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  • American Geophysical Union  (96)
  • Wiley  (46)
  • Springer Nature  (13)
  • American Institute of Physics  (2)
  • Seismological Society of America  (1)
  • 1
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    Mantle transition zone structure beneath the Changbai volcano: Insight into deep slab dehydration and hot upwelling near the 410-km discontinuity (2016)
    Wiley
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    Publication Date: 2016-07-15
    Description: We study the detailed mantle transition zone structure beneath the active Changbai intraplate volcano in Northeast China using a receiver-function method. A total of 3005 teleseismic receiver functions recorded by 70 broadband stations are obtained by using a common-conversion-point stacking method. For conducting the time-to-depth conversion, we use a three-dimension velocity model of the study region so as to take into account the influence of structural heterogeneities. Our results reveal significant depth variations of the 410, 520 and 660-km discontinuities. A broad depression of the 410 km discontinuity and a low-velocity anomaly are revealed beneath the Changbai volcano, which may reflect a large-scale hot mantle upwelling around the 410 km discontinuity with a positive Clapeyron slope. The 520 km discontinuity is identified clearly and its uplift occurs above the stagnant Pacific slab. We also find a prominent depression of the 660 km discontinuity, which is elongated along the trend of deep-earthquake clusters in a range of 39°N ~ 44°N latitude, and the depression area has a lateral extent of about 400 km. Because the 520 and 660 km discontinuities correspond to positive and negative Clapeyron slopes, respectively, we think that the 520 uplift and the 660 depression are caused by the cold subducting Pacific slab. A part of the Pacific slab may have penetrated into the lower mantle and so caused the large-scale 660 depression in front of the deep-earthquake clusters. Our results also reveal a part of the upper boundary of the subducting Pacific slab in the mantle transition zone.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 2
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    On the trade-off between seismic anisotropy and heterogeneity: Numerical simulations and application to Northeast Japan (2015)
    Wiley
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    Publication Date: 2015-04-09
    Description: P-wave tomography has been recently used to study 3-D azimuthal and radial anisotropy of subduction zones and continental regions. However, the fundamental issue about the trade-off between the isotropic and anisotropic structures is still unclear. In this study, we investigate this issue systematically with comprehensive synthetic tests. Our results indicate that good ray coverage in the azimuth (for azimuthal anisotropy) and incidence (for radial anisotropy) is required for determining reliable anisotropic models. The isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence. We therefore plot ray-azimuth and ray-incidence ellipses at every grid nodes and propose to use the normalized length of the short-axis (NLS; i.e., the ratio of the short-axis and long-axis lengths) for estimating the ray coverage quantitatively. Applying our novel approach to a large number of high-quality arrival-time data of local shallow and intermediate-depth earthquakes, we obtained new tomographic images of 3-D P-wave azimuthal and radial anisotropy in Northeast Japan. Both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region, whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab. Our results show dominant trench-normal and vertical-fast anisotropy in the mantle wedge while trench-parallel and horizontal-fast anisotropy in the subducting slab, which indicates different dynamics in different domains of the subduction zone.
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    Topics: Geosciences , Physics
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  • 3
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    Seismic tomography and anisotropy of the Helan-Liupan tectonic belt: Insight into lower crustal flow and seismotectonics (2016)
    Wiley
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    Publication Date: 2016-03-26
    Description: We determined detailed 3-D P- and S-wave velocity (Vp, Vs) and Poisson's ratio (σ) images as well as P-wave azimuthal anisotropy in the crust and uppermost mantle beneath the Helan-Liupan tectonic belt (HLTB) and adjacent regions. The data set used in this study consists of 38,880 P-wave and 35,117 S-wave arrival times from 5,028 local earthquakes recorded by 66 seismic stations in the study area during 1980 to 2014. Obvious low-Vp and low-σ anomalies are revealed in the lower crust beneath the Qilian Orogenic Belt and Western Qinling, which we interpret as a weakened zone mainly caused by water and capable of ductile flow on a geological timescale. Our P-wave anisotropy results suggest that the flow direction in the lower crust is nearly parallel to the direction of the geodetic crustal motion and that of the upper mantle flow beneath the study region. Most of the 26 large historical earthquakes (1125-1954) in the study region occurred in the boundary zones where Vp, Vs and σ change drastically over a short distance. Beneath the source areas of the large historical earthquakes, fluid-related low-velocity zones exist widely in the lower crust. The fluids result from dehydration of hydrous minerals in the deeper crust and uppermost mantle beneath the northeastern Tibetan Plateau. When the fluids migrate upward to the active faults, the fault-zone friction is reduced and so large crustal earthquakes can be triggered. Our present results shed new light on the seismogenesis and geodynamics of the northeastern Tibetan Plateau and adjacent areas.
