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At least 1,405 records · Page 78Linked to original sources
USGS2018

Geoelectric hazard maps for the Pacific Northwest

Maps of extreme value, horizontal component geoelectric field amplitude are constructed for the Pacific Northwest United States (and parts of neighboring Canada). Multidecade long geoelectric field time series are calculated by convolving Earth surface impedance tensors from 71 discrete magnetotelluric survey sites across the region with historical 1‐min (2‐min Nyquist) geomagnetic variation time series obtained from two nearby observatories. After fitting statistical models to 1‐min geoelectric amplitudes realized during magnetic storms, extrapolations are made to estimate threshold amplitudes that are only exceeded, on average, once per century. One hundred‐year geoelectric exceedance amplitudes range from 0.06 V/km at a survey site in western Washington State to 9.47 V/km at a site in southeast British Columbia; 100‐year geoelectric exceedance amplitudes equal 7.10 V/km at a site north of Seattle and 2.28 V/km at a site north of Portland. Systematic and random errors are estimated to be less than 20%, much less than site‐to‐site differences in geoelectric amplitude that arise from site‐to‐site differences in surface impedance. Maps of 100‐year exceedance amplitudes are compared with the peak geoelectric amplitudes realized during the March 1989 magnetic superstorm; it is noted that some storms of relatively modest intensity can generate localized geoelectric fields of relatively high amplitude. The geography of geoelectric hazard across the Pacific Northwest is closely related to known geologic and tectonic structures.

Pacific Northwest↗
USGS2018

Developing hydro-meteorological thresholds for shallow landslide initiation and early warning

Consistent relations between shallow landslide initiation and associated rainfall characteristics remain difficult to identify, due largely to the complex hydrological and geological processes causing slopes to be predisposed to failure and those processes that subsequently trigger failures. Considering the importance of hillslope hydrology for rainfall-induced landsliding, we develop and test a method for identifying hybrid hydro-meteorological thresholds to assess landslide initiation potential. We outline a series of steps for using a landslide inventory in combination with triggering rainfall and antecedent wetness to identify empirical thresholds that can inform landslide early warning systems. The method is semi-automated but remains flexible enough to allow threshold developers to consider data inputs and various performance metrics with different priorities for balancing failed versus false alarms. We demonstrate the utility of our approach for two monitoring sites near Seattle, Washington and in Portland, Oregon, USA, to develop daily bilinear thresholds within a two-dimensional parameter space, which rely on accurate 24 h forecasts, measured recent rainfall and in situ soil saturation. Although there were no prior landslide thresholds for Portland, our new hybrid threshold for the Seattle area outperforms established rainfall-only thresholds for the same region. Introducing subsurface hydrologic monitoring into landslide initiation thresholds has the potential to greatly improve early warning capabilities and help reduce losses.

Oregon, Washington↗
USGS2020

Systematic characterization of morphotectonic variability along the Cascadia convergent margin: Implications for shallow megathrust behavior and tsunami hazards

Studies of recent destructive megathrust earth­quakes and tsunamis along subduction margins in Japan, Sumatra, and Chile have linked forearc mor­phology and structure to megathrust behavior. This connection is based on the idea that spatial varia­tions in the frictional behavior of the megathrust influence the tectono-morphological evolution of the upper plate. Here we present a comprehen­sive examination of the tectonic geomorphology, outer wedge taper, and structural vergence along the marine forearc of the Cascadia subduction zone (offshore northwestern North America). The goal is to better understand geologic controls on outer wedge strength and segmentation at spatial scales equivalent to rupture lengths of large earthquakes (≥M 6.7), and to examine potential linkages with shallow megathrust behavior. We use cross-margin profiles, spaced 25 km apart, to characterize along-strike variation in outer wedge width, steepness, and structural vergence (measured between the toe and the outer arc high). The width of the outer wedge varies between 17 and 93 km, and the steepness ranges from 0.9° to 6.5°. Hierarchical cluster analysis of outer wedge width and steepness reveals four distinct regions that also display unique patterns of structural ver­gence and shape of the wedge: Vancouver Island, British Columbia, Canada (average width, linear wedge, seaward and mixed vergence); Washington, USA (higher width, concave wedge, landward and mixed vergence); northern and central Oregon, USA (average width, linear and convex wedge, mixed and seaward vergence); and southern Oregon and northern California, USA (lower width, convex wedge, seaward and mixed vergence). Variabil­ity in outer wedge morphology and structure is broadly associated with along-strike megathrust segmentation inferred from differences in oceanic asthenospheric velocities, patterns of episodic tremor and slow slip, GPS models of plate locking, and the distribution of seismicity near the plate interface. In more detail, our results appear to delin­eate the extent, geometry, and lithology of dynamic and static backstops along the margin. Varying backstop configurations along the Cascadia mar­gin are interpreted to represent material-strength contrasts within the wedge that appear to regulate the along- and across-strike taper and structural vergence in the outer wedge. We argue that the morphotectonic variability in the outer wedge may reflect spatial variations in shallow megathrust behavior occurring over roughly the last few million years. Comparing outer wedge taper along the Cascadia margin to a global compilation suggests that observations in the global catalog are not accurately representing the range of hetero­geneity within individual margins and highlights the need for detailed margin-wide morphotectonic analyses of subduction zones worldwide.

