Fig. 1

a Tectonic setting of Hokkaido (modified after Kita et al. (2012), originally from Iwasaki et al. (2004), and Kimura (1994), and the Geological Survey of Japan 1992). The main plot is an enlarged image of the squared area in the inset map. The pink zone indicates the Kuril forearc. Dotted lines indicate tectonic boundaries. Thick lines with triangles denote Quaternary and Neogene thrust faults (green: Ishikari-teichi-toen fault zone; thin blue: Yubari-dake fault (Ito 2000); red: Hidaka main thrust; blue: Tokachi Plain faults: purple: Urahoro fault; black: other thrust faults). Green- and gray-shaded zones indicate the Hidaka metamorphic belt and the Kamuikotan metamorphic belt, respectively. Yellow stars indicate relocated hypocenters in the present study. Black stars indicate the hypocenters of the 1970 Hidaka and 1982 Urakawa-oki earthquakes. Triangles indicate sites of active volcanoes. The blue dashed line shows the location of the contact zone of the slab upper surface (Kita et al. 2010) with the anomalously deepened crustal materials. b Schematic diagram of the 2018 M6.7 event and the structure beneath south-central Hokkaido. c Schematic diagram of large inland events and the structure of the collision between the Kuril and NE Japan forearcs beneath the Hidaka collision zone