Radial and Azimuthal Anisotropy Tomography of the NE Japan Subduction Zone: Implications for the Pacific Slab and Mantle Wedge Dynamics

We investigate slab and mantle structure of the NE Japan subduction zone from P wave azimuthal and radial anisotropy using travel time tomography. Trench normal E-W-trending azimuthal anisotropy (AA) and radial anisotropy (RA) with VPV > VPH are found in the mantle wedge, which supports the existence of small-scale convection in the mantle wedge with flow-induced LPO of mantle minerals. In the subducting Pacific slab, trench parallel N-S-trending AA and RA with VPH > VPV are obtained. Considering the effect of dip of the subducting slab on apparent anisotropy, we suggest that both characteristics can be explained by the presence of laminar structure, in addition to AA frozen-in in the subducting plate prior to subduction. Plain Language Summary There is increasing importance and interest in seismic anisotropy because it can provide crucial constraints on the lithospheric structure as well as the nature of dynamics of mantle flow. In this study, we performed two types of anisotropic tomography analyses using the same data set and estimated three-dimensional P wave azimuthal and radial anisotropy structures beneath NE Japan. These tomography analyses show that mantle wedge of the subduction zone is characterized by E-W-trending azimuthal anisotropy and radial anisotropy with VPV > VPH. On the other hand, N-S-trending azimuthal anisotropy and radial anisotropy with VPH > VPV are shown in the Pacific slab. Assuming flow-induced lattice preferred orientation of mantle minerals, the observed mantle wedge anisotropy can be explained by 3-D mantle flow with small-scale convection. Also, we evaluated the effect of dip of the slab that is anisotropic and carefully considered resulting apparent radial and azimuthal anisotropies in our tomography. Then, the anisotropy in the Pacific slab can be explained by a combination of positive radial anisotropy due to laminar scatterers in the oceanic plate and azimuthal anisotropy frozen-in during the formation of an oceanic plate whose fast direction is parallel to the ancient spreading direction.