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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 1,243 records · Page 69Linked to original sources
USGS1995

Preliminary map of peak horizontal ground acceleration for the Hanshin-Awaji earthquake of January 17, 1995, Japan - Description of Mapped Data Sets

The Hanshin-Awaji earthquake (also known as the Hyogo-ken Nanbu and the Great Hanshin earthquake) provided an unprecedented set of measurements of strong ground shaking. The measurements constitute the most comprehensive set of strong- motion recordings yet obtained for sites underlain by soft soil deposits of Holocene age within a few kilometers of the crustal rupture zone. The recordings, obtained on or near many important structures, provide an important new empirical data set for evaluating input ground motion levels and site amplification factors for codes and site-specific design procedures world wide. This report describes the data used to prepare a preliminary map summarizing the strong motion data in relation to seismicity and underlying geology (Wentworth, Borcherdt, and Mark., 1995; Figure 1, hereafter referred to as Figure 1/I). The map shows station locations, peak acceleration values, and generalized acceleration contours superimposed on pertinent seismicity and the geologic map of Japan. The map (Figure 1/I) indicates a zone of high acceleration with ground motions throughout the zone greater than 400 gal and locally greater than 800 gal. This zone encompasses the area of most intense damage mapped as JMA intensity level 7, which extends through Kobe City. The zone of most intense damage is parallel, but displaced slightly from the surface projection of the crustal rupture zone implied by aftershock locations. The zone is underlain by soft-soil deposits of Holocene age.

Open-File Report↗
USGS1997

Offset-vertical seismic profiling for marine gas hydrate exploration: Is it a suitable technique? First results from ODP Leg 164

Walkaway vertical seismic profiles were acquired during Ocean Drilling Project (ODP) Leg 164 at the Blake Ridge to investigate seismic properties of hydrate-bearing sediments and the zone of free gas beneath them. An evaluation of compressional (P-) wave arrivals Site 994 indicates P-wave anisotrophy in the sediment column. We identified several shear (S-) wave arrivals in the horizontal components of the geophone array in the borehole and in data recorded with an ocean bottom seismometer deployed at the seafloor. S-waves were converted from P-waves at several depth levels in the sediment column. One of the most prominent conversion points appears to be the bottom simulating reflector (BSR). It is likely that other conversion points are located in the zone of low P-wave reflectivity above the BSR. Modeling suggests that a change of the shear modulus is sufficient to cause significant shear conversion without a significant normal-incidence P-wave reflection.

North Carolina, South Carolina↗
USGS1999

Seismic subduction of the Nazca Ridge as shown by the 1996-97 Peru earthquakes

By rupturing more than half of the shallow subduction interface of the Nazca Ridge, the great November 12, 1996 Peruvian earthquake contradicts the hypothesis that oceanic ridges subduct aseismically. The mainshock's rupture has a length of about 200 km and has an average slip of about 1.4 m. Its moment is 1.5 x 1028 dyne-cm and the corresponding M(w) is 8.0. The mainshock registered three major episodes of moment release as shown by a finite fault inversion of teleseismically recorded broadband body waves. About 55% of the mainshock's total moment release occurred south of the Nazca Ridge, and the remaining moment release occurred at the southern half of the subduction interface of the Nazca Ridge. The rupture south of the Nazca Ridge was elongated parallel to the ridge axis and extended from a shallow depth to about 65 km depth. Because the axis of the Nazca Ridge is at a high angle to the plate convergence direction, the subducting Nazca Ridge has a large southwards component of motion, 5 cm/yr parallel to the coast. The 900-1200 m relief of the southwards sweeping Nazca Ridge is interpreted to act as a 'rigid indenter,' causing the greatest coupling south of the ridge's leading edge and leading to the large observed slip. The mainshock and aftershock hypocenters were relocated using a new procedure that simultaneously inverts local and teleseismic data. Most aftershocks were within the outline of the Nazca Ridge. A three-month delayed aftershock cluster' occurred at the northern part of the subducting Nazca Ridge. Aftershocks were notably lacking at the zone of greatest moment release, to the south of the Nazca Ridge. However, a lone foreshock at the southern end of this zone, some 140 km downstrike of the mainshock's epicenter, implies that conditions existed for rupture into that zone. The 1996 earthquake ruptured much of the inferred source zone of the M(w) 7.9-8.2 earthquake of 1942, although the latter was a slightly larger earthquake. The rupture zone of the 1996 earthquake is immediately north of the seismic gap left by the great earthquakes (M(w) ~8.8-9.1) of 1868 and 1877. The M(w) 8.0 Antofagasta earthquake of 1995 occurred at the southern end of this great seismic gap. The M(w) 8.2 deep-focus Bolivian earthquake of 1994 occurred directly downdip of the 1868 portion of that gap. The recent occurrence of three significant earthquakes on the periphery of the great seismic gap of the 1868 and 1877 events, among other factors, may signal an increased seismic potential for that zone.

