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  • 1
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    Backarc spreading and mantle wedge flow beneath the Japan Sea: insight from Rayleigh-wave anisotropic tomography (2016)
    Oxford University Press
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    Publication Date: 2016-08-27
    Description: We present the first high-resolution Rayleigh-wave phase-velocity azimuthal anisotropy tomography of the Japan subduction zone at periods of 20–150 s, which is determined using a large number of high-quality amplitude and phase data of teleseismic fundamental-mode Rayleigh waves. The obtained 2-D anisotropic phase-velocity models are then inverted for a 3-D shear-wave velocity azimuthal anisotropy tomography down to a depth of ~300 km beneath Japan. The subducting Pacific slab is imaged as a dipping high-velocity zone with trench-parallel fast-velocity directions (FVDs) which may indicate the anisotropy arising from the normal faults produced at the outer-rise area near the Japan trench axis, overprinting the slab fossil fabric, whereas the mantle wedge generally exhibits lower velocities with trench-normal FVDs which reflect subduction-driven corner flow and anisotropy. Depth variations of azimuthal anisotropy are revealed in the big mantle wedge beneath the Japan Sea, which may reflect past deformations in the Eurasian lithosphere related to backarc spreading during 21 to 15 Ma and complex current convection in the asthenosphere induced by active subductions of both the Pacific and Philippine Sea plates.
    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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  • 2
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    Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics (2015)
    Oxford University Press
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    Publication Date: 2015-02-13
    Description: We determined the first high-resolution P - and S -wave attenuation ( Qp and Qs ) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t * data measured from P - and S -wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high- Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low- Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous–Cenozoic accretionary wedge. Obvious low- Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low- Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high- Q zone. Prominent low- Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low- Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.
    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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  • 3
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    Seismic heterogeneity and anisotropy of the southern Kuril arc: insight into megathrust earthquakes (2013)
    Oxford University Press
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    Publication Date: 2013-07-05
    Description: Detailed 3-D P - and S -wave velocity ( V p and V s ) structures and P -wave anisotropy of the crust and upper mantle under the southern Kuril arc from the Kuril–Japan trench to the eastern margin of Japan Sea are investigated using a large number of high-quality arrival-time data from local earthquakes. The suboceanic earthquakes used in the tomographic inversion are relocated precisely using sP depth phase data, which are collected from three-component seismograms recoded by the dense Japanese seismic network. Our results show that three prominent high-velocity (high- V ) zones separated by low-velocity (low- V ) anomalies exist in the megathrust zone under the forearc region. These high- V zones coincide with areas with large coseismic slips of great megathrust earthquakes as well as the areas with large slip deficit on the plate interface, whereas the low- V anomalies are generally consistent with the afterslip distribution of the 2003 Tokachi-oki megathrust earthquake ( M w 8.0). We think that these high- V zones probably represent asperities (strongly coupled areas) in the megathrust zone, and the low- V anomalies around the asperities may contain fluids, which play an important role in the nucleation of the megathrust earthquakes, in addition to the stress accumulation. Our results also show an obvious boundary between the Northeast Japan arc and the Kuril arc, especially at the Hidaka collision zone. This boundary extends southwards to the source areas of the 1952 and 2003 Tokachi-oki earthquakes ( M w 8.1 and 8.0), which may have contributed to forming the asperity for the two megathrust events.
    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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  • 4
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    Upper and lower plate controls on the great 2011 Tohoku-oki earthquake (2018)
    American Association for the Advancement of Science (AAAS)
    Details
    Publication Date: 2018-06-21
    Description: The great 2011 Tohoku-oki earthquake [moment magnitude ( M w ) 9.0)] is the best-documented megathrust earthquake in the world, but its causal mechanism is still in controversy because of the poor state of knowledge on the nature of the megathrust zone. We constrain the structure of the Tohoku forearc using seismic tomography, residual topography, and gravity data, which reveal a close relationship between structural heterogeneities in and around the megathrust zone and rupture processes of the 2011 Tohoku-oki earthquake. Its mainshock nucleated in an area with high seismic velocity, low seismic attenuation, and strong seismic coupling, probably indicating a large asperity (or a cluster of asperities) in the megathrust zone. Strong coseismic high-frequency radiations also occurred in high-velocity patches, whereas large afterslips took plate in low-velocity areas, differences that may reflect changes in fault friction and lithological variations. These structural heterogeneities in and around the Tohoku megathrust originate from both the overriding and subducting plates, which controlled the nucleation and rupture processes of the 2011 Tohoku-oki earthquake.
    Electronic ISSN: 2375-2548
    Topics: Natural Sciences in General
    Published by American Association for the Advancement of Science (AAAS)
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