British Columbia, California, Oregon, Washington↗
USGS2008

Development and Evaluation of a Riparian Buffer Mapping Tool

Land use and land cover within riparian areas greatly affect the conditions of adjacent water features. In particular, riparian forests provide many environmental benefits, including nutrient uptake, bank stabilization, steam shading, sediment trapping, aquatic and terrestrial habitat, and stream organic matter. In contrast, residential and commercial development and associated transportation infrastructure increase pollutant and nutrient loading and change the hydrologic characteristics of the landscape, thereby affecting both water quality and habitat. Restoring riparian areas is a popular and cost effective restoration technique to improve and protect water quality. Recognizing this, the Chesapeake Executive Council committed to restoring 10,000 miles of riparian forest buffers throughout the Chesapeake Bay watershed by the year 2010. In 2006, the Chesapeake Executive Council further committed to 'using the best available...tools to identify areas where retention and expansion of forests is most needed to protect water quality'. The Chesapeake Bay watershed encompasses 64,000 square miles, including portions of six States and Washington, D.C. Therefore, the interpretation of remotely sensed imagery provides the only effective technique for comprehensively evaluating riparian forest protection and restoration opportunities throughout the watershed. Although 30-meter-resolution land use and land cover data have proved useful on a regional scale, they have not been equally successful at providing the detail required for local-scale assessment of riparian area characteristics. Use of high-resolution imagery (HRI) provides sufficient detail for local-scale assessments, although at greater cost owing to the cost of the imagery and the skill and time required to process the data. To facilitate the use of HRI for monitoring the extent of riparian forest buffers, the U.S. Forest Service and the U.S. Geological Survey Eastern Geographic Science Center funded the development of a prototype semiautomated image classification tool, RBMapper, that is designed for use by technicians with limited image processing training. This document provides an overview of the RBMapper tool, includes instructions on how to obtain the RBMapper tool and tutorial datasets, and contains a summary evaluation of the tool

Open-File Report↗
USGS2011

Estimate of tephra accumulation probabilities for the U.S. Department of Energy's Hanford Site, Washington

In response to a request from the U.S. Department of Energy, we estimate the thickness of tephra accumulation that has an annual probability of 1 in 10,000 of being equaled or exceeded at the Hanford Site in south-central Washington State, where a project to build the Tank Waste Treatment and Immobilization Plant is underway. We follow the methodology of a 1987 probabilistic assessment of tephra accumulation in the Pacific Northwest. For a given thickness of tephra, we calculate the product of three probabilities: (1) the annual probability of an eruption producing 0.1 km 3 (bulk volume) or more of tephra, (2) the probability that the wind will be blowing toward the Hanford Site, and (3) the probability that tephra accumulations will equal or exceed the given thickness at a given distance. Mount St. Helens, which lies about 200 km upwind from the Hanford Site, has been the most prolific source of tephra fallout among Cascade volcanoes in the recent geologic past and its annual eruption probability based on this record (0.008) dominates assessment of future tephra falls at the site. The probability that the prevailing wind blows toward Hanford from Mount St. Helens is 0.180. We estimate exceedance probabilities of various thicknesses of tephra fallout from an analysis of 14 eruptions of the size expectable from Mount St. Helens and for which we have measurements of tephra fallout at 200 km. The result is that the estimated thickness of tephra accumulation that has an annual probability of 1 in 10,000 of being equaled or exceeded is about 10 centimeters. It is likely that this thickness is a maximum estimate because we used conservative estimates of eruption and wind probabilities and because the 14 deposits we used probably provide an over-estimate. The use of deposits in this analysis that were mostly compacted by the time they were studied and measured implies that the bulk density of the tephra fallout we consider here is in the range of 1,000-1,250 kg/m 3 . The load of 10 cm of such tephra fallout on a flat surface would therefore be in the range of 100-125 kg/m 2 ; addition of water from rainfall or snowmelt would provide additional load.

