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At least 937 records · Page 52Linked to original sources

Introduction to the special section northeast Japan: A case history of subduction

Subduction-related tectonic events such as strike-slip faulting, uplift of high-pressure metamorphic rocks in the forearc, back arc spreading, arc magmatism, and continental collisions have shaped northeast Japan, making it a case history for subduction. Much of the record of these events is preserved and can be used to reconstruct the tectonic history of the region. Although many geological, geophysical, and geochemical data have been collected in Japan for the last 30 years, interpretation of these data into a plate tectonic context did not really begin until about 10 years ago. Much of the data and interpretation on northeast Japan have been published in Japanese or as individual local studies. The primary goal of this special section is to present new investigations covering a wide range of topics and scales and place them in a tectonic framework. The papers in this special section focus on three main subjects: Mesozoic aspects of northeast Japan; the relation of Japan Sea opening with the tectonics and geochemistry of northeast Japan in the Tertiary; and the geophysical setting. The purpose of this overview is to link these subjects and provide a tectonic framework for north- east Japan and eastern Asia since the Mesozoic.

Journal of Geophysical Research B: Solid Earth↗

U.S. Geological Survey science for the Wyoming Landscape Conservation Initiative—2014 annual report

This is the seventh report produced by the U.S. Geological Survey (USGS) for the Wyoming Landscape Conservation Initiative (WLCI) to detail annual activities conducted by the USGS for addressing specific management needs identified by WLCI partners. In FY2014, there were 26 projects, including a new one that was completed, two others that were also completed, and several that entered new phases or directions. The 26 projects fall into several categories: (1) synthesizing and analyzing existing data to identify current conditions on the landscape and using the data to develop models for projecting past and future landscape conditions; (2) monitoring indicators of ecosystem conditions and the effectiveness of on-the-ground habitat projects; (3) conducting research to elucidate the mechanisms underlying wildlife and habitat responses to changing land uses; (4) managing and making accessible the large number of databases, maps, and other products being developed; and (5) coordinating efforts among WLCI partners, helping them use USGS-developed decision-support tools, and integrating WLCI outcomes with future habitat enhancement and research projects. The new (completed) project was the development and publication of a public outreach piece for visitors of Fossil Butte National Monument. The final product was a USGS Fact Sheet that capitalized on previously collected elk-monitoring data to interpret the ecology of the Monument’s elk population and the importance of the Monument’s habitats to this highly visible wildlife species. One of the completed projects entailed developing and evaluating a synthetic approach to high-resolution satellite imagery for use in effectiveness monitoring, which culminated in a journal article. The other completed project was a coalescing of two similar tasks under data and information management that pertain to Web application development and the development of outreach and graphic products into a single integrated project that focuses on developing and maintaining/upgrading Web applications and other tools for visualizing, mapping, and using geospatial data. Major accomplishments for FY2014 included several publications, including Part B of an energy resources map that (with Part A) depicts coal, wind, oil, gas, oil shale, uranium, and solar energy production in the WLCI region. Two published works associated with sage-grouse included a Wildlife Monograph on prioritizing species’ habitats across large landscapes, multiple seasons, and novel areas (using sage-grouse in Wyoming as an example), and a USGS Data Series report that includes both the data used in the habitat-prioritization models and the habitat prioritization models developed for sage-grouse. Our Science Team also published a framework for conducting large, collaborative projects that rely on geospatial data, and a paper that describes the efficacy of fusing satellite data collected at various resolutions for measuring and monitoring vegetation changes. These products are all invaluable tools for maximizing the efficiency and effectiveness of managing species of concern, conducting future landscape-scale assessments, and monitoring status and trends of landscape conditions. Other highlights of FY2014 included a renewed effort to gather and analyze wildlife and habitat status and trend data for the WLCI Interagency Monitoring Database (IAMD) to assess long-term trends and cumulative effects associated with land-use and climate changes. Water-monitoring efforts included drilling four new groundwater-monitoring wells in the Green and New Fork River basins near the proposed