(3)Earthquakes in the eastern margin of the Sea of Japan (Japan Sea)


The coastal area of the Sea of Japan from Hokkaido to the Tohoku Region is referred to as the "eastern margin of the Sea of Japan (Japan Sea)". Several large earthquakes have occurred here recently, and significant damage has resulted due to tsunami, seismic ground motion, and ground liquefaction. Because the focal regions of these earthquakes have been near land, in some cases tsunami have hit shore immediately after or, in some case, several minutes after an earthquake has occurred.

One of these earthquakes was the 1993 Earthquake off the Southwest Coast of Hokkaido (M 7.8). The 1983 Central Sea of Japan Earthquake (M 7.7) was located south of the 1993 earthquake, while the 1940 earthquake off the coast of Cape Kamui (Kamui Misaki), (M 7.5) occurred just north of here, off the coast of Shakotan Peninsula (Syakotan Hanto). Off the coast of Rumoi located in the northeast of Cape Kamui, earthquakes occurred in 1947 (M 6.7) and in 1959 (M 6.2). Most of these earthquakes were accompanied by tsunami. Further north, an earthquake occurred off the southwest coast of Sakhalin (M 6.9) in 1971.

Since the mechanisms of these earthquakes were reversed faults compressed in an east-west direction. This is the basis for recent speculation that a plate boundary exists in the "eastern margin of the Sea of Japan (Japan Sea)". Some hold that the phenomenon can be explained even if no plate boundary exists, but no firm conclusions have been reached [See 4-1 (3)]. Existing data indicates that no large earthquakes have taken place in the open sea to the west of Hokkaido from Teuri Island and Yagishiri Island to Rishiri Island and Rebun Island, which lie between the source regions for the previously-mentioned earthquake in the open sea off the coast of Rumoi and the earthquake off the southwest coast of Sakhalin. Some believe that this area is a seismic gap.