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Showing papers by "United States Geological Survey published in 1996"


Journal ArticleDOI
28 Sep 1996-Nature
TL;DR: In this article, it was shown that some microorganisms found in soils and sediments are able to use humic substances as an electron acceptor for the anaerobic oxidation of organic compounds and hydrogen.
Abstract: HUMIC substances are heterogeneous high-molecular-weight organic materials which are ubiquitous in terrestrial and aquatic environments. They are resistant to microbial degradation1 and thus are not generally considered to be dynamically involved in microbial metabolism, especially in anoxic habitats. However, we show here that some microorganisms found in soils and sediments are able to use humic substances as an electron acceptor for the anaerobic oxidation of organic compounds and hydrogen. This electron transport yields energy to support growth. Microbial humic reduction also enhances the capacity for microorganisms to reduce other, less accessible electron acceptors, such as insoluble Fe(III) oxides, because humic substances can shuttle electrons between the humic-reducing microorganisms and the Fe(III) oxide. The finding that microorganisms can donate electrons to humic acids has important implications for the mechanisms by which microorganisms oxidize both natural and contaminant organics in anaerobic soils and sediments, and suggests a biological source of electrons for humics-mediated reduction of contaminant metals and organics.

1,651 citations


Book ChapterDOI
TL;DR: In this paper, the authors present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to North Atlantic.
Abstract: We present estimates of total nitrogen and total phosphorus fluxes in rivers to the North Atlantic Ocean from 14 regions in North America, South America, Europe, and Africa which collectively comprise the drainage basins to the North Atlantic. The Amazon basin dominates the overall phosphorus flux and has the highest phosphorus flux per area. The total nitrogen flux from the Amazon is also large, contributing 3.3 Tg yr-1 out of a total for the entire North Atlantic region of 13.1 Tg yr-1. On a per area basis, however, the largest nitrogen fluxes are found in the highly disturbed watersheds around the North Sea, in northwestern Europe, and in the northeastern U.S., all of which have riverine nitrogen fluxes greater than 1,000 kg N km-2 yr-1.

1,614 citations


01 Jan 1996
TL;DR: In this paper, the amount of water outgassed from Mars by impact erosion and hydrodynamic escape is estimated to be between 6 to 160 m. The two sets of estimates may be reconciled if early in its history, Mars lost part of its atmosphere.
Abstract: Estimates of the amount of water outgassed from Mars, based on the composition of the atmosphere, range from 6 to 160 m, as compared with 3 km for the Earth. In contrast, large flood features, valley networks, and several indicators of ground ice suggest that at least 500 m of water have outgassed. The two sets of estimates may be reconciled if early in its history, Mars lost part of its atmosphere by impact erosion and hydrodynamic escape.

910 citations


Book
11 Jan 1996
TL;DR: In this paper, the amount of water outgassed from Mars by impact erosion and hydrodynamic escape is estimated to be between 6 to 160 m. The two sets of estimates may be reconciled if early in its history, Mars lost part of its atmosphere.
Abstract: Estimates of the amount of water outgassed from Mars, based on the composition of the atmosphere, range from 6 to 160 m, as compared with 3 km for the Earth. In contrast, large flood features, valley networks, and several indicators of ground ice suggest that at least 500 m of water have outgassed. The two sets of estimates may be reconciled if early in its history, Mars lost part of its atmosphere by impact erosion and hydrodynamic escape.

885 citations


Journal ArticleDOI
28 Mar 1996-Nature
TL;DR: At an active volcano, long-period seismicity reflects pressure fluctuations resulting from unsteady mass transport in the sub-surface plumbing system, and hence provides a glimpse of the internal dynamics of the volcanic edifice as mentioned in this paper.
Abstract: At an active volcano, long-period seismicity (with typical periods in the range 02–2 s) reflects pressure fluctuations resulting from unsteady mass transport in the sub-surface plumbing system, and hence provides a glimpse of the internal dynamics of the volcanic edifice When this activity occurs at shallow depths, it may signal the pressure-induced disruption of the steam-dominated region of the volcano, and can accordingly be a useful indicator of impending eruption

832 citations


Journal ArticleDOI
TL;DR: In this paper, the relationship among vegetation, processes, and landforms is described for representative streams of four areas of the United States: high-gradient streams of the humid east, coastal-plain streams, Great Plains streams, and stream channels of the southwestern United States.