Normally Pressured Lance Formation energy development, and continued data collection at established water-monitoring sites. Three additional wells were sampled as part of the Wyoming Groundwater Monitoring Network, bringing the total to 19 Network wells sampled in the WLCI region since 2010. Combined, these water-monitoring efforts can help to identify potential changes in water quality or levels that may result from land-use changes. Major terrestrial monitoring accomplishments included processing satellite imagery from 1985−2010 to develop a historical perspective of long-term vegetation changes, which can serve as a basis for monitoring current and future trends in sagebrush steppe. Such data are crucial tools for agencies tasked with sage-grouse management and conservation. The USGS WLCI Science Team also continued monitoring and testing methods for evaluating WLCI habitat treatments designed to promote aspen regeneration and enhance sage-grouse habitat, and to assess how those treatments influence invasive species distributions and ungulate herbivory. Highlights included analyzing field data collected to elucidate the relationships between sage-grouse habitat use and the proximity of energy infrastructure, and using new instruments to measure productivity responses of aspen woodlands to various factors. Numerous FY2014 accomplishments specifically addressed agency needs to manage and conserve Wyoming’s wildlife species of concern. A pygmy rabbit habitat model and Wyoming distribution map were completed to identify factors associated with rabbit habitat occupancy. Previous work on sage-grouse population dynamics was expanded to better understand the factors that drive long-term viability of sage-grouse populations and to develop a tool that helps to identify key factors limiting sage-grouse persistence in Wyoming. Field work and data analyses continued for elucidating the relationships between sagebrush songbird abundance and productivity, the intensity of energy development, and community dynamics of nest predators. For the mule deer study, mixed mountain shrublands important to migrating and wintering mule deer were mapped and delivered to WLCI partners. Additionally, the relationships between energy development and crucial winter habitat for mule deer were evaluated, and a new phase of work was implemented to better understand relationships between plant phenology and mule deer migration movements. Finally, initial analyses of data collected to evaluate fish-community composition in relation to habitat quality indicate that water quality, as measured by concentrations of hydrocarbons, water temperature, and others parameters, has been diminished in subwatersheds with higher levels of energy development. Overall, the outcomes and products of these wildlife studies contribute significantly to the information and tools needed for addressing effects of land-use changes on Wyoming’s species of concern. Finally, capabilities of the WLCI Web site and the USGS ScienceBase infrastructure were maintained and upgraded to help ensure access to and efficient use of all the WLCI data, products, assessment tools, and outreach materials that have been developed. Of particular note is the completion of three Web applications developed for mapping (1) the 1900−2008 progression of oil and gas development;(2) the predicted distributions of Wyoming’s Species of Greatest Conservation Need; and (3) the locations of coal and wind energy production, sage-grouse distribution and core management areas, and alternative routes for transmission lines within the WLCI region. Collectively, these applications tools provide WLCI planners and managers with powerful tools for better understanding the distributions of wildlife species and potential alternatives for energy development.

Wyoming↗

Aeromagnetic and radio echo ice-sounding measurements over the Dufek intrusion, Antarctica

A combined aeromagnetic and radio echo ice-sounding survey (4200 km of traverse) made in 1978 in Antarctica over the Dufek layered mafic intrusion of Jurassic age suggests a minimum area of about 50,000 km 2 , making it comparable in size with the Bushveld Complex of Africa. Comparisons of the magnetic and subglacial topographic profiles illustrate the usefulness of this combination of methods in studying bedrock geology beneath ice-covered areas. Rocks are exposed in only 3% of the inferred area of the intrusion. Magnetic anomalies measured a few hundred meters above outcrops of the intrusion range in peak-to-trough amplitude from ∼50 nT over the lowermost exposed portion of the section in the Dufek Massif to ∼3600 nT over the uppermost part of the section in the Forrestal Range. Theoretical magnetic anomalies, computed from models based on the subice topography fitted to the highest-amplitude observed magnetic anomalies, required normal and reversed magnetizations ranging from 10 −4 to 10 −2 emu/cm 3 having directions and magnetizations consistent with measurements previously made on oriented samples. This result is interpreted as indicating that the Dufek intrusion cooled through the Curie isotherm during one or more reversals of the earth's magnetic field.

