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At least 271 records · Page 15Linked to original sources
USGS2014

Inundation of a barrier island (Chandeleur Islands, Louisiana, USA) during a hurricane: Observed water-level gradients and modeled seaward sand transport

Large geomorphic changes to barrier islands may occur during inundation, when storm surge exceeds island elevation. Inundation occurs episodically and under energetic conditions that make quantitative observations difficult. We measured water levels on both sides of a barrier island in the northern Chandeleur Islands during inundation by Hurricane Isaac. Wind patterns caused the water levels to slope from the bay side to the ocean side for much of the storm. Modeled geomorphic changes during the storm were very sensitive to the cross-island slopes imposed by water-level boundary conditions. Simulations with equal or landward sloping water levels produced the characteristic barrier island storm response of overwash deposits or displaced berms with smoother final topography. Simulations using the observed seaward sloping water levels produced cross-barrier channels and deposits of sand on the ocean side, consistent with poststorm observations. This sensitivity indicates that accurate water-level boundary conditions must be applied on both sides of a barrier to correctly represent the geomorphic response to inundation events. More broadly, the consequence of seaward transport is that it alters the relationship between storm intensity and volume of landward transport. Sand transported to the ocean side may move downdrift, or aid poststorm recovery by moving onto the beach face or closing recent breaches, but it does not contribute to island transgression or appear as an overwash deposit in the back-barrier stratigraphic record. The high vulnerability of the Chandeleur Islands allowed us to observe processes that are infrequent but may be important at other barrier islands.

Louisiana↗
USGS2013

Mw 8.6 Sumatran earthquake of 11 April 2012: rare seaward expression of oblique subduction

The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ~20-50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004.

Geology↗
USGS2007

Geologic framework studies of South Carolina's Long Bay from Little River Inlet to Winyah Bay, 1999-2003: Geospatial data release

The northern South Carolina coast is a heavily developed region that supports a thriving tourism industry, large local populations and extensive infrastructure (Figure 1). The economic stability of the region is closely tied to the health of its beaches: primarily in providing support for local tourism and protection from storm events. Despite relatively low long-term shoreline erosion rates, and the implied stability of the beaches, the economic impact of storm events to coastal communities has been costly. For example, Hurricane Hugo made landfall on the central South Carolina coast in 1989. High winds and storm surge inflicted roughly $6 billion in property loss and damages, and Hugo remains the costliest storm event in South Carolina history. Localized erosion, commonly occurring around tidal inlets and erosion "hot spots", has also proved costly. Construction and maintenance of hard structures and beach nourishment, designed to mitigate the effects of erosion, have become annual or multi-annual expenditures. Providing a better understanding of the physical processes controlling coastal erosion and shoreline change will allow for more effective management of coastal resources. In 1999, the U.S. Geological Survey (USGS), in partnership with the South Carolina Sea Grant Consortium (SCSGC), began a study to investigate inner continental shelf and shoreface processes. The objectives of the USGS/SCSGC cooperative program are: 1) to provide a regional synthesis of the shallow geologic framework underlying the shoreface and inner continental shelf, and to define its role in coastal evolution and modern beach behavior; 2) to identify and model the physical processes affecting coastal ocean circulation and sediment transport, and to define their role in shaping the modern shoreline; and 3) to identify sediment sources and transport pathways in order to develop a regional sediment budget. This report contains the geospatial data used to define the geologic framework offshore of the northern South Carolina coast. The digital data presented herein accompany USGS Open-File Reports OFR 2004-1013 and OFR 2005-1345, describing the stratigraphic framework and modern sediment distribution within Long Bay, respectively. Direct on-line links to these publications are available within 'References' on the navigation bar to the left. Additional links to other publications and web sites are also available.

