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At least 505 records · Page 28Linked to original sources
USGS2006

Remote sensing studies of the Dionysius region of the Moon

The Dionysius region is located near the western edge of Mare Tranquillitatis and is centered on Dionysius crater, which exhibits a well-developed dark ray system. Proposed origins for these dark rays included impact melt deposits and dark primary ejecta. The region also contains extensive deposits of Cayley-type light plains. Clementine multispectral images and a variety of spacecraft photography were utilized to investigate the composition and origin of geologic units in the Dionysius region. The portions of the dark rays for which spectral and chemical data were obtained are composed of mare debris contaminated with minor amounts of highland material. Both five-point spectra and values of the optical maturity (OMAT) parameter indicate that the dark rays are dominated by mare basalts, not glassy impact melts. The high-albedo rays associated with Dionysius exhibit FeO and TiO2 values that are lower than those of the adjacent dark ray surfaces and OMAT values that indicate that bright ray surfaces are not fully mature. The high-albedo rays are bright largely because of the contrast in albedo between ray material containing highlands-rich ejecta and the adjacent mare-rich surfaces. The mafic debris ejected by Dionysius was derived from a dark, iron-rich unit exposed high on the inner wall of the crater. This layer probably represents a mare deposit that was present at the surface of the preimpact target site. With one possible exception, there is no evidence for buried mare basalts associated with Cayley plains in the region.

Journal of Geophysical Research E: Planets↗
USGS2006

Radiative transfer modeling of dust-coated Pancam calibration target materials: Laboratory visible/near-infrared spectrogoniometry

Laboratory visible/near-infrared multispectral observations of Mars Exploration Rover Pancam calibration target materials coated with different thicknesses of Mars spectral analog dust were acquired under variable illumination geometries using the Bloomsburg University Goniometer. The data were fit with a two-layer radiative transfer model that combines a Hapke formulation for the dust with measured values of the substrate interpolated using a He-Torrance approach. We first determined the single-scattering albedo, phase function, opposition effect width, and amplitude for the dust using the entire data set (six coating thicknesses, three substrates, four wavelengths, and phase angles 3??-117??). The dust exhibited single-scattering albedo values similar to other Mars analog soils and to Mars Pathfinder dust and a dominantly forward scattering behavior whose scattering lobe became narrower at longer wavelengths. Opacity values for each dust thickness corresponded well to those predicted from the particles sizes of the Mars analog dust. We then restricted the number of substrates, dust thicknesses, and incidence angles input to the model. The results suggest that the dust properties are best characterized when using substrates whose reflectances are brighter and darker than those of the deposited dust and data that span a wide range of dust thicknesses. The model also determined the dust photometric properties relatively well despite limitations placed on the range of incidence angles. The model presented here will help determine the photometric properties of dust deposited on the MER rovers and to track the multiple episodes of dust deposition and erosion that have occurred at both landing sites. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2005

Evolving force balance at Columbia Glacier, Alaska, during its rapid retreat

Changes in driving and resistive stresses play an essential role in governing the buoyancy forces that are important controls on the speed and irreversibility of tidewater glacier retreats. We describe changes in geometry, velocity, and strain rate and present a top-down force balance analysis performed over the lower reach of Columbia Glacier. Our analysis uses new measurements and estimates of basal topography and photogrammetric surface velocity measurements made between 1977 and 2001, while assuming depth-independent strain. Sensitivity tests show that the method is robust and insensitive to small changes in the calculation parameters. Spatial distributions of ice speed show little correspondence with driving stress. Instead, spatial patterns of ice speed exhibit a nonlinear correspondence with basal drag. Primary resistance to flow comes from basal drag, but lateral drag becomes increasingly more important throughout the retreat, which may account for observed increases in speed. Maximum basal drag is always located in a prominent constriction located ~12 km upstream from the preretreat terminus. Once the terminus retreated into deep water off the terminal moraine marking the modern maximum extent, the upstream location of this maximum basal drag helped to promote thinning and decrease effective pressure in the lower region by limiting replenishing ice flow from upstream. An increase in both ice velocity and calving resulted, initiating what appears to be an irreversible retreat. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Acoustic bed velocity and bed load dynamics in a large sand bed river

