4.5 Implications to the foreshock–mainshock–aftershock
sequence
The results of previous studies suggested that many earthquake swarms
are caused by the movements of crustal fluids (e.g., Mogi, 1989;
Italiano et al., 2001; Fischer and Horálek, 2003; Parotidis et al.,
2003; Bianco et al., 2004; Yukutake et al., 2011; Shelly et al., 2016;
Yoshida et al., 2016a; Ruhl et al., 2016; De Barros et al., 2019). The
results of the present study suggest that the generation mechanism of
the foreshock activity is the same as that of earthquake swarms, that
is, a temporary increase in background seismicity rate due to increasing
pore pressure and aseismic slip. The whole sequence of the Kagoshima Bay
seismicity can be understood as the transition from swarm activity to
the mainshock–aftershock sequence.
The 2008 Mogul earthquake swarm, Nevada, may be a similar example. This
sequence was also initiated by swarm activity but shifted to a
mainshock–aftershock sequence after the occurrence of the M4.9
mainshock. The upward migration of the earthquakes suggests that
fault-valve behavior is involved in the occurrence of this earthquake
sequence (Ruhl et al., 2016). The aftershock activities of the 2014
ML 4.8, Ubaye earthquake, France (De Barros et al.,
2019), and the foreshock and aftershock activities of the 2017 M5.2
Akita-Daisen event can also be understood as transitions from swarm
activity to mainshock–aftershock sequences (Yoshida et al., 2020b).
Similarly, aseismic slip may have caused the foreshocks and mainshock of
the 2011 M9 Tohoku-Oki earthquake (Kato et al., 2012); 2014 Iquique Mw
8.1 earthquake, Chile (Kato & Nakagawa, 2014); and 2009 M6.3 L’Aquila
earthquake (Borghi et al., 2016). It is likely that pore pressure
migration and aseismic slip propagation occasionally coexist (Waite &
Smith, 2002; Ross et al., 2017a; Yoshida & Hasegawa, 2018; De Barros et
al., 2020) and contribute to the increase in the background seismicity
rate. Such aseismic processes may also cause mainshock–aftershock
activity without notable foreshocks. The 2019 M6.7 Yamagata-Oki
earthquake, NE Japan, may be an example. The earthquake occurred in the
stress shadow of the 2011 Tohoku-Oki earthquake and exhibited an upward
aftershock migration (Yoshida et al., 2020b). These observations suggest
that the monitoring of aseismic processes is crucial to understanding
the seismic activity.