#gamma#-ray Prospecting for Locating of an Active Fault on the Alluvial Plain

摘要

The Shinji Fault runs through central Shimane Peninsula, southwest Japan, and cuts Miocene strata. Part of the fault has been verified as active right-lateral. Gamma ray prospecting to detect the exact position of the fault beneath fluvial sediments was carried out in the Minami-Kobu district, where trench survey confirmed its active nature, and in the Noza-Nishi district where a distinct photo lineament representing the fault can be observed.In both districts survey lines about 100 meters long were set up at nearly perpendicular to the fault trace at localities where surface and moisture conditions were similar. Gamma rays were measured for five minutes with a NaI scintillation counter at stations at intervals of three meters along the survey lines. Prospecting was carried out only on days when rain was not falling. Measurements were taken beginning after 11:00 am, because gamma rays count rates are unstable in the early morning, due to vapor escaping from the ground. Prospecting was carried out four times at the same points between July and December.Repeat measurements were taken monthly at several control sites over 1 year to determine seasonal and daily variations. From these data, deviations of the ratios of counts of ~(214) Bi to ~(208) T1 greater or less than 15 percent from the mean were defined as anomalies. On plots of average values and anomalies of four measurements in both districts, positive anomalies always appear at stations beneath which the fault is thought to exist. The few negative anomalies which appeared at times at some other stations can be explained as background noise.Previously, gamma rays prospecting has been thought to be of limited use for detecting exact positions of active faults when overlain by recent fluvial sediments. However, our study shows that such prospecting is a cost-effective and simple method for accurately locating active faults when carried out as described above.