There is frequent seismic activity in southwestern Ibaraki Prefecture occurring at the upper surface of the subducting Philippine Sea Plate (a in the middle of the figure, depth is roughly 50 km), and at the upper surface of the subducting Pacific Plate (b in the middle of the figure, depth is roughly 70 km). There is also frequent seismic activity in northwestern Chiba (Tiba) Prefecture occurring from the upper surface of the subducting Pacific Plate to within the plate (c in the middle of the figure, depth is roughly 80 km). Aftershock activity is seen for the 1987 Earthquake off the East Coast of Chiba Prefecture (M 6.7) near Kujukuri hama Beach .

[Compiled from data from the National Research Institute for Earth Science and Disaster Prevention]