Understanding Backazimuth Dependence of Shear Wave Splitting Patterns of S Waves Transmitted Through Strongly Anisotropic Slabs

Recent studies have proposed strong evidence of high seismic intra-slab anisotropy (~25% shear anisotropy) in the vicinity of deep earthquakes (focal depth > 60km) to explain the cause of the observed apparent non-double-couple radiation patterns. If this hypothesis is true, a dipping slab containing strong anisotropy also predicts that transmitted shear wave splitting (SWS) patterns should depend on incident angles and backazimuths. Traditionally, the contribution of the intra-slab anisotropy was ignored in interpreting SWS results because the path length in the slab is relatively short compared to the entire ray path. However, if the intra-slab shear anisotropy is strong, it may contribute significantly to the SWS. After analyzing teleseismic waveforms of around 600 events recorded by ~600 Hi-Net stations in Japan, we obtained >6,000 high-quality SWS measurements for S, ScS and SKS phases. We investigated SWS patterns using the earthquake focal depth, event backazimuth and modeling. We found that the delay time between the fast and slow S waves can vary significantly from about 0 s to ~3 s for different earthquakes, even for the same recording station. The measured fast S polarization direction and the delay time have a complex but systematic relation with respect to the source location and focal depth, likely due to anisotropy present at the source and weakening with depth. We observed a spatial variation of SWS across Japan even for the same event, thus illustrating the influence of the slab on SWS that cannot be accounted for by source-side anisotropy. To explain the observations, we perform seismic wave modeling using the propagator matrix method and show that a 20km anisotropic layer in the slab can cause delay times on the order of ~1.0 s and a systematic rotation of the fast S polarization axis. These results indicate that for a dipping anisotropic slab, the SWS patterns from teleseismic earthquakes may depend on source location (and depth), the slab anisotropy, and the angle of incidence with which the wave hits the slab. Our results can provide important constraints for the slabs in which deep earthquakes are generated.