Historical data and research into tsunami show that several earthquakes have occurred along a north-south direction in this area. The 1940 Earthquake off the Coast of Shakotan Peninsula (Syakotan Hanto) (M 7.5) and the 1964 Niigata Earthquake (M 7.5) were earthquakes that occurred on reverse faults with compression in an east-west direction. In addition, compression is being exerted in an east-west direction over a wide area extending from Hokkaido to the Chubu region. This led to the formulation of a theory in the early 1980s that the eastern margin of the Sea of Japan was a convergent plate boundary. Two large earthquakes later occurred in this area: the 1983 Central Sea of Japan Earthquake (M 7.7), and the 1993 Earthquake off the Southwest Coast of Hokkaido (M 7.8). Research into the nature and causes of these earthquakes is currently under way. Some hold that the pattern of earthquakes here can be explained even if the eastern margin of the Sea of Japan is not a plate boundary; however, no firm conclusions have drawn as yet.

There is less seismic activity in this area than occurs offshore in the Pacific Ocean. If we limit the time of seismic activity to the period of the recent several decades, we find that a succession of large earthquakes has occurred in the open sea from Hokkaido to Niigata Prefecture. The southern third of the stretch of sea off the coast of Akita and Yamagata Prefectures between the source region of the 1983 Central Sea of Japan Earthquake and the 1964 Niigata Earthquake encompassed part of the source region of the 1833 Earthquake off the Coast of Shonai (Shonai-Oki Earthquake) (M 7 1/2). No earthquakes are known to have occurred in the northern most two-thirds of this area, however. Therefore, some have advanced the theory that this area is a seismic gap.