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

[SEM14-P10] Re-analysis of the time-series data obtained by the Network-MT survey in the Kii Peninsula, southwestern Japan, for constructing the 3-D electrical resistivity structure

*Akira Watanabe¹, Makoto Uyeshima¹, Satoru Yamaguchi², Yoshiya Usui¹, Hideki Murakami³, Tsutomu Ogawa¹, Naoto Oshiman⁴, Ryokei Yoshimura⁴, Koki Aizawa⁵, Ichiro Shiozaki⁶, Takafumi Kasaya⁷ (1.Earthquake Research Institute, The University of Tokyo, 2. Graduate School of Science, Osaka City University, 3.Kochi University, 4.Disaster Prevention Research Institute, Kyoto University, 5.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 6.Graduate School of Engineering, Tottori University, 7.Japan Agency for Marine-Earth Science and Technology)

Keywords:MT method, Network-MT method, electrical resistivity , Kii Peninsula, time-series analysis

The Kii Peninsula in the forearc region of southwest Japan has distinct structural and tectonic features characterized by the subducting Philippine Sea slab, high seismicity in the crust, Deep Low-frequency Tremors (DLTs), high surface heat flow, and high-temperature hot springs. Therefore, various geophysical surveys have been carried out on the Peninsula, including electromagnetic surveys. Some conventional MT surveys (New Energy and Industrial Technology Development Organization, 1994; Fuji-ta et al., 1997; Umeda et al., 2003; Kinoshita et al., 2018) and the Network-MT (NMT) survey (Yamaguchi et al., 2018) have been performed. The NMT method (Uyeshima et al., 2001; Uyeshima, 2007) is characterized by employing a commercial telephone network to measure voltage differences with long dipole lengths ranging from 10 to several tens of kilometers. This method has three advantages; the first is wide spatial-coverage (e.g., covering almost the entire Kii Peninsula), the second is wide-period range, especially for the longer period (from 10 s to 50000 s), and the third is better quality data in terms of high S/N ratio and less susceptibility to static effects. Yamaguchi et al. (2009) deployed 55 Nets with 3-5 electrodes for each Net and measured voltage differences at 10 s intervals for 50 – 400 days for each Net. Magnetic fields were also measured at three stations in the survey area. Using this data, Yamaguchi et al. (2009) showed a 2-D resistivity structure along a line crossing the central part of the Kii Peninsula. However, a 3-D model analysis is necessary to reveal the regional and deep structure of this region because the coastline and bathymetry are 3-D, and the strike of the igneous rocks (the Kumano acidic rocks) is not concordant with the direction of the subducting Philippine Sea slab.
Prior to determining the 3-D model, we reanalyzed whole NMT data obtained by Yamaguchi et al. (2009) using the BIRRP code(Chave and Thomson, 2004) under many different conditions, such as measurement time, data length, combination of electrodes, and intensity of the magnetic field disturbance.
In this presentation, we show how the respective conditions above improved the MT response functions. In addition, we show the spatial distribution of the best MT response functions in the Kii Peninsula.