Anisotropic Velocity Structure of Seismic Waves in Japan Subduction Zone

Abstract

Seismic anisotropy is critical to infer geodynamics such as mantle convection, plate tectonics, and the evolutionary processes of the crust. S-wave splitting analysis has been extensively used in the anisotropy study of the Earth’s interior, including Japan subduction zone, but the resolution of the splitting data is too poor to investigate the detailed vertical structure. To obtain 3D anisotropic velocity structure beneath the Japan Islands, anisotropic tomography has been applied with P-wave travel-time data obtained by seismic networks deployed over the Japan Islands. The resultant 3D anisotropic structure shows that the P-wave anisotropy exists not only in the upper crust and mantle wedge but also in the lower crust and subducting slabs, where anisotropy has not been thoroughly studied. Although the P-wave anisotropic structure is basically consistent with S-wave anisotropy obtained from S-wave analyses, disagreement between P-and S-wave anisotropies is seen in the upper mantle beneath northeast Japan and the upper crust of the region along the Itoigawa-Shizuoka Tectonic Line. This result requires further studies to construct an anisotropy model that allows disagreement between P-and S-wave anisotropies, and/or revisiting the S-wave anisotropy structure with a sophisticated method that could provide a high-resolution structure of S-wave anisotropy.