Figure 1 . (a ) Map showing southern Kyushu. Inverted triangles indicate the seismic stations. We used arrival time data obtained at both blue and green stations. We analyzed waveform data recorded at the green stations. The black square shows the study area. (b ) Hypocenter distribution of earthquakes that occurred in Kagoshima Bay from January 1, 2003, to April 8, 2018, and their focal mechanisms. The hypocenters and focal mechanisms were extracted from the JMA unified catalog. The red square is defined as “the area surrounding the mainshock hypocenter” in this study. The numbers above the focal mechanisms indicate the JMA magnitude of each earthquake. (c ) M–T diagram and cumulative number of \(M_{\text{JMA}}\ \geq\ 1.0\)earthquakes that occurred in the area surrounding the mainshock hypocenter (i.e., red square in Fig. 1b) prior to the mainshock. The vertical red line denotes the mainshock. (d ) Aftershock occurrence rate of events with a magnitude\(M_{\text{JMA}}\ \geq 1.0\)(blue) and corresponding M–T diagram (gray). The inset shows the correlation between the aftershock occurrence rate and time on a log-log scale. The occurrence rate was estimated by calculating the reciprocal of the time during which 10 events with \(M_{\text{JMA}}\ \geq 1.0\)occurred.
In this study, we examined the physical processes that controlled the ML 5.3 Kagoshima Bay earthquake sequence in Kyushu, southern Japan. First, we determined the hypocenters and focal mechanisms of this earthquake sequence and delineated the fault structure. We also estimated the size of the mainshock and examined its relationship with the fore- and aftershocks to obtain a comprehensive view of this foreshock–mainshock–aftershock sequence. We then examined the spatiotemporal characteristics of the intense fore- and aftershocks to extract information about the aseismic phenomena governing this earthquake sequence. Finally, by integrating the observations, we established a model that can be used to explain the occurrence and characteristics of the foreshock–mainshock–aftershock sequence associated with the 2017 M5.3 Kagoshima Bay earthquake.