Three-dimensional P- and S-wave velocity structures beneath Japan

Meteorological Research Institute, Tsukuba 305-0052, Japan; Department of Geophysics, Tohoku University, Sendai 980-8578, Japan; Earthquake Research Institute, The University of Tokyo, Bunkyo 113-0032, Japan; Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan; Kyoto University, Kyoto 611-0011, Japan; Chiba University, Chiba 263-8522, Japan; Hokkaido University, Sapporo 060-0810, Japan
Physics of The Earth and Planetary Interiors (Impact Factor: 2.38). 05/2008; DOI: 10.1016/j.pepi.2008.04.017

ABSTRACT We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30–60 km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate.

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    ABSTRACT: A high-resolution P-wave tomography of the crust and mantle down to 700 km depth beneath the Japan Islands is determined using a large number of high-quality arrival-time data from local earthquakes and teleseismic events simultaneously. The tomography shows that the Philippine Sea slab is subducting aseismically down to 430 km depth under southwest Japan, though the seismicity within the slab ends at 180 km depth. A low-velocity (low-V) zone in the mantle wedge under Tohoku and Kyushu is found to extend westward from the volcanic front to the backarc under the Japan Sea and East China Sea. Significant low-V anomalies are revealed in the deep portion of the mantle wedge (400-500 km depth) above the Pacific slab under southwest Japan, which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the Pacific slab. Low-V anomalies appear at 420-700 km depths beneath the Pacific slab under eastern Japan, which may reflect hot mantle upwelling associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.
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