P-wave anisotropic tomography beneath Northeast Japan

We determined a 3-D P-wave anisotropic tomography beneath Northeast Japan by using first P-arrival times. The travel-time inversion is parameterized with an isotropic component and two anisotropic parameters for each grid node by assuming P-wave azimuthal anisotropy with hexagonal symmetry axis distributed horizontally. The geometries of the Conrad and Moho discontinuities and the upper boundary of the subducting Pacific plate are taken into account in the model. Our extensive synthetic tests show that the anisotropic velocity inversion method is reliable. Our results show that low-velocity zones exist below active volcanoes in the crust, in the fore-arc mantle wedge and in the central portion of the mantle wedge above the high-velocity Pacific slab, which are similar to the previous tomographic images. Anisotropic velocity variations are revealed in the crust, mantle wedge and the subducting Pacific slab. The anisotropic patterns are complex and different in the upper and lower crust. In the mantle wedge, the fast direction of P-velocity is generally trench-parallel in the fore-arc area, while it becomes trench-normal in the back-arc area, which agree well with the shear-wave splitting results. The trench-parallel fast direction is revealed beneath the volcanic front, which may suggest the existence of complex 3-D mantle flows in the mantle wedge. The fast-velocity direction in the subducting Pacific slab is mostly N–S, which is likely due to the original fossil anisotropy of the Pacific plate formed at the mid-ocean ridge. Our result also suggests that the anisotropy is weak in the upper crust and the average amplitude of the velocity anisotropy in the mantle is no larger than 4%.