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USGS2020

Evaluation of seismic hazard models with fragile geologic features

We provide an overview of a 2019 workshop on the use of fragile geologic features (FGFs) to evaluate seismic hazard models. FGFs have been scarcely utilized in the evaluation of seismic hazard models, despite nearly 30 yr having passed since the first recognition of their potential value. Recently, several studies have begun to focus on the implementation of FGFs in seismic hazard modeling. The workshop was held to capture a “snapshot” of the state‐of‐the‐art in FGF work and to define key research areas that would increase confidence in FGF‐based evaluation of seismic hazard models. It was held at the annual meeting of the Southern California Earthquake Center on 8 September 2019, and the conveners were Mark Stirling (University of Otago, New Zealand) and Michael Oskin (University of California, Davis). The workshop attracted 44 participants from a wide range of disciplines. The main topics of discussion were FGF fragility age estimation (age at which an FGF achieved its current fragile geometry), fragility estimation, FGF‐based evaluation of seismic hazard models, and ethical considerations relating to documentation and preservation of FGFs. There are now many scientists working on, or motivated to work on, FGFs, and more types of FGFs are being worked on than just the precariously balanced rock (PBR) variety. One of the ideas presented at the workshop is that fragility ages for FGFs should be treated stochastically rather than assuming that all share a common age. In a similar vein, new studies propose more comprehensive methods of fragility assessment beyond peak ground acceleration and peak ground velocity‐based approaches. Two recent studies that apply PBRs to evaluate probabilistic seismic hazard models use significantly different methods of evaluation. Key research needs identified from the workshop will guide future, focused efforts that will ultimately facilitate the uptake of FGFs in seismic hazard analysis.

Seismological Research Letters↗

USGS2020

Nodal seismograph recordings of the 2019 Ridgecrest Earthquake Sequence

The 2019 Ridgecrest, California earthquake sequence included M w 6.4 and Mw 7.1 earthquakes that occurred on successive days beginning on 4 July 2019. These two largest earthquakes of the sequence occurred on orthogonal faults that ruptured the Earth’s surface. To better evaluate the 3D subsurface fault structure, ( P - and S -wave) velocity, 3D and temporal variations in seismicity, and other important aspects of the earthquake sequence, we recorded aftershocks and ambient noise using up to 461 three-component nodal seismographs for about two months, beginning about one day after the M w 7.1 mainshock. The ~ 30,000 M w ≥1 earthquakes that were recorded on the dense arrays provide an unusually large volume of data with which to evaluate the earthquake sequence. This report describes the recording arrays and is intended to provide metadata for researchers interested in evaluating various aspects of the 2019 Ridgecrest earthquake sequence using the nodal data set.

California↗

USGS2020

Latency of waveform data delivery from the Southern California Seismic Network during the 2019 Ridgecrest earthquake sequence and its effect on ShakeAlert

The occurrence of the 4–6 July 2019 M w 6.4 and M w 7.1 Ridgecrest earthquake sequence provided the first full‐scale test of the network and telemetry readiness of the Southern California Seismic Network (SCSN), to support the ShakeAlert earthquake early warning (EEW) system in California. ShakeAlert is a U.S. Geological Survey (USGS)‐led collaboration to detect earthquakes and, when possible, to alert the public before the arrival of the strongest shaking. The SCSN performed well in its regional monitoring role for both the 4 July M w 6.4 and the 6 July M w 7.1 earthquakes. In the EEW role, it provided timely delivery of 5 s of P ‐wave data to ShakeAlert, which issued its first alert 6.9 s after origin time. Data delivery at peak data volumes for many stations exhibited some latency, and, as a consequence, some data arrived too late for analysis by one of the EEW algorithms. We find that the average link bandwidth for each station was sufficient, because all waveform data were delivered automatically to the archive, but link capacity for many stations was insufficient for peak demand. We describe the performance of the data telemetry for the sequence, including cellular, radio, hybrid, and backhaul systems. Cellular‐based telemetry systems maintained low latency throughout strong shaking and after, but some stations, even at great distances, experienced subsequent brief increases in latency. Performance of radio links depended mostly on the signal strength of the link, with short‐distance direct shots to high‐bandwidth backhaul systems showing no latency impact, whereas stations on some long distance or marginal quality links suffered latencies of tens or hundreds of seconds. Improvements are being implemented to move telemetry links onto USGS and partner high‐bandwidth microwave systems, and to reduce dependency on less robust long‐distance radio shots.

