Across-fault distributions of radon concentrations in soil gas for different tectonic environments

Abstract

The radon concentration in soil gas at the crustal surface is closely related to the development of local fractures. Therefore, the spatial variation in soil-gas radon concentrations across faults differs between fault zones located in different tectonic environments. This study investigates the Beiluntai thrust fault and the Haiyuan strike-slip fault in west China, which are assumed to be typical examples of the different types of faults. Radon concentrations in soil gas were measured at various locations around these faults, and the spatial variation in these concentrations was characterized along fault-perpendicular profiles. For normal fault, our results indicate that concentrations are usually highest at the fault, and decrease gradually away from the fault in either direction. For thrust, however, concentrations increase in the hanging wall approaching the fault, but decrease sharply at the fault. For strike-slip fault with extension across the fault, soil-gas radon concentrations are relatively high at the fault, and decrease gradually with distance from the fault. For strike-slip fault with contraction across the fault, soil-gas radon concentrations are relatively low at the fault, and increase gradually with distance from the fault.