Fig. 2

a Seismic stations used in this study. Temporary and permanent stations are shown in crosses and in squares, respectively. A red star indicates the epicenter of the main shock of the 2018 Hokkaido Eastern Iburi earthquake. A rectangle indicates an area shown in Figs. 3, 4, 5. Thin solid lines are active faults (Research Group for Active Faults in Japan 1991). b P wave velocity structures. A red line is based on Kasahara et al (1994) and a black line is based on Iwasaki et al (2004) and Katsumata et al (2006)