6 Characteristics of seismic activity in the Chubu region


The destructive earthquakes in the Chubu region include great earthquakes of M 8 or so that have occurred repeatedly in the past in the Pacific Ocean and large earthquakes of M 7 to 8 that have occurred on land. The great earthquakes that have occurred in the Pacific Ocean have been characterized by strong seismic ground motion over a broad area and a large tsunami, and have caused significant damage. Since the Meiji period (1868), these have included the 1944 Tonankai Earthquake (M 7.9) and the 1946 Nankai Earthquake (M 8.0). Those that have occurred in shallow locations on land include the 1891 Nobi Earthquake (M 8.0), which caused more than 7,000 fatalities, and the 1948 Fukui Earthquake (M 7.1) that occurred directly underneath an urban area. On the Sea of Japan (Japan Sea) side of the region, the 1964 Niigata Earthquake (M 7.5) caused damage both from seismic ground motion and the accompanying tsunami. Damage is also known to have been caused by earthquakes in surrounding areas, such as the Great Kanto Earthquake (M 7.9) that occurred along the Sagami Trough in 1923, and by tsunami that originate overseas, such as the 1960 Chile Earthquake Tsunami. Fig.6-1 and Fig.6-2 show the primary destructive earthquakes known to have occurred in the Chubu region. (Refer to Fig.5-3 for earthquakes in the Izu area.)

Generally, there are three types of earthquakes in the Chubu region. The first type occurs near the plate boundary that is subducting toward land from the Suruga Trough and the Nankai Trough in the Pacific Ocean. The second type occurs at relatively shallow locations on land, at depths less than about 20 km. The third type occurs in the eastern margin of the Sea of Japan, near the area off Niigata Prefecture. The area from the Chubu region to the Kinki region relatively active for large earthquakes occurring in shallow locations on land in the Japanese archipelago.

The Philippine Sea Plate is approaching the Chubu region from the southeast at a speed of about 4 - 5 cm per year. The Philippine Sea Plate is subducting underneath the Chubu region from the Suruga Trough and Nankai Trough. Seismic activity from this subduction is observed down to a depth of few tens of kilometers (Fig.6-3). Also, the Izu Peninsula (Izu Hanto) on the Philippine Sea Plate is colliding with the Japanese archipelago at the eastern side of the Suruga Trough. The Pacific Plate, which is subducting at the Japan Trench east off the Kanto region, extends to a location deep under the Chubu region. The depth of the upper surface of this plate ranges from 150-400 km (Fig.6-4). One theory holds that there is a plate boundary that links the Suruga Trough and the Sagami Trough from the eastern margin of the Sea of Japan (near a location in the sea off the coast of Niigata Prefecture in the Chubu region) and passes through a location close to the Itoigawa-Shizuoka tectonic line(Itoigawa Shizuoka-Kozosen), a large boundary in terms of geological structure (details to follow).

The Chubu region has some extremely steep mountains, including the Hida, Kiso, and Akaishi Mountains. These mountains lie on the western side of the Itoigawa-Shizuoka tectonic line. There are many active faults in this area, at the boundary between mountains and basins, or mountains and hilly areas or plains (Fig.6-5). The Chubu region also has some of the largest crustal movement in the Japanese archipelago, and there is a pronounced contraction that generally ranges from an east-west to a northwest-southeast direction (Fig.6-6).

Relatively large earthquakes occur at shallow locations on these active faults in the Chubu region. The Nobi Earthquake (M 8.0), for example, occurred in the Nobi fault zone, which includes the Neodani fault. This was an extremely large earthquake of the type that occurs at shallow locations on land. Many M 7 or so earthquakes are known from historical documents. There are also cases, however, in which large destructive earthquakes have occurred in areas where there is no known active fault.

There are active earthquake swarms in the eastern Izu Peninsula and surrounding area, as well as in western Nagano Prefecture. Active swarms were also observed in the past in Matsushiro (Matusiro), Nagano City.

No destructive earthquakes have occurred along the Nankai Trough in the Pacific Ocean since the 1946 Nankai Earthquake (M 8.0). On land, the 1984 Western Nagano Prefecture Earthquake (Nagano-ken Seibu Earthquake) (M 6.8) near Mt. Ontake caused substantial damage. Frequent seismic activity is observed in this area. On the Izu Peninsula and surrounding area, there have been M 6-7 earthquakes as well as repeating swarm activity since the latter half of the 1970s. More recently, there was frequent swarm activities off the east coast of the Izu Peninsula (Ito City ) from May to June 1993 and September to October 1995, as well as in October 1996 and March 1997. An earthquake of M 6.6 occurred in 1993 off the coast of Noto Peninsula (Noto Hanto), which caused injuries and damaged houses. In the eastern margin of the Sea of Japan, the 1983 Central Sea of Japan Earthquake (Nihonkai Chubu Earthquake) (M 7.7) and the 1993 Earthquake off the Southwest Coast of Hokkaido (Hokkaido Nansei-Oki Earthquake) (M 7.8) generated a tsunami that caused damage on the Sea of Japan coast.