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At least 217 records · Page 12Linked to original sources
USGS2002

Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high-resolution seismic tomography

The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new high-resolution 3-D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc-parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high-angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin.

Washington↗
USGS2021

Sentinel-2 and WorldView-3 atmospheric correction and signal normalization based on ground-truth spectroradiometric measurements

Remote sensing satellite Earth Observing Systems (EOS) provide a variety of products for monitoring Earth surface processes at varying spatial and spectral resolutions. Combining information from high and medium spatial resolution images is valuable for monitoring ground cover and vegetation status in cropland, grassland, forests, and other natural settings. However, coupling information from different EOS requires compensating for atmospheric and view angle effects before integrating comparable surface reflectance (SR) values. The objectives of this study were i) to assess how different atmospheric constituents affect the atmospheric correction results in Sentinel-2 and WorldView-3 imagery, ii) to establish a relationship with field spectra measurements, and iii) to develop an empirical approach to ensure that SR values extracted from different EOS can be normalized for use in monitoring vegetation and land cover status. We compared surface reflectance values derived from Sentinel-2 images corrected with Sen2Cor, MODTRAN or FLAASH atmospheric correction approaches for the visible-to-near infrared regions. Additionally, this information was compared to SR values extracted from WorldView-3 imagery acquired from the same dates and location (Central Spain) and corrected with MODTRAN and FLAASH approaches. Assessment of the atmospheric correction was conducted by comparing satellite image SR with ground-truth spectra acquired with a FieldSpec hand-held spectroradiometer. The results emphasized the importance of using common atmospheric parameters collected from ancillary data sources (i.e. MODIS Atmosphere & Land products) to ensure a reliable SR comparison. When compared to field-collected spectral data, SR from corrected Sentinel-2 push-broom imagery showed a reliable match (<4% difference in the visible bands and <0.52% difference in the near infrared bands). However, SR imagery from the pointable WorldView-3 instrument showed significant deviation, likely resulting from the effects of steep off-nadir acquisition angles (24.6° to 39.1°) combined with surface anisotropy. The magnitude and sign of the deviation in SR differed depending on the vegetation type, wavelength and sun-surface-sensor geometry. Therefore, it was necessary to account for angular effects to ensure reliable comparisons of imagery from the different EOS. In this study, an empirical angular correction approach was developed based on calibrating each WorldView-3 band against the ground-truth spectra. This correction allowed for the accurate signal normalization of WorldView-3 and Sentinel-2 imagery SR in the visible-to-near infrared regions.

ISPRS Journal of Photogrammetry and Remote Sensing↗
USGS2012

Equivalent strike-slip earthquake cycles in half-space and lithosphere-asthenosphere earth models

By virtue of the images used in the dislocation solution, the deformation at the free surface produced throughout the earthquake cycle by slippage on a long strike-slip fault in an Earth model consisting of an elastic plate (lithosphere) overlying a viscoelastic half-space (asthenosphere) can be duplicated by prescribed slip on a vertical fault embedded in an elastic half-space. For the case in which each earthquake ruptures the entire lithosphere (thickness H ), the half-space equivalent slip rate is as follows: Depth interval 0- H , slip identical to that in lithosphere-asthenosphere model (i.e., abrupt coseismic slip and no subsequent slip); depth interval (2 n −1) H to (2 n +1)H ( n = 1,2,…), slip rate uniform in space and dependent upon time as F n ( t ) exp (− t /τ a ) where F n is a ( n - 1) degree polynomial in t , τ a is twice the asthenosphere relaxation time (η/μ), and t is measured from the instant after the preceding earthquake. The slip rate averaged over the seismic cycle in each depth interval equals the secular rate of relative plate motion. For reasonable values of τ a , slip rates below 5 H do not vary much from that mean value and can be treated as constant. Thus the surface deformation due to the earthquake cycle in the lithosphere-asthenosphere model can be calculated very simply from the half-space model with time-dependent slip in the two depth intervals H −3 H and 3 H −5 H , and uniform slip at a rate equal to the secular relative plate velocity below depth 5 H . Inversion of 1973–1988 geodetic measurements of deformation across the segment of the San Andreas fault in the Transverse Ranges north of Los Angeles for the half-space equivalent slip distribution suggests no significant slip on the fault above 30 km and a uniform slip rate of 36 mm/yr below 30 km. One equivalent lithosphere-asthenosphere model would have a 30-km thick lithosphere and an asthenosphere relaxation time greater than 33 years, but other models are possible.

