Showing papers by "United States Geological Survey published in 1995"

Potential theory in gravity and magnetic applications

Abstract: Introduction 1. The potential 2. Consequences of the potential 3. Newtonian potential 4. Magnetic potential 5. Magnetization 6. Spherical harmonic analysis 7. Regional gravity fields 8. The geomagnetic field 9. Forward method 10. Inverse method 11. Fourier-domain modeling 12. Transformations A. Review of vector calculus B. Subroutines C. Review of sampling theory D. Conversion of units.

Near‐source ground motion and its effects on flexible buildings

Abstract: Occurrence of large earthquakes close to cities in California is inevitable. The resulting ground shaking will subject buildings in the near-source region to large, rapid displacement pulses which are not represented in design codes. The simulated Mw7.0 earthquake on a blind-thrust fault used in this study produces peak ground displacement and velocity of 200 cm and 180 cm/sec, respectively. Over an area of several hundred square kilometers in the near-source region, flexible frame and base-isolated buildings would experience severe nonlinear behavior including the possibility of collapse at some locations. The susceptibility of welded connections to fracture significantly increases the collapse potential of steel-frame buildings under strong ground motions of the type resulting from the Mw7.0 simulation. Because collapse of a building depends on many factors which are poorly understood, the results presented here regarding collapse should be interpreted carefully.

Mapping Seismic Hazard in the Central and Eastern United States

Abstract: INTRODUCTION The U.S. Geological Survey (USGS) has been publishing probabilistic seismic hazard maps for the United States since 1976 (e.g., Algermissen and Perkins, 1976; Algermissen et al., 1990). We are preparing new national maps for the 1997 edition of the NEHRP Recommended Provisions for the Development of Seismic Regulations for New Buildings , published by the Building Seismic Safety Council (NEHRP stands for National Earthquake Hazards Reduction Program). The USGS hazard maps are to be the basis for design value maps for buildings to be included in the Provisions . We are conducting a series of regional workshops to discuss the methodology and input to the maps. As of this writing, workshops have been held in the Pacific Northwest, the northeastern U.S., and northern California. We intend to make maps showing ground motions with 10% probability of exceedance (PE) in 50, 100, and 250 years. These correspond to return times of 475,...

Large Arctic Temperature Change at the Wisconsin-Holocene Glacial Transition

Abstract: Analysis of borehole temperature and Greenland Ice Sheet Project II ice-core isotopic composition reveals that the warming from average glacial conditions to the Holocene in central Greenland was large, approximately 15°C. This is at least three times the coincident temperature change in the tropics and mid-latitudes. The coldest periods of the last glacial were probably 21°C colder than at present over the Greenland ice sheet.

An empirical model of the phytoplankton chlorophyll : carbon ratio‐the conversion factor between productivity and growth rate

Abstract: We present an empirical model that describes the ratio of phytoplankton chlorophyll a to carbon, Chl : C, as a function of temperature, daily irradiance, and nutrient-limited growth rate. Our model is based on 219 published measurements of algal cultures exposed to light-limited or nutrient-limited growth conditions. We illustrate an approach for using this estimator of Chl : C to calculate phytoplankton population growth rate from measured primary productivity. This adaptive Chl : C model gives rise to interactive light-nutrient effects in which growth efficiency increases with nutrient availability under low-light conditions. One implication of this interaction is the enhancement of phytoplankton growth efficiency, in addition to enhancement of biomass yield, as a response to eutrophication.

Seismic Evidence for an Earthquake Nucleation Phase

Abstract: Near-source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event. This phase was observed for the 30 earthquakes having moment magnitudes 2.6 to 8.1, and the size and duration of this phase scale with the eventual size of the earthquake. During the nucleation phase, moment release was irregular and appears to have been confined to a limited region of the fault. It was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone. These observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake.

Large-Scale Atmospheric Forcing of Recent Trends toward Early Snowmelt Runoff in California

Abstract: Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April–June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude...

Bioremediation of organic and metal contaminants with dissimilatory metal reduction

Abstract: Dissimilatory metal reduction has the potential to be a helpful mechanism for both intrinsic and engineered bioremediation of contaminated environments. Dissimilatory Fe(III) reduction is an important intrinsic process for removing organic contaminants from aquifers contaminated with petroleum or landfill leachate. Stimulation of microbial Fe(III) reduction can enhance the degradation of organic contaminants in ground water. Dissimilatory reduction of uranium, selenium, chromium, technetium, and possibly other metals, can convert soluble metal species to insoluble forms that can readily be removed from contaminated waters or waste streams. Reduction of mercury can volatilize mercury from waters and soils. Despite its potential, there has as yet been limited applied research into the use of dissimilatory metal reduction as a bioremediation tool.

