[S-CG53] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG53-P16] Electrical resistivity structure around the long-term Slow Slip Events beneath the Bungo Channel region, southwest Japan, by three-dimensional wideband magnetotelluric inversion

*Ryokei Yoshimura¹, Ken'ichi Yamazaki¹, Yasuo Ogawa², Jun Nakagawa¹, Shingo Kawasaki¹, Shintaro Komatsu¹, Itaru Yoneda¹, Yuhei Ouchi³, Tomohisa Okazaki³, Atsushi Suzuki², Zenshiro Saito², Yoshiya Usui², Koki Aizawa⁴, Mitsuru Utsugi³, Masahiro Teraishi¹ (1.Disaster Prevention Research Institute, Kyoto University, 2.Tokyo Institute of Technology, 3.Kyoto University, 4.Kyushu University)

Keywords:Resistivity Structure, Slow Slip Event, Bungo Channel

Recent geodetic observations detect recurrent slow slip events (SSEs), which occurred beneath the Bungo Channel and southwest Shikoku Island, with interval of approximately 6 years (e.g. GSI, 2010). To reveal a large-scale three-dimensional resistivity structure around this SSEs region, we carried out wideband magnetotelluric (MT) surveys around the western part of Shikoku Island. As of June, 2016, MT surveys were performed at 31 sites by using Phoenix wideband MT instruments. In the most of sites, high quality MT responses were estimated using the BIRRP code (Chave and Thomson, 2004) for the period range 300 Hz to 10,000 sec (Yoshimura et al., 2016). In addition, we used 8 more MT and telluric data obtained for different purposes; 6 sites from the opposite side of the Bungo Channel, namely the eastern part of Kyushu Island, measured by Metronix ADU and NT System Design ELOG systems (Aizawa et al., 2017) and 2 sites from the region of the central part of SSEs measured by Phoenix MTU system (Okazaki et al., 2017). These additional data were also reprocessed by the BIRRP code. In this study, we totally used 38 sites for a three-dimensional inversion.

Using obtained MT responses, we constructed a three-dimensional resistivity model around the SSE region. We inverted the impedance tensor and the vertical magnetic transfer function by the “femtic” inversion code developed by Usui (2015). The “femtic” inversion code employs the edge-based finite element method for unstructured tetrahedral elements and estimates the subsurface resistivity structure and the distortion tensor for each observation site. The main features of the obtained three-dimensional model are 1) a moderate conductive zone in the central part of SSEs 2) whose trenchward extension shows more conductive and 3) conductive zone surrounding the SSEs regions. These results suggest that the lateral electrical heterogeneity could have controlled the slip distributions of SSEs along the upper boundary of the Philippine Sea slab.