Geology topics

Robert A. Williams

Publications and source records attributed to Robert A. Williams.

At least 19 recordsLinked to original sources

USGS2025

Update to a management-focused population viability analysis for North Atlantic right whales

We provide an update to the recently published population viability analysis for North Atlantic right whales ( Eubalaena glacialis ). The update includes improvements to the reproduction modeling and also shares additional context given evidence of reduced mortality indicated by recent population monitoring. Projections from the analysis are used to quantify simulated population sizes across 100 years and resulting quasi-extinction probabilities (falling below 50 mature females that have proven ability to reproduce) to compare hypothetical scenarios related to management of threats and changing environmental conditions. Under a status quo scenario reflecting conditions of 2019, prior to the enactment of new regulations by the U.S. and Canada, the North Atlantic right whale population would be expected to continue to fall, with a median decline of 88 percent (95% projection interval, –98 percent to –45 percent change) and a probability of falling below 50 proven females (i.e., quasi-extinction) of 0.988 at 100 years. In hypothetical scenarios that fully remove each of the three primary threats to right whales one at a time, removal of the entanglement threat alone reduces the probability of falling below 50 proven females in 100 years to 0.070; removal of the vessel strike threat alone reduces it to 0.522; and a return to historical prey abundance patterns (pre-2010), but with both human-related threats still in place, reduces it to 0.524. Although additional baseline scenarios were explored to examine the potential effects of recent regulations, we found that the most up-to-date mortality rates (2020–2022) are similar to those simulated under a 70% reduction in severe entanglement injury compared to rates estimated during 2013–2019. If management measures implemented in the U.S. and Canada continue to reduce mortality, the estimated probability of falling below 50 proven females in 100 years is 0.234. Our model continues to provide a tool for assessing North Atlantic right whale recovery.

NOAA Technical Memorandum↗

USGS2025

A multistate capture-recapture model to estimate reproduction of North Atlantic right whales

The recent steep decline of the endangered North Atlantic right whale Eubalaena glacialis can be attributed to high mortality combined with low reproduction. While the former is a clear result of anthropogenic activity, the latter involves more complexity. Evidence suggests that both short-term fluctuations in prey availability and long-term decline in health are responsible for depressed right whale calving rates. To facilitate an assessment of extinction risk, we developed a multistate capture-recapture model that estimated the probability of calving using extensive sightings data from 1990-2019. The model estimated sub-lethal effects of severe injury on calving probability and modeled temporal variability in calving as related to indices of prey availability ( Calanus spp. biomass) and an apparent regime shift. The average annual probability of calving for known-breeding females, given average prey conditions, decreased from 0.217 [95% CI: 0.162, 0.281] to 0.142 [95% CI: 0.067, 0.252] after the 2010 regime shift. The model indicated strong evidence of a relationship between calving probability and the prey index from the eastern Gulf of Maine, although this relationship effectively disappeared after 2010; moderate evidence for a relationship with prey from the southwest Gulf of St. Lawrence remained. Weak evidence of reduced calving probability due to severe injury resulted from low sample sizes, given increased mortality for individuals observed with severe injuries. The regime effect is hypothesized to be capturing a long-term decline in health due to a combination of decreasing habitat quality resulting from climate change and potentially chronic sublethal injuries (e.g. entanglements). Our reproduction model provides demographic parameter estimates that can be used in population projections for North Atlantic right whales, although uncertainty remains in the mechanisms responsible for recent declines in calving.