Development of a practical technology for rapid quantification of bed load transport in large rivers would represent a revolutionary advance for sediment monitoring and the investigation of fluvial dynamics. Measurement of bed load motion with acoustic Doppler current profiles (ADCPs) has emerged as a promising approach for evaluating bed load transport. However, a better understanding of how ADCP data relate to conditions near the stream bed is necessary to make the method practical for quantitative applications. In this paper, we discuss the response of ADCP bed velocity measurements, defined as the near-bed sediment velocity detected by the instrument's bottom-tracking feature, to changing sediment-transporting conditions in the lower Missouri River. Bed velocity represents a weighted average of backscatter from moving bed load particles and spectral reflections from the immobile bed. The ratio of bed velocity to mean bed load particle velocity depends on the concentration of the particles moving in the bed load layer, the bed load layer thickness, and the backscatter strength from a unit area of moving particles relative to the echo strength from a unit area of unobstructed bed. A model based on existing bed load transport theory predicted measured bed velocities from hydraulic and grain size measurements with reasonable success. Bed velocities become more variable and increase more rapidly with shear stress when the transport stage, defined as the ratio of skin friction to the critical shear stress for particle entrainment, exceeds a threshold of about 17. This transition in bed velocity response appears to be associated with the appearance of longer, flatter bed forms at high transport stages.

Journal of Geophysical Research F: Earth Surface↗
USGS2007

Suspended sediment transport in an ephemeral stream following wildfire

We examine the impacts of a stand-clearing wildfire on the characteristics and magnitude of suspended sediment transport in ephemeral streams draining the burn area. We report the results of a monitoring program that includes 2 years of data prior to the Cerro Grande fire in New Mexico, and 3 years of postfire data. Suspended sediment concentration (SSC) increased by about 2 orders of magnitude following the fire, and the proportion of silt and clay increased from 50% to 80%. For a given flow event, SSC is highest at the flood bore and decreases monotonically with time, a pattern evident in every flood sampled both before and after the fire. We propose that the accumulation of flow and wash load at the flow front is an inherent characteristic of ephemeral stream flows, due to amplified momentum losses at the flood bore. We present a new model for computing suspended sediment transport in ephemeral streams (in the presence or absence of wildfire) by relating SSC to the time following the arrival of the flood bore, rather than to instantaneous discharge. Using this model and a rainfall history, we estimate that in the 3 years following the fire, floods transported in suspension a mass equivalent to about 3 mm of landscape lowering across the burn area, 20% of this following a single rainstorm. We test the model by computing fine sediment delivery to a small reservoir in an adjacent watershed, where we have a detailed record of postfire sedimentation based on repeat surveys. Systematic discrepancies between modeled and measured sedimentation rates in the reservoir suggest rapid reductions in suspended sediment delivery in the first several years after the fire.

Journal of Geophysical Research F: Earth Surface↗
USGS2006

Effect of tree line advance on carbon storage in NW Alaska

We investigated the size, distribution, and temporal dynamics of ecosystem carbon (C) pools in an area of recent tree line advance, northwest Alaska. Repeat aerial photographs show forest cover increased ???10% in our study area since 1949. We sampled C pools of four principal ecosystem types, tussock tundra, shrub tundra, woodland, and forest, all located on a 600-800 year old river terrace. Significant differences between ecosystem C pools, both above ground and below ground existed. Tundra sites store >22.2 kg C/m2, shrub tundra sites and woodland sites store 9.7 kg C/m2 and 14.3 kg C/m2, respectively, and forest sites store 14.4 kg C/m2. Landscape variation of total ecosystem C was primarily due to organic soil C and was secondarily due to C stored in trees. Soil C/N profiles of shrub tundra sites and woodland sites showed similarities with forest site soils at surface and tundra site soils at depth. We hypothesize that tundra systems transformed to forest systems in this area under a progression of permafrost degradation and enhanced drainage. On the basis of C pool estimates for the different ecosystem types, conversion of tundra sites to forest may have resulted in a net loss of > 7.8 kg C/m2, since aboveground C gains were more than offset by belowground C losses to decomposition in the tundra sites. Tree line advance therefore might not increase C storage in high-latitude ecosystems and thus might not, as previously suggested, act as a negative feedback to warming. Key to this hypothesis and to its projection to future climate response is the fate of soil carbon upon warming and permafrost drainage. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗
USGS2006

