Determining fluid migration and pore pressure changes within the Earth is key to understanding earthquake occurrences. We investigated the spatiotemporal characteristics of intense fore- and aftershocks of the 2017 ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern Japan, to examine the physical processes governing this earthquake sequence. The results show that the foreshock hypocenters moved upward on a sharply defined plane with steep dip. The mainshock hypocenter was located at the edge of a seismic gap formed by foreshocks along the plane. This spatial relationship suggests that the mainshock ruptured this seismic gap. The corner frequency of the mainshock supports this hypothesis. The aftershock hypocenters migrated upward along several steeply dipped planes. The aftershock activity slightly differs from the simple mainshock–aftershock type, suggesting that aseismic processes controlled this earthquake sequence. We established the following hypothesis: First, fluids originating from the subducting slab migrated upward and intruded into the fault plane, reducing the fault strength and causing a foreshock sequence and potentially aseismic slip. The continuous decrease in the fault strength associated with an increase in the pore pressure and the increase in shear stress associated with aseismic slip and foreshocks caused the mainshock in an area with relatively high fault strength. The change in the pore pressure associated with post-failure fluid discharge contributed to aftershocks, causing the upward migration of the earthquake. These observations demonstrate the importance of considering fluid movement at depth not only earthquake swarms but also foreshock—mainshock–aftershock sequences.