Three-dimensional structure of P-wave anisotropy in the presence of small-scale convection in the mantle wedge

Abstract

As a possible explanation of "hot fingers" in the mantle wedge below the Tohoku region, Japan, the existence of small-scale convection has been proposed. In this study, we performed numerical calculations around the subduction zone in 3D with composite rheology (i.e., a combination of linear and non-linear rheology) and found that small-scale convection could arise when the rheology determined from laboratory experiments is considered. We also calculated 3D structure of expected P-wave anisotropy for the case with and without small-scale convection based on a theory of LPO development and an assumption that anisotropy is approximately represented as hexagonal symmetry, and found that the fast axis of the P-wave propagation projected on the horizontal cross-sections is nearly in the same direction as that of plate motion in many places. It implies that effects of large-scale mantle flow associated with subducting slab is still dominant even in the presence of small-scale convection in mantle wedge. However, in vertical crosssections, the projected fast axis of the P-wave propagation could tilt vertically while that without small-scale convection is almost horizontal. Therefore, future seismological studies that determine the fast direction of P-wave propagation in 3D would give us critical information on the possible existence of small-scale convection in the mantle wedge.