Journal of Geophysical Research↗
USGS1997

The Phanerozoic impact cratering rate: Evidence from the farside of the Moon

The relatively recent (<1 b.y.) flux of asteroids and comets forming large craters on the Earth and Moon may be accurately recorded by craters with bright rays on the Moon's farside. Many previously unknown farside rayed craters are clearly distinguished in the low-phase-angle images returned by the Clementine spacecraft. Some large rayed craters on the lunar nearside are probably significantly older than 1 Ga; rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity. Most of the farside crust has a more homogeneous composition and only immature rays are visible. The size-frequency distribution of farside rayed craters is similar to that measured for Eratosthenian craters (up to 3.2 b.y.) at diameters larger than 15 km. The areal density of farside rayed craters matches that of a corrected tabulation of nearside Copernican craters. Hence the presence of bright rays due to immature soils around large craters provides a consistent time-stratigraphic basis for defining the base of the Copernican System. The density of large craters less than ∼3.2 b.y. old is ∼3.2 times higher than that of large farside rayed craters alone. This observation can be interpreted in two ways: (1) the average cratering rate has been constant over the past 3.2 b.y. and the base of the Copernican is ∼1 Ga, or (2) the cratering rate has increased in recent geologic time and the base of the Copernican is less than 1 Ga. We favor the latter interpretation because the rays of Copernicus (800–850 m.y. old) appear to be very close to optical maturity, suggesting that the average Copernican cratering rate was ∼35% higher than the average Eratosthenian rate. Other lines of evidence for an increase in the Phanerozoic (545 Ga) cratering rate are (1) the densities of small craters superimposed on Copernicus and Apollo landing sites, (2) the rates estimated from well-dated terrestrial craters (≤120 m.y.) and from present-day astronomical observations, and (3) the Proterozoic rate suggested by the crater record of Australia. The hypothesis most consistent with several key observations is that the cratering rate has increased by ∼2x during the past ∼300 m.y‥

Journal of Geophysical Research E: Planets↗
USGS2022

On the use of high-resolution and deep-learning seismic catalogs for short-term earthquake forecasts: Potential benefits and current limitations

Enhanced earthquake catalogs provide detailed images of evolving seismic sequences. Currently, these data sets take some time to be released but will soon become available in real time. Here, we explore whether and how enhanced seismic catalogs feeding into established short-term earthquake forecasting protocols may result in higher predictive skill. We consider three enhanced catalogs for the 2016–2017 Central Italy sequence, featuring a bulk completeness lower by at least two magnitude units compared to the real-time catalog and an improved hypocentral resolution. We use them to inform a set of physical Coulomb Rate-and-State (CRS) and statistical Epidemic-Type Aftershock Sequence (ETAS) models to forecast the space-time occurrence of M3+ events during the first 6 months of the sequence. We track model performance using standard likelihood-based metrics and compare their skill against the best-performing CRS and ETAS models among those developed with the real-time catalog. We find that while the incorporation of the triggering contributions from new small magnitude detections of the enhanced catalogs is beneficial for both types of forecasts, these models do not significantly outperform their respective near real-time benchmarks. To explore the reasons behind this result, we perform targeted sensitivity tests that show how (a) the typical spatial discretizations of forecast experiments ( 2 km) hamper the ability of models to capture highly localized secondary triggering patterns and (b) differences in earthquake parameters (i.e., magnitude and hypocenters) reported in different catalogs can affect forecast evaluation. These findings will contribute toward improving forecast model design and evaluation strategies for next-generation seismic catalogs.

Journal of Geophysical Research--Solid Earth↗
USGS2009

Trace-element record in zircons during exhumation from UHP conditions, North-East Greenland Caledonides

