Joint Interpretation of Geophysical Results and Geological Observations for Detecting Buried Active Faults: the Case of the “Il Lago” Plain (Pettoranello del Molise, Italy)

We carried out a study across an active normal fault system at the “Il Lago” Plain (Pettoranello del Molise, Southern Italy) by newly acquired geophysical data and literature geological data. We used low frequency GPR methods combined with seismic tomography methods. The usefulness of traditional GPR techniques in geological studies is affected by the limited obtainable depth of investigation. Previous studies indeed detected only the shallowest portions of faults. Conversely, our study demonstrates the optimal potential of combined seismic and deep GPR surveys for investigating buried faults at depth of tens of meters (down to 60 m in this study). This can be specifically applied to young structures that, despite their probable seismogenic potential, have not yet developed mature geomorphic features, or are buried under thick sequences of recent deposits. Even though the “Il Lago” Plain is a small basin within the Bojano fault system, our study added insights into the MPFS, recognizing four buried sub-vertical antithetic and synthetic normal faults that form horst and graben structures below the plain filling. GPR and seismic data analysis, allowed inferring, from different clues and evidence, the spatial continuity of fault systems previously unknown. This result is crucial given the seismotectonic importance of the Bojano basin, for improving the knowledge of the Holocene paleoseismological history. The geophysical evidence of Late Pleistocene to Holocene displacement beneath “Il Lago” Plain strongly suggests that recent sedimentation rates at this site are larger than the vertical component of normal faulting rate. A similar result was documented in the San Gregorio Magno Plain, where paleoseismic evidence of Holocene surface faulting was studied with exploratory trenching across the 1980 earthquake surface ruptures. Therefore, our investigations do not rule out that the 1805 surface faulting also affected the “Il Lago” Plain, as suggested by historical literature.