Journal of Geophysical Research Solid Earth↗

Seismic characteristics of central Brazil crust and upper mantle: A deep seismic refraction study

[1] A two‐dimensional model of the Brazilian central crust and upper mantle was obtained from the traveltime interpretation of deep seismic refraction data from the Porangatu and Cavalcante lines, each approximately 300 km long. When the lines were deployed, they overlapped by 50 km, forming an E‐W transect approximately 530 km long across the Tocantins Province and western São Francisco Craton. The Tocantins Province formed during the Neoproterozoic when the São Francisco, the Paranapanema, and the Amazon cratons collided, following the subduction of the former Goiás ocean basin. Average crustal V P and V P /V S ratios, Moho topography, and lateral discontinuities within crustal layers suggest that the crust beneath central Brazil can be associated with major geological domains recognized at the surface. The Moho is an irregular interface, between 36 and 44 km deep, that shows evidences of first‐order tectonic structures. The 8.05 and 8.23 km s −1 P wave velocities identify the upper mantle beneath the Porangatu and Cavalcante lines, respectively. The observed seismic features allow for the identification of (1) the crust has largely felsic composition in the studied region, (2) the absence of the mafic‐ultramafic root beneath the Goiás magmatic arc, and (3) block tectonics in the foreland fold‐and‐thrust belt of the northern Brasília Belt during the Neoproterozoic. Seismic data also suggested that the Bouguer gravimetric discontinuities are mainly compensated by differences in mass distribution within the lithospheric mantle. Finally, the Goiás‐Tocantins seismic belt can be interpreted as a natural seismic alignment related to the Neoproterozoic mantle domain.

Journal of Geophysical Research B: Solid Earth↗

A guide to creating an effective big data management framework

Many agencies and organizations, such as the U.S. Geological Survey, handle massive geospatial datasets and their auxiliary data and are thus faced with challenges in storing data and ingesting it, transferring it between internal programs, and egressing it to external entities. As a result, these agencies and organizations may inadvertently devote unnecessary time and money to convey data without existing or outdated standards. This research aims to evaluate the components of data conveyance systems, such as transfer methods, tracking, and automation, to guide their improved performance. Specifically, organizations face the challenges of slow dispatch time and manual intervention when conveying data into, within, and from their systems. Conveyance often requires skilled workers when the system depends on physical media such as hard drives, particularly when terabyte transfers are required. In addition, incomplete or inconsistent metadata may necessitate manual intervention, process changes, or both. A proposed solution is organization-wide guidance for efficient data conveyance. That guidance involves systems analysis to outline a data management framework, which may include understanding the minimum requirements of data manifests, specification of transport mechanisms, and improving automation capabilities.

Journal of Big Data↗

Holocene geologic slip rate for the Banning strand of the southern San Andreas Fault, southern California

Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25 ± 5 m from its source by the Banning strand to between 5.1 ± 0.4 ka (95% confidence interval (CI)) and 6.4 + 3.7/−2.1 ka (95% CI) using U-series dating of pedogenic carbonate clast coatings and 10 Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9 + 2.3/−1.6 mm/yr (median, 95% CI) to 4.9 + 1.0/−0.9 mm/yr (median, 95% CI). This rate represents only 25–35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U-series and 10 Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms.