Geomorphic control of landscape carbon accumulation

We use the CREEP process-response model to simulate soil organic carbon accumulation in an undisturbed prairie site in Iowa. Our primary objectives are to identify spatial patterns of carbon accumulation, and explore the effect of erosion on basin-scale C accumulation. Our results point to two general findings. First, redistribution of soil carbon by erosion results in a net increase in basin-wide carbon storage relative to a noneroding environment. Landscape-average mean residence times are increased in an eroding landscape owing to the burial/preservation of otherwise labile C. Second, field observations taken along a slope transect may overlook significant intraslope variations in carbon accumulation. Spatial patterns of modeled deep C accumulation are complex. While surface carbon with its relatively short equilibration time is predictable from surface properties, deep carbon is strongly influenced by the landscape's geomorphic and climatic history, resulting in wide spatial variability. Convergence and divergence associated with upland swales and interfluves result in bimodal carbon distributions in upper and mid slopes; variability in carbon storage within modeled mid slopes was as high as simulated differences between erosional shoulders and depositional valley bottoms. The bimodality of mid-slope C variability in the model suggests that a three-dimensional sampling strategy is preferable over the traditional two-dimensional analog or "catena" approach. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗
USGS2006

Modeling physical and biogeochemical controls over carbon accumulation in a boreal forest soil

Boreal soils are important to the global C cycle owing to large C stocks, repeated disturbance from fire, and the potential for permafrost thaw to expose previously stable, buried C. To evaluate the primary mechanisms responsible for both short- and long-term C accumulation in boreal soils, we developed a multi-isotope (12,14C) Soil C model with dynamic soil layers that develop through time as soil organic matter burns and reaccumulates. We then evaluated the mechanisms that control organic matter turnover in boreal regions including carbon input rates, substrate recalcitrance, soil moisture and temperature, and the presence of historical permafrost to assess the importance of these factors in boreal C accumulation. Results indicate that total C accumulation is controlled by the rate of carbon input, decomposition rates, and the presence of historical permafrost. However, unlike more temperate ecosystems, one of the key mechanisms involved in C preservation in boreal soils examined here is the cooling of subsurface soil layers as soil depth increases rather than increasing recalcitrance in subsurface soils. The propagation of the 14C bomb spike into soils also illustrates the importance of historical permafrost and twentieth century warming in contemporary boreal soil respiration fluxes. Both 14C and total C simulation data also strongly suggest that boreal SOM need not be recalcitrant to accumulate; the strong role of soil temperature controls on boreal C accumulation at our modeling test site in Manitoba, Canada, indicates that carbon in the deep organic soil horizons is probably relatively labile and thus subject to perturbations that result from changing climatic conditions in the future. Copyright 2006 by the American Geophysical Union.

Journal of Geophysical Research G: Biogeosciences↗
USGS2006

Effects of permafrost melting on CO2 and CH4 exchange of a poorly drained black spruce lowland

[1] Permafrost melting is occurring in areas of the boreal forest region where large amounts of carbon (C) are stored in organic soils. We measured soil respiration, net CO 2 flux, and net CH 4 flux during May–September 2003 and March 2004 in a black spruce lowland in interior Alaska to better understand how permafrost thaw in poorly drained landscapes affects land‐atmosphere CO 2 and CH 4 exchange. Sites included peat soils underlain by permafrost at ∼0.4 m depth (permafrost plateau, PP), four thermokarst wetlands (TW) having no permafrost in the upper 2.2 m, and peat soils bordering the thermokarst wetlands having permafrost at ∼0.5 m depth (thermokarst edges, TE). Soil respiration rates were not significantly different among the sites, and 5‐cm soil temperature explained 50–91% of the seasonal variability in soil respiration within the sites. Groundcover vegetation photosynthesis (calculated as net CO 2 minus soil respiration) was significantly different among the sites (TW > TE > PP), which can be partly attributed to the difference in photosynthetically active radiation reaching the ground at each site type. Methane emission rates were 15 to 28 times greater from TW than from TE and PP. We modeled annual soil respiration and groundcover vegetation photosynthesis using soil temperature and radiation data, and CH 4 flux by linear interpolation. We estimated all sites as net C gas sources to the atmosphere (not including tree CO 2 uptake at PP and TE), although the ranges in estimates when accounting for errors were large enough that TE and TW may have been net C sinks.

