Figure 8. Temporal evolution of hypocenters in the (a ) latitude, (b) longitude, and (c ) depth directions. (d ) Temporal evolution of the aftershock hypocenters in the depth direction. The red crosses and yellow curve in (d ) indicate the depth above which the shallowest 10% of the hypocenters are located (D10) for every bin with 400 events based on the occurrence time. The circle size corresponds to the JMA magnitude. The yellow star indicates the hypocenter of the mainshock.
Figures 7d–f show the distribution of aftershock hypocenters colored based on the occurrence time of each event. Figure 8d shows the temporal evolution of the aftershock hypocenters as a function of the depth. The temporal evolution of the aftershock hypocenters in both the latitudinal and longitudinal directions is shown in Fig. S7. Because the spatial distribution of the aftershocks is complex, the spatiotemporal features of the aftershocks are more difficult to determine than those of the foreshocks. Overall, the aftershock hypocenters move upward with time, as shown in Fig. 8d, which depicts the depths above which the shallowest 10% of the hypocenters are located (D10) for each bin containing 400 events, as denoted by the red curve. Although earthquakes occur in a relatively deep region immediately after the mainshock, the upper limit of the seismic depth (D10) gradually moves in the shallow direction, that is, the hypocenters gradually move to the shallower part with time after the mainshock.