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  • 1
    ISSN: 1420-9136
    Keywords: Key words: 3-D velocity structure, circular ray tracing, San Andreas Fault, seismicity.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —The three-dimensional P-wave velocity structure of the Bear Valley region of central California is determined by applying a circular ray-tracing technique to 1735 P-wave arrivals from 108 locally recorded earthquakes. Comparison of the results obtained from one-dimensional and laterally varying starting models shows that many of the features in the structure determined are fairly insensitive to the choice of the starting model. Velocities associated with the Gabilan granites southwest of the San Andreas Fault are slightly higher than those in the Franciscan formation to the northeast, and these two features are separated in the southern part of the region by a narrow fault zone with very low velocities. In the southeastern part of the region, where the Gabilan granites do not abut the San Andreas Fault, the low velocities of the fault zone cross over to the southwestern side of the fault. They also appear to extend to depths of at least 15km, thus locally reversing the contrast across the San Andreas Fault that prevails farther to the northwest. In the northwestern part of the region, the low velocities of the fault zone split and follow the surface traces of the San Andreas and Calaveras Faults, but do not appear to extend to depths much deeper than about 6km. There also appears to be a well-defined contrast in structure in the middle of the Santa Clara Valley, suggesting the existence of a fault in the basement of the valley that may be a southern extension of the Sargent Fault into this region. Relocated hypocenters beneath the San Andreas Fault cluster in a zone that dips about 80° southwest and intersects the surface trace of the fault in the southern part of region.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [s.l.] : Nature Publishing Group
    Nature 358 (1992), S. 144-147 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Azimuthal anisotropy is a useful indicator of deformation in the mantle because the principal constituent of the mantle, olivine, is highly anisotropic and its lattice-preferred orientation is sensitive to the direction of flow1"6. Many recent studies of azimuthal anisotropy are based on ...
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-246X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the microearthquakes recorded occurred to the east of the mountain belt: on the Huaytapallana fault in the Eastern Cordillera and beneath the western margin of the sub-Andes. Thus the sub-Andes appear to be the physiographic province subject to the most intense seismic deformation. Focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub-Andes appear to be as deep as 40-50 km. The fault-plane solutions of events in the sub-Andean margin show thrust faulting on steep planes oriented roughly north-south, similar to that observed in teleseismic earthquakes studied using body wave modelling. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a NE-SW trend that agrees with the fault scarp and the east-dipping nodal plane of two large earthquakes that occurred on this fault on 1969 July 24 and October 1. Microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with recent observations showing that beneath Peru the slab first dips at an angle of about 30° to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault-plane solutions of intermediate-depth microearthquakes have horizontal T axes oriented east-west suggesting slab pull is the dominant force in the downgoing slab.
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  • 4
    Publication Date: 2017-10-02
    Description: The vast majority of the microearthquakes recorded occurred to the east: on the Huaytapallana fault in the Eastern Cordillera or in the western margin of the sub-Andes. The sub-Andes appear to be the physiographic province subjected to the most intense seismic deformation. Focal depths for the crustal events here are as deep as 50 km, and the fault plane solutions, show thrust faulting on steep planes oriented roughly north-south. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a northeast-southwest trend that agrees with the fault scarp and the east dipping nodal plane of two large earthquakes that occurred on this fault in 1969. The recorded microearthquakes of intermediate depth show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with the suggestion that beneath Peru the slab first dips at an angle of 30 deg to a depth of 100 km and then flattens following a quasi-horizontal trajectory. Fault plane solutions of intermediate depth microearthquakes have horizontal T axes oriented east-west.
