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At least 1,171 records · Page 65Linked to original sources
USGS2019

Offshore shallow structure and sediment distribution, Point Sur to Point Arguello, central California

This publication consists of three map sheets that display shallow geologic structure, along with sediment distribution and thickness, for an about 225-km-long offshore section of the central California coast between Point Sur and Point Arguello. Each map sheet includes three maps, at scales of either 1:150,000 or 1:200,000, as well as a set of figures that contain representative high-resolution seismic-reflection profiles. The maps and seismic-reflection surveys cover most of the continental shelf in this region. In addition, the maps show the locations of the shelf break and the 3-nautical-mile limit of California’s State Waters. The seismic-reflection data, which are the primary dataset used to develop the maps, were collected to support the California Seafloor Mapping Program and U.S. Geological Survey Offshore Geologic Hazards projects. In addition to the three map sheets, this publication includes geographic information system data files of interpreted faults, folds, sediment thicknesses, and depths-to-base of sediment. The faults and folds shown on the maps have been locally simplified as appropriate for the map scales. The right-lateral San Gregorio–Hosgri Fault (SGHF) is the most significant structure in the map area. On a regional scale, the SGHF is part of a 400-km-long, right-lateral fault system that extends northwestward from Point Arguello to the area offshore of San Francisco, where it merges with the San Andreas Fault. From north to south in this part of central California, the SGHF lies offshore between the south flank of Point Sur and the north flank of Point Piedras Blancas, then comes onshore at Point Piedras Blancas, before heading offshore again between the south flank of Point Piedras Blancas and Point Arguello. Cumulative fault offset along the SGHF is as much as 150 to 160 km, decreasing to the south by transferring slip on to northwest-striking faults that converge with the SGHF both onland and offshore from the east. In the map area, the offshore-converging faults include the Los Osos Fault, the Shoreline–Point Buchon Fault, the Casmalia Fault, and the Lions Head Fault. Quaternary sediments and bedrock underlie the shelf. On the seismic-reflection profiles, we divide Quaternary shelf sediments into two units. Characterizing the younger, upper unit is a focus of this publication. This unit is inferred to have been deposited on the shelf in the last about 21,000 years during the sea-level rise that followed the last major lowstand and the Last Glacial Maximum (LGM). This upper unit overlies a transgressive surface of erosion, a commonly angular, wave-cut unconformity, and is generally characterized by low-amplitude, continuous to moderately continuous, diffuse, subparallel, generally flat reflections. Maps in this publication show both the thickness of this upper sediment unit and the depth to the base of the sediment unit. Within the map region, 11 different “domains” of post-LGM shelf sediment are delineated on the basis of sediment thickness and coastal geomorphology. Maximum sediment thickness is in the southern part of the region, offshore of the mouths of the Santa Ynez and Santa Maria Rivers. Minimum sediment thickness is found offshore of prominent rocky points, including Point Buchon and Piedras Blancas. Mean sediment thickness for the entire shelf in the map area between Point Sur and Point Arguello is 12.2 m, and total sediment volume is 24.7 million cubic meters.

California↗
USGS2000

Geologic map and map database of the Palo Alto 30' x 60' quadrangle, California

This digital map database, compiled from previously published and unpublished data, and new mapping by the authors, represents the general distribution of bedrock and surficial deposits in the mapped area. Together with the accompanying text file (pamf.ps, pamf.pdf, pamf.txt), it provides current information on the geologic structure and stratigraphy of the area covered. The database delineates map units that are identified by general age and lithology following the stratigraphic nomenclature of the U.S. Geological Survey. The scale of the source maps limits the spatial resolution (scale) of the database to 1:62,500 or smaller.

California↗
USGS1924

The San Juan Canyon, southeastern Utah: A geographic and hydrographic reconnaissance

This report, which describes the San Juan Canyon, San Juan River and the tributary streams and the geography and to some extent the geology of the region, presents information obtained by me during the descent of the river with the Trimble party in 1921. The exploration of the canyon, which was financed jointly by the United States Geological Survey and the Southern California Edison Co., had as its primary object the mapping and study of the San Juan in connection with proposed power and storage projects along this and Colorado rivers.1 The exploration party was headed by K. W. Thimble, topographic engineer of the United States Geological Survey. Other members of the party were Robert N. Allen, Los Angeles, Calif., recorder; H. E. Blake, jr., Monticello, Utah, and Hugh Hyde, Salt Lake City, Utah, rodmen; Bert Loper, Green River, Utah, boatman; Heber Christensen, Moab, Utah, cook; and H. D. Miser, geologist. Wesley Oliver, of Mexican Hat, Utah, served as packer for the party and brought mail and provisions by pack train twice a month to specified accessible places west of Goodridge.