California↗

USGS2021

Forecasting induced earthquake hazard using a hydromechanical earthquake nucleation model

In response to the dramatic increase in earthquake rates in the central United States, the U.S Geological Survey began releasing 1 yr earthquake hazard models for induced earthquakes in 2016. Although these models have been shown to accurately forecast earthquake hazard, they rely purely on earthquake statistics because there was no precedent for forecasting induced earthquakes based upon wastewater injection data. Since the publication of these hazard models, multiple physics‐based methods have been proposed to forecast earthquake rates using injection data. Here, we use one of these methods to generate earthquake hazard forecasts. Our earthquake hazard forecasts are more accurate than statistics‐based hazard forecasts. These results imply that fluid injection data, where and when available, and the physical implications of fluid injection should be included in future induced earthquake hazard forecasts.

Kansas, Oklahoma↗

USGS2020

Ensemble ShakeMaps for magnitude 9 earthquakes on the Cascadia Subduction Zone

We develop ensemble ShakeMaps for various magnitude 9 ( ⁠ M "> M M 9) earthquakes on the Cascadia megathrust. Ground‐shaking estimates are based on 30 M "> M M 9 Cascadia earthquake scenarios, which were selected using a logic‐tree approach that varied the hypocenter location, down‐dip rupture limit, slip distribution, and location of strong‐motion‐generating subevents. In a previous work, Frankel et al. (2018) used a hybrid approach (i.e., 3D deterministic simulations for frequencies < 1    Hz "> < 1 Hz <1 Hz and stochastic synthetics for frequencies > 1    Hz "> > 1 Hz >1 Hz ⁠ ) and uniform site amplification factors to create broadband seismograms from this set of 30 earthquake scenarios. Here, we expand on this work by computing site‐specific amplification factors for the Pacific Northwest and applying these factors to the ground‐motion estimates derived from Frankel et al. (2018) . In addition, we use empirical ground‐motion models (GMMs) to expand the ground‐shaking estimates beyond the original model extent of Frankel et al. (2018) to cover all of Washington State, Oregon, northern California, and southern British Columbia to facilitate the use of these ensemble ShakeMaps in region‐wide risk assessments and scenario planning exercises. Using this updated set of 30 M "> M M 9 Cascadia earthquake scenarios, we present ensemble ShakeMaps for the median, 2nd, 16th, 84th, and 98th percentile ground‐motion intensity measures. Whereas traditional scenario ShakeMaps are based on a single hypothetical earthquake rupture, our ensemble ShakeMaps take advantage of a logic‐tree approach to estimating ground motions from multiple earthquake rupture scenarios. In addition, 3D earthquake simulations capture important features such as strong ground‐motion amplification in the Pacific Northwest’s sedimentary basins, which are not well represented in the empirical GMMs that compose traditional scenario ShakeMaps. Overall, our results highlight the importance of strong‐motion‐generating subevents for coastal sites, as well as the amplification of long‐period ground shaking in deep sedimentary basins, compared with previous scenario ShakeMaps for Cascadia.

