Fig. 10

Comparison of surface-to-borehole spectral ratios of vertical component acceleration recordings (V-to-V) at site IBUH01 during weak-motions (mean of the weak-motions, black line), mainshock (red line), and an aftershock (blue line). The gray lines denote the range of plus–minus one standard deviation from the mean of weak-motion spectral ratios. The aftershock (Mj = 5.4, focal depth = 40 km, epicentral distance = 24.3 km, back azimuth = 142.6°) occurred on September 6, 2018, at 06:11 JST, about 3 h later than the mainshock