Directional site effects in a non-volcanic gas emission area (Mefite d’Ansanto, southern Italy): Evidence of a local transfer fault transversal to large NW–SE extensional faults?

• We measure ambient noise close to a non-volcanic natural emission of CO2 rich gases.
• We examine polarization of ambient vibrations to infer fracture orientation.
• We compute the predominant lineament directions using images from Aster GDEM.
• We cross-check polarization and lineament orientations.
• We relate observations to a transfer fault between the regional NW–SE normal faults.

The technique of the wavefield polarization is applied to ambient vibrations recorded in the Mefite d’Ansanto area, an important non-volcanic natural emission of low temperature CO2 enriched gases. Twenty-five measurements were performed in the study area, eleven near the emission site and the other fourteen in different sites within an area of 5 km. Polarization is assessed both in the frequency and time domain through the individual-station horizontal-to-vertical spectral ratio and covariance-matrix analysis, respectively. We find a significant tendency of ground motion in the gas emission area to be polarized in the horizontal plane, with a N115° predominant trend. This polarization tends to disappear while moving far from the site. According to previous papers in other study areas, such a directional effect is likely caused by fault-induced fractures and tends to be orthogonal to the fracture strike. The predominant NW–SE regional faulting does not fit the N115° polarization direction. To explain observations, we propose an interpretation in terms of a NE–SW oriented, local transfer fault as inferred from the lineament analysis. The intersection of the damage zone of this fault with the regional NW–SE normal fault system could easily be the responsible for the gas emissions since it favors a locally increased crustal weakness.