Journal of Geophysical Research G: Biogeosciences↗
USGS2007

Crustal deformation of the Yellowstone-Snake River Plain volcano-tectonic system-Campaign and continuous GPS observations, 1987-2004

The Yellowstone-Snake River Plain tectonomagmatic province resulted from Late Tertiary volcanism in western North America, producing three large, caldera-forming eruptions at the Yellowstone Plateau in the last 2 Myr. To understand the kinematics and geodynamics of this volcanic system, the University of Utah conducted seven GPS campaigns at 140 sites between 1987 and 2003 and installed a network of 15 permanent stations. GPS deployments focused on the Yellowstone caldera, the Hebgen Lake and Teton faults, and the eastern Snake River Plain. The GPS data revealed periods of uplift and subsidence of the Yellowstone caldera at rates up to 15 mm/yr. From 1987 to 1995, the caldera subsided and contracted, implying volume loss. From 1995 to 2000, deformation shifted to inflation and extension northwest of the caldera. From 2000 to 2003, uplift continued to the northwest while caldera subsidence was renewed. The GPS observations also revealed extension across the Hebgen Lake fault and fault-normal contraction across the Teton fault. Deformation rates of the Yellowstone caldera and Hebgen Lake fault were converted to equivalent total moment rates, which exceeded historic seismic moment release and late Quaternary fault slip-derived moment release by an order of magnitude. The Yellowstone caldera deformation trends were superimposed on regional southwest extension of the Yellowstone Plateau at up to 4.3 ± 0.2 mm/yr, while the eastern Snake River Plain moved southwest as a slower rate at 2.1 ± 0.2 mm/yr. This southwest extension of the Yellowstone-Snake River Plain system merged into east-west extension of the Basin-Range province. Copyright 2007 by the American Geophysical Union.

Idaho, Oregon, Utah, Wyoming↗
USGS2007

Cosmogenic 10Be and 36Cl geochronology of offset alluvial fans along the northern Death Valley fault zone: Implications for transient strain in the eastern California shear zone

The northern Death Valley fault zone (NDVFZ) has long been recognized as a major right-lateral strike-slip fault in the eastern California shear zone (ECSZ). However, its geologic slip rate has been difficult to determine. Using high-resolution digital topographic imagery and terrestrial cosmogenic nuclide dating, we present the first geochronologically determined slip rate for the NDVFZ. Our study focuses on the Red Wall Canyon alluvial fan, which exposes clean dextral offsets of seven channels. Analysis of airborne laser swath mapping data indicates ???297 ?? 9 m of right-lateral displacement on the fault system since the late Pleistocene. In situ terrestrial cosmogenic 10Be and 36C1 geochronology was used to date the Red Wall Canyon fan and a second, correlative fan also cut by the fault. Beryllium 10 dates from large cobbles and boulders provide a maximum age of 70 +22/-20 ka for the offset landforms. The minimum age of the alluvial fan deposits based on 36Cl depth profiles is 63 ?? 8 ka. Combining the offset measurement with the cosmogenic 10Be date yields a geologic fault slip rate of 4.2 +1.9/-1.1 mm yr-1, whereas the 36Cl data indicate 4.7 +0.9/-0.6 mm yr-1 of slip. Summing these slip rates with known rates on the Owens Valley, Hunter Mountain, and Stateline faults at similar latitudes suggests a total geologic slip rate across the northern ECSZ of ???8.5 to 10 mm yr-1. This rate is commensurate with the overall geodetic rate and implies that the apparent discrepancy between geologic and geodetic data observed in the Mojave section of the ECSZ does not extend north of the Garlock fault. Although the overall geodetic rates are similar, the best estimates based on geology predict higher strain rates in the eastern part of the ECSZ than to the west, whereas the observed geodetic strain is relatively constant. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Space geodetic observation of expansion of the San Gabriel Valley, California, aquifer system, during heavy rainfall in winter 2004-2005

[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in 2004–2005. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Earthquake-by-earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard

Boreholes and high-resolution seismic reflection data collected across the forelimb growth triangle above the central segment of the Puente Hills thrust fault (PHT) beneath Los Angeles, California, provide a detailed record of incremental fold growth during large earthquakes on this major blind thrust fault. These data document fold growth within a discrete kink band that narrows upward from ???460 m at the base of the Quaternary section (200-250 m depth) to <150 m at 2.5 m depth, with most growth during the most recent folding event occurring within a zone only ???60 m wide. These observations, coupled with evidence from petroleum industry seismic reflection data, demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kink bands, thereby enabling us to develop a paleoseismic history of the underlying blind thrust fault. The borehole data reveal that the youngest part of the growth triangle in the uppermost 20 m comprises three stratigraphically discrete growth intervals marked by southward thickening sedimentary strata that are separated by intervals in which sediments do not change thickness across the site. We interpret the intervals of growth as occurring after the formation of now-buried paleofold scarps during three large PHT earthquakes in the past 8 kyr. The intervening intervals of no growth record periods of structural quiescence and deposition at the regional, near-horizontal stream gradient at the study site. Minimum uplift in each of the scarp-forming events, which occurred at 0.2-2.2 ka (event Y), 3.0-6.3 ka (event X), and 6.6-8.1 ka (event W), ranged from ???1.1 to ???1.6 m, indicating minimum thrust displacements of ???2.5 to 4.5 m. Such large displacements are consistent with the occurrence of large-magnitude earthquakes (Mw > 7). Cumulative, minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulative thrust displacement of ???7 to 10.5 m. These values yield a minimum Holocene slip rate for the PHT of ???0.9 to 1.6 mm/yr. The borehole and seismic reflection data demonstrate that dip within the kink band is acquired incrementally, such that older strata that have been deformed by more earthquakes dip more steeply than younger strata. Specifically, strata dip 0.4?? at 4 m depth, 0.7?? at 20 m depth, 8?? at 90 m, 16?? at 110 m, and 17?? at 200 m. Moreover, structural restorations of the borehole data show that the locus of active folding (the anticlinal active axial surface) does not extend to the surface in exactly the same location from earthquake to earthquake. Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting a component of fold growth by kink band migration. The incremental acquisition of bed dip in the growth triangle may reflect some combination of fold growth by limb rotation in addition to kink band migration, possibly through a component of trishear or shear fault bend folding. Alternatively, the component of limb rotation may result from curved hinge fault bend folding, and/or the mechanical response of loosely consolidated granular sediments in the shallow subsurface to folding at depth. Copyright 2007 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Mechanical properties of sand, silt, and clay containing tetrahydrofuran hydrate

The mechanical behavior of hydrate-bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes, drilling and coring operations, and the analysis of certain small-strain properties of these sediments (for example, seismic velocities). This study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 MPa confining pressure on sand, crushed silt, precipitated silt, and clay specimens with closely controlled concentrations of synthetic hydrate. The results show that the stress-strain behavior of hydrate-bearing sediments is a complex function of particle size, confining pressure, and hydrate concentration. The mechanical properties of hydrate-bearing sediments at low hydrate concentration (probably < 40% of pore space) appear to be determined by stress-dependent soil stiffness and strength. At high hydrate concentrations (>50% of pore space), the behavior becomes more independent of stress because the hydrates control both stiffness and strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination, cementing particles together, and filling the pore space. The cementation contribution to the shear strength of hydrate-bearing sediments decreases with increasing specific surface of soil minerals. The lower the effective confining stress, the greater the impact of hydrate formation on normalized strength.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Hydrogeologic controls imposed by mechanical stratigraphy in layered rocks of the Chateauguay River Basin, a U.S.-Canada transborder aquifer

[1] The Ch&acirc;teauguay River Basin delineates a transborder watershed with roughly half of its surface area located in northern New York State and half in southern Qu&eacute;bec Province, Canada. As part of a multidisciplinary study designed to characterize the hydrogeologic properties of this basin, geophysical logs were obtained in 12 wells strategically located to penetrate the four major sedimentary rock formations that constitute the regional aquifers. The layered rocks were classified according to their elastic properties into three primary units: soft sandstone, hard sandstone, and dolostone. Downhole measurements were analyzed to identify fracture patterns associated with each unit and to evaluate their role in controlling groundwater flow. Fracture networks are composed of orthogonal sets of laterally extensive, subhorizontal bedding plane partings and bed-delimited, subvertical joints with spacings that are consistent with rock mechanics principles and stress models. The vertical distribution of transmissive zones is confined to a few select bedding plane fractures, with soft sandstone having the fewest (one per 70-m depth) and hard sandstone the most (five per 70-m depth). Bed-normal permeability is examined using a probabilistic model that considers the lengths of flow paths winding along joints and bedding plane fractures. Soft sandstone has the smallest bed-normal permeability primarily because of its wide, geomechanically undersaturated joint spacing. Results indicate that the three formations have similar values of bulk transmissivity, within roughly an order of magnitude, but that each rock unit has its own unique system of groundwater flow paths that constitute that transmissivity.

