Mapping P-wave anisotropy of the Honshu arc from Japan Trench to the back-arc

Abstract

We determined a 3-D P-wave anisotropic tomography beneath the entire Honshu arc using about 448,000 high-quality P-wave arrival times from 18,335 local earthquakes that occurred beneath the Northeast (NE) Japan land area and the fore-arc area under the Pacific Ocean. Our results show that low-velocity (low-V) zones exist beneath the active arc volcanoes in the crust and in the central portion of the mantle wedge above the subducting Pacific slab. The low-V anomalies are related to the arc magmatism. Low-V zones are also revealed in the fore-arc area, which are probably caused by large volumes of water releasing upwards from dehydration of the subducting oceanic crust and sediments. The anisotropic amplitude in the upper crust is weaker than that in other portions under NE Japan. In the mantle wedge, the fast velocity direction (FVD) is generally trench-normal in back-arc area, which may reflect that the olivine a axis aligns with the transport direction induced by the slab-driven corner flow. The FVD becomes trench-parallel in the central portion of the fore-arc mantle wedge, which is possibly induced by the olivine of B-type fabric in the slab-driven corner flow. The FVD shows trench-parallel in the low-V zones in the fore-arc mantle wedge close to the upper boundary of the Pacific slab, which may reflect the B-type olivine fabric dominating in those areas, or it may be induced by the dextral shearing of the overlying crust. The trench-parallel FVD is also revealed beneath the volcanic front, indicating complex 3-D mantle flows in the mantle wedge. The FVD in the subducting Pacific slab is mostly trench-parallel, which may reflect one or more of the following possibilities: (1) the original fossil anisotropy of the Pacific plate formed at the mid-ocean ridge, (2) cracks within the slab, and (3) the olivine fabric transition due to the changes in water content, stress, and temperature.