Fig. 4

Aftershock activity (M ≥ 1.5) of the main shock (iii, 6-Sep-2018) and the ETAS estimate. The ETAS model is fitted during the target interval from 0.1 day after the M6.7 main shock (iii) (2nd vertical dashed line from the left) until 23.4 elapsed days (1-Oct-2018; 3rd vertical dashed line). The estimation period and estimated parameters are printed in black. The background rate of the aftershock activity is fixed (μ = 0.228). This value is the MLE of the ETAS μ in the interval before the M6.7 earthquake (iii); this fit is plotted by thick dashed red curve, with the estimation period and estimated parameters printed in gray. Theoretical cumulative curves were then extrapolated to the last period. The empirical cumulative curve (black) is superimposed by estimated and extrapolated curve (red). The blue circles show the depths of the hypocenter against transformed time. See Additional file 1: Table S4 for the fitting configurations and results of model comparisons