Fig. 3

Distribution of initial (gray circles) and relocated hypocenters (red circles) in the present study in the period of September 6 to October 1 (3782 events). a Map view, b E–W strike, and c N–S strike vertical cross-sections. The initial and relocated mainshock hypocenters are indicated by gray and yellow stars, respectively. In a, the green line and the thick gray lines, respectively, denote the Ishikari Lowland eastern edge fault system (Ishikari-teichi-toen fault zone) and active faults. In a, the thin green dashed line denotes the Atsuma fault by Matsuno and Ishida (1960). The thin gray lines denote the Kamui-Kotan metamorphic belt. In b, the green triangle indicates the location of the Ishikari-teichi-toen fault zone