Two types of earthquakes occur near plate boundaries in the offshore area of the Pacific Ocean and other locations: first, the interplate earthquakes caused by slipping at the boundary of the subducting Pacific Plate and the plate on land; and second, those earthquakes that occur within the subducting Pacific Plate.

1) Interplate earthquakes caused by the subducting Pacific Plate

Great interplate earthquakes caused by the subducting Pacific Plate have occurred along the Chishima Trench and other locations. This ground motion has caused damage to areas located on the Pacific Ocean side of Hokkaido, and sent earthquake-generated tsunami crashing onto the Pacific Ocean coast.

Examples of this type of earthquake in the 19th century include the 1843 Earthquake off the Coast of Kushiro (M 7.5), the 1856 Earthquake off the east Coast of Aomori Prefecture (M 7.5), and the 1894 Earthquake off the Coast of Nemuro (M7.9). More recent earthquakes include the 1952 Earthquake off the Coast of Tokachi (Tokachi-Oki Earthquake) (M 8.2), the 1968 Earthquake off the East of Aomori Prefecture (Tokachi-Oki Earthquake) (M 7.9), and the 1973 Earthquake off the Coast of Nemuro Peninsula (M 7.4). These earthquakes occurred within a relatively short period of time, and their source regions did not overlap. A series of large earthquakes of this type are thought to occur repeatedly in the sea along the Chishima Trench in the Pacific Ocean at intervals ranging from several decades to 100 years.

These earthquakes can be explained in the following way. Strain accumulates over a period of several decades to 100 years in the area adjoining the Chishima Trench due to the subducting Pacific Plate. As the area in eastern Hokkaido facing the Chishima Trench compresses in a northwest-southeast direction, crustal movement is observed, with subducting of the coastal area in a stationary condition. This phenomenon is thought to be related to the accumulation of strain (Fig.3-4, Fig.3-5). After this strain has approached its limit, it is released by a series of interplate earthquakes. As a result, a series of large earthquakes occurs along the trench in a short time with no overlapping of focal regions. When the series of interplate earthquakes has finished, large earthquakes do not occur again until sufficient strain has reaccumulated. This cycle is expected to continue in the future. Great earthquakes within subducting Pacific Plate also occur in this area, such as the 1994 Earthquake off the East Coast of Hokkaido (M 8.1). It is difficult to forecast the occurrence of large earthquakes using only the interval between large earthquake series.

So-called "tsunami earthquakes" ("slow earthquakes" or "low-frequency earthquakes") also occur in this region. The characteristics of these earthquakes include fault movement less intense than that of ordinary earthquakes, and tsunami larger in comparison to the ground motion felt on land. Measured by ground motion, the 1975 Earthquake off the East Coast of Hokkaido registered M 7.0. However, estimated by the size of the tsunami, which is thought to roughly express the extent of fault movement, the magnitude of this earthquake is greater than 7.5.

2) Earthquakes within the subducting Pacific Plate

Large earthquakes occur inside the subducting Pacific Plate underneath the Hokkaido region. The 1993 Earthquake off the Coast of Kushiro (Kushiro-Oki Earthquake) (M7.8) occurred in a somewhat deeper area of roughly 100 km, and was caused by fault movement on a roughly horizontal fault. Also, the 1994 Earthquake off the East Coast of Hokkaido (M 8.1) was in a somewhat shallower area of about 20 km. This was caused by fault movement on a reversed fault at a large inclination. Further, an earthquake (M 7.7) occurred inside the Pacific Plate directly beneath the Kunashiri Channel at a depth of about 150 km. Of the earthquakes that occurred in the area adjoining the Hokkaido region, the 1933 Sanriku Earthquake (M 8.1) on a normal fault occurred in a very shallow location (about 20 km) near a trench. This earthquake was in the Pacific Plate as it was beginning to subduct. [For details, refer to 4-2 (1).] This earthquakes was accompanied by large tsunami and caused damage to areas along the Pacific Ocean coast of Hokkaido.