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At least 829 records · Page 46Linked to original sources
USGS2026

Simulation-based scenario ShakeMaps for large magnitude (MW6.5+) crustal earthquakes on the Seattle, Tacoma, and southern Whidbey Island faults, Washington, USA

Scenario ground‐motion maps based on empirical ground‐motion models (GMMs) provide a rapid and generally reliable means of estimating the amplitude and distribution of earthquake shaking. However, because GMMs are designed for broad applicability, they often rely on simplified representations of Earth structure, which can limit their accuracy in regions with complex source, path, and site effects. This can substantially impact the accuracy of predicted shaking in areas like western Washington State, where deep, interconnected basin structure exerts a strong influence on seismic‐wave propagation. In this study, we present a new suite of simulation‐based scenario ShakeMaps that characterize ground shaking from large‐magnitude ( ⁠ M W 6.5–7.5) earthquakes on the Seattle, Tacoma, and southern Whidbey Island faults. These maps are developed using results from recent 3D wave propagation simulations ( Stone et al. , 2022 , 2023 , 2025 ) that incorporate realistic rupture geometries, variable slip distributions, and a regional 3D seismic velocity model with shallow soils. Broadband ground motions are estimated by combining the low‐frequency (<1 Hz) deterministic seismograms from these studies with high‐frequency (1–10 Hz) stochastic seismograms. Simulated ground motions are corrected to account for the enforced minimum shear‐wave velocity and nonlinear site response. The resulting ShakeMaps represent median ground‐shaking estimates derived from multiple rupture scenarios with varying slip distributions and hypocenter locations for each fault. To extend ShakeMap coverage beyond the simulation domain (i.e., into eastern Washington, northern Oregon, and southwestern British Columbia), we scale GMM‐based ground‐motion estimates using amplification patterns observed in the simulations. These new ShakeMaps reveal the substantial influence of deep basin structure on shaking intensity, underscoring the importance of considering crustal structure complexity in regional hazard assessments for the Pacific Northwest.

Washington↗
USGS2026

Efficient physics‐informed ground‐motion simulations with reduced‐order models: CyberShake implications and high‐resolution site terms for southern San Andreas fault earthquakes

Recent advances in Probabilistic Seismic Hazard Analysis (PSHA) leverage physics‐based ground‐motion simulations to estimate seismic hazard, such as the CyberShake project. However, computational costs quickly escalate when performing PSHA for numerous faults or sites and can become prohibitively expensive. To reduce computational demands, CyberShake uses reciprocity and interpolates physics‐informed corrections from simulations conducted at fewer locations, but the accuracy of these interpolations remains poorly quantified. To quantify the interpolation accuracy, we derive high‐resolution, frequency‐dependent site terms for southern California and compare them with interpolated site terms using the CyberShake approach. We accomplish this by performing a set of earthquake point‐source simulations distributed along the nonplanar fault geometry for the southern San Andreas fault (SSAF) extending from Bombay Beach to Lake Hughes. Using SeisSol, we simulate three minutes of viscoelastic seismic wave propagation for these sources and store the horizontal‐component Green’s functions for 480,000 sites. We then use a scientific machine learning approach based on interpolated proper orthogonal decomposition to construct an accurate reduced‐order model of the Green’s functions to efficiently predict effective amplitude spectra (EAS) for finite‐source rupture models of SSAF earthquakes. Using minimum curvature interpolation with tension, as used in CyberShake, we compare the interpolated site terms against our high‐resolution site terms. We identify local discrepancies with EAS differing by up to a factor of approximately three. Furthermore, we identify locations where unexpectedly high or low ground motions are missed when using the interpolated dataset for these earthquakes. We estimate that our approach may be used within CyberShake to reduce the time‐to‐solution by a factor of 336 for the entire earthquake rupture forecast. Our analysis of physics‐based site terms provides more insight into the seismic hazard due to SSAF ruptures and guides future developments by combining high‐performance computing and reduced‐order modeling techniques for PSHA.

