SEARCH · Geology Reports

Results for “Interfaces”

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

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 451 records · Page 25Linked to original sources

USGS2009

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.

Icarus↗

USGS2004

Field investigation into unsaturated flow and transport in a fault: Model analyses

Results of a fault test performed in the unsaturated zone of Yucca Mountain, Nevada, were analyzed using a three-dimensional numerical model. The fault was explicitly represented as a discrete feature and the surrounding rock was treated as a dual-continuum (fracture-matrix) system. Model calibration against seepage and water-travel-velocity data suggests that lithophysal cavities connected to fractures can considerably enhance the effective fracture porosity and therefore retard water flow in fractures. Comparisons between simulation results and tracer concentration data also indicate that matrix diffusion is an important mechanism for solute transport in unsaturated fractured rock. We found that an increased fault-matrix and fracture-matrix interface areas were needed to match the observed tracer data, which is consistent with previous studies. The study results suggest that the current site-scale model for the unsaturated zone of Yucca Mountain may underestimate radionuclide transport time within the unsaturated zone, because an increased fracture-matrix interface area and the increased effective fracture porosity arising from lithophysal cavities are not considered in the current site-scale model. ?? 2004 Published by Elsevier B.V.

Journal of Contaminant Hydrology↗

USGS2018

Research note: Mapping spatial patterns in sewer age, material, and proximity to surface waterways to infer sewer leakage hotspots

Identifying areas where deteriorating sewer infrastructure is in close proximity to surface waterways is needed to map likely connections between sewers and streams. We present a method to estimate sewer installation year and deterioration status using historical maps of the sewer network, parcel-scale property assessment data, and pipe material. Areas where streams were likely buried into the sewer system were mapped by intersecting the historical stream network derived from a 10-m resolution digital elevation model with sewer pipe locations. Potential sewer leakage hotspots were mapped by identifying where aging sewer pipes are in close proximity (50-m) to surface waterways. Results from Pittsburgh, Pennsylvania (USA), indicated 41% of the historical stream length was lost or buried and the potential interface between sewers and streams is great. The co-location of aging sewer infrastructure (>75 years old) near stream channels suggests that 42% of existing streams are located in areas with a high potential for sewer leakage if sewer infrastructure fails. Mapping the sewer-stream interface provides an approach to better understand areas were failing sewers may contribute a disproportional amount of nutrients and other pathogens to surface waterways.

Pennsylvania↗

USGS2019

2D micromodel study of clogging behavior of fine-grained particles associated with gas hydrate production in NGHP-02 gas hydrate reservoir sediments

Fine-grained particles (fines) commonly coexist with coarse-grained sediments that host gas hydrate. These fines can be mobilized by liquid and gas flow during gas hydrate production. Once mobilized, fines can clog pore throats and reduce reservoir permeability. Even where particle sizes are smaller than pore-throat sizes, clogs can form due to clusters of fines. For certain types of fines, particularly swelling clays, cluster sizes depend on pore-fluid chemistry, which changes as pore-fluid freshens during gas hydrate dissociation. Fines can also be concentrated by a moving gas/liquid interface, increasing the chances of pore-throat clogging regardless of fines type. To test the relative significance of these clogging mechanisms, 2D micromodel experiments have been conducted with different pore-throat widths (20, 40, 60 and 100 m), single-phase pore-fluids (deionized water and 2M-sodium-chloride solution), and moving gas/liquid interfaces on specimens from Sites NGHP-02-09 and NGHP-02-16 (NGHP-02: National Gas Hydrate Program Expedition 02) as well as a selection of pure fines (silica silt, mica, calcium carbonate, diatoms, kaolin, and bentonite). Clogging depended on the ratio of particle-to-pore throat size, and also on pore-fluid chemistry because the pore-fluid chemistry changes effectively increased or decreased the fines cluster size relative to the pore-throat width. These interactions can be predicted based on the fines electrical sensitivity (defined by Jang and Santamarina, 2016). The fine-grained sediment component (grain size < 75 m) from the primary gas hydrate reservoir layers at Sites NGHP-02-09 and -16 show clogging via blocking or size exclusion (sieving) due to the large particles. Clogs also formed due to bridging or blocking by clusters of the smaller particles. Clogging generally occurred for pore-water sediment concentrations so low (0.2% by mass or less), that it was difficult to resolve the enhanced clogging in the presence of the gas/liquid meniscus.

