4 Characteristics of seismic activity in the Tohoku region


One well-known destructive earthquake in the Tohoku region was the 1896 Meiji Sanriku Earthquake (M 8 1/2) that occurred off the coast of Sanriku. The combined earthquake and tsunami resulted in more than 20,000 fatalities. Other earthquakes occurring at sea since the Meiji period (1868) include the 1933 Sanriku Earthquake (M 8.1) and the 1983 Central Sea of Japan (Japan Sea) Earthquake (Nihonkai Chubu Earthquake) (M 7.7), and those occurring inland include the 1894 Shonai Earthquake (M 7.0) and the 1896 Rikuu Earthquake (M 7.2). They caused damage both from seismic ground motion and tsunami. There are fewer historical accounts of older earthquakes in the Tohoku region than there are for the Kinki and other regions. Many destructive earthquakes have occurred in this region both on land and at sea, however. The oldest known earthquake here was the 830 earthquake at Dewa (M 7 - 7.5). Damage is known to have been caused in the Tohoku region by earthquakes and tsunami in surrounding areas. One destructive earthquake that occurred in this surrounding area was the 1993 Earthquake off the Southwest Coast of Hokkaido (Hokkaido Nansei-Oki Earthquake) (M 7.8). Tsunami generated by earthquakes in foreign countries have also caused damage, such as the 1960 Chile Earthquake Tsunami. Fig.4-1 and Fig.4-2 show the known primary destructive earthquakes in the Tohoku region.

Seismic activity in the Tohoku region can be roughly divided into three types: first, earthquakes that occur in the Pacific Ocean near the plate boundary that subducts from the Japan Trench to the land; second, earthquakes that occur on land in shallow locations less than 20 km underground; and third, earthquakes in the eastern margin of the Sea of Japan (Japan Sea). Of these three types, the earthquakes that are large and strike with great frequency are those in the Pacific Ocean.

The Pacific Plate approaches the Tohoku region from the east-southeast at a rate of about 8 cm a year. This plate subducts toward Tohoku from the Japan Trench. The seismic activity that accompanies this subduction occurs at a depth of about 150-200 km underground on the Sea of Japan side of the Tohoku region. This activity can be detected as far underground as 600 km below the area near the southern edge of the maritime region of Russia (Fig.4-3, Fig.2-5). The land is being compressed in an east-west direction in the eastern margin of the Sea of Japan. Some have theorized that the plate has begun subducting along a row of earthquakes, running in a north-south direction in this region.

The topography of the Tohoku region is characterized by alternating mountain ranges and lowlands running in a north-south direction, particularly the Ou Mountains (Ou Sanmyaku) and the Dewa Mountains (Dewa Santi) area. The primary active faults of the Tohoku region lie at the boundaries of the mountains and the lowlands, and most of these run in a north-south direction. Most are reverse faults, indicating that the area is uniformly being compressed in an east-west direction. Observations of crustal movement, however, show that many more areas are being extended than compressed. The manner and extent of crustal movement have local characteristics, however, and overall the Tohoku region is not uniformly being compressed in an east-west direction (Fig.4-5). Most of the large destructive inland earthquakes occur on identified active faults, but some earthquakes occur in areas where no active faults are known. Earthquake swarms are known to occur in some locations. Fig.4-4 provides bird's eye views of the topography and distribution of the active faults in the Tohoku region from the northeast and the northwest.

There has not been any destructive earthquake in the Pacific Ocean near the Tohoku region and surrounding areas since the 1978 Eathquake off the Coast of Miyagi Prefecture (Miyagi-ken-Oki Earthquake) (M 7.4). The Earthquake far off the Coast of Sanriku (Sanriku Haruka-Oki Earthquake) occurred in 1994 (M 7.5), however, and caused damage primarily in Aomori Prefecture. There have not been any recent destructive earthquakes of note on land, but some damage was caused by seismic activity (maximum M 5.9) in 1996 near the border of Miyagi and Akita Prefectures. On the eastern margin of the Sea of Japan, the 1983 Central Sea of Japan Earthquake (M 7.7) and the 1993 Earthquake off the Southwest Coast of Hokkaido (M 7.8) caused substantial damage, primarily on the Sea of Japan coast.