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.