Seismic anisotropy beneath the Japan subduction zone 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. In this study we attempt to better characterize the geometry and strength of anisotropy in the mantle wedge beneath northern Honshu and Hokkaido, which overlie the subducting Pacific plate. Our previous shear wave splitting measurements indicate that anisotropy in the mantle wedge beneath the Japan subduction zone is highly complex, with splitting patterns that exhibit both dramatic spatial variations and a strong dependence on frequency (Wirth and Long, 2010). In order to complement the splitting data set and characterize these lateral variations in anisotropy more completely, we are currently analyzing teleseismic receiver functions (RFs) from a subset of broadband F-net stations in northern Honshu and Hokkaido using the multitaper correlation receiver function estimator (Park and Levin, 2000). Observations of coherent P-to-SV converted energy at time delays of ~8 sec are consistent with conversions originating at the top of the slab. We also observe significant spatial variations in the character of transverse RFs which are consistent with lateral variations in structure. In northern Honshu, P-to-SH converted energy originating from sharp gradients in structure just above the subducting slab show a strong sin(2θ) amplitude pattern with backazimuth, providing clear evidence for the presence of anisotropy. Based on our receiver function analysis and in conjunction with forward modeling of synthetic seismograms, we are currently developing models for the depths, thicknesses, and strengths of the anisotropic layers in the mantle wedge beneath northern Honshu.