South Carolina↗
USGS2004

Modeling a three-dimensional river plume over continental shelf using a 3D unstructured grid model

River derived fresh water discharging into an adjacent continental shelf forms a trapped river plume that propagates in a narrow region along the coast. These river plumes are real and they have been observed in the field. Many previous investigations have reported some aspects of the river plume properties, which are sensitive to stratification, Coriolis acceleration, winds (upwelling or downwelling), coastal currents, and river discharge. Numerical modeling of the dynamics of river plumes is very challenging, because the complete problem involves a wide range of vertical and horizontal scales. Proper simulations of river plume dynamics cannot be achieved without a realistic representation of the flow and salinity structure near the river mouth that controls the initial formation and propagation of the plume in the coastal ocean. In this study, an unstructured grid model was used for simulations of river plume dynamics allowing fine grid resolution in the river and in regions near the coast with a coarse grid in the far field of the river plume in the coastal ocean, in the vertical, fine fixed levels were used near the free surface, and coarse vertical levels were used over the continental shelf. The simulations have demonstrated the uniquely important role played by Coriolis acceleration. Without Coriolis acceleration, no trapped river plume can be formed no matter how favorable the ambient conditions might be. The simulation results show properties of the river plume and the characteristics of flow and salinity within the estuary; they are completely consistent with the physics of estuaries and coastal oceans.

Conference Paper↗
USGS2012

IOOS modeling subsystem: vision and implementation strategy

Numerical modeling is vital to achieving the U.S. IOOS® goals of predicting, understanding and adapting to change in the ocean and Great Lakes. In the next decade IOOS should cultivate a holistic approach to coastal ocean prediction, and encourage more balanced investment among the observing, modeling and information management subsystems. We believe the vision of a prediction framework driven by observations, and leveraging advanced technology and understanding of the ocean and Great Lakes, would lead to a new era for IOOS that would not only produce more powerful information, but would also capture broad community support, particularly from the general public, thus allowing IOOS to develop into the comprehensive information system that was envisioned at the outset.

Book↗
USGS2011

Equilibrium shoreline response of a high wave energy beach

Four years of beach elevation surveys at Ocean Beach, San Francisco, California, are used to extend an existing equilibrium shoreline change model, previously calibrated with fine sand and moderate energy waves, to medium sand and higher-energy waves. The shoreline, characterized as the cross-shore location of the mean high water contour, varied seasonally by between 30 and 60 m, depending on the alongshore location. The equilibrium shoreline change model relates the rate of horizontal shoreline displacement to the hourly wave energy E and the wave energy disequilibrium, the difference between E and the equilibrium wave energy that would cause no change in the present shoreline location. Values for the model shoreline response coefficients are tuned to fit the observations in 500 m alongshore segments and averaged over segments where the model has good skill and the estimated effects of neglected alongshore sediment transport are relatively small. Using these representative response coefficients for 0.3 mm sand from Ocean Beach and driving the model with much lower-energy winter waves observed at San Onofre Beach (also 0.3 mm sand) in southern California, qualitatively reproduces the small seasonal shoreline fluctuations at San Onofre. This consistency suggests that the shoreline model response coefficients depend on grain size and may be constant, and thus transportable, between sites with similar grain size and different wave climates. The calibrated model response coefficients predict that for equal fluctuations in wave energy, changes in shoreline location on a medium-grained (0.3 mm) beach are much smaller than on a previously studied fine-grained (0.2 mm) beach. Copyright ?? 2011 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗
USGS1980

