GNGTS 2013 - Atti del 32° Convegno Nazionale

after the 1963 failure, confirm the hypothesis of a rigid roto-translation of the collapsed mass. The large anomalous thickness of the c - d -e sequence is probably caused by some detachment phenomena occurred along existing or newly developed high-angle discontinuities (Rossi and Semenza, 1965; Martinis, 1978) during the failure. The profile also show some low angle detachment planes (see for example at the x-coordinate interval 200-300 m) that could explain the anomalous thickness of the c , d and e units in depth. The resistivity distribution along profile ERT5 (Fig. 2), crossing lobe B, ismore homogeneous and it’s more and less comparable with the post-failure geological map of Rossi and Semenza (1965). The outcrops are represented by an undifferentiated unit a covering almost the entire lobe B (Fig. 1) and by some minor exposures of the unit b . The general structure appears to be folded up into an anticline with some major displacements. In this case also the geometry of the bottom layers of lithological unit a’’ was utilized as the prominent geophysical marker to define the settings. In the reference section unit a’ is not exposed and hence there are no indications about its possible geophysical response. Carloni and Mazzanti (1964) suggest a total thickness of about 80-90 for the undifferentiated unit a with a maximum thickness of unit a’’ around 50 m. According to several other authors (Rossi and Semenza, 1965; Martinis, 1978; Hendron and Patton, 1985) unit a’ is similar to unit a’’ with the sole difference of the presence of the interbedded clays in the base layers. The two units should then exhibit a comparable geophysical signature. The increase of the resistivity in the deeper portion of the profile (where unit a’ was expected) is rather difficult to explain. There are possible explanations: the upper part of unit a’ is more resistive than expected; more realistically unit a’’ overthrusts the resistive terrains belonging to the c - d units in the deeper portion of profile. Several high and low angle discontinuities are also visible in the profile. These planes disrupt the former continuity of the geological layers generating an ensemble of pop-up like structures. Unfortunately on this lobe the sliding surface is not constrained by post-landslide data and also during the failure occurred a partial overlap of lobe B on the Massalezza lobe. The deeper portion of the geophysical image could be then really complex. Fig. 2 – 2-D Resistivity inversion images along tomography ERT 1 (top) and ERT 5 (bottom) (see Fig. 1). The investigated landslide mass could be then considered dry. 194 GNGTS 2013 S essione 3.3