Utah↗
USGS2018

Land subsidence along the California Aqueduct in west-central San Joaquin Valley, California, 2003–10

Extensive groundwater withdrawal from the unconsolidated deposits in the San Joaquin Valley caused widespread aquifer-system compaction and resultant land subsidence from 1926 to 1970—locally exceeding 8.5 meters. The importation of surface water beginning in the early 1950s through the Delta-Mendota Canal and in the early 1970s through the California Aqueduct resulted in decreased groundwater pumping, recovery of water levels, and a reduced rate of compaction in some areas of the San Joaquin Valley. However, drought conditions during 1976–77, 1987–92, and drought conditions and operational reductions in surface-water deliveries during 2007–10 decreased surface-water availability, causing pumping to increase, water levels to decline, and renewed compaction. Land subsidence from this compaction has reduced freeboard and flow capacity of the California Aqueduct, Delta-Mendota Canal, and other canals that deliver irrigation water and transport floodwater. The U.S. Geological Survey, in cooperation with the California Department of Water Resources, assessed more recent land subsidence near a 145-kilometer reach of the California Aqueduct in the west-central part of the San Joaquin Valley as part of an effort to minimize future subsidence-related damages to the California Aqueduct. The location, magnitude, and stress regime of land-surface deformation during 2003–10 were determined by using data and analyses associated with extensometers, Global Positioning System surveys, Interferometric Synthetic Aperture Radar, spirit-leveling surveys, and groundwater wells. Comparison of continuous Global Positioning System, shallow-extensometer, and groundwater-level data indicated that most of the compaction in this area took place beneath the Corcoran Clay, the primary regional confining unit. The integration of measurements strengthens confidence in individual measurement methods and provides the information at spatial and temporal scales that water managers need to design and implement groundwater sustainability plans in compliance with California’s Sustainable Groundwater Management Act. Measurements of land-surface deformation during 2003–10 indicated that the parts of the California Aqueduct closest to the Coast Ranges in the west-central part of the San Joaquin Valley were fairly stable or minimally subsiding on an annual basis; some areas show seasonal periods of subsidence and uplift that resulted in little or no longer-term elevation loss. Many groundwater levels in these areas did not reach historical lows during 2003–10, indicating that deformation nearest the Coast Ranges was likely primarily elastic. Land-surface deformation measurements indicated that some parts of the California Aqueduct that traverse farther from the Coast Ranges toward the valley center subsided. Some parts of the California Aqueduct subsided locally, but generally the California Aqueduct is within part of a 12,000-square-kilometer area affected by 25 millimeters or more of subsidence during 2008–10, with maxima in Madera County, south of the town of El Nido near the San Joaquin River and the Eastside Bypass (540 millimeters), and in Tulare County, west of the town of Pixley (345 millimeters). Interferometric Synthetic Aperture Radar-derived subsidence maps for various periods during 2003–10 show that the area of maximum active subsidence (that is, the largest rates of subsidence) shifted from its historical (1926–70) location southwest of the town of Mendota to these areas nearer the valley center. Calculations indicated that the subsidence rate doubled in 2008 in parts of the study area. Water levels declined during 2007–10 in many shallow and deep wells in the most rapidly subsiding areas, where water levels in many deep wells reached their historical lows, indicating that subsidence measured during this period was largely inelastic. Continued groundwater-level and land-subsidence monitoring in the San Joaquin Valley is important because (1) operational- and drought-related reductions in surface-water deliveries since 1976 have resulted in increased groundwater pumping and associated water-level declines and land subsidence, (2) land use and associated pumping continue to change throughout the valley, and (3) subsidence management is stipulated in the Sustainable Groundwater Management Act. The availability of surface water remains uncertain; even during record-setting precipitation years, such as 2010–11, water deliveries fell short of requests and groundwater pumping was required to meet the irrigation demand. In some areas, the infrastructure is not available to supply surface water, and groundwater is the only source of water. Because of the expected continued demand for water and the limitations and uncertainty of surface-water supplies, groundwater pumping and associated land subsidence remains a concern. Spatially detailed information on land subsidence is needed to minimize future subsidence-related damages to the California Aqueduct and other infrastructure in the San Joaquin Valley, as well as alterations to natural resources such as stream gradients, water depths, and water temperatures. The integration of data on land-surface elevation, subsurface deformation, and water levels—particularly continuous measurements—enables the analysis of aquifer-system response to groundwater pumping, which in turn, enables estimation of the preconsolidation head and calculation of aquifer-system storage properties. This information can be used to improve numerical model simulations of groundwater flow and aquifer-system compaction and allow for consideration of land subsidence in the evaluation of water resource management alternatives and compliance with the Sustainable Groundwater Management Act.

