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Influence of a Dipping Anisotropic Slab on Shear Wave Splitting in Japan

Description: 

Strong intra-slab seismic anisotropic fabric of laminated type (~25% shear anisotropy) within the vicinity of deep subduction earthquakes (focal depth > 60 km) was proposed to explain the cause of the large non-double-couple radiation patterns. If it is true, a predicted consequence is that the dipping anisotropic slab should produce shear wave splitting (SWS). Due to the dipping geometry, the measured splitting parameters (delay time and polarization directions) should depend on incident angles of the S-wave ray paths with respect to the slab. We evaluate this prediction by analyzing teleseismic waveforms of around ~600 global earthquakes recorded by a dense network coverage (>800 Hi-net stations) in Japan. After acquiring >8,000 high-quality SWS measurements for various S phases (S, ScS and SKS), we explore SWS patterns using the event backazimuth, focal depth, and forward seismic modeling. Our measured results indicate that the delay time between the fast and slow shear waves vary significantly up to ~3 s but also with numerous null measurements (~950 observations). In addition, the measured fast S polarization direction has a complex but systematic relation with respect to the source location and depth, likely due to source-side anisotropy weakening with event depth. We also notice a spatial variation of the measured SWS patterns across Japan for the same earthquake, showing the influence of Pacific and Philippine Sea slabs on SWS which cannot be attributed to source-side anisotropy. Finally, we perform seismic forward modeling using the propagator matrix method to explain these measurements. We find that a 20km thick anisotropic layer in the slab can cause similar delay times and a systematic rotation of the fast S polarization axis. These outcomes show that in teleseismic SWS analyses, we need to consider the dipping anisotropic slab, not just the sub/supra-slab anisotropy and earthquake source side anisotropy. Our approaches and analyses provide new tools for understanding slab anisotropic structure, important for deep earthquake generation and recycling of volatiles back into the Earth mantle.

Session: Structure and Properties of Subducting Slabs and Deep Earthquakes

Type: Oral

Date: 4/19/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Sharmila Appini

Student Presenter: Yes

Invited Presentation: 

Authors