Seismic imaging of Northwest Pacific and East Asia: New insight into volcanism, seismogenesis and geodynamics

Recent studies on seismic imaging of the Northwest Pacific and East Asian region are reviewed. High-resolution tomographic images reveal significant lateral heterogeneities in the crust and upper mantle, which shed new light on interplate and intraplate volcanism, earthquake mechanism, and mantle dynamics. Significant recent advances in seismic imaging are tomographic inversions for 3-D distribution of seismic anisotropy and attenuation in the crust and upper mantle, which provide important new information on lithospheric deformation, mantle convection, melts and fluids associated with plate subductions and mantle dynamics. The results show that intraplate volcanism in NE Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone. The subducting Pacific and Philippine Sea slabs exhibit mainly trench-parallel fast-velocity directions (FVDs) of azimuthal anisotropy, which may reflect frozen-in lattice-preferred orientation of aligned anisotropic minerals and/or shape-preferred orientation, such as transform faults formed at the mid-ocean ridge and normal faults produced at the outer-rise area near the trench axis. Trench-normal FVDs are generally revealed in the mantle wedge under the volcanic front and back-arc region, which may reflect corner flows in the mantle wedge due to the plate subduction and dehydration. Trench-normal FVDs also appear in the subslab mantle, which may reflect asthenospheric shear deformation associated with the overlying slab subduction. The nucleation of large earthquakes, including both crustal and megathrust events, is controlled by structural heterogeneities in and around seismogenic faults. The cause of deep-focus earthquakes is a subject of ongoing debate. A recent example is the 2015 Bonin deep earthquake (M 7.9,-670 km depth), which was probably caused by joint effects of several factors, including the Pacific slab?s fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and ambient mantle. The key to future advances in seismic imaging is instrumentation. Gradual deployment of seismometers on seafloors and in those less instrumented land areas will be the most important task for seismologists from now. For this purpose, international cooperation of seismologists in different countries will be necessary