Endangered Species Research↗

USGS2024

Quantifying uncertainty in anthropogenic causes of injury and mortality for an endangered baleen whale

Understanding the causes of mortality for a declining species is essential for developing effective conservation and management strategies, particularly when anthropogenic activities are the primary threat. Using a competing hazards framework allows for robust estimation of the cause-specific variation in risk that may exist across multiple dimensions, such as time and individual. Here, we estimated cause-specific rates of severe injury and mortality for North Atlantic right whales ( Eubalaena glacialis ), a critically endangered species that is currently in peril due to human-caused interactions. We developed a multistate capture–recapture model that leveraged 30 years of intensive survey effort yielding sightings of individuals with injury assessments and necropsies of carcass recoveries. We examined variation in the hazard rates of severe injury and mortality due to entanglements in fishing gear and vessel strikes as explained by temporal patterns and the age and reproductive status of the individual. We found strong evidence for increased rates of severe entanglement injuries after 2013 and for females with calves, with consequently higher marginal mortality. The model results also suggested that despite vessel strikes causing a lower average rate of severe injuries, the higher mortality rate conditional on injury results in significant total mortality risk, particularly for females resting from a recent calving event. Large uncertainty in the estimation of carcass recovery rate for vessel strike deaths permeated into the apportionment of mortality causes. The increased rates of North Atlantic right whale mortality in the last decade, particularly for reproducing females, has been responsible for the severe decline in the species. By apportioning the human-caused threats using a quantitative approach with estimation of relevant uncertainty, this work can guide development of conservation and management strategies to facilitate species recovery. Our approach is relevant to other monitored populations where cause-specific injuries from multiple threats can be observed in live and dead individuals.

Ecosphere↗

USGS2023

A management-focused population viability analysis for North Atlantic right whales

The North Atlantic right whale ( Eubalaena glacialis ) is among the most endangered whale species in the world and has been in decline since 2010. Considerable effort is directed toward its recovery by striving to remove threats. In this report, we describe the development of a population viability analysis for right whales that is designed to assess the current status, evaluate the contributions of various threats, and explore the management interventions needed to achieve recovery. The individual-based model that underlies this analysis accounts for age- and stagespecific survival and reproductive rates, the effects of severe injury from entanglement or vessel strike, and future changes in prey availability and accessibility. Several new or updated empirical analyses supplied parameter estimates, and parametric uncertainty was carefully incorporated into the model results. We find that under the status quo conditions of 2019, prior to the enactment of new regulations by the U.S. and Canada after 2020, the North Atlantic right whale population would be expected to continue to fall, with a median decline of 75% in 100 years (95% projection interval, –98% to +9% change) and a probability of falling below 50 proven females of 0.934 in 100 years. If the recently enacted regulations reduce entanglement risk by 25%, however, the population would be expected to decrease by 42% over 100 years (95% projection interval –92% to +154% change), with a risk of falling below 50 proven females in 100 years of 0.705. If, instead, the recently enacted regulations reduce entanglement risk by 50%, the population would be expected to increase by 52% in 100 years (95% projection interval –83% to +497% change), with a probability of falling below 50 proven females of 0.349. Of the 3 primary threats explored in this analysis, the risk of entanglement contributes the most to the long-term risk of quasi-extinction, followed closely by the risk of vessel strike, and much more distantly by a decrease in prey availability. In hypothetical scenarios that fully remove one threat at a time, removal of the entanglement threat alone reduces the probability of falling below 50 proven females in 100 years from 0.934 to 0.053; removal of the vessel strike threat alone reduces it to 0.343; and a return to higher prey conditions, but with both human-related threats still in place, reduces it to 0.875. We explored a wide range of management intervention scenarios that changed the rate of entanglement risk (e.g., endline reductions, closures, implementation of ropeless/on-demand gear); the effect of entanglement (through use of weak rope technology); the rate of vessel traffic increase over time; and the severity of vessel strike risk through speed restrictions. We found, for example, that reducing entanglement risk alone by 25% reduces the risk of quasi-extinction from 0.934 to 0.705; reducing vessel strike risk alone by 25% reduces the risk of quasi-extinction from 0.934 to 0.846; but the combination of reducing both entanglement risk and vessel strike risk by 25% reduces the risk of quasi-extinction to 0.528. This model and the results it produced are meant to represent an assessment of the current status of North Atlantic right whales using the best available scientific and commercial data and state-of-the-art analytical tools. Our knowledge of the future of the right whale population, however, has limitations. We have endeavored to fully incorporate uncertainty into this model, but there are many areas for continued improvement. We view this model as a living tool that can be improved, adapted, and extended as new data, new methods, and new questions arise.

