Frequency-dependent shear wave splitting beneath the Japan and Izu-Bonin subduction zones

Abstract

Despite its importance for our understanding of physical processes associated with subduction, the geometry of mantle flow in subduction zones remains poorly understood, particularly in the mantle wedge above subducting slabs. Constraints on mantle flow and deformation can be obtained by measurements of shear wave splitting, a valuable tool used to characterize the geometry and strength of seismic anisotropy. A complete characterization of shear wave splitting is particularly important for understanding the mantle wedge beneath Japan, which overlies multiple subduction zones with complex slab morphologies; previous studies indicate that the upper mantle beneath Japan exhibits complicated anisotropy that manifests itself in complex splitting patterns. To characterize better the geometry of mantle anisotropy beneath Japan, we analyzed direct S waves from local earthquakes originating in the subducting slabs for evidence of shear wave splitting using data from 54 broadband seismic stations in Japan's F-net array. In addition, both local S and teleseismic SKS phases were examined using data from four F-net stations in the Izu-Bonin arc. In order to characterize any frequency dependence of splitting parameters that may indicate the presence of complex anisotropy, we carried out our splitting analysis in two different frequency bands (0.02–0.125 Hz and 0.125–0.5 Hz). Our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath Japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency.