Estimation of active-fault shape using radon concentration in soil gas and numerical simulation

Radon prospecting using the α-scintillation counter method has been executed in the Futagawa fault area, situated in the southeastern Kumamoto plain, to clarify geometrical features of the fault. The Futagawa fault has proven to be active by the aerophotograph interpretations and field investigations. Along six lines perpendicular to the fault, 602 points are set at 2.5-m intervals to pump up soil gas from the borehole of 65-cm depth. A theory for calculating number of radon in the soil gas, which considers the gas condition with respect to the radioactive equilibrium, is applied to evaluate the true 222Rn concentration at each measurement point. The obtained concentrations are higher than those observed in the areas without active faults. It should be noted that several points on the fault system are found to have young soil gas under the nonequilibrium condition. Two characteristics of the radon concentrations can be pointed out as the follows: the first is good correlation between the concentrations and the locations of lineaments, and the second is asymmetric shape of the concentration anomalies. Based on a numerical simulation using the finite difference method for the diffusion and movement of radon, shape models of the Futagawa fault are constructed by fitting the calculated radon concentrations to the measured ones and, consequently, the fault is interpreted as normal fault type dipping northward with the range from 55deg to 76deg. The origin of the radon in the study area is also discussed using the radioactive measurement datasets of the samples taken from the borehole cores that penetrate the fault. (author)