抄録
We estimated three-dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern Japan by applying the double-difference tomography method to arrival time data of earthquakes obtained by the dense nationwide seismic network (Kiban-network). Alow S-wave velocity and high Vp/Vs layer shallowly dipping toward the subduction direction of the slab and several km thick has been clearly imaged at locations immediately above the top of intraslab seismicity in a wide area from Tokai to Kyushu. Comparison with the location of the upper surface of the Philippine Sea slab estimated from seismic refraction surveys at four survey lines shows that this low S-wave velocity and high Vp/Vs layer corresponds to the crust of the Philippine Sea slab. This indicates that many of intraslab earthquakes are occurring in the slab mantle, suggesting that the location of the upper plate interface previously estimated only from spatial distribution of intraslab seismicity is not correct. Based on the presently obtained location of the low S-wave velocity and high Vp/Vs layer and hypocenter distribution of relocated intraslab earthquakes, we estimated the configuration of the upper surface of the Philippine Sea slab in the whole area of southwestern Japan.
