Lavic Lake had not previously been mapped. However, our field investigations have identified ancient, subdued fault scarps along portions of the 1999 rupture zone in this area. It thus appears that the entire segment of the Lavic Lake Fault that was involved in the 1999 event had ruptured in the past. As is typical for most faults within the Eastern California Shear Zone, the rate of movement along the Lavic Lake Fault may be quite slow (<1 mm/yr) and should produce earthquakes only infrequently. This event is a reminder that faults that have ruptured in late Quaternary time, but that lack evidence of Holocene displacement, can still produce earthquakes in this low-slip-rate tectonic setting. Additionally, the Hector Mine earthquake is noteworthy for a couple of other reasons. First, it clearly produced triggered seismicity over much of southern California, from the rupture zone toward the south-southwest in particular. Second, as we will discuss, the event may provide new data and insight into recently developed paradigms concerning earthquake interactions and the role of static stress changes. Questions such as these will, of course, be the subject of extensive detailed analyses in years to come. Fortunately, the Hector Mine sequence will provide one of the best data sets obtained to date for a significant earthquake in the United States. Because it occurred when major upgrades to both the regional seismic network (TriNet) and the regional geodetic network (SCIGN) were well underway, the Earth science community will have abundant high-quality data with which to explore the important and interesting questions that have been raised. In this paper, we present and discuss the basic data and preliminary results from the Hector Mine earthquake.

California↗

USGS2014

Continuing megathrust earthquake potential in Chile after the 2014 Iquique earthquake

The seismic gap theory identifies regions of elevated hazard based on a lack of recent seismicity in comparison with other portions of a fault. It has successfully explained past earthquakes (see, for example, ref. 2) and is useful for qualitatively describing where large earthquakes might occur. A large earthquake had been expected in the subduction zone adjacent to northern Chile which had not ruptured in a megathrust earthquake since a M ~8.8 event in 1877. On 1 April 2014 a M 8.2 earthquake occurred within this seismic gap. Here we present an assessment of the seismotectonics of the March–April 2014 Iquique sequence, including analyses of earthquake relocations, moment tensors, finite fault models, moment deficit calculations and cumulative Coulomb stress transfer. This ensemble of information allows us to place the sequence within the context of regional seismicity and to identify areas of remaining and/or elevated hazard. Our results constrain the size and spatial extent of rupture, and indicate that this was not the earthquake that had been anticipated. Significant sections of the northern Chile subduction zone have not ruptured in almost 150 years, so it is likely that future megathrust earthquakes will occur to the south and potentially to the north of the 2014 Iquique sequence.

Nature↗

USGS2018

Ground motions from the 7 and 19 September, 2017 Tehuantepec and Puebla-Morelos, Mexico, earthquakes

The 2017 M "> M 8.2 Tehuantepec and M "> M 7.1 Puebla‐Morelos earthquakes were deep inslab normal‐faulting events that caused significant damage to several central‐to‐southern regions of Mexico. Inslab earthquakes are an important component of seismicity and seismic hazard in Mexico. Ground‐motion prediction equations (GMPEs) are an integral part of seismic hazard assessment as well as risk and rapid‐response products. This work examines the observed ground motions from these two events in comparison to the predicted median ground motions from four GMPEs. The residuals between the observed and modeled ground motions allow us to study regional differences in shaking, the effects of each earthquake, and basin effects in Mexico City, Puebla, and Oaxaca. We find that the ground motions from these two earthquakes are generally well modeled by the GMPEs. However, the Tehuantepec event shows larger than expected ground motions at greater distances and longer periods, which suggests a waveguide effect from the subduction zone geometry. Finally, Mexico City and the cities of Puebla and Oaxaca exhibit very large ground motions, indicative of well‐known site and basin effects that are much stronger than the basin terms included in some of the GMPEs. Simple and rapid ground‐motion parameter estimates that include site effects are key for hazard and real‐time risk assessments in regions such as Mexico, where the vast majority of the population lives in areas where the aforementioned effects are relevant. However, GMPEs based on site correction terms dependent on topographic slope proxies underestimate, at least in the three cities tackled in this work, the observed amplification. Therefore, there is a need to improve models of seismic amplification in basins that could be included in GMPEs.