Coesite-bearing zircon formed at ultrahigh-pressure (UHP) conditions share general characteristics of eclogite-facies zircon with trace-element signatures characterized by depleted heavy rare earth elements (HREE), lack of an Eu anomaly, and low Th/ U ratios. Trace-element signatures of zircons from the Caledonian UHP terrane in North-East Greenland were used to examine the possible changes in signature with age during exhumation. Collection and interpretation of age and trace-element analyses of zircon from three samples of quartzofeldspathic gneiss and two leucocratic intrusions were guided by core vs. rim zoning patterns as imaged by cathodoluminesence. Change from igneous to eclogite-facies metamorphic trace-element signature in protolith zircon is characterized by gradual depletion of HREE, whereas newly formed metamorphic rims have flat HREE patterns and REE concentrations that are distinct from the recrystallized inherited cores. The signature associated with eclogite-facies metamorphic zircon is observed in coesite-bearing zircon formed at 358 ?? 4 Ma, metamorphic rims formed at 348 ?? 5 Ma during the initial stages of exhumation, and metamorphic rims formed at 337 ?? 5 Ma. Zircons from a garnet-bearing granite emplaced in the neck of an eclogite boudin and a leucocratic dike that cross-cuts amphibolite-facies structural fabrics have steeply sloping HREE patterns, variably developed negative Eu anomalies, and low Th/U ratios. The granite records initial decompression melting and exhumation at 347 ?? 2 Ma and later zircon rim growth at 329 ?? 5. The leucocratic dike was likely emplaced at amphibolite-facies conditions at 330 ?? 2 Ma, but records additional growth of compositionally similar zircon at 321 ??2 Ma. The difference between the trace-element signature of metamorphic zircon in the gneisses and in part coeval leucocratic intrusions indicates that the zircon signature varies as a function of lithology and context, thus enhancing its ability to aid in the interpretation of U-Pb data and track the exhumation history of UHP terranes. The differences may reflect variation in elemental availability through breakdown reactions in quartzofeldpathic gneiss vs. availability during melt production and/or crystallization. UHP rocks in North-East Greenland began exhumation by 347 ?? 2 Ma, were still at HP eclogite-facies conditions at 337 ?? 5 Ma and were at amphibolite-facies conditions by 330 ?? 2 Ma. ?? 2009 E. Schweizerbart'sche Verlagsbuchhandlung.

European Journal of Mineralogy↗
USGS2018

A 30-m landsat-derived cropland extent product of Australia and China using random forest machine learning algorithm on Google Earth Engine cloud computing platform

Mapping high resolution (30-m or better) cropland extent over very large areas such as continents or large countries or regions accurately, precisely, repeatedly, and rapidly is of great importance for addressing the global food and water security challenges. Such cropland extent products capture individual farm fields, small or large, and are crucial for developing accurate higher-level cropland products such as cropping intensities, crop types, crop watering methods (irrigated or rainfed), crop productivity, and crop water productivity. It also brings many challenges that include handling massively large data volumes, computing power, and collecting resource intensive reference training and validation data over complex geographic and political boundaries. Thereby, this study developed a precise and accurate Landsat 30-m derived cropland extent product for two very important, distinct, diverse, and large countries: Australia and China. The study used of eight bands (blue, green, red, NIR, SWIR1, SWIR2, TIR1, and NDVI) of Landsat-8 every 16-day Operational Land Imager (OLI) data for the years 2013–2015. The classification was performed by using a pixel-based supervised random forest (RF) machine learning algorithm (MLA) executed on the Google Earth Engine (GEE) cloud computing platform. Each band was time-composited over 4–6 time-periods over a year using median value for various agro-ecological zones (AEZs) of Australia and China. This resulted in a 32–48-layer mega-file data-cube (MFDC) for each of the AEZs. Reference training and validation data were gathered from: (a) field visits, (b) sub-meter to 5-m very high spatial resolution imagery (VHRI) data, and (c) ancillary sources such as from the National agriculture bureaus. Croplands versus non-croplands knowledge base for training the RF algorithm were derived from MFDC using 958 reference-training samples for Australia and 2130 reference-training samples for China. The resulting 30-m cropland extent product was assessed for accuracies using independent validation samples: 900 for Australia and 1972 for China. The 30-m cropland extent product of Australia showed an overall accuracy of 97.6% with a producer’s accuracy of 98.8% (errors of omissions = 1.2%), and user’s accuracy of 79% (errors of commissions = 21%) for the cropland class. For China, overall accuracies were 94% with a producer’s accuracy of 80% (errors of omissions = 20%), and user’s accuracy of 84.2% (errors of commissions = 15.8%) for cropland class. Total cropland areas of Australia were estimated as 35.1 million hectares and 165.2 million hectares for China. These estimates were higher by 8.6% for Australia and 3.9% for China when compared with the traditionally derived national statistics. The cropland extent product further demonstrated the ability to estimate sub-national cropland areas accurately by providing an R 2 value of 0.85 when compared with province-wise cropland areas of China. The study provides a paradigm-shift on how cropland maps are produced using multi-date remote sensing. These products can be browsed at www.croplands.org and made available for download at NASA’s Land Processes Distributed Active Archive Center (LP DAAC) https://www.lpdaac.usgs.gov/node/1282 .

