抄録
The seismicity in and near Hokkaido, Japan during the period from 1976 to 1980 has been studied by means of the telemeter observation system having nine high-gain seismograph stations.
The highest activity of shallow microearthquakes (0 to 60km depth) is in Pacific coast from the coast of Urakawa to east off the Shimokita peninsula. In northern part of this area great earthquake with M7 or greater has not occurred for 50 years. In southern part of this area the largest aftershock (M7.5) of the 1968 Tokachi-Oki earthquake (M7.9) occurred. Seismicities in the aftershock areas of the 1968 Tokachi-Oki earthquake and the 1973 Nemuro-Oki earthquake (M7.4) are inactive, while the number of microearthquakes in the aftershock area of the 1952 Tokachi-Oki earthquake (M8.1) has an increasing tendency since 1979. Teshikaga-Akan area of eastern Hokkaido, in which nine shallow earthquakes with magnitudes 5 or 6 occurred during the period from 1938 to 1967 shows low seismic activity during the period of observation.
The accurate location of microearthquakes in deep seismic zone shows two dipping zones 25-30km apart beneath the area from the southwest to the middle of Hokkaido. Beneath southern Hokkaido (the northern side of the Northeast Japan arc), the seismicity of the upper zone is more active than that of the lower zone, while beneath central Hokkaido (the western side of the Kurile arc), the seismicity of the lower zone is more active than that of the upper zone.
