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

[SEM14-04] Two-dimensional electrical resistivity structure beneath the northern Kanto region

*Yoshiya Usui¹, Makoto Uyeshima¹, Shin'ya Sakanaka², Takao Koyama¹, Masahiro Ichiki³, Yusuke Yamaya⁴ (1.Earthquake Research Institute, the University of Tokyo, 2.Faculty of Engineering and Resource Science, Akita University, 3.Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, 4.Renewable Energy Research Center, Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology)

Keywords:electrical resistivity structure, fluid distribution, magnetotelluric method, Kanto region, subduction zone, magma supply system

The tectonics of the Kanto region in central Japan is dominated by the dual subduction of the Philippine Sea (PHS) slab and the Pacific (PAC) slab. The PHS slab subducts under the continental Okhotsk plate, and the PAC subducts below the PHS and the continental plates. It is thought that the interaction between the two slabs plays an important role in the magma generation and the seismogenic process under the region. Geochemical study based on isotopic ratios of volcanic rocks (Nakamura et al. 2018) suggested that aseismic PHS slab extends to the north of seismically determined slab edge and perturbs mantle flow, enhancing the flux of the slab-derived fluid to the northern margin of the Kanto region. In order to elucidate the transport of the slab-derived fluid and its relationship with volcanic and seismic activities, it is important to reveal the subsurface fluid distribution by conducting an electromagnetic induction survey that delineates the subsurface electrical resistivity structure. Since the electrical resistivity of subsurface 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. Therefore, the authors performed a magnetotelluric survey around the northern margin of the Kanto region. In the central part of the survey line, there are active volcanos, Takaharayama and Hiuchigatake. The volcanic rocks of Takaharayama show a high contribution of the fluid originating from both the PHS and PAC slabs (Nakamura et al. 2018). After we determined the impedance tensor, the vertical magnetic transfer function, and the inter-station horizontal magnetic transfer functions, we estimated the two-dimensional resistivity structure. In this study, we show the resultant resistivity structure and discuss the subsurface fluid distribution as well as its relationship with the volcanic and seismic activities around the study area.