California↗
USGS2004

Earthquake scenario and probabilistic ground-shaking hazard maps for the Albuquerque-Belen-Santa Fe, New Mexico, corridor

New Mexico's population is concentrated along the corridor that extends from Belen in the south to Española in the north and includes Albuquerque and Santa Fe. The Rio Grande rift, which encompasses the corridor, is a major tectonically, volcanically, and seismically active continental rift in the western U.S. Although only one large earthquake (moment magnitude (M) ≥ 6) has possibly occurred in the New Mexico portion of the rift since 1849, paleoseismic data indicate that prehistoric surface-faulting earthquakes of M 6.5 and greater have occurred on aver- age every 400 yrs on many faults throughout the Rio Grande rift. We have developed a series of nine scenario and probabilistic hazard maps that portray the ground shaking that could occur in the Albuquerque-Belen-Santa Fe corridor from future earthquakes in New Mexico. These maps, at a scale of 1:500,000, display color-contoured ground-motion values in terms of the parameters of peak horizontal acceleration and horizontal spectral accelerations at 0.2 and 1.0 second (sec) periods. The maps depict surficial ground shaking and incorporate the site-response effects at locations underlain by unconsolidated sediments. The scenario maps are for a M 7.0 earthquake rupturing the Sandia-Rincon faults, which are adjacent to and dip west beneath Albuquerque. The probabilistic maps are for the two annual exceedance probabilities of building code relevance, 10% and 2% exceedance probabilities in 50 yrs (corresponding to return periods of 500 and 2,500 yrs, respectively). We included 57 Quaternary faults, all located within the Rio Grande rift, in the probabilistic seismic hazard analysis. These faults were characterized in terms of their geometry, rupture behavior (including possible segmentation), maximum expected earthquake magnitude, recurrence model, probability of activity, and slip rate. Preferred maximum magnitude values for these faults ranged from M 6.1 to 7.4 and preferred slip rates from 0.01 to approximately 0.12 mm/yr. Regional source zones and Gaussian smoothing of the historical seismicity were also included in the probabilistic hazard analysis to account for the hazard from background earthquakes (M ≤ 6.5). A numerical ground-motion modeling approach and empirical attenuation relation- ships appropriate for extensional tectonic regimes were used to compute the scenario earthquake and probabilistic ground motions on rock. Amplification factors were then used to modify the rock motions and hence to incorporate site response into the hazard maps. These factors were based on three generalized geologic site-response categories (hard rock, soft rock, and firm/stiff soil) and were adopted from similar California-based categories because insufficient subsurface geologic and geotechnical data are available for the map area. The resulting hazard maps indicate that from both scenario and probabilistic perspectives, the ground-shaking hazard in the Albuquerque–Belen–Santa Fe corridor from future earthquakes could be severe, damaging, and potentially disastrous. In the event of a M 7.0 earthquake occurring on the Sandia–Rincon faults, ground shaking as characterized by peak ground acceleration could reach 0.7 g in much of the eastern half of the Albuquerque metropolitan area. (1 g = 980 cm/sec, the rate of gravitational acceleration.) These high ground motions will be attributable to the city’s location directly over the Sandia–Rincon faults and the amplifying effect of the unconsolidated sediments within the Albuquerque Basin. These levels of ground shaking will probably result in severe damage to traditional adobe construction and even to modern buildings. Long- period ground motions (> 1.0 sec), which are significant to long and tall structures (e.g., tall buildings, long bridges, and highway overpasses), will also be high (> 1.0 g). Injuries and loss of life will be likely. For the 500- and 2,500-yr return period maps, the highest peak accelerations are predicted to be at the damaging levels of 0.3 g and 0.6 g, respectively. All maps show dramatically the frequency-dependent amplification of unconsolidated sediments in the basins along the Rio Grande valley (e.g., Albuquerque Basin). The pattern of amplification and deamplification is clearly a function of the distribution of unconsolidated sediments. These maps are not intended to be a substitute for site-specific studies for engineering design nor to replace standard maps commonly referenced in building codes. Rather, we hope that these maps will be used as a guide by government agencies; the engineering, urban planning, emergency preparedness, and response communities; and the general public as part of an overall program to reduce earthquake risk and losses in New Mexico.

