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At least 415 records · Page 23Linked to original sources

USGS1997

Färoe-Iceland Ridge Experiment: 1. Crustal structure of northeastern Iceland

Results from the Färoe-Iceland Ridge Experiment (FIRE) constrain the crustal thickness as 19 km under the Northern Volcanic Zone of Iceland and 35 km under older Tertiary areas of northeastern Iceland. The Moho is defined by strong P wave and S wave reflections. Synthetic seismogram modeling of the Moho reflection indicates mantle velocities of at least 8.0 km/s beneath the Tertiary areas of northeastern Iceland and at least 7.9 km/s beneath the neovolcanic zone. Crustal diving rays resolve the structure of the upper and lower crust. Surface P wave velocities are 1.1–4.0 km/s in Quaternary rocks and are rather higher, 4.4–4.7 km/s, in the Tertiary basalts that outcrop elsewhere. The highest crustal P wave velocities observed directly from diving rays are 7.1 km/s, from rays that turn at 24 km depth. Velocities of 7.35 km/s at the base of the crust are inferred from extrapolation of the lower crustal velocity gradient (0.024 s −1 ). A Poisson's ratio of approximately 0.27, equivalent to an S wave to P wave travel time ratio of 1.78, is measured throughout the crust east of the neovolcanic zone. The Poisson's ratio and the steep Moho topography (in places up to 30° from the horizontal) indicate that the entire crust outside the neovolcanic zone is cool (<800°C). Gravity data are well matched by a velocity/density conversion of our seismic crustal model and indicate a region of low mantle density beneath the neovolcanic zone, believed to be due to elevated mantle temperatures. The crustal thickness in the neovolcanic zone is consistent with geochemical estimates of the melt generation, placing constraints on the flow within the Iceland mantle plume.

Journal of Geophysical Research B: Solid Earth↗

USGS2021

Coastal Tree-Ring Records for Paleoclimate and Paleoenvironmental Applications in North America

For more than a century, tree-ring research has identified relationships between climatic and ecological conditions and tree growth to describe past environments and constrain future ecosystem vulnerabilities. Tree-ring records are frequently used as environmental proxies that extend knowledge of past climate and ecology on millennial scales. Many of the most pressing global change questions facing North America concern the rate of climate change and vulnerability of ecosystems and society along the coast. The opportunities and applications in dendrochronology continue to grow with advancing methodologies, faster computational ability, and the cost-reduction of many chemical and anatomical analyses. Here, we propose that many pressing global change questions that affect coastal communities can be addressed using dendrochronological techniques. We review coastal tree-ring studies that demonstrate the utility and potential for future tree-ring studies in the northeastern, southeastern, northwestern, and southwestern North American coasts. Additionally, we show that tree-ring chronologies along the coast give insight into local and regional climate phenomena that are distinct from nearby, inland tree-ring chronologies of the same species. Lastly, we identify opportunities for coastal dendrochronology and encourage the collection of more tree-ring records that are directly impacted by coastal phenomena.

Quaternary Science Reviews↗

USGS2003

Geology of the continental margin beneath Santa Monica Bay, Southern California, from seismic-reflection data

We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in3 generator-injector air gun, to show the geologic structure of the continental shelf and slope and of the deep-water, Santa Monica and San Pedro Basins. The goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults. These data reveal that northwest of the Palos Verdes Peninsula, the Palos Verdes Fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise. Other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface. However, other major faults in the study area, such as the Dume and San Pedro Basin Faults, were active recently, as indicated by an arched seafloor and offset shallow sediment. Rocks under the lower continental slope are deformed to differing degrees on opposite sides of Santa Monica Canyon. Northwest of this canyon, the continental slope is underlain by a little-deformed sediment apron; the main structures that deform this apron are two lower-slope anticlines that extend toward Point Dume and are cored by faults showing reverse or thrust separation. Southeast of Santa Monica Canyon, lower-slope rocks are deformed by a complex arrangement of strike-slip, normal, and reverse faults. The San Pedro Escarpment rises abruptly along the southeast side of Santa Monica Canyon. Reverse faults and folds underpinning this escarpment steepen progressively southeastward. Locally they form flower structures and cut downward into basement rocks. These faults merge downward with the San Pedro Basin fault zone, which is nearly vertical and strike slip. The escarpment and its attendant structures diverge from this strike-slip fault zone and extend for 60 km along the margin, separating the continental shelf from the deep-water basins. The deep-water Santa Monica Basin has large extent but is filled with only a thin (less than 1.5-km) section of what are probably post-Miocene rocks and sediment. Extrapolating ages obtained from Ocean Drilling Program site 1015 indicates that this sedimentary cover is Quaternary, possibly no older than 600 ka. Folds and faults along the base of the San Pedro Escarpment began to form during 8-13 ka ago. Refraction-velocity data show that high-velocity rocks, probably the Catalina Schist or Miocene volcanic rocks, underlie the sedimentary section. The San Pedro Basin developed along a strike-slip fault, widens to the southeast, and is deformed by faults having apparent reverse separation and by folds near Redondo Canyon and the Palos Verdes Peninsula.

