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.