Increased radon-222 in soil gas because of cumulative seismicity at active faults

Abstract

This study demonstrates how the radon-222 ({222}Rn) concentration of soil gas at an active fault is sensitive to cumulative recent seismicity by examining seven active faults in western Japan. The {222}Rn concentration was found to correlate well with the total earthquake energy within a 100-km radius of each fault. This phenomenon can probably be ascribed to the increase of pore pressure around the source depth of {222}Rn in shallow soil caused by frequently induced strain. This increase in pore pressure can enhance the ascent velocity of {222}Rn carrier gas as governed by Darcy's law. Anomalous {222}Rn concentrations are likely to originate from high gas velocities, rather than increased accumulations of parent nuclides. The high velocities also can yield unusual young gas under the radioactive nonequilibrium condition of short elapsed time since {222}Rn generation. The results suggest that ongoing seismicity in the vicinity of an active fault can cause accumulation of strain in shallow fault soils. Therefore, the {222}Rn concentration is a possible gauge for the degree of strain accumulation.