Bulletin of the Seismological Society of America↗

USGS1972

Active faults, probable active faults, and associated fracture zones, San Mateo County, California

As used on this map, the term active fault is defined as a fracture or fracture zone several miles long along which the two sides either are moving relative to one another and parallel to the fracture or are judged likely to move in such fashion within the useful life of existing man-made structures. The judgment that a fault is active may be based on: 1) historic records of surface faulting and earthquake activity, 2) topographic evidence that demonstrates repeated and systematic displacements along the fault trace, 3) systematic displacement of Holocene or Pleistocene geologic units along the fault trace, 4) a linear pattern of small-earthquake activity either along the surface trace of the fault or parallel to it and within 2 miles of the surface trace, or 5) current and measurable systematic displacement across the surface trace of the fault. Most active faults shown on the map are recognized as active on the basis of two or more of these criteria.

California↗

USGS1968

Map showing recently active breaks along the Hayward fault zone and the southern part of the Calaveras fault zone, California

This map is one of a set that shows the traces of the inferred most recently active faults of the San Andreas and related fault zones of the San Andreas fault systems (Cromwell, 1962.) It was compiled primarily to provide information for those concerned with land use and development on or near these fault zones. The map shows the locations of all known or suspected fault traces formed by recent tectonic displacements of the ground surface along the Hayward and the southern part of the Calaveras fault zones. The displacements may have been due to violent rupture during earthquakes, to imperceptible movement not accompanied by felt earthquakes (slippage or "creep"), or to some combination of both. Map users should keep in mind that the lines shown on the map are primarily guides for locating fault traces on the ground and are not necessarily located with the precision needed for every engineering or land-use project.

California↗

USGS2023

Magnitude conversion and earthquake recurrence rate models for the central and eastern United States

Development of Seismic Source Characterization (SSC) models, which is an essential part of Probabilistic Seismic Hazard Analyses (PSHA), can help forecast the temporal and spatial distribution of future damaging earthquakes (𝑀 w ≥ 5) in seismically active regions. Because it is impossible to associate all earthquakes with known faults, seismic source models for PSHA often include sources of diffuse seismicity in which future earthquake scenarios are not localized on mapped faults. These sources of diffuse seismicity are referred to as area source zones, distributed seismicity zones, or just source zones. During the early years of PSHA studies, it was assumed that earthquakes in seismotectonic zones have (1) uniform spatial distribution, (2) Poisson temporal distribution, and (3) exponential magnitude distribution (NRC, 2012). In seismically active regions (e.g., the Western United States), where active faults are readily identified, models of the spatial distribution of earthquakes include both the fault source geometries and the distributed seismicity (background) source zones. Source characterization of active faults is complemented by paleoseismic studies with estimates of earthquake magnitudes, dates of occurrences, and slip rates, which provide important information for PSHA studies. In the Central and Eastern United States (CEUS) very few Quaternary-active faults have the requisite information for use in PSHA (i.e., fault geometry and dimensions, event rates or slip rates, etc.), and we lack knowledge about the causative faults for most observed seismicity in the region. As a result, area source zones are frequently used in site-specific PSHA in the CEUS to represent diffuse seismicity that cannot be associated with faults. However, there are examples of active fault sources in the CEUS, such as the Meers fault, the Cheraw fault, and New Madrid region, where individual faults can be characterized. The source characterization models for background seismicity are based, to a large extent, on an assumption that spatial distribution of historical and recorded seismicity will not change substantially for time periods of interest for PSHA (approximately the next 50-100 years for engineered structures). Furthermore, studies such as those by Kafka (2007, 2009) found a correlation between the locations of small- to moderate-magnitude earthquakes and the locations of large-magnitude earthquakes, indicating that we can, with some level of confidence, use the spatial pattern of smaller earthquakes to forecast the future pattern of damaging earthquakes. Within background seismicity zones, the earthquake rate forecast is developed using spatial smoothing of the small to moderate magnitude events in earthquake catalogs. Different methodologies are used for this purpose and can predict varying distributions of seismicity rates. This in turn affects the results of a seismic hazard analysis. The U.S. Geological Survey (USGS) and Nuclear Regulatory Commission (NRC) use different methods for computing spatially smoothed seismicity rates in the CEUS; the USGS uses kernel-based spatial smoothing methods in developing the National Seismic Hazard Model (NSHM), and the method adopted in the Central and Eastern United States Seismic Source Characterization (CEUS-SSC) project is used when evaluating seismic hazard for nuclear power plant siting. These methods are described and the impact on seismic hazard are evaluated in this Research Information Letter (RIL). Another important input to estimating the rate of distributed seismicity is event magnitudes listed in earthquake catalogs. A substantial source of uncertainty in catalogs is the magnitude assigned to a given earthquake. Numerous different magnitude types exist, with each magnitude type computed in a different way. Therefore, for the sake of consistency, both the CEUS-SSC and the USGS NSHM have attempted to assemble a complete catalog with a uniform magnitude determination. To this end, moment magnitude, 𝑀 w , which is a physics-based measurement, has been adopted as the standard. However, 𝑀 w was not computed routinely until the past few decades. To address this issue, the CEUS-SSC conducted extensive analyses to determine conversion equations from which to take a routinely computed network (e.g., 𝑀 L or 𝑚 bLg ) and convert it into 𝑀 w . Another issue with using 𝑀 w is that it becomes increasingly difficult to compute for earthquakes with 𝑀 less than ~4. This study investigates the effects of moment magnitude estimation and spatial smoothing methods on estimation of the earthquake rate forecast and on seismic hazard. We investigate the validity of the magnitude conversion equations and their associated uncertainties by applying them to a case study for induced earthquakes in southern Kansas and northern Oklahoma, and summarize the use of the decay of the seismic coda to estimate 𝑀 w for small earthquakes (𝑀 w < 4. Furthermore, the study documents a comparison and assessment of background seismicity smoothing methods implemented by the USGS for the NSHM and used by the CEUS-SSC for siting nuclear facilities based on probabilistic seismic hazard estimates from multiple source zones in the CEUS and for multiple sites.