Pure and Applied Geophysics↗
USGS1991

Circum-Pacific seismic potential: 1989-1999

The seismic potential for 96 segments of simple plate boundaries around the circum-Pacific region is presented in terms of the conditional probability for the occurrence of either large or great interplate earthquakes during the next 5, 10, and 20 years (i.e., 1989-1994, 1989-1999 and 1989-2009). This study represents the first probabilistic summary of seismic potential on this scale, and involves the comparison of plate boundary segments that exhibit varying recurrence times, magnitudes, and tectonic regimes. Presenting these data in a probabilistic framework provides a basis for the uniform comparison of seismic hazard between these differing fault segments, as well as accounting for individual variations in recurrence time along a specific fault segment, and uncertainties in the determination of the average recurrence time. The definition of specific segments along simple plate boundaries relies on the mapping of earthquake rupture zones as defined by the aftershock distributions of prior large and great earthquakes, and historic descriptions of felt intensities and damage areas. The 96 segments are chosen to represent areas likely to be ruptured by "characteristic" earthquakes of a specified size or magnitude. The term characteristic implies repeated breakage of a plate boundary segment by large or great earthquakes whose source dimensions are similar from cycle to cycle. This definition does not exclude the possibility that occasionally adjacent characteristic earthquake segments may break together in a single, larger event. Conversely, a segment may also break in a series of smaller ruptures. Estimates of recurrence times and conditional probabilities for characteristic earthquakes along segments of simple plate boundaries are based on 1) the historic and instrumental record of large and great earthquake occurrence; 2) paleoseismic evidence of recurrence from radiometric dating of Holocene features produced by earthquakes; 3) direct calculations of recurrence time from the size of the most recent characteristic event and the long-term rates of plate motion assuming the validity of the time-predictable model for earthquake recurrence; and 4) the application of a lognormal distribution for the recurrence times of large and great earthquakes. Time-dependent estimates of seismic potential are based on a physical model of earthquake occurrence which assumes that the probability for an earthquake is low immediately following the occurrence of a characteristic earthquake and increases with time as the stress on the fault segment recovers the stress drop of the event. This study updates earlier work on seismic gaps by explicitly including both recurrence time information and the temporal proximity to the next event as factors in describing earthquake hazards. Currently, 11 out of 96 regions have a high (i.e., ???50%) probability of recurrence during the next 10 years and are characterized by either fairly short (i.e., less than 30-40 years) recurrence times or long elapsed times relative to the average recurrence time. The majority of these segments are located in the southwest Pacific (Vanuatu, New Guinea, and Tonga). When a longer time window is considered (e.g., 20 years or 1989-2009), 30 out of 96 regions have a high potential. Many of these regions are located near areas of high population density. These determinations do not preclude rupture of other fault segments, with less than a 50% chance in 10 or 20 years, or large and great earthquakes in areas we have not studied in detail. While this study has summarized the seismic potential for a large number of regions around the circum-Pacific, there are still a number of geographic and seismotectonic regions that need to be considered, including Indonesia, the Philippines, New Zealand, and the countries that surround the Caribbean basin. ?? 1991 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS2026

Spatial and temporal geochemical variations of lava flows and tephra deposits from the December 2020 to September 2024 eruptions of Kīlauea volcano