Open-File Report↗
USGS2017

Suspended sediment, turbidity, and stream water temperature in the Sauk River Basin, western Washington, water years 2012-16

The Sauk River is a federally designated Wild and Scenic River that drains a relatively undisturbed landscape along the western slope of the North Cascade Mountain Range, Washington, which includes the glaciated volcano, Glacier Peak. Naturally high sediment loads characteristic of basins draining volcanoes like Glacier Peak make the Sauk River a dominant contributor of sediment to the downstream main stem river, the Skagit River. Additionally, the Sauk River serves as important spawning and rearing habitat for several salmonid species in the greater Skagit River system. Because of the importance of sediment to morphology, flow-conveyance, and ecosystem condition, there is interest in understanding the magnitude and timing of suspended sediment and turbidity from the Sauk River system and its principal tributaries, the White Chuck and Suiattle Rivers, to the Skagit River. Suspended-sediment measurements, turbidity data, and water temperature data were collected at two U.S. Geological Survey streamgages in the upper and middle reaches of the Sauk River over a 4-year period extending from October 2011 to September 2015, and at a downstream location in the lower river for a 5-year period extending from October 2011 to September 2016. Over the collective 5-year study period, mean annual suspended-sediment loads at the three streamgages on the upper, middle, and lower Sauk River streamgages were 94,200 metric tons (t), 203,000 t, and 940,000 t streamgages, respectively. Fine (smaller than 0.0625 millimeter) total suspended-sediment load averaged 49 percent at the upper Sauk River streamgage, 42 percent at the middle Sauk River streamgage, and 34 percent at the lower Sauk River streamgage.

Washington↗
USGS1978

Spectrographic and chemical analyses of whole-rock and insoluble-residue samples, Rolla 1 degree x 2 degrees quadrangle, Missouri: Drill holes no. 18, 19, and 20

Geochemical studies of the Rolla, Mo., 1° x 2° quadrangle were begun in 1977 as part of a joint multidisciplinary study of the quadrangle by the U.S. Geological Survey and the Division of Geology and Land Survey, Missouri Department of Natural Resources. The study is to access the mineral resource potential of the area by integrated geologic, geochemical, and geophysical studies. The geochemical work to date has been directed at the characterization of the sedimentary and igneous rocks in the quadrangle through spectrographic and chemical analyses of whole-rock samples and dilute-hydrochloric-acid insoluble-residue samples of whole rock from widely spaced drill holes. Sixty-two drill holes have been selected for study from the sample library of the Missouri Division of Geology and Land Survey. None of the holes are company-confidential, none intersect economically significant mineralized ground, and only a few are located in known ore-bearing trends. The analytical results for drill hole no. 18, Missouri log numbers 24244 and 20974, drill hole no. 19, Missouri log numbers 20967 and 23863, and for drill hole no. 20, Missouri log number 20154, are given in this report. Drill hole no. 18 is located in sec. 13, T. 33 N., R. 1 W., in Reynolds County, drill hole no. 19 is located in sec. 2, T. 34 N., R. 1 W. in Iron County, and drill hole no. 20 is located in sec. 16, T. 35 N., R. 1 W. in Washington County. Data for the whole-rock samples for drill hole no. 18 are listed in table 1, and data for the insoluble-residue samples for drill hole no. 18 are listed in table 2.

Missouri↗
USGS1976

Relations among surficial materials, light intensity, and sycamore-seed germination along the Potomac River near Washington, D.C.

Seed of sycamore, a common tree on river flood plains, germinate in mineral soil exposed to high light intensities. Germination rates are low on surfaces covered with leaf litter, and seedlings die when shaded by closely spaced herbaceous plants. All germination rates were higher when seed were kept moist. Surflcial materials and light values in this study are similar to those on flood plains. Results were derived from counting seedlings after planting 1,000 seeds in each of several flats filled with different kinds of flood-plain and upland soils.

