Abstract
Weekly observation to monitor radon content changes in the soil gas using a track etch method has been carried on, on the lines across the several faults such as Fujigawa, Himenoyu and Inatori-Omineyama faults since May 1978, as a part of the earthquake prediction research program. This conventional method measures relative radon concentration in the soil gas by counting the number of tracks per cm2·day recorded on a small piece of cellulose nitrate film which is sensitive to α-ray radiation. Examination of the weekly track data on the lower reaches of Fuji river revealed that:
1. Temporal change in the number of tracks is related well to that in the radon concentration in the water of the randon observation well nearby.
2. It is suggested that a truck etch method can be a simple alternative method for the observation of radon concentration in the water by deep well, in some cases.
Studies of the weekly track data at Himenoyu and Inatori-Omineyama faults in the Izu peninsla verified that:
1. Temporal changes in the number of tracks at Himenoyu and Inatori-Omineyama faults are related to that of the daily number of earthquakes, that is, the increase in the number of tracks follows after the increase in the number of earthquakes.
2. However, the pattern of the increase and decrease in the number of tracks is different between the two faults. The difference seems to be caused by a difference in the maturity between the two faults which are ascending paths of radon.
3. For the purpose of earthquake prediction, the observationsite should be selected on an active fault of the highest maturity.
