Mantle anisotropy beneath northwest Pacific subduction zones

Abstract

To assess the location, strength, and orientation of seismic anisotropy in the southern Kuril, Japan, and Izu‐Bonin subduction zones, we analyzed shear wave splitting in local S phases and teleseismic SKS phases. Fast directions from these phases are roughly parallel to the direction of absolute Pacific plate motion (∼WNW) in Izu‐Bonin, roughly parallel to the strike of the trench (∼NNE) in central Honshu, and roughly N in the southern Kuril back arc in the vicinity of Sakhalin Island. Assuming that the orientation and strength of anisotropy does not vary with depth, modeling of splitting times from local S and teleseismic phases recorded at Sakhalin Island requires that the maximum depth of anisotropy lies between 480 km and 950 km. In contrast, splitting times for local S phases from events in Izu‐Bonin rule out anisotropy in the transition zone. All the data are consistent with a model in which the lower transition zone (520–660 km) and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone (410–520 km), possibly due to preferred orientation of β spinel. Assuming that splitting in the upper mantle is produced by preferred orientation of olivine, the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones. The relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow, but that flow correlated with upper plate deformation must also exist.