NOAA Technical Memorandum↗

USGS2018

2018 one‐year seismic hazard forecast for the central and eastern United States from induced and natural earthquakes

This article describes the U.S. Geological Survey (USGS) 2018 one‐year probabilistic seismic hazard forecast for the central and eastern United States from induced and natural earthquakes. For consistency, the updated 2018 forecast is developed using the same probabilistic seismicity‐based methodology as applied in the two previous forecasts. Rates of earthquakes across the United States M ≥ 3.0 "> M ≥ 3.0 grew rapidly between 2008 and 2015 but have steadily declined over the past 3 years, especially in areas of Oklahoma and southern Kansas where fluid injection has decreased. The seismicity pattern in 2017 was complex with earthquakes more spatially dispersed than in the previous years. Some areas of west‐central Oklahoma experienced increased activity rates where industrial activity increased. Earthquake rates in Oklahoma (429 earthquakes of M ≥ 3 "> M ≥ 3 and 4 M ≥ 4 "> M ≥ 4 ), Raton basin (Colorado/New Mexico border, six earthquakes M ≥ 3 "> M ≥ 3 ), and the New Madrid seismic zone (11 earthquakes M ≥ 3 "> M ≥ 3 ) continue to be higher than historical levels. Almost all of these earthquakes occurred within the highest hazard regions of the 2017 forecast. Even though rates declined over the past 3 years, the short‐term hazard for damaging ground shaking across much of Oklahoma remains at high levels due to continuing high rates of smaller earthquakes that are still hundreds of times higher than at any time in the state’s history. Fine details and variability between the 2016–2018 forecasts are obscured by significant uncertainties in the input model. These short‐term hazard levels are similar to active regions in California. During 2017, M ≥ 3 "> M ≥ 3 earthquakes also occurred in or near Ohio, West Virginia, Missouri, Kentucky, Tennessee, Arkansas, Illinois, Oklahoma, Kansas, Colorado, New Mexico, Utah, and Wyoming.

Seismological Research Letters↗

USGS2018

Integrate urban‐scale seismic hazard analyses with the U.S. National Seismic Hazard Model

For more than 20 yrs, damage patterns and instrumental recordings have highlighted the influence of the local 3D geologic structure on earthquake ground motions (e.g., M "> M M 6.7 Northridge, California, Gao et al. , 1996 ; M "> M M 6.9 Kobe, Japan, Kawase, 1996 ; M "> M M 6.8 Nisqually, Washington, Frankel, Carver, and Williams, 2002 ). Although this and other local‐scale features are critical to improving seismic hazard forecasts, historically they have not been explicitly incorporated into the U.S. National Seismic Hazard Model (NSHM, national model and maps), primarily because the necessary basin maps and methodologies were not available at the national scale. Instead, the U.S. Geological Survey (USGS), its partners, and external groups developed urban seismic hazard maps (urban models and maps) that consider detailed site effects in local areas (e.g., Wong et al. , 2002 ; Cramer et al. , 2006 ; Frankel et al. , 2007 ; Graves et al. , 2011 ). The disconnect between the urban and national hazard models, however, means that the national models, which underlie U.S. building codes and other applications, do not make use of all of the scientific results informing earthquake ground‐shaking hazards. We recommend that future U.S. national seismic hazard assessment make use of all available regional information, including that in urban models. In this column, we describe the roles of and differences between the urban and national models, and discuss the obstacles to and benefits of integrating the urban models with the national model. Future progress on this issue will require further coordination and implementation efforts between the USGS and external partners.

