Radon content of groundwater as an earthquake precursor: Evaluation of worldwide data and physical basis

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Abstract
The properties of a worldwide data set of 91 radon (222Rn) anomalies (the frequency of occurrence, the precursor time interval, and the distribution of peak amplitudes) are correlated with earthquake data such as the respective magnitude and epicentral distance. These anomalies were reported as precursors to earthquakes in the United States, USSR, China, Japan, and Iceland. Although the data set is incomplete and limited by experimental deficiencies, several consistent patterns emerge. Radon anomalies from different tectonic regions show similar patterns. The radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude. Anomalies preceding large earthquakes (M ≥ 6) are frequently observed at a distance of 100 to 500 km. These distances are larger than several times the rupture dimensions of the future earthquakes. The time from the onset of an anomaly to the time of the earthquake (the precursor time) increases with magnitude but decreases with distance between epicenter and radon station. In addition, radon anomalies are observed more frequently prior to large earthquakes than prior to small ones, indicating that the preparation zone increases in size as magnitude increases. The peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude, the farther away the largest amplitudes tend to occur. The preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone. The outer radius of this annulus scales with the earthquake magnitude. Model calculations indicate that strain fields of at most 10−6 to 10−8 strain caused the radon anomalies. If these strains are divided by the appropriate precursor time, minimum strain rates from 10−7 day−l to 10−10 day−1 are obtained. Such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies. Large changes in the orientation of the local strain field, however, could occur and affect the local stress intensity factor. Since changes in the stress intensity factor can result in stress corrosion, the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater.