(3)Earthquakes in the eastern margin of the Sea of Japan
Several large earthquakes have recently occurred extending north and south along the Sea of Japan coast of the Tohoku region. These include the 1983 Central Sea of Japan Earthquake (Nihonkai Chubu Earthquake) (M 7.7) and the 1993 Earthquake off the Southwest Coast of Hokkaido (Hokkaido Nansei-Oki Earthquake) (M 7.8). Some seismologists have advanced the theory that a plate boundary exists along the eastern margin of the Sea of Japan (Fig.2-17, broken line). Therefore, we will classify and describe earthquakes in this area as earthquakes in the eastern margin of the Sea of Japan. Large earthquakes in this area occur in relatively shallow locations. Therefore, the crustal movement at the sea floor (upheaval and subsidence) often results in tsunami. The hypocenter in these cases is near land, so coastal areas near the earthquake source experience strong ground motion and the arrival of a tsunami several minutes later. Further, tsunami sometimes occur at coastal regions far from the hypocenter. The Sea of Japan is surrounded by land, so the transmission of the tsunami has a very complex process. In some cases, the tsunami reaches its maximum height several hours after the earthquake.