Seismological Research Letters↗

USGS2015

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

The late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley illustrates the dynamic nature of the North American–Pacific plate boundary and its effect on basin and landscape development. Prior to early Miocene time, the area that became Santa Clara Valley consisted of eroding Franciscan complex basement structurally interleaved in places with Coast Range ophiolite and Mesozoic Great Valley sequence, and locally overlapped by Paleogene strata. During early to middle Miocene time, this landscape was flooded by the sea and was deformed locally into deeper depressions such as the Cupertino Basin in the southwestern part of the valley. Marine deposition during the middle and late Miocene laid down thin deposits in shallow water and thick deeper-water deposits in the Cupertino Basin. During this sedimentation, the San Andreas fault system encroached into the valley, with most offset partitioned onto the San Andreas fault southwest of the valley and the southern Calaveras–Silver Creek–Hayward fault system in the northeastern part of the valley. A 6-km-wide right step between the Hayward and Silver Creek faults formed the 40-km-long Evergreen pull-apart basin along the northeastern margin of the valley, leaving a basement ridge between it and the Cupertino Basin. The Silver Creek fault was largely abandoned ca. 2.5 Ma in favor of a compressional left step between the Calaveras and Hayward fault, although some slip continued to at least mid-Quaternary time. Gravity, seismic, stratigraphic, and interferometric synthetic aperture radar (InSAR) data indicate no other major San Andreas system faults within the central block between the present-day range-front faults bounding the valley and the Silver Creek fault. Sometime between 9 and 4 Ma (9 and 1 Ma for the central block), the area rose above sea level, and a regional surface of erosion was carved into the Mesozoic and Tertiary rocks. Alluvial gravels were deposited on this surface along the margins of the valley beginning ca. 4 Ma, but they may not have prograded onto the central block until ca. 1 Ma, because no older equivalents of the Pliocene–Quaternary Santa Clara gravels have been found there. Thus, either the central block was high enough relative to the surrounding areas that Santa Clara gravels were never deposited on it, or any Santa Clara gravels deposited there were stripped away before ca. 1 Ma. Analysis of alluvium on the central block implies a remarkably uniform, piston-like, subsidence of the valley of ∼0.4 mm/yr since ca. 0.8 Ma, possibly extending north to northern San Francisco Bay. Today, the central block continues to subside, the range-front reverse faults are active, and the major active faults of the San Andreas system are mostly outside the valley.

California↗

USGS2015

Seismic-hazard maps for the conterminous United States, 2014

The maps presented here provide an update to the 2008 data contained in U.S Geological Survey Scientific Investigations Map 3195 ( http://pubs.usgs.gov/sim/3195/ ).Probabilistic seismic-hazard maps were prepared for the conterminous United States for 2014 portraying peak horizontal acceleration and horizontal spectral response acceleration for 0.2- and 1.0-second periods with probabilities of exceedance of 10 percent in 50 years and 2 percent in 50 years. All of the maps were prepared by combining the hazard derived from spatially smoothed historical seismicity with the hazard from fault-specific sources. The acceleration values contoured are the random horizontal component. The reference site condition is firm rock, defined as having an average shear-wave velocity of 760 m/s in the top 30 meters corresponding to the boundary between NEHRP (National Earthquake Hazards Reduction program) site classes B and C. This data set represents the results of calculations of hazard curves for a grid of points with a spacing of 0.05 degrees in latitude and longitude. The grid of points was contoured to produce the final representation of the seismic hazard. These maps are intended to summarize the available quantitative information about seismic ground motion hazard for the conterminous United States from geologic and geophysical sources.

Scientific Investigations Map↗

USGS2015

Physical subdivision and description of the water-bearing sediments of the Santa Clara Valley, California

