Temporal Variation of Soil Gas Radon Associated with Seismic Activity: A Case Study in NW Greece

Soil gas radon concentrations were continuously monitored from November 2016 to May 2018, close to an active fault zone in the area of Ioannina (Northwestern Greece) that gave rise to intense seismic swarms with magnitudes up to 5.3 on the Richter scale, during October 2016. Meteorologic parameters (soil and air temperature, atmospheric pressure, wind speed and rainfall) were simultaneously obtained, and their contribution to radon fluctuations was examined by partial correlation and cross-correlation analysis. Soil temperature and atmospheric pressure were found to be the parameters controlling radon concentrations, and their effect was reduced using multiple linear regression analysis. During the monitoring period, 11 spike-like anomalies were identified in the residual radon time series using the 2 σ deviation criterion. The duration of the anomalies varied from < 1 day to approximately 5 days. Earthquakes of local magnitudes M L > 2.5, occurring within a distance of 100 km from the monitoring site, were collected and filtered by applying Dobrovolsky’s radius approach. Most of the observed radon anomalies were likely associated with seismic events, and the precursor time ranged roughly from 2 to 15 days.