Ocean eddy structure by satellite radar altimetry required for iceberg towing

Models for the towing of large tabular icebergs give towing speeds of 0.5 knots to 1.0 knots relative to the ambient near surface current. Recent oceanographic research indicates that the world oceans are not principally composed of large steady-state current systems, like the Gulf Stream, but that most of the ocean momentum is probably involved in intense rings, formed by meanders of the large streams, and in mid-ocean eddies. These rings and eddies have typical dimensions on the order of 200 km with dynamic height anomalies across them of tens-of-centimeters to a meter. They migrate at speeds on the order of a few cm/sec. Current velocities as great as 3 knots have been observed in rings, and currents of 1 knot are common. Thus, the successful towing of icebergs is dependent on the ability to locate, measure, and track ocean rings and eddies. To accomplish this systematically on synoptic scales appears to be possible only by using satelliteborne radar altimeters. Ocean current and eddy structures as observed by the radar altimeters on the GEOS-3 and Seasat-1 satellites are presented and compared. Several satellite programs presently being planned call for flying radar altimeters in polar or near-polar orbits in the mid-1980 time frame. Thus, by the time tows of large icebergs will probably be attempted, it is possible synoptic observations of ocean rings and eddies which can be used to ascertain their location, size, intensity, and translation velocity will be a reality.

Cold Regions Science and Technology↗
USGS2023

A 600-kyr reconstruction of deep Arctic seawater δ18O from benthic foraminiferal oxygen isotopes and ostracode Mg/Ca paleothermometry

The oxygen isotopic composition of benthic foraminiferal tests ( δ 18 O b ) is one of the pre-eminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ 18 O b applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ 18 O b variability compared to open-ocean records. Here we evaluate the controls on Arctic δ 18 O b by using ostracode Mg / Ca "> paleothermometry to generate a composite record of the δ 18 O of seawater ( δ 18 O sw ) from 12 sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) that covers the last 600 kyr based on biostratigraphy and orbitally tuned age models. Results show that Arctic δ 18 O b was generally higher than open-ocean δ 18 O b during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial–interglacial Arctic δ 18 O b range resulted in part from the opposing effect of temperature, with intermediate to deep Arctic warming during glacials counteracting the whole-ocean δ 18 O sw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ 18 O b , we find that the intermediate to deep Arctic experienced large ( ≥1 ‰) variations in local δ 18 O sw , with generally higher local δ 18 O sw during interglacials and lower δ 18 O sw during glacials. Both the magnitude and timing of low local δ 18 O sw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ 18 O sw in the intermediate to deep Arctic Ocean during glaciations reflected weaker upper-ocean stratification and more efficient transport of low- δ 18 O sw Arctic surface waters to depth by mixing and/or brine rejection.

Climate of the Past↗
USGS1973

Mantle convection and volcanic periodicity in the pacific; Evidence from Hawaii

The thermal-feedback theory of mantle melting proposed by Shaw in 1969 is found to be quantitatively consistent with data pertaining to the evolution of the Hawaiian Ridge. Applicable rate factors are estimated from relations between lava volumes and position along the ridge given in this paper and the radio-metric age distributions given by Jackson and others in 1972. Rate curves derived from these data provide a new method of age extrapolation or interpolation; results indicate that previous methods used to estimate the age of the Hawaiian-Emperor Bend are in error. No definite age is established, but calculations suggest an age greater than 50 m.y. Much more extensive radiometric data are required to define kinematic relations between the Hawaiian Ridge and Emperor Seamount chain. It appears to be firmly established from the work of Jackson and others and from the present study that the evolution of the Hawaiian Ridge has been episodic, with episodes of several different time scales. Average growth rates of the entire ridge system are divided into two regimes with a discontinuity at a position roughly 1,000 km northwest of Kilauea; the estimated age of this discontinuity is about 10 m.y. Other episodes relate to the durations of eruptive sequences along individual or contiguous lines of volcanoes within the en échelon set of locus lines defined by Jackson and others. The latest of these episodes, beginning about 6 m.y. ago, is marked by accelerating volume rates of eruption and accelerating rates of ridge propagation; this episode appears to be approaching a culminating stage represented by the present activity of Kilauea Volcano. The calculated rate of eruption of Kilauea (0.11 km 3 per yr) is virtually identical with a rate independently estimated by Swanson in 1972 using different data. Calculated durations for older locus lines are generally greater than 6 m.y., but major time overlaps occur that are not adequately understood. Episodic behavior of shorter durations also exists relative to growth of individual shields or to synchronous activity on neighboring shields (for example, Mauna Loa and Kilauea). Some of these shorter term effects are partly explained in terms of isostatic factors acting on the lithosphere and asthenosphere. The longer episodes are explained in terms of variations of melting rates in the asthenosphere, governed by viscous heating produced by the interaction of lithosphere translation and both vertical and horizontal shear flows in the subjacent mantle. Accelerations of eruption and propagation rates are explained by melting instabilities in the upper zones of the asthenosphere as a result of thermal feedback. During the latest melting episode, shear stresses in the asthenosphere derived from the rate data as interpreted by the thermal feedback model are in the range 100 to 200 bars; apparent viscosities range from 2 × 10 21 to 4 × 10 20 poise, decreasing with increasing melting rate. In general, a thermomechanical model is shown to be consistent with the idea that oceanic melting spots can be fixed relative to the deep mantle, although this invariance is not completely established. The thermal plume model of Morgan is not definitely ruled out but does not seem to be required for internally consistent interpretations of oceanic chains of volcanism. It is concluded that motion vectors of the Pacific plate cannot be inferred directly from rates of propagation of volcanic chains, because these rates reflect local, not average, relative velocities of lithosphere versus mantle flow. During growth of the Hawaiian Ridge, propagation speeds calculated on the basis of rate data for the southeastern Hawaiian Islands ranged from less than 1 cm per yr near the Hawaiian-Emperor Bend to nearly 30 cm per yr at the present ridge front.

