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At least 163 records · Page 9Linked to original sources
USGS2006

Integration of P- and SH-wave high-resolution seismic reflection and micro-gravity techniques to improve interpretation of shallow subsurface structure: New Madrid seismic zone

Shallow high-resolution seismic reflection surveys have traditionally been restricted to either compressional (P) or horizontally polarized shear (SH) waves in order to produce 2-D images of subsurface structure. The northernmost Mississippi embayment and coincident New Madrid seismic zone (NMSZ) provide an ideal laboratory to study the experimental use of integrating P- and SH-wave seismic profiles, integrated, where practicable, with micro-gravity data. In this area, the relation between "deeper" deformation of Paleozoic bedrock associated with the formation of the Reelfoot rift and NMSZ seismicity and "shallower" deformation of overlying sediments has remained elusive, but could be revealed using integrated P- and SH-wave reflection. Surface expressions of deformation are almost non-existent in this region, which makes seismic reflection surveying the only means of detecting structures that are possibly pertinent to seismic hazard assessment. Since P- and SH-waves respond differently to the rock and fluid properties and travel at dissimilar speeds, the resulting seismic profiles provide complementary views of the subsurface based on different levels of resolution and imaging capability. P-wave profiles acquired in southwestern Illinois and western Kentucky (USA) detect faulting of deep, Paleozoic bedrock and Cretaceous reflectors while coincident SH-wave surveys show that this deformation propagates higher into overlying Tertiary and Quaternary strata. Forward modeling of micro-gravity data acquired along one of the seismic profiles further supports an interpretation of faulting of bedrock and Cretaceous strata. The integration of the two seismic and the micro-gravity methods therefore increases the scope for investigating the relation between the older and younger deformation in an area of critical seismic hazard. ?? 2006 Elsevier B.V. All rights reserved.

Tectonophysics↗
USGS2009

Interpreting the tectonic evolution of Pacific Rim margins using plate kinematics and slab window volcanism

The possibility that slab windows might form in the wake of ridge subduction was first conceptualized in the late 1970s as earth scientists explored the implications of plate tectonic theory in three dimensions. Acceptance of slab-window occurrence, however, has been a long time coming (McCrory and Wilson, this issue). With records of both current and past slab-window transients now firmly established through detailed studies of volcanism and tectonism tied to kinematic plate reconstructions, we are able to exploit these spatially and temporally restricted events as paleogeographic markers. Slab-window research encompasses and integrates a broad range of disciplines including earthquake seismology, plate kinematics, volcanic geochemistry, lithospheric dynamics, seismic tomography, and structural geology. In this special issue we use this powerful new tool to constrain and interpret the Cenozoic evolution of Pacific Rim subduction margins from Alaska, to Patagonia, to Antarctica, to Tonga.

Tectonophysics↗
USGS2001

Geologic structures related to New Madrid earthquakes near Memphis, Tennessee, based on gravity and magnetic interpretations

New inversions of gravity and magnetic data in the region north of memphis. Tennessee, and south of latitude 36?? define boundaries of regional structures and igneous complexes in the upper crust. Microseismicity patterns near interpreted boundaries suggest that igneous complexes influence the locations of microseismicity. A weak seismicity cluster occurs near one intrusion (Covington pluton), at the intersection of the southwest margin of the Missouri batholith and the southeast margin of the Reelfoot rift. A narrow seismicity trend along the Reelfoot rift axis becomes diffuse near a second intrusion (Osceola intrusive complex) and changes direction to an area along the northwest flank of the intrusion. The axial seismicity trend also contains a tight cluster of earthquakes located just outside the Osceola intrusive complex. The mechanical explanation of the two seismicity patterns is uncertain, but the first cluster may be caused by stress concentration due to the high elastic stiffness and strength of the Covington intrusion. The spatially changing seismicity pattern near the Osceola complex may be caused by the preceding factors plus interaction with faulting along the rift axis. The axial seismicity strand itself is one of several connected and interacting active strands that may produce stress concentrations at strand ends and junctions. The microseismicity clusters at the peripheries of the two intrusions lead us to conclude that these stress concentrations or stressed volumes may be locations of future moderate to large earthquakes near Memphis. Published by Elsevier Science B.V.