605 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that the olivine → spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone, and that wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km.
Abstract: Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher-pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine → spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge-shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer-grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes should cease at depths near 700 km, because the seismogenic phase transformations in the slab are completed or can no longer occur. Substantial metastability is expected only in old, cold slabs, consistent with the observed restriction of deep earthquakes to those settings. Earthquakes should be restricted to the cold cores of slabs, as in any model in which the seismicity is temperature controlled, via the distribution of metastability. However, the geometries of recent large deep earthquakes pose a challenge for any such models. Transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics, including focal mechanisms indicating localized shear failure rather than implosive deformation, are so similar to those of shallow earthquakes. Finally, metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab. Such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and/or are detached from subducting slabs. Metastability in downgoing slabs could have considerable geodynamic significance. Metastable wedges would reduce the negative buoyancy of slabs, decrease the driving force for subduction, and influence the state of stress in slabs. Heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage, causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity. Because wedge formation should occur only for fast subducting slabs, it may act as a “parachute” and contribute to regulating plate speeds. Wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel, favoring the subsequent sinking of the slab into the lower mantle.

524 citations


Journal ArticleDOI
TL;DR: In this article, the authors studied the distribution of rare earth elements in hydrogenetic marine ferromanganese (Fe-Mn) crusts from the Central Pacific Ocean and found that the rare earth properties of the seafloor are similar to those of the terrestrial environment.

497 citations


Journal ArticleDOI
TL;DR: In the field of geotechnical analysis of sediment deformation, the most important criterion is that a seismic liquefaction origin requires widespread, regional development of features around a core area where the effects are most severe.

457 citations


Journal ArticleDOI
29 Nov 1996-Science
TL;DR: Analytical analysis shows that the observed enhancement is localized to the permanently shadowed regions of the lunar south pole, and that observations of periodically solar-illuminated lunar surfaces, including the north pole, yielded no enhancement.
Abstract: During the Clementine 1 mission, a bistatic radar experiment measured the magnitude and polarization of the radar echo versus bistatic angle, beta, for selected lunar areas. Observations of the lunar south pole yield a same-sense polarization enhancement around beta = 0. Analysis shows that the observed enhancement is localized to the permanently shadowed regions of the lunar south pole. Radar observations of periodically solar-illuminated lunar surfaces, including the north pole, yielded no such enhancement. A probable explanation for these differences is the presence of low-loss volume scatterers, such as water ice, in the permanently shadowed region at the south pole.

425 citations


Journal ArticleDOI
TL;DR: In this paper, a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines, is presented.
Abstract: We present a rupture model of the Northridge earthquake, determined from the joint inversion of near-source strong ground motion recordings, P and SH teleseismic body waves, Global Positioning System (GPS) displacement vectors, and permanent uplift measured along leveling lines. The fault is defined to strike 122° and dip 40° to the south-southwest. The average rake vector is determined to be 101°, and average slip is 1.3 m; the peak slip reaches about 3 m. Our estimate of the seismic moment is 1.3 ± 0.2 × 10^(26) dyne-cm (potency of 0.4 km3). The rupture area is small relative to the overall aftershock dimensions and is approximately 15 km along strike, nearly 20 km in the dip direction, and there is no indication of slip shallower than about 5 to 6 km. The up-dip, strong-motion velocity waveforms are dominated by large S-wave pulses attributed to source directivity and are comprised of at least 2 to 3 distinct arrivals (a few seconds apart). Stations at southern azimuths indicate two main S-wave arrivals separated longer in time (about 4 to 5 sec). These observations are best modeled with a complex distribution of subevents: The initial S-wave arrival comes from an asperity that begins at the hypocenter and extends up-dip and to the north where a second, larger subevent is centered (about 12 km away). The secondary S arrivals at southern azimuths are best fit with additional energy radiation from another high slip region at a depth of 19 km, 8 km west of the hypocenter. The resolving power of the individual data sets is examined by predicting the geodetic (GPS and leveling) displacements with the dislocation model determined from the waveform data, and vice versa, and also by analyzing how well the teleseismic solution predicts the recorded strong motions. The general features of the geodetic displacements are not well predicted from the model determined independently from the strong-motion data; likewise, the slip model determined from geodetic data does not adequately reproduce the strong-motion characteristics. Whereas a particularly smooth slip pattern is sufficient to satisfy the geodetic data, the strong-motion and teleseismic data require a more heterogeneous slip distribution in order to reproduce the velocity amplitudes and frequency content. Although the teleseismic model can adequately reproduce the overall amplitude and frequency content of the strong-motion velocity recordings, it does a poor job of predicting the geodetic data. Consequently, a robust representation of the slip history and heterogeneity requires a combined analysis of these data sets.