District of Columbia, Virginia↗
USGS2016

Demonstration of the Cascadia G‐FAST geodetic earthquake early warning system for the Nisqually, Washington, earthquake

A prototype earthquake early warning (EEW) system is currently in development in the Pacific Northwest. We have taken a two‐stage approach to EEW: (1) detection and initial characterization using strong‐motion data with the Earthquake Alarm Systems (ElarmS) seismic early warning package and (2) the triggering of geodetic modeling modules using Global Navigation Satellite Systems data that help provide robust estimates of large‐magnitude earthquakes. In this article we demonstrate the performance of the latter, the Geodetic First Approximation of Size and Time (G‐FAST) geodetic early warning system, using simulated displacements for the 2001 M w 6.8 Nisqually earthquake. We test the timing and performance of the two G‐FAST source characterization modules, peak ground displacement scaling, and Centroid Moment Tensor‐driven finite‐fault‐slip modeling under ideal, latent, noisy, and incomplete data conditions. We show good agreement between source parameters computed by G‐FAST with previously published and postprocessed seismic and geodetic results for all test cases and modeling modules, and we discuss the challenges with integration into the U.S. Geological Survey’s ShakeAlert EEW system.

Washington↗
USGS1984

Availability of ground water from the alluvial aquifer on the Nisqually Indian Reservation, Washington

A digital model using finite-difference techniques was constructed to simulate ground-water flow in an alluvial aquifer on the Nisqually Indian Reservation. The maximum long-term rate of pumping from individual wells, based on available data, is about 0.75 cubic feet per second (340 gallons per minute). Data on the extent, hydraulic conductivity, saturated thickness of the alluvial aquifer, and quality of the ground water was obtained primarily from the more than 22 test holes drilled for this project. The test holes ranged in depth from about 10 to 100 feet. The saturated thickness of the alluvium was found to range generally from about 10 to 60 feet in the area investigated. The water table is usually less than 10 feet below land surface. The hydraulic conductivity of the aquifer was determined to range from 8.5 to 170 feet per day. The leakage coefficient of the river bed material was determined to be about 0.06 foot per day. Rainfall recharge to the aquifer is about 10 inches per year. A U.S. Geological Survey two-dimensional digital computer model was calibrated to simulate ground-water flow in the alluvial aquifer (area investigated is about 1.1 square miles). The calibrated model simulated measured water levels in the alluvial aquifer to within about 1 foot at 13 of 17 test well locations throughout the model area and within 2 feet at 16 of 17 test well locations. When pumping from the alluvial aquifer was simulated with the computer model it was found that 90 to 100 percent of the water pumped from wells was derived from induced recharge from the Nisqually River into the aquifer and (or) reduced discharge from the aquifer to the Nisqually River. Wells drilled for a large demand use such as a fish hatchery supply will achieve the highest yield if they are placed close to the Nisqually River and in the areas of greatest saturated thickness and highest permeability of the aquifer. (USGS)

Water-Resources Investigations Report↗
USGS1985

Sedimentation in a blast-zone lake at Mount St. Helens, Washington—Implications for varve formation

Sediment collected in traps in a newly formed lake in the blast-impact area at Mount St. Helens recorded a sediment yield that is about two orders of magnitude greater than for comparable basins with vegetation and similar precipitation. Most sediment was mobilized by storms and runoff at the onset of the wet season. The sedimentation response to strongly seasonal precipitation, in the absence of vegetation, produced turbidites and graded annual couplets. The style of sedimentation suggests an alternate mechanism for the formation of long sequences of graded clastic varves.

