Complex Uppermost Mantle Structure and Deformation Beneath the Northwest Pacific Region

We used a newly developed Pn tomography method to obtain high-resolution uppermost mantle velocity and anisotropy structures beneath the Northwest Pacific region. The observed Pn velocities are consistent with the local tectonic background, where high Pn velocities are observed beneath the Japan Trench area and Songliao Basin, and low Pn velocities beneath the Kuril Islands, Japan Archipelago-Izu Islands, Kyushu Island, Changbaishan-Jingpohu volcanoes, Korea Peninsula, and Japan Basin. The new Pn velocity image outlines the subducting slabs along the trenches and the young seafloor within the Japan Basin. Our results also support the existence of hot upwelling feeding the Changbaishan, Jingpohu, and Chuga-Ryong volcanoes, where small-scale mantle convection may exist below the Northeast China region. Further east, both trench-parallel anisotropy below arcs and trench-perpendicular anisotropy within the back-arc region suggest subduction-dominant mantle flow, where anisotropy may be attributable to the lattice-preferred orientation of olivine induced by flow-related strain. The highly accurate uppermost mantle velocity and anisotropy structures provide crucial information outlining the complex dynamic processes near convergent plate boundaries.