A thick Quaternary alluvial section fills a sedimentary basin beneath the Santa Clara Valley, California, located within the San Andreas Fault system at the south end of San Francisco Bay. This section consists of an upper sequence about 1,000 feet thick containing eight sedimentary cycles and a lower fine-grained unit as thick as several hundred feet. Together these constitute the Quaternary Santa Clara Basin. The section overlies an irregular unconformity with more than 1,200 feet of relief cut into the underlying bedrock. This stratigraphy is determined through study of new wells and seismic reflection profiles, together with a sample of the many thousands of water wells in the valley. It represents a major change and improvement in understanding of the basin, particularly with regard to the upper cyclic sequence, which forms a large groundwater system that is an important resource in the San Francisco Bay region. Each of the eight sedimentary cycles consists of a coarse-grained bottom interval overlain by a fine-grained top, with the coarse bottom forming a permeable sheet that is more or less continuous around the basin and the fine top forming a similarly extensive, relatively impermeable confining layer. This stratigraphic organization contrasts with most previous views, which have considered the coarse sediment in the basin to occur as scattered, discrete lenses and (or) sinuous channel sands, all embedded in a predominantly fine-grained section. Temperature logs in several wells demonstrate that the fine cycle tops do limit vertical movement of groundwater, although this may not be the case where those tops are thin to perhaps locally absent around parts of the basin margin. Age control has been obtained from previous work, in which the sedimentary cycles were correlated with the marine oxygen isotope record and the ages of two deeper Quaternary unconformities were estimated, and from detailed paleomagnetic study of cores from the new wells by E.A. Mankinen. Despite careful search of the cores, very few fossils were found, and none that are helpful in subdividing the section. No tephra (volcanic ash) was recovered, and the few carbon samples found and dated radiometrically are limited to the upper 120 feet of the section. The upper cyclic section ranges in age from 0 to somewhat older than 718 thousand years (ka), and the lower fine-grained section lies between unconformities with estimated ages of 950 and 1500 ka. Reflections in the seismic profiles indicate that layering in the basin is subparallel to the ground surface, and this fact, together with the continuous stratigraphic detail provided by geophysical logs of the new wells, allows the confident interwell correlation required to delineate the sedimentary cycles. The sequence of layers within any one cycle tends to persist laterally between the wells in the dataset, which are spaced 1 to 3 km apart, with most changes occurring gradually. The eight cycles, in contrast, tend to differ from each other in the details of their internal organization. Maps and cross sections show the elevations of cycle boundaries and the underlying bedrock surface, the varying thicknesses of the cycles and of their fine tops and coarse bottoms, and the aggregate thickness of coarse layers in those bottom intervals. Coarse sediment is more abundant toward some parts of the basin margin and in the southern part of the basin. Cycle boundary surfaces are relatively smooth, and their shapes are consistent with having been intercycle topographic surfaces. The underlying bedrock surface has a relief of more than 1,200 feet and deepens toward the center of the basin and the west edge of the fault-bounded Evergreen Basin, which is concealed beneath the east side of the Quaternary basin. The absence of consistent abrupt changes in thicknesses or boundary elevations across the basin or in cross section indicates that the interior of the basin is largely unfaulted, with the Silver Creek strand of the San Andreas system at the west edge of the Evergreen Basin being the sole exception. The east and west margins of the Santa Clara Basin, in contrast, are marked by reverse and thrust fault systems.

California↗

USGS2014

Ground-motion site effects from multimethod shear-wave velocity characterization at 16 seismograph stations deployed for aftershocks of the August 2011 Mineral, Virginia earthquake

We characterize shear-wave velocity versus depth (Vs profile) at 16 portable seismograph sites through the epicentral region of the 2011 M w 5.8 Mineral (Virginia, USA) earthquake to investigate ground-motion site effects in the area. We used a multimethod acquisition and analysis approach, where active-source horizontal shear (SH) wave reflection and refraction as well as active-source multichannel analysis of surface waves (MASW) and passive-source refraction microtremor (ReMi) Rayleigh wave dispersion were interpreted separately. The time-averaged shear-wave velocity to a depth of 30 m (Vs30), interpreted bedrock depth, and site resonant frequency were estimated from the best-fit Vs profile of each method at each location for analysis. Using the median Vs30 value (270–715 m/s) as representative of a given site, we estimate that all 16 sites are National Earthquake Hazards Reduction Program (NEHRP) site class C or D. Based on a comparison of simplified mapped surface geology to median Vs30 at our sites, we do not see clear evidence for using surface geologic units as a proxy for Vs30 in the epicentral region, although this may primarily be because the units are similar in age (Paleozoic) and may have similar bulk seismic properties. We compare resonant frequencies calculated from ambient noise horizontal:vertical spectral ratios (HVSR) at available sites to predicted site frequencies (generally between 1.9 and 7.6 Hz) derived from the median bedrock depth and average Vs to bedrock. Robust linear regression of HVSR to both site frequency and Vs30 demonstrate moderate correlation to each, and thus both appear to be generally representative of site response in this region. Based on Kendall tau rank correlation testing, we find that Vs30 and the site frequency calculated from average Vs to median interpreted bedrock depth can both be considered reliable predictors of weak-motion site effects in the epicentral region.

