GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 - POSTER GNGTS 2023 Fig. 3 – ERT across the supposed fault scarp of Piano de Rossi (DD, Dipolo-Dipolo; WS, Wenner- Schlumberger; length 590 m; see Fig. 2). This preliminary elaboration, without topography, shows very well the subvertical contact between bodies with different resistivity just below the fault scarp. Warmer colors indicate the alluvial fan gravels, thickened in the hanging-wall; colder colors likely indicate the outcropping Pliocene sand and clays. The offset of the bottom of the paleofan reaches 25 m, twice as much as that of the surface. This may imply that the alluvial fan piled up following the downthrown of the MFF hanging-wall, i.e., during the strongest activity of this fault, the movement of which provided the mechanism that maintained the high gradient necessary for fan development. At the same time, this explain why the fan thickened in its hanging-wall. The 11 m offset of the top-surface can be interpreted as the residual fault activity recorded by the fan, when it was already extinct and suspended above the surrounding landscape. Considering the age indicated in Robustelli and Muto (2017), we can conclude that the fault activity itself ended at the beginning of the Middle Pleistocene. Conclusions From all the above, we can conclude that the presumed activity of the western fault system of the Crati basin, and particularly of its master fault, is not supported by any geological data, but is in fact contradicted by the ground evidence. These testify to a strong activity of the MFF between the Lower and Middle Pleistocene, ended at the latest during the Middle Pleistocene, from when it either no longer moved at the surface or, unlikely, it has moved at negligible rates, two orders of magnitude less than the Apennine active faults. This conclusion is not at odd with the historical seismicity of the area, and in particular with the sequence of events that affected the Crati Valley progressively from north to south in the 18th-19th centuries. In fact, the magnitude of these events (estimated from macroseismic data close to Mw~6) corresponds to the lower limit of occurrence of coseismic rupture at the surface, as observed in the most recent earthquakes of the Apennines.