Seismic velocity azimuthal anisotropy of the Japan subduction zone: Constraints from P and S wave traveltimes

Abstract

AbstractWe present 3‐D images of azimuthal anisotropy tomography of the crust and upper mantle of the Japan subduction zone, which are determined using a large number of high‐quality P and S wave arrival time data of local earthquakes and teleseismic events. A tomographic method for P wave velocity azimuthal anisotropy is modified and extended to invert S wave traveltimes for 3‐D S wave velocity azimuthal anisotropy. A joint inversion of the P and S wave data is conducted to constrain the 3‐D azimuthal anisotropy of the Japan subduction zone. Our results show that the subducting Pacific and Philippine Sea (PHS) slabs exhibit mainly trench‐parallel fast‐velocity directions (FVDs), which may reflect frozen‐in lattice‐preferred orientation of aligned anisotropic minerals formed at the mid‐ocean ridge as well as shape‐preferred orientation such as normal faults produced at the outer‐rise area near the trench axis. Trench‐normal FVDs are generally revealed in the mantle wedge, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench‐normal FVDs are also visible in the subslab mantle, which may reflect the subducting asthenosphere underlying the slabs. Our results also reveal toroidal mantle flows in and around a window (hole) in the PHS slab beneath SW Japan, suggesting that the occurrence of the PHS slab window may have caused a complex flow pattern in the mantle wedge above the Pacific slab.