Kīlauea volcano underwent dramatic morphological changes in 2018. That year recorded the end of the 35-year-long eruption of Puʻuʻōʻō (1983–2018) and 10-year-long (2008–2018) Halemaʻumaʻu lava lake and emplacement of the ~4-month-long lower East Rift Zone lava flows that coincided with ~500 m of summit caldera collapse. Starting on December 20, 2020, eruptions resumed at Kīlauea’s summit. There were five summit eruptions between December 2020 and September 2023, which ranged in duration from more than a year to as short as a week. Following these summit eruptions, seismicity and deformation increased in the upper Southwest Rift Zone in 2024, culminating in a ~8.5-h-long eruption in this region on June 3, 2024. Increased seismicity and deformation then shifted to the upper and middle East Rift Zone and after several months culminated in an eruption just west of, and within, Nāpau Crater in the middle East Rift Zone from September 15 to 20, 2024. Despite vast morphological changes at Kīlauea’s summit, the geochemical compositions (i.e., whole rock and glass) that erupted from December 2020 to September 2023 are all remarkably similar to each other. Whole-rock compositions appear distinct from the preceding 2008–2018 Halemaʻumaʻu lava lake and phase 3 (i.e., summit or uprift-derived mafic lavas) of the 2018 lower East Rift Zone lava flows, although glass compositions appear to have more overlap with 2018 lower East Rift Zone glasses. The June 3, 2024, upper Southwest Rift Zone spatter and lava flows exhibit a dramatic enrichment in whole-rock MgO that is not recorded in glass, which reflects accumulation of olivine (e.g., antecrysts or xenocrysts) during dike emplacement, and is consistent with the abundance of olivine in the lava flows (5–10%). June 2024 Southwest Rift Zone whole-rock and glass compositions overlap with those erupted at the summit from December 2020 to September 2023, whereas some whole-rock trace (i.e., Sc, Sr, and Zr) and major elements (i.e., CaO) are suggestive of mixing with a magmatic component that had fractionated plagioclase and pyroxene and/or a new parental magma influencing the summit reservoir system. The September 15–20, 2024, eruption at Nāpau Crater in the middle East Rift Zone involved the most differentiated magma since eruptive activity resumed in December 2020, with its magma fractionating olivine + plagioclase + pyroxene. The September 15–20, 2024, composition resembles Puʻuʻōʻō lava flows that erupted in, or near, Nāpau Crater in 1983 (episode 1), 1997 (episode 54), and 2011 (episode 59), with episode 59 having a compositional cluster that is most similar to that of the September 2024 lava flows. The data presented and provided herein open new research perspectives for long-term analyses of geochemical variations following caldera collapse at Kīlauea volcano and facilitate comparisons with other basaltic caldera systems worldwide.

Hawaii↗
USGS2015

Pore-pressure sensitivities to dynamic strains: observations in active tectonic regions

Triggered seismicity arising from dynamic stresses is often explained by the Mohr-Coulomb failure criterion, where elevated pore pressures reduce the effective strength of faults in fluid-saturated rock. The seismic response of a fluid-rock system naturally depends on its hydro-mechanical properties, but accurately assessing how pore-fluid pressure responds to applied stress over large scales in situ remains a challenging task; hence, spatial variations in response are not well understood, especially around active faults. Here I analyze previously unutilized records of dynamic strain and pore-pressure from regional and teleseismic earthquakes at Plate Boundary Observatory (PBO) stations from 2006 through 2012 to investigate variations in response along the Pacific/North American tectonic plate boundary. I find robust scaling-response coefficients between excess pore pressure and dynamic strain at each station that are spatially correlated: around the San Andreas and San Jacinto fault systems, the response is lowest in regions of the crust undergoing the highest rates of secular shear strain. PBO stations in the Parkfield instrument cluster are at comparable distances to the San Andreas fault (SAF), and spatial variations there follow patterns in dextral creep rates along the fault, with the highest response in the actively creeping section, which is consistent with a narrowing zone of strain accumulation seen in geodetic velocity profiles. At stations in the San Juan Bautista (SJB) and Anza instrument clusters, the response depends non-linearly on the inverse fault-perpendicular distance, with the response decreasing towards the fault; the SJB cluster is at the northern transition from creeping-to-locked behavior along the SAF, where creep rates are at moderate to low levels, and the Anza cluster is around the San Jacinto fault, where to date there have been no statistically significant creep rates observed at the surface. These results suggest that the strength of the pore pressure response in fluid-saturated rock near active faults is controlled by shear strain accumulation associated with tectonic loading, which implies a strong feedback between fault strength and permeability: dynamic triggering susceptibilities may vary in space and also in time.

