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

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 proxy of geodynamic processes is not yet resolved unambiguously. The Sopronbánfalva Geodynamic Observatory in Hungary is one of the few places where the temporal variations of the high radon concentration and the 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 domain on the basis of seven-year long data series measured in years from 2009 till 2015. The relationship between rock deformation and radon concentration were 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^-1 – 10^-2 kBq nstr^-1, while the concentration variations bear more considerable similarity and relation to the temperature variations. The theoretical tidal potential at the location of the measurement site and tidal components computed from strain and radon concentration data were compared with each other. Spectral analysis of the theoretical tidal, strain and radon concentration 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 tidal evaluation also proved the lack of the principal lunar semidiurnal M2 and diurnal O1 tidal waves, which have the strongest effect on the deformation of the solid Earth, but they are explicit components in the theoretical tidal and rock strain variations. These results does not reveal any connection between radon concentration variations and Earth’s tide induced rock strain at the measurement site and the tidal components appearing in the radon concentration are presumably due to the random variation of the weather.

This research was funded by the Hungarian National Research Fund (OTKA) under project K 109060.