抄録
The Urakawa-Oki earthquake (M=7.1) occurred offshore of Urakawa, Hokkaido, Japan, on March 21, 1982. In order to investigate the aftershock activity of this event, we deployed four ocean bottom seismographs (OBS's) off Urakawa. During an observation period from March 29 to April 6, the OBS network detected more than 4, 500 earthquakes. Magnitude range of these events is from -2.0 to 5.0.
Aftershock distribution of the Urakawa-Oki earthquake was determined on the basis of the OBS and land observations. In the analysis, we selected about 250 aftershocks with M≥2.0, because the seismic signals from these events were recorded clearly both by the OBS's and the land stations around the aftershock area. Hypocenter determination was carried out using a method of inversion analysis. Most of the aftershocks were located in a region of 42°00'-42°20'N and 142°25'-142°40'E. These events were distributed in a depth range of 10-30km. The characteristic dimensions of the aftershock area were estimated as 35km×25km. The events in the southern part(south of 42°15'N) were distributed on a plane with an area of 20-25km×10-15km dipping 20-30° northward. In the northern part of the aftershock area, we found another trend of aftershocks which inclined southward with a high angle of 60-70°. The events in this group were located on a plane with a relatively narrow area of 10km×10-15km. The aftershock distribution obtained in the present analysis suggests that the fracture mechanism of the Urakawa-Oki earthquake was very complicated.
