GNGTS 2018 - 37° Convegno Nazionale

188 GNGTS 2018 S essione 1.2 are several slip planes in the active fault zone with different offsets along strike; this suggests that the linkage process is not completed and that the main fault is still formed by a series of segments. On the contrary, at the final stages the zone is largely dominated by a main fault (Fossen, 2010; Peacock et al. , 1991). This peculiar evolution of the fault zones will affect the shallow distribution of radon that can show anomalies of various shape and intensity. Fig. 2 shows the isochrones contour map (continuous lines) expressed in TWT (interval 50 and 100 ms) of the alluvial and lacustrine deposits (Cavinato et al., 2002) overlapping the image color map of radon concentrations in soil gas. As reported in Cara et al., 2011, assuming a Vp of 2,000 m/s the values of TWT can be interpreted as thickness of sedimentary cover in meters. The cover thickness is then used to estimate the different offsets (light green area in fig. 2) along the main faults. Fig. 2 highlights that radon spot and/or linear anomalies (> 30 kBq m -3 ) occur along the strikes of the main faults (buried and exposed) that intersect the basin. We focused the study on the spatial distribution of radon anomalies along the SBMF because it shows the highest offsets. Assuming that the highest thicknesses measured along the SBGMF correspond to the major offsets, NW-SE profiles of the cover thickness and of the radon concentrations have been elaborated along the fault strike (Fig. 3a). The black segments along the SBGMF represents the fault segments to which corresponds a thickness above the threshold value of 500 ms calculated using a normal probability plot of TWT values. Figure 3a shows a significant coincidence between the peaks of the two variables. In particular, the presence of 4 peaks above the threshold value of 500 ms suggests that the SBGMF shows high and low gas-permeable zone along strike, while low thickness values suggest the presence of fault tips. The thickness decreases along the fault toward SE. Radon profile shows the major peaks in correspondence of the highest offsets, and radon signal also decreases toward SE (Fig. 3a). The presence of high radon values at the starting point of the profile is caused by the intersection of the SBMF with the ACF to the north Fig. 2 - Map of the 222 Rn distribution across the Fucino plain. Linear radon anomalies occur along the main faults (buried and exposed) crossing the basin. The map also shows the isochrones (in ms) of the lacustrine deposits reworked by Cavinato et al. , 2002. The isochrones map allows the calculation of the fault offsets (green area) along the SBGMF thus suggesting the presence of several faults segments. Highest radon anomalies occur in correspondence of these recognized offsets.