Hawaii↗
USGS2007

Break-up of Gondwana and opening of the South Atlantic: Review of existing plate tectonic models

The opening history of both the Weddell Sea and South Atlantic Ocean is critical to understanding the break- up of Gondwana and the evolution of Antarctica since Early Jurassic times. The dispersal of the Gondwanide fragments is important to understanding the development of past ocean circulation. Unfortunately the tectonics of the Weddell Sea region is difficult to resolve because of the paucity of data coverage and the region’s inherent complexity. Although considerable progress has been achieved in the past 20 years with new marine aeromagnetic data and satellite derived gravity anomaly maps, there are still several models in consideration which differ in crustal ages and schemes of opening. In this paper we present a review of four of those models. Focusing on poles of rotation, synthetic isochrons and flowlines, we proceed backward in time beginning at Chron 34 (83.5 Ma), as it is relatively well defined ocean wide, and present crustal age maps that display the estimated trace of the South America-Antarctica-Africa triple junction for each model. We also plot reconstructions at four selected epochs for all models using the same projection and scale to facilitate comparison. The diverse simplifying assumptions that need to be made in every case regarding plate fragmentation to account for the numerous syn-rift basins and periods of stretching are strong indicators that rigid plate tectonics is too simple a model for the present problem.

Open-File Report↗
USGS2017

Uncertainties in forecasting the response of polar bears to global climate change

Several sources of uncertainty affect how precisely the future status of polar bears ( Ursus maritimus ) can be forecasted. Foremost are unknowns about the future levels of global greenhouse gas emissions, which could range from an unabated increase to an aggressively mitigated reduction. Uncertainties also arise because different climate models project different amounts and rates of future warming (and sea ice loss)—even for the same emission scenario. There are also uncertainties about how global warming could affect the Arctic Ocean’s food web, so even if climate models project the presence of sea ice in the future, the availability of polar bear prey is not guaranteed. Under a worst-case emission scenario in which rates of greenhouse gas emissions continue to rise unabated to century’s end, the uncertainties about polar bear status center on a potential for extinction. If the species were to persist, it would likely be restricted to a high-latitude refugium in northern Canada and Greenland—assuming a food web also existed with enough accessible prey to fuel weight gains for surviving onshore during the most extreme years of summer ice melt. On the other hand, if emissions were to be aggressively mitigated at the levels proposed in the Paris Climate Agreement, healthy polar bear populations would probably continue to occupy all but the most southern areas of their contemporary summer range. While polar bears have survived previous warming phases—which indicate some resiliency to the loss of sea ice habitat—what is certain is that the present pace of warming is unprecedented and will increasingly expose polar bears to historically novel stressors.

