Subslab seismic anisotropy and mantle flow at the SW edge of the Ryukyu subduction zone

The SW edge of the Ryukyu subdution zone is at the subduction-collision junction of the Phillipine plate and the Eurasin lithosphere. Previous splitting results have shown a large scale trench-parallel flow in the subslab mantle of the Ryukyu subduction zone, but how these mantle flow interact with the Eurasin lithosphere mantle remains unknown. We present the shear wave splitting measurements at the SW edge of the Ryukyu subduction zone for SKS coming from remote events and S from slab events received by Australian stations, both sampling the mantle behind the Ryukyu slab. Splittings at Australian stations were corrected. Delay times for S, ranging from 1 to 4.5 s, are larger than that for SKS averaged on about 1.1 s. Fast directions to first order parallel the trench, being consistent with previous observations. A more complicated pattern is seen toward the SW edge of the Rykyu subduction zone, where the fast direction is transitional rotating from trench-parallel to trench-oblique/-perpendicular,. The pattern at the western boundary of measurement reflects the fabric of the orogenic belt of Taiwan, while the rotation away from trench-parallel occurs distant from the Eurasian lithosphere, indicating its mantle dynamic control. Synthetic waveforms for shear-wave splitting is employed to interpret how the subslab asthenosphere deform along with the subduction to shape the observed pattern.