5 Conclusions
The results of previous studies suggested that many earthquake swarms have been caused by the movement of crustal fluids (e.g., Mogi, 1989; Fischer and Horálek, 2003; Parotidis et al., 2003; Bianco et al., 2004; Yukutake et al., 2011; Chen et al., 2012; Shelly et al., 2016; Yoshida et al., 2016a; Ruhl et al., 2016; De Barros et al., 2019). In the present study, the intense foreshock–mainshock–aftershock sequence of the 2017 M5.3 Kagoshima Bay earthquake was examined. The results show that the whole sequence can be explained by upward fluid movement: (1) most foreshocks were located on a single plane with a steep dip to the east and migrated along the plane. This foreshock migration can be interpreted as a reflection of fluid movement and possibly triggered aseismic slip on the plane; (2) The hypocenter of the mainshock was located at the edge of a seismic gap with a size comparable to that of the source of the mainshock rupture. This suggests that the mainshock rupture was due to the slip of this seismic gap and the seismic gap was a large seismogenic patch with higher fault strength, which finally ruptured due to the increase in the pore pressure and aseismic slip in the surrounding areas; and (3) Aftershocks occurred on several planes with a steep dip to the east and moved from deeper to shallower regions. The upward migration can be interpreted as a reflection of post-failure fluid discharge. Thus, the overall sequence of the 2017 M5.3 Kagoshima Bay earthquake can be explained by upward fluid movement, as presumed by the fault-valve model (Sibson, 1992).
The whole sequence of the Kagoshima Bay seismicity can be understood as the transition from swarm activity to a mainshock–aftershock sequence. The results of the present study suggest that the generation mechanism of the foreshock activity is the same as that of the earthquake swarms, that is, a temporary increase in the background seismicity rate due to increasing pore pressure and aseismic slip. Aseismic processes sometimes cause a large earthquakes that is followed by numerous aftershocks; the foreshock–mainshock–aftershock sequence may such a sequence.