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At least 181 records · Page 10Linked to original sources
USGS2011

Imaging the San Andreas fault with explosion and earthquake sources

Mounting evidence suggests that fault zone heterogeneity may play a crucial role in the localization of rupture in earthquakes [ Aki , 1995]. The heterogeneity can take several forms: spatial variations in physical properties (elastic properties, pore fluid pressure, etc.) or complexity in the fault surface (bends, offsets, etc.). High-resolution, three-dimensional models of the P and S wave velocity ( V p and V s ) structure and accurate hypocenters provide tools for studying the effects of fault zone heterogeneity on rupture localization. A dense, passive array of 48 seismic instruments was deployed south of Hollister, California, from mid-November 1994 to late May 1995. The array obtained local-earthquake seismograms for high-resolution, three-dimensional imaging of the ( V p and V s ) structure in the San Andreas fault zone. In mid-May, anactive seismic experiment was also carried out. Figure 1 shows a 7-km wide, 3- to 4-km deep zone of very low V p imaged during the experiment. The zone is bounded on the southwest by the San Andreas fault and the adjacent Northern Gabilan Range. The San Andreas fault appears to be approximately vertical to at least 5 km depth (the limit of good model resolution), contrary to an existing model based on gravity data.

California↗
USGS2011

Images of crust beneath southern California will aid study of earthquakes and their effects

The Whittier Narrows earthquake of 1987 and the Northridge earthquake of 1991 highlighted the earthquake hazards associated with buried faults in the Los Angeles region. A more thorough knowledge of the subsurface structure of southern California is needed to reveal these and other buried faults and to aid us in understanding how the earthquake-producing machinery works in this region.

California↗
USGS2011

Old ice in rock glaciers may provide long-term climate records

Anyone who spends much time above the treeline has probably seen rock glaciers and paused to wonder about them. Their curious and occasionally spectacular forms (Figure 1) occur in alpine and polar regions throughout the world, yet much remains uncertain about how they develop. A core of ice recently recovered from a rock glacier in the Absaroka Mountains of northwestern Wyoming vividly illustrates several important aspects about rock glaciers. At least some rock glaciers are a form of debris-covered glacier, and original isotopic stratigraphy may be preserved within their ice. Perhaps most interesting of all, the core of some rock glaciers is composed of layered ice that can be drilled and recovered, and some of this ice is exceptionally old.

Eos, Transactions, American Geophysical Union↗
USGS2011

Aeromagnetic survey over U.S. to advance geomagnetic research

A proposed high‐altitude survey of the United States offers an exciting and cost effective opportunity to collect magnetic‐anomaly data. Lockheed Martin Missile and Space Company is considering funding a reimbursable ER‐2 aircraft (Figure 1) mission to collect synthetic aperture radar (SAR) imagery at an altitude of about 21 km over the conterminous United States and Alaska. The collection of total and vector magnetic field data would be a secondary objective of the flight. Through this “piggyback approach,” the geomagnetic community would inherit invaluable magnetic data at a nominal cost. These data would provide insight on fundamental tectonic and thermal processes and give a new view of the structural and lithologic framework of the crust and upper mantle.

Eos, Transactions, American Geophysical Union↗
USGS2011

Seismograms offer insight into Oklahoma City bombing

The terrorist bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995, generated seismic waves that were recorded on two permanent seismographs about 7 and 26 km away from the bombing. The seismogram recorded at 26 km shows two low-frequency wave trains, discrete sets of oscillatory signals, that begin about 10 s apart. Public release of this record prompted speculation that each wave train was caused by a different energy source. On May 23, 1995, the U.S. Geological Survey monitored the demolition of the bomb-ravaged Federal Building with portable seismographs (Figure 1). Two wave trains were picked up again. The recordings indicate that the wave trains during both the bombing and demolition represent seismic waves traveling at different velocities. We conclude that the two wave trains recorded during the bombing are consistent with a single impulsive energy source.

Oklahoma↗
USGS2011

No: The L.A. array is not ready for prime time

Although much interest will focus upon the temporal behavior of observed deformation, the principal justification for the SCIGN array is that within a 5‐year interval it will provide an accurate and detailed determination of the velocity field in the Los Angeles basin that can be used to identify the active faults and estimate their secular slip rates. Obviously, the accuracy of the measurements will determine the success of the SCIGN array in reaching its objective. Over the past several years, Duncan Agnew, Hadley Johnson, and Frank Wyatt have developed arguments that quantify the accuracy likely to be obtained in the measurements and the accuracy that will be required to resolve slip rates on individual faults. In view of those arguments I do not believe that the SCIGN strategy can accomplish its objective. Instead, a mix of annual and continuous GPS surveys may be a more cost‐effective way to accomplish what actually can be achieved.

California↗
USGS2011

Imperfect science: Uncertainty, diversity, and experts

Seismic safety issues related to nuclear reactors in the eastern United States pose special challenges to the Earth and engineering sciences, given the severe consequences that can attend even very infrequent earthquakes. To deal with low-probability, potentially damaging ground motions, two major probabilistic seismic hazard analyses were conducted in the 1980s for nuclear reactors in the eastern United States, that part of the country east of the Rocky Mountains. The first study was performed by the Lawrence Livermore National Laboratory (LLNL) [ Bernreuter et al ., 1989] and was supported by the U.S. Nuclear Regulatory Commission (USNRC). The second was commissioned by the Seismicity Owners Group of the Electric Power Research Institute (EPRI) , [1989]. These studies generally agreed in terms of median hazard estimates, but mean hazard estimates at individual sites varied considerably, in several cases by 2 orders of magnitude or more.