ISPRS Journal of Photogrammetry and Remote Sensing↗
USGS1996

Along-axis variability in crustal accretion at the Mid-Atlantic Ridge: Results from the OCEAN study

The OCEAN experiment is an integrated geophysical study of a region of the Cape Verde abyssal plain that formed at 140 Ma. Deep seismic reflection and ocean bottom hydrophone (OBH) refraction data were acquired along lines parallel and perpendicular to the paleoridge axis trend identified from a detailed magnetic anomaly survey. The igneous basement is overlain by about 1.3 km of sediment which enables improved imaging of intracrustal structure beyond that possible near the Mid-Atlantic Ridge axis. We describe the results of a 150-km long profile oriented parallel to magnetic anomalies M15 and M16, along which deep seismic reflection data collected by the British Institutions Reflection Profiling Syndicate are complemented by refraction data constrained by four OBHs. The line spans an entire spreading segment between two fracture zones; the northern of which has an offset of 40 km and the other (central) has an offset of only 10 km. Away from the fracture zones, the mean igneous crustal thickness is 7.2 km; near both fracture zones, thinning of up to 4 km is observed, giving a mean igneous crustal thickness over the whole segment of approximately 6.5 km. Differences are seen between the two fracture zones in their seismic velocity structure, in the associated basement topography, and in the presence of a strong reflection extending into the mantle beneath the northern fracture zone. The boundary between oceanic layers 2 and 3 correlates with variably coherent normal incidence reflections and a change in the character of the reflectivity. A number of planar reflections up to 10 km in length are present within the middle and lower crust, dipping outward from beneath low-amplitude basement highs at ∼15°; these appear to be present only within layer 3. The Moho has several expressions in the reflection data, including isolated reflection events, a local increase in reflected amplitudes, and a downward decrease in coherent reflections. At the center of the segment there is a zone at the base of the crust within which both high- and low-velocity materials are present. This zone shows an enhanced level of discontinuous normal incidence reflectivity and may represent an initial fractionation event as melt was emplaced at the spreading ridge.

Journal of Geophysical Research B: Solid Earth↗
USGS2017

Automated cropland mapping of continental Africa using Google Earth Engine cloud computing

The automation of agricultural mapping using satellite-derived remotely sensed data remains a challenge in Africa because of the heterogeneous and fragmental landscape, complex crop cycles, and limited access to local knowledge. Currently, consistent, continent-wide routine cropland mapping of Africa does not exist, with most studies focused either on certain portions of the continent or at most a one-time effort at mapping the continent at coarse resolution remote sensing. In this research, we addressed these limitations by applying an automated cropland mapping algorithm (ACMA) that captures extensive knowledge on the croplands of Africa available through: (a) ground-based training samples, (b) very high (sub-meter to five-meter) resolution imagery (VHRI), and (c) local knowledge captured during field visits and/or sourced from country reports and literature. The study used 16-day time-series of Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) composited data at 250-m resolution for the entire African continent. Based on these data, the study first produced accurate reference cropland layers or RCLs (cropland extent/areas, irrigation versus rainfed, cropping intensities, crop dominance, and croplands versus cropland fallows) for the year 2014 that provided an overall accuracy of around 90% for crop extent in different agro-ecological zones (AEZs). The RCLs for the year 2014 (RCL2014) were then used in the development of the ACMA algorithm to create ACMA-derived cropland layers for 2014 (ACL2014). ACL2014 when compared pixel-by-pixel with the RCL2014 had an overall similarity greater than 95%. Based on the ACL2014, the African continent had 296 Mha of net cropland areas (260 Mha cultivated plus 36 Mha fallows) and 330 Mha of gross cropland areas. Of the 260 Mha of net cropland areas cultivated during 2014, 90.6% (236 Mha) was rainfed and just 9.4% (24 Mha) was irrigated. Africa has about 15% of the world’s population, but only about 6% of world’s irrigation. Net cropland area distribution was 95 Mha during season 1, 117 Mha during season 2, and 84 Mha continuous. About 58% of the rainfed and 39% of the irrigated were single crops (net cropland area without cropland fallows) cropped during either season 1 (January-May) or season 2 (June-September). The ACMA algorithm was deployed on Google Earth Engine (GEE) cloud computing platform and applied on MODIS time-series data from 2003 through 2014 to obtain ACMA-derived cropland layers for these years (ACL2003 to ACL2014). The results indicated that over these twelve years, on average: (a) croplands increased by 1 Mha/yr, and (b) cropland fallows decreased by 1 Mha/year. Cropland areas computed from ACL2014 for the 55 African countries were largely underestimated when compared with an independent source of census-based cropland data, with a root-mean-square error (RMSE) of 3.5 Mha. ACMA demonstrated the ability to hind-cast (past years), now-cast (present year), and forecast (future years) cropland products using MODIS 250-m time-series data rapidly, but currently, insufficient reference data exist to rigorously report trends from these results.