Journal of Geophysical Research B: Solid Earth↗
USGS2021

Spatiotemporal clustering of great earthquakes on a transform fault controlled by geometry

Minor changes in geometry along the length of mature strike-slip faults may act as conditional barriers to earthquake rupture, terminating some and allowing others to pass. This hypothesis remains largely untested because palaeoearthquake data that constrain spatial and temporal patterns of fault rupture are generally imprecise. Here we develop palaeoearthquake event data that encompass the last 20 major-to-great earthquakes along approximately 320 km of the Alpine Fault in New Zealand with sufficient temporal resolution and spatial coverage to reveal along-strike patterns of rupture extent. The palaeoearthquake record shows that earthquake terminations tend to cluster in time near minor along-strike changes in geometry. These terminations limit the length to which rupture can grow and produce two modes of earthquake behaviour characterized by phases of major ( M w 7–8) and great ( M w > 8) earthquakes. Physics-based simulations of seismic cycles closely resemble our observations when parameterized with realistic fault geometry. Switching between the rupture modes emerges due to heterogeneous stress states that evolve over multiple seismic cycles in response to along-strike differences in geometry. These geometric complexities exert a first-order control on rupture behaviour that is not currently accounted for in fault-source models for seismic hazard.

South Island↗
USGS2016

Responses of two tall buildings in Tokyo, Japan, before, during, and after the M9.0 Tohoku earthquake of 11 March 2011

The 11 March 2011 M 9.0 Tohoku earthquake generated significant long duration shaking that propagated hundreds of kilometers from the epicenter and affected urban areas throughout much of Honshu. Recorded responses of tall buildings at several hundred km from the epicenter of the main shock and other events show tall buildings were affected by long-period motions of events at distant sources. This study presents behavioral aspects of 29-story and 30-story neighboring buildings in the Shinjuku area of Tokyo, Japan, as inferred from records retrieved from a sparse array of accelerometers deployed in the superstructures, at ground and 100 m below the ground level over a time interval covering before, during, and after the main shock. Such long-period effects are common in several regions of Japan as well as in the United States and in other seismically active countries. Permanent shifts in fundamental frequencies are observed. Drift ratios indicate possible structural nonlinear behavior occurred during the main shock. The need to consider risks to built environments from distant sources, including those in neighboring countries, is emphasized.

Tokyo↗
USGS2004

Two stages of deformation and fluid migration in the west-central Brooks Range fold and thrust belt, Northern Alaska

