[S-CG58] 岩石―流体相互作用の新展開：表層から沈み込み帯深部まで

[SCG58-06] 地震波異方性から見たスラブ内の含水亀裂と地震発生

★招待講演

*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：地震波異方性、スラブ内地震、含水クラック、地震波トモグラフィー

Water entering subduction zones is a crucial incentive for large earthquakes and arc magmas. Plate-bending associated normal faults in the outer-rise area before subduction are invaded by seawater and serpentinized to form hydrated faults and cracks. Subduction of the hydrated faults and cracks with the oceanic lithosphere can be an important transport route for seawater into the deep mantle. However, geophysical evidence for the existence of these hydrated faults and cracks beneath the forearc is still lacking. Recently we determined high-resolution P-wave anisotropic tomography of the Tohoku forearc by using a great number of arrival-time data of local earthquakes recorded at both the onshore Hi-net and offshore S-net seismic stations (Wang et al., 2022). Our updated tomographic method can determine both 3-D isotropic velocity heterogeneity and 3-D anisotropy with tilting symmetry axis (Wang & Zhao, 2021). Trench-parallel fast-velocity planes of anisotropy are revealed in the subducting Pacific slab, which intersect the slab upper surface at high angles (~45-90 degrees), reflecting aligned hydrated faults and cracks in the slab. Ruptures of the hydrated faults may cause large intraslab earthquakes. The hydrated-crack associated water entering a large near-trench asperity in the megathrust zone could have triggered the great 2011 Tohoku-oki earthquake (Mw 9.0). The 16 March 2022 Fukushima–oki earthquake (M7.4) is the largest one among the forearc intraslab events in Japan since 2000. We determined detailed 3-D images of P and S wave velocity and Poisson’s ratio in source zones of four large intraslab events (M ≥7.0) during 2003–2022 in NE Japan by inverting arrival-time data recorded at both onshore and offshore seismic stations (Zhao et al., 2022; Wang et al., 2023). Aftershocks of the intraslab earthquakes are mainly distributed in gaps of high-velocity bodies with high Poisson’s ratio and at the upper ~20 km depth of the subducting slab. Our results indicate that the large intraslab events were caused by rupturing of buried hydrated faults that formed at the outer rise and dehydration embrittlement on the fault planes.

References
Wang, Z., D. Zhao (2021). 3D anisotropic structure of the Japan subduction zone. Science Advances 7, eabc9620.
Wang, Z., D. Zhao, X. Chen (2022). Seismic anisotropy and intraslab hydrated faults beneath the NE Japan forearc. Geophys. Res. Lett. 49, e2021GL097266.
Wang, Z., D. Zhao, X. Chen (2023). Fine structure of the subducting slab and the 2022 M 7.4 Fukushima–oki intraslab earthquake. Seismol. Res. Lett. 94, 17–25.
Zhao, D., Y. Katayama, G. Toyokuni (2022). The Moho, slab and tomography of the East Japan forearc derived from seafloor S-net data. Tectonophysics 837, 229452.