GNGTS 2018 - 37° Convegno Nazionale

GNGTS 2018 S essione 1.2 189 where the highest radon values were measured (Fig. 2). In correspondence of the first fault segment, radon peaks occur at the fault tips suggesting the presence of fracturing zones, as occur during the linkage process. The other radon peaks occur in correspondence of the highest offset peaks suggesting that in this cases the fault zone is still dominated by fracturing (Fig. 3b). The spatial variability of Rn and thickness values are also analysed by using unidirectional variograms to verify a spatial cyclicity of the peak values (Fig. 3c). The variograms are modelled with hole effect models that highlight variance peaks with a ciclycity of about 2000 m for radon, and a cyclicity of about 4000 m for the thickness. The cyclicity of the spatial distribution of offsets and radon peaks along the fault strike may confirm the presence of distinct segments of the SBMF with a typical geometry, e.g. relay ramps, suggesting that the linkage process is not completed. The double cyclicity of the cover thickness with respect to the radon values can be interpreted in terms of the evolution of the faults. In fact, as the most fractured area is usually located at the fault tip where relay ramps and secondary faults can be developed, this would result in a symmetrical and opposite distribution of the radon anomalies with respect to the progress of the offset (Fossen and Rotevatn , 2016). Conclusions. Radon has received much attention as an effective geophysical tracer of buried faults even in basins with high thickness of unconsolidated cover materials. Linear radon anomalies along the fault strike constitute the main distribution pattern suggesting the extension of the fault domain. However, the intensity of radon anomalies can vary along the fault strike due to permeability and/or porosity changes (i.e., geometry and maturity). In particular, radon distribution appears to be controlled by the highest offsets in the case of early stage of the fault and by presence of highly fractured transfer zones at the tips of the fault segments during the linkage process. This strict link between Rn and fault processes suggests the relation between the stress-strain changes along seismogenic faults and the anomalous signals in geogenic Rn time series. However, results have not been sufficiently conclusive so that a certain dissatisfaction has Fig. 3 - Radon and offset profiles along the SBMF (a); shallow radon signal and evolution of the fault zone. 1. Uniform extensive regime generating a series of en-echelon faults in the sedimentary cover, 2. that link up to a non-planar large fault (b); unidirectional variograms calculated for radon concentrations and isochrones values along the SBGMF strike (c).