GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 CF-Dip, 1195 m-long, orthogonal to the Mt. Cefalone fault) and WNW-ESE (profile CF-Strike 1315-m long, nearly fault-parallel). The line CF-Dip is tied to the Mt. Cefalone fault scarp since prohibitive topographic conditions prevented investigating the fault footwall. Seismic data were processed to obtain stack and depth-converted reflection sections with advanced techniques able to cope with laterally heterogeneous structures. Seismic reflection data illuminated the subsurface down to a maximum depth of 700-800 m. First-arrival traveltimes were inverted with a robust non-linear code to obtain multi-scale P-wave velocity models resolved down to about 300 m depth. The preliminary results can be summarized as follows. Profile CF-Dip indicates a clear syn-tectonic NE thickening of the alluvial and outwash glacial deposits in the hangingwall of Mt. Cefalone fault, with a pre-Quaternary limestone bedrock deepening northeastward down to about 300-350 m depth. Close to the bounding fault, laterally continuous reflectors showing on-lap terminations pass to SW-dipping steep and discontinuous events which suggest slope coarse deposits possibly faulted. Profile CF-Strike is tied to three boreholes published by Giraudi et al (2011) and shows fine details of the lacustrine infill sequence interfingering with clastic alluvial and glacial deposits, suggesting that the basin may be as deep as 600 m in this part. Seismic refraction tomography provides valuable information on the geometry of the carbonate substratum (very-high Vp regions), glacial and fan deposits (high-Vp) and alluvial/lacustrine sequences (intermediate to very-low Vp), thus facilitating the interpretation of the seismic reflection sections. The high-resolution ERT surveys were performed using 96 electrodes, 10-m-spaced, adopting both Wenner-Schlumberger and dipole-dipole configurations for data inversion; two ERTs were co-located with the seismic profiles. We used a roll-along acquisition geometry to cover a total length of 1200 and 1400 m. A third profile, 950-m-long, was purposely acquired orthogonally to profile CF-Dip to constrain the thickness of outwash glacial deposits. The results indicate that the infill material of the basin contains thick conductive regions in the southern part, related to lacustrine sequences, while in the central part thick resistive bodies dominate, due to massive moraines and widespread fans (see also presentation by Sapia et al., this session). The surveys were paired with tens of single-station seismic measurements, using 5-sec sensors recording for variable lengths of a few hours. Data were processed using standard techniques (horizontal to vertical spectral ratio HVSR) and more sophisticated ones (cross-correlation techniques). Preliminary results indicate that the HVSR function shows peaks around 0.5-0.6 Hz in the southern part of the basin, and peaks around 1-2 Hz in the central part close to the bounding fault, consistent with a basin deepening and a prevalence of lacustrine fine deposits to the south.