Book chapter↗
USGS2009

Continuous Tidal Streamflow and Gage-Height Data for Bass and Cinder Creeks on Kiawah Island, South Carolina, September 2007

A three-dimensional model of Bass and Cinder Creeks on Kiawah Island, South Carolina, was developed to evaluate methodologies for determining fecal coliform total maximum daily loads for shellfish waters. To calibrate the model, two index-velocity sites on the creeks were instrumented with continuous acoustic velocity meters and water-level sensors to compute a 21-day continuous record of tidal streamflows. In addition to monitoring tidal cycles, streamflow measurements were made at the index-velocity sites, and tidal-cycle streamflow measurements were made at the mouth of Bass Creek and on the Stono River to characterize the streamflow dynamics near the ocean boundary of the three-dimensional model at the beginning, September 6, 2007, and end, September 26, 2007, of the index-velocity meter deployment. The maximum floodtide and ebbtide measured on the Stono River by the mouth of Bass Creek for the two measurements were -155,000 and 170,000 cubic feet per second (ft3/s). At the mouth of Bass Creek, the maximum floodtide and ebbtide measurements during the 2 measurement days were +/-10,200 ft3/s. Tidal streamflows for the 21-day deployment on Bass Creek ranged from -2,510 ft3/s for an incoming tide to 4,360 ft3/s for an outgoing tide. On Cinder Creek, the incoming and outgoing tide varied from -2,180 to 2,400 ft3/s during the same period.

South Carolina↗
USGS2004

An operation manual for a time-series, storm-activated suspended sediment sampler deployed in the coastal ocean: function, maintenance, and testing procedures

This manual describes the operation and testing procedures for two models of a multi-port suspended sediment sampler that are moored in the coastal ocean and that collect samples on a programmable time schedule that can be interrupted to collect during a storm. The ability to sense and collect samples before, during, and after the height of a storm is a unique feature of these instruments because it provides samples during conditions when it is difficult or impossible to sample from a surface ship. The sensors used to trigger storm sampling are a transmissometer or a pressure sensor. The purpose of such samples is to assess composition and concentration of sediment resuspended from the seafloor during storms and subsequently transported within the coastal system. Both light transmission and the standard deviation of pressure from surface waves correlate with the passage of major storms. The instruments successfully identified the onset of storms and collected samples before, during, and after the storm maximum as programmed. The accuracy of determining suspended matter concentrations collected by the sediment sampler has not been fully evaluated. Preliminary laboratory tests using a suspension of muddy sediment collected in a near-bottom sediment trap yielded excellent results. However in laboratory tests with different sediment types, the suspended matter concentrations determined with these samplers became less accurate with increasing average grain size. Future calibration work is necessary and should be conducted in a facility that ideally has a water depth of at least 30 feet to prevent cavitation of the pump that draws sea water through the filters. The test facility should also have the capability for adding suspended matter of known composition and concentration to a fixed volume of seawater that is well mixed.

Open-File Report↗
USGS1998

Origin of the Columbia River basalts: Melting model of a heterogeneous plume head