central and eastern United States↗

USGS1975

Reconnaissance engineering geology of the Ketchikan area, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

The Alaska earthquake of March 27, 1964, dramatically emphasized the need for engineering geologic studies of urban areas in seismically active regions. A reconnaissance study of the Ketchikan area in southeastern Alaska is part of a program to evaluate earthquake and other geologic hazards in most of the larger Alaska coastal communities. These evaluations in the Ketchikan area should provide broad guidelines useful in city and land-use planning. Ketchikan, which had a population of approximately 7,000 in 1970, is built on the southwestern end of Revillagigedo Island along the northeastern coastline of Tongass Narrows. Altitudes reach 1,000 feet (305 m) within half a mile (0.8 km) of the coast and near-vertical cliffs characterize the terrain in places. The climate is predominantly marine. Average precipitation is approximately 152 inches (386 cm). The Ketchikan area was covered by glacier ice at least once and probably several times during the Pleistocene Epoch. The present topography, characterized by elongate lakes, U-shaped valleys, fiords, inlets, and passages, clearly reflects the effects of glaciation. The presence of emergent marine deposits, at least 300 feet (91 m) above sea level, shows that the land has been uplifted relative to sea level since the last deglaciation of the region. Bedrock is exposed or is near the surface throughout most of the mapped area. The bedrock consists chiefly of metamorphic rocks. In a few places these rocks have been intruded by igneous rocks. Exposed metamorphic rocks are mostly thinly foliated schists and phyllites, metamorphosed to greenschist facies. Foliation generally strikes northwest with moderate to steep dips to the northeast. Most of the rock is fairly competent and near-vertical cuts tend to be stable. The more indurated metamorphic rock can be used for riprap, but more durable blocks generally can be obtained from the igneous rock. The surficial deposits have been divided into the following map units on the basis of their time of deposition, mode of origin, and grain size: (1) undifferentiated drift (Qd), (2) elevated marine deposits (Qm), (3) stream alluvium (Qa), (4) fan-delta deposits (Qf), and (5) modern beach deposits (Qb). Manmade fill (f) also is mapped as a separate unit. Muskeg, colluvium, and offshore deposits are not included as map units but are discussed in the report under the heading "Surficial deposits (not shown on map)." The undifferentiated drift deposits consist mostly of till or other diamictons, generally less than 25 feet (7.6 m) thick. Exposed elevated marine deposits (Qm) generally consist of sand and gravel less than 5 feet (1.5 m) thick. Stream alluvium (Qa) is chiefly sand, gravel, cobbles, and boulders probably everywhere less than 15 feet (4.6 m) thick. Fan-delta deposits (Qf) consist mostly of loose sand, gravel, and boulders as much as 50 feet (15 m) thick. Modern beach deposits (Qb) are mostly loose sand and gravel generally less than 10 feet (3 m) thick. Two basically different types of manmade fill are present: (1) large fills along the waterfront, commonly 5 to 15 feet (1.5-4.6 m) thick, consisting of silt, sand, gravel, rock, and diverse other materials, and (2) fills, generally less than 10 feet (3 m) thick and consisting of sand, gravel, or crushed rock, placed inland from the waterfront and used as pads for buildings and parking areas. Fairly thick deposits of muskeg may be present in the southeastern part of the mapped area but have not been examined in the field. Colluvial deposits, locally 5 to 8 feet (1.5-2.4 m) thick, consist mostly of decomposing bedrock fragments. Offshore deposits are poorly known; near-shore loose sand and gravel rest on a sloping bedrock surface. Southeastern Alaska lies within the circum-Pacific seismic belt that rims the northern Pacific Basin and has been tectonically active since at least early Paleozoic time. Large-scale faulting has been common. The two most prominent fault systems in southeastern Alaska and surrounding regions are (1) the Denali fault system, and (2) the Fairweather-Queen Charlotte Islands fault system. Of the two, the Fairweather-Queen Charlotte Islands fault system is the more active and of most significance in relation to the Ketchikan area. Ketchikan lies within