California↗

USGS2013

The geologic records of dust in the Quaternary

Study of geologic records of dust composition, sources and deposition rates is important for understanding the role of dust in the overall planetary radiation balance, fertilization of organisms in the world’s oceans, nutrient additions to the terrestrial biosphere and soils, and for paleoclimatic reconstructions. Both glacial and non-glacial processes produce fine-grained particles that can be transported by the wind. Geologic records of dust flux occur in a number of depositional archives for sediments: (1) loess deposits; (2) lake sediments; (3) soils; (4) deep-ocean basins; and (5) ice sheets and smaller glaciers. These archives have several characteristics that make them highly suitable for understanding the dynamics of dust entrainment, transport, and deposition. First, they are often distributed over wide geographic areas, which permits reconstruction of spatial variation of dust flux. Second, a number of dating methods can be applied to sediment archives, which allows identification of specific periods of greater or lesser dust flux. Third, aeolian sediment particle size and composition can be determined so that dust source areas can be ascertained and dust transport pathways can be reconstructed. Over much of the Earth’s surface, dust deposition rates were greater during the last glacial period than during the present interglacial period. A dustier Earth during glacial periods is likely due to increased source areas, greater aridity, less vegetation, lower soil moisture, possibly stronger winds, a decreased intensity of the hydrologic cycle, and greater production of dust-sized particles from expanded ice sheets and glaciers.

Aeolian Research↗

USGS1987

Quaternary silicic pyroclastic deposits of Atitlán Caldera, Guatemala

Atitlán caldera has been the site of several silicic eruptions within the last 150,000 years, following a period of basalt/andesite volcanism. The silicic volcanism began with 5–10 km 3 of rhyodacites, erupted as plinian fall and pyroclastic flows, about 126,000 yr. B.P. At 85,000 yr. B.P. 270–280 km 3 of compositionally distinct rhyolite was erupted in the Los Chocoyos event which produced widely dispersed, plinian fall deposits and widespread, mobile pyroclastic flows. In the latter parts of this eruption rhyodacite and minor dacite were erupted which compositionally resembled the earliest silicic magmas of the Atitlán center. As a result of this major eruption, the modern Atitlán (III) caldera formed. Following this event, rhyodacites were again erupted in smaller (5–13 km 3 ) volumes, partly through the lake, and mafic volcanism resumed, forming three composite volcanoes within the caldera. The bimodal mafic/silicic Atitlán volcanism is similar to that which has occurred elsewhere in the Guatemalan Highlands, but is significantly more voluminous. Mafic lavas are thought to originate in the mantle, but rise, intrude and underplate the lower crust and partly escape to the surface. Eventually, silicic melts form in the crust, possibly partly derived from underplated basaltic material, rise, crystallize and erupt. The renewed mafic volcanism could reflect either regional magmato-tectonic adjustment after the large silicic eruption or the onset of a new cycle.

Journal of Volcanology and Geothermal Research↗

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↗

USGS2012

The growth of geological structures by repeated earthquakes: 2, Field examples of continental dip-slip faults

