4 Discussion
Our results show that: (1) the foreshocks of the 2017 M5.3 Kagoshima Bay earthquake sequence occurred on a single plane with a steep dip to the east, whereas aftershocks occurred on several more complex planar structures, (2) the foreshock hypocenters formed a seismic gap, and (3) the foreshock and aftershock hypocenters exhibit clear migration behaviors. In this section, we integrated these observations and propose a simple model that can explain the occurrence of the foreshock–mainshock–aftershock sequence of the 2017 M5.3 Kagoshima Bay earthquake based on the upward fluid movement, which is similar to the fault-valve model proposed by Sibson (1992).