    Keywords: GEOPHYSICS
    Type: NAS 1.26:175445 , NASA-CR-175445
    Format: application/pdf
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  • 5
    Publication Date: 2011-08-19
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 90; 7771-779
    Format: text
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  • 6
    Publication Date: 2011-08-18
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 11; 38-41
    Format: text
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  • 7
    Publication Date: 2011-08-18
    Description: Because the contortion in the seismic zone in southern Peru is aligned approximately parallel to the direction of relative plate motion, rather than perpendicular to the coast of Peru, the position of the contortion need not migrate with respect to the overriding South American plate as the Nazca plate subducts beneath it, and the flow in the surrounding asthenosphere could be in a steady state. In addition, the position of the contortion defines the northern boundary of the volcanic arc in southern Peru. The inference that a wedge of asthenospheric material must overlie the downgoing slab for subduction-related volcanism to occur is thereby strengthened.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 89; 6139-615
    Format: text
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  • 8
    Publication Date: 2014-08-18
    Description: The Tien Shan is the largest active intracontinental orogenic belt on Earth. To better understand the processes causing mountains to form at great distances from a plate boundary, we analyse passive source seismic data collected on 40 broad-band stations of the MANAS project (2005–2007) and 12 stations of the permanent KRNET seismic network to determine variations in crustal thickness and shear wave speed across the range. We jointly invert P - and S -wave receiver functions with surface wave observations from both earthquakes and ambient noise to reduce the ambiguity inherent in the images obtained from the techniques applied individually. Inclusion of ambient noise data improves constraints on the upper crust by allowing dispersion measurements to be made at shorter periods. Joint inversion can also reduce the ambiguity in interpretation by revealing the extent to which various features in the receiver functions are amplified or eliminated by interference from multiples. The resulting wave speed model shows a variation in crustal thickness across the range. We find that crustal velocities extend to ~75 km beneath the Kokshaal Range, which we attribute to underthrusting of the Tarim Basin beneath the southern Tien Shan. This result supports the plate model of intracontinental convergence. Crustal thickness elsewhere beneath the range is about 50 km, including beneath the Naryn Valley in the central Tien Shan where previous studies reported a shallow Moho. This difference apparently is the result of wave speed variations in the upper crust that were not previously taken into account. Finally, a high velocity lid appears in the upper mantle of the Central and Northern part of the Tien Shan, which we interpret as a remnant of material that may have delaminated elsewhere under the range.
    Keywords: Geodynamics and Tectonics
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
    Publication Date: 2014-07-13
    Description: We carry out a joint inversion of surface wave dispersion curves and teleseismic shear wave arrival times across the Tibetan collision zone, from just south of the Himalaya to the Qaidam Basin at the northeastern margin of the plateau, and from the surface to 600 km depth. The surface wave data consist of Rayleigh-wave group dispersion curves, mainly in the period range from 10 to 70 s, with a maximum of 2877 source–receiver pairs. The body wave data consist of more than 8000 S -wave arrival times recorded from 356 telesesmic events. The tomographic images show a ‘wedge’ of fast seismic velocities beneath central Tibet that starts underneath the Himalaya and reaches as far as the Bangong–Nujiang Suture (BNS). In our preferred interpretation, in central Tibet the Indian lithosphere underthrusts the plateau to approximately the BNS, and then subducts steeply. Further east, Indian lithosphere appears to be subducting at an angle of ~45°. We see fast seismic velocities under much of the plateau, as far as the BNS in central Tibet, and as far as the Xiangshuihe–Xiaojiang Fault in the east. At 150 km depth, the fast region is broken by an area ~300 km wide that stretches from the northern edge of central Tibet southeastwards as far as the Himalaya. We suggest that this gap, which has been observed previously by other investigators, represents the northernmost edge of the Indian lithosphere, and is a consequence of the steepening of the subduction zone from central to eastern Tibet. This also implies that the fast velocities in the northeast have a different origin, and are likely to be caused by lithospheric thickening or small-scale subduction of Asian lithosphere. Slow velocities observed to the south of the Qaidam suggest that the basin is not subducting. Finally, we interpret fast velocities below 400 km as subducted material from an earlier stage of the collision that has stalled in the transition zone. Its position to the south of the present subduction is likely to be due to the relative motion of India to the northeast.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 10
    Publication Date: 1996-04-01
    Description: Seismograms of earthquakes and explosions recorded at local, regional, and teleseismic distances by a small-aperture, dense seismic array located on Pinyon Flat, in southern California, reveal large (+ or -15 degrees ) backazimuth anomalies. We investigate the causes and implications of these anomalies by first comparing the effectiveness of estimating backazimuth with an array using three different techniques: the broadband frequency-wavenumber (BBFK) technique, the polarization technique, and the beamforming technique. While each technique provided nearly the same direction as a most likely estimate, the beamforming estimate was associated with the smallest uncertainties. Backazimuth anomalies were then calculated for the entire data set by comparing the results from beamforming with backazimuths derived from earthquake locations reported by the Anza and Caltech seismic networks and the Preliminary Determination of Epicenters (PDE) Bulletin. These backazimuth anomalies have a simple sinelike dependence on azimuth, with the largest anomalies observed from the southeast and northwest directions. Such a trend may be explained as the effect of one or more interfaces dipping to the northeast beneath the array. A best-fit model of a single interface has a dip and strike of 20 degrees and 315 degrees , respectively, and a velocity contrast of 0.82 km/sec. Application of corrections computed from this simple model to ray directions significantly improves locations at all distances and directions, suggesting that this is an upper crustal feature. We confirm that knowledge of local structure can be very important for earthquake location by an array but also show that corrections computed from simple models may not only be adequate but superior to those determined by raytracing through smoothed laterally varying models.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences
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