Bay of Bengal↗

USGS2019

Numerical simulations of depressurization-induced gas production from an interbedded turbidite gas hydrate-bearing sedimentary section in the offshore India: Site NGHP-02-16 (Area-B)

The recent National Gas Hydrate Program Expedition 02 (NGHP-02) identified the existence of gas hydrate-bearing sand reservoirs at a number of sites in the offshore of India including Site NGHP-02-16 in Area-B of the Krishna-Godavari Basin. The architecture of that gas hydrate accumulation is characterized by thin, gas hydrate-bearing, high quality sand layers interbedded with mud layers within a turbidite interval. The lowest gas hydrate-bearing layer contacting a thinly-interbedded saline aquifer designates the base of the gas hydrate stability zone (BGHSZ). The proximity of the BGHSZ and the average temperature around 20oC make the reservoir a favorable target for hydrate destabilization by means of the depressurization method. The results of the reservoir simulations indicate high gas production potential from this marine gas hydrate deposit with manageable concomitant water production using a well completion design that hydraulically isolates layers with water-saturated sands. Using a detailed geological input model, the predicted cumulative gas rates reach 3.1 x 104 m3/day (1.1 mmscf/day) after 90 days of continuous depressurization and demonstrate sustained production rates of 3.0 x 104 m3/day (1.0 mmscf/day) after 5 years of production. The interbedded nature of this gas hydrate occurrence promotes the development of horizontal dissociation interfaces between gas hydrate-bearing sand and mud layers. As a result, non-uniform gas production along the horizontal interfaces becomes a primary determinant of reservoir performance. Simulation cases have been executed to determine the impact of the uncertainty in in situ reservoir permeability and the manner in which intrinsic permeability dynamically changes during dissociation in response to the imposed effective stress increase. The cases where effective permeability is initially low and compaction is highly sensitive to stress result in the least favorable production predictions.

Journal of Marine and Petroleum Geology↗

USGS2004

Probabilistic seismic hazard analysis for Sumatra, Indonesia and across the Southern Malaysian Peninsula

The ground motion hazard for Sumatra and the Malaysian peninsula is calculated in a probabilistic framework, using procedures developed for the US National Seismic Hazard Maps. We constructed regional earthquake source models and used standard published and modified attenuation equations to calculate peak ground acceleration at 2% and 10% probability of exceedance in 50 years for rock site conditions. We developed or modified earthquake catalogs and declustered these catalogs to include only independent earthquakes. The resulting catalogs were used to define four source zones that characterize earthquakes in four tectonic environments: subduction zone interface earthquakes, subduction zone deep intraslab earthquakes, strike-slip transform earthquakes, and intraplate earthquakes. The recurrence rates and sizes of historical earthquakes on known faults and across zones were also determined from this modified catalog. In addition to the source zones, our seismic source model considers two major faults that are known historically to generate large earthquakes: the Sumatran subduction zone and the Sumatran transform fault. Several published studies were used to describe earthquakes along these faults during historical and pre-historical time, as well as to identify segmentation models of faults. Peak horizontal ground accelerations were calculated using ground motion prediction relations that were developed from seismic data obtained from the crustal interplate environment, crustal intraplate environment, along the subduction zone interface, and from deep intraslab earthquakes. Most of these relations, however, have not been developed for large distances that are needed for calculating the hazard across the Malaysian peninsula, and none were developed for earthquake ground motions generated in an interplate tectonic environment that are propagated into an intraplate tectonic environment. For the interplate and intraplate crustal earthquakes, we have applied ground-motion prediction relations that are consistent with California (interplate) and India (intraplate) strong motion data that we collected for distances beyond 200 km. For the subduction zone equations, we recognized that the published relationships at large distances were not consistent with global earthquake data that we collected and modified the relations to be compatible with the global subduction zone ground motions. In this analysis, we have used alternative source and attenuation models and weighted them to account for our uncertainty in which model is most appropriate for Sumatra or for the Malaysian peninsula. The resulting peak horizontal ground accelerations for 2% probability of exceedance in 50 years range from over 100% g to about 10% g across Sumatra and generally less than 20% g across most of the Malaysian peninsula. The ground motions at 10% probability of exceedance in 50 years are typically about 60% of the ground motions derived for a hazard level at 2% probability of exceedance in 50 years. The largest contributors to hazard are from the Sumatran faults.