Journal of Geophysical Research B: Solid Earth↗

Lithospheric signature of late Cenozoic extension in electrical resistivity structure of the Rio Grande rift, New Mexico, USA

We present electrical resistivity models of the crust and upper mantle from two‐dimensional (2‐D) inversion of magnetotelluric (MT) data collected in the Rio Grande rift, New Mexico, USA. Previous geophysical studies of the lithosphere beneath the rift identified a low‐velocity zone several hundred kilometers wide, suggesting that the upper mantle is characterized by a very broad zone of modified lithosphere. In contrast, the surface expression of the rift (e.g., high‐angle normal faults and synrift sedimentary units) is confined to a narrow region a few tens of kilometers wide about the rift axis. MT data are uniquely suited to probing the depths of the lithosphere that fill the gap between surface geology and body wave seismic tomography, namely the middle to lower crust and uppermost mantle. We model the electrical resistivity structure of the lithosphere along two east‐west trending profiles straddling the rift axis at the latitudes of 36.2 and 32.0°N. We present results from both isotropic and anisotropic 2‐D inversions of MT data along these profiles, with a strong preference for the latter in our interpretation. A key feature of the anisotropic resistivity modeling is a broad (~200‐km wide) zone of enhanced conductivity (<20 Ωm) in the middle to lower crust imaged beneath both profiles. We attribute this lower crustal conductor to the accumulation of free saline fluids and partial melt, a direct result of magmatic activity along the rift. High‐conductivity anomalies in the midcrust and upper mantle are interpreted as fault zone alteration and partial melt, respectively.

New Mexico↗

Paleoseismic investigations in the Santa Cruz mountains, California: Implications for recurrence of large-magnitude earthquakes on the San Andreas fault

Trenching, microgeomorphic mapping, and tree ring analysis provide information on timing of paleoearthquakes and behavior of the San Andreas fault in the Santa Cruz mountains. At the Grizzly Flat site alluvial units dated at 1640–1659 A.D., 1679–1894 A.D., 1668–1893 A.D., and the present ground surface are displaced by a single event. This was the 1906 surface rupture. Combined trench dates and tree ring analysis suggest that the penultimate event occurred in the mid-1600 s, possibly in an interval as narrow as 1632–1659 A.D. There is no direct evidence in the trenches for the 1838 or 1865 earthquakes, which have been proposed as occurring on this part of the fault zone. In a minimum time of about 340 years only one large surface faulting event (1906) occurred at Grizzly Flat, in contrast to previous recurrence estimates of 95–110 years for the Santa Cruz mountains segment. Comparison with dates of the penultimate San Andreas earthquake at sites north of San Francisco suggests that the San Andreas fault between Point Arena and the Santa Cruz mountains may have failed either as a sequence of closely timed earthquakes on adjacent segments or as a single long rupture similar in length to the 1906 rupture around the mid-1600 s. The 1906 coseismic geodetic slip and the late Holocene geologic slip rate on the San Francisco peninsula and southward are about 50–70% and 70% of their values north of San Francisco, respectively. The slip gradient along the 1906 rupture section of the San Andreas reflects partitioning of plate boundary slip onto the San Gregorio, Sargent, and other faults south of the Golden Gate. If a mid-1600 s event ruptured the same section of the fault that failed in 1906, it supports the concept that long strike-slip faults can contain master rupture segments that repeat in both length and slip distribution. Recognition of a persistent slip rate gradient along the northern San Andreas fault and the concept of a master segment remove the requirement that lower slip sections of large events such as 1906 must fill in on a periodic basis with smaller and more frequent earthquakes.

Journal of Geophysical Research B: Solid Earth↗

The evolution, approval and implementation of the U.S. Geological Survey Science Data Lifecycle Model

This paper details how the United States Geological Survey (USGS) Community for Data Integration (CDI) Data Management Working Group developed a Science Data Lifecycle Model, and the role the Model plays in shaping agency-wide policies. Starting with an extensive literature review of existing data Lifecycle models, representatives from various backgrounds in USGS attended a two-day meeting where the basic elements for the Science Data Lifecycle Model were determined. Refinements and reviews spanned two years, leading to finalization of the model and documentation in a formal agency publication . The Model serves as a critical framework for data management policy, instructional resources, and tools. The Model helps the USGS address both the Office of Science and Technology Policy (OSTP) for increased public access to federally funded research, and the Office of Management and Budget (OMB) 2013 Open Data directives, as the foundation for a series of agency policies related to data management planning, metadata development, data release procedures, and the long-term preservation of data. Additionally, the agency website devoted to data management instruction and best practices (www2.usgs.gov/datamanagement) is designed around the Model’s structure and concepts. This paper also illustrates how the Model is being used to develop tools for supporting USGS research and data management processes.

