3 Characteristics of seismic activity in the Hokkaido region


Destructive earthquakes in the Hokkaido region frequently occur at sea, as with the 1952 Earthquake off the Coast of Tokachi (Tokachi-Oki Earthquake) (M 8.2) and the 1993 Earthquake off the Southwest Coast of Hokkaido (Hokkaido Nansei-Oki Earthquake) (M 7.8). The damage caused by these earthquakes resulted both from ground motion and tsunami. The Earthquake off the Southwest Coast of Hokkaido was particularly devastating. Damage due to the tsunami and other factors resulted in 230 dead or missing. Many inland earthquakes have also caused local destruction, though the earthquakes themselves were not as severe. Earthquakes and tsunami have also caused damage in the areas surrounding Hokkaido. Examples are the 1968 Earthquake off the Coast of Tokachi (M7.9), which occurred off the east coast of Aomori Prefecture, and the 1983 Central Sea of Japan (Japan Sea) Earthquake (Nihonkai Chubu Earthquake) (M 7.7), which occurred off the west coast of Aomori and Akita Prefectures. The latter was accompanied by a tsunami that caused additional destruction. Further, tsunami caused by earthquakes in foreign countries have also resulted in tsunami disaster, such as the 1960 Chile Earthquake Tsunami. Historical accounts on earthquakes in Hokkaido before the Meiji period (the late 19th century) is scanty compared to that available for other regions in Japan. Therefore, it is not as easy to acquire details on the characteristics of seismic activity here as it is for other regions. Fig.3-1 shows the known primary destructive earthquakes in Hokkaido.

Seismic activity in Hokkaido can be roughly divided into three types: first, earthquakes that occur in the Pacific Ocean near the plate boundary subducting toward land from the Chishima(Kuril) Trench and the Japan Trench; second, earthquakes that occur in shallow locations on land less than 20 km underground; third, those earthquakes that occur on the eastern margin of the Sea of Japan (Japan Sea). Among the earthquakes that occur on land, there are also M 7 earthquakes that occur in deeper locations from 20-40 km underground, particularly in the southern Hidaka Mountains (Hidaka Sanmyaku ) and the offshore area of Urakawa.

The Pacific Plate is approaching Hokkaido from the east-southeast at a rate of about 8 cm a year. This plate is subducting toward Hokkaido from the Chishima Trench and the Japan Trench. The seismic activity that accompanies this subducting force can be observed to a depth of 400-500 km underground near southern Sakhalin (Fig.3-2). The Pacific Plate almost directly faces the land at the Japan Trench, and is subducting at a rather steep angle at the Chishima Trench. The earthquakes in the southern Hidaka Mountains are thought to be related to this angular subducting force. Some have theorized that the plate has begun to subduct in the area along the eastern margin of the Sea of Japan (Japan Sea).

Crustal deformation can be observed accompanying the subducting Pacific Plate in the eastern part of Hokkaido facing the Chishima Trench. The ground tends to extend in a southwestern Hokkaido, while the earth is compressing in other regions, generally from a west-northwest and east-southeast direction to an east-west direction (Fig.3-4).

The central area of Hokkaido includes the Hidaka Mountains, which are surrounded by two broad plains, the Ishikari and Tokachi Plains (Isikari and Tokati Heiya). The primary active faults lie from north to south along the edge of these plains, but there are fewer active faults here than in other areas. There are many volcanoes in the areas southwest of Sapporo and Tomakomai, where earthquake swarms are known to occur. Active faults in this area lie in a north-south direction. Many volcanoes are also found from the area near Mt. Taisetsu in central Hokkaido to Shiretoko Peninsula (Siretoko Hanto) in the east, and earthquake swarms also occur here. The active faults in this region are found near the Shiretoko Peninsula. There have been several M 6 earthquakes with slipping of the ground in the area surrounding Lake Kussharo (Kussyaro Ko) near Teshikaga. In the area north of Asahikawa, the mountainous areas have gentler slopes, such as Teshio Mountains (Tesio Santi) and Soya Hills (Soya Kyuryo). There are few active faults here. Fig.3-3 provides bird's eye views of the topography and distribution of the active faults in the Hokkaido region from the southeast and the northwest.

After the 1973 Earthquake off the Coast of Nemuro Peninsula (Nemuro-hanto-Oki Earthquake) (M 7.4), there were few destructive earthquakes in the Pacific Ocean off the coast of Hokkaido and the surrounding areas. This changed with the 1993 Earthquake off the Coast of Kushiro (M 7.8), followed by the 1994 Earthquake off the East Coast of Hokkaido (Hokkaido Toho-Oki Earthquake) (M 8.1). These earthquakes caused damage near the eastern part of Hokkaido. After the 1967 Earthquake near Teshikaga (M 6.5), no destructive earthquakes occurred for some time in shallow land areas. In the area of the Sorachi branch administrative office, both the 1986 earthquake near Numata (M 5.3) and the 1995 earthquake east of Mt. Shokanbetsu (M 5.7) did cause some slight damage, and destructive earthquakes have occurred in somewhat deeper locations near the southern Hidaka Mountains. One such was the 1982 Earthquake off the Coast of Urakawa (Urakawa-Oki Earthquake) (M 7.1), which caused damage primarily in the area of the Hidaka branch administrative office. In the eastern margin of the Sea of Japan (Japan Sea), the 1993 Earthquake off the Southwest Coast of Hokkaido (M 7.8) caused the most fatalities in Hokkaido since the start of the Meiji period in 1868. The tsunami resulting from the 1983 Central Sea of Japan (Japan Sea) Earthquake (M 7.7) also caused a number of fatalities and destroyed several buildings. Finally, earthquake swarms began in the area off the coast of Matsumae in October 1995, with the largest earthquake in the swarm having a magnitude of 4.4.