Book ChapterDOI
TL;DR: In this article, several poroelastic techniques employed in the study of earthquake-related hydrologic phenomena are presented. But, although chemistry plays an important role in earthquake hydrology, only mechanical and fluid-dynamic effects can be modeled using poro-elasticity.
Abstract: Publisher Summary This chapter presents several poroelastic techniques employed in the study of earthquake-related hydrologic phenomena. The well water levels can be used to measure tectonic strain over a limited frequency band that depends critically on the degree of aquifer confinement. Although chemistry plays an important role in earthquake hydrology, only mechanical and fluid-dynamic effects can be modeled using poroelasticity. The chapter focuses on hydrologic data related to earthquakes and their quantitative interpretation. Some examples of the earthquake-related hydrologic phenomena are provided. These examples include the 1964 Alaska earthquake, the 1975 Haicheng, China, earthquake, and the 1976 Tangshan, China, earthquake. This chapter describes how the water table drainage affects the time-varying aquifer response to strain imposed by earth tides, barometric pressure, or earthquakes. A graph illustrating the relationship among the depth below the water table, hydraulic diffusivity, and time scale of pore pressure dissipation to the water table is presented The question of whether the hydrologic phenomena precedes earthquakes is also addressed in the chapter.

Journal ArticleDOI
TL;DR: In this article, the second most frequently detected chemical in samples of shallow ambient groundwater from urban areas that were collected during 1993-1994 as part of the U.S. Geological Survey's National Water-Quality Assessment program.
Abstract: The 1990 Clean Air Act Amendments require fuel oxygenates to be added to gasoline used in some metropolitan areas to reduce atmospheric concentrations of carbon monoxide or ozone. Methyl tert-butyl ether (MTBE) is the most commonly used fuel oxygenate and is a relatively new gasoline additive. Nevertheless, out of 60 volatile organic chemicals analyzed, MTBE was the second most frequently detected chemical in samples of shallow ambient groundwater from urban areas that were collected during 1993-1994 as part of the U.S. Geological Survey's National Water-Quality Assessment program. Samples were collected from five drinking water wells, 12 springs, and 193 monitoring wells in urban areas. No MTBE was detected in drinking water wells. At a reporting level of 0.2 μg/L, MTBE was detected most frequently in shallow groundwater from urban areas (27% of 210 wells and springs sampled in eight areas) as compared to shallow groundwater from agricultural areas (1.3% of 549 wells sampled in 21 areas) or deeper groundwater from major aquifers (1.0% of 412 wells sampled in nine areas). Only 3% of the shallow wells sampled in urban areas had concentrations of MTBE that exceed 20 μg/L, which is the estimated lower limit of the United States Environmental Protection Agency draft drinking water health advisory. Because MTBE is persistent and mobile in groundwater, it can move from shallow to deeper aquifers with time. In shallow urban groundwater, MTBE generally was not found with benzene, toluene, ethylbenzene, or xylene (BTEX) compounds, which commonly are associated with gasoline spills. This disassociation causes uncertainty as to the source of MTBE. Possible sources of MTBE in groundwater include point sources, such as leaking storage tanks, and non-point sources, such as recharge of precipitation and stormwater runoff.