A strong test of our understanding of the earthquake cycle is the ability to reproduce extant fault-bounded geological structures, such as basins and ranges, which are built by repeated cycles of deformation. Along strike-slip faults, the coseismic and interseismic deformation can be nearly equal in magnitude and opposite in sign, resulting in little permanent deformation except for the fault offset. For dip-slip faults, portions of the crust are lifted and dropped, and so buoyancy forces are exerted. The seismic and interseismic deformations do not balance, and structures grow and become subject to erosion and deposition. We consider three examples for which the structure and fault geometry are well known: the White Wolf reverse fault in California, site of the 1952 Kern County M=7.3 earthquake, the Lost River normal fault in Idaho, site of the 1983 Borah Peak M=7.0 earthquake, and the Cricket Mountain normal fault in Utah, site of Quaternary slip events. Basin stratigraphy and seismic reflection records are used to profile the structure, and coseismic deformation measured by leveling surveys is used to estimate the fault geometry. To reproduce these structures, we add the deformation associated with the earthquake cycle (the coseismic slip and postseismic relaxation) to the flexure caused by the observed sediment load, treating the crust as a thin elastic plate overlying a fluid substrate. The cumulative deformation is principally dependent on the elastic plate thickness, modestly sensitive to the sediment-substrate density difference, and insensitive to the fluid viscosity for the 4- to 8-Ma structures. We deduce a longterm flexural rigidity of 2–15 × 10 19 Nm; this is equivalent to an elastic plate thickness of 2–4 km for a Young's modulus of 2.5 × 10 10 Nm −2 . This value is found where independent estimates of the elastic thickness from the coherence between surface topography and gravity yield values of about 4 km, but where coseismic fault slip extends to a depth of 10–15 km. Thus much of the seismogenic crust must weaken substantially during the life of active faults, causing the fault-bounded basins to narrow over time.

Journal of Geophysical Research Solid Earth↗

USGS2025

Late Quaternary pollen record from southwest Seward Peninsula, western Alaska, and the vegetation history of central Beringia

Pollen analysis of samples from a coastal exposure near Teller, southwestern Seward Peninsula, Alaska, provides a record of vegetation and climate spanning the Last Glacial Maximum (LGM) through the Holocene. The site is near the center of the former Bering Land Bridge (BLB). The oldest pollen-bearing sediment unit is a loess deposit of LGM age, with pollen assemblages that closely resemble LGM assemblages from other key sites in central Beringia spanning 16° of latitude. These fossil assemblages represent vegetation composed primarily of grasses, sedges, Artemisia , willows, and forbs and are interpreted to represent steppe–tundra, associated with dry climates and summer temperatures cooler than at present. LGM mosses did not accumulate insulating layers of peat; the summer active soil layer was deeper than at present. Permafrost with ice wedges and loess deposition were widespread. A regional transition from steppe–tundra vegetation to a dwarf shrub–sedge–moss mesic-to-wetland vegetation began in central Beringia with the onset of Bølling–Allerød (B-A) warming at 14,700 cal yr BP. Warming events of the B-A and early Holocene resulted in widespread development of thermokarst terrain on the BLB and on ice-rich terrain in Western Alaska. Mesic climates and vegetation developed on the BLB during the marine transgression and because of B-A and early Holocene warming. Early Holocene warming allowed some boreal forest species such as alders to begin colonizing Western Alaska from the interior.

Alaska↗

USGS2023

Magnitude conversion and earthquake recurrence rate models for the central and eastern United States

