Shear-wave splitting in southwestern Japan and its implications for mantle flow

We investigate shear-wave splitting beneath SW Japan using local S phases to constrain a spatial pattern of mantle flow generated by the subduction of the Pacific and Philippine Sea plates. This region includes the arc- arc junction, where the direction of the plate motion is changed relative to the trench axis, and consequently, the oblique subduction is taking place beneath the Izu-Bonin arc. In addition, two oceanic plates are subducted, probably resulting in a complicated upper mantle structure. Therefore this region is an excellent natural laboratory for studying the effect of local slab geometry and the subduction of two slabs on mantle corner flow. A detailed investigation of shear-wave splitting in subduction zones will provide important information concerning tectonic deformation and dynamic process of the mantle wedge. Waveforms of 536 intermediate-depth earthquakes recorded at 442 seismic stations were used, and 1,926 splitting parameters, the leading shear-wave polarization direction (fast direction) and delay time between two split waves, were obtained. In the north part of Kanto where only the Pacific slab is subducted, fast directions in the fore-arc side are N-S, while those in the back-arc side are NW-SE, which is consistent with the results in NE Japan. We found that fast directions are generally E-W in the entire SW Japan and delay times varies from 0.3 sec to 0.7 sec. The direction of E-W is oblique both to relative plate motions of the Pacific and Philippine Sea slabs relative to the overriding continental plate. However, apparent N-S polarized fast directions are observed only around the Norikura Volcanic chain, central Japan, which was pointed out in previous studies [e.g., Hiramatsu et al., 1998]. A recent seismic tomography by Nakajima and Hasegawa [2006] revealed that the dip angle of the Philippine Sea slab changes rapidly beneath the volcanic chain and the slab reaches depth of 200 km. The region with N-S polarization directions coincides well with the region where the dip of the Philippine Sea slab becomes steeper. These observations might suggest that the N-S polarization direction is responsible for anisotropy generated by the subduction of the Philippine Sea slab.