Next, we will examine the distribution of active faults (Fig.6-5). The Itoigawa-Shizuoka tectonic line is both a large tectonic boundary, as previously mentioned, and a highly active fault that lies in Nagano and Yamanashi Prefectures. The active fault along the Shinanogawa River is the only large active fault known to exist on the eastern side of this tectonic line in the Chubu region. However, there are many active faults with high activity (A and B classes) on the western side of this tectonic line such as the Atotsugawa fault, the Adera fault zone, the Inadani fault zone, the Nobi fault zone including the Neodani fault and the Yoro-Kuwana-Yokkaichi fault zone. These active faults lie at the boundary between mountainous areas and basins or between mountainous areas and hilly areas or plains. Most of these faults run in either northeast-southwest or northwest-southeast directions. Accumulated fault movements are reflection of compression in an east-west direction in that area. It is known from historical accounts that the 1858 Hietsu Earthquake (M 7.0-7.1) occurred on the Atotsugawa fault, and the 1586 Tensho Earthquake (M 7.8) is thought to have occurred in the area extending from the Shokawa River (Syokawa) fault zone to the Adera fault zone. Most of the active faults in this area are strike-slip faults or strike-slip faults with reverse-fault components. The area surrounding the Izu Peninsula corresponds to the boundary between the Philippine Sea Plate and the land plate. Active faults with an A class activity are known to exist here, including the Fujikawa River-mouth (Fujikawa-kako) fault zone and the Kannawa/Kouzu-Matsuda fault zone.

The Chubu region experiences the greatest crustal movement of any region in the Japanese archipelago. There is distinct compression from northwest-southeast to west-northwest-east-southeast in the Kiso and Akaishi Mountains just west of the Itoigawa-Shizuoka tectonic line. Moving further west to the Hida highlands (Hida Koti) and the Ryohaku Mountains (Ryohaku Santi), the compression is in a nearly east-west direction (Fig.6-6).

The presence of steep mountains and many active faults and the extensive crustal movement are all interrelated, suggesting that a strong compression beneath this region is working in a northwest-southeast to east-west direction. The factors behind this force are thought to be the result of complex interrelated activity between the Philippine Sea Plate, the Pacific Plate, and the inland plate. However, the details of this activity are not well understood.

Other than those in the area surrounding the Izu Peninsula, the major destructive earthquakes occurring in shallow locations on land since the Meiji Period (1868) include the 1891 Nobi Earthquake (M 8.0), the 1945 Mikawa Earthquake (M 6.8), the 1948 Fukui Earthquake (M 7.1), and the 1984 Western Nagano Prefecture Earthquake (M 6.8). The Nobi Earthquake occurred on the Nobi fault zone and the Gifu-Ichinomiya fault zone. During the Mikawa Earthquake, there appeared offset at the surface on the Fukozu fault. Visible fault offset on the earth's surface was not present during the Fukui Earthquake or the Western Nagano Prefecture Earthquake, however. Also, the 1952 Earthquake off the Coast of Daishoji (Daishoji-Oki Earthquake) (M 6.5), the 1963 Earthquake off the Coast of Cape Echizen (Echizen-misaki-Oki Earthquake) (M 6.9), and the 1993 Earthquake off the Coast of Noto Peninsula (Noto-hanto-Oki Earthquake) (M 6.6) occurred in the area from the Noto Peninsula (Noto Hanto) to the west in the Sea of Japan. They are thought to be of a type identical to those occurring in shallow locations on land. There are active faults with a B class activity in the Noto Peninsula and other areas close to the coast in the Hokuriku region. These include the Ouchigata fault zone, the Tonami Plain (Tonami-heiya) fault zone, and the Morimoto-Togashi fault zone. Many of these faults are reverse faults.

The Itoigawa-Shizuoka tectonic line is a highly active fault from the vicinity of Hakuba in Nagano Prefecture to near the western margin of the Kofu Basin (Kohu Bonti), it also referred to as the Itoigawa-Shizuoka tectonic line fault zone. A comprehensive evaluation conducted by the Headquarters for Earthquake Research Promotion based on the results of active fault survey shows it is quite likely that an M 8 earthquake with a source region from Hakuba to the vicinity of Kobuchizawa within this fault zone occurred about 1,200 years ago. Based on the history of fault activity, it also is thought likely that an M 8 level (M 7 1/2-8 1/2) earthquake could occur at any time in the next several hundred years in this fault zone, an area that includes the Gofukuji fault near Matsumoto. The Median Tectonic Line (Chuo-kozosen) is an important tectonic boundary from a viewpoint of geological structure [for details, refer to 8-1 (2)]. One section of this tectonic line in the Chubu region is not a highly active fault.

East of the Itoigawa-Shizuoka tectonic line, the Shinano-gawa fault zone and others stretch from north of Nagano City to the Echigo Plain (Etigo Heiya). The activity for these faults are A or B classes. The 1847 Zenkoji (Zenkoji Temple) Earthquake (M 7.4) occurred in the Shinano-gawa fault zone in Nagano Prefecture, producing fault offset on the earth's surface that extended for about 40 km. On the northern side of the Shinano-gawa fault zone, an active fold zone, in which the stratum gradually curves due to east-west compression, runs north and south. The active fold zone is related to earthquakes that occur in extremely shallow locations here.

Earthquakes in the M 6 to M 7 range, swarm activity, and submarine volcanic eruptions have occurred since the 1970s in the area surrounding Izu Peninsula. There are relatively new volcanoes on this peninsula, and the stratum has not undergone significant deformation. Therefore, the geological characteristics differ substantially from those of surrounding areas, including the Tanzawa Mountains (Tanzawa Santi) and the Fujikawa River basin. Izu Peninsula is considered part of the Philippine Sea Plate, but the earthquakes that occur here are the same type as those that occur in shallow locations on land. Destructive earthquakes have occurred here fairly recently. Examples are the 1974 Earthquake off the Coast of Izu Peninsula (Izu-hanto-Oki Earthquake) (M 6.9) and the 1978 Earthquake Near the Coast of Izu Oshima (Izu Oshima Kinkai Earthquake) (M 7.0). A survey of crustal movements in the area around Ito in the northeastern Izu Peninsula indicates extension of the crustal movement in a northeast-southwest direction believed to accompany volcanic activity. There has been an upheaval of the land here by about 50 cm over the past 20 years. In July 1989, severe swarm activity occurred with submarine eruptions 3 km off the coast of Ito City. After that brisk swarm activity here has been recurring. Previously, the 1930 North Izu Earthquake (Kita-Izu Earthquake) (M 7.3) occurred in the Kitaizu (North Izu) fault zone on the northern Izu Peninsula. Violent swarm activity (Maximum M 5.9) occurred here from February to May 1930, followed by the 1930 North Izu Earthquake in November.

The intervals of seismic activity for most active faults are 1,000 years or more; however, this does not indicate that no earthquake can occur even if no earthquake has occurred here since the beginning of history.

Swarm activity frequently occurs in the vicinity of volcanoes. Some activity of this type includes that in the open sea east of the Izu Peninsula, as well as near the Hakone volcano, the southern base of Mt. Ontake, and the southwestern base of Mt. Norikura. There were large eruptions of water during the latter period of swarm activity in Matsushiro in Nagano Prefecture that occurred from 1965 to 1968.