Detecting time series anomalies using hybrid methods applied to Radon signals recorded in caves for possible correlation with earthquakes

Anomalies in the Radon activity concentration time series recorded in five European caves (Czech Republic, Slovakia, Slovenia) are detected using three hybrid methods: (1) multiple linear regression and autoregressive integrated moving average statistical methods, (2) Empirical Mode Decomposition with Support Vector Regression techniques and (3) the Singular Spectrum Analysis composed with a predicting methodology. Results coming from the three methods are compared and the best hybrid method is selected based on statistical evaluation criteria of the uncertainty. Radon anomalies occur ± 30 days from earthquake occurrence, selected according to the Dobrovolsky’s earthquake preparation zone formula and to seismic events (with magnitude ≥ 4) occurred in the neighboring European Countries to the monitoring caves. The anomalies detection furnishes results consistent across the used methodologies, as proven by the calculation of a statistical parameter that search the presence of anomalies coming from the hybrid methods within ± 30 days from earthquake event.