Virginia↗

USGS2014

Documentation for the 2014 update of the United States national seismic hazard maps

The national seismic hazard maps for the conterminous United States have been updated to account for new methods, models, and data that have been obtained since the 2008 maps were released (Petersen and others, 2008). The input models are improved from those implemented in 2008 by using new ground motion models that have incorporated about twice as many earthquake strong ground shaking data and by incorporating many additional scientific studies that indicate broader ranges of earthquake source and ground motion models. These time-independent maps are shown for 2-percent and 10-percent probability of exceedance in 50 years for peak horizontal ground acceleration as well as 5-hertz and 1-hertz spectral accelerations with 5-percent damping on a uniform firm rock site condition (760 meters per second shear wave velocity in the upper 30 m, V S30 ). In this report, the 2014 updated maps are compared with the 2008 version of the maps and indicate changes of plus or minus 20 percent over wide areas, with larger changes locally, caused by the modifications to the seismic source and ground motion inputs.

Open-File Report↗

USGS2014

Response and recovery lessons from the 2010-2011 earthquake sequence in Canterbury, New Zealand

The impacts and opportunities that result when low-probability moderate earthquakes strike an urban area similar to many throughout the US were vividly conveyed in a one-day workshop in which social and Earth scientists, public officials, engineers, and an emergency manager shared their experiences of the earthquake sequence that struck the city of Christchurch and surrounding Canterbury region of New Zealand in 2010-2011. Without question, the earthquake sequence has had unprecedented impacts in all spheres on New Zealand society, locally to nationally--10% of the country's population was directly impacted and losses total 8-10% of their GDP. The following paragraphs present a few lessons from Christchurch.

Canterbury↗

USGS2013

Vs30 and spectral response from collocated shallow, active- and passive-source Vs data at 27 sites in Puerto Rico

Shear‐wave velocity (V S ) and time‐averaged shear‐wave velocity to 30 m depth (V S30 ) are the key parameters used in seismic site response modeling and earthquake engineering design. Where V S data are limited, available data are often used to develop and refine map‐based proxy models of V S30 for predicting ground‐motion intensities. In this paper, we present shallow V S data from 27 sites in Puerto Rico. These data were acquired using a multimethod acquisition approach consisting of noninvasive, collocated, active‐source body‐wave (refraction/reflection), active‐source surface wave at nine sites, and passive‐source surface‐wave refraction microtremor (ReMi) techniques. V S ‐versus‐depth models are constructed and used to calculate spectral response plots for each site. Factors affecting method reliability are analyzed with respect to site‐specific differences in bedrock V S and spectral response. At many but not all sites, body‐ and surface‐wave methods generally determine similar depths to bedrock, and it is the difference in bedrock V S that influences site amplification. The predicted resonant frequencies for the majority of the sites are observed to be within a relatively narrow bandwidth of 1–3.5 Hz. For a first‐order comparison of peak frequency position, predictive spectral response plots from eight sites are plotted along with seismograph instrument spectra derived from the time series of the 16 May 2010 Puerto Rico earthquake. We show how a multimethod acquisition approach using collocated arrays compliments and corroborates V S results, thus adding confidence that reliable site characterization information has been obtained.