New Mexico↗
USGS2012

Lg and Rg waves on the California regional networks from the December 23, 1985 Nahanni earthquake

We investigate Lg and Rg propagation in California using the central and southern California regional networks. Approximately 550 stations constitute these two short-period networks providing a dense coverage of almost the entire state. The waveforms recorded from the December 23, 1985, Nahanni, Canada, earthquake are used to construct three profiles along the propagation path (almost N-S) and three perpendicular to the propagation path (almost E-W) to look at the nature of propagation of these two types of surface waves. Groups of records from stations in various geological and tectonic provinces in California are also examined in order to establish regional characteristics of the surface waves. We find that the propagation characteristics of Lg differ from those of Rg across California; Lg waves are apparently more sensitive to crustal heterogeneities. The most striking observations are the similarity of coda for both the Lg and the Rg waves within geologic provinces and the marked difference in coda between regions. These differences are seen in the amplitudes, coda duration, shape of the energy envelope, frequency content, and sharpness of the phase initiation. In general, a decrease in the Moho depth near the Pacific Coast is correlated with a decrease in the surface wave amplitude, especially at higher frequencies (0.15–0.2 Hz). Most interesting is the association of the San Andreas fault with abrupt changes in the wave train amplitudes. The surface waves are amplified in the vicinity of the fault zone and then decrease in amplitude after the zone is crossed. In the Coast Ranges, amplitudes are low and waveform coherence is poor. The Rg phase dominates the record in the Sierra Nevada, and both surface waves are amplified by the thick sedimentary sequence of the Great Valley.

Journal of Geophysical Research↗
USGS2023

Crustal block-controlled contrasts in deformation, uplift, and exhumation in the Santa Cruz Mountains, California, USA, imaged through apatite (U-Th)/He thermochronology and 3-D geological modeling

Deformation along strike-slip plate margins often accumulates within structurally partitioned and rheologically heterogeneous crustal blocks within the plate boundary. In these cases, contrasts in the physical properties and state of juxtaposed crustal blocks may play an important role in accommodation of deformation. Near the San Francisco Bay Area, California, USA, the Pacific−North American plate-bounding San Andreas fault bisects the Santa Cruz Mountains (SCM), which host numerous distinct, fault-bounded lithotectonic blocks that surround the San Andreas fault zone. In the SCM, a restraining bend in the San Andreas fault (the SCM bend) caused recent uplift of the mountain range since ca. 4 Ma. To understand how rheologic heterogeneity within a complex fault zone might influence deformation, we quantified plausible bounds on deformation and uplift across two adjacent SCM lithotectonic blocks on the Pacific Plate whose stratigraphic and tectonic histories differ. This was accomplished by combining 31 new apatite (U-Th)/He ages with existing thermochronological datasets to constrain exhumation of these two blocks. Additionally, surface exposures of the latest Miocene to late Pliocene Purisima Formation interpreted in 18 structural cross sections spanning the SCM allowed construction and restoration of Pliocene deformation in a three-dimensional geologic model. We found that rock uplift and deformation concentrated within individual Pacific Plate lithotectonic blocks in the SCM. Since 4 Ma, maximum principal strain computed for the more deformed block adjacent to the fault exceeded that computed for the less deformed block by at least 375%, and cumulative uplift has been more spatially extensive and higher in magnitude. We attribute the difference in uplift and deformation between the two blocks primarily to contrasts in lithotectonic structure, which resulted from diverging geologic histories along the evolving plate boundary.

California↗
USGS2009

Neogene Gas Total Petroleum System -- Neogene Nonassociated Gas Assessment Unit of the San Joaquin Basin Province: Chapter 22 in Petroleum systems and geologic assessment of oil and gas in the San Joaquin Basin Province, California

The Neogene Nonassociated Gas Assessment Unit (AU) of the Neogene Total Petroleum System consists of nonassociated gas accumulations in Pliocene marine and brackish-water sandstone located in the south and central San Joaquin Basin Province (Rudkin, 1968). Traps consist mainly of stratigraphic lenses in low-relief, elongate domes that trend northwest-southeast. Reservoir rocks typically occur as sands that pinch out at shallow depths (1,000 to 7,500 feet) within the Etchegoin and San Joaquin Formations. Map boundaries of the assessment unit are shown in figures 22.1 and 22.2; this assessment unit replaces the Pliocene Nonassociated Gas play 1001 (shown by purple line in fig. 22.1) considered by the U.S. Geological Survey (USGS) in its 1995 National Assessment (Beyer, 1996). The AU is drawn to include all existing fields containing nonassociated gas accumulations in the Pliocene to Pleistocene section, as was done in the 1995 assessment, but it was greatly expanded to include adjacent areas believed to contain similar source and reservoir rock relationships. Stratigraphically, the AU extends from the topographic surface to the base of the Etchegoin Formation (figs. 22.3 and 22.4). The boundaries of the AU explicitly exclude gas accumulations in Neogene rocks on the severely deformed west side of the basin and gas accumulations in underlying Miocene rocks; these resources, which primarily consist of a mixture of mostly thermogenic and some biogenic gas, are included in two other assessment units. Lillis and others (this volume, chapter 10) discuss the geochemical characteristics of biogenic gas in the San Joaquin Basin Province. Primary fields in the assessment unit are defined as those containing hydrocarbon resources greater than the USGS minimum threshold for assessment—3 billion cubic feet (BCF) of gas; secondary fields contain smaller volumes of gas but constitute a significant show of hydrocarbons. Although 12 fields meet the 3 BCF criterion for inclusion in the AU, only 5 fields were considered at the time of assessment.