Chateauguay River Basin↗
USGS2007

Structure of the California Coast Ranges and San Andreas Fault at SAFOD from seismic waveform inversion and reflection imaging

[1] A seismic reflection and refraction survey across the San Andreas Fault (SAF) near Parkfield provides a detailed characterization of crustal structure across the location of the San Andreas Fault Observatory at Depth (SAFOD). Steep-dip prestack migration and frequency domain acoustic waveform tomography were applied to obtain highly resolved images of the upper 5 km of the crust for 15 km on either side of the SAF. The resulting velocity model constrains the top of the Salinian granite with great detail. Steep-dip reflection seismic images show several strong-amplitude vertical reflectors in the uppermost crust near SAFOD that define an ∼2-km-wide zone comprising the main SAF and two or more local faults. Another prominent subvertical reflector at 2–4 km depth ∼9 km to the northeast of the SAF marks the boundary between the Franciscan terrane and the Great Valley Sequence. A deep seismic section of low resolution shows several reflectors in the Salinian crust west of the SAF. Two horizontal reflectors around 10 km depth correlate with strains of seismicity observed along-strike of the SAF. They represent midcrustal shear zones partially decoupling the ductile lower crust from the brittle upper crust. The deepest reflections from ∼25 km depth are interpreted as crust-mantle boundary.

Journal of Geophysical Research B: Solid Earth↗
USGS2007

Forecast experiment: do temporal and spatial b value variations along the Calaveras fault portend M ≥ 4.0 earthquakes?

The power law distribution of earthquake magnitudes and frequencies is a fundamental scaling relationship used for forecasting. However, can its slope (b value) be used on individual faults as a stress indicator? Some have concluded that b values drop just before large shocks. Others suggested that temporally stable low b value zones identify future large-earthquake locations. This study assesses the frequency of b value anomalies portending M ≥ 4.0 shocks versus how often they do not. I investigated M ≥ 4.0 Calaveras fault earthquakes because there have been 25 over the 37-year duration of the instrumental catalog on the most active southern half of the fault. With that relatively large sample, I conducted retrospective time and space earthquake forecasts. I calculated temporal b value changes in 5-km-radius cylindrical volumes of crust that were significant at 90% confidence, but these changes were poor forecasters of M ≥ 4.0 earthquakes. M ≥ 4.0 events were as likely to happen at times of high b values as they were at low ones. However, I could not rule out a hypothesis that spatial b value anomalies portend M ≥ 4.0 events; of 20 M ≥ 4 shocks that could be studied, 6 to 8 (depending on calculation method) occurred where b values were significantly less than the spatial mean, 1 to 2 happened above the mean, and 10 to 13 occurred within 90% confidence intervals of the mean and were thus inconclusive. Thus spatial b value variation might be a useful forecast tool, but resolution is poor, even on seismically active faults.

California↗
USGS2007

Three-dimensional P wave velocity model for the San Francisco Bay region, California

[1] A new three-dimensional P wave velocity model for the greater San Francisco Bay region has been derived using the double-difference seismic tomography method, using data from about 5,500 chemical explosions or air gun blasts and approximately 6,000 earthquakes. The model region covers 140 km NE-SW by 240 km NW-SE, extending from 20 km south of Monterey to Santa Rosa and reaching from the Pacific coast to the edge of the Great Valley. Our model provides the first regional view of a number of basement highs that are imaged in the uppermost few kilometers of the model, and images a number of velocity anomaly lows associated with known Mesozoic and Cenozoic basins in the study area. High velocity ( V p > 6.5 km/s) features at ∼15-km depth beneath part of the edge of the Great Valley and along the San Francisco peninsula are interpreted as ophiolite bodies. The relocated earthquakes provide a clear picture of the geometry of the major faults in the region, illuminating fault dips that are generally consistent with previous studies. Ninety-five percent of the earthquakes have depths between 2.3 and 15.2 km, and the corresponding seismic velocities at the hypocenters range from 4.8 km/s (presumably corresponding to Franciscan basement or Mesozoic sedimentary rocks of the Great Valley Sequence) to 6.8 km/s. The top of the seismogenic zone is thus largely controlled by basement depth, but the base of the seismogenic zone is not restricted to seismic velocities of ≤6.3 km/s in this region, as had been previously proposed.

California↗