The Brooks Range is a north-directed fold and thrust belt that forms the southern boundary of the North Slope petroleum province in northern Alaska. Field-based studies have long recognized that large-magnitude, thin-skinned folding and thrusting in the Brooks Range occurred during arc-continent collision in the Middle Jurassic to Early Cretaceous (Neocomian). Folds and thrusts, however, also deform middle and Upper Cretaceous strata of the Colville foreland basin and thus record a younger phase of deformation that apatite fission-track data have shown to occur primarily during the early Tertiary (~60 and ~45 Ma). A structural and kinematic model that reconciles these observations is critical to understanding the petroleum system of the Brooks Range fold and thrust belt. New interpretations of outcrop and regional seismic reflection data indicate that from the modern mountain front northward to near the deformation front under the coastal plain, the basal thrust detachment for the orogen is located in the Jurassic and Lower Cretaceous Kingak Shale in the upper part of the regionally extensive, gently south-dipping, north-derived Mississippian to Early Cretaceous Ellesmerian sequence. The frontal part of the orogen lies in middle Cretaceous foreland basin strata and consists of a thin-skinned fold belt at the deformation front and a fully developed passive-roof duplex to the south. Near the mountain front, the orogen is composed of a stacked series of allochthons and thrust duplexes and associated Neocomian syntectonic deposits that are unconformably overlain by proximal foreland basin strata. The foreland basin strata and underlying deformed rocks are truncated by a younger generation of folds and thrusts. Vitrinite reflectance and stable isotope compositions of veins provide evidence of two fluid events in these rocks, including an earlier higher temperature (~250-300°C) event that was buffered by limestone and a younger, lower temperature (~150°C) event that had distinctly lower δ 13 C values as a result of oxidation of organic matter and/or methane. Zircon fission-track data from the host rocks of the veins show that the higher temperature fluid event occurred at 160-120 Ma, whereas the lower temperature event probably occurred at about 60-45 Ma. It is proposed that the Brooks Range consists of two superposed contractional orogens that used many of the same mechanically incompetent stratigraphic units (e.g., Kayak Shale, Kingak Shale) as sites of thrust detachment. The older orogen formed in a north-directed arc-continent collisional zone that was active from 160 to 120 Ma. This deformation produced a thin-skinned deformational wedge that is characterized by far-traveled allochthons with relatively low structural relief, because it involved a thin (1-4-km [0.6-2.5-mi]-thick) stratigraphic section. Deeper parts of the deformational wedge are envisioned to have contained relatively high-temperature fluids that presumably migrated from or through limestone-rich source areas in the underlying autochthon or from deeper parts of the orogen. The younger orogen, which formed initially at about 60 Ma and reactivated at 45 Ma, produced a thrust belt and frontal triangle zone with low amounts of shortening and relatively high structural relief, because it involved a structural section 5-10 km (3-6 mi) thick. Fluids associated with this deformation were relatively of lower temperature and suggest that hydrocarbon migration occurred at this time. We conclude that hydrocarbon generation from Triassic and Jurassic source strata and migration into stratigraphic traps occurred primarily by sedimentary burial principally at 100-90 Ma, between the times of the two major episodes of deformation. Subsequent sedimentary burial caused deep stratigraphic traps to become overmature, cracking oil to gas, and initiated some new hydrocarbon generation progressively higher in the section. Structural disruption of the traps in the early Tertiary released sequestered hydrocarbons. The hydrocarbons remigrated into newly formed structural traps, which formed at higher structural levels or were lost to the surface. Because of the generally high maturation of the Colville basin at the time of the deformation and remigration, most of the hydrocarbons available to fill traps were gas.

Alaska↗
USGS2017

Geomorphology, denudation rates, and stream channel profiles reveal patterns of mountain building adjacent to the San Andreas fault in northern California, USA

Relative horizontal motion along strike-slip faults can build mountains when motion is oblique to the trend of the strike-slip boundary. The resulting contraction and uplift pose off-fault seismic hazards, which are often difficult to detect because of the poor vertical resolution of satellite geodesy and difficulty of locating offset datable landforms in active mountain ranges. Sparse geomorphic markers, topographic analyses, and measurement of denudation allow us to map spatiotemporal patterns of uplift along the northern San Andreas fault. Between Jenner and Mendocino, California, emergent marine terraces found southwest of the San Andreas fault record late Pleistocene uplift rates between 0.20 and 0.45 mm yr –1 along much of the coast. However, on the northeast side of the San Andreas fault, a zone of rapid uplift (0.6–1.0 mm yr –1 ) exists adjacent to the San Andreas fault, but rates decay northeastward as the coast becomes more distant from the San Andreas fault. A newly dated 4.5 Ma shallow-marine deposit located at ∼500 m above sea level (masl) adjacent to the San Andreas fault is warped down to just 150 masl 15 km northeast of the San Andreas fault, and it is exposed at just 60–110 masl to the west of the fault. Landscape denudation rates calculated from abundance of cosmogenic radionuclides in fluvial sediment northeast of, and adjacent to, the San Andreas fault are 0.16–0.29 mm yr –1 , but they are only 0.03–0.07 mm yr –1 west of the fault. Basin-average channel steepness and the denudation rates can be used to infer the erosive properties of the underlying bedrock. Calibrated erosion rates can then be estimated across the entire landscape using the spatial distribution of channel steepness with these erosive properties. The lower-elevation areas of this landscape that show high channel steepness (and hence calibrated erosion rate) are distinct from higher-elevation areas with systematically lower channel steepness and denudation rates. These two areas do not appear to be coincident with lithologic contacts. Assuming that changes in rock uplift rates are manifest in channel steepness values as an upstream-propagating kinematic wave that separates high and low channel steepness values, the distance that this transition has migrated vertically provides an estimate of the timing of rock uplift rate increase. This analysis suggests that rock uplift rates along the coast changed from 0.3 to 0.75 mm yr –1 between 450 and 350 ka. This zone of recent, relatively rapid crustal deformation along the plate boundary may be a result of the impingement of relatively strong crust underlying the Gualala block into the thinner, weaker oceanic crust left at the western margin of the North American plate by the westward migration of the subduction zone prior to establishment of the current transform plate boundary. The warped Pliocene marine deposits and the presence of a topographic ridge support the patterns indicated by the channel steepness analyses, and further indicate that the zone of rapid uplift may herald elevated off-fault seismic hazard if this uplift is created by periodic stick-slip motion on contractional structures.