the northwest trend of the Gravina-Nutzotin belt of fault thrusting. The trends of at least some of the linear fiords near the mapped area are controlled by faults. However, it is not known whether a major fault extends up Tongass Narrows offshore from Ketchikan. Between 1899 and 1970, five earthquakes having magnitudes of 8 or greater occurred in or near southeastern Alaska or in adjacent offshore areas; three have occurred having magnitudes of between 7 and 8, at least eight with magnitudes of between 6 and 7, 15 with magnitudes of between 5 and 6, and about 140 have been recorded with magnitudes of less than 5 or of unassigned magnitudes. All of the earthquakes with magnitudes greater than 8, and a large proportion of the others, appear to be related to the Fair-weather-Queen Charlotte Islands fault system or to the connecting Chugach-St. Elias fault to the northwest. Within a 50-mile (80-km) radius of Ketchikan, epicenters of three earthquakes with magnitudes of 5 or less have been recorded. Within a radius of 100 miles (160 km), 10 epicenters have been recorded, two with magnitudes between 6 and 7 and eight with magnitudes of 5 or less. Although no instrumentally recorded earthquakes had epicenters in the mapped area, at least 32 earthquakes that had epicenters elsewhere were felt or possibly felt in Ketchikan. Most of these earthquakes probably had epicenters along the Queen Charlotte Islands fault. Ketchikan is tentatively assigned by me to seismic zone 2. This is a zone in which magnitudes of the largest expectable earthquake would range from 4.5 to 6.0 and where moderate damage could be expected. Large earthquakes of magnitude 8 or greater, however, can be expected to occur from time to time along the Queen Charlotte Islands fault. Ground motion from these earthquakes, although attenuated with distance, may still be sufficiently strong at Ketchikan to cause substantial damage. Possible future earthquake effects include: (1) land-level changes caused by local faulting or by large-scale regional deformation, (2) ground shaking, (3) compaction, (4) liquefaction, (5) subaerial and submarine sliding, (6) water-sediment ejection and ground fracturing, (7) reaction of sensitive and quick clays, and (8) effects of tsunamis, seiches, and other abnormal water waves. Although land-level changes due to local faulting are unlikely, large-scale regional deformation may cause uplift or subsidence in Ketchikan. Adverse effects would be confined mainly to the waterfront area. This area also would be most heavily damaged if Ketchikan were strongly shaken by an earthquake. Nonengineered, loose, manmade fills and fan-delta deposits in this area probably would be subject to the strongest shaking. These deposits probably also are most subject to compaction, liquefaction, sliding, and water-sediment ejection. Earthquake effects expectably would be considerably fewer and less severe for the part of Ketchikan upslope from the harbor area because bedrock is at or near the surface in large parts of the area. No sensitive clays have been identified but, if present, they probably are confined to the till and other diamicton deposits in the northeastern part of the mapped area. Tsunami waves are not expected to have a local generation source. Those arriving from a distant source, although potentially highly destructive, probably would be greatly attenuated before arriving at Ketchikan. Seiche waves may develop on lakes near the mapped area and possibly cause failure of earth-fill dams. Destructive waves generated by earthquake-induced local submarine sliding appear to be unlikely in the Ketchikan area. Geologic hazards in the area that are not caused by earthquakes are believed to be relatively minor. They include: (1) landsliding and subaqueous sliding, and (2) flooding. Only minor landsliding has occurred in the mapped area, but the potential for sliding may increase as the city expands and heavily timbered areas are cleared, with attendant accelerated erosion and mass wasting. The greatest potential for subaqueous sliding is along the shoreline, where fairly thick fan-delta deposits rest on a sloping bedrock surface. Periodic flooding has occurred on some creeks in the mapped area and can be expected to occur from time to time in the future. In order that more accurate evaluations of geologic hazards can be made in the future, several recommendations are made for additional studies.

