Investigation of the relationship between rock strain and radon concentration in the tidal frequency-range

Changes in the radon gas concentration can precede geodynamic processes associated with tectonic, volcanic activities and earthquakes. For this reason the relationship between rock strain and radon concentration is an important scientific issue to be answered. According to the complexity of the radon emanation process influenced by environmental effects, the interpretation of radon concentration variation as a possible precursor of geodynamic processes is not yet resolved unambiguously. The Sopronbánfalva Geodynamic Observatory in Hungary is one of the few places where radon concentration and rock strain variations are simultaneously monitored. The object of this study is to investigate the connection between indoor radon concentration and rock strain in the tidal frequency-range on the basis of seven-year long data series measured in years from 2009 till 2015. The relationship between rock deformation and radon concentration was investigated together with the temperature and barometric pressure effects. It was found that the strain induced radon concentration variations are in the order of 10-100 Bq nstr−1, while the concentration variations bear more considerable similarity and relation to the temperature and barometric variations. The theoretical tide at the location of the measurement site and tidal components computed from strain, radon concentration, barometric pressure and temperature data were compared with each other. Spectral and tidal analysis of data demonstrated that only the thermally induced solar components S1 and S2 are present in the radon concentration but their amplitudes hardly exceed the spectral noise level. The principal lunar semidiurnal M2 and diurnal O1 tidal waves cause the largest rock strain variations. The lack of the O1 and M2 constituents in the radon concentration confirms the fact that the detected S1 and S2 tidal components appear due to the barometric tide and the daily variations of the temperature and barometric pressure.