California↗
USGS2026

Helping California get ready for when the mud hits the fan

With frequent wildfires, powerful rainstorms, steep terrain, and dense population, California is particularly vulnerable to postfire hazards like flooding and debris flow. Major postfire flooding and debris-flow events can be more costly than the fire itself, causing dozens of fatalities and widespread damage (Chawner, 1934; Lancaster et al., 2021). Smaller events occur nearly every year in California and can locally damage structures and roads, affect water quality and storage, and result in injury or loss of life (e.g., Dow et al., 2024; Rodriguez, 2025; Swanson et al., 2024; Figure 1). To mitigate these threats, the U.S. Geological Survey (USGS) has developed tools to assist local, state, and federal agencies in rapidly assessing postfire debris-flow risk across California’s diverse geology and geography. This article summarizes that effort and describes ongoing work by the USGS and partners to advance these methods and help California better prepare for future wildfires.

Caliornia↗
USGS2008

Miocene Total Petroleum System -- Southeast Stable Shelf Assessment Unit of the San Joaquin Basin Province: Chapter 13 in Petroleum systems and geologic assessment of oil and gas in the San Joaquin Basin Province, California

The confirmed stratigraphic and structural-stratigraphic Southeast Stable Shelf Assessment Unit (AU) of the Miocene Total Petroleum System (San Joaquin Basin Province) comprises all hydrocarbon accumulations within the geographic limits of the AU. Traps typically display low dip angles, gentle folds, and normal faults. Reservoirs, which range in age from fractured Mesozoic basement rocks to Holocene nonmarine rocks, are mainly Oligocene to Miocene sandstones from the uppermost slope and adjacent shelf of the San Joaquin Basin, shallow marine shelf sandstones mainly of Miocene age, and nonmarine sandstones and conglomerates mostly of Pliocene- Pleistocene age. Faults have relatively small vertical displacements. Map boundaries of the assessment unit are shown in figures 13.1 and 13.2; this assessment unit replaces the Southeast Stable Shelf play 1002 considered by the U.S. Geological Survey (USGS) in its 1995 National Assessment (Beyer, 1996). Stratigraphically, the AU extends from the uppermost crystalline basement to the topographic surface (fig. 13.3). The AU is bounded on the west by the approximate location of the shelfslope break of the San Joaquin Basin in late Miocene time, thus excluding reservoirs in the deep-water Stevens sand of Eckis (1940). The eastern boundary of the AU is the edge of onlap of Neogene sedimentary sequences on crystalline basement rocks of the Sierra Nevada. The northern AU boundary is placed at the approximate northern extent of oils in shelf-facies reservoirs known to be sourced by the Miocene Total Petroleum System. This northern boundary explicitly excludes the Deer Creek and Jasmin fields, which were included in the corresponding earlier (1995) USGS play (Beyer, 1996), but which are now known to contain oil generated from Eocene source rocks. The White Wolf Fault bounds the AU on the south.

California↗
USGS2021

Kelp forest monitoring at Naval Base Ventura County, San Nicolas Island, California—Fall 2019, sixth annual repor