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

Abstract: The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo. 11 refs., 4 figs., 1 tab.

Chemical and Isotopic Methods for Quantifying Ground‐Water Recharge in a Regional, Semiarid Environment

Abstract: The High Plains aquifer underlying the semiarid Southern High Plains of Texas and New Mexico, USA was used to illustrate solute and isotopic methods for evaluating recharge fluxes, runoff, and spatial and temporal distribution of recharge. The chloride mass-balance method can provide, under certain conditions, a time-integrated technique for evaluation of recharge flux to regional aquifers that is independent of physical parameters. Applying this method to the High Plains aquifer of the Southern High Plains suggests that recharge flux is approximately 2% of precipitation, or approximately 11 ± 2 mm/y, consistent with previous estimates based on a variety of physically based measurements. The method is useful because long-term average precipitation and chloride concentrations in rain and ground water have less uncertainty and are generally less expensive to acquire than physically based parameters commonly used in analyzing recharge. Spatial and temporal distribution of recharge was evaluated by use of δ2H, δ18O, and tritium concentrations in both ground water and the unsaturated zone. Analyses suggest that nearly half of the recharge to the Southern High Plains occurs as piston flow through playa basin floors that occupy approximately 6% of the area, and that macropore recharge may be important in the remaining recharge. Tritium and chloride concentrations in the unsaturated zone were used in a new equation developed to quantify runoff. Using this equation and data from a representative basin, runoff was found to be 24 ± 3 mm/y; that is in close agreement with values obtained from water-balance measurements on experimental watersheds in the area. Such geochemical estimates are possible because tritium is used to calculate a recharge flux that is independent of precipitation and runoff, whereas recharge flux based on chloride concentration in the unsaturated zone is dependent upon the amount of runoff. The difference between these two estimates yields the amount of runoff to the basin.

Response of high-rise and base-isolated buildings to a hypothetical mw 7.0 blind thrust earthquake.

Abstract: High-rise flexible-frame buildings are commonly considered to be resistant to shaking from the largest earthquakes. In addition, base isolation has become increasingly popular for critical buildings that should still function after an earthquake. How will these two types of buildings perform if a large earthquake occurs beneath a metropolitan area? To answer this question, we simulated the near-source ground motions of a M(w) 7.0 thrust earthquake and then mathematically modeled the response of a 20-story steel-frame building and a 3-story base-isolated building. The synthesized ground motions were characterized by large displacement pulses (up to 2 meters) and large ground velocities. These ground motions caused large deformation and possible collapse of the frame building, and they required exceptional measures in the design of the base-isolated building if it was to remain functional.

Forest-killing diffuse CO2 emission at Mammoth Mountain as a sign of magmatic unrest

Abstract: Mammoth Mountain, in the western United States, is a large dacitic volcano with a long history of volcanism that began 200 kyr ago 1 and produced phreatic eruptions as recently as 500± 200 yr BP (ref. 2). Seismicity, ground deformation and changes in fumarole gas composition suggested an episode of shallow dyke intrusion in 1989-90 (refs 3, 4). Areas of dying forest and incidents of near asphyxia in confined spaces, first reported in 1990, prompted us to search for diffuse flank emissions of magmatic CO 2 , as have been described at Mount Etna 5 and Vulcano 6 . Here we report the results of a soil-gas survey, begun in 1994, that revealed CO 2 concentrations of 30-96% in a 30-hectare region of killed trees, from which we estimate a total CO 2 flux of ≥1,200 tonnes per day. The forest die-off is the most conspicuous surface manifestation of magmatic processes at Mammoth Mountain, which hosts only weak fumarolic vents and no summit activity. Although the onset of tree kill coincided with the episode of shallow dyke intrusion, the magnitude and duration of the CO 2 flux indicates that a larger, deeper magma source and/or a large reservoir of high-pressure gas is being tapped.

Permeability enhancement in the shallow crust as a cause of earthquake-induced hydrological changes

Abstract: CHANGES in hydrology, usually involving increases in stream and spring flow, occur in response to large earthquakes. These changes have been attributed to two very different mechanisms: the expulsion of water from the upper or middle crust due to elastic compression1, or near-surface permeability enhancements2–4. If the former mechanism is correct, then sampling streams and springs affected by earthquakes may provide information about the nature of fluids at depth. Alternatively, if the changes in hydrology reflect only shallow processes, then the behaviour of these fluids provides insight into the rheological response of the shallow crust to earthquakes. Studies following the 17 October 1989 Loma Prieta earthquake in California provided a wealth of information regarding changes in stream and spring flow, groundwater flow and stream chemistry in the region around the earthquake epicentre1-4. Here we show that both the initial hydrological response and the hydrology of the region several years after the earthquake are more readily explained by earthquake-induced enhancements of permeability in the shallow crust that are persistent and widespread.

Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

Abstract: Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for ...

Modes of occurrence of environmentally· sensitive trace elements in coal

Abstract: Coal utilisation, especially coal combustion, causes the release of many inorganic elements into the environment. Various pollution control procedures and devices are used by the coal industry to minimise the release of these elements. Selective mining and coal cleaning procedures reduce the amount of the inorganic constituents in coal prior to combustion. Electrostatic precipitators, chemical additives, baghouses, and flue-gas scrubbers reduce particulate and element emissions after combustion.

Seasonal Land-Cover Regions of the United States

Abstract: An interesting controversy has recently arisen regarding the relative effectiveness of map-based versus route-based environmental learning. Although spatial theory suggests that map-based (survey) knowledge should be more effective, recent research shows some contrary results. In this paper we describe an experiment in which subjects learn either from a map or from a computer-simulated walk through an environment. Tests of route reproduction, cue location, orientation, and directional knowledge are undertaken. We are unable to show convincing statistical evidence of the superiority of map learning, although descriptive measures of performance on our tasks all pointed to map superiority. Some evidence is obtained that geographic training helps performance tasks. No differences based on sex could be statistically determined, although some evidence pointed to higher performance overall by geographically trained females. A final discussion focuses on the role that the configuration of features in the...

Molecular and isotopic tracers used to examine sources of organic matter and its incorporation into the food webs of San Francisco Bay

Abstract: Multiple indicators (Chl a, C : N ratios, [6L3C]POC, and two classes of lipid biomarker compounds- sterols and phospholipid ester-linked fatty acids) were used to evaluate spatial and temporal variations in the origin of particulate organic matter (POM) in the San Francisco Bay (SFB) estuary. Comparisons were made between the northern and southern subestuaries of SFB, as well as along the salinity gradient of northern SFB. Two sample types were collected-seston, which was used to characterize the bulk POM, and tissues of the suspension-feeding bivalve Potamocorbula amurensis -in order to evaluate the assimilable portion of the POM. Samples were collected around biological and physical events (phytoplankton blooms and freshwater inflow) thought to be the primary mechanisms controlling temporal variability in organic matter sources. Seston samples indicate that phytoplankton sources of POM are important throughout the entire SFB system, with additional inputs of organic matter from bacterial and terrestrial vascular plant sources delivered to the northern region. Analysis of biomarker compounds in- P. amurensis tissues indicates that phytoplankton supply a large fraction of the assimilable carbon to clams throughout SFB, although isotopic analysis of clam tissues suggests that the origin of this reactive carbon varies spatially and that freshwater algae are an important source of reactive organic matter to clams living in northern SFB.

A new mechanism for calcium loss in forest-floor soils

Abstract: CALCIUM is the fifth most abundant element in trees, and is an essential component for wood formation and the maintenance of cell walls. Depletion of Ca from the rooting zone can result in acidification of soil1 and surface water2 and possibly growth decline and dieback of red spruce3,4. During the past six decades, concentrations of root-available Ca (exchangeable and acid-extractable forms) in forest-floor soils have decreased in the northeastern United States5,6. Both net forest growth and acid deposition have been put forth as mechanisms that can account for this Ca depletion5,6. Here, however, we present data collected in red spruce forests in the northeastern United States that are inconsistent with either of these mechanisms. We propose that aluminium, mobilized in the mineral soil by acid deposition, is transported into the forest floor in a reactive form that reduces storage of Ca, and thus its availability for root uptake. This results in potential stress to trees and, by increasing the demand for Ca, also decreases neutralization of drainage waters, thereby leading to acidification of lakes and streams.

Partition of nonpolar organic pollutants from water to soil and sediment organic matters.

Abstract: The partition coefficients (K oc ) of carbon tetrachloride and,1,2-dichlorobenzene between normal soil/sediment organic matter and water have been determined for a large set of soils, bed sediments, and suspended solids from the United States and the People's Republic of China. The K oc values for both solutes are quite invariant either for the soils or for the bed sediments; the values on bed sediments are about twice those on soils. The similarity of K oc values between normal soils and between normal bed sediments suggests that natural organic matters in soils (or sediments) of different geographic origins exhibit comparable polarities and possibly comparable compositions. The results also suggest that the process that converts eroded soils into bed sediments brings about a change in the organic matter property. The difference between soil and sediment K oc values provides a basis for identifying the source of suspended solids in river waters. The very high K oc values observed for some special soils and sediments are diagnostic of severe anthropogenic contamination