Engineering Geology↗
USGS2002

An interpretation of differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network monitoring program

Differences between field and laboratory pH values reported by the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) monitoring program from 1984 through 1986 are investigated. Median differences in hydrogen ion concentration between laboratory and field pH determinations at sites averaged -4.6 μ equiv/L in natural precipitation samples on an annual basis. The median difference found in external quality assurance samples analyzed during the same time period was -11 μ equiv/L. The results suggest a systematic bias in pH values reported by the NADP/NTN network. The bias appears to have a fixed component of approximately -7 μ equiv/L, which can be attributed to the sampling bucket and lid, and a seasonal and regional component that ranges from +4 to -22 μ equiv/L at the 10th and 90th percentiles. Differences were found to be independent of sample pH and sample volume. The magnitude of the bias has implications for the interpretation of previously published pH and hydrogen ion concentration and deposition values in the western United States.

Environmental Science and Technology↗
USGS2002

Dynamic interpretation of slug tests in highly permeable aquifers

Considerable progress has been made in developing a theoretical framework for modeling slug test responses in formations with high hydraulic conductivity K . However, several questions of practical significance remain unresolved. Given the rapid and often oscillatory nature of test responses, the traditional hydrostatic relationship between the water level and the transducer‐measured head in the water column may not be appropriate. A general dynamic interpretation is proposed that describes the relationship between water level response and transducer‐measured head. This theory is utilized to develop a procedure for transforming model‐generated water level responses to transducer readings. The magnitude of the difference between the actual water level position and the apparent position based on the transducer measurement is a function of the acceleration and velocity of the water column, test geometry, and depth of the transducer. The dynamic approach explains the entire slug test response, including the often‐noted discrepancy between the actual initial water level displacement and that measured by a transducer in the water column. Failure to use this approach can lead to a significant underestimation of K when the transducer is a considerable distance below the static water level. Previous investigators have noted a dependence of test responses on the magnitude of the initial water level displacement and have developed various approximate methods for analyzing such data. These methods are re‐examined and their limitations clarified. Practical field guidelines are proposed on the basis of findings of this work. The soundness of the dynamic approach is demonstrated through a comparison of K profiles from a series of multilevel slug tests with those from dipole‐flow tests performed in the same wells.

Water Resources Research↗
USGS2008

Observations and interpretation of fundamental mode Rayleigh wavefields recorded by the Transportable Array (USArray)

Broadband recordings of the dense Transportable Array (TA) in the western United States provide unparalleled detailed images of long-period seismic surface wavefields. With 400 stations spanning most of the western United States, wavefronts of fundamental mode Rayleigh waves may be visualized coherently across the array at periods ???40 s. In order to constrain the Rayleigh wave phase velocity structure in the western United States, I assemble a data set of vertical component seismograms from 53 teleseismic events recorded by the TA from April 2006 to October 2007. Complex amplitude spectra from these recordings at peni ods 27-100 s are interpreted using the multiplane wave tomographic method of Friederich and Wielandt (1995) and Pollitz (1999). This analysis yields detailed surface wave phase velocity and three-dimensional shear wave velocity patterns across the North American plate boundary zone, elucidating the active processes in the highly heterogeneous western U.S. upper mantle.