California↗
USGS2026

High‐resolution surface deformation and slip distribution observations for the 2023 Kahramanmaraş, Türkiye, earthquake sequence help constrain the rupture process

Splay, or branch, faults are a common geometric feature of earthquake surface ruptures and may provide constraints on the rupture behavior of an earthquake. The 2023 M w 7.8 Pazarcık and M w 7.5 Elbistan, Türkiye, earthquakes are examples of ruptures with multiple small splays, and the Pazarcık earthquake nucleated on a splay fault, the Narlı fault, before rupturing bilaterally on the East Anatolian fault (EAF). Here, we present 3‐m‐resolution surface displacement from subpixel correlation of Planet Dove optical images for the entirety of both ruptures with corresponding surface slip distributions. For a 30‐km‐long study region spanning the Narlı‐EAF intersection, we compare surface slip derived from five data sets with different resolutions (on‐the‐ground, WorldView, Planet Dove, Sentinel‐2, and Sentinel‐1) to elucidate complementary information. In addition, we integrate information from the surface expression of faulting with published dynamic rupture simulations and rupture process studies to constrain a rupture evolution for the Pazarcık earthquake that is consistent across data sets. This work highlights the complementary nature of disparate surface slip data sets and the role that high‐resolution surface displacement information, including from fault splays, can play in constraining nonunique rupture models and refining understanding of the earthquake rupture process.

Bulletin of the Seismological Society of America↗
USGS2026

Simulations of earthquakes in California with multiple velocity models

Estimating seismic hazard using 3D earthquake simulations of moderate‐to‐large magnitude events is becoming a reality. In this study, we further this endeavor by simulating seismic wavefields for five earthquakes in California, United States, and evaluating their pseudospectral acceleration at periods of 2, 5, and 10 s. We compare results from the recently developed U.S. Geological Survey (USGS) National Crustal Model with three well‐established and validated regional community velocity models (CVMs): two CVMs for Southern California (CVMS4.26.M01 and CVMH15.1.0) and the USGS San Francisco Bay region 3D CVM. A minimum shear‐wave velocity of 500 m/s is imposed in all four of the velocity models. We use finite‐difference calculations to simulate the earthquakes and specify a grid and timestep capable of resolving periods down to 1 s. For our simulations, we find that for each earthquake, the available velocity models produce broadly similar amplitudes and patterns of earthquake ground motions. Use of the 3D velocity models generally perform better than ergodic ground‐motion models, and variability in the results between earthquakes may indicate a combination of factors including uncertainty in earthquake magnitude and source parameterization, signal‐to‐noise ratio in the observed time series, and the lack of fault damage zones, topography, and other features in the computational seismic velocity models. Of particular note, we find that for most cases, averaging the ground motions for all available velocity models outperforms the ground‐motion results from any single velocity model. Our results indicate that the 3D models considered in this study can be sufficiently independent and have enough predictive power that their combination could be used to improve estimates of earthquake ground motions.

California↗
USGS2026

Testing characteristic magnitude distributions in modern PSHA models

The characteristic magnitude distribution hypothesis predicts a higher rate of large earthquakes than a Gutenberg–Richter extrapolation of the small‐earthquake rate would imply. Characteristic magnitude distributions have been commonly applied to faults in probabilistic seismic hazard analysis (PSHA), and in modern models they can emerge from the way short‐term seismicity constraints are combined with long‐term geologic and geodetic constraints. We test the characteristic magnitude distribution hypothesis by comparing the fault‐based magnitude distributions from the 2023 update to the National Seismic Hazard Model (NSHM23) in the Western United States with observed seismicity over the past 93 yr. We find that observed magnitude distributions fall outside the model‐predicted confidence bounds in regions where NSHM23 produces characteristic magnitude distributions: in these regions, the model predicts higher rates of large earthquakes than are observed. An analysis of the earlier California model (Uniform California Earthquake Rupture Forecast, version 3) also reveals discrepancies between the modeled and observed magnitude distributions. In addition, we find that observed magnitude distributions near modeled faults are not significantly different from those in background regions. These results challenge the prevalence of characteristic magnitude distributions in fault‐based seismic hazard models and call for a reassessment of how disparate data sets are integrated in PSHA.

western United States↗
USGS2026

ETAS-positive: Extracting unbiased epidemic-type aftershock models from incomplete earthquake catalogs

Earthquake catalogs are inherently incomplete, and short-term fluctuations in catalog completeness can strongly bias estimates of seismicity parameters. Here, we extend the positive statistics framework recently introduced for estimating Gutenberg–Richter a and b values to the Epidemic-Type Aftershock Sequence (ETAS) model. The approach focuses on complete magnitude–time intervals defined by pairs of earthquakes in which the second event is larger than the first. Comprehensive synthetic tests demonstrate that the proposed ETAS + method retrieves nearly unbiased parameter estimates even from highly incomplete catalogs, whereas parameters obtained using classical estimation approaches are strongly biased. Application to Southern California reveals a similar trend, with the productivity scaling parameter α approaching the b value, consistent with self-similar earthquake triggering within estimation uncertainties. Owing to its robustness against catalog incompleteness and computational efficiency, the ETAS + method provides a powerful new tool for seismicity analysis and earthquake forecasting.

