(2)Shallow inland earthquakes (Depths of 20 km or less)


There is a close connection between the topography of the Tohoku region and the distribution of active faults. The topography of the Tohoku region can be divided into several areas, each running in a north-south direction. On the Pacific Ocean side, the gently sloping Kitakami highlands (Kitakami Koti) and the Abukuma highlands (Abukuma Koti) extend from north to south. On the western side of these highlands, the Kitakami and Abukuma Rivers flank a lowland area, and the steep Ou Mountains cut through the central part of the Tohoku region from north to south. The Ou Mountains contain many volcanoes, including Mt. Iwate, Mt. Kurikoma, and Mt. Zao. In addition, there is a mountainous area that includes Dewa, running north to south in the western part of the area adjoining the Sea of Japan. This mountainous area flanks several basins, including the Yokote and Yamagata Basins (Yokote and Yamagata Bonti). Several plains run roughly north to south along the Sea of Japan coast, including the Akita Plain (Akita Heiya) and the Shonai Plain (Syonai Heiya).

The primary active faults in the Tohoku region are distributed in a roughly north to south direction, and lie at the boundaries of mountainous areas and basins, plains, and lowlands, as well as along the Sea of Japan coast. Most of these active faults are reverse faults with an activity of B class. So far, the fault movement has been in a direction that makes the mountainside higher. This characteristic indicates that compression is being applied to the active faults in an east-west direction. Crustal movement in this area, however, indicates that locations with prominent extension outnumber those with compression. Also, there are local differences in the characteristics and extent of this movement. The subducting Pacific Plate is not believed simply to be causing compression in an east-west direction (Fig.4-5). Old rock is widely distributed in the Kitakami and Abukuma highlands, and the geologic characteristics of this area are relatively stable. There is slight crustal movement in this area, and a few active faults.

Many earthquakes are known to have occurred in shallow locations on land, mostly 15-20 km deep. Destructive earthquakes of this type after the Meiji Period (1868) include the 1894 Shonai Earthquake (M 7.0), the 1896 Rikuu Earthquake (M 7.2), the 1900 earthquake in northern Miyagi Prefecture (M 7.0), and the 1914 Akita Senboku Earthquake (M 7.1; also known as the Kowakubi Earthquake). The 1896 Rikuu Earthquake is thought to have occurred on the eastern margin of the Yokote-Basin (Yokote-bonchi-toen) fault zone and the 1894 Shonai Earthquake is thought to have occurred on the eastern margin of Shonai Plain (Shonai-heiya-toen) fault zone. In contrast, the active faults corresponding to the 1900 earthquake in northern Miyagi Prefecture and the 1914 Akita Senboku Earthquake are not yet discovered. Thus large earthquakes have occurred even in locations where active faults are not known to exist. Destructive earthquakes from an older period include those that occurred in the coastal area of Akita Prefecture, such as the 1694 earthquake near Noshiro (M 7.0) and the 1804 Kisakata Earthquake (M 7.0), and those in the interior, such as the 1611 earthquake at Aizu (M 6.9) and the 1766 earthquake at Tsugaru (M 7 1/4). Earthquakes of M 6 or so, smaller than those previously described, also occur even in locations other than active fault zones, and sometimes cause damage locally. In addition, the intervals of activity for most active faults are 1,000 years or longer. Therefore, even if none are known today, this does not indicate that no earthquakes occur here however.

Earthquake swarms are known to occur in several places in the Tohoku region. Most of these earthquakes occur near volcanoes. The magnitudes of the individual earthquakes that comprise the earthquake swarms are usually M 4 or less; an individual earthquake of M 5 or stronger rarely occurs, and damage occurs locally. The duration of earthquake swarm activity is usually short, from one to three months. However, there are cases of this activity lasting longer than one year.