[S-IT21] Innovation through the Integration of Solid Earth Science and Materials Science

[SIT21-P02] Waveform inversion for localized 3-D transversely isotropic structure in the lowermost mantle beneath the western Pacific using Thai Seismic Array (TSAR) data

*Yuki Suzuki¹, Kenji Kawai², Robert J. Geller², Satoru Tanaka³, Weerachai Siripunvaraporn⁴, Songkhun Boonchaisuku⁴, Noisagool Sutthipong⁴, Yasushi Ishihara³, Taweoon Kim³ (1.Department of Earth and Planetary Science School of Science Tokyo Institute of Technology, 2.Department of Earth and Planetary Science School of Science University of Tokyo, 3.Japan Agency for Marine-Earth Science and Technology, 4.Faculty of Science, Mahidol University)

Keywords:Lowermost mantle, Anisotropy, Waveform inversion, Pacific LLSVP, Plume cluster, D″ region

Previous tomographic studies found a large-scale low S-velocity province (LLSVP) in the lowermost mantle beneath the Pacific. However, due to a lack of resolution it remained unclear whether the LLSVP consists of clusters of small-scale low-velocity anomalies or large-scale anomalies. In a previous study (Suzuki et al., PEPI, 2020) we inverted a new dataset from the Thai Seismic Array (TSAR; Tanaka et al., BERI, 2019) to infer the isotropic 3-D S-velocity structure beneath the western Pacific. We found a high-velocity anomaly extending vertically to 400 km above the core-mantle boundary (CMB) beneath the Philippine Sea and small-scale low-velocity patches with a diameter of ~300 km at the CMB beneath New Guinea. We hypothesized that a vertically continuous high-velocity anomaly could be the sinking Izanagi paleoslab, which subducted ~200 Ma, and interpreted small-scale low-velocity anomalies as a plume cluster that forms at the western margin of the Pacific LLSVP.
To obtain further geodynamical information, in this study we conduct waveform inversion for transversely isotropic structure. We infer high-resolution 3-D models of the S-velocity perturbation δVs and the anisotropy parameter δξ in the lowermost 400 km of the mantle. The inferred anisotropic structure is due to the deformation-induced alignment of the crystal caused by mantle flow for either Mg-perovskite, Mg-post-perovskite, ferropericlase, or a combination thereof because of the high-stress condition in the boundary layer of the mantle convection. When we assume the dominant glide system of each mineral under the lowermost mantle conditions given by previous experimental results, the inferred anisotropic structure shows that vertical flow (upwelling plumes and downwelling of paleoslabs) is dominant in the lowermost mantle beneath the western Pacific.