Journal of Geophysical Research B: Solid Earth↗
USGS2008

Mapping and interpretation of Sinlap crater on Titan using Cassini VIMS and RADAR data

Only a few impact craters have been unambiguously detected on Titan by the Cassini-Huygens mission. Among these, Sinlap is the only one that has been observed both by the RADAR and VIMS instruments. This paper describes observations at centimeter and infrared wavelengths which provide complementary information about the composition, topography, and surface roughness. Several units appear in VIMS false color composites of band ratios in the Sinlap area, suggesting compositional heterogeneities. A bright pixel possibly related to a central peak does not show significant spectral variations, indicating either that the impact site was vertically homogeneous, or that this area has been recovered by homogeneous deposits. Both VIMS ratio images and dielectric constant measurements suggest the presence of an area enriched in water ice around the main ejecta blanket. Since the Ku-band SAR may see subsurface structures at the meter scale, the difference between infrared and SAR observations can be explained by the presence of a thin layer transparent to the radar. An analogy with terrestrial craters in Libya supports this interpretation. Finally, a tentative model describes the geological history of this area prior, during, and after the impact. It involves mainly the creation of ballistic ejecta and an expanding plume of vapor triggered by the impact, followed by the redeposition of icy spherules recondensed from this vapor plume blown downwind. Subsequent evolution is then driven by erosional processes and aeolian deposition. Copyright 2008 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗
USGS2026

A roadmap for identifying and interpreting physical processes and national water model prediction bias associated with baseflow index regimes across the contiguous United States

Understanding how groundwater–surface water interactions shape streamflow variability is critical for diagnosing low flow behavior and prediction bias in continental scale hydrologic models. We present a process informed framework that links observed baseflow (BF) dynamics, watershed attributes, and National Water Model (NWM) performance across the contiguous United States. Using daily observed streamflow from 797 reference quality streamgages, we developed monthly baseflow index (BFI) signatures using a streamgage specific, calibrated digital filter. Hierarchical clustering of these signatures identified seven distinct BFI regimes capturing regional and seasonal variability. We evaluated NWM v3.0 retrospective streamflow performance within each regime using multiple hydrograph and flow duration curve-based metrics. Model skill varied systematically across regimes: mixed flow systems were simulated most accurately, while predominantly BF dominated and quickflow dominated regimes exhibited substantially poorer performance. Across nearly all regimes, the NWM underestimated observed BFI magnitude and frequently failed to reproduce seasonal BF patterns, indicating systematic biases in simulated low flow contributions. To relate these regimes to potential process controls, we trained a Random Forest classifier using static watershed attributes and applied Shapley Additive Explanations to identify features most strongly associated with each regime. Results highlight regionally varying influences, including the dominant role of snow fraction and seasonal runoff timing in snow dominated basins and the importance of evapotranspiration and aridity in quickflow dominated systems. Collectively, these findings demonstrate how hydrologic signatures combined with interpretable machine learning can diagnose regime specific model biases and generate process-based hypotheses about limitations in large scale hydrologic prediction systems.

contiguous United States↗
USGS2012

Enhanced visualization for the interpretation of Magellan radar data: Supplement to the Magellan special issue

The differences of radar data from more familiar photographic images, coupled with the unique geologic characteristics of Venus's surface, can make interpretation of the standard Magellan data products difficult for both planetary scientists and nonspecialists. We describe a set of digital processing techniques for transforming individual remote‐sensing datasets in order to make the information they contain more apparent, and for combining multiple datasets of diverse resolution and information content into synthetic images that allow the viewer to explore correlations among the data. The creation of derived data products by techniques like those we describe is becoming increasingly widespread and important, both for scientific research and for use in educational and media presentations. We apply our processing techniques to Magellan synthetic aperture radar images, altimetry, and microwave emissivity measurements of four areas that exemplify some of the most important styles of geologic activity on Venus.

Journal of Geophysical Research E: Planets↗
USGS2010

Role of physical heterogeneity in the interpretation of small-scale laboratory and field observations of bacteria, microbial-sized microsphere, and bromide transport through aquifer sediments

