Shear-wave splitting beneath the southwestern Kurile arc and northeastern Japan arc: A new insight into mantle return flow

Shear-wave splitting in the southwestern part of the Kurile arc and the northeastern (NE) Japan arc is investigated using the waveforms from local earthquakes. For both arcs observed shear-wave splitting shows clear evidence for a striking rotation of fast direction across the arc, suggesting the different feature of anisotropy between the fore-arc and back-arc sides. Trench-parallel fast directions are observed in the fore-arc side, which is consistent with the anisotropy expected from the deformation of the B-type olivine fabric as well as the anisotropy in the slab. Fast directions observed in the back-arc side show approximately E-W and ESE-WNW in the NE Japan arc and N-S in the southwestern Kurile arc, which are characterized by the local maximum-dip direction of the subducted Pacific plate not by the direction of plate convergence. This relationship suggests that the direction of the mantle return flow is governed by the local slab geometry.