Interplate coupling and seismotectonics under the fore-arc regions of Japan

Three-dimensional P- and S-wave velocities (<i<v</i<<sub<P</sub<, <i<v</i<<sub<S</sub<), Poisson's ratio (<i<σ</i<), crack-density (<i<ε</i<) and bulk-sound velocity (<i<v</i<<sub<<i<ϕ</i<</sub<) structures along the slab upper boundary beneath the fore-arc regions were determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in Japan. The velocity and Poisson's ratio images provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous low velocity and high Poisson's ratio associated with slab dehydration under the fore-arc areas. Most great thrust earthquakes (<i<M</i< > 7.5) occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas, suggesting strong interplate coupling (asperities) with slab subduction. On the other hand, prominent low-velocity anomalies were revealed along the slab upper boundary, which may reflect weak coupled or decoupled patches (aseismicity) of the plates due to serpentinization of the fore-arc mantle wedge. The crack-density and bulk-sound velocity images, calculated from the corresponding velocity models, indicate that the interplate coupling in northeastern Japan is different from that under central and southwestern Japan owing to differences between the tectonic backgrounds of the subduction system, such as the geological age, thermal regime and dipping angle of the oceanic plates. A comparison between fluid-related anomalies of Japan, Cascadia, Chile, and Costa Rica subduction zones suggests that seismic mantle may be common in fore-arc settings and these reflect similar 3-D seismic structures relatively to fluid liberating processes. We consider that the fluid-bearing anomalies along the interface of the subducting slab, attributing to processes such as slab dehydration and serpentinization of the fore-arc mantle, are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the fore-arc regions in Japan.