California, Oregon, Washington↗

USGS2021

Quick and dirty (and accurate) 3-D paleoseismic trench models using coded scale bars

Structure‐from‐motion (SfM) modeling has dramatically increased the speed of generating geometrically accurate orthophoto mosaics of paleoseismic trenches, but some aspects of this technique remain time and labor intensive. Model accuracy relies on control points to establish scale, reduce distortion, and orient 3D models. Traditional SfM methods use total station or Global Navigation Satellite System (GNSS) surveys to constrain models, but collecting control points along a vertical trench wall is often inhibited by poor line of sight to the survey sensor or limited sky view and requires many hours in the field and office. We used physical scale bars printed with coded targets to constrain SfM models of a dusty, 46‐m‐long trench excavation across the Teton fault (Wyoming, U.S.A.). We present a workflow for generating quick and accurate 3D SfM models and orthophoto mosaics and compare the effectiveness of using scale bar, GNSS, and total‐station control in the models. Our results show that the scale bar model deviates from total station survey points by an average of 3.1 cm (maximum of 5.3 cm). In addition, the scale‐bar model only deviates an average of 1.7 cm (maximum 3.5 cm) when compared to the best model alternative, the SfM model controlled by the total station survey. Scale bars eliminate several hours needed to collect and incorporate control points from total station or GNSS surveys and significantly simplify the workflow, at the cost of slightly increased 3D model and orthophoto mosaic error. Our results further suggest that trench models can be constrained with at least four physical scale bars, but using five to six physical scale bars provides redundant control for field deployment and model optimization. The scale bar method for paleoseismic trenches proves to be portable and fast, minimizes the need for specialized survey equipment, and maintains model accuracy needed for mapping trench walls.

Seismological Research Letters↗

USGS2020

Monitoring network changes during the 2018 Kīlauea Volcano eruption

In the summer of 2018, Kīlauea Volcano underwent one of its most significant eruptions in the past few hundred years. The volcano’s summit and East Rift Zone magma system partially drained, resulting in a series of occasionally explosive partial caldera collapses, and widespread lava flows in the lower East Rift Zone. The Hawaiian Volcano Observatory (HVO) operates a robust permanent monitoring network of about 250 stations, recording a variety of real‐time data streams: seismic (short‐period, broadband, strong‐motion), infrasound, Global Navigation Satellite Systems (GNSS), tilt, camera, laser rangefinder, and gas geochemistry. During the eruption, HVO staff quickly established 35 new temporary monitoring stations, to better constrain evolving volcanic hazards. The partial collapses of the caldera threatened to disrupt important telemetry links in the HVO monitoring network, and a major effort was undertaken in the midst of the eruption crisis to reroute radio telemetry and maintain continuity of data flow. In the process, a new data center was established in Hilo, to mitigate a long‐standing potential single point of failure at the HVO facility. Over the course of the eruption from May through August, lava, ashfall, wildfire, and cliff collapse destroyed or disabled 36 stations. Thousands of earthquakes damaged the main HVO facility at Uēkahuna Bluff, causing staff to evacuate the building and relocate observatory operations in the midst of the eruption response, adding more complexity to the response effort. Throughout these events, the HVO team maintained the monitoring network, provided timely information to the public and emergency managers, and collected valuable scientific data to better understand Kīlauea Volcano.

Hawaii↗

USGS2021

Seismic monitoring during crises at the NEIC in support of the ANSS

Over the past two decades, the U.S. Geological Survey (USGS) National Earthquake Information Center (NEIC) has overcome many operational challenges. These range from minor disruptions, such as power outages, to significant operational changes, including system reconfiguration to handle unique earthquake sequences and the need to handle distributed work during a pandemic. Our ability to overcome crises is built on the development and implementation of a continuity of operations plan, well‐designed infrastructure, adaptive software systems, experienced staff, and extensive collaboration. The NEIC does not operate in a vacuum but benefits from contributions of United States and international seismic networks. Similarly, the overall resilience of earthquake monitoring in the United States and around the globe benefits from the NEIC’s role as the national center for the Advanced National Seismic System (ANSS). Here, we highlight significant adaptations the NEIC has made in the face of crises. We discuss the COVID‐19 pandemic, which represents the most significant operational crisis to impact the NEIC. The NEIC has maintained continuous operations during the ongoing COVID‐19 pandemic by shifting from a fully onsite operations center to a distributed hybrid of onsite and telework staffing. We then discuss cases in which the NEIC has supported regional monitoring in the face of significant crises. In 2018, the NEIC assisted the Hawaiian Volcano Observatory with the Kīlauea volcano eruption by responding to large events, implementing contingency monitoring procedures, and calculating moment magnitudes for the low‐frequency caldera collapses. Impacts of a crisis extend beyond the immediate response and often require a significant postevent assessment and a rebuilding phase. After the 2017 Hurricane Maria, the NEIC, the USGS National Strong‐Motion Program, and the USGS Albuquerque Seismological Laboratory worked with the Puerto Rico Seismic Network and the Puerto Rico Strong‐Motion program to assess, plan, and implement upgrades at sites that experienced storm damage.

