Upper mantle anisotropy beneath Japan from shear wave splitting

In the southern part of the F-net array, along the Ryukyu arc, we find that fast directions are consistently trench-parallel, with splitting times of 1 s or more. Moving northward along the array, the measured splitting patterns become more complicated, with significant variations in apparent splitting parameters that indicate complex anisotropic structure. Additionally, measured fast directions vary significantly over short length scales, and stations separated by less than 100 km often exhibit very different splitting behavior. This increase in complexity of anisotropic structure coincides geographically with the complicated slab morphology of the subducting Pacific plate. At stations on Hokkaido, to the north, and Kyushu, to the south, we see some evidence that the fast direction of anisotropy may rotate from trench-parallel close to the trench to subduction-parallel further away from the trench, which may correspond either to a change in stress conditions and/or volatile content, or to a change in flow regime.