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    Topics: Geosciences , Physics
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  • 4
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    Seismic Anisotropy Evidence for Ductile Deformation of the Forearc Lithospheric Mantle in Subduction Zones (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Seismic anisotropy provides important information on the structure and geodynamics of the Earth. The forearc mantle wedge in subduction zones mainly exhibits trench‐parallel azimuthal anisotropy globally, which is inconsistent with the model of olivine a axis aligning with the slab‐driven corner flow. Its formation mechanism is currently unclear. Here we present high‐resolution 3‐D P wave anisotropic tomography of the Tohoku subduction zone. We suggest that ductile deformation of the forearc lithospheric mantle of the overriding plate induces the trench‐parallel azimuthal anisotropy and positive radial anisotropy (i.e., horizontal velocity 〉 vertical velocity) in Tohoku. Our results provide the first seismic anisotropic evidence for the slab‐mantle decoupling at a common depth of ~70 km. On the basis of the high‐resolution seismic images, we propose a geodynamic model suggesting that the forearc mantle wedge anisotropy is produced via ductile deformation of dry olivine or hydrous antigorite lithospheric mantle, which accords well with the trench‐parallel shear wave splitting measurements dominant in subduction zones globally.
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
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  • 5
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    Structural Heterogeneity and Anisotropy in the Source Zone of the 2018 Eastern Iburi Earthquake in Hokkaido, Japan (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We investigate 3‐D seismic structures (Vp, Vs, and Poisson's ratio) and Vp azimuthal anisotropy in the source area of the 2018 Eastern Iburi earthquake (M 6.7) in Hokkaido, Japan. Its mainshock occurred at the edge of a high‐Vp (2–4%) seismogenic zone. Significant low‐Vs (−1% to −3%) and high Poisson's ratio (2–7%) anomalies are imaged in and below the source zone and extend to the upper surface of the subducting Pacific slab, most likely reflecting ascending fluids released by the slab dehydration. A high consistency between the fault plane and the low‐Vs and high Poisson's ratio anomalies indicates that the fluids may have entered the fault and affected the rupture nucleation. A high‐V (1–3%) anomaly is revealed in the fore‐arc mantle wedge and connects with the high‐V seismogenic zone, probably reflecting a lithospheric fragment and contributing to cool down the mantle wedge. Complex seismic anisotropy is revealed in the crust in and around the source area, which may reflect complicated stress regime and strong structural heterogeneities there.
    Print ISSN: 2169-9313
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  • 6
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    Mantle Dynamics of Western Pacific and East Asia: New Insights from P Wave Anisotropic Tomography (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Seismic anisotropy records past and present tectonic deformations and provides important constraints for understanding the structure and dynamics of the Earth's interior. In this work, we use tremendous amounts of high‐quality P wave arrival times from local and regional earthquakes to determine a high‐resolution tomographic model of 3‐D P wave azimuthal anisotropy down to 1,000‐km depth beneath East Asia. Our results show that trench‐parallel fast‐velocity directions (FVDs) are visible in the shallow portion of the subducting Pacific slab (〈80 km), whereas the deeper portion of the Pacific slab mainly exhibits trench‐normal FVDs, except for the stagnant slab in the mantle transition zone (MTZ) where obvious NE‐SW FVDs are revealed. The FVDs in the subslab mantle change from a subduction‐parallel trend at depths of 80–400 km to a subduction‐normal trend in the MTZ. Large‐scale low‐velocity anomalies are revealed beneath the Philippine Sea plate where the FVD is NE‐SW. The FVDs along the Izu‐Bonin arc and in a slab gap exhibit a striking anticlockwise toroidal trend. All these features may reflect complex 3‐D flows in the mantle wedge due to tearing and dehydration processes of the subducting Pacific slab. The subducting Pacific slab is split at ~300‐km depth under the Bonin arc and then penetrates into the lower mantle, whereas under East Asia the Pacific slab becomes stagnant in the MTZ and reaches the North‐South Gravity Lineament in China. The intraplate volcanoes in East Asia are caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 7
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    Tomography, Seismotectonics, and Mantle Dynamics of Central and Eastern United States (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract We study the 3‐D P wave velocity structure of the crust and mantle down to 1,000‐km depth beneath the central and eastern United States. A 3‐D velocity model is obtained by conducting a joint inversion of 236,670 arrival times of local earthquakes and 870,455 relative traveltime residuals of teleseismic events recorded by the EarthScope/USArray Transportable Array. Significant low‐velocity (low‐V) anomalies are revealed in the crust beneath the eastern arm of the Midcontinent Rift and the Triassic Basins along the East Coast, whereas a prominent high‐velocity (high‐V) anomaly is visible beneath the Llano Uplift in central Texas. The stable North American Craton exhibits high‐V anomalies at depths of 65–250 km. Low‐V anomalies exist along the eastern and southern margins of the craton, which may reflect relatively thin lithosphere there. A prominent low‐V anomaly is revealed at depths of 50–200 km beneath the New Madrid Seismic Zone, which is bounded by high‐V anomalies to its southeast and northwest. This feature reflects a weak lithosphere surrounded by relatively strong cratonic regions and stress concentration caused intraplate seismicity in the New Madrid region. Two high‐V bodies appear in the mantle transition zone (410‐ to 660‐km depths) beneath the Interior Low Plateaus, the central Great Plains, and the Central Lowland, which may reflect the subducted Farallon plate or delaminated lithosphere. At depths of 800–1,000 km, a high‐V anomaly is visible beneath the southeast United States, which may be the subducted Hess Rise conjugate.