Seismological Research Letters↗

USGS2021

Coseismic fault slip and afterslip associated with the M5.7 March 18, 2020 Magna, Utah, earthquake

The 2020 Magna, Utah, earthquake produced observable crustal deformation over a ∼ 100 km 2 area around the southeast margin of Great Salt Lake, but it did not produce any surface rupture. To obtain a detailed picture of the fault slip, we combine strong motion seismic waveforms with GPS static oﬀsets and Interferometric Synthetic Aperture Radar (InSAR) observations to obtain kinematic and static slip models of the event. We sample the regional seismic waveﬁeld with 3-component records from 68 stations of the University of Utah Seismograph Stations network. We ﬁnd that coseismic slip and afterslip, with predominantly normal slip, distributed on a shallowly west-dipping plane, possibly augmented by afterslip on a steeply northeast-dipping plane, best ﬁts the joint dataset. The west-dipping plane locates near previously inferred sources of interseismic creep at depth. Hence the earthquake may have occurred on the downdip ex-tension of the Wasatch fault and activated further slip (afterslip) at shallow depth east of the hypocenter. This inferred afterslip may have driven the vigorous aftershock activity that was concentrated east of the hypocenter.

Utah↗

USGS2020

Rupture process of the M6.5 Stanley, Idaho, earthquake inferred from seismic waveform and geodetic data

The 2020 M 6.5 Stanley, Idaho, earthquake produced rupture in the north of the active Sawtooth fault in the northern basin and range at depth, without any observable surface rupture. Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data yield several millimeters of static offsets out to ∼100 km from the rupture and up to ∼0.1 m of near‐field crustal deformation. We combine the GPS and InSAR data with long‐period regional seismic waveforms to derive models of kinematic slip and afterslip. We find that the coseismic rupture is complex, likely involving up to 2 m combined left‐lateral strike slip and normal slip on a previously unidentified ∼south‐southeast‐striking fault. This slip is predominantly left‐lateral strike slip, different from the dominant east‐northeast–west‐northwest normal faulting of the region. At least one ∼northeast‐trending fault, likely associated with the Trans‐Challis fault system, is inferred to have accommodated a few decimeters of right‐lateral afterslip, consistent with vigorous aftershock activity at depth along northeast‐trending lineations.

