Shear-wave splitting at the edge of the Ryukyu subduction zone

Abstract

Intraslab events recorded by ocean-bottom seismometers in the Okinawa trough provide an extended depiction of shear wave splitting in the southwest section of the Ryukyu subduction zone. At 100–200km from the western edge of the subduction system, we observed trench-normal fast polarization direction in the back arc compatible with 2D slab- or rifting-driven corner flow. Towards the edge, the fast directions are sub-parallel to the trench in the arc—back arc region, and rotate to trench-normal within 50km of the edge. Splitting constrained by land stations with paths mostly in the mantle wedge exhibits similar trench-normal fast directions in the subduction edge zone. Further inland, the dominant component of fast directions becomes roughly parallel to the Taiwan orogenic fabric. Splitting of P-to-S phases converted at the Moho of the Okinawa trough and of S phases from shallow events suggest that crustal anisotropy may affect the measured splitting, but the observed pattern reflects predominantly mantle anisotropy. The variation in splitting along the Okinawa trough cannot be explained by a B-type–A-type olivine fabric transition in the mantle wedge. It may indicate the presence of an along-arc flow in the mantle wedge towards the edge where it is blocked and deflected by the Eurasian lithosphere. This scenario bolsters previous studies suggesting a significant impact of Eurasian lithosphere on the dynamics of the Ryukyu subduction system.