Washington↗
USGS2006

National Assessment of Shoreline Change Part 3: Historical Shoreline Change and Associated Coastal Land Loss Along Sandy Shorelines of the California Coast

Beach erosion is a chronic problem along many open-ocean shores of the United States. As coastal populations continue to grow and community infrastructures are threatened by erosion, there is increased demand for accurate information regarding past and present trends and rates of shoreline movement. There is also a need for a comprehensive analysis of shoreline movement that is consistent from one coastal region to another. To meet these national needs, the U.S. Geological Survey is conducting an analysis of historical shoreline changes along open-ocean sandy shores of the conterminous United States and parts of Hawaii and Alaska. One purpose of this work is to develop standard repeatable methods for mapping and analyzing shoreline movement so that periodic updates regarding coastal erosion and land loss can be made nationally that are systematic and internally consistent. In the case of this study, the shoreline being measured is the boundary between the ocean water surface and the sandy beach. This report on the California Coast represents the first of two reports on long-term sandy shoreline change for the western U.S., the second of which will include the coast of the Pacific NW, including Oregon and Washington. A report for the Gulf of Mexico shoreline was completed in 2004 and is available at: http://pubs.usgs.gov/of/2004/1043/. This report summarizes the methods of analysis, interprets the results, provides explanations regarding long-term and short-term trends and rates of change, and describes how different coastal communities are responding to coastal erosion. Shoreline change evaluations are based on comparing three historical shorelines digitized from maps, with a recent shoreline derived from lidar (Light Detection and Ranging) topographic surveys. The historical shorelines generally represent the following periods: 1800s, 1920s-1930s, and 1950s-1970s, whereas the lidar shoreline is from 1998-2002. Long-term rates of change are calculated using all four shorelines (1800s to lidar shoreline), whereas short-term rates of change are calculated for only the most recent period (1950s-1970s to lidar shoreline). The rates of change presented in this report represent past conditions and therefore are not intended for predicting future shoreline positions or rates of change. Due to the geomorphology of the California Coast (rocky coastline instead of beach) as well as to data gaps in some areas, this report presents beach erosion rates for 45% of California's 1100 km of coast. The average rate of long-term shoreline change for the State of California was 0.2?0.1 m/yr, an accretional trend. This is based on shoreline change rates averaged from 14,562 individual transects, of which 40% were eroding. Of the transects on which the shoreline was eroding, the long-term erosion rates were generally lowest in Southern California where coastal engineering projects have greatly altered the natural shoreline movement. On a regional scale, long-term accretion rates were either equal to (Central California) or greater than (Northern and Southern California) the long-term erosion rates, yielding the net accretional trend for the entire state. This accretional trend is most likely due to changes in the large volumes of sediment that are added to the system from large rivers and to the impact from coastal engineering and beach nourishment projects. The average rate of short-term shoreline change for the state was erosional. The net short-term rate as averaged along 16,142 transects was -0.2?0.4 m/yr. Of the transects used to measure short-term change, 66% had erosional trends. In addition erosion rates were higher in the short-term period, possibly related to the localized artificial nourishment that occurred over much of the 20th century but that has recently slowed or stopped (Flick, 1993; Wiegel, 1994). Short-term accretion rates were highest in Northern California where the overall magnitudes of shoreline change are systematically higher than in Central and Southern California. The most stable (low erosion and accretion rates) California beaches were most commonly found in Central California. Seawalls and/or riprap revetments have been constructed in all three sections of California, although many of these structures were built to protect houses and infrastructures from the erosion of coastal cliffs and bluffs rather than to protect against long-term beach erosion. California permits shoreline stabilization structures where homes, buildings or other community infrastructure are imminently threatened by erosion. A second California report that is following this publication will include analyses and reports on long-term coastal cliff erosion, as this hazard is of equal or greater concern to coastal communities in many areas along the California Coast.

Open-File Report↗
USGS2011

Rapid, low-cost photogrammetry to monitor volcanic eruptions: An example from Mount St. Helens, Washington, USA

We describe a low-cost application of digital photogrammetry using commercially available photogrammetric software and oblique photographs taken with an off-the-shelf digital camera to create sequential digital elevation models (DEMs) of a lava dome that grew during the 2004–2008 eruption of Mount St. Helens (MSH) volcano. Renewed activity at MSH provided an opportunity to devise and test this method, because it could be validated against other observations of this well-monitored volcano. The datasets consist of oblique aerial photographs (snapshots) taken from a helicopter using a digital single-lens reflex camera. Twelve sets of overlapping digital images of the dome taken during 2004–2007 were used to produce DEMs and to calculate lava dome volumes and extrusion rates. Analyses of the digital images were carried out using photogrammetric software to produce three-dimensional coordinates of points identified in multiple photos. The evolving morphology of the dome was modeled by comparing successive DEMs. Results were validated by comparison to volume measurements derived from traditional vertical photogrammetric surveys by the US Geological Survey Cascades Volcano Observatory. Our technique was significantly less expensive and required less time than traditional vertical photogrammetric techniques; yet, it consistently yielded volume estimates within 5% of the traditional method. This technique provides an inexpensive, rapid assessment tool for tracking lava dome growth or other topographic changes at restless volcanoes.

Washington↗
USGS1973

Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Tholeiitic basalt flows and breccias of Miocene age in western Oregon and Washington form three distinct stratigraphic units. Each unit was erupted from coastal vents marked by dikes and sills of the same composition as associated extrusive rocks. The three coastal basalt units are interbedded with predominantly marine sedimentary rocks of middle to late Miocene age. These units are here named, from older to younger, the Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout. The three units can be distinguished by their petrographic characteristics. The Depoe Bay Basalt is nonporphyritic; Cape Foulweather Basalt has sparse large labradorite phenocrysts; and Pack Sack basalt has labradorite phenocrysts with numerous pyroxene and glass inclusions as well as augite and olivine phenocrysts. Chemical analyses of basalts from these three units show that each has a distinct and uniform composition. The Depoe Bay Basalt is characterized by high SiO 2 content; the Cape Foulweather Basalt has high content of total iron, TiO 2 , and P 2 O 5 ; and Pack Sack basalt is marked by relatively high MgO and CaO content. The Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout on the coast occur in the same stratigraphic order and are essentially the same ages as three basalt units that erupted on the Columbia Plateau. The plateau-derived units are the Yakima and late-Yakima petrographic types of Waters (1961) and the Pomona flow of Schmincke (1967). The virtual identity in chemical composition of the Depoe Bay Basalt and Yakima-type basalt, the Cape Foulweather Basalt and the late-Yakima–type basalt, and the Pack Sack basalt and the Pomona basalt flow indicate that each pair is consanguineous. Fissure vents for the plateau basalt are located in eastern Oregon and Washington and western Idaho more than 500 km east of the coastal vent areas. Thus, a regional mechanism of magma generation or emplacement is required. Three models of magma genesis considered in this report are: (1) partial melting of the subducted Juan de Fuca plate; (2) partial melting along a nearly horizontal shear zone at the base of the American plate; and (3) partial melting within the asthenosphere and fractionation during ascent of the magma.

Oregon, Washington↗
USGS2004

Spatial and temporal variation of Cenozoic surface elevation in the Great Basin and Sierra Nevada

The surface uplift of mountain belts caused by tectonism plays an important role in determining the long-term climate evolution of the Earth. However, the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography, tectonics, and climate change on geologic time-scales. Here, we present a δ 18 O and δD record of authigenic minerals for the northern Great Basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western United States during the Cenozoic. Authigenic calcite, smectite, and chert δ 18 O values suggest the northern Great Basin region experienced ~2km of surface uplift between the middle Eocene and early Oligocene followed by ~1 to 2km of surface subsidence in the southern Great Basin and/or Sierra Nevada since the middle Miocene. These data when combined with previously published work show that the surface uplift history varied in both space and time. Surface uplift migrated from north to south with high elevations in southern British Columbia and northeastern Washington in the middle Eocene and development of surface uplift in north and central Nevada in the Oligocene. This pattern of north to south surface uplift is similar to the timing of magmatism in the western Cordillera, a result that supports tectonic models linking magamtism with removal of mantle lithosphere and/or a subducting slab.

California, Idaho, Nevada, Oregon, Utah↗
USGS1968

Preliminary results of geological, geochemical, and geophysical studies in part of the Virginia City quadrangle, Nevada

Geological, geochemical, and geophysical studies in the Comstock Lode district and adjoining parts of the Virginia Range near Virginia City, Nev., have resulted in recognition of two geophysical anomalies and several geochemical anomalies in an area north of Virginia City. The geophysical anomalies were found during an induced-polarization survey carried out to aid in tracing the Comstock fault, the principal structure localizing the bonanza silver-gold deposits of the Comstock Lode district, in an area of intensely altered rock and alluvial cover about 5 miles north of Virginia City. Geochemical anomalies showing mercury in excess of 5 ppm (parts per million) were found in altered rocks along the Cornstock fault near Cornwall Knob about 5 miles north of Virginia City and in the Washington Hill area, 6 miles farther north.