The U.S. Geological Survey conducts ecological monitoring of rocky subtidal communities at four permanent sites around San Nicolas Island. The sites—Nav Fac 100, West End, Dutch Harbor, and Daytona 100—were based on ones that had been monitored since 1980 by the U.S. Geological Survey and, in cooperation with the U.S. Navy, were combined or expanded in 2014 for better comparability with monitoring programs conducted at the other California Channel Islands. At the sites, we counted a suite of kelps and invertebrates on benthic band transects, measured bottom cover of algae and sessile invertebrate species in quadrats, and counted and sized fish on swimming transects. Holdfast diameter and number of stipes of giant kelp ( Macrocystis pyrifera ) were recorded on these transects and size data were collected for urchins, sea stars, and shelled mollusks. Bottom temperatures were recorded at hourly intervals by archival data loggers that were deployed at the sites. Typically, this monitoring work is conducted semi-annually, in fall and spring. Because the spring 2020 trip was cancelled due to the Coronavirus Disease 2019 pandemic, this report focuses primarily on data collected in fall 2019 and makes comparisons with data collected in previous years, beginning in fall 2014. The sites are distributed around the island and differ in their physical and ecological characteristics. Nav Fac 100, situated on the north side of San Nicolas Island, has a relatively low benthic profile. The invasive brown alga Sargassum horneri was first observed at this site in 2015. West End, to the southwest of the island, also lacks much bottom relief but has more crevice habitat associated with boulders. For almost three decades, West End has been a focal point for the small, but growing, population of southern sea otters ( Enhydra lutris nereis ) at the island. Dutch Harbor, on the south side, has many high relief rocky reefs and had the greatest fish and non-motile invertebrate densities. Daytona 100, on the southeast side, has moderate relief and has remained a patchwork of kelp and sea urchin dominated areas. There were no major changes at the sites since spring 2019, but some trends observed during the last few years continued whereas others changed. Red urchins continued a declining trend (observed during the last 4 years) at Daytona 100. The wavy turban snail ( Megastraea undosa ) began to increase rapidly at Nav Fav 100 in 2015 and has subsequently been increasing at the other sites as well, after more than a decade of very low numbers at all sites. Sea star wasting syndrome, which has devastated multiple species of sea stars along the Pacific coast of North America, affected most species at San Nicolas Island in the year prior to the fall 2014 sampling. Since then, there has been a reduction in the number of bat stars ( Patiria miniata ), and very few sea stars of other species have been observed. There has been a slight recovery of P. miniata since 2016 but little sign of change in other species. All the sites had a slight decline in the densities of purple urchins following an increase during the previous 2 years. Long-term data are presented to illustrate trends and changes during almost four decades of monitoring this dynamic system.

Naval Base Ventura County, San Nicolas Island↗
USGS2008

Miocene Total Petroleum System -- Lower Bakersfield Arch Assessment Unit of the San Joaquin Basin Province: Chapter 14 in Petroleum systems and geologic assessment of oil and gas in the San Joaquin Basin Province, California

The Lower Bakersfield Arch Assessment Unit (AU) of the Miocene Total Petroleum System (San Joaquin Basin Province) is primarily defined by the distribution of hydrocarbons generated from biosiliceous shale of the Monterey Formation and by the distribution of basinal-facies sandstones of the Stevens sand of Eckis (1940; hereafter referred to as Stevens sand). Traps are principally stratigraphic and structural/stratigraphic, with most discovered accumulations occurring in deep-sea channel, fan, and braided submarine channel deposits of the late Miocene Stevens sand. Smaller and fewer accumulations are found in older sandstones such as the Vedder and Jewett Sands of Oligocene to Miocene age. Compared to the west side of the basin, the AU is largely unstructured, except for localized down-to-the-basin normal faults. Map boundaries of the assessment unit are shown in figures 14.1 and 14.2; this assessment unit supersedes the Lower Bakersfield Arch play 1003 considered by the U.S. Geological Survey (USGS) in the 1995 National Assessment (Beyer, 1996). Stratigraphically, the AU extends from the uppermost crystalline basement to the topographic surface (fig. 14.3). The AU is bounded on the east and north by the limit of basinal- facies sandstones of the Stevens sand; this eastern boundary corresponds to the approximate location of the shelf-slope break of the San Joaquin Basin in late Miocene time. The western boundary of the AU is the approximate eastern limit of structural deformation on the basin’s west side. The White Wolf Fault bounds the AU on the south.

California↗
USGS2025

New evidence for eolian activity and mammoths on Santa Rosa Island prior to the Last Glacial Maximum

Sea-level fluctuations due to the growth and decay of continental ice sheets of the Quaternary exert a strong influence on geologic processes along coastlines. The California Channel Islands are no exception to this, and many studies have been conducted that focus on the extremes of these glacial-interglacial cycles, such as the last glacial period (marine isotope stage [MIS] 2) and the last interglacial period (MIS 5). Far less attention has been paid to intermediate time periods between these extremes, such as MIS 4. Here, we present two very different geologic records from this time period that show how sea-level change affected the nature of sedimentation on the northern shore of Santa Rosa Island. On the northwestern coast of Santa Rosa Island, thick eolian sediments accumulated between ~80 ka and ~45 ka due to a lowered sea level that exposed carbonate-rich skeletal sands that had accumulated during the last interglacial period. On the central part of the northern coast of Santa Rosa Island, thick alluvial sediments were deposited between 80 ka and 47 ka, similar to the time of eolian sedimentation to the west. A mammoth tusk discovered within these deposits is only the third stratigraphically controlled mammoth fossil on the Channel Islands that dates prior to MIS 2. Its discovery adds to the evidence that mammoths migrated to the Channel Islands from mainland California prior to the Last Glacial Maximum.