ISPRS Journal of Photogrammetry and Remote Sensing↗
USGS2015

Evaluating a satellite-based seasonal evapotranspiration product and identifying its relationship with other satellite-derived products and crop yield: A case study for Ethiopia

Satellite-derived evapotranspiration anomalies and normalized difference vegetation index (NDVI) products from Moderate Resolution Imaging Spectroradiometer (MODIS) data are currently used for African agricultural drought monitoring and food security status assessment. In this study, a process to evaluate satellite-derived evapotranspiration (ETa) products with a geospatial statistical exploratory technique that uses NDVI, satellite-derived rainfall estimate (RFE), and crop yield data has been developed. The main goal of this study was to evaluate the ETa using the NDVI and RFE, and identify a relationship between the ETa and Ethiopia’s cereal crop (i.e., teff, sorghum, corn/maize, barley, and wheat) yields during the main rainy season. Since crop production is one of the main factors affecting food security, the evaluation of remote sensing-based seasonal ETa was done to identify the appropriateness of this tool as a proxy for monitoring vegetation condition in drought vulnerable and food insecure areas to support decision makers. The results of this study showed that the comparison between seasonal ETa and RFE produced strong correlation ( R 2 > 0.99) for all 41 crop growing zones in Ethiopia. The results of the spatial regression analyses of seasonal ETa and NDVI using Ordinary Least Squares and Geographically Weighted Regression showed relatively weak yearly spatial relationships ( R 2 < 0.7) for all cropping zones. However, for each individual crop zones, the correlation between NDVI and ETa ranged between 0.3 and 0.84 for about 44% of the cropping zones. Similarly, for each individual crop zones, the correlation ( R 2 ) between the seasonal ETa anomaly and de-trended cereal crop yield was between 0.4 and 0.82 for 76% (31 out of 41) of the crop growing zones. The preliminary results indicated that the ETa products have a good predictive potential for these 31 identified zones in Ethiopia. Decision makers may potentially use ETa products for monitoring cereal crop yields and early warning of food insecurity during drought years for these identified zones.

International Journal of Applied Earth Observation↗
USGS1996

The upper mantle structure of the central Rio Grande rift region from teleseismic P and S wave travel time delays and attenuation

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki-Christofferson-Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE-SW to NNE-SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north-south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low-velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW-ESE), possibly heralding future extension across the Jemez lineament and transition zone.

Journal of Geophysical Research B: Solid Earth↗
USGS2026

Subsurface structure across the Tacoma Basin, Washington State, using trans-dimensional Bayesian inversion of fundamental mode spatial autocorrelation data

Spatial autocorrelation (SPAC), the azimuthal average of the normalized cross-correlation between equidistant station pairs deployed in a 2-D array, is widely used to image the subsurface structure. However, the rigorous estimate of subsurface structure and its uncertainties as a function of depth using SPAC data is challenging due to the nonlinear relation between the SPAC data and Earth structure as well as the trade-off between depth and velocity. Additionally, data noise is strongly correlated due to data processing (e.g. filtering, stacking from multiple time segments and azimuthal averaging). Most studies do not account for the correlated noise and fix the ratio of compressional-wave velocity ( V P ) to shear-wave velocity ( V s ) (i.e. V P / V s ratio) and the number of layers, both of which are typically unknown. To address these challenges, we develop a hierarchical trans-dimensional Bayesian inversion of fundamental mode of SPAC data that properly accounts for the correlated data noise, samples the V P / V s ratio and relaxes the number of layers (i.e. model parametrization) to be unknown in the inversion. We further examine the limitation of using only fundamental modes in the inversion. Our synthetic experiments show that the inversion recovers an incorrect model unless we sample the correlated noise and V P / V s ratio in the inversion. The inversion is then applied to SPAC data acquired at 19 sites across the Tacoma basin in Washington State to characterize the V s and the time-averaged V s over 30-m depth ( V s 30 ). Our results show that the V s 30 varies from ∼200 to 800 m s −1 . The V s 30 within the basin is higher in the middle and lower on the east and west sides. We find that these V s 30 values vary with geologic unit. The uncertainties for V s 30 are within 20 m s −1 in average except for the most eastern site TB28. Additionally, the uncertainties are greater for deeper depths beneath most of the sites as the sensitivity decreases as a function of depth. The Vs structure as a function of depth is also complex beneath some sites, possibly because the SPAC curves are affected by higher order Rayleigh modes that are not considered in the inversion. To better constrain the deeper V s structure, V s 30 and/or other average measures of V s over depth, additional constraints from complementary data, such as ellipticity or geologic data are needed. Moreover, our synthetic experiments show that higher order modes can have significant effect in the inversion results, particularly when there is a low-velocity layer.

