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  • 1
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    In:  Science, Warszawa, AGU, vol. 300, no. 5624, pp. 1424-1427, pp. 2091, (ISSN: 1340-4202)
    Publication Date: 2003
    Keywords: Deep seismic sounding (espec. cont. crust) ; Velocity depth profile ; China ; ConvolutionE ; CRUST ; earth mantle ; Tomography ; Tectonics
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  • 2
    Publication Date: 2015-05-19
    Description: Azimuthal anisotropy derived from multi-mode Rayleigh wave tomography in China exhibits depth-dependent variations in Tibet, which can be explained as induced by the Cenozoic India-Eurasian collision. In west Tibet, the E-W fast polarization direction at depths 〈100 km is consistent with the accumulated shear strain in the Tibetan lithosphere, whereas the N-S fast direction at greater depths is aligned with Indian plate motion. In northeast Tibet, depth-consistent NW-SE directions imply coupled deformation throughout the whole lithosphere, possibly also involving the underlying asthenosphere. Significant anisotropy at depths of 225 km in southeast Tibet reflects sublithospheric deformation induced by northward and eastward lithospheric subduction beneath the Himalaya and Burma, respectively. The multi-layer anisotropic surface wave model can explain some features of SKS splitting measurements in Tibet, with differences probably attributable to the limited backazimuthal coverage of most SKS studies in Tibet and the limited horizontal resolution of the surface wave results.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
    Publication Date: 2013-01-16
    Description: [1]  We present a detailed 3-D P -wave velocity model obtained by first-arrival travel-time tomography with seismic refraction data in the segment boundary of the Sumatra subduction zone across Simeulue Island, and an image of the top of the subducted oceanic crust extracted from depth-migrated multi-channel seismic reflection profiles. We have picked P -wave first arrivals of the air-gun source seismic data recorded by local networks of ocean-bottom seismometers, and inverted the travel-times for a 3-D velocity model of the subduction zone. This velocity model shows an anomalous zone of intermediate velocities between those of oceanic crust and mantle that is associated with raised topography on the top of the oceanic crust. We interpret this feature as a thickened crustal zone in the subducting plate with compositional and topographic variations, providing a primary control on the upper plate structure and on the segmentation of the 2004 and 2005 earthquake ruptures.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
    Publication Date: 2012-11-08
    Description: Using fundamental mode Rayleigh waves from the INDEPTH-IV and Namche-Barwa seismic experiments for periods between 20 and 143 s, we have investigated the lithospheric structure beneath eastern Tibet. We have found a ∼200-km-wide high velocity body, starting at ∼60 km depth and roughly centered beneath the Bangong-Nijuang Suture, which is most likely a piece of the underthrusting Indian continental lithosphere. The sub-horizontal underthrusting of the Indian lithosphere beneath eastern Tibet appears to be accompanied by its lateral tearing into at least two fragments, and subsequent break-off of the westernmost portion at ∼91°E-33°N. The uppermost mantle low velocity zone we observe beneath the N. Qiangtang and Songpan-Ganzi terranes is most probably due to warmer and thinner lithosphere relative to southern Tibet. We attribute the low velocity zones concentrated along the northern and southern branches of the eastern Kunlun fault at lithospheric depths to strain heating caused by shearing. The azimuthal fast directions at all periods up to 143 s (∼200 km peak sensitivity depth) beneath the N. Qiangtang and Songpan-Ganzi terranes are consistent, suggesting vertically coherent deformation between crust and uppermost mantle. Furthermore, the low velocity zone below the Kunlun Shan reaching down to 〉200 km argues against a present southward continental subduction along the southern margin of Qaidam Basin.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
    Publication Date: 2012-04-13
    Description: SUMMARY We present a series of 1-D shear velocity models for the Sumatran Forearc and Arc derived from Rayleigh wave group dispersion in noise correlation functions from vertical and pressure records from an onshore–offshore seismic deployment. The 1-D models represent the crustal structure of the downgoing Indian Plate, the accretionary prism and the arc. There is a progression in shear velocity across the forearc to the arc associated with thickening of the accretionary prism and the development of an arc crust. The velocity structure inferred for the upper 20 km based on path averages between stations on the accretionary prism has velocities consistent with a thick sediment package in agreement with estimates of depth to the plate boundary determined from active source experiments. We also find low Indian Plate shear velocities, 〈4 km s −1 to 25 km depth beneath our station locations on the downgoing plate. These low seismic velocities are consistent with at least 14–24 per cent serpentinization of the oceanic crust and upper mantle of the downgoing plate. This high degree of serpentinization, may weaken the plate interface and explain the segmentation observed in the great Sumatran thrust earthquakes if the serpentinization is localized. The success of this study suggests that future onshore–offshore seismic deployments will be able to utilize this method.
    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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  • 6
    Publication Date: 2012-04-15
    Description: SUMMARY We present a series of 1-D shear velocity models for the Sumatran Forearc and Arc derived from Rayleigh wave group dispersion in noise correlation functions from vertical and pressure records from an onshore–offshore seismic deployment. The 1-D models represent the crustal structure of the downgoing Indian Plate, the accretionary prism and the arc. There is a progression in shear velocity across the forearc to the arc associated with thickening of the accretionary prism and the development of an arc crust. The velocity structure inferred for the upper 20 km based on path averages between stations on the accretionary prism has velocities consistent with a thick sediment package in agreement with estimates of depth to the plate boundary determined from active source experiments. We also find low Indian Plate shear velocities, 〈4 km s −1 to 25 km depth beneath our station locations on the downgoing plate. These low seismic velocities are consistent with at least 14–24 per cent serpentinization of the oceanic crust and upper mantle of the downgoing plate. This high degree of serpentinization, may weaken the plate interface and explain the segmentation observed in the great Sumatran thrust earthquakes if the serpentinization is localized. The success of this study suggests that future onshore–offshore seismic deployments will be able to utilize this method.
