Fig. 3
a Black curve represents the time-lapse change in the estimated 4- to 20-Hz energy release rate from the origin time of the April 14 M JMA6.5 event to 10 days after the origin of the April 16 M JMA7.3 event. The red line represents the threshold energy release rate (4.1 × 103 J/s) above which the amplitude of the observed envelope is considered to be mostly excited by earthquakes. b Zoomed energy release rate around the origin times (0 s) of the April 14 (black) and the April 16 (red) events. The secondary peak of the April 16 event that appeared at 34 s corresponds to the earthquake triggered in Oita prefecture