In order to study the origin of the Grande Ronde basalts (GRs) erupted in the climax stage of the Columbia River basalts (CRBs), we carried out high pressure melting experiments on four of the most primitive rock compositions representing the Yakima group of the CRBs. The voluminous GRs (constituting >80 vol% of CRBs) are totally aphyric basaltic andesites. GRs show very narrow and coherent chemical trends both in major and trace elements as well as isotopes. The silica-rich GRs (SiO 2 = 52–56 wt%) can be produced by direct partial melting of a MORB like source material (CRB72-31) at ∼2 GPa or ∼70 km depth. By 30–50% partial melting of the CRB72-31, the entire compositional range of the GRs can be produced in a narrow temperature interval (1300–1350°C) at ∼2 GPa. The aluminous clinopyroxene that appears in the above melting range is consistent as the major controlling phase of the GR trends. The partial melts are very similar to the GRs except for Al 2 O 3 and FeO which could be due to the mismatch in the source rock composition. Judging from the variation in REE, involvement of garnet in GR magma genesis can be ruled out. Small amounts of plagioclase (10–30 wt%) may be present in the partial melting residue. Judging from REE patterns and Nd isotopes of the GRs, the source rock should be unfractionated in REE. Based on the melting experiments, a heterogeneous plume model is proposed for the initial stage of the Yellowstone hot spot. Large lithologically distinct blobs of old oceanic crust components were included in the plume head. The GR magmas were produced by partial melting of the oceanic crust components at the bottom of the North American lithosphere. Similar melting processes of basalt/peridotite composite source may be operating in other LIPs (large igneous provinces). The GR type genuine oceanic crust derived melts may be seen where the ambient peridotite remains under subsolidus conditions. Volume and temperature of mantle plumes may have been overestimated, because contributions from the recycled oceanic crust is so large and the current mantle melting models concern only peridotite source.

Idaho, Nevada, Oregon↗
USGS2007

Predicting seabed burial of cylinders by wave-induced scour: Application to the sandy inner shelf off Florida and Massachusetts

A simple parameterized model for wave-induced burial of mine-like cylinders as a function of grain-size, time-varying, wave orbital velocity and mine diameter was implemented and assessed against results from inert instrumented mines placed off the Indian Rocks Beach (IRB, FL), and off the Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The steady flow scour parameters provided by Whitehouse (1998) for self-settling cylinders worked well for predicting burial by depth below the ambient seabed for O (0.5 m) diameter mines in fine sand at both sites. By including or excluding scour pit infilling, a range of percent burial by surface area was predicted that was also consistent with observations. Rapid scour pit infilling was often seen at MVCO but never at IRB, suggesting that the environmental presence of fine sediment plays a key role in promoting infilling. Overprediction of mine scour in coarse sand was corrected by assuming a mine within a field of large ripples buries only until it generates no more turbulence than that produced by surrounding bedforms. The feasibility of using a regional wave model to predict mine burial in both hindcast and real-time forecast mode was tested using the National Oceanic and Atmospheric Administration (NOAA, Washington, DC) WaveWatch 3 (WW3) model. Hindcast waves were adequate for useful operational forcing of mine burial predictions, but five-day wave forecasts introduced large errors. This investigation was part of a larger effort to develop simple yet reliable predictions of mine burial suitable for addressing the operational needs of the U.S. Navy. ?? 2007 IEEE.

IEEE Journal of Oceanic Engineering↗
USGS2008

Variations in Community Exposure and Sensitivity to Tsunami Hazards on the Open-Ocean and Strait of Juan de Fuca Coasts of Washington