Eos, Transactions, American Geophysical Union↗
USGS2011

Toward a comprehensive catalog of global historical seismicity

The United States Geological Survey (USGS) and the Colorado School of Mines (CSM) have initiated a project to locate more accurately all earthquakes recorded by instruments during the period 1900 to 1963. Seismicity for this period (hereafter referred to as historical seismicity, following Lee et al . [1988]) is still poorly understood, even for basic parameters such as earthquake locations (see Figure 1). In some cases this is the result of inherent limitations in the distribution, response characteristics, and timing of the instruments. However, locations for most of the pre-1964 earthquakes are poorly determined simply because modern data analysis techniques have yet to be applied to the available arrival-time observations, which are mainly preserved as printed bulletins and not in a computer-ready digital format. The arduous task of hand-entering these data has prevented the systematic analysis and relocation of historical seismicity.

Eos, Transactions, American Geophysical Union↗
USGS2006

Streams in Catskill Mountains still susceptible to acid rain

Precipitation in North America has become less acidic over the past 2 decades because of reduced power plant emissions and compliance with the Clean Air Act Amendments [ Sirois , 19937rsqb;. The 1990 Clean Air Act Amendments were developed to reduce the acidity of sensitive surface waters, which are primarily in upland forested environments, where acidified waters and associated high aluminum concentrations are toxic to many species of aquatic flora and fauna [ Schindler et al. , 1989]. Our studies show that in spite of less acidic precipitation, the buffering capacity of streams in upland forests of the Catskill Mountains in southeastern New York has not increased in recent years. These data suggest that long-term leaching by acid rain has lowered exchangeable calcium ion concentrations in the soil in upland areas, where the underlying, slow-weathering bedrock provides an inadequate supply of cations to neutralize acidity.

New York↗
USGS2006

Major improvements in progress for Southern California Earthquake Monitoring

Major improvements in seismic and strong-motion monitoring networks are being implemented in southern California to better meet the needs of emergency response personnel, structural engineers, and the research community in promoting earthquake hazard reduction. Known as the TriNet project, the improvements are being coordinated by the California Institute of Technology (Caltech), the U.S. Geological Survey (USGS), and the California Division of Mines and Geology (CDMG) of the state's Department of Conservation. Already the ambitious instrument and system development project has started to record and disseminate ground motions from a spatially dense and robust network of high quality seismographs.

Eos, Transactions, American Geophysical Union↗
USGS2006

Satellite radar interferometry measures deformation at Okmok Volcano

The center of the Okmok caldera in Alaska subsided 140 cm as a result of its February– April 1997 eruption, according to satellite data from ERS-1 and ERS-2 synthetic aperture radar (SAR) interferometry. The inferred deflationary source was located 2.7 km beneath the approximate center of the caldera using a point source deflation model. Researchers believe this source is a magma chamber about 5 km from the eruptive source vent. During the 3 years before the eruption, the center of the caldera uplifted by about 23 cm, which researchers believe was a pre-emptive inflation of the magma chamber. Scientists say such measurements demonstrate that radar interferometry is a promising spaceborne technique for monitoring remote volcanoes. Frequent, routine acquisition of images with SAR interferometry could make near realtime monitoring at such volcanoes the rule, aiding in eruption forecasting.

Eos, Transactions, American Geophysical Union↗
USGS2011

Seismic survey probes urban earthquake hazards in Pacific Northwest

A multidisciplinary seismic survey earlier this year in the Pacific Northwest is expected to reveal much new information about the earthquake threat to U.S. and Canadian urban areas there. A disastrous earthquake is a very real possibility in the region. The survey, known as the Seismic Hazards Investigation in Puget Sound (SHIPS), engendered close cooperation among geologists, biologists, environmental groups, and government agencies. It also succeeded in striking a fine balance between the need to prepare for a great earthquake and the requirement to protect a coveted marine environment while operating a large airgun array.

Washington↗
USGS2011

Debating complexity in modeling

Complexity in modeling would seem to be an issue of universal importance throughout the geosciences, perhaps throughout all science, if the debate last year among groundwater modelers is any indication. During the discussion the following questions and observations made up the heart of the debate. As scientists trying to understand the natural world, how should our effort be apportioned? We know that the natural world is characterized by complex and interrelated processes. Yet do we need to explicitly incorporate these intricacies to perform the tasks we are charged with? In this era of expanding computer power and development of sophisticated preprocessors and postprocessors, are bigger machines making better models? Put another way, do we understand the natural world better now with all these advancements in our simulation ability? Today the public's patience for long-term projects producing indeterminate results is wearing thin. This increases pressure on the investigator to use the appropriate technology efficiently. On the other hand, bringing scientific results into the legal arena opens up a new dimension to the issue: to the layperson, a tool that includes more of the complexity known to exist in the real world is expected to provide the more scientifically valid answer.

Eos, Transactions, American Geophysical Union↗
USGS2011

Airborne laser study quantifies El Niño-induced coastal change

Winter storms during the 1997–1998 El Niño caused extensive changes to the beaches and cliffs of the west coast of the United States, a NASA-NOAA-USGS investigation using a scanning airborne laser has found. For example, near Pacifica in central California, the cliff eroded locally as much as 10–13 m landward during the El Niño winter, at least 40 times the long term average erosion rate. However, only several hundred meters away the cliff was stable. This variability in cliff response may be related to differences in local beach changes where an accreting beach protected part of the cliff and an eroding beach exposed another part to attack by waves.

Pacific Coast↗