Tectonophysics↗

USGS2026

GST-1: A high-resolution global sediment thickness model

Global Sediment Thickness 1 (GST-1) is a high-resolution sedimentary thickness model calculated on a 0.125° x 0.125° grid. It modifies the sediment thickness of the 1° x 1° Earth Crustal Model 1 (ECM1) by means of 3D inversions of free air gravity anomalies. GST-1 is calculated by performing structural inversions on high-density contrasts across two crustal boundaries: the sediment – basement interface and the crystalline crust – upper mantle interface. The inversions are calculated in each of ten overlapping 3D models that span the globe. These ten models are merged to obtain the GST-1 global model, providing an eight-fold increase in lateral spatial resolution in comparison with ECM1 and CRUST 1.0. Our sediment thickness model exploits the nearly continuous sampling of gravity data when compared to the irregular, sparse sampling of seismic refraction data. Sediment thickness values in GST-1 are in excellent agreement with independently derived cross sections from well-studied sedimentary basins, and within expected resolution limits of seismic refraction data. GST-1 offers a robust, high resolution global model of sedimentary thickness to support studies of sedimentary basins.

Tectonophysics↗

USGS2001

Reconstructing the rise of recent coastal anoxia; molybdenum in Chesapeake Bay sediments

Sporadic, direct observations over a 50 yr period inadequately characterize the history of seasonal hypoxia and anoxia in Chesapeake Bay, alarge estuary threatened by eutrophication. Here, we undertake a reconstruction of 20th century oxygen depletion in this estuary using Mo concentrations in 210Pb-dated sediments; Cu concentrations are used to control for anthropogenic influences. Cores from the central channel display mild Mo enrichments above crustal backgrounds (up to 5 ??g/g) and strong Cu enrichments (up to 35 ??g/g). Temporally, Cu enrichment (mostly anthropogenic) began earlier and stabilized in the last two thirds of the 20th century. In contrast, Mo enrichment has grown during the last two thirds of the century. Molybdenum enrichment is mostly hydrogenic, except in a section of the channel that receives additional Mo from erosion of Early Miocene shore deposits. Two geochemical mechanisms promote Mo enrichment: Manganese refluxing concentrates dissolved MoO24- at the sediment-water interface and sulfide substitution into MoO24- produces thiomolybdates, which can be fixed by particles. The Mo enrichment mechanisms operate primarily during periods when bottom waters are anoxic and thiomolybdate formation can occur near the sediment-water interface. This implies a temporal coupling between water-column anoxia and Mo fixation even though fixation occurs only within sediments. The Mo enrichment profiles suggest that Chesapeake Bay has experienced growing O2 depletion since the first half of the 20th century, but especially after 1960. Copyright ?? 2001 Elsevier Science Ltd.

Geochimica et Cosmochimica Acta↗

USGS1998

New approach to analyzing soil-building systems

A new method of analyzing seismic response of soil-building systems is introduced. The method is based on the discrete-time formulation of wave propagation in layered media for vertically propagating plane shear waves. Buildings are modeled as an extension of the layered soil media by assuming that each story in the building is another layer. The seismic response is expressed in terms of wave travel times between the layers, and the wave reflection and transmission coefficients at layer interfaces. The calculation of the response is reduced to a pair of simple finite-difference equations for each layer, which are solved recursively starting from the bedrock. Compared with commonly used vibration formulation, the wave propagation formulation provides several advantages, including the ability to incorporate soil layers, simplicity of the calculations, improved accuracy in modeling the mass and damping, and better tools for system identification and damage detection.A new method of analyzing seismic response of soil-building systems is introduced. The method is based on the discrete-time formulation of wave propagation in layered media for vertically propagating plane shear waves. Buildings are modeled as an extension of the layered soil media by assuming that each story in the building is another layer. The seismic response is expressed in terms of wave travel times between the layers, and the wave reflection and transmission coefficients at layer interfaces. The calculation of the response is reduced to a pair of simple finite-difference equations for each layer, which are solved recursively starting from the bedrock. Compared with commonly used vibration formulation, the wave propagation formulation provides several advantages, including the ability to incorporate soil layers, simplicity of the calculations, improved accuracy in modeling the mass and damping, and better tools for system identification and damage detection.