Alaska↗

USGS1996

Calculation of earthquake rupture histories using a hybrid global search algorithm: Application to the 1992 Landers, California, earthquake

A method is presented for the simultaneous calculation of slip amplitudes and rupture times for a finite fault using a hybrid global search algorithm. The method we use combines simulated annealing with the downhill simplex method to produce a more efficient search algorithm then either of the two constituent parts. This formulation has advantages over traditional iterative or linearized approaches to the problem because it is able to escape local minima in its search through model space for the global optimum. We apply this global search method to the calculation of the rupture history for the Landers, California, earthquake. The rupture is modeled using three separate finite-fault planes to represent the three main fault segments that failed during this earthquake. Both the slip amplitude and the time of slip are calculated for a grid work of subfaults. The data used consist of digital, teleseismic P and SH body waves. Long-period, broadband, and short-period records are utilized to obtain a wideband characterization of the source. The results of the global search inversion are compared with a more traditional linear-least-squares inversion for only slip amplitudes. We use a multi-time-window linear analysis to relax the constraints on rupture time and rise time in the least-squares inversion. Both inversions produce similar slip distributions, although the linear-least-squares solution has a 10% larger moment (7.3 ?? 1026 dyne-cm compared with 6.6 ?? 1026 dyne-cm). Both inversions fit the data equally well and point out the importance of (1) using a parameterization with sufficient spatial and temporal flexibility to encompass likely complexities in the rupture process, (2) including suitable physically based constraints on the inversion to reduce instabilities in the solution, and (3) focusing on those robust rupture characteristics that rise above the details of the parameterization and data set.

Physics of the Earth and Planetary Interiors↗

USGS2004

Preliminary report on the 22 December 2003, M 6.5 San Simeon, California earthquake

