Fig. 3

a Frequency distribution of the focal depths of earthquakes in the Wakayama swarm (red) and in the areas surrounding the Wakayama swarm (gray). The hypocenter distributions are shown in Fig. 2a. Average focal depths and their associated standard deviations are indicated by the circle and error bars, respectively. b Magnitude–frequency distribution of the earthquakes in the Wakayama swarm (red) and in the surrounding areas of the Wakayama swarm (gray). Estimated b values for the M ≥ 1 earthquakes are shown