一軸圧縮過程のウエスタリー花崗岩から放出されるラドン濃度の変動 (1)

抄録

In order to understand mechanisms of radon accumulation in the soil gas around active faults and radon anomaly in groundwater or soil gas near epicenters before and after large earthquakes, we investigated variations of radon emanation from Westerly granites under uniaxial compression. Creep tests were carried out under multi-stress levels by increasing the stress stepwise and by applying constant stresses intermittently, to clarify the relations between stress, radon emanation, acoustic emission and strain.
The results of the experiments are as follows:
1) Accompanying acoustic emission, radon emanation began to increase around 140MPa of axial stress which corresponds to 70% of fracture stress (200MPa) of Westerly granite. Radon emanation increased over this stress, corresponding to every increase of axial stress. This suggests that gaseous radon appears outside the rock sample, passing through the channels formed by the network of stress-induced cracks inside the rock sample.
2) After the axial stress was removed, stressed rock samples that formerly showed increase of radon were still emanating the same amount of radon. This irreversible increase of radon emanation may indicate that the crack network remains inside the rock sample after removing the stress.
3) Remarkable increase of radon emanation appeard after the utimate fracture, which may be caused by increase of fracture surfaces and development of crack networks.
4) The α-ray intensity-time curves of the air around the rock sample indicated that most of radon consists of ²²⁰Rn (Thoron) by comparing the observed curves to the calculated curves for various initial ratios of ²²²Rn (Radon, 3.825 day half-life) and ²²⁰Rn (52 second half-life).
The above results indicating the strong relation between cracks and radon emanation suggest that high radon concentration around active faults may be caused by gaseous radon accumulation through the channels of dense networks of crack, joint and fault.