Language：English   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

We synthesized information from recent high-resolution tomographic studies of large crustal earthquakes which occurred in the Japanese Islands during 1995-2008. Prominent anomalies of low-velocity and high Poisson's ratio are revealed in the crust and uppermost mantle beneath the mainshock hypocenters, which may reflect arc magma and fluids that are produced by a combination of subducting slab dehydration and corner flow in the mantle wedge. Distribution of 164 crustal earthquakes (M 5.7-8.0) that occurred in Japan during 1885-2008 also shows a correlation with the distribution of low-velocity zones in the crust and uppermost mantle. A qualitative model is proposed to explain the geophysical observations recorded so far in Japan. We consider that the nucleation of a large earthquake is not entirely a mechanical process, but is closely related to the subduction dynamics and physical and chemical properties of materials in the crust and upper mantle; in particular, the arc magma and fluids.

DOI： 10.1111/j.1440-1738.2009.00694.x

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Temperature and water content anomalies just above the 660-km discontinuity under the Japan Islands are estimated from seismological observations. Two sets of seismological observations of P-wave velocity perturbations and depth variations of the 660-km discontinuity are used, which are (1) long-wavelength (similar to 500 km) variations from seismic tomography based on a grid parameterization and waveform analysis of ScS reverberations and (2) moderate-wavelength (similar to 150 km) variations from seismic tomography with a block parameterization and receiver-function analysis. To estimate temperature and water content anomalies, partial derivatives of velocity and depth variations with respect to temperature and water content determined by mineral physics studies are used. Under Southwest Japan, low temperature and high water-content anomalies are obtained from both sets of seismological observations, which have already been found by a previous study. Under Northeast Japan, however, there are discrepancies between the results estimated from the two data sets, possibly due to the different resolution scales. This discrepancy is dismissed when examining distributions of the temperature and water content anomalies. These anomalies can be grouped into two geographical locations, one group under Southwest Japan, the other under Northeast Japan. The two groups are clearly separated and exhibit common features in both sets of the seismological observations. The grouping is interpreted by a compositional difference between the subducting Pacific slab under Southwest Japan and a normal mantle under Northeast Japan. (C) 2009 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

DOI： 10.1016/j.gr.2009.04.009

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Language：English   Publisher：ELSEVIER SCIENCE BV  

High-resolution 3-D P-wave velocity structure at depths of 0-400 km was determined to investigate the structural heterogeneities and their implications for mountain building, plate collision and tectonics benefit Taiwan by simultaneously inverting a large number of arrival-time data from local and teleseismic earthquakes. In the joint seismic tomography, 227,258 absolute arrival times from 8982 local and regional earthquakes and 18,869 relative arrival-time data collected from 3179 releseismic events were used. Our tomographic images provide direct geophysical evidence for a tectonic model proposed by previous studies and revealed some new features of structural heterogeneity related to the subducted Eurasian lithosphere. The seismic images show that the Philippine Sea slab descends northwestward from the Ryukyu Trer down to a depth of 200 kin, showing agreement with the previous seismic, geochemical and geophysical studies. Low-velocity anomalies beneath the Kueishan volcano are revealed at depths of 0-65 kin in No Taiwan, indicating its close relationship to the subducting process of the Philippine Sea slab. The Eurasian lithosphere is generally imaged as a high-velocity zone with a thickness of 65-80 km and it has subducted down to a depth of 300 km under South Taiwan, whilst it is not visible beneath North Taiwan. Low-velocity anomalies above the subducted continental lithosphere are observed at depths of 0-200 km, which might reflect, at least part of the subducted continental crust. Our tomographic results indicate that the plate convergence of the Eurasian plate varies from subduction beneath South Taiwan to collision with t Philippine Sea slab under North Taiwan. These features of the structural heterogeneities in the crust and upper mantle suggest that the mountain building process in the central part of Taiwan, arc magmatism northeast offshore region and seismotectonics are mainly attributed to the subduction-collision configuration at the boundary between the Eurasian continental lithosphere and the subducting oceanic lithosphere of the Philippine Sea slab. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tecto.2009.07.018

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Analyses of reflected seismic phases PcP and ScP from the core-mantle boundary (CMB) enable us to constrain the physical properties of the ultralow velocity zones (ULVZs) at the base of the mantle. The CMB region beneath the western and southern Pacific is investigated in detail by using a large amount of waveform data recorded by the short-period array stations. Clear arrivals of postcursor to ScP (ScsP) are detected from several event-receiver pairs, which sample the CMB regions beneath Philippine-Kalimantan and East of Australia, indicating the existence of ULVZs under the regions. No ScP precursors (SdP and SPcP) are found in our data set except for a few events with a large epicentral distance over 45 degrees. No additional arrivals related to PcP are found in the region where ScsP is observed in the ScP wavelet. Elastic properties of the ULVZ are precisely constrained by waveform modeling of both PcP and ScP which sample the CMB region under Philippine-Kalimantan. Different ULVZ structures are found between the northeastern and southwestern parts of the Philippine-Kalimantan: the ratio of shear-to-compressional velocity reductions is 2.0-2.5 and the ratio of density-to-shear velocity changes is -0.5 to -0.33 in the northeastern part, while minimal changes in compressional velocity and large shear velocity decrease in the southwestern part are found. A certain degree of density increases and shear velocity decreases are also observed in the ULVZ beneath East of Australia. Increase in density and decrease in shear velocity throughout our observation imply that the iron incorporation into the lowermost mantle minerals may generate the ULVZ anomaly. However, other mechanisms such as partial melting can explain the structural difference between the northeastern and southwestern parts of the ULVZ beneath the Philippine-Kalimantan region. (c) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.pepi.2007.07.005

