4.2 Seismic gap of the foreshock and aftershock sequence in the mainshock fault plane
The doughnut-like pattern of the foreshocks (Fig. 6) is similar to the “Mogi doughnut” (Mogi, 1969). It has been reported that aftershocks do not occur in the mainshock region (e.g., Mendoza & Hartzell, 1988; Das & Henry, 2003; Woessner et al., 2006; Asano et al., 2011; Ebel & Chambers, 2016; Yoshida et al., 2016b and 2020a; Ross, 2017b, 2018; Wetzler et al., 2018), which is likely because the shear stress was released during the mainshock. In Figure 6a, the size of the seismic gap is compared with the estimated size of the fault related to the mainshock. Because the centroid location of the mainshock was not determined, we assumed the centroid is located by a few hundred meters into the shallower region such that the mainshock centroid is located in the center of the seismic gap shown in the figure. The fault size of the mainshock is similar to that of the seismic gap. This is consistent with the hypothesis that the mainshock rupture occurred in the seismic gap of the foreshock and aftershock activities. A similar spatial separation of the mainshock and fore- and aftershocks in the rupture area was also reported for a recent M5.2 intraplate earthquake that occurred in Akita, NE Japan (Yoshida et al., 2020).
The seismic gap in the foreshock activity may originate from the spatial heterogeneities in the frictional and material properties along the fault plane. The fault strength of the mainshock rupture area may have been higher than that of the surrounding area, as proposed in the asperity model of Lay & Kanamori (1981). Foreshocks can be understood as failures of small seismogenic patches in the surrounding stable area. Alternatively, the area may have been covered by an impermeable medium, hindering fluid intrusion. The occurrences of foreshocks and aseismic slip increased the shear stress in the future source region of the mainshock rupture. The mainshock occurred in this region due to the gradually increasing pore pressure and shear stress.