The effect of physical variability upon the relative transport behavior of microbial-sized microspheres, indigenous bacteria, and bromide was examined in field and flow-through column studies for a layered, but relatively well sorted, sandy glaciofluvial aquifer. These investigations involved repacked, sieved, and undisturbed aquifer sediments. In the field, peak abundance of labeled bacteria traveling laterally with groundwater flow 6 m downgradient from point of injection was coincident with the retarded peak of carboxylated microspheres (retardation factor, RF = 1.7) at the 8.8 m depth, but preceded the bromide peak and the retarded microsphere peak (RF = 1.5) at the 9.0 m depth. At the 9.5 m depth, the bacterial peak was coincident with both the bromide and the microsphere peaks. Although sorption appeared to be a predominant mechanism responsible for immobilization of microbial-sized microspheres in the aquifer, straining appeared to be primarily responsible for their removal in 0.6-m-long columns of repacked, unsieved aquifer sediments. The manner in which the columns were packed also affected optimal size for microsphere transport, which in one experiment was near the size of the small (∼2 μm) groundwater protozoa (flagellates). These data suggest that variability in aquifer sediment structure can be important in interpretation of both small-scale field and laboratory experiments examining microbial transport behavior.

Water Resources Research↗
USGS2010

Interpretation of surface flux measurements in heterogeneous terrain during the Monsoon '90 experiment

A network of 9-m-tall surface flux measurement stations were deployed at eight sparsely vegetated sites during the Monsoon '90 experiment to measure net radiation, Q , soil heat flux, G , sensible heat flux, H (using eddy correlation), and latent heat flux, λ E (using the energy balance equation). At four of these sites, 2-m-tall eddy correlation systems were used to measure all four fluxes directly. Also a 2-m-tall Bowen ratio system was deployed at one site. Magnitudes of the energy balance closure ( Q + G + H + λ E ) increased as the complexity of terrain increased. The daytime Bowen ratio decreased from about 10 before the monsoon season to about 0.3 during the monsoons. Source areas of the measurements are developed and compared to scales of heterogeneity arising from the sparse vegetation and the topography. There was very good agreement among simultaneous measurements of Q with the same model sensor at different heights (representing different source areas), but poor agreement among different brands of sensors. Comparisons of simultaneous measurements of G suggest that because of the extremely small source area, extreme care in sensor deployment is necessary for accurate measurement in sparse canopies. A recently published model to estimate fetch is used to interpret measurements of H at the 2 m and 9 m heights. Three sites were characterized by undulating topography, with ridgetops separated by about 200–600 m. At these sites, sensors were located on ridgetops, and the 9-m fetch included the adjacent valley, whereas the 2-m fetch was limited to the immediate ridgetop and hillside. Before the monsoons began, vegetation was mostly dormant, the watershed was uniformly hot and dry, and the two measurements of H were in close agreement. After the monsoons began and vegetation fully matured, the 2-m measurements of H were significantly greater than the 9-m measurements, presumably because the vegetation in the valleys was denser and cooler than on the ridgetops and hillsides. At one lowland site with little topographic relief, the vegetation was more uniform, and the two measurements of H were in close agreement during peak vegetation. Values of λ E could only be compared at two sites, but the 9-m values were greater than the 2-m values, suggesting λ E from the dense vegetation in the valleys was greater than elsewhere.

Water Environment Research↗
USGS1998

Effects of topography on the interpretation of the deformation field of prominent volcanoes - Application to Etna

We have investigated the effects of topography on the surface-deformation field of volcanoes. Our study provides limits to the use of classical half-space models. Considering axisymmetrical volcanoes, we show that interpreting ground-surface displacements with half-space models can lead to erroneous estimations of the shape of the deformation source. When the average slope of the flanks of a volcano exceeds 20°, tilting in the summit area is reversed to that expected for a flat surface. Thus, neglecting topography may lead to misinterpreting an inflation of the source as a deflation. Comparisons of Mogi's model with a three-dimensional model shows that ignoring topography may lead to an overestimate of the source-volume change by as much as 50% for a slope of 30°. This comparison also shows that the depths calculated by using Mogi's solution for prominent volcanoes should be considered as depths from the summit of the edifices. Finally, we illustrate these topographic effects by analyzing the deformation field measured by radar interferometry at Mount Etna during its 1991–1993 eruption. A three-dimensional modeling calculation shows that the flattening of the deflation field near the volcano's summit is probably a topographic effect.

