Observational Study of Variable Shear Wave Splitting Patterns At West Pacific Subduction Zones



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In the recent past, strong evidence of high seismic intra-slab anisotropy (~25% shear anisotropy) is proposed in the vicinity of deep earthquakes (focal depth > 60km) to explain the cause of their observed apparent non-double-couple radiation patterns. It also predicts that shear wave splitting (SWS) patterns should depend on earthquake locations and backazimuths. In my thesis, I have focused on the measurement of shear wave splitting patterns in a West Pacific subduction zone to observe the spatial variation and dependence of backazimuth. 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 cannot be neglected. Specifically, I have worked on SWS measurements for teleseismic earthquakes covering a wide range of backazimuths, recorded by stations above subduction zone. After analyzing the seismic waveforms of around 600 teleseismic events recorded by 455 Hi-Net stations in Japan, I obtained >5,000 high quality SWS measurements for S, ScS and SKS phases. I investigated SWS patterns on 1) the earthquake focal depth, 2) event backazimuth, and 3) modeling. I found that the delay time between the fast and slow S waves can vary significantly from about 0 s to ~3 s, even for the same station. Majority of my splitting measurements show a trench-parallel or trench-perpendicular trend corresponding to Pacific and Philippine Sea slabs. However, the measured fast S polarization direction and the delay time have a complex relation with respect to the source location and focal depth, which is challenging to interpret. This might indicate that for a dipping anisotropic slab, the SWS patterns of the shear waves coming from teleseismic earthquakes are dependent on source location, the slab anisotropy, and the angle of incidence with which the wave hits the slab. Seismic wave modeling using propagator matrix method showed that a 20km anisotropic layer in the slab can cause 1.0 s in delay time and systematic rotation of the fast S polarization axis.



Shear wave splitting, Seismic Anisotropy, Deep Earthquakes, Subduction Zones