Journal ArticleDOI
TL;DR: The degree of conservation of sequences flanking microsatellite repeat motifs and the utility of heterologous PCR primers for analyses in closely related taxa was tested using 10 salmonid species from four genera, suggesting broad application in salmonid research.
Abstract: Twenty-two variable number of tandem repeat microsatellite dinucleotide repeat ((GA)n and (CA)n) loci were cloned from sockeye salmon (Oncorhynchus nerka) partial genomic libraries. Characteristics and optimal polymerase chain reaction (PCR) conditions were defined for each locus. The degree of conservation of sequences flanking microsatellite repeat motifs and the utility of heterologous PCR primers for analyses in closely related taxa was tested using 10 salmonid species from four genera. Nearly all microsatellite primers produce amplification products in multiple species, suggesting broad application in salmonid research. The utility of these loci for population genetic studies was tested using individuals (N = 83) from three spawning populations of chinook salmon (Oncorhynchus tshawytscha ) from the Yukon River, Yukon Territories. Twelve of 16 loci screened were polymorphic (mean heterozygosity = 0.254). Genetic distance estimates between populations were concordant with results from a previous allozyme survey of these same populations. Discussions of the utility of microsatellite markers in salmonid population genetic research are presented in light of recently described statistical methodologies based on mutational properties and interallelic differences in repeat score. Resume : A partir de banques genomiques partielles de saumons sockeye (Oncorhychus nerka), on a clone 22 loci de microsatellite sequences repetees en tandem dinucleotidiques ((GA)n et (CA)n). Les caracteristiques et les conditions optimales pour la PCR ont ete definies pour chaque locus. Le degre de conversion des sequences qui flanquent les motifs repetes des microsatellites et l'utilite des amorces heterologues de la PCR pour des analyses dans des taxons etroitement apparentes ont ete evalues avec 10 especes de salmonides appartenant a quatre genres. Presque toutes les amorces de microsatellites ont donne des produits d'amplification chez plusieurs especes, ce qui laisse supposer qu'elles sont applicables a grande echelle dans les recherches sur les salmonides. On a evalue l'utilite de ces loci dans les etudes genetiques des populations, en utilisant des individus ( N = 83) provenant de trois populations de saumons quinnat (Oncorhychus tshawytscha ) en cours de reproduction, vivant dans le fleuve Yukon (Territoires du Nord-ouest). Douze des 16 loci examines etaient polymorphes (heterozygotie moyenne = 0,254). Les estimations de distance genetique entre les populations concordaient avec les resultats obtenus au cours d'une etude allozymatique precedente des memes populations. On examine l'utilite de marqueurs a microsatellites dans les recherches genetiques de populations de salmonides, a la lumiere de methodes statistiques recemment decrites, basees sur les proprietes mutationnelles et les differences inter-alleliques des resultats obtenus avec les sequences repetees. (Traduit par la Redaction)

Journal ArticleDOI
TL;DR: In this paper, the physical structure of benthic habitats was reduced by direct removal of biogenic (e.g., sponges, hydrozoans, bryozoans, amphipod tubes, holothurians, shell aggregates) and sedimentary (i.e., sand waves, depressions).
Abstract: Fishing gear alters seafloor habitats, but the extent of these alterations, and their effects, have not been quantified extensively in the northwest Atlantic. Understanding the extent of these impacts, and their effects on populations of living marine resources, is needed to properly manage current and future levels of fishing effort and fishing power. For example, the entire U.S. side of the Gulf of Maine was impacted annually by mobile fishing gear between 1984 and 1990, based on calculations of area swept by trawl and dredge gear. Georges Bank was imparted three to nearly four times annually during the same period. Studies at three sites in the Gulf of Maine (off Swans Island, Jeffreys Bank, and Stellwagen Bank) showed that mobile fishing gear altered the physical structure (=complexity) of benthic habitats. Complexity was reduced by direct removal of biogenic (e.g., sponges, hydrozoans, bryozoans, amphipod tubes, holothurians, shell aggregates) and‐ sedimentary (e.g., sand waves, depressions)...

Journal ArticleDOI
TL;DR: In this paper, a numerical model was proposed to predict aluminosilicate dissolution rates as a function of changes in residual mineral abundance, grain size distributions, and mineral surface areas with time.

Journal ArticleDOI
27 Sep 1996-Science
TL;DR: In this paper, a slow, constant-volume heating of water ice plus methane gas mixtures forms methane clathrate hydrate by a progressive reaction that occurs at the nascent ice/liquid water interface.
Abstract: Slow, constant-volume heating of water ice plus methane gas mixtures forms methane clathrate hydrate by a progressive reaction that occurs at the nascent ice/liquid water interface. As this reaction proceeds, the rate of melting of metastable water ice may be suppressed to allow short-lived superheating of ice to at least 276 kelvin. Plastic flow properties measured on clathrate test specimens are significantly different from those of water ice; under nonhydrostatic stress, methane clathrate undergoes extensive strain hardening and a process of solid-state disproportionation or exsolution at conditions well within its conventional hydrostatic stability field.

