P-wave anisotropy tomography of central Japan: Insight into subduction dynamics

Abstract

A P-wave anisotropic tomography under central Japan was determined using a large number of arrival-time data from 22,244 earthquakes recorded by 770 seismic stations on the Japan Islands. The results show that the anisotropy exists widely beneath the study area except for the central portion of the mantle wedge. The fast-velocity directions (FVDs) of the mantle wedge are generally trench-parallel under the fore-arc area and trench-normal under the back-arc area, which may be attributed to the lattice-preferred orientation (LPO) of olivine changing from B-type under the fore-arc area to A-type under the back-arc area with the variation of water content and the occurrence of mantle flow. The subducting Philippine Sea plate (PHS) beneath Tokai is revealed as a high-velocity anomaly dipping northward with NEE to NE–SW FVDs which are consistent with the spreading direction of the PHS during its formation and are inferred as the fossil anisotropy of the PHS. The anisotropy of the upper portion of the Pacific slab under central Japan is also revealed in this study. The FVDs of the upper portion of the Pacific slab are sub-parallel to the Japan Trench and Izu-Bonin Trench at ~60–120km depths and become almost perpendicular to the trenches below 100–120km depth. After comparisons with the Mesozoic magnetic anomaly lineations and the principle stresses in the Pacific slab, we propose that the fossil FVDs of the Pacific slab beneath central Japan were already rebuilt by the principle stresses in the slab.