P wave anisotropic tomography of the Nankai subduction zone in Southwest Japan

Abstract

The active subduction of the young Philippine Sea (PHS) plate and the old Pacific plate has resulted in significant seismic heterogeneity and anisotropy in Southwest (SW) Japan. In this work we determined a detailed 3‐D P wave anisotropic tomography of the crust and upper mantle beneath SW Japan using ∼540,000 P wave arrival times from 5,249 local earthquakes recorded by 1095 stations. The PHS slab is imaged clearly as a high‐velocity (high‐V) anomaly which exhibits considerable lateral variations. Significant low‐velocity (low‐V) anomalies are revealed above and below the PHS slab. The low‐V anomalies above the PHS slab may reflect the upwelling flow in the mantle wedge and the PHS slab dehydration, and they form the source zone of the arc volcanoes in SW Japan. The low‐V zones under the PHS slab may reflect the upwelling flow in the big mantle wedge above the Pacific slab. The anisotropy in the crust and upper mantle is complex. In Kyushu, the P wave fast velocity direction (FVD) is generally trench‐normal in the mantle wedge under the back‐arc, which is consistent with the corner flow driven by the PHS slab subduction. The FVD is trench‐parallel in the subducting PHS slab under Kyushu. We think that the intraslab seismicity is a potential indicator to the slab anisotropy. That is, the PHS slab with seismicity has kept its original fossil anisotropy formed at the mid‐ocean ridge, while the aseismic PHS slab has reproduced the anisotropy according to its current deformation.