Multiple layers of seismic anisotropy and a low-velocity region in the mantle wedge beneath Japan: Evidence from teleseismic receiver functions

A complete characterization of seismic anisotropy can yield powerful constraints on mantle flow and deformation. This is particularly important for the mantle wedge above subducting slabs, where the geometry of mantle flow remains poorly understood. We seek to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northeast Honshu and Hokkaido, both of which overlie the subducting Pacific plate. Previous studies indicate that upper mantle anisotropy in the Japan subduction zone is highly complex and exhibits dramatic spatial variations. To provide complementary constraints on the along strike variations in anisotropy, we analyze teleseismic receiver functions from stations of the broad-band F-net array using the multitaper correlation receiver function estimator. Backazimuthal variations in P-to-SH converted energy provide clear evidence for complicated anisotropic structure in the mantle wedge beneath northeast Honshu and Hokkaido. In northeast Honshu, forward modeling of receiver functions using synthetic seismograms suggests the presence of an anisotropic layer in the forearc mantle wedge above the subducting slab and a second anisotropic layer beneath the crust of the overriding plate. We also see evidence for a region of low (isotropic) velocity in the central part of the wedge beneath NE Honshu. Comparisons between transverse component receiver functions at stations located in NE Honshu and Hokkaido reveal striking differences, providing further evidence for along strike variation in anisotropic structure in the mantle wedge beneath Japan.