Seismic activity inclining downward from the upper right (southeast) to the lower left (northwest) is along the subducting Philippine Sea Plate. From the distribution of these earthquakes, it is apparent that the Philippine Sea Plate is subducting under the Chugoku-Shikoku region. The angle of subduction in profile A is slight, but it ranges from moderate to steep in profile B, and deeper seismic activity is seen here.

[Compiled from data from Tokyo, Kyoto, and Kochi Universities]