Development of Seismic Source Characterization (SSC) models, which is an essential part of Probabilistic Seismic Hazard Analyses (PSHA), can help forecast the temporal and spatial distribution of future damaging earthquakes (𝑀 w ≥ 5) in seismically active regions. Because it is impossible to associate all earthquakes with known faults, seismic source models for PSHA often include sources of diffuse seismicity in which future earthquake scenarios are not localized on mapped faults. These sources of diffuse seismicity are referred to as area source zones, distributed seismicity zones, or just source zones. During the early years of PSHA studies, it was assumed that earthquakes in seismotectonic zones have (1) uniform spatial distribution, (2) Poisson temporal distribution, and (3) exponential magnitude distribution (NRC, 2012). In seismically active regions (e.g., the Western United States), where active faults are readily identified, models of the spatial distribution of earthquakes include both the fault source geometries and the distributed seismicity (background) source zones. Source characterization of active faults is complemented by paleoseismic studies with estimates of earthquake magnitudes, dates of occurrences, and slip rates, which provide important information for PSHA studies. In the Central and Eastern United States (CEUS) very few Quaternary-active faults have the requisite information for use in PSHA (i.e., fault geometry and dimensions, event rates or slip rates, etc.), and we lack knowledge about the causative faults for most observed seismicity in the region. As a result, area source zones are frequently used in site-specific PSHA in the CEUS to represent diffuse seismicity that cannot be associated with faults. However, there are examples of active fault sources in the CEUS, such as the Meers fault, the Cheraw fault, and New Madrid region, where individual faults can be characterized. The source characterization models for background seismicity are based, to a large extent, on an assumption that spatial distribution of historical and recorded seismicity will not change substantially for time periods of interest for PSHA (approximately the next 50-100 years for engineered structures). Furthermore, studies such as those by Kafka (2007, 2009) found a correlation between the locations of small- to moderate-magnitude earthquakes and the locations of large-magnitude earthquakes, indicating that we can, with some level of confidence, use the spatial pattern of smaller earthquakes to forecast the future pattern of damaging earthquakes. Within background seismicity zones, the earthquake rate forecast is developed using spatial smoothing of the small to moderate magnitude events in earthquake catalogs. Different methodologies are used for this purpose and can predict varying distributions of seismicity rates. This in turn affects the results of a seismic hazard analysis. The U.S. Geological Survey (USGS) and Nuclear Regulatory Commission (NRC) use different methods for computing spatially smoothed seismicity rates in the CEUS; the USGS uses kernel-based spatial smoothing methods in developing the National Seismic Hazard Model (NSHM), and the method adopted in the Central and Eastern United States Seismic Source Characterization (CEUS-SSC) project is used when evaluating seismic hazard for nuclear power plant siting. These methods are described and the impact on seismic hazard are evaluated in this Research Information Letter (RIL). Another important input to estimating the rate of distributed seismicity is event magnitudes listed in earthquake catalogs. A substantial source of uncertainty in catalogs is the magnitude assigned to a given earthquake. Numerous different magnitude types exist, with each magnitude type computed in a different way. Therefore, for the sake of consistency, both the CEUS-SSC and the USGS NSHM have attempted to assemble a complete catalog with a uniform magnitude determination. To this end, moment magnitude, 𝑀 w , which is a physics-based measurement, has been adopted as the standard. However, 𝑀 w was not computed routinely until the past few decades. To address this issue, the CEUS-SSC conducted extensive analyses to determine conversion equations from which to take a routinely computed network (e.g., 𝑀 L or 𝑚 bLg ) and convert it into 𝑀 w . Another issue with using 𝑀 w is that it becomes increasingly difficult to compute for earthquakes with 𝑀 less than ~4. This study investigates the effects of moment magnitude estimation and spatial smoothing methods on estimation of the earthquake rate forecast and on seismic hazard. We investigate the validity of the magnitude conversion equations and their associated uncertainties by applying them to a case study for induced earthquakes in southern Kansas and northern Oklahoma, and summarize the use of the decay of the seismic coda to estimate 𝑀 w for small earthquakes (𝑀 w < 4. Furthermore, the study documents a comparison and assessment of background seismicity smoothing methods implemented by the USGS for the NSHM and used by the CEUS-SSC for siting nuclear facilities based on probabilistic seismic hazard estimates from multiple source zones in the CEUS and for multiple sites.

central and eastern United States↗

USGS2023

Regional variability in peatland burning at mid-to high-latitudes during the Holocene

Northern peatlands store globally-important amounts of carbon in the form of partly decomposed plant detritus. Drying associated with climate and land-use change may lead to increased fire frequency and severity in peatlands and the rapid loss of carbon to the atmosphere. However, our understanding of the patterns and drivers of peatland burning on an appropriate decadal to millennial timescale relies heavily on individual site-based reconstructions. For the first time, we synthesise peatland macrocharcoal records from across North America, Europe, and Patagonia to reveal regional variation in peatland burning during the Holocene. We used an existing database of proximal sedimentary charcoal to represent regional burning trends in the wider landscape for each region. Long-term trends in peatland burning appear to be largely climate driven, with human activities likely having an increasing influence in the late Holocene. Warmer conditions during the Holocene Thermal Maximum (∼9–6 cal. ka BP) were associated with greater peatland burning in North America's Atlantic coast, southern Scandinavia and the Baltics, and Patagonia. Since the Little Ice Age, peatland burning has declined across North America and in some areas of Europe. This decline is mirrored by a decrease in wider landscape burning in some, but not all sub-regions, linked to fire-suppression policies, and landscape fragmentation caused by agricultural expansion. Peatlands demonstrate lower susceptibility to burning than the wider landscape in several instances, probably because of autogenic processes that maintain high levels of near-surface wetness even during drought. Nonetheless, widespread drying and degradation of peatlands, particularly in Europe, has likely increased their vulnerability to burning in recent centuries. Consequently, peatland restoration efforts are important to mitigate the risk of peatland fire under a changing climate. Finally, we make recommendations for future research to improve our understanding of the controls on peatland fires.