Idaho↗

USGS2020

The normal faulting 2020 Mw5.8 Lone Pine, Eastern California earthquake sequence

The 2020 M w "> M w 5.8 Lone Pine earthquake, the largest earthquake on the Owens Valley fault zone, eastern California, since the nineteenth century, ruptured an extensional stepover in that fault. Owens Valley separates two normal‐faulting regimes, the western margin of the Great basin and the eastern margin of the Sierra Nevada, forming a complex seismotectonic zone, and a possible nascent plate boundary. Foreshocks began on 22 June 2020; the largest M w "> M w 4.7 foreshock occurred at ∼ 6    km "> ∼6 km depth, with primarily normal faulting, followed ∼ 40    hr "> ∼ 40 hr later on 24 June 2020 by an M w "> M w 5.8 mainshock at ∼ 7    km "> ∼ 7 km depth. The sequence caused overlapping ruptures across a ∼ 0.25    km 2 "> ∼0.25 km 2 area, extended to ∼ 4    km 2 "> ∼ 4 km 2 , and culminated in an ∼ 25    km 2 "> ∼25 km 2 aftershock area. The mainshock was predominantly normal faulting, with a strike of 330° (north‐northwest), dipping 60°–65° to the east‐northeast. Comparison of background seismicity and 2020 Ridgecrest aftershock rates showed that this earthquake was not an aftershock of the Ridgecrest mainshock. The M w "> M w M w – m B "> – m B relationship and distribution of ground motions suggest typical rupture speeds. The aftershocks form a north‐northwest‐trending, north‐northeast‐dipping, 5 km long distribution, consistent with the rupture length estimated from analysis of regional waveform data. No surface rupture was reported along the 1872 scarps from the 2020 M w "> M w 5.8 mainshock, although, the dipping rupture zone of the M w "> M w 5.8 mainshock projects to the surface in the general area. The mainshock seismic energy triggered rockfalls at high elevations ( ⁠ > 3.0    km "> > 3.0 km ⁠ ) in the Sierra Nevada, at distances of 8–20 km, and liquefaction along the western edge of Owens Lake. Because there were ∼ 30 % "> ∼ 30 % fewer aftershocks than for an average southern California sequence, the aftershock forecast probabilities were lower than expected. ShakeAlert, the earthquake early warning system, provided first warning within 9.9 s, as well as subsequent updates.

California↗

USGS2021

Which earthquake accounts matter?

Earthquake observations contributed by human observers provide an invaluable source of information to investigate both historical and modern earthquakes. Commonly, the observers whose eyewitness accounts are available to scientists are a self‐selected minority of those who experience a given earthquake. As such these may not be representative of the overall population that experienced shaking from the event. Eyewitness accounts can contribute to modern science only if they are recorded in the first place and archived in an accessible repository. In this study, we explore the extent to which geopolitics and socioeconomic disparities can limit the number of earthquake observers whose observations can contribute to science. We first revisit a late nineteenth‐century earthquake in the central United States in 1882 that provides an illustrative example of an event that has been poorly characterized due to a reliance on English‐language archival materials. For modern earthquakes, we analyze data collected for recent earthquakes in California and India via the online “Did You Feel It?” (DYFI) system. In California, online data‐collection systems appear to be effective in gathering eyewitness accounts from a broad range of socioeconomic groups. In India, however, responses to the DYFI system reveal a strong bias toward responses from urban areas as opposed to rural settlements, as well a bias with literacy rate. The dissimilarity of our results from modern earthquakes in the United States and India provides a caution that, in some parts of the world, contributed felt reports can still potentially provide an unrepresentative view of earthquake effects, especially if online data collection systems are not designed to be broadly accessible. This limitation can in turn potentially shape our understanding of an earthquake’s impact and the characterization of seismic hazard.

Arkansas, Oklahoma, Texas↗

USGS2021

Field response and surface rupture characteristics of the 2020 M6.5 Monte Cristo Range earthquake, central Walker Lane, Nevada

The M 6.5 Monte Cristo Range earthquake that occurred in the central Walker Lane on 15 May 2020 was the largest earthquake in Nevada in 66 yr and resulted in a multidisciplinary scientific field response. The earthquake was the result of left‐lateral slip along largely unmapped parts of the Candelaria fault, one of a series of east–northeast‐striking faults that comprise the Mina deflection, a major right step in the north–northwest structural grain of the central Walker Lane. We describe the characteristics of the surface rupture and document distinct differences in the style and orientation of fractures produced along the 28 km long rupture zone. Along the western part of the rupture, left‐lateral and extensional displacements occurred along northeasterly and north‐striking planes that splay off the eastern termination of the mapped Candelaria fault. To the east, extensional and right‐lateral displacements occurred along predominantly north‐striking planes that project toward well‐defined Quaternary and bedrock faults. Although, the largest left‐lateral displacement observed was ∼ 20 cm ⁠ , the majority of displacements were < 5 cm and were distributed across broad zones up to 800 m wide, which are not likely to be preserved in the geologic record. The complex pattern of surface rupture is consistent with a network of faults defined in the shallow subsurface by aftershock seismicity and suggests that slip partitioning between east‐striking left‐lateral faults and north to northwest‐striking right‐lateral faults plays an important role in accommodating northwest‐directed transtension in the central Walker Lane.