The M w 6.5 San Simeon earthquake struck the central California coast on 22 December 2003 at 19:15:56 UTC (11:15:56 am local time.) The epicenter was located 11 km northeast of the town of San Simeon, and 39 km west-northwest of Paso Robles ( Figure 1 ), as reported by the California Integrated Seismic Network (CISN, the California region of the Advanced National Seismic System [ANSS]). The mainshock nucleated at 35.702°N, 121.108°W and a depth of 7.1 km, and the rupture propagated unilaterally to the southeast. The strong directivity of the rupture resulted in a concentration of damage and aftershock activity to the southeast of the hypocenter. The worst earthquake damage occurred in Paso Robles, where two people died in the collapse of an unreinforced masonry building. The accurate and rapid earthquake information provided in near real-time by CISN/ANSS to the Governor's Office of Emergency Services made it possible to focus emergency response in the source area, although the earthquake was felt from San Francisco to Los Angeles. The San Simeon earthquake occurred on a reverse fault striking northwest and most likely dipping to the northeast. Although motion along the Pacific-North America plate boundary in California is dominantly strike-slip, there is a small compressional component through central California. Repeated thrust earthquakes such as the San Simeon event accommodate this compression and build the Coast Ranges. Other recent thrust earthquakes in central California include the 1983 Coalinga (M 6.4) and the 1985 Kettleman Hills (M 6.0) earthquakes. Prior earthquakes in the vicinity of the San Simeon event include a M 5-6 earthquake in 1853, a M 5.7 earthquake in 1906, and the M L 6.2 Bryson earthquake of 1952 ( Figure 1 ) ( McLaren and Savage, 2001 .) The San Simeon earthquake occurred on a previously unknown blind thrust fault. No surface rupture associated with the earthquake has been identified. A number of roads, including State Highway 46, buckled due to the earthquake, but this deformation appears mainly to be failure of road fill due to ground shaking and not the result of tectonic surface rupture. Extrapolation of the fault plane to the surface would roughly align with the surface trace of the Oceanic Fault, but this is thought to be a vertical strike-slip fault. Two models for the kinematics of the region have previously been proposed. The first is a fault-propagation fold model developed by Namson and Davis ( 1990 ) for the Santa Lucia mountains ∼30 km to the southeast of the San Simeon sequence. The mainshock geometry is similar to, although more steeply dipping than, the main blind thrust of this model, implying that this model may be applicable to the San Simeon region as well. The second is the model of McLaren and Savage ( 2001 ), in which the region is dominated by strike-slip faulting with shortening on high-angle reverse faults. This model also may be applicable, although the dip of the San Simeon mainshock is shallower than predicted. The San Simeon earthquake was followed by a vigorous aftershock sequence, with 165 events above M 3 reported by CISN within the first week of the mainshock. Although the event triggered many aftershocks, it did not significantly impact the seismicity rates of other nearby faults such as the San Andreas Fault and the San Simeon-Hosgri fault zone. The only triggered seismicity seems to be a few small events within the mainshock coda at the Geysers geothermal area, north of San Francisco. The San Simeon earthquake did, however, trigger shallow creep on the San Andreas Fault at Parkfield and hydrologic changes in hot springs in Paso Robles.

California↗

USGS1974

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

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

Circular↗

USGS2018

Quantifying uncertainty in cumulative surface slip along the Cucamonga Fault, a crustal thrust fault in southern California

Studies of historic earthquake ground surface ruptures show that displacements along strike are spatially variable. As a result, latest Quaternary slip rates developed from a spatially restricted set of cumulative displacement measurements may not accurately represent fault velocity. Here we examine the uncertainties associated with slip on the Cucamonga Fault, which is part of a network of faults that have generated damaging historical earthquakes in and around Los Angeles, California. Numerous scarps along its ~25‐km length are well expressed on alluvial fans. We make 310 measurements of vertical separation across the scarps using lidar data. We show that the dispersion of the vertical separations cannot be explained by our best estimates of analytical uncertainties alone. Additional epistemic uncertainties are required. We find that the magnitude of the required epistemic uncertainty is typically larger than analytical uncertainty by a factor of 3 and typically about 22% of the maximum vertical separation. These relationships appear to hold at several spatial scales. We examine three potential sources of epistemic uncertainty and find that none among surface age uncertainty, fault dip, and anthropogenic landscape alteration is likely sufficient to explain the overdispersion of the data, which suggests differences in cumulative strain along the strike of the fault. We calculate a range of dip‐slip rates between 0.4 and 2.6 mm/year. In light of our results, we suggest that future thrust‐fault slip‐rate studies adopt an epistemic uncertainty of 22% of the maximum value for vertical separation measurements, unless there are sufficient data to demonstrate otherwise.