Quaternary Science Reviews↗

USGS2009

Processes that initiate turbidity currents and their influence on turbidites: A marine geology perspective

How the processes that initiate turbidity currents influence turbidite deposition is poorly understood, and many discussions in the literature rely on concepts that are overly simplistic. Marine geological studies provide information on the initiation and flow path of turbidity currents, including their response to gradient. In case studies of late Quaternary turbidites on the eastern Canadian and western U.S. margins, initiation processes are inferred either from real-time data for historical flows or indirectly from the age and contemporary paleogeography, erosional features, and depositional record. Three major types of initiation process are recognized: transformation of failed sediment, hyperpycnal flow from rivers or ice margins, and resuspension of sediment near the shelf edge by oceanographic processes. Many high-concentration flows result from hyperpycnal supply of hyperconcentrated bedload, or liquefaction failure of coarse-grained sediment, and most tend to deposit in slope conduits and on gradients < 0.5° at the base of slope and on the mid fan. Highly turbulent flows, from transformation of retrogressive failures and from ignitive flows that are triggered by oceanographic processes, tend to cannibalize these more proximal sediments and redeposit them on lower gradients on the basin plain. Such conduit flushing provides most of the sediment in large turbidites. Initiation mechanism exerts a strong control on the duration of turbidity flows. In most basins, there is a complex feedback between different types of turbidity-current initiation, the transformation of the flows, and the associated slope morphology. As a result, there is no simple relationship between initiating process and type of deposit.

Journal of Sedimentary Research↗

USGS2003

Slip rate and earthquake recurrence along the central Septentrional fault, North American-Caribbean plate boundary, Dominican Republic

The Septentrional fault zone (SFZ) is the major North American-Caribbean, strike-slip, plate boundary fault at the longitude of eastern Hispaniola. The SFZ traverses the densely populated Cibao Valley of the Dominican Republic, forming a prominent scarp in alluvium. Our studies at four sites along the central SFZ are aimed at quantifying the late Quaternary behavior of this structure to better understand the seismic hazard it represents for the northeastern Caribbean. Our investigations of excavations at sites near Rio Cenovi show that the most recent ground-rupturing earthquake along this fault in the north central Dominican Republic occurred between A.D. 1040 and A.D. 1230, and involved a minimum of ???4 m of left-lateral slip and 2.3 m of normal dip slip at that site. Our studies of offset stream terraces at two locations, Rio Juan Lopez and Rio Licey, provide late Holocene slip rate estimates of 6-9 mm/yr and a maximum of 11-12 mm/yr, respectively, across the Septentrional fault. Combining these results gives a best estimate of 6-12 mm/yr for the slip rate across the SFZ. Three excavations, two near Tenares and one at the Rio Licey site, yielded evidence for the occurrence of earlier prehistoric earthquakes. Dates of strata associated with the penultimate event suggest that it occurred post-A.D. 30, giving a recurrence interval of 800-1200 years. These studies indicate that the SFZ has likely accumulated elastic strain sufficient to generate a major earthquake during the more than 800 years since it last slipped and should be considered likely to produce a destructive future earthquake.

Journal of Geophysical Research B: Solid Earth↗

USGS2012

Is the extent of glaciation limited by marine gas-hydrates?

Methane may have been released to the atmosphere during the Quaternary from Arctic shelf gas-hydrates as a result of thermal decomposition caused by climatic warming and rising sea-level; this release of methane (a greenhouse gas) may represent a positive feedback on global warming [Revelle, 1983; Kvenvolden, 1988a; Nisbet, 1990]. We consider the response to sea-level changes by the immense amount of gas-hydrate that exists in continental rise sediments, and suggest that the reverse situation may apply—that release of methane trapped in the deep-sea sediments as gas-hydrates may provide a negative feedback to advancing glaciation. Methane is likely to be released from deep-sea gas-hydrates as sea-level falls because methane gas-hydrates decompose with pressure decrease. Methane would be released to sediment pore space at shallow sub-bottom depths (100's of meters beneath the seafloor, commonly at water depths of 500 to 4,000 m) producing zones of markedly decreased sediment strength, leading to slumping [Carpenter, 1981; Kayen, 1988] and abrupt release of the gas. Methane is likely to be released to the atmosphere in spikes that become larger and more frequent as glaciation progresses. Because addition of methane to the atmosphere warms the planet, this process provides a negative feedback to glaciation, and could trigger deglaciation.

