1) Interplate earthquakes caused by the subducting Pacific Plate

Large earthquakes of M 7-8 sometimes occur near the Chishima (Kuril) Trench, Japan Trench, and the Izu-Ogasawara Trench, where the Pacific Plate is subducting. Large earthquakes such as these are generated by fault movement in which the edge of the plate on land is dragged along as the Pacific Plate subducts, and the plate on land finally pushes up when the stress has reached its limit (Fig.2-19). Examples of this type of earthquake are the 1968 Tokachi-Oki Earthquake (Earthquake off the Coast of Tokachi) (M 7.9) and the more recent 1994 Sanriku Haruka-Oki Earthquake (Earthquake far off the Coast of Sanriku) (M 7.5). This type of earthquake is called an "interplate earthquake" or a "plate boundary earthquake". When the fault length reaches 100 km or longer, the earthquake's magnitude climbs to roughly 8. Also, the crustal movement (upheaval and subsidence) at sea floor accompanying this type of large earthquake causes the seawater to either rise or fall, and tsunami tend to occur as a result. Therefore, coastal areas facing plate boundaries, particularly areas near their hypocenter, often experience strong ground motion caused by interplate earthquakes and tsunami that arrive immediately after the earthquake. The tsunami sometimes arrives and causes damage in coastal areas far away from the hypocenter, even when the ground motion is weak. Fig.2-20 shows the source area of tsunami caused by earthquakes occurring near plate boundaries.

The crustal movement (upheaval and subsidence) at the sea floor also can generate tsunami when the slip along the fault plane occurs gradually at one section of the plate boundary. In this instance, the slip occurs gradually, so the seismic waves - or more accurately, the perceptible seismic waves over a short period - are comparatively small. Therefore, we feel only a slight ground motion. The extent of the crustal movement at the sea floor that creates tsunami, however, does not depend on the speed of the slip. Rather, it depends on the size of the fault movement (the area and extent of the slip). Therefore, even if the perceived ground motion is relatively weak, a large tsunami may occur if there is extensive fault movement. Thus, the earthquakes that generate large tsunami even when human perception ground motion is weak are called "tsunami earthquakes" , "slow earthquakes" or "low-frequency earthquakes". One such earthquake was the Meiji Sanriku Earthquake of 1896. This earthquake is estimated at roughly M 7 from the strength of the short-period ground motion. Actually, however, it is determined to have been M 8 1/2 based on the scale of the estimated fault movement.

Tsunami scale depends on the scale of the fault movement as well as the inclination of the fault plane and the direction of slip. The closer the fault plane inclination is to the vertical, or the greater the dip-slip components, the more efficiently the sea water is raised or lowered and the larger the tsunami becomes.

2) Interplate earthquakes caused by the subducting Philippine Sea Plate

Large earthquakes occur from the Sagami Trough and Suruga Trough to the Nankai Trough and the vicinity of the Nansei Islands Trench, where the Philippine Sea Plate is subducting. Frequently, great earthquakes on the order of M 8 occur here. Typical examples of these earthquakes are the 1923 Kanto Earthquake (M 7.9) which caused the Great Kanto Earthquake Disaster, the 1944 Tonankai Earthquake (M 7.9), and the 1946 Nankai Earthquake (M 8.0). All of these were interplate earthquakes that occurred at the boundary of the Philippine Sea Plate and the land plate. Historical data makes clear that great earthquakes of M 8 frequently occur in the Suruga Trough and the Nankai Trough. It is also known that a series of earthquakes with adjoining source regions tend to occur here within several-year periods. If a series of earthquakes is considered as one event, historical accounts show that this type of event has taken place nine times. [For details, refer to 8-1 (1) 1)]

These M 7 - 8 earthquakes are generally accompanied by tsunami caused by crustal movement of the sea floor(upheaval and subsidence). Therefore, the occurrence of earthquakes of this type results in strong ground motion and the immediate arrival of a tsunami, in the coastal areas facing the plate boundaries or especially in sites close to the hypocenter. Even coastal regions far away from the hypocenter sometimes suffer damage from tsunami, even when the ground motion is weak.

Sometimes, there is gradual fault movement (tsunami earthquake, slow earthquake, or low-frequency earthquake) that only creates tsunami without strong ground motion, even with the subducting Philippine Sea Plate.

3) Earthquakes within subducting plates

Extensive collapse can occur within subducting plates from sea trenches or elsewhere, and this sometimes causes large earthquakes. When large earthquakes of this type occur in relatively shallow underground areas, they are accompanied by tsunami. For example, the 1933 Sanriku Earthquake (M 8.1) and the 1994 Hokkaido Toho-Oki Earthquake (Earthquake off the East Coast of Hokkaido) (M 8.1) were earthquakes of this type, and both were accompanied by tsunami.

A deep earthquake will be generated when rupture occurs within a subducting plate. Even earthquakes with a deep hypocenter of 100-200 km will generate strong ground motion at the earth's surface if the earthquake is large enough. For example, with the 1993 Kushiro-Oki Earthquake (Earthquake off the Coast of Kushiro) (M 7.8), a seismic intensity of 6 (JMA scale) was observed at Kushiro, and significant damage was sustained even though the earthquake occurred in the Pacific Plate at a depth of about 100 km.