Hypocenter distributions of cluster 4 and neighboring clusters (a) and their time histories (b). (a) The light brown shaded region is a schematic illustration of a high-density rock body (Kodaira et al.2006). The black arrow indicates the current convergence direction of the PHS plate. The other notation is the same as in Figure 10. (b) Time histories of each cluster. The color and symbols assigned to each cluster are the same as in Figure 12a. The numbers of earthquakes within each cluster is indicated at the right side of each row.