California↗
USGS2017

Effects of deep basins on structural collapse during large subduction earthquakes

Deep sedimentary basins are known to increase the intensity of ground motions, but this effect is implicitly considered in seismic hazard maps used in U.S. building codes. The basin amplification of ground motions from subduction earthquakes is particularly important in the Pacific Northwest, where the hazard at long periods is dominated by such earthquakes. This paper evaluates the effects of basins on spectral accelerations, ground-motion duration, spectral shape, and structural collapse using subduction earthquake recordings from basins in Japan that have similar depths as the Puget Lowland basin. For three of the Japanese basins and the Puget Lowland basin, the spectral accelerations were amplified by a factor of 2 to 4 for periods above 2.0 s. The long-duration subduction earthquakes and the effects of basins on spectral shape combined, lower the spectral accelerations at collapse for a set of building archetypes relative to other ground motions. For the hypothetical case in which these motions represent the entire hazard, the archetypes would need to increase up to 3.3 times its strength to compensate for these effects.

Earthquake Spectra↗
USGS2016

Processing of multichannel seismic reflection data acquired in 2013 for seismic investigations of gas hydrates in the Gulf of Mexico

As part of a cooperative effort among the U.S. Geological Survey (USGS), the U.S. Department of Energy, and the U.S. Department of the Interior Bureau of Ocean Energy Management, two grids of two-dimensional multichannel seismic reflection data were acquired in the Gulf of Mexico over lease blocks Green Canyon 955 and Walker Ridge 313 between April 18 and May 3, 2013. The purpose of the data acquisition was to fill knowledge gaps in an ongoing study of known gas hydrate accumulations in the area. These data were initially processed onboard the recording ship R/V Pelican for more quality control during the recording. The data were subsequently processed in detail by the U.S. Geological Survey in Denver, Colorado, in two phases. The first phase was to create a “kinematic” dataset that removed extensive noise present in the data but did not preserve relative amplitudes. The second phase was to create a true relative amplitude dataset that included noise removal and “wavelet” deconvolution that preserved the amplitude information. This report describes the processing techniques used to create both datasets.

Open-File Report↗
USGS2009

Introduction to paleoenvironments of Bear Lake, Utah and Idaho, and its catchment

