Shear-wave polarization anisotropy in the mantle wedge above the subducting Pacific plate

Abstract

Research of azimuthal anisotropy in the mantle wedge above subduction zones opens up a new source of information about rising magma in the region, because the physical property estimated from shear-wave anisotropy is related to dynamic processes inside the earth. Regional variation in shear-wave polarization is investigated from a teleseismic event and some local deep earthquakes using a spatially dense seismic array of the NIED located in the central part of Japan. A characteristic pattern is found on the spatial distribution of shear-wave polarization data obtained from ScS waves. Large and small values of shear-wave polarization anisotropy are observed at seismic stations located to the west and east of the volcanic front, respectively. The anisotropic zone with maximum velocity in N-S direction is localized in the wedge portion between the surface and subducting slab using the data of the local deep events. The location of the anisotropic zone corresponds with a low- Q zone obtained from seismic tomography data. We conclude that the azimuthal anisotropic zone in the mantle wedge is caused by melt-filled cracks, which is consistent with models obtained by petrological studies.