(1)Earthquakes near the plate boundary off the Coast of Pacific Ocean


The Pacific Plate is subducting toward the Tohoku region from the Japan Trench off the east coast of the Tohoku region (Fig.4-3).

There are two types of earthquakes that occur near the plate boundaries off the Pacific Coast: first, the interplate earthquakes that are caused by the slipping movement at the boundary of the subducting Pacific Plate and the plate on land; second, earthquakes that occur within the subducting Pacific Plate. Most known destructive earthquakes have been interplate earthquakes.

1) Interplate earthquakes caused by the subducting Pacific Plate

Many destructive earthquakes of this type are known, including the 1896 Meiji Sanriku Earthquake (M 8 1/2), the 1968 Earthquake off the Coast of Tokachi (Tokachi-Oki Earthquake) (M7.9), the 1978 Earthquake off the Coast of Miyagi Prefecture (M 7.4), and the 1994 Earthquake far off the Coast of Sanriku (M 7.5). Many of these earthquakes occur at relatively shallow locations underneath the sea floor. In most cases, the crustal movement on the sea floor caused by fault movement accompanies a tsunami. The tsunami height grows larger in proportion to the shallowness of the water, and therefore becomes generally high when it reaches the coast. The tsunami height is also determined by the shape of a bay and other factors. The hypocenter will be slightly deeper for earthquakes of this type which occur closer to shore because the Pacific Plate inclines and subducts in the direction of land. Therefore, tsunami occurring with this type of earthquake are not usually high. The ground motion can cause substantial damage on land, however, because the focal region is close to land. This happened during the Earthquake off the Coast of Miygai Prefecture and the Earthquake far off the Coast of Sanriku.

The area off the Pacific coast of the Tohoku region is one of the most seismically active regions around Japan. The characteristics of how earthquakes occur here differ depending on the location. For example, great earthquakes of M 8 sometimes occur in the sea off the coast of Aomori and Miyagi Prefectures. These include the 1896 Meiji Sanriku Earthquake (M 8 1/2) and the 1968 Earthquake off the Coast of Tokachi (M 7.9). The type of seismic activity that occurs in the open sea off Fukushima is of a different nature, however. While there have been several earthquakes with M 7 or so , such as the 1938 Earthquake off the East Coast of Fukushima Prefecture (Fukushima-ken Toho-Oki Earthquake) (M7.5), no earthquakes with M 8 or so are known to have occurred. Also, earthquakes of roughly the same magnitude have occurred every about 40 years since 1855 near the same sea location, the most recent one was the 1978 Earthquake off the Coast of Miyagi Prefecture (M 7.4).

So-called "tsunami earthquakes" ("slow earthquakes" or "low-frequency earthquakes") also occur in this region. The characteristics of these earthquakes are relatively slow fault movement. In this case, tsunami is large, although people do not feel strong ground motion. Typical example was the 1896 Meiji Sanriku Earthquake (M 8 1/2). The fault movement during that earthquake was clearly much greater than that estimated from the ground motion. Thus, there are some earthquakes for which it is difficult to ascertain the existence of a tsunami or to assess its size from perceptible ground motion only.

2) Earthquakes within the subducting Pacific Plate

The 1933 Sanriku Earthquake (M 8.1) is a destructive earthquake of this type known to have occurred in this region. This earthquake occurred on a normal fault in a shallow location near the Japan Trench, from which the Pacific Plate is beginning to subduct directly under the land plate. The tsunami caused upward of 3,000 fatalities. Among earthquakes which occur within subducting plates, there are also destructive earthquakes other than those occurring in shallow locations on normal faults. In the Chishima Trench (Tisima Trench), the 1993 Earthquake off the Coast of Kushiro (Kushiro-Oki Earthquake) (M 7.8) occurred in a somewhat deeper area of roughly 100 km, and was caused by the fault movement on a roughly horizontal fault. Also, the 1994 Earthquake off the East Coast of Hokkaido (Hokkaido Toho-Oki Earthquake) (M 8.1) was in a somewhat shallower area of about 20 km deep. This was caused by the fault movement of a reverse fault at a high inclination. Therefore, the hypocentral depth and the nature of the fault movement vary for earthquakes that occur within subducting plates.

Recently, the 1987 earthquake (M 6.6, depth about 70 km) caused slight damage in northern Iwate Prefecture.