SPATIAL RADON ANOMALIES ON ACTIVE FAULTS IN CALIFORNIA

Radon emanation has been observed to be anomalously high along active faults in many parts of the world. We tested this relationship by cond ucting and repeating soil-air radon surveys with a portable radon mete r across several faults in California. The results confirm the existen ce of fault-associated radon anomalies, which show characteristic feat ures that may be related to fault structures but vary in time due to o ther environmental changes, such as rainfall. Across two creeping faul ts in San Juan Bautista and Hollister, the radon anomalies showed prom inent double peaks straddling the fault-gouge zone during dry summers, but the peak-to-background ratios diminished after significant rain f all during winter. Across a locked segment of the San Andreas fault ne ar Olema, the anomaly has a single peak located several meters southwe st of the slip zone associated with the 1906 San Francisco earthquake. Across two fault segments that ruptured during the magnitude 7.5 Land ers earthquake in 1992, anomalously high radon concentration was found in the fractures three weeks after the earthquake. We attribute the f ault-related anomalies to a slow vertical gas how in or near the fault zones. Radon generated locally in subsurface soil has a concentration profile that increases three orders of magnitude from the surface to a depth of several meters; thus an upward flow that brings up deeper a nd radon-richer soil air to the detection level can cause a significan tly higher concentration reading. This explanation is consistent with concentrations of carbon dioxide and oxygen, measured in soil-air samp les collected during one of the surveys. Copyright (C) 1996 Published by Elsevier Science Ltd