California↗

USGS2004

Earthquake nucleation by transient deformations caused by the M = 7.9 Denali, Alaska, earthquake

The permanent and dynamic (transient) stress changes inferred to trigger earthquakes are usually orders of magnitude smaller than the stresses relaxed by the earthquakes themselves, implying that triggering occurs on critically stressed faults. Triggered seismicity rate increases may therefore be most likely to occur in areas where loading rates are highest and elevated pore pressures, perhaps facilitated by high-temperature fluids, reduce frictional stresses and promote failure. Here we show that the 2002 magnitude M = 7.9 Denali, Alaska, earthquake triggered wide-spread seismicity rate increases throughout British Columbia and into the western United States. Dynamic triggering by seismic waves should be enhanced in directions where rupture directivity focuses radiated energy, and we verify this using seismic and new high-sample GPS recordings of the Denali mainshock. These observations are comparable in scale only to the triggering caused by the 1992 M = 7.4 Landers, California, earthquake, and demonstrate that Landers triggering did not reflect some peculiarity of the region or the earthquake. However, the rate increases triggered by the Denali earthquake occurred in areas not obviously tectonically active, implying that even in areas of low ambient stressing rates, faults may still be critically stressed and that dynamic triggering may be ubiquitous and unpredictable.

Nature↗

USGS2011

Recent faulting in western Nevada revealed by multi-scale seismic reflection

The main goal of this study is to compare different reflection methods used to image subsurface structure within different physical environments in western Nevada. With all the methods employed, the primary goal is fault imaging for structural information toward geothermal exploration and seismic hazard estimation. We use seismic CHIRP (a swept-frequency marine acquisition system), weight drop (an accelerated hammer source), and two different vibroseis systems to characterize fault structure. We focused our efforts in the Reno metropolitan area and the area within and surrounding Pyramid Lake in northern Nevada. These different methods have provided valuable constraints on the fault geometry and activity, as well as associated fluid movement. These are critical in evaluating the potential for large earthquakes in these areas, and geothermal exploration possibilities near these structures.

Nevada↗

USGS2019

Ground-motion attenuation in the Sacramento-San Joaquin delta, California, from 14 Bay Area earthquakes, including the 2014 M 6.0 South Napa earthquake

Peak ground motions (acceleration and velocity) radiated by earthquakes in the San Francisco Bay area and recorded within the Sacramento–San Joaquin Delta generally attenuate faster with distance than the Next Generation Attenuation-West2 ground-motion prediction equations (GMPEs). We evaluate the attenuation for a wide set of paths into the Delta by analyzing recorded ground motions from fourteen 4 ≤ M < 7 earthquakes located on major Bay area faults: the San Andreas, Calaveras, Hayward, West Napa, and Green Valley faults. We select stations within azimuthal ranges of 38°–114° into the Delta and calculate the residuals of the peak ground motions relative to the Boore et al. (2014) GMPEs. We then fit the natural log of these peak ground acceleration and peak ground velocity residuals for each earthquake to the function a−krγ, in which a is an event term and krγ is the differential attenuation. Although there is some variation in the differential attenuation obtained for each earthquake, the peak ground motions from most of the 14 events attenuate faster than predicted by the Boore et al. (2014) GMPEs. The differential attenuation does not appear to depend on azimuth or magnitude of the earthquake; however, earthquake depth may have an effect. Our results suggest that attenuation models for the Delta can be significantly improved through regionalization, although this regionalization will increase the model complexity and the epistemic uncertainty.

California↗

USGS2016

A rare moderate‐sized (Mw 4.9) earthquake in Kansas: Rupture process of the Milan, Kansas, earthquake of 12 November 2014 and its relationship to fluid injection

The largest recorded earthquake in Kansas occurred northeast of Milan on 12 November 2014 ( M w 4.9) in a region previously devoid of significant seismic activity. Applying multistation processing to data from local stations, we are able to detail the rupture process and rupture geometry of the mainshock, identify the causative fault plane, and delineate the expansion and extent of the subsequent seismic activity. The earthquake followed rapid increases of fluid injection by multiple wastewater injection wells in the vicinity of the fault. The source parameters and behavior of the Milan earthquake and foreshock–aftershock sequence are similar to characteristics of other earthquakes induced by wastewater injection into permeable formations overlying crystalline basement. This earthquake also provides an opportunity to test the empirical relation that uses felt area to estimate moment magnitude for historical earthquakes for Kansas.

