Anisotropy in the subducting slab: Observations from Philippine Sea plate events in Taiwan

In the southernmost Ryukyu subduction zone, slab-guiding behavior from intermediate-depth earthquakes is well documented with a low-frequency (<2 Hz) first P arrival followed by sustained high-frequency (3-10 Hz) wave trains. Such waves developed by propagating along a long path within the slab are expected to have high sensitivity to anisotropy within the slab. We determine shear wave splitting parameters from 178 intraplate events that are deeper than 100 km. The possible slab-anisotropy-associated polarization pattern shows the fast direction at N65°E and delay time of 0.13-0.45 s. This is stronger than the previously documented crust effect (<0.1 s), similar to the mantle wedge effect (0.28 s in average), but weaker than the upper mantle effect (1.3 s in average) in Taiwan. The fast axis reflects the fossil spreading direction of Philippine Sea plate with minor clockwise rotation due to the collision to Eurasian plate.