Journal of eScience Librarianship↗

Long-term monotonic trends in water budget components in the contiguous United States: Insights from two hydrologic models

Characterizing changes to water availability for domestic, industrial, agricultural, and other uses is essential to support water management. To better quantify these changes, the U.S. Geological Survey and National Science Foundation National Center for Atmospheric Research produced two hydrologic models simulating water budget components from 1980 to 2021 over the contiguous United States (CONUS). Both hydrologic models were driven by a common atmospheric forcing dataset and aggregated to common spatial and temporal scales, which enables a novel evaluation of congruency between the models. We present annual and seasonal trends in six water budget components (precipitation, evapotranspiration, streamflow, groundwater recharge, soil saturation, and snow water equivalent) based on the Mann–Kendall test for monotonic trend and Theil-Sen slope estimate for the water year 1983–2021 period for ~86,000 catchments in CONUS. Additional components and metrics from our analysis pipeline are available in an associated published dataset, which contains more than 46 million trend results. The water budget trends showed broad agreement with prior observational and modeling studies that indicate increasing trends in the northeast and decreasing trends in southwestern CONUS. We found the seasonal variability in water budget trends was greatest in the southern, central, and northwest CONUS. These findings support integrated trend assessments when coupled with trends in water quality and use.

Contiguous United States↗

Crustal magmatism and anisotropy beneath the Arabian Shield - A cautionary tale

Volcanism in Saudi Arabia includes a historic eruption close to the holy city of Al Madinah. As part of a volcanic hazard assessment of this area, magnetotelluric (MT) data were collected to investigate the structural setting, the distribution of melt within the crust, and the mantle source of volcanism. Interpretation of a new 3‐D resistivity model includes a shallow graben beneath thin lava fields (Harrats), a melt‐free upper crust, and decompression melting in the asthenosphere below thin lithosphere. Within the lower crust the model images elongate conductivity anomalies, one of which was attributed in a previous MT study to melt. The regional MT data, combined with perspective from geology and geophysical modeling, suggest the lower crust is anisotropic with no interconnected melt zones. These divergent interpretations have distinct hazard implications and highlight the importance of large survey aperture and anisotropic modeling to MT studies of volcanic regions. Lower‐crustal anisotropy extends beyond the Harrat, with the most conductive direction oriented N10°E and a factor of 3–5, determined from 2‐D anisotropic inversion, between the most and least conductive directions. The enhanced conductivity is likely due to interconnected grain boundary graphite, while the anisotropy direction reflects either frozen‐in fabric from Neoproterozoic stabilization of the Arabian Shield or modern ductile deformation driven by channelized asthenospheric flow coupled through a thin rigid mantle lid. Asthenospheric melt is interpreted to transect the crust primarily through diking, with limited melt storage and short residence times in the crust.

Arabian Shield↗

Reconstructing the velocity and deformation of a rapid landslide using multiview video

Noncontact measurements of spatially varied ground surface deformation during landslide motion can provide important constraints on landslide mechanics. Here, we present and test a new method for extracting measurements of rapid landslide surface displacement and velocity (accelerations of approximately 1 m/s 2 ) using sequences of stereo images obtained from a pair of inexpensive, stationary 4K video cameras with nominal frame rates of 29.97 Hz. The method combines elements of Structure from Motion with those of optical flow to extract data on 3‐D evolution of the ground surface during slope failure. We apply the method to an experiment at the U.S. Geological Survey debris‐flow flume in which a high‐speed, liquefying landslide was triggered by gradually adding water to a 6‐m 3 prism of loosely packed sediment on a 31° slope. Strip‐scanning lidar measurements made during the experiment corroborate our video‐based measurements, but the latter encompassed the entire landslide surface and were much lower in cost. Our video‐based measurements enabled computation of depth‐integrated landslide dilation/contraction rates. The range of computed rates was within the ranges inferred from independent measurements of evolving pore water pressures and reasonable estimates of the hydraulic permeability of the sediment. Dilation and contraction rates play a crucial role in landslide mechanics. The dilation and contraction we observe contradict the incompressible flow assumption used in many studies that have employed noncontact methods to infer landslide properties.

