The lateral variation of the shear-wave splitting values just above the subducting ridge

In Tokai region, central part of Japan, the Philippine Sea plate is descending to the NW direction. The configuration of the subducting slab has been revealed by the seismic tomography and refraction/reflection studies. Those studies suggested that the top of the slab was not smooth. A clear image of the subducting ridges was figured out. The characteristic of the asperity is one of the important topics to know the mechanism of the plate boundary earthquakes. There are many discussions for the relationship between the subducting sea mount and asperity. The Tokai region is one of the good fields to know the relationship. The location and configurations of the subducting ridges have been revealed by the refraction and reflection studies. The subducting ridge is located just beneath the land area. If the subducting ridge causes the stress concentration, the spatial pattern of the stress distribution will be changed. It will be detectable by the shear wave splitting analysis at the seismic stations above the ridge. In this area, slow-slip was also reported. The Tokai area is a very good field to know the whole image of the plate coupling. We did temporal seismic observation with about 70 seismic stations in Tokai region. The seismic stations are located as a linear array trending to NW direction which is consistent with the direction of subduction of the Philippine Sea plate. The length of the array was about 100 km. It was operated from April, 2008 to September, 2008. We studied shear wave splitting in this area using the array. The array is located just above the subducting ridge. The spatial variation of the shear-wave splitting values was obtained. The maximum stress direction of the area is NW-SE. The polarization direction of the area with the direction of NW-SE was obtained. That is very consistent with the stress field of the area. The tame lag values at the south are little bit larger than that of north. But, we could not see large lateral variation of the polarization direction and time-lag values along the profile line. We could not find out any close relationship between the topography of the subducting ridge and spatial variation of shear-wave splitting. It is expected that the effect of the subducting ridge to the stress pattern in the crust seems to be small.