[S-CG61] Dynamics in mobile belts

[SCG61-P05] Revisiting electrical resistivity structure beneath the southern Tohoku district and estimating geofluid volume fraction from resistivity and P-wave velocity models

Naoyuki Wakao¹, *Masahiro Ichiki¹, Aoi Motoyama¹, Yasuo Ogawa^1,2, Makoto Uyeshima³, Kohichi Asamori⁴, Toshihiro Uchida⁵, Hideaki Hideaki^3,6, Takao Koyama³, Shin'ya Sakanaka⁷, Yusuke Yamaya^3,5, Koki Aizawa^3,8, Songhun Boonchaisuk⁹, Toshiki Kaida¹, Tomomi Okada¹ (1.Graduate School of Science, Tohoku University, 2.Institute of Integrated Research, Institute of Science Tokyo, 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 and Development Inc., 7.Graduate School of International Resource Science, Akita University, 8.Graduate School of Science, Kyushu University, 9.Mahidol University)

Keywords:electrical resistivity, geofluids, seismic swarms, seismic tomography, Fault-valve model

We integrated wideband magnetotelluric (MT) data observed in the southern Tohoku district to model the crustal resistivity structure (e.g., Motoyama et al., 2020 JpGU; Wakao et al., 2024 JpGU). The observation sites cover Aizu-Yonezawa area beneath which seismic swarms occurred after the 2011 Tohoku-Oki Earthquake (Okada et al., 2015) and Azuma Volcanoes. However, the subsurface resistivity model did not show alignments of conductors to the seismic swarms and active volcanoes. In this study, we added new data observed at 18 wideband MT stations around Azuma Volcanoes (Ichiki et al., 2021) and remodeled the crustal electrical resistivity distribution beneath the southern Tohoku district using the inversion code with tetrahedral element finite element method (Usui, 2015). To restrict the presence of fluids, we have developed an original inversion code to quantify the crustal geofluid volume fraction and salinity from electrical resistivity and seismic P-wave velocity models. The geofluid inversion code assumes the fluids do not contain melts and is formulated using the NaCl aq. resistivity experimental result (Sinmyo & Keppler, 2017).
The updated electrical resistivity model indicates the seismic swarms are located at the boundary between the high and low resistivity zones, and a low resistivity body is imaged in the lower crust beneath the boundary. However, the model shows no columnar low resistivity body connected to the seismic swarms, viz., no image of fluid pathways. We applied the geofluid inversion to the seismic swarm region's resistivity and seismic P-wave velocity. As depth goes more profound, the water content increases at a depth of 12 km and attains 3.8 vol.% at a depth of 19 km. The depth at which the water content increased corresponds to the lower boundary of the swarm earthquake region and the depth where S-wave reflection surfaces are concentrated, which is interpreted as the shallowest depth of the impermeable seal region in the Fault-valve model proposed by Sibson (1990; 2020).
A columnar low resistivity body lies in the upper crust beneath Azuma Volcanoes, and the conductor continuously and broadly extends westward and northward in the lower crust. The westward-extending low resistivity body is estimated to be composed of melt with a volume fraction of 1.8 – 6.8 vol.% and a water content of 8 wt%. Therefore, the seismic activity around this magma reservoir is interpreted to have been caused by the dehydration of large amounts of water from melts saturated with water and undergoing solidification.