[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-P19] A crustal electrical resistivity model beneath Aizu-Yonezawa-Azuma district estimated from a broadband magnetotelluric observation network over southern Tohoku, NE Japan

*Naoyuki Wakao¹, Aoi Motoyama¹, Masahiro Ichiki¹, Yasuo Ogawa², Makoto Uyeshima³, Koichi Asamori⁴, Toshihiro Uchida⁵, Hideaki Hase⁶, Takao Koyama³, Shin'ya Sakanaka⁷, Yusuke Yamaya⁵, Koki Aizawa⁸, Songkhun Boonchaisuk⁹, Toshiki Kaida¹ (1.Graduate school of science, Tohoku university, 2.Institute of innovative research, Tokyo institute of technology, 3.Earthquake research institute, The university of Tokyo, 4.Japan atomic energy agency, 5.National institute of advanced industrial science and technology, 6.Geothermal energy research & development Co., Ltd, 7.Graduate school of international resource sciences, Akita university, 8.Graduate school of science, Kyusyu university, 9.Mahidol university, Thailand)

Aizu-Yonezawa area, NE Japan is where the seismic swarms turned active after the 2011 Tohoku-Oki Great Earthquake in the upper crust, and hypocenter migration has been confirmed in this swarm area (Okada et al., 2015). Moreover, deep low-frequency earthquakes (DLFEs) occur in the lower crust to the uppermost mantle (20-40 km depth) beneath Azuma Volcano. These results indicate fluid reservoirs and transportation in the crust beneath this area. We integrated broadband MT data observed in the southern Tohoku district to model the crustal resistivity structure (e.g. Motoyama et al., 2020 JpGU; Wakao et al., 2023 JpGU). However, the integrated data set had a spatial observation gap from Aizu-Yonezawa district to Azuma Volcano. The resistivity model estimated from the data set showed a high resistivity of about 1000 Ωm in the swarm area, while the model indicated a conductive body at the depths of 15-40 km below sea level (bsl), the boundary of which the DLFEs occurred (Motoyama et al., 2020). In this study, we added new data observed at 18 broadband MT stations around Azuma Volcano (Ichiki et al., 2021) and remodeled the crustal electrical resistivity model beneath the southern Tohoku district using the inversion code with tetrahedral element finite element method (Usui, 2015). A checkerboard resolution test shows the addition of the new stations substantially improved the reproducibility of the checkerbox boundary, in particular, at the depth of 20 to 40 km bsl. (Wakao et al., 2023 JpGU). The updated electrical resistivity model indicates the swarms are located at the boundary of the resistive and conductive zone, and a minimal resistivity of the deep conductive zone is about 5 Ωm. Furthermore, the S-wave reflectors are observed just above the conductive zone at the depths of 5-20 km (Suzuki, 2018 Master Thesis, Tohoku University), which suggests dehydrated fluid from the deep magma reservoir moves to the shallow, and the swarms occur at the boundary of the resistive and conductive zone. The updated resistivity model reveals continuous vertical conductor from the lower to upper crust beneath Azuma Volcano, and the conductor probably connects the conductive zone beneath Aizu-Yonezawa area. The vertically continuous conductor beneath Azuma Volcano was not detected by Ichiki et al. (2021). We are underestimating the confidence interval of the resistivity value of the conductive body in the lower crust and plan to estimate water content using the confidence interval.