Geophysical Research Letters↗

USGS2006

Surficial sediment character of the Louisiana offshore continental shelf region: A GIS compilation

The Louisiana coastal zone, comprising the Mississippi River delta plain stretching nearly 400 km from Sabine Pass at the Texas border east to the Chandeleur Islands at the Mississippi border, represents one of North America’s most important coastal ecosystems in terms of natural resources, human infrastructure, and cultural heritage. At the same time, this region has the highest rates of coastal erosion and wetland loss in the Nation due to a complex combination of natural processes and anthropogenic actions over the past century. Comparison of historical maps dating back to 1855 and recent aerial photography show the Louisiana coast undergoing net erosion at highly variable rates. Rates have increased significantly during the past several decades. Earlier published statewide average shoreline erosion rates were >6 m/yr; rates have increased recently to >10 m/yr. The increase is attributable to collective action of storms, rapid subsidence, and pervasive man-made alterations of the rivers and the coast. In response to the dramatic landloss, regional-scale restoration plans are being developed by a partnership of federal and state agencies for the delta plain that have the objectives of maintaining the barrier islands, reducing wetland loss, and enhancing the natural sediment delivery processes. There is growing awareness that the sustainability of coastal Louisiana's natural resources and human infrastructure depends on the successful restoration of natural geologic processes. Critical to the long term success of restoration is scientific understanding of the geologic history and processes of the coastal zone region, including interactions between the rivers, wetlands, coast, and inner shelf. A variety of geophysical studies and mapping of Late Quaternary sedimentary framework and coastal processes by U.S. Geological Survey and other scientists during the past 50 years document that the Louisiana delta plain is the product of a complex history of cyclic delta switching by the Mississippi River and its distributaries over the past ~10,000 years that resulted in laterally overlapping deltaic depocenters. The interactions among riverine, coastal, and inner shelf processes have been superimposed on the Holocene transgression resulting in distinctive landforms and sedimentary sequences. Four Holocene shelf-phase delta complexes have been identified using seismic reflection data and vibracores. Each delta complex is bounded by transgressive surfaces. Following each cycle of deposition and abandonment, the delta lobes undergo regional subsidence and marine reworking that forms transgressive coastal systems and barrier islands. Ultimately, the distal end of each of the abandoned delta lobes is marked by submerged marine sand bodies representing drowned barriers. These sand bodies (e.g. Ship Shoal, Outer Shoal, Trinity Shoal, Tiger Shoal, St. Bernard Shoal) offer the largest volumes and highest quality sand for beach nourishment and shoreline and wetlands restoration. These four large sand shoals on inner continental shelf, representing the reworked remnants of former prograded deltaic headlands that existed on the continental shelf at lower sea level, were generated in the retreat path of the Mississippi River delta plain during the Holocene transgression. Penland and others (1989) have shown these sand bodies represent former shoreline positions associated with lower still stands in sea level. Short periods of rapid relative sea-level rise led to the transgressive submergence of the shorelines which today can be recognized at the -10 m to -20 m isobaths on the Louisiana continental shelf. Trinity Shoal and Ship Shoal represent the -10 m middle-to-late Holocene shoreline trend, whereas Outer Shoal and the St. Bernard Shoals define the -20 m early Holocene shoreline trend (Penland and others, 1989). Collectively, these sand shoals constitute a large volume of high quality sandy sediment potentially suitable for barrier island nourishment and coastal restoration. The USGS has actively supported coastal and wetlands geologic research for the past two decades in partnership with universities (e.g., Louisiana State University, University of New Orleans), state agencies (e.g. Louisiana Geological Survey, Louisiana Department of Natural Resources), and private organizations (Williams and others, 1992a,b; Williams and Cichon, 1993; List and others, 1994). These studies have focused on regional-scale mapping of coastal and wetland change and developing a better understanding of the processes that cause coastal erosion and wetlands loss, particularly the rapid deterioration of Louisiana's barrier islands, estuaries, and wetlands environments. With a better understanding of these processes, the ability to model and predict erosion and wetlands loss will improve. More accurate predictions will, in turn, allow for proper management of coastal resources. Improved predictions will also allow for better assessments of the utility of different restoration alternatives.