California↗
USGS2026

3D broadband ground motion simulations of Cascadia megathrust earthquakes: Full-margin scenarios incorporating variable magnitudes and frontal thrust splay faulting

Great megathrust earthquakes (∼ M w 8–9) that rupture along the Cascadia Subduction Zone pose significant shaking and tsunami hazards to the U.S. Pacific Northwest. Although these large earthquakes are documented in the geologic record, none have been recorded with modern instrumentation, complicating regional seismic hazard assessments. To improve constraints on seismic hazard estimates for future earthquakes, we develop 3D broadband (0–10 Hz) ground‐motion simulations for 36 full‐margin megathrust earthquake scenarios through the Cascadia Coastlines and Peoples Hazards Research Hub. These simulations expand on the M9 Project ( Frankel et al. , 2018 ; Wirth, Frankel, et al. , 2018 ) by incorporating a broader range of rupture parameters—variable magnitudes, splay faulting, and geodetically informed slip distributions. Benchmarking these simulations against the Next Generation Attenuation Subduction (NGA‐Sub) ground‐motion models shows reasonable fits for non‐basin sites, although simulations predict stronger shaking at short (0–2 s) and long (6–10 s) periods. NGA‐Sub basin terms improve fits to synthetic shaking within the Seattle Basin but underpredict amplitudes by a factor of 2 at periods of 2–5 s. We also evaluate how rupture characteristics influence regional inland shaking variability and basin response in the Puget Lowlands. Larger magnitude scenarios generally cause stronger inland shaking due to farther inland extents of slip and larger strong‐motion‐generating fault patches but show little difference in basin amplification within the Puget Lowlands compared to smaller magnitude events. Along‐dip hypocenter location strongly influences overall inland shaking, with offshore (updip) hypocenters generating on average ∼45% higher shaking at long periods (>6 s) compared to onshore (downdip) hypocenters due to rupture directivity effects. Offshore splay faults minimally affect inland shaking but alter offshore deformation important for coupled ground‐shaking and tsunami scenarios. Finally, we show that rupture directivity increases amplification within the Seattle Basin through the generation and interaction of higher order mode surface waves, an effect of 3D basin geometry that cannot be captured using 1D approximations.

British Columbia, California, Oregon, Washington↗
USGS2022

DevelNet: Earthquake detection on Develocorder films with deep learning: Application to the rangely earthquake control experiment

There exists over a century of instrumental seismic data; however, most seismograms recorded before the 1980s are only available in analog form. Although analog seismograms are of great value, they are underutilized due to the difficulties of making quantitative measurements on the original media and in converting them to digital time series. In this study, we present an alternative workflow, based on deep learning, to reconstruct an earthquake catalog from images of analog data without conversion to vector time series. We trained a convolutional neural network—DevelNet, using synthetic analog data to detect earthquakes on scanned multichannel Develocorder film images. We then developed an image‐based processing workflow to measure arrival times, locate, and determine the magnitudes of earthquakes in the data. We demonstrate the performance of this approach on two years of continuous Develocorder film recordings from the Rangely earthquake control experiment in the mid‐1970s. Our approach detects twice the number of events reported in the original catalog ( Raleigh et al. , 1976 ). This demonstrates that DevelNet efficiently detects earthquakes from Develocorder film scans, performs consistently over time, and is robust to changes in network geometry. Our locations generally agree with the original study, although the automatically measured arrival times are less precise than manual reading, leading to increased location scatter. Our automatic workflow of Develocorder films rivals the performance of skilled analysts in earthquake detection, but with minimal human intervention. This image‐based processing offers a new approach for effectively and efficiently extracting earthquake information from analog seismic data.

Bulletin of the Seismological Society of America↗
USGS1968

Earthquake of August 19, 1966, Varto Area, eastern Turkey

The earthquake of August 19, 1966, centered near Varto in eastern Turkey. It occurred at 12:22:09.6 G. M. T. and had a Richter magnitude of about 7. Intensities reached IX. Reportedly, 2,529 people were killed, 1,500 injured, and 19,013 buildings were demolished or heavily damaged. Surface fractures in three parallel fracture zones trending N65-70°W display both right-lateral strike slip and extension normal to the fractures. The trend and sense of displacement relate the fracturing to the North Anatolia fault system. Examples of geologic factors influencing damage include: (1) degree of saturation of the alluvium, (2) proximity to landslides, and (3) relation to surface faulting.

