NATURE OF SOIL GAS RADON RELEASE AND EARTHQUAKE PREDICTION

The elastic rebound was believed by Reid (1906) to be the immediate cause of earthquake. The theory has been confirmed over the years. The maximum deformation of rock units, therefore, occurs right before the spring back or rebound. Following the arguments of National Academy of Sciences (1976), there is no change in soil gas radon level during the initial energy accumulation stage. Radon levels start to increase when stress exceed one half of the rock strength. Spike-like anomalies start to develop when micro-fractures start to form and groundwater is quickly moving into the fractured and expanded rocks. The groundwater charging purges radon accumulated from radium decay in the micro-fractured rock matrix. This leads to a quick and intermittent with a subsequent short period of low releases of radon.

The spectra of continuous monitoring of soil gas radon with an improved solid-state detector placed in an active fault zone in south central Taiwan appear to support these theoretical predictions. The observation is a quick raise in radon level about a couple of weeks before a noticeable earthquake, and a peak precursor one to seven days before the occurrence. Noticeable earthquakes occur frequently in Taiwan that permits us to recognize stressed and relaxed states of the rock formations by using the recorded radon spectrum. With additional radon detecting units, a stress domain could also be determined. In addition, radon is found to correlate with total soil gas in release rate from measurements of gas bubbles out of a pond in a fault zone. Radon correlates with CH₄ and C₂H₆ in release rate as well. However, radon has larger fluctuations over time than CH₄and C₂H₆,and thus a better precursor. Other gases measured, including CO₂, H₂, He, H₂O, N₂, O₂ and Ar, are not sensitive to earthquakes. An identical radon monitoring system is placed over North American craton in Akron for comparison. Soil gas radon level is low and variation over time is small when compared with the recordings in Taiwan.