Seismic anisotropy near source region in subduction zones around Japan

Abstract

Broad-band seismographs have been distributed widely over the Japanese islands and have provided us with high-quality digital waveform data in recent years. We have investigated splitting of ScS waves from deep earthquakes in the Kuril and Izu-Bonin subduction zones recorded on the STS seismograms. We used ScS waves from 13 events observed in Japan and recognized shear-wave splitting which is mainly caused by anisotropy in the lithospheric slab. The anisotropy was confirmed around the source region by comparing ScS-wave splitting from deep earthquakes in different regions and at different depths with S-wave splitting from deep events just beneath central Japan. We have found that the anisotropic regions exist within the subducting slab, although they are distributed locally in and around the source, possibly as patches with a diameter of 100 km or less. Discrepancy in the direction of the fast polarized shear waves between two nearby events is observed in the Izu-Bonin subduction zone: the splitting of the shallower event (289 km) shows its fast polarization direction to be parallel to the fossil motion of the Pacific plate, but that of the deeper event (361 km) immediately beneath is parallel to the current motion. These observations suggest that the change in shear-wave splitting with depth comes from the modification of preferred orientation of minerals depending on depth. We propose in this study that the reorientation of minerals (olivine) occurs owing to the change in physical conditions associated with the phase transformation of olivine (α phase) to modified spinel (β phase).