Near Varto in eastern Turkey↗
USGS1968

A method for estimating the uncertainty of seismic velocities measured by refraction techniques

Time residuals from 7 5-km segments of 18 crustal seismic-refraction profiles in the Basin and Range province are used to investigate the validity of the linear-regression model and to make large sample estimates of the variance in the travel time distributions. A formula for unbiased estimates of velocity uncertainty is derived, assuming a linear trend with distance for the variances of the travel-time distributions. If the recording units are symmetric about the center of the recording interval, this formula is equivalent to the one derived assuming the variances are equal. At the 95-per cent confidence level the chi-squared test implied 84 per cent of the time-residual samples were inconsistent with the hypothesis that their parent populations had Gaussian distributions. If the number of recording locations expceeds 8, confidence limits computed without the Gaussian assumption suggest the departures from normality are not significant for velocity uncertainty estimates. The large sample estimates of the time-residual populations may be applicable to other areas. This evidence motivated the development of a method, requiring very little numerical calculation, for estimating uncertainties in velocities. The method requires, in addition to the large sample estimates of the travel time variances, information on the quality of the data, the location of the recording interval, and the number of recording units. The method is useful for the design of new experiments and independent estimates of uncertainty reported in the literature.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1970

Effects of local geology on ground motion near San Francisco Bay

Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, California. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of March 22, 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. For sites underlain by a layer of younger bay mud or artificial fill, maximum horizontal ground velocities generally increased with thickness of the layer and were as much as ten times greater than those recorded on nearby bedrock. The maximum vertical velocities for these sites were between 1 and 3.5 times greater. Spectral amplification curves clearly define a “dominant ground period” of about 1 second for sites underlain by younger bay mud. For sites underlain by older, more consolidated sediments, no clearly defined “dominant ground period” was found. Maximum ground velocities for the older bay sediment sites were about twice those recorded on bedrock.

California↗
USGS1971

Geological aspects of the May 31, 1970, Perú earthquake

Geological effects of the destructive May 31, 1970, Perú earthquake (Ms = 7.7) extended over roughly 65,000 km2 of west-central Perú. Earthquake-triggered slope failures of all types that occurred throughout the mountainous parts of the region extensively damaged transportation routes and irrigation canals and temporarily dammed some rivers and lakes. The geologically most important and spectacular of these, a cataclysmic debris avalanche from between 5,500 and 6,400 m altitude on the north peak of Huascarán Mountain, buried the city of Yungay and part of Ranrahirca (elevation about 2,500 m) with a loss of more than about 18,000 lives. The debris avalanche involved 50-100 million m3 of rock, ice, snow, and soil that traveled 14.5 km from the source to Yungay at an average velocity of between 280 and 335 km/hr. Exceptionally rapid movement of the avalanche is indicated by eyewitness accounts, by topographic irregularities as high as 140 m that were overridden, and locally by boulders weighing several tons that were hurled as much as 1,000 m beyond its margins. A pulse of muddy water from the debris avalanche that swept down the Río Santa 160 km to the sea inundated farms and small settlements, buried highway and railroad routes, and destroyed the diversion dam and access bridge to a major hydroelectric plant.

Bulletin of the Seismological Society of America↗
USGS1971

The earthquake sequence near Danville, California, 1970

Several thousand small earthquakes, with magnitudes ranging up to 4, occurred near Danville, California, during May, June, and July 1970. Seven temporary seismographs were installed near the epicentral region to augment an existing telemetered network within 1 day after the first felt earthquake. The dense concentration of 10 seismographs within 12 km of the seismic activity and the existence of a reversed seismic-refraction profile through the region permitted a very detailed study of this earthquake sequence. Over 400 events with magnitude greater than 0.5 were located with an average accuracy of ±1 km. The main earthquake region was approximately spherical: 1 km in radius, 6 km deep, and centered about 5 km southeast of Danville, 40 km east of San Francisco. Fault-plane solutions indicate that faulting associated with the earthquakes was of the right-lateral strike-slip type, with a strike of N 35°W.

