伊豆半島及び富士川周辺におけるαトラック法による活断層調査

抄録

Application of a track etch method, for the mapping and detection of active faults and the evaluation of their activities has been carried out at 10 sites for the purpose of the earthquake research.
The method is applied conventionally for the measurement of relative random concentration in the soil gas by counting the number of tracks per cm²·day on a small piece of plastic film (cellulose nitrate) which is sensitive to α-ray radiation.
As the results of the track measurements on many survey lines crossing 10 active faults including earthquake faults in the areas of the Izu Peninsula and those in the lower reachs of the Fuji river, it was clarified that:
1. The peak of the track number appears just above the fault line generally, and sometimes shifts from it slightly.
2. The line connecting peaks on the several survey lines is parallel to the strike of of fault.
3. Relative position between the peak and the fault line on the surface suggests the type of the fault, normal or reverse.
4. The number of tracks counted on the Inatori-Omineyama and Sengenyama earthquake faults, which occured by Izu-Oshima-Kinkai Earthquake in January 1978, was anomalously high as much as several times of that on the existing active faults. This fact indicates that the supply of radon contained gas from the deep crust through fault plane would be activated by the crustal disturbance.
In addition to the above, weekly observation to monitor changes in radon content in soil gas using a track etch method has been carried on across the several faults such as Inatori-Omineyama, Himenoyu and Fujikawa faults for the purpose of the earthquake prediction, by present authers.