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.