Increase in radon emission due to rock failure: An experimental study

Radon anomalies are commonly observed prior to dynamic failure in the crust and are interpreted as cracking of the medium, thus attracting considerable attention in understanding the precursory phenomena of earthquakes and volcanic activity. In this study we have compared the starting radon emissions from low porosity crystalline lava (phonolite) samples with those from damaged and failed samples. The damaged sample was loaded up to just beyond the end of the linear elastic phase, as evidenced by the output of AE energy, the increase in total porosity and a decrease in P-wave and S-wave velocity relative to the intact sample. Whereas, the failed sample showed deformation behaviour characteristically brittle with increasing values of AE output and porosity as the sample approached macroscopic failure. Radon measurements have evidenced that dilatational microcracking of deformed sample produced no significant variation in radon emanation with respect to the intact sample. In contrast, after macroscopic failure, radon emanation drastically increased. Therefore, major finding from this study is that, in the case of low porosity and relatively high strength crystalline lavas, the development of a macroscopic fracture provides new large exhaling surface resulting in a substantial increase in radon emission rate.