Washington↗
USGS2000

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100 m deep, and the base of the cores is only ∼670 ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ∼2–3 Ma.

Lake Baikal Rift↗
USGS2012

Teleseismic tomography of the compressional wave velocity structure beneath the Long Valley region, California

In 1982 and 1984 the U.S. Geological Survey used several seismic networks, totaling over 90 stations, to record teleseismic P waves and measure travel time residuals in an area centered on the Long Valley caldera. We inverted the travel time residuals to obtain a three-dimensional image of the velocity structure with resolution of 5–6 km to depths of 70 km beneath the array. Direct inversion of these data indicates that the 2- to 4-km-thick low-velocity caldera fill contaminates the signal from any midcrustal velocity anomalies beneath the caldera. Thus two methods were used to strip the effects of the upper crust from the travel time residuals: (1) ray tracing through upper crustal velocity models provided by seismic refraction experiments and gravity surveys, and (2) an iterative stripping scheme using the inversion itself. The methods produce essentially identical results and adequately remove the effects of the shallowest crustal structures, including the caldera fill and hydrothermal alteration effects. The resulting “stripped” models show two well-resolved midcrustal low-velocity bodies in the Long Valley region. The first body is centered between 7 and 20 km depth beneath the resurgent dome of the Long Valley caldera and has a volume of 150–600 km 3 . The second, with a similar volume, is centered between 10 and 20 km depth beneath the Mono Craters, about 10 km north of Long Valley. Velocity contrasts in both of these bodies are about 6–10%, and the features are interpreted as silicic magma chambers. This experiment does not preclude the presence of additional pockets of magma smaller than 5 km across in the upper crust, particularly beneath the resurgent dome of the caldera (which would be removed with the stripping methods). It is likely that the midcrust anomaly beneath the resurgent dome is a remnant of the caldera forming magma chamber. Upper mantle velocities are lower than average beneath Mono Craters and higher than average beneath Long Valley. We tentatively interpret these anomalies as mantle partial melt beneath the Mono Craters and refractory upper mantle residuum beneath Long Valley. The high eruptive rate of the Mono Craters and these upper mantle structures suggest that the focus of volcanism is shifting north from Long Valley to the Mono Craters.

Journal of Geophysical Research Solid Earth↗
USGS2012

Global map of eolian features on Mars

Ten basic categories of eolian features on Mars were identified from a survey of Mariner 9 and Viking orbiter images. The ten features mapped are (1) light streaks (including frost streaks), (2) dark streaks, (3) sand sheets or splotches, (4) barchari dunes, (5) transverse dunes, (6) crescentic dunes, (7) anomalous dunes, (8) yardangs, (9) wind grooves, and (10) deflation pits. The features were mapped in groups, not as individual landforms, and recorded according to their geographic positions and orientations on maps of 1:12.5 million or 1:25 million scale. In the north polar region, light and dark streaks indicate winds from the west and northwest. Frost streaks show northeast and northwest winds. Barchan dunes show southwest and west winds. Transverse dunes show east and west winds. Local erosional features show winds from the northwest or southeast and northeast or southwest. In the middle and low northern latitudes, streaks show northeast winds; yardangs and deflation pits show mostly easterly and northeasterly winds. In the low southern latitudes, light streaks and dunes record northwest and northeast winds; dark streaks record southeast winds. In high southern latitudes, most streaks and dunes record southeast and east winds. In the south polar region, light and frost streaks record southwest and northwest winds, whereas dark streaks and transverse dunes show southeast winds. The patterns recorded for ephemeral features conform to global conditions of strong southern spring and summer (northern fall and winter) wind circulation. Erosional features in bedrock indicate long-term and perhaps ancient wind trends, whereas depositional features may record relatively more recent winds. Wind directions indicated by yardangs, pits, and other erosional features seldom correspond to those shown by streaks. These erosional features indicating winds of different directions than those that formed the streaks may have been carved when the effective regional winds on Mars were different due to relative hemisphere insolation differences, which are related to obliquity variations. Alternatively, many yardangs and pits may be largely controlled by structure and carved along surface structural patterns or features that channeled both ancient and present-day winds. Deflation pits in the mantled terrain may contain the best record of ancient wind trends. These pits seem to have stratigraphically related orientations. Different stratigraphic units composing the mantle contain wind erosion pits that possibly record large-scale wind direction changes because of long-term changes in the obliquity of the Mars.