Seismological Research Letters↗

USGS2014

Diverse rupture modes for surface-deforming upper plate earthquakes in the southern Puget Lowland of Washington State

Earthquake prehistory of the southern Puget Lowland, in the north-south compressive regime of the migrating Cascadia forearc, reflects diverse earthquake rupture modes with variable recurrence. Stratigraphy and Bayesian analyses of previously reported and new 14 C ages in trenches and cores along backthrust scarps in the Seattle fault zone restrict a large earthquake to 1040–910 cal yr B.P. (2σ), an interval that includes the time of the M 7–7.5 Restoration Point earthquake. A newly identified surface-rupturing earthquake along the Waterman Point backthrust dates to 940–380 cal yr B.P., bringing the number of earthquakes in the Seattle fault zone in the past 3500 yr to 4 or 5. Whether scarps record earthquakes of moderate (M 5.5–6.0) or large (M 6.5–7.0) magnitude, backthrusts of the Seattle fault zone may slip during moderate to large earthquakes every few hundred years for periods of 1000–2000 yr, and then not slip for periods of at least several thousands of years. Four new fault scarp trenches in the Tacoma fault zone show evidence of late Holocene folding and faulting about the time of a large earthquake or earthquakes inferred from widespread coseismic subsidence ca. 1000 cal yr B.P.; 12 ages from 8 sites in the Tacoma fault zone limit the earthquakes to 1050–980 cal yr B.P. Evidence is too sparse to determine whether a large earthquake was closely predated or postdated by other earthquakes in the Tacoma basin, but the scarp of the Tacoma fault was formed by multiple earthquakes. In the northeast-striking Saddle Mountain deformation zone, along the western limit of the Seattle and Tacoma fault zones, analysis of previous ages limits earthquakes to 1200–310 cal yr B.P. The prehistory clarifies earthquake clustering in the central Puget Lowland, but cannot resolve potential structural links among the three Holocene fault zones.

Washington↗

USGS2011

Triggered surface slips in southern California associated with the 2010 El Mayor-Cucapah, Baja California, Mexico, earthquake

The April 4, 2010 ( M w 7.2), El Mayor-Cucapah, Baja California, Mexico, earthquake is the strongest earthquake to shake the Salton Trough area since the 1992 ( M w 7.3) Landers earthquake. Similar to the Landers event, ground-surface fracturing occurred on multiple faults in the trough. However, the 2010 event triggered surface slip on more faults in the central Salton Trough than previous earthquakes, including multiple faults in the Yuha Desert area, the southwestern section of the Salton Trough. In the central Salton Trough, surface fracturing occurred along the southern San Andreas, Coyote Creek, Superstition Hills, Wienert, Kalin, and Imperial Faults and along the Brawley Fault Zone, all of which are known to have slipped in historical time, either in primary (tectonic) slip and/or in triggered slip. Surface slip in association with the El Mayor-Cucapah earthquake is at least the eighth time in the past 42 years that a local or regional earthquake has triggered slip along faults in the central Salton Trough. In the southwestern part of the Salton Trough, surface fractures (triggered slip) occurred in a broad area of the Yuha Desert. This is the first time that triggered slip has been observed in the southwestern Salton Trough. Triggered slip in the Yuha Desert area occurred along more than two dozen faults, only some of which were recognized before the April 4, 2010, El Mayor-Cucapah earthquake. From east to northwest, slip occurred in seven general areas: (1) in the Northern Centinela Fault Zone (newly named), (2) along unnamed faults south of Pinto Wash, (3) along the Yuha Fault (newly named), (4) along both east and west branches of the Laguna Salada Fault, (5) along the Yuha Well Fault Zone (newly revised name) and related faults between it and the Yuha Fault, (6) along the Ocotillo Fault (newly named) and related faults to the north and south, and (7) along the southeasternmost section of the Elsinore Fault. Faults that slipped in the Yuha Desert area include northwest-trending right-lateral faults, northeast-trending left-lateral faults, and north-south faults, some of which had dominantly vertical offset. Triggered slip along the Ocotillo and Elsinore Faults appears to have occurred only in association with the June 14, 2010 ( M w 5.7), aftershock. This aftershock also resulted in slip along other faults near the town of Ocotillo. Triggered offset on faults in the Yuha Desert area was mostly less than 20 mm, with three significant exceptions, including slip of about 50–60 mm on the Yuha Fault, 40 mm on a fault south of Pinto Wash, and about 85 mm on the Ocotillo Fault. All triggered slips in the Yuha Desert area occurred along preexisting faults, whether previously recognized or not.

California↗