沈み込み帯での地震波異方性とかんらん石の格子選択配向

Abstract

Seismic anisotropy is a powerful tool for detecting the geometry and style of deformation in the Earth's interior, as it primarily reflects the deformation-induced preferred orientation of anisotropic crystals. Although seismic anisotropy in the oceanic upper mantle is generally parallel to the plate motion, the propagation direction of the fast shear-wave is oriented subparallel to the trench axis in several subduction zones. In this study, the distribution of seismic anisotropy in the mantle wedge is inferred from deformation mechanism: a lattice-preferred orientation and seismic anisotropy are generated by deformation via dislocation creep, but not by diffusion creep or frictional sliding. Based on the thermal structure beneath northeast Japan, deformation throughout most of the mantle wedge is inferred to be controlled by diffusion creep, and the region of dislocation creep is limited to a thin layer of 10-20 km thickness within a region of relatively high stress and low temperature located above the subducting plate and beneath the island arc crust. The variation of the seismic anisotropy in subduction systems is probably due to the change of active slip-system in olivine as a consequence of chemical and physical properties in the mantle wedge.