The Moho, slab and tomography of the East Japan forearc derived from seafloor S-net data

Abstract

We investigate the 3-D P and S wave velocity structure of the East Japan forearc using a large number of high-quality arrival-time data of local earthquakes recorded by a permanent seafloor seismic network (S-net) and the land-based Kiban seismic network. Geometries of the subducting slab boundary and the Conrad and Moho discontinuities under the forearc are refined by referring to results of previous studies including active-source seismic soundings and seismicity located by the S-net. The obtained P and S wave velocity images are quite similar to each other, suggesting that they reflect robust features of the 3-D seismic structure. The subducting Pacific slab exhibits high velocities, whereas significant low-velocity (low-V) anomalies are revealed in the overlying Okhotsk plate beneath the forearc, which may reflect accretionary materials containing abundant fluids. The subducting oceanic crust atop the Pacific slab is revealed clearly as a thin low-V layer extending from the trench axis down to ~100 km depth under the volcanic front. Our results of earthquake relocation using the 3-D velocity model indicate that the double seismic zone in the subducting Pacific slab occurs from the trench axis down to ~180 km depth. The occurrence and focal mechanisms of the double seismic zone are caused by slab deformation during its subduction, such as slab bending or unbending, as well as other processes including hydration, dehydration, compositional variations, and phase changes in the subducting slab.