The origin and detection of spike-like anomalies in soil gas radon time series

Abstract

Desorption of radon is of short duration but longer than telluric noises and is usually accomplished in less than one minute based on our binary event counting with a 16 second interval. Adapting an hourly counting time schedule, telluric noises as well as transient variation of radon anomalies of a few hours duration can be excluded but not the spike-like anomalies of a few minutes duration. The time series of soil gas radon, as recorded continuously in a redesigned ditch located inside an active fault zone, has sporadic spike-like anomalies. The anomalies could represent desorbed radon from shallow sources as observed from a similar detecting system located near a construction site. Radon release due to compressive stress is related to compressive back-filling activities. Spike-like anomalies, on the other hand, are related to caisson auguring shear activities. Charge transfer related to electrokinetic potential phenomena may have resulted in the sudden radon releases and caused spike-like anomalies. The timing of the anomalies provides good information on the timing of electokinetic potential change as a result of outward propagation of stress from hypocenter. With multiple monitoring stations, the location, timing, and magnitude of the incoming earthquake can then be calculated or graphically derived.