Soil gas survey for tracing seismogenic faults: A case study in the Fucino basin, central Italy

Soil-gas (He, Rn, CO2, and CH4) surveys were performed to test their sensitivity for locating fault or fracture systems when masked by unconsolidated lithologies and to investigate the gas-bearing properties of seismogenic faults. The Fucino basin (Central Italy) was chosen as a test site because it displays a network of surface and shallow-buried active faults within the valley floor which were partially reactivated during the 1915 Avezzano earthquake (Ms 7.0). The highest radon values were found aligned along the most important faults, especially along the San Benedetto-Gioia dei Marsi fault (SBGMF) to the east and in the northwestern sector where the Avezzano-Celano fault (ACF) meets the northwestern prolongation of the 1915 coseismic rupture. Moderately anomalous values of radon occur along the faults located in the depression of the historical lake (Trasacco fault, TF, and Ortucchio fault, OF). Anomalous values of CO2, Rn, and He were detected north of Borgo Ottomila in a zone where coseismic scarps and liquefaction phenomena were responsible for a pervasive brittle deformation during the 1915 earthquake. Highest helium values prevail in the western part of the plain, in correspondence with a horst structure bounded by the TF and considered to be the extension of the Vallelonga-Trasacco ridge. Results of this research confirm the possible usefulness of soil gas methods for locating some tectonic faults when covered by unconsolidated deposits. The study provides some clues on the spatial influence of tectonics and geology on deeply-seated gas migration toward the surface.