Circular↗
USGS2008

Estimates of Nutrient Loading by Ground-Water Discharge into the Lynch Cove Area of Hood Canal, Washington

Low dissolved oxygen concentrations in the waters of Hood Canal threaten marine life in late summer and early autumn. Oxygen depletion in the deep layers and landward reaches of the canal is caused by decomposition of excess phytoplankton biomass, which feeds on nutrients (primarily nitrogen compounds) that enter the canal from various sources, along with stratification of the water column that prevents mixing and replenishment of oxygen. Although seawater entering the canal is the largest source of nitrogen, ground-water discharge to the canal also contributes significant quantities, particularly during summer months when phytoplankton growth is most sensitive to nutrient availability. Quantifying ground-water derived nutrient loads entering an ecologically sensitive system such as Hood Canal is a critical component of constraining the total nutrient budget and ultimately implementing effective management strategies to reduce impacts of eutrophication. The amount of nutrients entering Hood Canal from ground water was estimated using traditional and indirect measurements of ground-water discharge, and analysis of nutrient concentrations. Ground-water discharge to Hood Canal is variable in space and time because of local geology, variable hydraulic gradients in the ground-water system adjacent to the shoreline, and a large tidal range of 3 to 5 meters. Intensive studies of ground-water seepage and hydraulic-head gradients in the shallow, nearshore areas were used to quantify the freshwater component of submarine ground-water discharge (SGD), whereas indirect methods using radon and radium geochemical tracers helped quantify total SGD and recirculated seawater. In areas with confirmed ground-water discharge, shore-perpendicular electrical resistivity profiles, continuous electromagnetic seepage-meter measurements, and continuous radon measurements were used to visualize temporal variations in ground-water discharge over several tidal cycles. The results of these field investigations show that ground-water discharge into the Lynch Cove area of Hood Canal is highly dynamic and strongly affected by the large tidal range. In areas with a steep shoreline and steep hydraulic gradient, ground-water discharge is spatially concentrated in or near the intertidal zone, with increased discharge during low tide. Topographically flat areas with weak hydraulic gradients had more spatial variability, including larger areas of seawater recirculation and more widely dispersed discharge. Measured total-dissolved-nitrogen concentrations in ground water ranged from below detection limits to 2.29 milligrams per liter and the total load entering Lynch Cove was estimated to be approximately 98 ? 10.3 metric tons per year (MT/yr). This estimate is based on net freshwater seepage rates from Lee-type seepage meter measurements and can be compared to estimates derived from geochemical tracer mass balance estimates (radon and radium) of 231 to 749 MT/yr, and previous water-mass-balance estimates (14 to 47 MT/ yr). Uncertainty in these loading estimates is introduced by complex biogeochemical cycles of relevant nutrient species, the representativeness of measurement sites, and by energetic dynamics at the coastal aquifer-seawater interface caused by tidal forcing.

Scientific Investigations Report↗
USGS2010

High-resolution seismic reflection imaging of growth folding and shallow faults beneath the Southern Puget Lowland, Washington State

Marine seismic reflection data from southern Puget Sound, Washington, were collected to investigate the nature of shallow structures associated with the Tacoma fault zone and the Olympia structure. Growth folding and probable Holocene surface deformation were imaged within the Tacoma fault zone beneath Case and Carr Inlets. Shallow faults near potential field anomalies associated with the Olympia structure were imaged beneath Budd and Eld Inlets. Beneath Case Inlet, the Tacoma fault zone includes an ∼350-m wide section of south-dipping strata forming the upper part of a fold (kink band) coincident with the southern edge of an uplifted shoreline terrace. An ∼2 m change in the depth of the water bottom, onlapping postglacial sediments, and increasing stratal dips with increasing depth are consistent with late Pleistocene to Holocene postglacial growth folding above a blind fault. Geologic data across a topographic lineament on nearby land indicate recent uplift of late Holocene age. Profiles acquired in Carr Inlet 10 km to the east of Case Inlet showed late Pleistocene or Holocene faulting at one location with ∼3 to 4 m of vertical displacement, south side up. North of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad Tacoma fault zone. Seismic reflection profiles across part of the Olympia structure beneath southern Puget Sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding. Directly beneath one of these faults, a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw, suggesting strike-slip motion. Deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset, consistent with strike-slip faulting. These faults and folds indicate that the Tacoma fault and Olympia structure include active structures with probable postglacial motion.

Washington↗