Geophysical Research Letters↗
USGS2012

An interpretation of episodic slip on the Calaveras fault near Hollister, California

Data from a precision multi-wavelength distance measuring (MWDM) instrument, located near Hollister, California, have been analyzed in terms of strike-slip faulting in the region covered by the network. Four episodes of deformation that are readily identifiable in the MWDM data for the year following September 1975 can be modeled as slip on the Calaveras fault. Creepmeter data tend to support this interpretation, since the creep events and the observed episodes of deformation appear to be correlated in a temporal sense. Lower bounds on the moment for each episode of slip can be determined from the MWDM data by the technique of linear programing. The results indicate that aseismic slip is the dominant mechanism for strain release, since the combined moment of 1.16 × 10 24 dyn cm for the four episodes far exceeds the moment of all of the earthquakes that occurred during the year. Creepmeter data and geologic evidence that are taken in conjunction with the lower bound of the moment indicate that the depth of aseismic slip for some of the episodes of fault slip could extend to below the greatest depth of earthquake hypocenters for this region of central California.

Journal of Geophysical Research Solid Earth↗
USGS2012

Interpretation of gravity data in a complex volcano-tectonic setting, southwestern Nevada

This regional gravity study, based on an irregular 2-km data grid, was conducted during the past few years at Yucca Mountain, southern Nye County, Nevada, as part of a program to locate a suitable repository for high-level nuclear waste. About 100 surface rock samples, three borehole gamma-gamma logs, and one borehole gravity study provide excellent density control. A nearly linear increase in density of 0.26 g/cm 3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain. Isostatic and 2.0-g/cm 3 Bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects, respectively. The Bare Mountain gravity high, with an isostatic anomaly maximum of 48 mGal, is connected with a greater gravity high over the Funeral Mountains, to the southwest; together, these highs result from a continuous block of dense, metamorphosed Precambrian and Paleozoic rocks that stretches across much of the Walker Lane from the east edge of Death Valley to Bare Mountain. The Calico Hills gravity high appears more likely to originate from a northeast trending buried ridge of Paleozoic rocks that extends southwestward beneath Busted Butte, 5 km southeast of the proposed repository, where two- and three-dimensional modeling indicates that the pre-Cenozoic rocks lie less than 1000 m beneath the surface. Tuff, at least 4000 m thick, fills a large steep-sided depression in the pretuff rocks beneath Yucca Mountain and Crater Flat. The gravity low and the thick tuff section lie within a large collapse area that includes the Crater Flat-Timber Mountain-Silent Canyon caldera complexes. Gravity lows in Crater Flat itself are interpreted to coincide with the source areas of the Prow Pass Member, the Bullfrog Member, and the Tram Member of the Crater Flat Tuff; these source areas add nearly 350 km 2 to the previously recognized extent of the local caldera complexes. Southward extension of the broad gravity low associated with Crater Flat into the Amargosa Desert is evidence for sector graben-type collapse segments related to the formation of the Timber Mountain caldera and superimposed on the other volcanic and extensional structures within Crater Flat.

Journal of Geophysical Research Solid Earth↗
USGS2012

Active faulting and deformation of the Coalinga anticline as interpreted from three-dimensional velocity structure and seismicity

