Seismic velocity structure, VP/VS ratio and anisotropy beneath Northeast Japan: implications for water migration in subduction zones

We determined three-dimensional tomographic images of P-wave velocity (V-P), S-wave velocity (V-S), V-P/V-S ratio, and P-wave anisotropy structures down to 150 km depth beneath Northeast Japan using a large number of first P- and S-wave arrival times of local shallow and intermediate-depth earthquakes. Our results show that zones of low-velocity and high V-P/V-S ratio exist beneath volcanoes in the lower crust and mantle wedge, which accords with the distribution of low-frequency microearthquakes (M<2.5). The low-velocity and high V-P/V-S anomalies in the lower crust and mantle wedge may be associated with fluids released from dehydration of the subducting Pacific slab and hot upwelling melts related to arc magmatism. The double seismic zone in the subducting Pacific slab is possibly induced by dehydration embrittlement of hydrous minerals. High V-P/V-S ratio anomalies are not dominant in the entire double seismic zone, which may be affected by the amounts of hydrous minerals in the subducting slab. Trench-parallel azimuthal anisotropy and positive radial anisotropy are dominant in the subducting slab and the overlying mantle wedge under the fore-arc, which are induced mainly by B-type olivine fabrics due to relatively low temperature and increasing water content from the slab dehydration. Our results indicate that the information of both seismic velocity and anisotropy is of great importance for understanding water migration in subducting zones.