Analyzing the 222Rn/220Rn Ratio in a Seismic Area: A Reliable Method to Understand the Development of Active Structural Discontinuities in Earthquake Surveillance and Sustainability

Studies on the individuation of surface and buried faults in seismic areas using geochemical methods can be considered a valid approach for improving sustainability in the risk assessment framework. Appropriate scientific knowledge of structural geology and its evolution pre/during/post seismic events can play a fundamental role in human safety and resilience. The Abruzzo region (central Italy) underwent to a Mw 6.3 seismic event, in April 2009, that interested L’Aquila city (the county seat of the region) and many villages in the surrounding area. A first soil gas survey including radon (222Rn) and thoron (220Rn) measurements was accomplished soon after the main shock, in an area of ~24 km2 a few kilometers away from L’Aquila city. Results highlighted the spatial influence of the active tectonic on gas migration towards the surface. The area was investigated again in spring 2016, both to evaluate the natural degassing during a period without further meaningful earthquakes and to verify the presence of faults supposed after the previous survey results. Comparing data from the two surveys, a variation in the 222Rn/220Rn ratio was observed, suggesting different sources (deep or superficial) of gas degassing strictly correlated with the temporal variation in soil vertical permeability. Furthermore, the results infer a new structural system different from that known in the literature.