Foreshocks and Aftershocks of an Earthquake East of Hachijo-jima, December 4, 1972

On December 4, 1972, an earthquake of M=7.2 occurred near the aftershock area of an earlier earthquake east of Hachijo-jima, February 29, 1972. We carried out aftershock observations at Hachijo-jima. We determined hypocenters of foreshocks. Clearly, three foreshocks occurred from 3 days to 5 hours before the main shock. The locations for three foreshocks are inside and eastside of the aftershock area for the earthquake, December 4. The main shock occurred at the eastward boundary of aftershock area for this, and then aftershock expanded to westward. The aftershock area is connected to the westside boundary of that for the earthquake, February 29. The size of aftershock area in December, 1972, is about 60km×40km, and larger than that of aftershock in March, 1972. The aftershocks occurred on the plane steeply inclining from north to south. In order to interpret the occurrence of these two earthquakes east of Hachijo-jima, we proposed a model.