Soil Dynamics and Earthquake Engineering↗

USGS1999

Uranium biogeochemistry across the redox transition zone of a permanently stratified fjord: Framvaren, Norway

During August 1995, the vertical concentration profile of dissolved and particulate uranium exhibited strong non-conservative characteristics in the upper 30 m of Framvaren Fjord. There was a pronounced peak in both particulate (> 0.2 ??m; 1.09 nM) and dissolved (< 0.2 ??m; 17.06 nM) uranium in the finely stratified waters at the O2/H2S interface which is positioned well within the euphotic zone at about 20-21 m. Such concentration maxima at the redox boundary are also observed for dissolved organic carbon (DEC), Sr and Ba. Dissolved U levels seen in the water column from 18 m down to 30 m exceeded the high salinity (salinity = 35) U concentrations (13.63 ?? 0.84 nM; Chen, J.H., Edwards, R.L., Wasserburg, G.L., 1986. 238U, 234U and 232Th in seawater. Earth Planet Sci. Lett. 80, 241-251.) observed uniformly in the open ocean. A prolific population of S microbes (e.g., Chromatium, Chlorobium sp.) flourishes at the O2/H2S interface. The source of elevated U at the redox boundary must be due to microbial uptake and subsequent release processes rather than dilution from oceanic uranium. Uranium oxidation state determinations in waters from 1, 22 and 30 m depth reveal that reduced U(IV) is not present in significant abundance, and that the chemical and/or biological reduction of hexavalent uranium is largely inhibited. Our results suggest that U and other trace constituents such as DOC, Sr, Ba, Fe(II), Mn(II) are greatly modified by direct and indirect microbial transformation reactions which are most concentrated across the redox transition zone in Framvaren Fjord.

Marine Chemistry↗

USGS2015

Predicting redox conditions in groundwater at a regional scale

Defining the oxic-suboxic interface is often critical for determining pathways for nitrate transport in groundwater and to streams at the local scale. Defining this interface on a regional scale is complicated by the spatial variability of reaction rates. The probability of oxic groundwater in the Chesapeake Bay watershed was predicted by relating dissolved O 2 concentrations in groundwater samples to indicators of residence time and/or electron donor availability using logistic regression. Variables that describe surficial geology, position in the flow system, and soil drainage were important predictors of oxic water. The probability of encountering oxic groundwater at a 30 m depth and the depth to the bottom of the oxic layer were predicted for the Chesapeake Bay watershed. The influence of depth to the bottom of the oxic layer on stream nitrate concentrations and time lags (i.e., time period between land application of nitrogen and its effect on streams) are illustrated using model simulations for hypothetical basins. Regional maps of the probability of oxic groundwater should prove useful as indicators of groundwater susceptibility and stream susceptibility to contaminant sources derived from groundwater.

Maryland, New York, Pennsylvania, Virginia↗

USGS1999

The carbon cycle and biogeochemical dynamics in lake sediments

The concentrations of organic carbon (OC) and CaCO3 in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO3 accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO3 indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO2 produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO3 that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO3 over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO2 and accompanying increase in pH also may increase the production of CaCO3. However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO3 and therefore a decrease in the rain rate of CaCO3 to the sediment-water interface.