Bulletin of the Seismological Society of America↗

USGS2012

Seismic velocity model of the central United States (Version 1): Description and simulation of the 18 April 2008 Mt. Carmel, Illinois, Earthquake

We have developed a new three‐dimensional seismic velocity model of the central United States (CUSVM) that includes the New Madrid Seismic Zone (NMSZ) and covers parts of Arkansas, Mississippi, Alabama, Illinois, Missouri, Kentucky, and Tennessee. The model represents a compilation of decades of crustal research consisting of seismic, aeromagnetic, and gravity profiles; geologic mapping; geophysical and geological borehole logs; and inversions of the regional seismic properties. The density, P ‐ and S ‐wave velocities are synthesized in a stand‐alone spatial database that can be queried to generate the required input for numerical seismic‐wave propagation simulations. We test and calibrate the CUSVM by simulating ground motions of the 18 April 2008 M w 5.4 Mt. Carmel, Illinois, earthquake and comparing the results with observed records within the model area. The selected stations in the comparisons reflect different geological site conditions and cover distances ranging from 10 to 430 km from the epicenter. The results, based on a qualitative and quantitative goodness‐of‐fit (GOF) characterization, indicate that both within and outside the Mississippi Embayment the CUSVM reasonably reproduces: (1) the body and surface‐wave arrival times and (2) the observed regional variations in ground‐motion amplitude, cumulative energy, duration, and frequency content up to a frequency of 1.0 Hz. In addition, we discuss the probable structural causes for the ground‐motion patterns in the central United States that we observed in the recorded motions of the 18 April Mt. Carmel earthquake.

Illinois↗

USGS2012

The 2011 Virginia earthquake: what are scientists learning?

Nearly 1 year ago, on 23 August, tens of millions of people in the eastern United States and southeastern Canada were startled in the middle of their workday (1:51 P.M. local time) by the sudden onset of moderate to strong ground shaking from a rare magnitude (M) 5.8 earthquake in central Virginia. Treating the shaking as if it were a fire drill, millions of workers in Washington, D. C., New York City, and other eastern cities hurriedly exited their buildings, exposing themselves to potentially greater danger from falling bricks and glass; “drop, cover, and hold” would have been a better response. Fortunately, the strong shaking stopped after about 5 seconds and did not cause widespread severe damage or serious injuries.

Virginia↗

USGS2012

Characterization of intrabasin faulting and deformation for earthquake hazards in southern Utah Valley, Utah, from high-resolution seismic imaging

We conducted active and passive seismic imaging investigations along a 5.6-km-long, east–west transect ending at the mapped trace of the Wasatch fault in southern Utah Valley. Using two-dimensional (2D) P-wave seismic reflection data, we imaged basin deformation and faulting to a depth of 1.4 km and developed a detailed interval velocity model for prestack depth migration and 2D ground-motion simulations. Passive-source microtremor data acquired at two sites along the seismic reflection transect resolve S-wave velocities of approximately 200 m/s at the surface to about 900 m/s at 160 m depth and confirm a substantial thickening of low-velocity material westward into the valley. From the P-wave reflection profile, we interpret shallow (100–600 m) bedrock deformation extending from the surface trace of the Wasatch fault to roughly 1.5 km west into the valley. The bedrock deformation is caused by multiple interpreted fault splays displacing fault blocks downward to the west of the range front. Further west in the valley, the P-wave data reveal subhorizontal horizons from approximately 90 to 900 m depth that vary in thickness and whose dip increases with depth eastward toward the Wasatch fault. Another inferred fault about 4 km west of the mapped Wasatch fault displaces horizons within the valley to as shallow as 100 m depth. The overall deformational pattern imaged in our data is consistent with the Wasatch fault migrating eastward through time and with the abandonment of earlier synextensional faults, as part of the evolution of an inferred 20-km-wide half-graben structure within Utah Valley. Finite-difference 2D modeling suggests the imaged subsurface basin geometry can cause fourfold variation in peak ground velocity over distances of 300 m.