California↗
USGS1974

Effects of the Bear Valley and San Juan Bautista earthquakes of 1972 on Geodimeter line lengths

Measurements of lines of the California Geodimeter network in the vicinity of the epicenters of four 1972 earthquakes (magnitudes 4.7, 4.7, 4.8, and 5.1) along the San Andreas fault system indicate that no significant anomalous changes in line length preceded or accompanied those earthquakes. Within the precision of measurement, the data are consistent with a linear change in line length with time. Measurements of two 20-km-long lines made 2 days before and 1 day after a magnitude 4.7 earthquake showed no significant change even though the epicenter of the earthquake was only about 15 km from each of the three stations at the end points of the lines.

California↗
USGS1974

Geodetic determination of strain at the Nevada Test Site following the Handley event

Repeated surveys of a trilateration network (aperture greater than 20 km) centered on ground zero for the HANDLEY event, a nuclear explosion at the Nevada Test Site with yield in excess of 1 megaton, suggest that the explosion induced an east-west extension of the network by more than 50 mm. In the year following the detonation, this deformation reversed such that the final configuration represented a small east-west contraction from the pre-HANDLEY state. In the subsequent 2-year period, only minor deformation was detected. Thus, the overall response of Pahute Mesa may be described as stable. The explosion-induced deformation is thought to be partly due to slip on faults driven by the large-amplitude seismic waves from the explosion. The mechanism of the postshot relaxation is not understood.

Nevada↗
USGS1976

Effects of local geological conditions in the San Francisco Bay region on ground motions and the intensities of the 1906 earthquake

Measurements of ground motion generated by nuclear explosions in Nevada have been completed for 99 locations in the San Francisco Bay region, California. The recordings show marked amplitude variations in the frequency band 0.25 to 3.0 Hz that are consistently related to the local geological conditions of the recording site. The average spectral amplifications observed for vertical and horizontal ground motions are, respectively: (1, 1) for granite, (1.5, 1.6) for the Franciscan Formation, (3.0, 2.7) for the Santa Clara Formation, (3.3, 4.4) for alluvium, and (3.7, 11.3) for bay mud. Spectral amplification curves define predominant ground frequencies in the band 0.25 to 3.0 E for bay mud sites and for some alluvial sites. Amplitude spectra computed from recordings of seismic background noise at 50 sites do not generally define predominant ground frequencies. The intensities ascribed to various sites in the San Francisco Bay region for the California earthquake of April 18, 1906, are strongly dependent on distance from the zone of surface faulting and the geological character of the ground. Considering only those sites (approximately one square city block in size) for which there is good evidence for the degree of ascribed intensity, the intensities for 917 sites on Franciscan rocks generally decrease with the logarithm of distance as Intensity = 2.69 - 1.90 log (Distance in kilometers). (1) For sites on other geological units, intensity increments, derived from this empirical relation, correlate strongly with the Average Horizontal Spectral Amplifications (AHSA) according to the empirical relation Intensity Increment = 0.27 + 2.70 log (AHSA). (2) Average intensity increments predicted for the various geological units are −0.3 for granite, 0.2 for the Franciscan Formation, 0.6 for the Great Valley sequence, 0.8 for the Santa Clara Formation, 1.3 for alluvium, and 2.4 for bay mud. The maximum intensity map predicted on the basis of these data delineates areas in the San Francisco Bay region of potentially high intensity for large earthquakes on either the San Andreas fault or the Hayward fault. The map provides a crude form of seismic zonation for the region and may be useful for certain general types of land-use zonation.

California↗
USGS1977

Reflection and refraction of type-II S waves in elastic and anelastic media

The general theory of viscoelasticity, which accounts for elastic as well as anelastic linear behavior of materials, predicts that two types of S waves propagate in anelastic earth materials. The particle motion for an inhomogeneous plane S wave of type I is elliptical in the plane defined by the directions of propagation and attenuation, while the particle motion for an inhomogeneous plane S wave of type II is linear perpendicular to this plane. The general theory predicts that an S -wave incident upon a plane boundary perpendicular to the plane defined by the directions of propagation and attenuation generates S waves only of the same type. General characteristics of the type-II S waves reflected and refracted at plane anelastic boundaries are: (a) velocities and maximum attenuations which depend on the angle of incidence and frequency, (b) maximum energy flow at a different velocity and in a different direction than phase propagation, (c) energy flow across the boundary due to interaction of the incident and reflected waves. The general theory predicts these characteristics for the waves whenever a plane type-II S wave interacts with a plane anelastic boundary such as a soil-bedrock, crust-mantle, or core-mantle interface. None of these characteristics are predicted for the plane SH waves described by elasticity theory.

Bulletin of Seismological Society of America↗