Journal of Geophysical Research Solid Earth↗
USGS2017

Monitoring land surface albedo and vegetation dynamics using high spatial and temporal resolution synthetic time series from Landsat and the MODIS BRDF/NBAR/albedo product

Seasonal vegetation phenology can significantly alter surface albedo which in turn affects the global energy balance and the albedo warming/cooling feedbacks that impact climate change. To monitor and quantify the surface dynamics of heterogeneous landscapes, high temporal and spatial resolution synthetic time series of albedo and the enhanced vegetation index (EVI) were generated from the 500 m Moderate Resolution Imaging Spectroradiometer (MODIS) operational Collection V006 daily BRDF/NBAR/albedo products and 30 m Landsat 5 albedo and near-nadir reflectance data through the use of the Spatial and Temporal Adaptive Reflectance Fusion Model (STARFM). The traditional Landsat Albedo (Shuai et al., 2011) makes use of the MODIS BRDF/Albedo products (MCD43) by assigning appropriate BRDFs from coincident MODIS products to each Landsat image to generate a 30 m Landsat albedo product for that acquisition date. The available cloud free Landsat 5 albedos (due to clouds, generated every 16 days at best) were used in conjunction with the daily MODIS albedos to determine the appropriate 30 m albedos for the intervening daily time steps in this study. These enhanced daily 30 m spatial resolution synthetic time series were then used to track albedo and vegetation phenology dynamics over three Ameriflux tower sites (Harvard Forest in 2007, Santa Rita in 2011 and Walker Branch in 2005). These Ameriflux sites were chosen as they are all quite nearby new towers coming on line for the National Ecological Observatory Network (NEON), and thus represent locations which will be served by spatially paired albedo measures in the near future. The availability of data from the NEON towers will greatly expand the sources of tower albedometer data available for evaluation of satellite products. At these three Ameriflux tower sites the synthetic time series of broadband shortwave albedos were evaluated using the tower albedo measurements with a Root Mean Square Error (RMSE) less than 0.013 and a bias within the range of ±0.006. These synthetic time series provide much greater spatial detail than the 500 m gridded MODIS data, especially over more heterogeneous surfaces, which improves the efforts to characterize and monitor the spatial variation across species and communities. The mean of the difference between maximum and minimum synthetic time series of albedo within the MODIS pixels over a subset of satellite data of Harvard Forest (16 km by 14 km) was as high as 0.2 during the snow-covered period and reduced to around 0.1 during the snow-free period. Similarly, we have used STARFM to also couple MODIS Nadir BRDF Adjusted Reflectances (NBAR) values with Landsat 5 reflectances to generate daily synthetic times series of NBAR and thus Enhanced Vegetation Index (NBAR-EVI) at a 30 m resolution. While normally STARFM is used with directional reflectances, the use of the view angle corrected daily MODIS NBAR values will provide more consistent time series. These synthetic times series of EVI are shown to capture seasonal vegetation dynamics with finer spatial and temporal details, especially over heterogeneous land surfaces.

