Seismic tomography and anisotropy in the source area of the 2008 Iwate-Miyagi earthquake (M 7.2)

In order to understand the generation mechanism of the Iwate-Miyagi earthquake (M 7.2) that occurred on 14 June 2008 in Northeast Japan, we determined high-resolution 3D seismic velocity (Vp. Vs) and Poisson's ratio (sigma) structures as well as P-wave azimuthal anisotropy in and around the source area using 246,623 P-wave and 189,153 S-wave high-quality arrival times from 1941 aftershocks of the 2008 Iwate-Miyagi earthquake as well as 4878 shallow and intermediate-depth earthquakes in this area recorded by the dense High-Sensitivity Seismic Network (Hi-net) and the seismic stations of Tohoku University. Our results show that hypocenters of the mainshock and three large aftershocks are located in a boundary zone where both seismic velocity and Poisson's ratio change drastically in a short distance. A zone with pronounced low-velocity and high Poisson's ratio is revealed in the lower crust and uppermost mantle under the source area, which may reflect the arc magma and fluids ascending from the upper-mantle wedge. The P-wave azimuthal anisotropy is complex under the study area, which may also indicate the complex crustal structure there. These results indicate that the generation of the 2008 Iwate-Miyagi earthquake was influenced by the ascending arc magma and fluids associated with the dehydration reactions of the subducting Pacific slab under Northeast Japan. (C) 2010 Elsevier B.V. All rights reserved.