Subduction-driven mantle flow beneath and around the Philippine Sea Plate from seismic anisotropy

Jonathan Wolf , Frederik Link, Maureen Long, Ed Garnero, John D. West

Shear-wave splitting illuminates mantle flow and subduction zone dynamics but is typically inferred near stations or earthquakes, limiting studies in sparsely instrumented regions away from earthquakes. Where stations or earthquakes are present, fast splitting directions are often parallel to the nearest trench, which has yet to be fully understood and reconciled with geodynamic flow predictions. Here we use a novel technique that leverages surface-reflected PS and PPS waves to overcome limitations in coverage beneath the Philippine Sea Plate, which is surrounded on all sides by subducting slabs. Upper mantle deformation in this region, best explained by mantle flow below 250 km depth, is generally different from the absolute plate motion direction and instead shaped by the surrounding subduction zones. We interpret our observations as likely evidence for trench-parallel flow in the mantle wedge beneath the Ryukyu, Nankai, and parts of the Izu-Bonin subduction zones, as well as toroidal flow around the Ryukyu and Mariana slab edges. Beneath the Pacific Plate, near the Mariana subduction zone, flow is trench-perpendicular. Our results are generally consistent with flow patterns predicted by previous geodynamic models, and provide a new constraint on subduction-driven deformation in the region.