This work gives a clear picture of the geometry of aftershock seismicity in a large thrust earthquake. Interpretation of hypocenters and fault plane solutions, from the 1983 Coalinga, Coast Range California, earthquake sequence, in combination with the three-dimensional velocity structure shows that the active faulting beneath the fold primarily consists of a set of southwest dipping thrusts uplifting blocks of higher-velocity material. Above the main listric blind thrust there is a conjugate fault, steeply northeast dipping, that provides the western limit of the aftershocks within the Coalinga Anticline and that corresponds in location and spatial extent with the adjacent Pleasant Valley syncline. The character of the seismicity varies with the degree of previous deformation on each section of the anticline. Where the previous uplift was largest, the shallow seismicity shows secondary faulting on either side of the fold with orientations that correspond to the preexisting geologic structure. Diffuse seismicity characterizes the area with the least previous deformation. The mainshock rupture terminated where the fold trend was no longer uniform but had competing north and west trending features. The upward extent of the mainshock rupture ended at the approximate boundary between Franciscan and Great Valley Sequence rocks. Above that depth the main thrust appears to splay into a steeper segment and a near-horizontal segment. Thus the extent of rupture area is limited by the area of uniform structural orientation and by the variation in the type of material. With the three-dimensional velocity model each individual hypocenter moved slightly (0–2 km) in accord with the details of the surrounding velocity structure, so that secondary features in the seismicity pattern are more detailed than with a local one-dimensional model and station corrections. The overall character of the fault plane solutions was not altered by the three-dimensional model, but the more accurate ray paths did result in distinct changes. In particular, the mainshock has a fault plane dipping 30° southwest instead of the 23° obtained with the one-dimensional model.

Journal of Geophysical Research Solid Earth↗
USGS2014

Geophysical interpretation of ground‐water levels

The theory of rock‐pressure as a cause of artesian‐head dates back at least to early Grecian times. Thus the philosopher Thales, about 600 B.C., taught that the springs derive their water from the ocean through subterranean channels and that the water is lifted to the springs by rook‐pressure. The theory of rock‐pressure has had a number of recent advocates, chief among whom was the British geologist, J. W. Gregory, but apparently no effective attempt was made by any of them to apply critical data to the problem. In general, geologists and hydrologists have rejected rock‐pressure as a vague heresy and have assumed that the artesian formations function as perfectly rigid and inert containers, not recognizing the fact that the data in regard to the performance of wells appear to conflict with such an interpretation .

Eos, Transactions, American Geophysical Union↗
USGS2014

A graphic procedure in the geochemical interpretation of water‐analyses

This paper outlines certain fundamental principles in a graphic procedure which appears to be an effective tool in segregating analytical data for critical study with respect to sources of the dissolved constituents in waters, modifications in the character of a water as it passes through an area, and related geochemical problems. The procedure is based on a multiple‐trilinear diagram (Fig. 1) whose form has been evolved gradually and independently by the writer during the past several years through trial and modification of less comprehensive antecedent forms. Neither the diagram nor the procedure here described is a panacea for the easy solution of all geochemical problems. Many problems of interpretation can be answered only by intensive study of critical analytical data by other methods.

Eos, Transactions, American Geophysical Union↗
USGS2010

Genesis of the West Siberian Basin and its petroleum geology: A recent interpretation

A prominent Western specialist on the geology of the oil and gas deposits of Russia provides an interpretation of the genesis of the West Siberian basin, relying, in part, on most recent Russian studies as well as information made available in 1994 evaluating the reserves of Russia's most important producing province. From Late Carboniferous through Middle Jurassic time, the region of West Siberia passed through orogenic, rift, and early platform stages. A domal high was present in the region during the orogenic stage, arising from cratonization of the Ural-Mongolian fold belt. Early Triassic rifting was part of a global rifting event and was a precursor to the subsequent crustal sagging that produced the West Siberian basin. The Early-Middle Jurassic was a time of cyclical marine and continental deposition, the sea moving back and forth from the north. The Talinskoye oil field occurs in Lower-Middle Jurassic sandstones that have the form of a river channel that extends more than 200 km. The Priobskoye field is associated with a Lower Cretaceous clinoform that has been traced N–S for more than 300 km. It is suggested that: (a) the oil in the Lower Cretaceous Neocomian sandstones was sourced by bituminous clays that interfinger with these sandstones on the west; and (b) that Upper Cretaceous Cenomanian gas was sourced in part by deeply buried Paleozoics and by overlying Upper Cretaceous Turonian clays. Predicted discoveries in West Siberia include several thousand small fields with reserves of less than 10 million tons, 250 to 300 medium-sized fields, and several large fields with 30 to 100 million tons.

International Geology Review↗