Figure 5. Hypocenter distribution of the foreshocks. Red and
blue circles represent the hypocenters of foreshocks and aftershocks,
respectively, Blue circles represent the hypocenters of the precursory
activity. (a ) Map showing the hypocenters of the precursor
activity and aftershocks. (b ) Map showing only the hypocenters
of the precursory activity. The broken ellipse indicates the location of
the seismic gap. (c ) Cross-sectional views along vertical
sections A to I shown in (a ). The yellow star indicates the
hypocenter location of the mainshock.
The hypocenters of the foreshocks are not uniformly distributed in the
plane, but they are distributed in form of a doughnut, that is, a
seismic gap forms in the center of the plane (broken ellipse in Fig.
5b). To demonstrate this distribution, we estimated the lateral
distribution of the moment release on the fault (Fig. 6a) during the
foreshock sequence following Yoshida et al. (2020a). We computed the
seismic moment release of each earthquake by assuming that its magnitude
is equal to the moment magnitude. Subsequently, we summed the moment
release values of the points that were evenly spaced every 0.04 km by
using the earthquakes within the nearest grid cell. The result shown in
Fig. 6b indicates that the moment release of the foreshock sequence is
smaller (\({<10}^{11}\text{\ Nm}\)) in the region corresponding to the
seismic gap than in the surrounding region (\({>10}^{11}\text{\ Nm}\)).
The hypocenter of the mainshock is located at the edge of this seismic
gap. Figure 5 shows a comparison of the hypocenters of the foreshocks
and aftershocks. Although aftershocks occur inside the seismic gap based
on the map (Fig. 5a), they actually occur in shallower areas than the
foreshocks (Fig. 5c), that is, not within the seismic gap of the
foreshocks.