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.