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


A look at the topography of Hokkaido shows various mountainous regions lying from north to south in locations south of Asahikawa. These include the Hidaka Mountains, Yubari Mountains (Yubari Santi) and Mashike Mountains (Masike Santi). Also, there are broad plains adjoining these mountains, including the Tokachi Plain and the Ishikari Plain. The primary active faults in Hokkaido lie adjacent to the edge of these plains. They have activity classifications of B and C class, and are located in the Tokachi-plain (Tokachi-heiya) fault zone, the eastern margin of the Mashike Mountains (Mashike-sanchi-toen) fault zone, and the eastern margin of the Ishikari-lowlands (Ishikari-teichi-toen) fault zone. Most of these faults run in a north-south direction, and are classed as reverse faults. This indicates that the central area of Hokkaido is being compressed in an east-west direction. Observation of crustal movements shows that the ground is being compressed in a roughly west-northwest to east-southeast direction. These factors have led to speculation that the central area of Hokkaido is being compressed in an east-west direction by a force resulting from the subducting Pacific Plate.

The area near the Oshima Peninsula (Osima Hanto) in southwest Hokkaido is believed to be a topographical and geological extension of the Ou Mountains (Ou Sanmyaku) in the Tohoku District. There are active faults at the western margin of the Hakodate Plain (Hakodate-heiya-seien) fault zone in Hakodate Plain, and at the Kuromatsunai lowlands (Kuromatsunai-teichi) fault zone in the Kuromatsunai lowlands extending, from Suttsu (Suttu) on the Sea of Japan (Japan Sea) coast through to Uchiura Bay (Utiura Wan). These are believed to be reverse faults with activity of B or C class. An active fault is also known to exist in the area near the Shiretoko Peninsula (Siretoko Hanto).

While almost no active faults have been observed in the Sarobetsu Plain in the northern part of Hokkaido, pronounced deformations on terrace surfaces and in strata have been observed. There is significant crustal movement that indicates compression from a west-northwest to an east-southeast direction, but the reasons for this movement are not yet understood.

There is no record of large M 7-level earthquakes having their focal regions in shallow inland taking place in Hokkaido. This can be attributed to the scarcity of information on Hokkaido earthquakes before 1868. Also, there are no known examples of direct action of existing active faults. Since the interval of activity for most active faults is 1,000 years or more, this does not mean that earthquakes have not occurred here. Of the destructive earthquakes in the past, there is the 1938 earthquake at Lake Kussharo near Teshikaga (M 6.1), during which slip occurred at the surface. Earthquakes of M 5-6 have also struck here. Historical accounts indicate that the 1834 earthquake near the mouth of the Ishikari River (Isikari Gawa) (M 6.4) known as the Ishikari Earthquake, was of this type. In present-day Sapporo, many traces remain of the ground liquefaction thought to have been caused by this earthquake. Other events include the 1995 earthquake east of Mt. Shokanbetsu (M 5.7) in the area of the Sorachi branch administrative office, and the 1956 earthquake off the coast of Abashiri (M 6.3) that caused slight damage on the Okhotsk Sea coast of Hokkaido.

Earthquake swarms are known to occur in eastern and southwestern Hokkaido. Most of these earthquakes occur near volcanoes. The magnitude of individual earthquakes in the swarms is usually less than M 5. Individual earthquakes with M larger than 5 rarely occur and cause damage usually restricted to near the hypocenter. The earthquake swarms usually occur within a short period lasting one to three months, but some have been known to last for more than one year. Examples of recent primary earthquake swarms include the 1964 swarm at Rausu, where the largest earthquake was M 4.6; the swarm that have continued since 1989 in the northern part of the area of the Tokachi branch administrative office, whose largest earthquake was M 4.8; the swarm off the coast of Hakodate that occurred from 1978 to 1980, and whose largest earthquake was M 4.2; and the swarm off the coast of Matsumae that started in 1995 and are continuing, the largest of which had a magnitude of 4.4.