Evidence of past events and modeling of potential future events suggest that tsunamis are significant threats to communities on the open-ocean and Strait of Juan de Fuca coasts of Washington. Although potential tsunami-inundation zones from a Cascadia Subduction Zone (CSZ) earthquake have been delineated, the amount and type of human development in tsunami-prone areas have not been documented. A vulnerability assessment using geographic-information-system tools was conducted to document variations in developed land, human populations, economic assets, and critical facilities relative to CSZ-related tsunami-inundation zones among communities on the open-ocean and Strait of Juan de Fuca coasts of Washington (including Clallam, Jefferson, Grays Harbor, and Pacific Counties). The tsunami-inundation zone in these counties contains 42,972 residents (24 percent of the total study-area population), 24,934 employees (33 percent of the total labor force), and 17,029 daily visitors to coastal Washington State Parks. The tsunami-inundation zone also contains 2,908 businesses that generate $4.6 billion in annual sales volume (31 and 40 percent of study-area totals, respectively) and tax parcels with a combined total value of $4.5 billion (25 percent of the study-area total). Although occupancy values are not known for each site, the tsunami-inundation zone also contains numerous dependent-population facilities (for example, schools and child-day-care centers), public venues (for example, religious organizations), and critical facilities (for example, police stations and public-work facilities). Racial diversity of residents in tsunami-prone areas is low?89 percent of residents are White and 8 percent are American Indian or Alaska Native. Nineteen percent of the residents in the tsunami-inundation zone are over 65 years in age, 30 percent of the residents live on unincorporated county lands, and 35 percent of the households are renter occupied. Employees in the tsunami-inundation zone are largely in businesses related to health care and social assistance, accommodation and food services, and retail trade, reflecting businesses that cater to a growing retiree and tourist population. Community vulnerability, described here by exposure (the amount of assets in tsunami-prone areas) and sensitivity (the relative percentage of assets in tsunami-prone areas) varies among 13 incorporated cities, 7 Indian reservations, and 4 counties. The City of Aberdeen has the highest relative community exposure to tsunamis, whereas the City of Long Beach has the highest relative community sensitivity. Levels of community exposure and sensitivity to tsunamis are found to be related to the amount and percentage, respectively, of a community?s land that is in a tsunami-inundation zone. This report will further the dialogue on societal risk to tsunami hazards in Washington and help risk managers to determine where additional risk-reduction strategies may be needed.

Scientific Investigations Report↗
USGS2010

Synthesis of crustal seismic structure and implications for the concept of a slab gap beneath Coastal California

Compilation of seismic transects across the central and northern California Coast Ranges provides evidence for the widespread tectonic emplacement beneath the margin of a slab of partially subducted oceanic lithosphere. The oceanic crust of this lithosphere can be traced landward from the former convergent margin (fossil trench), beneath the Coast Ranges, to at least as far east as the Coast Range/Great Valley boundary. Comparison of measured shear and compressional wave velocities in the middle crust beneath the Hayward fault with laboratory measurements suggests that the middle crust is a diabase (oceanic crust). Both of these observations are consistent with recent models of the high heat flow and age progression of Neogene volcanism along the Coast Ranges based on tectonic emplacement (stalling) of young, hot oceanic lithosphere beneath the margin, but appear to contradict the major predictions of the slab-gap or asthenospheric-window model. Finally, the Neogene volcanism and major strike-slip faults in the Coast Ranges occur within the thickest regions (>14 km thick) of the forearc, suggesting that the locations of Cenozoic volcanism and faulting along the margin are structurally controlled by the forearc thickness rather than being determined by the location of a broad slab gap.

International Geology Review↗
USGS2009

The observed relationship between wave conditions and beach response, Ocean Beach, San Francisco, CA

Understanding how sandy beaches respond to storms is critical for effective sediment management and developing successful erosion mitigation efforts. However, only limited progress has been made in relating observed beach changes to wave conditions, with one of the major limiting factors being the lack of temporally dense beach topography and nearshore wave data in most studies. This study uses temporally dense beach topographic and offshore wave data to directly link beach response and wave forcing with generally good results. Ocean Beach is an open coast high-energy sandy beach located in San Francisco, CA, USA. From April 2004 through the end of 2008, 60 three-dimensional topographic beach surveys were conducted on approximately a monthly basis, with more frequent “short-term surveys during the winters of 2005-06 and 2006-07. Shoreline position data from the short-term surveys show good correlation with offshore wave height, period, and direction averaged over several days prior to the survey (mean R*=0.54 for entire beach). There is, however, considerable alongshore variation in model performance, with R- values ranging from 0.81 to 0.19 for individual sections of the beach. After wave height, the direction of wave approach was the most important factor in determining the response of the shoreline, followed by wave period. Our results indicate that an empirical predictive model of beach response to wave conditions at Ocean Beach is possible with frequent beach mapping and wave data, and that such a model could be useful to coastal managers.

California↗