Journal of Paleolimnology↗

USGS2001

Seismic mapping of shallow fault zones in the San Gabriel Mountains from the Los Angeles Region Seismic Experiment, southern California

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a profile (line 1) extending from Seal Beach, California, northeastward to the Mojave Desert and crossing the Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. In most shot gathers from the southern and central San Gabriel Mountains, clear secondary arrivals are seen that merge, or appear to merge, with first arrivals at three locations, including the location of the Vincent thrust fault, an exposed late Mesozoic/early Cenozoic megathrust. These secondary arrivals are interpretable as reflections in the shallow crust (<5 km depth) from a concave-upward interface that projects to the surface in the north near the Vincent thrust fault, is offset in its central part at the San Gabriel fault (an old branch of the San Andreas fault), and terminates in the south at 1 to 2 km depth at the southern mountain front. The velocity structure above and below this interface strongly suggests it is the Vincent thrust fault: intermediate velocities (6.2 km/s), consistent with mylonites overlying the Vincent thrust fault, are observed above it; lower velocities (5.8 km/s), consistent with the Pelona Schist underlying the Vincent thrust fault, are observed below it. Problems arise, however, in attempting to match this reflector to the exposed Vincent thrust fault, which is seen in outcrops east of line 1. The Vincent thrust fault is shallower than the reflector in most places. An unmapped structure (steep fault, monocline, or thrust fault) is required between line 1 and the outcrops that either drops the Vincent thrust fault down to the depths of the reflector or repeats the Vincent thrust fault beneath line 1 in the footwall of another thrust fault. An alternative interpretation of the reflector is a deep greenstone horizon within the Pelona Schist, although this alternative is not favored by the velocity structure. Copyright 2001 by the American Geophysical Union.

California↗

USGS2002

Strong interseismic coupling, fault afterslip, and viscoelastic flow before and after the Oct. 9, 1995 Colima-Jalisco earthquake: continuous GPS measurements from Colima, Mexico

Continuous GPS measurements from Colima, Mexico during 4/93-6/01, bracketing the Oct. 9, 1995 M = 8.0 Colima-Jalisco earthquake, provide new constraints on Rivera plate subduction mechanics. Modeling of margin-normal strain accumulation before the earthquake suggests the Rivera-North America subduction interface was fully locked. Transient postseismic motion from 10/ 95-6/97 is well fit by a model that includes logarithmically-decaying fault afterslip, elastic strain from shallow fault relocking, and possibly a minor viscoelastic response, but is fit poorly by models that assume a dominant Maxwell viscoelastic response of the lower crust and upper mantle, independent of the assumed viscosities. Landward, margin-normal motion since mid-1997 is parallel to but ??? 75% slower than the pre-seismic velocity. Afterslip alone fails to account for this slowdown. The viscoelastic response predicted by a FEM correctly resolves the remaining velocity difference within the uncertainties. Both processes thus offset elastic strain accumulating from the relocked subduction interface.

Colima↗

USGS2005

Magnitude and location of historical earthquakes in Japan and implications for the 1855 Ansei Edo earthquake

Japan Meteorological Agency (JMA) intensity assignments IJMA are used to derive intensity attenuation models suitable for estimating the location and an intensity magnitude Mjma for historical earthquakes in Japan. The intensity for shallow crustal earthquakes on Honshu is equal to -1.89 + 1.42MJMA - 0.00887?? h - 1.66log??h, where MJMA is the JMA magnitude, ??h = (??2 + h2)1/2, and ?? and h are epicentral distance and focal depth (km), respectively. Four earthquakes located near the Japan Trench were used to develop a subducting plate intensity attenuation model where intensity is equal to -8.33 + 2.19MJMA -0.00550??h - 1.14 log ?? h. The IJMA assignments for the MJMA7.9 great 1923 Kanto earthquake on the Philippine Sea-Eurasian plate interface are consistent with the subducting plate model; Using the subducting plate model and 226 IJMA IV-VI assignments, the location of the intensity center is 25 km north of the epicenter, Mjma is 7.7, and MJMA is 7.3-8.0 at the 1?? confidence level. Intensity assignments and reported aftershock activity for the enigmatic 11 November 1855 Ansei Edo earthquake are consistent with an MJMA 7.2 Philippine Sea-Eurasian interplate source or Philippine Sea intraslab source at about 30 km depth. If the 1855 earthquake was a Philippine Sea-Eurasian interplate event, the intensity center was adjacent to and downdip of the rupture area of the great 1923 Kanto earthquake, suggesting that the 1855 and 1923 events ruptured adjoining sections of the Philippine Sea-Eurasian plate interface.

