[S-SS07] Crustal Structure

[SSS07-P03] Three-dimensional resistivity modeling in the eastern Iburi district, Hokkaido

*Ryo Ito¹, Takeshi Hashimoto¹, Tomohiro Inoue¹, Makoto Uyeshima², Hiroshi Ichihara³, Yusuke Yamaya⁴ (1.Graduate School of Science, Hokkaido University, 2.Earthquake Research Institute, The University of Tokyo, 3.Graduate school of environmental studies, Nagoya University, 4.National Institute of Advanced Industrial Science and Technology)

A magnetotelluric (MT) survey was carried out in October to November in 2018 at 14 sites around the epicenter of the 2018 Hokkaido Eastern Iburi earthquake (September 6, 2018: Mw 6.6). We inverted full components of the MT impedance and tippers to estimate a three-dimensional (3-D) electrical resistivity model of this area by using the BIRRP (Chave and Thomson, 2004) and ModEM programs (Egbert & Kelbert, 2012). The main shock was located at a depth of nearly 40 km, and its aftershocks were concentrated in a depth range from 20 to 40 km. It is important to investigate the subsurface structure of the seismogenic zone of such an unusually deep intraplate earthquake. Previous studies have suggested that the low resistivity regions in the mid-crust under active faults are saturated with aqueous fluid, and that pore water pressure increases as the fluid migrates to the surroundings, driving earthquakes (e.g., Ogawa et al., 2001). A distinct low resistivity body was also found by Yamaya et al. (2017) at the depth of Ishikari-teichi-toen fault zone (ITFZ), the western neighbor of the target area of our study. They presumed that the pore space created by ductile deformation of the lower part of the thick sediment layer was saturated with fluid. In our 3-D model a low-resistivity layer of about 10 Ωm was found at a depth from the surface down to c.a. 10 km. Similar conductive thick sediments were reported in the previous studies that dealt with the MT data across the Hidaka collision zone (Ichihara et al., 2019), and that investigated the ITFZ (Yamaya et al., 2017). On the other hand, our 3-D model at present is not well constrained for the deeper part including the hypocenter region. In the next step, we are going to incorporate the data from the previous studies into our inversion model to better image an electrical structure in the deeper part.

Acknowledgments: M. Takada, K. Saito (HU), and M. Masuda (ERI, UT) contributed to the fieldwork and data acquisition in 2018. This study was supported by ERI JURP 2018-F2-04 for the MT instruments, and financially supported by MEXT KAKENHI Grant-in-Aid for Special Purposes #18K19952.