[S-CG50] 変動帯ダイナミクス

[SCG50-P01] 太平洋スラブとその周辺マントルの3次元地震波異方性

*趙 大鵬¹、Wang Zewei¹ (1.東北大学大学院理学研究科附属地震・噴火予知研究観測センター)

キーワード：沈み込み帯、異方性、地震波トモグラフィー

With the increasing number of seismic stations deployed and high-quality seismic data accumulated, body-wave anisotropic tomography has shown its power in revealing the 3-D anisotropic structure of subduction zones (e.g., Wang & Zhao, 2008, 2013; Zhao, 2015; Liu & Zhao, 2016, 2017; Ishise et al., 2018). To simplify the calculation, the previous studies often assume an anisotropy with a hexagonal symmetry for materials in the modeling space. This assumption is a good approximation for typical mantle minerals such as olivine. For a further simplification, the previous studies assumed the symmetry axis to be either horizontal or vertical for investigating azimuthal anisotropy or radial anisotropy. However, the assumption of horizontal or vertical symmetry axis is not reasonable at least in two cases that are common in subduction zones: (1) complex mantle flow with a flow direction neither parallel nor perpendicular to any of the modeling axes, and (2) intra-slab anisotropy affected by the geometric inclining of a subducting slab. Therefore, the horizontal or vertical symmetry axis assumption should be released for better revealing the intra-slab fabric alignment and the 3-D mantle flow field in subduction zones. Under a general hexagonal symmetry anisotropy assumption, we developed a new tomographic method (Wang & Zhao, 2021) to determine a high-resolution 3-D P-wave anisotropic model of the Japan subduction zone by inverting a great number of travel-time data of local and teleseismic events. As a result, the 3-D anisotropic structure in and around the dipping Pacific slab is firstly revealed. Significant anisotropy with a dipping symmetry axis also occurs in the mantle wedge and the sub-slab mantle associated with the slab subduction. Our results show that slab deformation plays an important role in both mantle flow and intra-slab fabric, and the widely observed trench-parallel anisotropy in the forearc from shear-wave splitting measurements is related to the intra-slab deformation during the outer-rise yielding of the subducting plate.

References
Ishise, M. et al. (2018). Geophys. Res. Lett. 45, 3923–3931.
Liu, X., D. Zhao (2016). J. Geophys. Res. 121, 5086-5115.
Liu, X., D. Zhao (2017). Earth Planet. Sci. Lett. 473, 33-43.
Wang, J., D. Zhao (2008). Phys. Earth Planet. Inter. 170, 115-133.
Wang, J., D. Zhao (2013). Geophys. J. Int. 193, 1166-1181.
Wang, Z.W., D. Zhao (2021). Science Advances 7, eabc9620.
Zhao, D. (2015). Multiscale seismic tomography. Springer, 304 pp.