    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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  • 7
    Publication Date: 2011-12-01
    Description: Tibetan plate overriding the Asian plate in central and northern Tibet Nature Geoscience 4, 870 (2011). doi:10.1038/ngeo1309 Authors: Wenjin Zhao, Prakash Kumar, James Mechie, Rainer Kind, Rolf Meissner, Zhenhan Wu, Danian Shi, Heping Su, Guangqi Xue, Marianne Karplus & Frederik Tilmann The southern boundary between India and the Tibetan Plateau represents a classical case of continental subduction, where the Indian continental lithosphere is subducted northwards beneath the Tibetan Plateau. At the northern boundary, southward subduction of Asian lithosphere beneath the Tibetan Plateau has also been proposed, but imaging has been hampered by inadequate data quality. Here we analyse the plate tectonic structure of the northern boundary between Tibet and Asia using the S receiver function technique. Our passive source seismic data build on, and extend further northwards, the existing geophysical data from the International Deep Profiling of Tibet and the Himalaya project. We detect, beneath central and northern Tibet, a relatively thin, but separate, Tibetan lithosphere overriding the flat, southward subducting Asian lithosphere. We suggest that this overriding Tibetan lithosphere helps to accommodate the convergence between India and Asia in central and northern Tibet. We conclude that the Tibetan–Himalayan system is composed of three major parts: the Indian, Asian and Tibetan lithospheres. In the south, the Indian lithosphere underthrusts Tibet. In central and northern Tibet a separate, thin Tibetan lithosphere exists, which is underthrust by the Asian lithosphere from the north.
    Print ISSN: 1752-0894
    Electronic ISSN: 1752-0908
    Topics: Geosciences
    Published by Springer Nature
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  • 8
    Publication Date: 2012-02-24
    Description: We use a backprojection method to determine dominant directions of ocean infragravity waves from 60–200 s period from cross correlations between 5 ocean bottom differential pressure gauges located off the coast of Sumatra. We observe infragravity waves arriving from all directions, but there is a dominant source direction that represents coherent propagation along the coast from the southeast or from the south. This study demonstrates the effectiveness of our projection technique, which may be applied to past and future seismic data to improve models of ocean infragravity wave generation and tsunami propagation.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
    Publication Date: 2019
    Description: Abstract Based on a large data set of local body wave travel times, we determined the first 3‐D model of azimuthal P wave anisotropic tomography of northern Chile in order to study the deformation in the subduction zone. Our results indicate different deformation patterns in the overriding lithosphere, mantle wedge, and the subducting Nazca slab as well as significant along‐arc variations. Radiating fast velocity directions (FVDs) around the rupture zone of the 2014 Iquique earthquake (Mw 8.2) are notable in the crust, which may reflect the specific crustal extension induced by a point‐like stress source in the form of a fully locked asperity at the plate interface. In most of the study region, FVDs in the mantle wedge are trench normal, which we interpret to have resulted from mantle wedge flow driven by the oceanic plate subduction. However, trench‐parallel FVDs are found beneath the northern segment, which may imply the occurrence of B‐type olivine fabrics in the cold forearc mantle. Strong along‐arc variations are also observed in the subducting slab, which may reflect intraslab heterogeneity and change in the slab geometry. In the northern segment, slab bending induces generally trench‐normal extension and leads to trench‐normal FVDs in the upper part of the slab. In contrast, in the southern segment, flat slab subduction and thus slab unbending dominate, producing trench‐normal compression and trench‐parallel FVDs instead. Along the subducting slab interface, FVDs change from trench‐parallel to trench‐normal in the downdip direction, which may indicate the first‐order transition from compression in the locking area to dominant shear in the decoupling area along the slab interface.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 10
    Publication Date: 2019
    Description: Abstract We use local and teleseismic earthquakes to analyze shear wave splitting within the Pamir‐Hindu Kush region, north of the western syntaxis of the India‐Asia collision zone. These two data sets allowed us to map the distribution of azimuthal anisotropy, to put constraints on the depth range where it is accumulated, and to deduce characteristics of ongoing deformation. From 1,073 SKS (core‐mantle refracted phases) measurements at 107 stations, we derived time delays of 0.7–2.25 s and dominantly ENE‐WSW oriented fast polarization directions. Fast polarization directions only deviate adjacent to the subducting slabs and major strike‐slip faults, aligning parallel to these structures. From 461 direct S measurements along a transect perpendicular to the Pamir seismic zone, we obtain fast directions parallel to those from SKS measurements but smaller delay times (average 0.4 s), which vary depending on depth. Time delays exhibit 0.1–0.3 s crustal contribution and increase to 0.8 s in a narrow domain coinciding with the inferred subcrustal contact of the two colliding plates. We find measurements from the same event‐station paths at different filter frequencies to be frequency‐independent, allowing a comparison with SKS results along the studied profile. The smaller average time delays of local events imply that the crust and uppermost mantle only make a minor contribution to the SKS splitting. Thus, the coherent fast direction pattern suggests a strain field dominated by the indentation of India and the escape of sublithospheric material north of the indenter. Crustal anisotropy is likely also controlled by this regional deformation pattern with locally highest strain rates closest to the continental subduction front.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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