Louisiana↗

USGS1974

Time of travel and dye dosage for an irrigation canal system near Duchesne, Utah

Fracturing and subsidence of the land surface in the Milford area oil Utah have resulted from the decline of water levels due to pumping in unconsolidated deposits of Quaternary age. To the writers’ knowledge, these are the first such effects of ground-water withdrawal reported in Utah. The fracturing is in an area about 1 mile (1.6 km) wide and 11 miles (18 km) long near Milford, in an unsaturated clay-silt zone (locally peaty at top) in the upper part of the principal ground-water reservoir. The fractures range in length from several feet to more than 100 feet (30 m), and their maximum measured depth in 1972 was 4 feet (1.2 m). Land subsidence in the Milford area is demonstrated by three lines of evidence: (1) collapse structures, (2) well casings that protrude higher above the land surface than when first placed in the borehole, and (3) lower elevations at National Ocean Survey (formerly U.S. Coast and Geodetic Survey) bench marks in 1970 than in 1908. This evidence shows that land subsidence in the Milford area is of two types, each having a different origin. One type has a near-surface origin in the clay-silt zone in the upper part of the principal groundwater reservoir, and the other is in the lower artesian aquifers of the principal ground-water reservoir. The amount of observed subsidence ranges from 0.05 foot (0.015 m) at the bench mark at Read to about 6 feet (1.8 m) at collapse structures in the Hay Springs area.

Utah↗

USGS1976

Fracturing and subsidence of the land surface caused by the withdrawal of ground water in the Milford area, Utah

Fracturing and subsidence of the land surface in the Milford area of Utah have resulted from the decline of water levels due to pumping in unconsolidated deposits of Quaternary age. To the writers' knowledge, these are the first such effects of ground-water withdrawal reported in Utah. The fracturing is in an area about 1 mile (1.6 km) wide and 11 miles (18 km) long near Milford, in an unsaturated clay-silt zone (locally peaty at top) in the upper part of the principal ground-water reservoir. The fractures range in length from several feet to more than 100 feet (30 m), and their maximum measured depth in 1972 was 4 feet (1.2 m). Land subsidence in the Milford area is demonstrated by three lines of evidence: (1) collapse structures, (2) well casings that protrude higher above the land surface than when first placed in the borehole, and (3) lower elevations at National Ocean Survey (formerly U.S. Coast and Geodetic Survey) bench marks in 1970 than in 1908. This evidence shows that land subsidence in the Milford area is of two types, each having a different origin. One type has a near-surface origin in the clay-silt zone in the upper part of the principal groundwater reservoir, and the other is in the lower artesian aquifers of the principal ground-water reservoir. The amount of observed subsidence ranges from 0.05 foot (0.015 m) at the bench mark at Read to about 6 feet (1.8 m) at collapse structures in the Hay Springs area.

Utah↗

USGS2014

Latest Quaternary paleoseismology and evidence of distributed dextral shear along the Mohawk Valley fault zone, northern Walker Lane, California

The dextral-slip Mohawk Valley fault zone (MVFZ) strikes northwestward along the eastern margin of the Sierra Nevada in the northern Walker Lane. Geodetic block modeling indicates that the MVFZ may accommodate ~3 mm/yr of regional dextral strain, implying that it is the highest slip-rate strike-slip fault in the region; however, only limited geologic data are available to constrain the system’s slip rate and earthquake history. We mapped the MVFZ using airborne lidar data and field observations and identified a site near Sulphur Creek for paleoseismic investigation. At this site, oblique dextral-normal faulting on the steep valley margin has created a closed depression that floods annually during spring snowmelt to form an ephemeral pond. We excavated three fault-perpendicular trenches at the site and exposed pond sediment that interfingers with multiple colluvial packages eroded from the scarp that bounds the eastern side of the pond. We documented evidence for four surface-rupturing earthquakes on this strand of the MVFZ. OxCal modeling of radiocarbon and luminescence ages indicates that these earthquakes occurred at 14.0 ka, 12.8 ka, 5.7 ka, and 1.9 ka. The mean ~4 kyr recurrence interval is inconsistent with slip rates of ~3 mm/yr; these rates imply surface ruptures of more than 10 m per event, which is geologically implausible for the subdued geomorphic expression and 60 km length of the MVFZ. We propose that unidentified structures not yet incorporated into geodetic models may accommodate significant dextral shear across the northern Walker Lane, highlighting the role of distributed deformation in this region.