In 1996 a group led by the late Kerry Kelts (University of Minnesota) and Robert Thompson (U.S. Geological Survey) acquired three piston cores (BL96-1, -2, and -3) from Bear Lake. The coring arose from their recognition of Bear Lake as a potential repository of long records of paleoenvironmental change. They recognized that the lake is located in an area that is sensitive to changes in regional climate patterns (Dean et al., this volume), that the lake basin is long lived (see Colman, 2006; Kaufman et al., this volume), and that, unlike many lakes in the Great Basin, Bear Lake was never dry during warm dry periods. Bear Lake lies in the northeastern Great Basin to the northeast of Great Salt Lake, just south of the Snake River drainage, and a short distance west of the Green River drainage that makes up part of the Upper Colorado River Basin (Fig. 1). Similarity among the historic Bear Lake and Great Salt Lake hydrographs and flows on the Green River indicates that the hydrology of Bear Lake reflects regional precipitation (Fig. 2). Therefore, paleorecords from Bear Lake are important to understanding past climate for a large region, including the Upper Colorado River Basin, the source of much of the water for the southwestern United States. Initially, paleoenvironmental studies of Bear Lake sediments focused on cores BL96-1, -2, and -3. Additional coring was conducted to elucidate the spatial distribution of sedimentary units and to extend the record back in time. The study was also expanded to include extensive study of the catchment, including the properties of catchment materials and the processes that could potentially affect the delivery of catchment materials to the lake. Cores BL96-1, -2, and -3 were taken with a Kullenburg piston corer along an east–west profile in roughly 50, 40, and 30 m of water, respectively (Table 1, Fig. 3). These three cores, each taken as a single 4- to 5-m-long segment, provide a nearly complete composite section from ca. 26 cal ka to the late Holocene. In 1998 a number of short gravity cores were taken from the uppermost water-rich sediments that were not sampled by the 1996 cores. During 2000, cores were taken with a percussion piston corer (manufactured by UWITEC) at three locations in and around Mud Lake and at two locations in the northern end of Bear Lake (Fig. 3). Cores acquired with the percussion corer comprise as many as three overlapping segments up to 2 m in length. In 2002, additional percussion piston cores and associated gravity cores of the uppermost sediments were acquired from five sites in the northern half of the lake. In conjunction with two of the cores collected in 2000, these cores form a north–south profile along a seismic line and span water depths from less than 10 m to ~40 m. Data from this profile provide much of the evidence for lake-level variations (Smoot and Rosenbaum, this volume). Finally, during 2000, two long cores, BL00-1D and -1E (collectively referred to here simply as BL00-1), were taken at a site near the depocenter during testing of the GLAD800 coring platform (Fig. 4; Dean et al., 2002). These cores provide a record back to ca. 220 ka.

Idaho;Utah;Wyoming↗
USGS2020

Kinematic rupture and 3D wave propagation simulations of the 2019 Mw 7.1 Ridgecrest, California, earthquake

We model the kinematic rupture process of the 2019 M w "> M w Mw 7.1 Ridgecrest, California, earthquake using numerical simulations to reproduce the elastodynamic wave field observed by inertial seismometers, high‐rate Global Navigation Satellite System stations, and borehole strainmeters. This was the largest earthquake in Southern California in 20 yr and was widely felt throughout the region. The M w "> M w Mw 7.1 mainshock was part of a large sequence of ∼ 30 , 000 "> ∼ 30 , 000 ∼30,000 aftershocks and was notably preceded by an M w "> M w Mw 6.4 foreshock by 34 hr on fault structures that were once poorly understood. A large number of seismic and geodetic instruments measured the rupture process for both events, with many stations located in the near field. Hence, this is a rare opportunity to better understand complex earthquake processes that arise in an immature fault zone using advanced computing. Of the kinematic rupture models that we tested, our preferred is the simplest one that reproduces signals recorded by the three different geophysical datasets; it is composed of four distinct ruptures that progressively migrate to the southeast with delayed initiation times, and typical rupture speeds. This type of model does a better job at matching the recorded ground motions and deformations than does one composed of a continuous rupture with very low‐rupture velocity, as proposed in other studies of this earthquake.

California↗
USGS1996

Selected hydrogeologic data from the Cedar Rapids Area, Benton and Linn counties, Iowa, October 1992 through March 1996

The city of Cedar Rapids, Iowa, obtains its water supply from shallow wells screened in the alluvial aquifer along the Cedar River. A cooperative study between the city of Cedar Rapids, Iowa, and the U.S. Geological Survey was started in March 1992 to assess the water quality and water quantity of the ground-water resource. This report summarizes selected hydrogeologic data collected from October 1992 through March 1996. Information collected includes water quality (major ions, nutrients, and pesticides), ground-water levels, multiprobe-instrument data (water levels, specific conductance, pH, water temperature, and dissolved oxygen monitored at 15-, 30-, or 60-minute intervals), well information (location, casing type, screen interval, and depth), and geophysical seismic-refraction and seismic-reflection data (estimated depth to bedrock and alluvial thickness along the Cedar River). Geologic, hydrologic, and water-quality data were collected from domestic, municipal, observation, and industrial wells and the Cedar River. Well-construction data for more than 300 wells in the Cedar Rapids area in Benton and Linn Counties, Iowa, were compiled primarily from records on file with the Iowa Department of Natural Resources, Geologic Survey Bureau (Iowa City).