Utah↗

USGS2011

Putting down roots in earthquake country: Your handbook for earthquakes in the Central United States

This handbook provides information to residents of the Central United States about the threat of earthquakes in that area, particularly along the New Madrid seismic zone, and explains how to prepare for, survive, and recover from such events. It explains the need for concern about earthquakes for those residents and describes what one can expect during and after an earthquake. Much is known about the threat of earthquakes in the Central United States, including where they are likely to occur and what can be done to reduce losses from future earthquakes, but not enough has been done to prepare for future earthquakes. The handbook describes such preparations that can be taken by individual residents before an earthquake to be safe and protect property.

Alabama, Arkansas, Illinois, Indiana, Kentucky, Mi↗

USGS2010

The Quaternary Silver Creek Fault Beneath the Santa Clara Valley, California

The northwest-trending Silver Creek Fault is a 40-km-long strike-slip fault in the eastern Santa Clara Valley, California, that has exhibited different behaviors within a changing San Andreas Fault system over the past 10-15 Ma. Quaternary alluvium several hundred meters thick that buries the northern half of the Silver Creek Fault, and that has been sampled by drilling and imaged in a detailed seismic reflection profile, provides a record of the Quaternary history of the fault. We assemble evidence from areal geology, stratigraphy, paleomagnetics, ground-water hydrology, potential-field geophysics, and reflection and earthquake seismology to determine the long history of the fault in order to evaluate its current behavior. The fault formed in the Miocene more than 100 km to the southeast, as the southwestern fault in a 5-km-wide right step to the Hayward Fault, within which the 40-km-long Evergreen pull-apart basin formed. Later, this basin was obliquely cut by the newly recognized Mt. Misery Fault to form a more direct connection to the Hayward Fault, although continued growth of the basin was sufficient to accommodate at least some late Pliocene alluvium. Large offset along the San Andreas-Calaveras-Mt Misery-Hayward Faults carried the basin northwestward almost to its present position when, about 2 Ma, the fault system was reorganized. This led to near abandonment of the faults bounding the pull-apart basin in favor of right slip extending the Calaveras Fault farther north before stepping west to the Hayward Fault, as it does today. Despite these changes, the Silver Creek Fault experienced a further 200 m of dip slip in the early Quaternary, from which we infer an associated 1.6 km or so of right slip, based on the ratio of the 40-km length of the strike-slip fault to a 5-km depth of the Evergreen Basin. This dip slip ends at a mid-Quaternary unconformity, above which the upper 300 m of alluvial cover exhibits a structural sag at the fault that we interpret as a negative flower structure. This structure implies some continuing strike slip on the Silver Creek Fault in the late Quaternary as well, with a transtensional component but no dip slip. Our only basis for estimating the rate of this later Quaternary strike slip on the Silver Creek Fault is to assume continuation of the inferred early Quaternary rate of less than 2 mm/yr. Faulting evident in a detailed seismic reflection profile across the Silver Creek Fault extends up to the limit of data at a depth of 50 m and age of about 140 ka, and the course of Coyote Creek suggests Holocene capture in a structural depression along the fault. No surface trace is evident on the alluvial plain, however, and convincing evidence of Holocene offset is lacking. Few instrumentally recorded earthquakes are located near the fault, and those that are near its southern end represent cross-fault shortening, not strike slip. The fault might have been responsible, however, for two poorly located moderate earthquakes that occurred in the area in 1903. Its southeastern end does mark an abrupt change in the pattern of abundant instrumentally recorded earthquakes along the Calaveras Fault-in both its strike and in the depth distribution of hypocenters-that could indicate continuing influence by the Silver Creek Fault. In the absence of convincing evidence to the contrary, and as a conservative estimate, we presume that the Silver Creek Fault has continued its strike-slip movement through the Holocene, but at a very slow rate. Such a slow rate would, at most, yield very infrequent damaging earthquakes. If the 1903 earthquakes did, in fact, occur on the Silver Creek Fault, they would have greatly reduced the short-term future potential for large earthquakes on the fault.

Open-File Report↗