International Journal of Applied Earth Observation↗
USGS2000

Three-dimensional seismic velocity structure of the San Francisco Bay area

Seismic travel times from the northern California earthquake catalogue and from the 1991 Bay Area Seismic Imaging Experiment (BASIX) refraction survey were used to obtain a three-dimensional model of the seismic velocity structure of the San Francisco Bay area. Nonlinear tomography was used to simultaneously invert for both velocity and hypocenters. The new hypocenter inversion algorithm uses finite difference travel times and is an extension of an existing velocity tomography algorithm. Numerous inversions were performed with different parameters to test the reliability of the resulting velocity model. Most hypocenters were relocated <2 km from their catalogue locations. Large lateral velocity variations at shallow (<4 km) depth correlate with known surface geology, including low-velocity Cenozoic sedimentary basins, high-velocity Cenozoic volcanic rocks, and outcrop patterns of the major Mesozoic geologic terranes. Salinian arc rocks have higher velocities than the Franciscan melange, which in turn are faster than Great Valley Sequence forearc rocks. The thickess of low-velocity sediment is defined, including >12 km under the Sacramento River Delta, 6 km beneath Livermore Valley, 5 km beneath the Santa Clara Valley, and 4 km beneath eastern San Pablo Bay. The Great Valley Sequence east of San Francisco Bay is 4-6 km thick. A relatively high velocity body exists in the upper 10 km beneath the Sonoma volcanic field, but no evidence for a large intrusion or magma chamber exists in the crust under The Geysers or the Clear Lake volcanic center. Lateral velocity contrasts indicate that the major strike-slip faults extend subvertically beneath their surface locations through most of the crust. Strong lateral velocity contrasts of 0.3-0.6 km/s are observed across the San Andreas Fault in the middle crust and across the Hayward, Rogers Creek, Calaveras, and Greenville Faults at shallow depth. Weaker velocity contrasts (0.1-0.3 km/s) exist across the San Andreas, Hayward, and Rogers Creek Faults at all other depths. Low spatial resolution evidence in the lower crust suggests that the top of high-velocity mafic rocks gets deeper from west to east and may be offset under the major faults. The data suggest that the major strike-slip faults extend subvertically through the middle and perhaps the lower crust and juxtapose differing lithology due to accumulated strike-slip motion. The extent and physical properties of the major geologic units as constrained by the model should be used to improve studies of seismicity, strong ground motion, and regional stress.

Journal of Geophysical Research B: Solid Earth↗
USGS1996

Crustal structure of a transform plate boundary: San Francisco Bay and the central California continental margin

Wide-angle seismic data collected during the Bay Area Seismic Imaging Experiment provide new glimpses of the deep structure of the San Francisco Bay Area Block and across the offshore continental margin. San Francisco Bay is underlain by a veneer (<300 m) of sediments, beneath which P wave velocities increase rapidly from 5.2 km/s to 6.0 km/s at 7 km depth, consistent with rocks of the Franciscan subduction assemblage. The base of the Franciscan at-15-18 km depth is marked by a strong wide-angle reflector, beneath which lies an 8- to 10-km-thick lower crust with an average velocity of 6.75??0.15 km/s. The lower crust of the Bay Area Block may be oceanic in origin, but its structure and reflectivity indicate that it has been modified by shearing and/or magmatic intrusion. Wide-angle reflections define two layers within the lower crust, with velocities of 6.4-6.6 km/s and 6.9-7.3 km/s. Prominent subhorizontal reflectivity observed at near-vertical incidence resides principally in the lowermost layer, the top of which corresponds to the "6-s reflector" of Brocher et al. [1994]. Rheological modeling suggests that the lower crust beneath the 6-s reflector is the weakest part of the lithosphere; the horizontal shear zone suggested by Furlong et al. [1989] to link the San Andreas and Hayward/Calaveras fault systems may actually be a broad zone of shear deformation occupying the lowermost crust. A transect across the continental margin from the paleotrench to the Hayward fault shows a deep crustal structure that is more complex than previously realized. Strong lateral variability in seismic velocity and wide-angle reflectivity suggests that crustal composition changes across major transcurrent fault systems. Pacific oceanic crust extends 40-50 km landward of the paleotrench but, contrary to prior models, probably does not continue beneath the Salinian Block, a Cretaceous arc complex that lies west of the San Andreas fault in the Bay Area. The thickness (10 km) and high lower-crustal velocity of Pacific oceanic crust suggest that it was underplated by magmatism associated with the nearby Pioneer seamount. The Salinian Block consists of a 15-km-thick layer of velocity 6.0-6.2 km/s overlying a 5-km-thick, high-velocity (7.0 km/s) lower crust that may be oceanic crust, Cretaceous arc-derived lower crust, or a magmatically underplated layer. The strong structural variability across the margin attests to the activity of strike-slip faulting prior to and during development of the transcurrent Pacific/North American plate boundary around 29 Ma. Copyright 1996 by the American Geophysical Union.

Journal of Geophysical Research B: Solid Earth↗