## 4 Discussion  
The obtained results of trends and residuals of \({}^{220}\)Rn in soils of OLB and MSA sites were compared to independent geo-indicator data used to depict the current phase of Campi Flegrei volcanic unrest, during the period 2011-2017 (Figure 4), characterized by ground uplift, increasing seismic activity and marked variation in the hydrothermal activity and geothermal fluid degassing. In particular, we considered the following data sets: (i) the ground deformation data synthetically represented by the maximum vertical displacement acquired by GPS networks [32]; (ii) the cumulative number of days with earthquakes since 2000, i.e., the background seismicity [32]; (iii) the major fumaradic tremor (RSAM) registered by the seismic station at of Pisciarelli fumarole [33]; (iv) the trend of CO\({}_{2}\) air concentration flux from Pisciarelli vent [6]; and (v) the temperature and pressure of the hydrothermal system [48]. As demonstrated by previous works in [20; 21; 22; 23; 32; 33; 48], the presently investigated geophysical-geochemical parameters are recognized as powerful indicators of the current unrest in the Campi Flegrei caldera. Fumaroles are in fact known to generate seismic tremors, which correlate with the variations in the ground shaking/deformation and in hydrothermal activity, controlled by the gas fluxes from the fumarolic vent, such as CO\({}_{2}\) concentration flux in air. From the comparison in Figure 4a, the thoron residuals (anomalies) at OLB and MSA sites mimic the prominent fumarolic tremor enhancement peaks, which practically coincide with the periods of highest seismic tremor of the hydrothermal area. This finding is especially significant, because the RSAM signal is directly controlled by the Pisciarelli emission. Figure 4b shows the correlation of the thoron trends at OLB and MSA sites with the following signals registered at Campi Flegrei caldera: ground deformation,  
Figure 3: (**a**) Anomalies of \({}^{222-220}\)Rn at OLB (first graph) and MSA (second graph) sites, obtained by subtracting the forecasted signals, using hybrid method MLR + EMD + SVR, from the raw recorded data, within a 95% CI. (**b**) Trends in \({}^{222-220}\)Rn at OLB and MSA sites, obtained by applying the MLR + EMD hybrid method. The accuracy of the measured data, i.e., the error bars, is an approximately constant 4%. the background seismicity and the trend of CO\({}_{2}\) air concentration flux computed in [6]. Actually, all these signals demonstrate a general similar increasing pattern. The results in Figure 4 suggest the high influence that the main geo-indicators of the current crisis at Campi Flegrei have on the thoron in the surface layer of the soil. Independently from the generation-propagation mechanisms of all the observed signals, they appear to be linked together. During long-term periods of volcanically induced crisis, characterized by continuous injections of magma that stall at very shallow levels, a volcano system passes through different stages of hydrothermal activity and degassing phenomena [49]. The dynamic changes in gas flow regime within the Campi Flegrei caldera system influence the RSAM signal, controlled by the Pisciarelli emission [50]. In particular, the increasing CO\({}_{2}\) degassing of Pisciarelli vent affects the increase in the fluid pressure (Figure 5b) of the Campi Flegrei hydrothermal system [19; 48]. Thus, the deformation of the soil and episodes of ground uplift, accompanied by correspondent seismic activity, grows, inducing an increase in the permeability and porosity of the media [32; 35; 51] that can induce an increase in the release of thoron on the surface and, therefore, the growing trend. Indeed, the CO\({}_{2}\) plays a key role in this process in controlling the migration and the transport of gas traces, such as \({}^{220}\)Rn and \({}^{222}\)Rn, towards the surface [52; 53; 54].