[S-EM14] Electric, magnetic and electromagnetic survey technologies and scientific achievements

[SEM14-P16] A regional crustal electrical resistivity structure analysis beneath southern Tohoku district, 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, Toshiki Kaida¹ (1.TOHOKU UNIVERSITY, 2.Volcanic Fluid Research Center, School of Science, 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 Department of Earth Resource Engineering and Environmental Science, Akita University, 8.Graduate school of science, Kyusyu university)

We have integrated broadband MT data observed in the southern Tohoku district, NE Japan to analyze the crustal electrical resistivity structure (Motoyama et al., 2019 JpGU; 2020 JpGU). However, the integrated data set has a spatial observation gap near the Fukushima-Yamagata prefectural border. The area beneath this gap region is where the seismic swarms turned active after the 2011 Tohoku-Oki Great Earthquake (Okada et al., 2015). An analysis of S-wave reflectors beneath the gap region reveals the S-wave reflectors at a depth of 5-15 km below sea level (bsl), which overlap with the hypocenters of the seismic swarms (Suzuki, 2018 Master Thesis, Tohoku University). This indicates the presence of fluids in the hypocentral area. However, the resistivity model estimated by Motoyama et al shows an average resistivity of 100-1000 Ωm in the hypocentral area, which does not indicate the presence of fluids. On the other hand, the resistivity model of the lower crust to uppermost mantle (15-40km depth bsl) beneath the observation gap region shows a prominent low resistivity body, at the boundary of which deep low-frequency earthquakes (DLFEs) occur. In this study, we added new 18 stations of broadband MT data, which were acquired to explore the subsurface resistivity structure beneath Azumayama volcano (Ichiki et al., 2021). After the assessment of the resolution improvement by adding the new observation data using the checkerboard resolution tests, we reanalyzed the crustal electrical resistivity structure beneath the southern Tohoku district.
We first checked the phase tensor characteristics. Square root of the phase tensor determinant, Φ_2, at a period of about 10 s suggests being resistive beneath the northeast side of Azumayama volcano and being conductive beneath the southwest side.
We tested two types of checkerboard models; a shallow checkerboard model consisting of a low resistivity block of 10 km per side placed at a depth of 5 km, and a deep checkerboard model consisting of a low resistivity block of 20 km thickness and 40 km per horizontal side placed at a depth of 20 km. The shallow checkerboard model result showed that the resolution of the swarm hypocentral area was not significantly improved, because there was no new observation site right above the hypocentral area. The deep checkerboard model result showed that the addition of the new observation sites improved the reproducibility of the checkerboard boundaries. Thus, we can expect a more reliable discussion of the relationship between the location of the lower crust conductor and the DLFE hypocenters. In the presentation, we will show the reanalyzed three-dimensional resistivity model using the inversion with tetrahedral element finite element method (Usui, 2015).