Analysis of radon measurements in relation to daily seismic activity rate of Vrancea region, Romania

Many previous research studies have shown how local, and even regional, earthquakes can significantly affect the release of radon in the soil. The aim of this research work is to investigate the relation between radon measurements and the daily seismic activity rate, and develop a methodology that allows estimating the seismic activity rate using just only radon measurements. To carry out this study, the earthquake catalogue of the Vrancea region (Romania) has been used to estimate the daily seismic activity rate during a given time period in which radon measurements were also recorded, from January 2016 to September 2020. The Vrancea zone represents the most active seismic zone in Europe and is located on the Eastern edge of the strongly bent Carpathian arc. In the case of the radon measurements, seasonal behaviours and linear trends due to non-seismic factors have been identified and subsequently removed. The discrete wavelet transform has been used to analyze the radon signal at two different scales: long and short periods. From the analysis carried out in the long-period scale, an approximate linear relationship has been obtained between the radon series and the daily seismic activity rate, which provides insights into the behaviour of the seismic activity in the study region with only the radon information. Besides, the study carried out reveals certain characteristics that could be used as precursors of earthquakes at different scales: weeks in the case of the estimated daily seismic activity rate, and days in the case of the short period signal obtained by the wavelet analysis. The results obtained for this region allow us to hope that the analysis of the radon time series can become an effective complement to conventional seismic analysis used in operational earthquake forecasting.en