California↗

USGS2000

Preliminary report on the 16 October 1999 M 7.1 Hector mine, California, earthquake

The M w 7.1 Hector Mine, California, earthquake occurred at 9:46 GMT on 16 October 1999. The event caused minimal damage because it was located in a remote, sparsely populated part of the Mojave Desert, approximately 47 miles east-southeast of Barstow, with epicentral coordinates 34.59°N 116.27°W and a hypocentral depth of 5 ± 3 km. Twelve foreshocks, M 1.9-3.8, preceded the mainshock during the previous twelve hours. All of these events were located close to the hypocenter of the mainshock. The Hector Mine earthquake occurred within the Eastern California Shear Zone (ECSZ). By virtue of its remote location, the societal impact of the Hector Mine earthquake was, fortunately, minimal in spite of the event's appreciable size. The ECSZ is characterized by high seismicity, a high tectonic strain rate, and a broad, distributed zone of north-northwest-trending faults (ECSZ; Figure 1 ; Dokka and Travis, 1990 ; Sauber et al., 1986 ; Sauber et al., 1994 ; Sieh et al., 1993 ). Data regarding the slip rates of faults within the ECSZ suggest that on the order of 15% of the Pacific-North American plate motion occurs along this zone ( Sauber et al., 1986 ; Wesnousky, 1986 ). Most of the faults in the ECSZ have low slip rates and long repeat times for major earthquakes, on the order of several thousands to tens of thousands of years. The occurrence of the Hector Mine earthquake within seven years and only about 30 km east of the 1992 M w 7.3 Landers earthquake suggests that the closely spaced surface faults in the ECSZ are mechanically related. The Hector Mine event involved rupture on two previously mapped fault zones—the Bullion Fault and an unnamed, more northerly-trending fault that is informally referred to in this paper as the Lavic Lake Fault (Dibblee, 1966 , 1967a , b ). Traces of the Bullion Fault exhibit evidence of Holocene displacement and were zoned as active in 1988 under California's Mquist-Priolo Earthquake Fault Zoning Act ( Hart and Bryant, 1997 ). The pattern of rupture along more than one named fault was also observed from the 1992 Landers earthquake ( Hauksson et al., 1993 ; Sieh et al., 1994). Much of the fault zone that produced the Hector Mine earthquake had been buried by relatively young stream deposits, and the fault scarps in bedrock have a subdued morphology. It appears that these faults have not experienced significant offset for perhaps 10,000 years or more ( Hart, 1987 ). Planned future investigations will refine the age of the last event on these faults. The portion of the Lavic Lake Fault that ruptured between the northern end of the Bullion Mountains and Lavic Lake had not previously been mapped. However, our field investigations have identified ancient, subdued fault scarps along portions of the 1999 rupture zone in this area. It thus appears that the entire segment of the Lavic Lake Fault that was involved in the 1999 event had ruptured in the past. As is typical for most faults within the Eastern California Shear Zone, the rate of movement along the Lavic Lake Fault may be quite slow (<1 mm/yr) and should produce earthquakes only infrequently. This event is a reminder that faults that have ruptured in late Quaternary time, but that lack evidence of Holocene displacement, can still produce earthquakes in this low-slip-rate tectonic setting. Additionally, the Hector Mine earthquake is noteworthy for a couple of other reasons. First, it clearly produced triggered seismicity over much of southern California, from the rupture zone toward the south-southwest in particular. Second, as we will discuss, the event may provide new data and insight into recently developed paradigms concerning earthquake interactions and the role of static stress changes. Questions such as these will, of course, be the subject of extensive detailed analyses in years to come. Fortunately, the Hector Mine sequence will provide one of the best data sets obtained to date for a significant earthquake in the United States. Because it occurred when major upgrades to both the regional seismic network (TriNet) and the regional geodetic network (SCIGN) were well underway, the Earth science community will have abundant high-quality data with which to explore the important and interesting questions that have been raised. In this paper, we present and discuss the basic data and preliminary results from the Hector Mine earthquake.

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