The mantle wedge anisotropy estimated by the shear-wave splitting analysis in central part of Japan

Abstract:

The 1891 Nobi earthquake was one of the largest inland earthquakes in Japan. We developed temporary seismic network at the area. We researched shear-wave splitting to obtain seismic image in the central part of Japan by the use of the seismic stations at the temporary seismic network and Hi-net. The deep earthquakes which occurred from 2009 to 2014 were used.

The results suggested remarkable lateral variation of the polarization directions. In the northeast of the fault area, the data showed NE-SW polarization directions. In the south of the fault area, the polarization direction was E-W. Those data were consistent with the results of previous researches of the area. However, the polarization directions around the 1891 Nobi earthquake were NW-SE. The data with spatially high dense network figured out fine scale lateral variation of polarization directions. The time delays of the splitting were larger than 0.5 sec. The crustal anisotropy had been studied in the area. The maximum crustal anisotropy in the area was around 0.1 sec. The large anisotropic area should be located in the mantle wedge.

The polarization direction in the northeastern part of the research area can be explained by the anisotropy which is caused by mantle flow related to the subduction of Philippine Sea plate. The polarization direction in the southern part of the research area can be explained by the mantle anisotropy caused by the flow related to the subduction of Pacific plate. The polarization direction with NW-SE, which was observed at the source area of the 1891 Nobi earthquake, is not consistent with both of the directions of the subducting oceanic plates. In the area, low velocity zone and high conductive zone were detected above the subducting slab. The zone characterized as low velocity and high conductivity seems to be related to the fluid dehydrated from slab. The observed large shear-wave splitting might be caused by the heterogeneous structure related to the fluid or magma.