Oregon↗

State of the data: Assessing the FAIRness of USGS data

In response to recent shifts towards open science that emphasize transparency, reproducibility, and access to research data, the US Geological Survey (USGS) conducted a study to assess the degree to which USGS data assets meet the FAIR data principles (Findable, Accessible, Interoperable, and Reusable). The USGS designed and applied a methodology for quantitative analysis of FAIR characteristics. A new rubric was derived from a crosswalk of existing FAIR evaluation frameworks and customized for the USGS. The rubric, consisting of 62 yes/no questions, was applied to 392 metadata records of USGS data products published between 1987 and 2022. Results were analyzed to show which FAIR characteristics were most and least present in the metadata and how these scores changed after the implementation of data policy requirements in 2016. Aggregated scores showed specific areas of strength and needed improvements. The greatest increases in FAIR scores over time were for elements that were required by new data policies, especially in the ‘Findable’ category. Based on the results, this paper presents strategies to further improve USGS alignment with FAIR. The suggested strategies are organized in four key areas: USGS data repository characteristics, training and communities of practice, data management policy considerations, and metadata standards, tools, and best practices.

Data Science Journal↗

Acoustic deterrence of bighead carp ( Hypophthalmichthys nobilis ) to a broadband sound stimulus

Recent studies have shown the potential of acoustic deterrents against invasive silver carp ( Hypophthalmichthys molitrix ). This study examined the phonotaxic response of the bighead carp ( H. nobilis ) to pure tones (500–2000 Hz) and playbacks of broadband sound from an underwater recording of a 100 hp outboard motor (0.06–10 kHz) in an outdoor concrete pond (10 × 5 × 1.2 m) at the U.S. Geological Survey Upper Midwest Environmental Science Center in La Crosse, WI. The number of consecutive times the fish reacted to sound from alternating locations at each end of the pond was assessed. Bighead carp were relatively indifferent to the pure tones with median consecutive responses ranging from 0 to 2 reactions away from the sound source. However, fish consistently exhibited significantly (P < 0.001) greater negative phonotaxis to the broadband sound (outboard motor recording) with an overall median response of 20 consecutive reactions during the 10 min trials. In over 50% of broadband sound tests, carp were still reacting to the stimulus at the end of the trial, implying that fish were not habituating to the sound. This study suggests that broadband sound may be an effective deterrent to bighead carp and provides a basis for conducting studies with wild fish.

Journal of Great Lakes Research↗

A prelanding assessment of the ice table depth and ground ice characteristics in Martian permafrost at the Phoenix landing site

We review multiple estimates of the ice table depth at potential Phoenix landing sites and consider the possible state and distribution of subsurface ice. A two-layer model of ice-rich material overlain by ice-free material is consistent with both the observational and theoretical lines of evidence. Results indicate ground ice to be shallow and ubiquitous, 2-6 cm below the surface. Undulations in the ice table depth are expected because of the thermodynamic effects of rocks, slopes, and soil variations on the scale of the Phoenix Lander and within the digging area, which can be advantageous for analysis of both dry surficial soils and buried ice-rich materials. The ground ice at the ice table to be sampled by the Phoenix Lander is expected to be geologically young because of recent climate oscillations. However, estimates of the ratio of soil to ice in the ice-rich subsurface layer suggest that that the ice content exceeds the available pore space, which is difficult to reconcile with existing ground ice stability and dynamics models. These high concentrations of ice may be the result of either the burial of surface snow during times of higher obliquity, initially high-porosity soils, or the migration of water along thin films. Measurement of the D/H ratio within the ice at the ice table and of the soil-to-ice ratio, as well as imaging ice-soil textures, will help determine if the ice is indeed young and if the models of the effects of climate change on the ground ice are reasonable.