California↗
USGS2022

Evolution of design ground motions in California: NEHRP 2009 to 2020

The U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) is used in construction codes, such as the National Earthquake Hazard Reduction Program (NEHRP) Provisions, to develop ground motions for structural and geotechnical design. When the NSHM is updated (e.g. changes to its earthquake rupture forecast or ground motion models), or the manner in which it is implemented in construction codes changes, design ground motion values can increase or decrease at a given location. This study presents an analysis of 21 sites in California to characterize how design ground motion values (SS, S1, SMS, SM1) have changed from the 2009 to 2015 to 2020 NEHRP Provisions. Four sites are presented in greater detail: two in Southern California and two in Northern California. The results show that significant changes in design ground motion values between NEHRP releases are explained by major changes in the NSHM science (e.g., inclusion of basin effects) and its use (e.g., for a broader range of site classes), but in ways that are difficult to ascertain beforehand. It is therefore likely that design ground motion values may continue to evolve between NSHM and construction code updates as the earthquake science and engineering field's understanding continues to advance.

California↗
USGS2018

Breaching of strike-slip faults and successive flooding of pull-apart basins to form the Gulf of California seaway from ca. 8–6 Ma

The geologic record of the formation of marine basins during continental rifting is uncommonly preserved. Using GIS-based paleotectonic maps, we show that marine basin formation in the Gulf of California–Salton trough oblique rift (Mexico and the United States) occurred in a stepwise manner as crustal thinning lowered elevations within the Gulf of California Shear Zone, and subsidence along strike-slip and transtensional faults linked isolated pull-apart basins. At 8 Ma, the earliest marine conditions in the Gulf of California were restricted to an embayment at its southern mouth. Farther north, the plate boundary was a set of continental strike-slip faults and linked pull-apart basins, similar to the modern Walker Lane in Nevada and California. By ca. 7 Ma, a series of marine incursions breached across strike-slip faults to the Pescadero and Farallon basins. Marine waters then breached a 75–100 km-long transtensional fault zone between the Farallon and Guaymas basins, with intermittent flooding that led to accumulation of extensive evaporite deposits in the Guaymas basin. Marine incursion north of the Guaymas basin via breaches across the Guaymas and Tiburón strike-slip faults and transtensional zones resulted in flooding of the northern >500 km of the oblique rift by 6.5–6.3 Ma. Thus, strike-slip and transtensional faulting promoted localization of plate boundary strain and guided punctuated marine flooding of the Gulf of California seaway. Inception of the narrow, 1500-km-long Gulf of California at ca. 6.3 Ma was followed by complete continental rupture in the Guaymas basin at ca. 6.0 Ma.

Geology↗
USGS2005

A new reconstruction of the Paleozoic continental margin of southwestern North America: Implications for the nature and timing of continental truncation and the possible role of the Mojave-Sonora megashear

Data bearing on interpretations of the Paleozoic and Mesozoic paleogeography of southwestern North America are important for testing the hypothesis that the Paleozoic miogeocline in this region has been tectonically truncated, and if so, for ascertaining the time of the event and the possible role of the Mojave-Sonora megashear. Here, we present an analysis of existing and new data permitting reconstruction of the Paleozoic continental margin of southwestern North America. Significant new and recent information incorporated into this reconstruction includes (1) spatial distribution of Middle to Upper Devonian continental-margin facies belts, (2) positions of other paleogeographically significant sedimentary boundaries on the Paleozoic continental shelf, (3) distribution of Upper Permian through Upper Triassic plutonic rocks, and (4) evidence that the southern Sierra Nevada and western Mojave Desert are underlain by continental crust. After restoring the geology of western Nevada and California along known and inferred strike-slip faults, we find that the Devonian facies belts and pre-Pennsylvanian sedimentary boundaries define an arcuate, generally south-trending continental margin that appears to be truncated on the southwest. A Pennsylvanian basin, a Permian coral belt, and a belt of Upper Permian to Upper Triassic plutons stretching from Sonora, Mexico, into westernmost central Nevada, cut across the older facies belts, suggesting that truncation of the continental margin occurred in the Pennsylvanian. We postulate that the main truncating structure was a left-lateral transform fault zone that extended from the Mojave-Sonora megashear in northwestern Mexico to the Foothills Suture in California. The Caborca block of northwestern Mexico, where Devonian facies belts and pre-Pennsylvanian sedimentary boundaries like those in California have been identified, is interpreted to represent a missing fragment of the continental margin that underwent ???400 km of left-lateral displacement along this fault zone. If this model is correct, the Mojave-Sonora megashear played a direct role in the Pennsylvanian truncation of the continental margin, and any younger displacement on this fault has been relatively small. ?? 2005 Geological Society of America.

