Fig. 4

Seismic-wave velocity structure and frequency–magnitude distribution. a Horizontal map of the P-wave velocity structure at 10 km depth, which was imaged by Matsubara et al. (2020). Black and yellow stars, blue circles, and gray lines are the same as in Fig. 1b. The black circles are the same as the red circles in Fig. 1b. b Horizontal map of the P-wave velocity structure at 15 km depth (Matsubara et al. 2020). See a for other details. c Cumulative frequency–magnitude distributions of the earthquakes that were recorded from 1998 and which occurred in the corresponding colored rectangular regions in a. The \(b\)-value is calculated using the formula of binned magnitude (Tinti and Mulargia 1987; Marzocchi and Sandri 2003). We define M1.9 as the cutoff magnitude considering the goodness of fit (Wiemer and Wyss 2000) and the temporary deficiency of aftershocks immediately after the 2019 Yamagata-Oki earthquake (Additional file 1: Fig. S1b). The uncertainty is the standard deviation estimated using the bootstrapping method