Fig. 1

Hypocenter map in the Eastern Iburi region with M ≥ 1.5. The three panels show a epicenters, b longitude vs. depth location, c depth vs. longitude, and d Hokkaido map with the Eastern Iburi region and a source model of the M8.0 Tokachi-Oki earthquake (i, 26-Sep-2003) that is listed in Table 1. Gray dots indicate the aftershocks of the M6.7 main shock (iii, 6-Sep-2018) up until March 16, 2019. Black dots indicate the micro-earthquakes before the main shock. Red circles indicate the micro-earthquakes that occurred in between events (ii, 1-Jul-2017) and (iii). Pink circles indicate the micro-earthquakes that occurred 24 days after the main shock (iii), over 4 days. Blue circles indicate the earthquakes after the M5.8 largest aftershock (iv, 21-Feb-2019). The triangle indicates the hypocenter of the main shock (iii). The sizes of disks reflect magnitudes. See Table 1 for the hypocenter coordinates and mechanisms of the cited earthquakes