[S-CG52] Dynamics in mobile belts

[SCG52-08] Three-dimensional electrical resistivity structure beneath the back-arc side of the southern Tohoku region

*Yoshiya Usui¹, Makoto Uyeshima¹, Hideaki Hase², Hiroshi Ichihara³, Koki Aizawa⁴, Takao Koyama¹, Shin'ya Sakanaka⁵, Tsutomu Ogawa¹, Yusuke Yamaya⁶, Tadashi Nishitani⁵, Koichi Asamori⁷, Yasuo Ogawa⁸, Ryokei Yoshimura⁹, Shinichi Takakura¹⁰, Masaaki Mishina¹¹, Yuichi Morita¹² (1.Earthquake Research Institute, the University of Tokyo, 2.Geothermal Energy Research & Development Co., Ltd., 3.Graduate School of Environmental Studies, Nagoya University, 4.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 5.Faculty of Engineering and Resource Science, Akita University, 6.Renewable Energy Research Center, Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, 7.Tono Geoscience Center, Japan Atomic Energy Agency, 8.Volcanic Fluid Research Center, Tokyo Institute of Technology, 9.Disaster Prevention Research Institute, Kyoto University, 10.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 11.Graduate School of Science, Tohoku University, 12.Center for Integrated Volcano Research, National Research Institute for Earth Science and Disaster Resilience)

Keywords:electrical resistivity structure, crustal fluid, magnetotelluric method, magma supply system, subduction zone, northeastern japan

In the back-arc area of the southern Tohoku region, there are several quaternary volcanos. In the vicinity of some of the volcanoes, deep low-frequency earthquakes occur, implying the transfer of melt or aqueous fluid. So as to reveal the magma supply system, it is important to reveal the distribution of the subsurface fluid. In addition, in the area, there are a number of active faults that have the potential to cause M7-class inland earthquakes. Because it is thought that aqueous fluid has an important role in generating inland earthquakes, understanding the fluid distribution also helps elucidate the earthquake generation process. An effective approach for imaging the subsurface fluid distribution is conducting an electromagnetic induction survey which delineates the subsurface electrical resistivity structure. Since the electrical resistivity of fluid is generally lower than that of dry rocks by more than several orders of magnitude, the electrical resistivity is sensitive to the interconnected fluid in subsurface rocks. Thus, the authors estimated the resistivity structure in the back-arc area of the southern Tohoku region by performing magnetotelluric surveys. Our previous study (Usui et al. 2021) indicated the probable presence of the conductive areas under Chokaisan and Gassan, which reach the lower crust, and suggested the possibility that the bottom of the seismogenic layer is locationally consistent with the boundary between shallow resistive layer and deep conductive layer. In addition, Asamori et al. (2011) revealed a conductive area under the Asahi Mountains, containing the fluids that originated from the mantle. In this study, using both datasets of the previous two studies, the authors estimated the 3-D resistivity structure with more high resolution. In this study, we show the resultant electrical resistivity structure and discuss the subsurface fluid distribution as well as its relationship with the volcanic and seismic activities around the study area.