Iowa↗
USGS2013

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

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

Darfield, El Mayor-Cucapah↗
USGS2015

The Mw6.0 24 August 2014 South Napa earthquake

The M w 6.0 South Napa earthquake, which occurred at 10:20 UTC 24 August 2014 was the largest earthquake to strike the greater San Francisco Bay area since the M w 6.9 1989 Loma Prieta earthquake. The rupture from this right‐lateral earthquake propagated mostly unilaterally to the north and up‐dip, directing the strongest shaking toward the city of Napa, where peak ground accelerations (PGAs) between 45% g and 61% g were recorded and modified Mercalli intensities (MMIs) of VII–VIII were reported. Tectonic surface rupture with dextral slip of up to 46 cm was observed on a 12.5 km long segment, some of which was along a previously mapped strand of the West Napa fault system, although the rupture extended to the north of the mapped Quaternary strand. Modeling of seismic and geodetic data suggests an average coseismic slip of 50 cm, with a maximum slip of about 1 m at depths of 10–11 km. We observed up to 35 cm of afterslip along the surface trace in the week following the mainshock, primarily along the southern half of the surface rupture that experienced relatively little coseismic offset. Relocation of the sparse aftershock sequence suggests en echelon southwest‐ and northeast‐dipping fault planes, reflective of the complex fault geometry in this region. The Napa basin and historic and late Holocene alluvial flood deposits in downtown Napa amplified the ground motions there. Few ground failures were mapped, reflecting the dry season (as well as a persistent drought that had lowered the groundwater table) and the short duration of strong shaking in the epicentral area.

California↗
USGS1983

Principles for selecting earthquake motions in engineering design of large dams

This report gives a synopsis of the various tools and techniques used in selecting earthquake ground motion parameters for large dams. It presents 18 charts giving newly developed relations for acceleration, velocity, and duration versus site earthquake intensity for near- and far-field hard and soft sites and earthquakes having magnitudes above and below 7. The material for this report is based on procedures developed at the Waterways Experiment Station. Although these procedures are suggested primarily for large dams, they may also be applicable for other facilities. Because no standard procedure exists for selecting earthquake motions in engineering design of large dams, a number of precautions are presented to guide users. The selection of earthquake motions is dependent on which one of two types of engineering analyses are performed. A pseudostatic analysis uses a coefficient usually obtained from an appropriate contour map; whereas, a dynamic analysis uses either accelerograms assigned to a site or specified respunse spectra. Each type of analysis requires significantly different input motions. All selections of design motions must allow for the lack of representative strong motion records, especially near-field motions from earthquakes of magnitude 7 and greater, as well as an enormous spread in the available data. Limited data must be projected and its spread bracketed in order to fill in the gaps and to assure that there will be no surprises. Because each site may have differing special characteristics in its geology, seismic history, attenuation, recurrence, interpreted maximum events, etc., as integrated approach gives best results. Each part of the site investigation requires a number of decisions. In some cases, the decision to use a 'least ork' approach may be suitable, simply assuming the worst of several possibilities and testing for it. Because there are no standard procedures to follow, multiple approaches are useful. For example, peak motions at a site may be obtained from several methods that involve magnitude of earthquake, distance from source, and corresponding motions; or, alternately, peak motions may be assigned from other correlations based on earthquake intensity. Various interpretations exist to account for duration, recurrence, effects of site conditions, etc. Comparison of the various interpretations can be very useful. Probabilities can be assigned; however, they can present very serious problems unless appropriate care is taken when data are extrapolated beyond their data base. In making deterministic judgments, probabilistic data can provide useful guidance in estimating the uncertainties of the decision. The selection of a design ground motion for large dams is based in the end on subjective judgments which should depend, to an important extent, on the consequences of failure. Usually, use of a design value of ground motion representing a mean plus one standard deviation of possible variation in the mean of the data puts one in a conservative position. If failure presents no hazard to life, lower values of design ground motion may be justified, providing there are cost benefits and the risk is acceptable to the owner. Where a large hazard to life exists (i.e., a dam above an urbanized area) one may wish to use values of design ground motion that approximate the very worst case. The selection of a design ground motion must be appropriate for its particular set of circumstances.

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