Fig. 2

Surface geology and depth-migrated section. a Surface geology. Hmr: Hidaka metamorphic rocks; Po: Poroshiri Ophiolite; Iz: Idon’nappu zone; Sr: Sorachi Group; Yz: Yezo Group; Kz: Kamuikotan zone; NIz: Nappe of Iz; PN: Poronai Group; M: Miocene strata; HMT: Hidaka Main Thrust; NOF: Nitarachi-Oshorobetsu fault; KGF: Kenomai-Gabari fault. b Depth-migrated section from 1998–2000 seismic reflection data using CRS/MDRS method (see the text for detailed explanation). Yellow allows show eastward dipping events beneath the HB. This domain is traced down to a 20–25-km depth. At depths of 25–40 km beneath the HMT, several uncoherent events are recognized (light blue arrows). West-verging events in the uppermost 15-km crust west of the HMT are also noticed (red arrows). A white arrow is the uppermost event dipping to the west. c Depth-migrated section from 1998–2000 seismic reflection data with our interpretations (red lines). “A” indicates the deeper image of the HMT. Uncoherent events around the bottom of the HMT are enclosed by a broken ellipse (“B”). West-verging events in the uppermost crust are indicated by “C”. Deep structures in the NE Japan Arc are shown by green lines (“D”). Red broken lines (“R1” to “R3”) are structural boundaries interpreted with aid of the velocity structure model from the refraction/wide-angle reflection data (see Fig. 5b)