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


A series of large earthquakes has occurred recently in the eastern margin of the Sea of Japan, north of Niigata Prefecture. These earthquakes have caused the damage by seismic ground motion and tsunami. The tsunami reaches the coast in an extremely short time since the source region of the earthquakes is close to land. [For details refer to 4-1(3).] One example of this type of earthquake in the Chubu region was the 1964 Niigata Earthquake (M 7.5).

A theory has recently been advanced that the eastern margin of the Sea of Japan is a plate boundary. [Refer to 2-4 (3) and 4-1 (3).] According to this theory, the extension of this plate boundary is thought to emerge somewhere in the area from the central to southern Niigata Prefecture to the vicinity of Toyama Bay (Toyama Wan) and then turns south. The plate boundary then crosses Honshu in a roughly north-south direction along the Itoigawa-Shizuoka tectonic line, and links the Suruga Trough and the Sagami Trough. Further, there is another idea that plate boundaries of this type have a width of a certain dimension.

There are no specific records of large earthquakes occurring off the coast of Itoigawa to the vicinity of Toyama Bay. However, one theory holds that this phenomenon can be explained even if the eastern margin of the Sea of Japan is not a plate boundary. However a firm conclusion has yet to be reached.