Most of the seismological phenomena, e.g., the arc magmatism, the large crustal and magathrust earthquakes and the low frequency tremors in the subduction zone are closely related to fluids released from the dehydration process of the subducting slab and/or the metamorphism reaction at the lowermost crust. The high-resolution velocity images together with other Physical parameters such as Poisson’s ratio, crack density and saturation rate estimated by the present studies show that more than 95% of the megathrust earthquakes are located at or near to the high-velocity areas, indicating strong interplate coupling, along the slab upper boundary under the forearc regions from Hokkaido to Kyushu. Fluids released from the dehydration of the subducting Philippine Sea (PHS) slab intermingling with the upwelling magmatic materials associated with the Okinawa Trough extension causes partial melting there and then leads to the Unzen volcanism. Such an interpretation is different from the previous studies that the Unzen volcanism is caused by a mantle plume beneath the East China Sea. Meanwhile, our study suggests that the non-volcanic low-frequency tremors are mainly caused by the dehydrated fluids migrating in the faults and/or by crack opening, closing and extending in the corner of the mantle wedge along the subducting PHS slab, possibly due to high fluid-pressure and high crack-density under the forearc region of the Nankai subduction zone. For the first time our study provides strong geophysical evidence for the Eurasian lithosphere subducting beneath South Taiwan from the surface down to a depth of 300 km since a previous geological study proposed it about 20 years ago. This velocity model suggests that the subducted Eurasian lithosphere colliding with the subducted PHS plate would have contributed to the mountain building, active seismicity and crustal deformation in the Taiwan Island.
問い合わせ先：土屋 卓久 TEL （089）927-8198
E-mail takut@sci.ehime-u.ac.jp