Journal ArticleDOI
01 Jul 1996-Nature
TL;DR: This article found no evidence of steppe tundra on the Bering land bridge and showed that much of the land bridge was above sea level and thus available for human and animal migration until 11,000 yr BP.
Abstract: UNDERSTANDING the environment of the Bering land bridge and determining the timing of late Wisconsin inundation are important for several areas of study. These include: (1) the timing of the re-establishment of circulation between Pacific and Atlantic Oceans; (2) the timing of development of a northern biotic refugium and the closing of the bridge to species immigration; (3) Palaeoindian migration routes; and (4) palaeotopographic data for atmospheric general circulation models1. Late Wisconsin palaeobotanical and fossil insect data from the central and northern sectors of the Bering land bridge indicate widespread mesic shrub tundra environments even during the last glacial maximum. Contrary to previous hypotheses, we found no evidence of steppe tundra on the land bridge. New accelerator mass spectrometer 14C dates show much of the land bridge was above sea level and thus available for human and animal migration until 11,000 yr BP. Insect evidence suggests that summer temperatures at that time were substantially warmer than now.

Journal ArticleDOI
05 Sep 1996-Nature
TL;DR: In this article, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes.
Abstract: EARTHQUAKES typically release stored strain energy on timescales of the order of seconds, limited by the velocity of sound in rock. Over the past 20 years, observations1–13 and laboratory experiments14 have indicated that rupture can also occur more slowly, with durations up to hours. Such events may be important in earthquake nucleation15 and in accounting for the excess of plate convergence over seismic slip in subduction zones. The detection of events with larger timescales requires near-field deformation measurements. In December 1992, two borehole strainmeters close to the San Andreas fault in California recorded a slow strain event of about a week in duration, and we show here that the strain changes were produced by a slow earthquake sequence (equivalent magnitude 4.8) with complexity similar to that of regular earthquakes. The largest earthquakes associated with these slow events were small (local magnitude 3.7) and contributed negligible strain release. The importance of slow earthquakes in the seismogenic process remains an open question, but these observations extend the observed timescale for slow events by two orders of magnitude.

Journal ArticleDOI
TL;DR: The isolation of five new strains which could obtain energy for growth by dissimilatory Fe(III) reduction from freshwater aquatic sediments, a pristine deep aquifer, and a petroleum-contaminated shallow aquifer demonstrates that Geobacter species are widely distributed in a diversity of sedimentary environments in which Fe( III) reduction is an important process.
Abstract: In an attempt to better understand the microorganisms responsible for Fe(III) reduction in sedimentary environments, Fe(III)-reducing microorganisms were enriched for and isolated from freshwater aquatic sediments, a pristine deep aquifer, and a petroleum-contaminated shallow aquifer. Enrichments were initiated with acetate or toluene as the electron donor and Fe(III) as the electron acceptor. Isolations were made with acetate or benzoate. Five new strains which could obtain energy for growth by dissimilatory Fe(III) reduction were isolated. All five isolates are gram-negative strict anaerobes which grow with acetate as the electron donor and Fe(III) as the electron acceptor. Analysis of the 16S rRNA sequence of the isolated organisms demonstrated that they all belonged to the genus Geobacter in the delta subdivision of the Proteobacteria. Unlike the type strain, Geobacter metallireducens, three of the five isolates could use H2 as an electron donor for Fe(III) reduction. The deep subsurface isolate is the first Fe(III) reducer shown to completely oxidize lactate to carbon dioxide, while one of the freshwater sediment isolates is only the second Fe(III) reducer known that can oxidize toluene. The isolation of these organisms demonstrates that Geobacter species are widely distributed in a diversity of sedimentary environments in which Fe(III) reduction is an important process.