Journal of Geophysical Research E: Planets↗

Paleomagnetic investigation of some volcanic rocks from the McMurdo volcanic province, Antarctica

Paleomagnetic data for lava flows from sporadic but long-lived eruptions in the McMurdo Sound region are combined with previously published geologic and geochronologic data to determine the general eruptive sequence of the area. Lava flows in the Walcott Bay area were erupted during the Gauss Normal, Matuyama Reversed, and Brunhes Normal Polarity Chrons. The youngest flows on Black Island probably erupted near the boundary between the Gilbert and Gauss chrons. The most recent activity was concentrated on the volcanic edifices of Mounts Morning and Discovery and on Ross Island where Mount Erebus is currently active. Combining virtual geomagnetic poles calculated for 39 units sampled during this study with those of eight flows that were published previously yields a mean paleomagnetic pole at 87.3°N, 317.3°E (α 95 = 6.3°). The ancient geomagnetic field dispersion about this mean pole is 23.5°, with upper and lower limits of 95% confidence equal to 27.4° and 20.5°, respectively. This value probably is a reasonable estimate of secular variation for the Antarctic continent during Pliocene and Pleistocene time.

Journal of Geophysical Research Solid Earth↗

Slicing up the San Francisco Bay Area: Block kinematics and fault slip rates from GPS-derived surface velocities

Observations of surface deformation allow us to determine the kinematics of faults in the San Francisco Bay Area. We present the Bay Area velocity unification (BA??VU??, "bay view"), a compilation of over 200 horizontal surface velocities computed from campaign-style and continuous Global Positioning System (GPS) observations from 1993 to 2003. We interpret this interseismic velocity field using a three-dimensional block model to determine the relative contributions of block motion, elastic strain accumulation, and shallow aseismic creep. The total relative motion between the Pacific plate and the rigid Sierra Nevada/Great Valley (SNGV) microplate is 37.9 ?? 0.6 mm yr-1 directed toward N30.4??W ?? 0.8?? at San Francisco (??2??). Fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data (notable right-lateral slip rates in mm yr-1: San Gregorio fault, 2.4 ?? 1.0; West Napa fault, 4.0 ?? 3.0; zone of faulting along the eastern margin of the Coast Range, 5.4 ?? 1.0; and Mount Diablo thrust, 3.9 ?? 1.0 of reverse slip and 4.0 ?? 0.2 of right-lateral strike slip). Slip on the northern Calaveras is partitioned between both the West Napa and Concord/ Green Valley fault systems. The total convergence across the Bay Area is negligible. Poles of rotation for Bay Area blocks progress systematically from the North America-Pacific to North America-SNGV poles. The resulting present-day relative motion cannot explain the strike of most Bay Area faults, but fault strike does loosely correlate with inferred plate motions at the time each fault initiated. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗

Post 12 m.y. rotation of southwest Washington

Paleomagnetic field directions from the basalt of Pack Sack Lookout are compared to those from the Pomona Member of the Saddle Mountains Basalt of the Columbia River Basalt Group. The Pomona crops out over a wide region on the Columbia Plateau east of the Cascade Range, and the basalt of Pack Sack Lookout crops out well to the west of the Cascades about 30 to 60 km east of the Washington coast. Our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12‐m.y.‐old flow that erupted in western Idaho and flowed to the Washington coast. The magnetic data further indicate that southwest Washington has undergone clockwise rotation with respect to the Columbia Plateau and stable North America. The data suggest that both a large‐scale regional rotation of ∼16° and locally complex small‐scale rotations exist, the two being present to different degrees in the eastern and western regions of southwest Washington. The Cascade Range appears to coincide with the tectonic boundary, separating rotated and unrotated regions of Washington state. Rotation of southwest Washington appears to have been associated with the rotation of large pieces of western Oregon and southern Washington, possibly as a result of either a ridge‐push force from the Basin and Range province or shear along the Pacific‐North America plate boundary.

Washington↗