Fig. 1

a Topographic map of Hokkaido Island. Elevation data are from the ETOPO1 global relief model (Amante and Eakins 2009). The red rectangle marks the study area. The locations of the Sorachi-Yezo belt (bounded east and west by dashed lines) and Kamuikotan zone (black hatched area) are from Ueda (2016). b Geological map of the study area based on that compiled by the Japanese Institute of Advanced Industrial Science and Technology (AIST 2020). Solid black lines are active faults (Sakai and Matsuoka 2019). The light blue dots are hypocenters (M_JMA > 0) compiled by the Japan Meteorological Agency (2019). Diamonds are MT recording sites; colors indicate the west to east zoning of the sites: red, western area; black (area of serpentinite outcrop) and green, central area; blue, eastern area. The cyan circle marks the borehole site of Ogura and Kamon (1992) and the cyan rectangle marks the location of the borehole and resistivity survey of Okazaki et al. (2011). The red dashed rectangle encloses the rupture zone of the SSE that occurred between July 2012 and January 2013 (Ohzono et al. 2015)