Journal of Geophysical Research B: Solid Earth↗

USGS2018

Direct observations of hydrologic exchange occurring with less‐mobile porosity and the development of anoxic microzones in sandy lakebed sediments

Quantifying coupled mobile/less‐mobile porosity dynamics is critical to the prediction of biogeochemical storage, release, and transformation processes in the zone where groundwater and surface water exchange. The recent development of fine‐scale geoelectrical monitoring paired with pore‐water sampling in groundwater systems enables direct characterization of hydrologic exchange between more‐ and less‐mobile porosity during tracer tests. We adapt this technique to sandy interface sediments at a groundwater flow‐through kettle lake. Tracer experiments were conducted within controlled‐head permeameters over a range of specified downward flow conditions over several days. Although the bed was predominantly composed of highly permeable sands and gravels, cobble inclusions created less‐mobile flow zones at the centimeter scale. Less‐mobile porosity fractions, residence times, and rates of exchange were inferred from paired bulk and fluid electrical conductivity data, without the need for inverse model calibration. The conservative solute experiments were paired with 15 NO 3 − and other reactive amendments, revealing anaerobic processes occurring at shallow sediment depths where pore‐water sampling indicated bulk‐oxic conditions. The average less‐mobile porosity residence times as evaluated with the geoelectrical method were on 1‐hr timescales, which appear to be biogeochemically important in the context of creating anoxic microzones within less‐mobile porosity of sandy interface sediments.

Water Resources Research↗

USGS2021

Constraints on the geometry of the subducted Gorda Plate with converted phases generated by local earthquakes

The largest slip in great megathrust earthquakes often occurs in the 10–30 km depth range, yet seismic imaging of the material properties in this region has proven difficult. We utilize a dense onshore-offshore passive seismic dataset from the southernmost Cascadia subduction zone where seismicity in the mantle of the subducted Gorda Plate produces S -to- P and P -to- S conversions generated within a few km of the plate interface. These conversions typically occur in the 10–20 km depth range at either the top or bottom of a ∼5 km thick layer with a high Vp/Vs that we infer to be primarily the subducted crust. We use their arrival times and amplitudes to infer the location of the top and bottom of the subducted crust as well as the velocity contrasts across these discontinuities. Comparing with both the Slab1.0 and the updated Slab2 interface models, the Slab2 model is generally consistent with the converted phases, while the Slab1.0 model is 1–2 km deeper in the 2–20 km depth range and ∼6–8 km too deep in the 10–20 km depth range between 40.25°N and 40.4°N. Comparing the amplitudes of the converted phases to synthetics for simplified velocity structures, the amplitude of the converted phases requires models containing a ∼5 km thick zone with at least a ∼10%–20% reduction in S wave velocity. Thus, the plate boundary is likely contained within or at the top of this low velocity zone, which potentially indicates a significant porosity and fluid content within the seismogenic zone.

California↗

USGS2012

Strain accumulation in western Washington

The Juan de Fuca plate is subducted beneath the North American plate off the coast of Washington at a rate of about 40 mm/yr N68°E. The average principal strain rates (extension reckoned positive) measured in northwestern Washington are as follows: Olympic peninsula 25 km south of Port Angeles from 1982 through 1990, and and near Seattle from 1972 through 1985, and . Both strain measurements are consistent with uniaxial contraction in the direction of plate convergence. Uplift rates inferred from tide gage recordings are about 4 mm/yr on the Pacific coast and near 0 mm/yr farther inland near Seattle. These deformation rates are consistent with a model of the Cascadia subduction zone in which the plate interface beneath the continental slope and outer continental shelf is locked but free to slip farther landward. The limited downdip extent of the locked segment of the plate interface is consistent with a shallow depth (∼20 km) of the isotherm (∼450°C) that defines the brittle‐ductile transition. Small thrust events diagnostic of seismic subduction should then occur only offshore and at shallow depths. The principal strain rates measured from 1972 through 1983 in the back arc region near Richland, Washington, are and .

Washington↗