    Print ISSN: 2169-9313
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  • 8
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    Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave travel times (2016)
    Wiley
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    Publication Date: 2016-07-01
    Description: We present 3-D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high-quality P - and S -wave arrival-time data of local earthquakes and teleseismic events. A tomographic method for P -wave velocity azimuthal anisotropy is modified and extended to invert S -wave travel times for 3-D S -wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3-D azimuthal anisotropy of the Japan subduction zone. Our results show that the subducting Pacific and Philippine Sea (PHS) slabs exhibit mainly trench-parallel fast-velocity directions (FVDs), which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals formed at the mid-ocean ridge as well as shape-preferred orientation such as normal faults produced at the outer-rise area near the trench axis. Trench-normal FVDs are generally revealed in the mantle wedge, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench-normal FVDs are also visible in the subslab mantle, which may reflect the subducting asthenosphere underlying the slabs. Our results also reveal toroidal mantle flows in and around a window (hole) in the PHS slab beneath SW Japan, suggesting that the occurrence of the PHS slab window may have caused a complex flow pattern in the mantle wedge above the Pacific slab.
    Print ISSN: 0148-0227
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  • 9
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    Stress field in the 2008 Iwate-Miyagi earthquake (M7.2) area (2011)
    Wiley
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    Publication Date: 2011-06-19
    Description: We determined the focal mechanism solutions (FMS) of 191 crustal earthquakes as well as the stress tensor in the source area of the 2008 Iwate-Miyagi earthquake (2008 IMEQ, M7.2) that occurred in the central portion of northeast (NE) Japan. The FMS and the stress tensors were determined by using both 1-D and 3-D velocity models, which exhibit almost the same results. The differences caused by the use of 1-D and 3-D models can be neglected when compared with the differences due to the different methods, which indicates that the FMS and the stress tensor determined with a 1-D model are accurate enough to study the crustal stress field in the study region. The obtained P axis (σ1) trends WNW-ESE subhorizontally, and the T axis (σ3) is oriented subvertically in a NNE-SSW belt perpendicular to σ1. The σ1 orientation is consistent with the motion of the Pacific plate relative to NE Japan, which indicates that the plate boundary forces dominate the intraplate stress regime. Both temporal and spatial variations of the stress field in the IMEQ source area are detected, which may be induced by the stress rotation accompanying the main shock and its aftershocks. The seismogenic faults in the study area are estimated to be very weak, which argues against the concept of strong crust. The faults may be weakened by the high-temperature magma and the fluids in the lower crust and uppermost mantle that intrude upward into the shallower crust.
    Electronic ISSN: 1525-2027
    Topics: Chemistry and Pharmacology , Geosciences , Physics
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  • 10
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    P-wave tomography and anisotropy beneath Southeast Asia: Insight into mantle dynamics (2015)
    Wiley
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    Publication Date: 2015-06-09
    Description: Southeast Asia is surrounded by subduction zones resulting from the interactions of several lithospheric plates. Its evolution has been also influenced by active tectonics due to the Indo-Asian collision in the Cenozoic. In this study, we use a large number of arrival-time data of local and regional earthquakes to determine 3-D P-wave tomography and azimuthal anisotropy in the mantle beneath SE Asia. High-velocity (high-V) anomalies representing the subducting slabs are clearly visible in the upper mantle and the mantle transition zone (MTZ). Low-velocity (low-V) zones with trench-normal anisotropy are revealed in the uppermost mantle, which indicate back-arc spreading or secondary mantle-wedge flow induced by the slab subduction. In contrast, trench-parallel anisotropy dominates in the deep upper-mantle and reflects structures either in the subducting slab or in the upper mantle surrounding the slab. The trench-parallel anisotropy is also significant in the lower MTZ, which may contribute to shear-wave splitting observations. A low-V body extending down to the lower mantle is visible under the Hainan volcano far away from the plate boundaries, suggesting that Hainan is a hotspot fed by a lower-mantle plume. The low-V body under Hainan is connected with low-V zones in the upper mantle under SE Tibet and Vietnam. Our P-wave anisotropy results reflect significant mantle flow existing in the asthenosphere from SE Tibet to Hainan and further southwestward to Vietnam. The present study, especially the 3-D P-wave anisotropy results, provide important new insight into mantle dynamics in SE Asia.
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