Nevada↗

USGS2021

A review of timing accuracy across the Global Seismographic Network

The accuracy of timing across a seismic network is important for locating earthquakes as well as studies that use phase‐arrival information (e.g., tomography). The Global Seismographic Network (GSN) was designed with the goal of having reported timing be better than 10 ms. In this work, we provide a brief overview of how timing is kept across the GSN and discuss how clock‐quality metrics are embedded in Standard for Exchange of Earthquake Data records. Specifically, blockette 1001 contains the timing‐quality field, which can be used to identify time periods when poor clock quality could compromise timing accuracy. To verify the timing across the GSN, we compare cross‐correlation lags between collocated sensors from 1 January 2000 to 1 January 2020. We find that the mean error is less than 10 ms, with much of the difference likely coming from the method or uncertainty in the phase response of the instruments. This indicates that timing across the GSN is potentially better than 10 ms. We conclude that unless clock quality is compromised (as indicated in blockette 1001), GSN data’s timing accuracy should be suitable for most current seismological applications that require 10 ms accuracy. To assist users, the GSN network operators have implemented a “gsn_timing” metric available via the Incorporated Research Institutions for Seismology Data Management Center that helps users identify data with substandard timing accuracy (the 10 ms design goal of the GSN).

Seismological Research Letters↗

USGS2021

An International Virtual Workshop on Global Seismology and Tectonics (IVWGST‐2020)

An International Virtual Workshop on Global Seismology and Tectonics (IVWGST‐2020) was organized by the Geoscience and Technology Division of Council of Scientific and Industrial Research—North East Institute of Science and Technology, Jorhat, India from 14 to 25 September 2020. This workshop predominantly catered to undergraduate, postgraduate, and Ph.D. students, scientists, and academicians from across the globe. The primary motive of IVWGST-2020 was to inspire the participating students, perturbed by the unprecedented situation brought about by the COVID-19 pandemic, with quality lecture sessions, so as to lift their spirits. The virtual workshop served as a conduit for the students and researchers to directly interact with several pioneers and prominent geoscience researchers from around the world. Lectures, via Microsoft Teams, were given by 15 eminent speakers from diverse geoscience forums and institutions, and were attended by more than 1000 participants, mostly students and researchers, from 30 different countries. This report briefly summarizes the agenda, describes our experiences hosting the virtual workshop, and documents the challenges faced.

Seismological Research Letters↗

USGS2021

Six decades of seismology at South Pole, Antarctica: Current limitations and future opportunities to facilitate new geophysical observations

Seismograms from the South Pole have been important for seismological observations for over six decades by providing (until 2007) the only continuous seismic records from the interior of the Antarctic continent. The South Pole, Antarctica station has undergone many updates over the years, including conversion to a digital recording station as part of the Global Seismographic Network (GSN) in 1991 and being relocated to multiple deep ( ⁠ > 250    m "> > 250 m >250 m ⁠ ) boreholes 8 km away from the station in 2003 (and renamed to Quiet South Pole, Antarctica [QSPA]). Notably, QSPA is the second most used GSN station by the National Earthquake Information Center to pick phases used to rapidly detect and locate earthquakes globally, and has been used for a variety of glaciological and oceanography studies. In addition, it is the only seismic station on the Earth where low‐frequency ( ⁠ < 5    mHz "> < 5 mHz <5 mHz ⁠ ), normal‐mode oscillations of the planet excited by large earthquakes can be recorded without influence from Earth’s rotation, and most of the direct effects of the solid Earth tide vanish. However, the current sensors are largely 1980s vintage, and, while able to make some lower‐frequency observations from earthquakes, the borehole sensors appear unable to resolve ambient ground motions at frequencies lower than 25 mHz due to instrument noise and contamination from magnetic field variations. Recently developed borehole sensors offer the potential to extend background noise observations to below 3 mHz, which would substantially improve the fidelity and scientific value of seismic observations at South Pole. Through collaboration with the IceCube Neutrino Observatory, the opportunity exists to emplace a modern very broadband seismometer near the base ( ⁠ > 2    km "> > 2 km >2 km depth) of the Antarctic ice cap, which could lead to unprecedented seismic observations at long periods and facilitate a broad spectrum of Earth science studies.

