United States Geological Survey

About: United States Geological Survey is a government organization based out in Reston, Virginia, United States. It is known for research contribution in the topics: Population & Groundwater. The organization has 17899 authors who have published 51097 publications receiving 2479125 citations. The organization is also known as: USGS & US Geological Survey.

Mechanism of the Chilean Earthquakes of May 21 and 22, 1960

Abstract: The Chilean earthquake sequence of May 21–22, 1960, was accompanied by linear zones of tectonic warping, including both uplift and subsidence relative to sea level. The region involved is more than 200 km wide and about 1000 km long, and lies along the continental margin between latitude 37° and 48° S. Significant horizontal strains accompanied the vertical movements in parts of the subsided zone for which triangulation data are available. Displacements were initiated near the northern end of the deformed region during the opening earthquake of the sequence (M s ≅ 7.5) on May 21 at 10h 02m 50s GMT and were extended over the remainder of the region during the culminating shock (M s ≅ 8.5) on May 22 at 19h llm 17s GMT. During the latter event, sudden uplift of adjacent portions of the continental shelf and much or all of the continental slope apparently generated the destructive tsunami that immediately followed the main shock. Available data suggest that the primary fault or zone of faulting along which displacement occurred probably is a complex thrust fault roughly 1000 km long and at least 60 km wide; it dips eastward at a moderate angle beneath the continental margin and intersects the surface on the continental slope. Dip slip required to satisfy the surface displacements is at least 20 m and perhaps as large as 40 m. There is some evidence that there was a minor component of right-lateral slip on the fault plane.

Diviner Lunar Radiometer Observations of Cold Traps in the Moon’s South Polar Region

Abstract: Diviner Lunar Radiometer Experiment surface-temperature maps reveal the existence of widespread surface and near-surface cryogenic regions that extend beyond the boundaries of persistent shadow. The Lunar Crater Observation and Sensing Satellite (LCROSS) struck one of the coldest of these regions, where subsurface temperatures are estimated to be 38 kelvin. Large areas of the lunar polar regions are currently cold enough to cold-trap water ice as well as a range of both more volatile and less volatile species. The diverse mixture of water and high-volatility compounds detected in the LCROSS ejecta plume is strong evidence for the impact delivery and cold-trapping of volatiles derived from primitive outer solar system bodies.

Crystallization, fractionation and solidification of the Tuolumne intrusive series, Yosemite National Park, California

Abstract: Study of the Tuolumne Intrusive Series, a concentric texturally and compositionally zoned plutonic sequence in the eastern part of Yosemite National Park, was undertaken to develop and test a model for the origin of comagmatic plutonic sequences in the Sierra Nevada batholith. The granitoid units that make up the sequence are progressively younger and more felsic inward. The bulk of the rocks are granodiorite, but the outermost formation is quartz diorite, and the innermost one is granite porphyry. The compositional gradient changes both gradually within formations and abruptly between them. The change is greatest in the outer 1 km and lower toward the center of the sequence. Hornblende and biotite, abundant in the marginal rocks, decrease rapidly inward for 1 km as K-feldspar and quartz increase, but farther inward, they decrease slowly. The most conspicuous chemical changes are shown by the elements that are enriched in the mafic minerals. The compositional zoning indicates that with decreasing temperature, the sequence solidified from the margins inward. Solidification was interrupted repeatedly by surges of fluid core magma. The magma eroded the adjacent solidifying rock, and it expanded the area of the magma chamber at the exposed level by crowding the wall and roof rocks outward and upward and by breaking through the solidifying carapace into the wall rocks. The compositional zonation resulted from crystal fractionation that could have involved (1) preferential accretion of crystalline material present in the magma to the margins of the magma chamber, thus displacing the melt phase progressively inward, and/or (2) downward settling of crystals, probably accompanied by upward movement of melt and volatiles; the residual magma solidifying to form the granitoids. Although either mechanism can explain the observed relations, they lead to very different interpretations of the composition of the magma when the first exposed granitoids solidified at the margins of the magma chamber and as the sequence solidified inward.

In Defense of Indices: The Case of Bird Surveys

Abstract: Indices to population size have come under increasing criticism in recent years, on the grounds that indices might not faithfully represent the entire population. Most criticisms involve surveys of birds, particularly those based on point counts, which is my focus here. A variety of quantitative methods have been developed to reduce the bias of point counts, such as distance sampling, multiple-observer surveys, and time-of-detection methods. I argue that these developments are valuable, in that they enhance understanding of the detection process, but that their practical application may well be limited, likely to intensive studies focusing on a small number of species. These quantitative methods are not generally applicable to extensive, multiple-species surveys. Although criticism of the thoughtless use of indices is welcome, their wholesale rejection is not.

Talc-bearing serpentinite and the creeping section of the San Andreas fault

Abstract: The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.