Fig. 1

Relocated aftershock distribution and the assumed fault model. Black circles represent the hypocentral locations of the aftershocks. Inset: tectonic setting of Hokkaido corner region. The red arrows and the dotted line indicate the direction of the relative movement of Kuril arc against the northern Japan island ark and the inferred border of them (Kita et al. 2012), respectively. a The map view of aftershock distribution. The red star indicates the observed mainshock epicenter. The red lines show the cross-sectional projection of the assumed fault at a depth of 34 km. The orange line shows the branch fault (Segment N) added in Case B. b The cross-sectional views for the sections i–v; the red and orange lines show the cross-sectional projection of the assumed fault surfaces on the middle of each section, respectively. The red star indicates the locations of the observed hypocenter and the assumed hypocenter in Case B, and the yellow star indicates the assumed nucleation location in Case A without the branch fault