Radon as an Earthquake Precursor – Methods for Detecting Anomalies

Radon is one of many geophysical and geochemical phenomena that can be considered to be an earthquake precursor. Due to the non-linear dependence of earthquakes’ initial conditions, the question about the predictability of earthquakes often arises (Geller, 1997). The successful prediction of earthquakes is yet to be accomplished, in terms of their magnitude, location and time, and much effort has been spent on this goal. The term “earthquake precursor” is used to describe a wide variety of geophysical and geochemical phenomena that reportedly precede at least some earthquakes (Cicerone et al., 2009). The observation of these types of phenomena is one recent research activity which has aimed at reducing the effects of natural hazards. Among the different precursors, geochemistry has provided some high-quality signals, since fluid flows in the Earth’s crust have a widely recognised role in faulting processes (Hickman et al., 1995). The potential of gas geochemistry in seismo-tectonics has been widely discussed by Toutain and Baubron (1999). In the late 1960s and early 1970s, reports from seismically active countries such as the former USSR, China, Japan and the USA (Ulomov & Mavashev, 1967; Wakita et al., 1980) indicated that concentrations of radon gas in the earth apparently changed prior to the occurrence of nearby earthquakes (Lomnitz, 1994). The noble gas radon (222Rn) originates from the radioactive transformation of 226Ra in the 238U decay chain in the Earth’s crust. Since radon is a radioactive gas, it is easy and relatively inexpensive to monitor instrumentally, and its short half-life (3.82 days) means that short-term changes in radon concentration in the earth can be monitored with a very good time resolution. Radon emanation from grains depends mainly on their 226Ra content and their mineral grain size, its transport in the earth being governed by geophysical and geochemical parameters (Etiope & Martinelli, 2002), while exhalation is controlled by hydrometeorological conditions. The stress-strain developed within the Earth’s crust before an earthquake leads to changes in gas transport and a rise of volatiles from the deep earth up to the surface (Ghosh et al., 2009; Thomas, 1988), resulting in anomalous changes in radon concentration. The mechanism of observed radon anomalies is still poorly understood, although several theories have been proposed (Atkinson, 1980; King, 1978; Lay et al., 1998; Martinelli, 1991). Over the past three decades, the occurrence of anomalous temporal