P-wave anisotropic tomography of the crust and upper mantle under Hokkaido, Japan

Abstract

We determined a 3-D anisotropic velocity structure beneath Hokkaido in northern Japan using a large number of first P-wave 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 results show that low-velocity zones exist beneath active arc volcanoes in the crust and in the central portion of mantle wedge above the high-velocity Pacific slab, which are similar to the previous tomographic images. P-wave anisotropy is revealed in the crust, mantle wedge and the subducting Pacific slab. In the upper crust, the anisotropy is possibly caused by microcracks and cracks. In the lower crust, the anisotropic structure is different from that in the upper crust and it may be affected considerably by the plastic flow deformation. In the mantle wedge, the fast-velocity direction (FVD) is generally trench-normal in most of the study area, while it becomes trench-parallel in the Pacific coast areas close to the trench, which agrees with the shear-wave splitting results. Beneath the volcanic front, the FVD is also trench-parallel, which may suggest the existence of complex 3-D mantle flow in the mantle wedge. The FVD in the subducting Pacific slab is nearly north–south, which likely keeps the original fossil anisotropy of the Pacific plate formed at the mid-ocean ridge or is affected by the olivine fabric transition from A-type to B-type fabric due to the changes in water content, stress, and temperature.