Seismological Research Letters↗

USGS2021

Seismic wave propagation and basin amplification in the Wasatch Front, Utah

Ground‐motion analysis of more than 3000 records from 59 earthquakes, including records from the March 2020 M w "> M w 5.7 Magna earthquake sequence, was carried out to investigate site response and basin amplification in the Wasatch Front, Utah. We compare ground motions with the Bayless and Abrahamson (2019 ; hereafter, BA18) ground‐motion model (GMM) for Fourier amplitude spectra, which was developed on crustal earthquake records from California and other tectonically active regions. The Wasatch Front records show a significantly different near‐source rate of distance attenuation than the BA18 model, which we attribute to differences in (apparent) geometric attenuation. Near‐source residuals show a period dependence of this effect, with greater attenuation at shorter periods ( ⁠ T < 0.5    s "> T < 0.5 s ) and a correlation between period and the distance over which the discrepancy manifests ( ⁠ ∼ 20 – 50    km "> ∼ 20 – 50 km ⁠ ). We adjusted the recorded ground motions for these regional path effects and solved for station site terms using linear mixed‐effects regressions, with groupings for events and stations. We analyzed basin amplification by comparing the site terms with the basin geometry and basin depths from two seismic‐velocity models for the region. Sites over the deeper parts of the sedimentary basins are amplified by factors of 3–10, relative to sites with thin sedimentary cover, with greater amplification at longer periods ( ⁠ T ≳ 1    s "> T ≳ 1 s ). Average ground‐motion variability increases with period, and long‐period variability exhibits a slight increase at the basin edges. These results indicate regional seismic wave propagation effects requiring further study, and potentially a regionalized GMM, as well as highlight basin amplification complexities that may be incorporated into seismic hazard assessments.

Utah↗

USGS2021

The liquefaction record of past earthquakes in the Central Virginia Seismic Zone, Eastern United States

Following the 2011 moment magnitude, M "> M M 5.7 Mineral, Virginia, earthquake, we conducted a search for paleoliquefaction features and found 41 sand dikes, sand sills, and soft‐sediment deformation features at 24 sites exposed in cutbanks along several rivers: (1) the South Anna River, where paleoliquefaction features were found in the epicentral area of the Mineral earthquake and farther downstream to the southeast; (2) the Mattaponi and Pamunkey Rivers east of the Fall Line, where liquefiable sediments are more common than in the epicentral area; and (3) the James River and Rivanna River–Stigger Creek, where a few sand dikes were found in the 1990s. Liquefaction features are grouped into two age categories based on dating of host sediment in which they occur and weathering characteristics of the features. A younger generation of features that formed during the past 350 yr are small, few in number, and appear to be limited to the James and Pamunkey Rivers. Though there are large uncertainties in their locations and magnitudes, one or more preinstrumental earthquakes, including the 1758, 1774, and 1875 events, likely caused these features. An older generation of liquefaction features that formed between 350 and 2800 yr ago are larger, more numerous, and more broadly distributed than the younger generation of features. Several earthquakes could account for the regional distribution of paleoliquefaction features, including one event of M "> M M 6.25–6.5 near Holly Grove, or two events of M "> M M 6.0 near Mineral and M "> M M 6.25 near Ashland. Amplification of ground motions in Coastal Plain sediment might have contributed to liquefaction along the Mattaponi and Pamunkey Rivers.

Seismological Research Letters↗