Frequency-dependent shear-wave splitting and multilayer anisotropy in northeast Japan

We analyzed carefully shear-wave splitting on 320 intermediate-depth earthquakes occurring in the subducting Pacific slab in different frequency bands to investigate the S-wave anisotropy and subduction dynamics under Northeast (NE) Japan. Our results show that the differential time between the fast and slow shear waves (δt) is definitely smaller (<0.2 s) in the high-frequency band than that (0.3-0.4 s) in the low-frequency band, and so the splitting parameters, especially δt, are strongly frequency-dependent. Although the δt is indubitably smaller under NE Japan than the other subduction zones regardless of the frequency band, nine large δt values (0.5-0.7 s) are detected, which indicates that the anisotropy is potentially strong in NE Japan. Both the P and S wave anisotropy results in NE Japan are consistent with a model of subduction-driven back-arc spreading and convection in the mantle wedge causing trench-normal fast orientations in the wedge and aligned faults and cracks in the subducting Pacific slab causing trench-parallel fast orientations in the slab. When an S wave travels through the area with the multilayer orthogonal anisotropies, some of its splitting would be cancelled and thus small δt is observed.