Special Paper of the Geological Society of America↗
USGS2019

#EarthquakeAdvisory: Exploring discourse between government officials, news media and social media during the Bombay Beach 2016 Swarm

Communicating probabilities of natural hazards to varied audiences is a notoriously difficult task. Many of these challenges were encountered during the 2016 Bombay Beach, California, swarm of ~100 2≤M≤4.3 earthquakes, which began on 26 September 2016 and lasted for several days. The swarm’s proximity to the southern end of the San Andreas fault caused concern that a larger earthquake could be triggered. Within 1–2 days, different forecast models were used to evaluate the likelihood of a larger event with two agencies (the U.S. Geological Survey [USGS] and the California Governor’s Office of Emergency Services) releasing probabilities and forecasts for larger earthquakes. Our research explores communication and news media efforts, as well as how people on a microblogging social media site (Twitter) responded to these forecasts. Our findings suggest that news media used a combination of information sources, basing their articles on what they learned from social media, as well as using information provided by government agencies. As the swarm slowed down, there is evidence of the continued interplay between news media and social media, with the USGS issuing revised probability reports and scientists from the USGS and other institutions participating in media interviews. In reporting on the swarm, news media often used language more generally than the scientists; terms such as probability, likelihood, chance, and possibility were used interchangeably. Knowledge of how news media used scientific information from the 2016 Bombay Beach forecasts can assist local, state, and federal agencies in developing effective communication strategies to respond to future earthquakes.

California↗
USGS2009

Late Cenozoic geology and lacustrine history of Searles Valley, Inyo and San Bernardino Counties, California

Searles Valley is an arid, closed basin lying 70 km east of the south end of the Sierra Nevada, California. It is bounded on the east and northeast by the Slate Range, on the west by the Argus Range and Spangler Hills, and on the south by the Lava Mountains; Searles (dry) Lake occupies the north-central part of the valley. During those parts of late Pliocene and Pleistocene time when precipitation and runoff from the east side of the Sierra Nevada into the Owens River were much greater than at present, a chain of as many as five large lakes was created, of which Searles Lake was third. The stratigraphic record left in Searles Valley when that lake expanded, contracted, or desiccated, is fully revealed by cores from beneath the surface of Searles (dry) Lake and partly recorded by sediments cropping out around the edge of the valley. The subsurface record is described elsewhere. This volume includes six geologic maps (scales: 1:50,000 and 1:10,000) and a text that describes the outcrop record, most of which represents sedimentation since 150 ka. Although this outcrop record is discontinuous, it provides evidence indicating the lake's water depths during each expansion, which the subsurface record does not. Maximum-depth lakes rose to the 2,280-ft (695 m) contour, the level of the spillway that led overflowing waters to Panamint Valley; that spillway is about 660 ft (200 m) above the present dry-lake surface. Several rock units of Tertiary and early Quaternary ages crop out in Searles Valley. Siltstone and sandstone of Tertiary age, mostly lacustrine in nature and locally deformed to near-vertical dips, are exposed in the southern part of the valley, as is the younger(?) upper Miocene Bedrock Spring Formation. Unnamed, mostly mafic volcanic rocks of probable Miocene or Pliocene age are exposed along the north and south edges of the basin. Slightly deformed lacustrine sandstones are mapped in the central-southwestern and southern parts of the study area. The Christmas Canyon Formation and deposits mapped as older gravel and older tufa are extensively exposed over much of the basin floor. The older gravel unit and the gravel facies of the Christmas Canyon Formation are boulder alluvial gravels; parts of these units are probably correlative. The lacustrine facies of the Christmas Canyon Formation includes the Lava Creek ash, which is dated at 0.64 Ma; the older tufa deposits may be equivalent in age to those sediments. Most of this study concerns sediments of the newly described Searles Lake Formation, whose deposition spanned the period between about 150 ka and 2 ka. Most of this formation is lacustrine in origin, but it includes interbedded alluvium. To extract as much geologic detail as possible, criteria were developed that permitted (1) intrabasin correlation of some thin outcrop units representative of only a few thousand years (or less), (2) identification of unconformities produced by subaerial erosion, (3) identification of unconformities produced by sublacustrine erosion, and (4) correlation of outcrop units with subsurface units. The Searles Lake Formation is divided into seven main units, many of which are subdivided on the five larger scale geologic maps. Units A (oldest), B, C, and D are dominantly lacustrine in origin. The Pleistocene-Holocene boundary is placed at the top of unit C. In areas that were a kilometer or more from shore at the time of deposition, deposits of units A,B, and C consist of fine, highly calcareous sand, silt, or clay; nearer to shore they consist of well-sorted coarse sand and gravel. Unit A has been locally subdivided into as many as four subunits, unit B into six subunits, and unit C into six subunits. The finer facies of units A, B, and C contain such high percentages of Caco3 that they are best described as marl. Sediments of unit C, and to a lesser extent those of unit B, are laminated with light- to white-colored layers of aragonite, calcite, or dolomite(?) that may repre

California↗