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.