Journal ArticleDOI
TL;DR: In this paper, the authors identified a 3.2-Hz resonance that is a transverse oscillation of the hill (a directional topographic effect), which was seen most strongly within 50 m of Station C00.
Abstract: The Northridge earthquake caused 1.78 g acceleration in the east-west direction at a site in Tarzana, California, located about 6 km south of the mainshock epicenter. The accelerograph was located atop a hill about 15-m high, 500-m long, and 130-m wide, striking about N78°E. During the aftershock sequence, a temporary array of 21 three-component geophones was deployed in six radial lines centered on the accelerograph, with an average sensor spacing of 35 m. Station C00 was located about 2 m from the accelerograph. We inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion. We identified a 3.2-Hz resonance that is a transverse oscillation of the hill (a directional topographic effect). The top/base amplification ratio at 3.2 Hz is about 4.5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill. This resonance is seen most strongly within 50 m of C00. Other resonant frequencies were also observed. A strong lateral variation in attenuation, probably associated with a fault, caused substantially lower motion at frequencies above 6 Hz at the east end of the hill. There may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill, causing particle motions (not spectral ratios) at the top of the hill to be rotated about 20° away from the direction transverse to the hill. The resonant frequency, but not the amplitude, of our observed topographic resonance agrees well with theory, even for such a low hill. Comparisons of our observations with theoretical results indicate that the 3D shape of the hill and its internal structure are important factors affecting its response. The strong transverse resonance of the hill does not account for the large east-west mainshock motions. Assuming linear soil response, mainshock east-west motions at the Tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill. Probable variations in surficial shear-wave velocity do not account for the observed differences among mainshock acceleration observed at Tarzana and at two different sites within 2 km of Tarzana.

Journal ArticleDOI
TL;DR: In this paper, a procedure was developed to obtain microscope images of regions of contact between roughened surfaces of transparent materials, while the surfaces are subjected to static loads or undergoing frictional slip.

Journal ArticleDOI
TL;DR: In the case of the Exxon Valdez oil spill, a variety of molecular and isotopic techniques were employed to identify various hydrocarbon sources and to distinguish quantitatively among mixed sources in the samples.

Journal ArticleDOI
TL;DR: In this paper, an analysis of Pliocene marine microfossils from 64 globally distributed stratigraphic sequences has been used to produce a middle PlIocene sea surface temperature reconstruction of Earth.

Journal ArticleDOI
30 Aug 1996-Science
TL;DR: In this article, Synthetic aperture radar interferometry revealed several centimeters of postseismic uplift in pull-apart structures and subsidence in a compressive jog along the Landers, California, 1992 earthquake surface rupture, with a relaxation time of 270 ± 45 days.
Abstract: Near-field strain induced by large crustal earthquakes results in changes in pore fluid pressure that dissipate with time and produce surface deformation. Synthetic aperture radar (SAR) interferometry revealed several centimeters of postseismic uplift in pull-apart structures and subsidence in a compressive jog along the Landers, California, 1992 earthquake surface rupture, with a relaxation time of 270 ± 45 days. Such a postseismic rebound may be explained by the transition of the Poisson's ratio of the deformed volumes of rock from undrained to drained conditions as pore fluid flow allows pore pressure to return to hydrostatic equilibrium.

Journal ArticleDOI
TL;DR: Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1-7 at 25°C.

Journal ArticleDOI
TL;DR: In this article, pyroclast structures are used to constrain degassing in basaltic lava fountains and the influence of vesiculation rate on magma fragmentation.

Journal ArticleDOI
TL;DR: In this paper, the authors present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr.
Abstract: Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.

Journal ArticleDOI
TL;DR: In this paper, it was shown that large-scale compound pahoehoe lava flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger (10-20 km3) phoehoe flows in Iceland.
Abstract: Extensive flows of the Columbia River Basalt (CRB) Group in Washington, Oregon, and Idaho are dominantly inflated compound pahoehoe sheet lavas. Early studies recognized that CRB lavas are compound pahoehoe flows, with textures suggesting low flow velocities, but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks. However, small volume (< 1 km³) compound pahoehoe flows on Kilauea, Hawai'i, demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger (10–20 km3) pahoehoe flows in Iceland. The vertical distribution of vesicles and other morphologic features within CRB lava flows indicate that they grew similarly by inflation. Small pahoehoe lobes at the base and top of many CRB pahoehoe lava flows indicate emplacement in a gradual, piecemeal manner rather than as a single flood. We propose that each thick CRB sheet flow was active for months to years and that each group of flows produced by a single eruption (a flow field) was emplaced slowly over many years.