GEOSTATISTICAL ANALYSIS OF SOIL GAS DATA IN A HIGH SEISMIC INTERMONTANE BASIN: FUCINO PLAIN, CENTRAL ITALY

Abstract

Numerous soil gas measurements of four gaseous species with very different geochemical behaviors were performed in the Fucino Basin, an area characterized by known and inferred structural discontinuities. A comprehensive statistical and geostatistical treatment of these data followed in order to provide insight into the spatial influence of tectonic discontinuities and geology on deep-seated gas migration toward the surface. The results yielded anomalies with different features, reflecting the different gas-bearing properties of the eastern seismogenic faults related to the 1915 earthquake (Mb = 7.0) and the hidden structural features occurring in the western side of the plain. In particular, this approach demonstrates that soil gas concentration (i.e., Rn and CO2) can identify the simpler normal faults of the eastern sector of the plain. In contrast, the more pervasive fracturing and faulting, as well as the occurrence of coarser deposits, on the western side of the area, make the location of faults less clear. The results show that gases migrate preferentially through zones of brittle deformation by advective processes, as suggested by the relatively high rate of migration needed to obtain anomalies of short-lived 222Rn in the soil pores. Furthermore, a geostatistical study of soil gas data was conducted to quantify the spatial domain of correlation and the gas-bearing properties of faults on the basis of shallow soil gas distribution (i.e., anisotropic behavior). The results provide a clear correlation between the shape and orientation of the anomalies and the different geometry of the faults recognized in the plain.