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Language：English   Publisher：ELSEVIER SCIENCE BV  

To better understand the generation of low frequency tremors (LFTs) that occurred in the Nankai subduction zone, detailed 3-D seismic velocity (Vp and Vs) and Poisson's ratio (a) structures are determined by inverting 181,576 P-wave and 159,517 S-wave arrival time data from 7572 earthquakes. We also estimated the crack density (e), saturation rate (C) and porosity (0) structures along the LFT belt using the inverted velocity data. Our results provide geophysical evidence for that the long-duration tremors are closely related to the dehydration reactions of the subducting Philippine Sea slab and in the lowermost crust. Low-V(p), low-Vs, high-sigma, high-epsilon, high-zeta and high porosity anomalies are imaged at depths of 25-45 km along the LFT belt, which may indicate the existence of fluids liberated by the dehydration of the descending slab and/or by the metamorphism in the lowermost crust. The interpretation accords well with the behavior of the subducted hydrous minerals which are expected to dehydrate at the temperature of 350-500 degrees C at depths of 35-50 km in Southwest Japan. Two prominent LFT gaps between Shikoku and Kii peninsula and between Kii peninsula and Tokai along the tremor belt are imaged with low-a and low-C anomalies due to the lack of dehydration reactions within those gaps. We conclude that the LFTs are caused by the dehydrated fluids migrating in the faults and/or by crack opening, closing and extending in the corner of mantle wedge along the subducting Philippine Sea slab, possibly due to the high pore fluid-pressure and high crack-density under the forearc region of the Nankai subduction zone. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2006.08.025

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Language：English   Publisher：ELSEVIER SCIENCE BV  

P-wave velocity images are determined under the Baikal rift zone in Siberia by using teleseismic tomography. Our results show prominent low-velocity anomalies in the upper mantle under the Baikal rift zone and high-velocity anomalies in the lithosphere under the Siberian craton. The low-velocity anomalies are interpreted as a mantle upwelling (plume) which has played an important role in the initiation and evolution of the Baikal rift zone. The rift formation may be,also controlled by other factors such as older (prerift) linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far-field forces caused by the India-Asia collision. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.epsl.2006.01.033

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Language：English   Publisher：Blackwell Publishing Ltd  

We estimate temperature anomalies and water content in the mantle transition zone from the depth of the "660-km discontinuity" and tomographically determined P-velocity anomalies. We assume a linear dependence of the discontinuity depths and P-velocity on temperature anomaly and water content. Beneath the Philippine Sea, where the Pacific plate is subducted, temperature anomalies are as low as -500K to -700 K within and near the stagnant Pacific slab and the water content is estimated to be in the range of 1-1.4 wt.%H2O. The west Philippine basin, away from the Pacific slab, does not have a significant temperature anomaly or water content. Beneath western Japan, where the Pacific slab is subducted, we obtain temperature anomalies up to -300 to -600 K and water content up to 1-1.5 wt.%H2O. Many problems remain to be solved for obtaining a definitive conclusion on the presence of water and quantitative estimates of temperature and water content. Estimates of the temperature anomaly and water content are highly sensitive to the input seismic parameters (the discontinuity depths and P-velocities) and the assumed dependence of the seismic parameters on temperature and water content determined from experimental studies. More accurate estimates of the seismic parameters and the experimental data measured under the pressure and temperature conditions of the mantle transition zone are necessary. Anelastic attenuation is probably enhanced by water, which might break down the linear dependence of the discontinuity depths and P-velocity on temperature anomaly and water content. A non-linear optimization approach using better seismic and experimental data may be required for obtaining more conclusive evidence for the presence of water.

DOI： 10.1029/167GM18

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Language：English  

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Language：English  

CiNii Books

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Language：English   Publisher：AMER GEOPHYSICAL UNION  

The short-period vertical-component data from the Fiji Is. earthquakes recorded by three seismological networks in Japan are analysed to investigate short-wavelength lateral heterogeneities in D" layer. A double-array stacking method, which allows us to combine event and receiver arrays, is applied to the records of each network. Three regions in the D" layer sampled by the three networks are located at horizontal intervals of a few hundred kilometers. In all three regions, the D" reflectors are detected. The height of the D" reflector above the CMB, which is estimated by the differential travel time between PdP (D" reflection) and P, changes from 270 km to 170 km. This result may suggest that some strong short-wavelength lateral heterogeneities exist in the D" layer beneath the southwestern Pacific.

DOI： 10.1029/1998GL900188

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Language：English   Publisher：American Geophysical Union  

The double-array (event- and receiver-array) stacking is applied to the P-wave data from 6 Fiji Is. events recorded by a local network of short-period seismographs in Japan to investigate VP structure in the lowermost mantle. A signal which may be interpreted as the reflection from a discontinuity at 170 km above the CMB (core-mantle boundary) is observed between P and PcP in the stacked record. As compared with the receiver-array stacking with a small number of stations applied to a single event, the double-array stacking is effective to detect the weak signal of PdP. Copyright 1996 by the American Geophysical Union.

DOI： 10.1029/96GL01564

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DOI： 10.1029/93GL01761

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DOI： 10.1029/93GL01761

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