GNGTS 2013 - Atti del 32° Convegno Nazionale

3D GEOPHYSICAL MODELLING OF THE VAJONT LANDSLIDE R. Francese, M. Giorgi, G. Böhm OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy Introduction and Motivations. The 1963 Vajont landslide, mostly because of its size and of the catastrophic effects, has been studied for several tens of years by many different authors (Selli and Trevisan, 1964; Rossi and Semenza, 1965; Martinis, 1978; Hendron and Patton, 1985; Semenza and Ghirotti, 2000). The majority of the studies focused on the triggering mechanisms (Kilburn and Petley, 2003) and on the post-failure geology. A comprehensive review is given by Semenza and Ghirotti (2005). Although the collapse has been largely studied some of the factors controlling the dynamic of the movement are still not completely clear. Among the major issues there is the high velocity of the sliding mass itself and the movement almost as a unitary rock block that caused such an unexpected large wave. A geophysical parameterization of the landslide body could result in a better insight in both in the understanding of the geometry settings and the topology of the collapsed units and in a better estimate of the changes of the elastic properties caused by the collapse. This last information could be used as a vital constrain in the recently developed collapse model. Very few geophysical data are presently available for the landslide body. Some measurements of the P- and S- wave velocity were undertaken on the northern scarp of the Monte Toc before the collapse but with conflicting results and no clear indications about the rock quality (Caloi and Spadea, 1960, 1961). After the collapse, on behalf of the Court of Belluno, was carried out a seismic campaign with P-wave velocity and borehole sonic measurements on the landslide body (Morelli and Carabelli, 1965). A new and comprehensive geophysical investigation, based on 2D and 3D seismic and resistivity imaging was then undertaken since the year 2011 and it’s still in progress. This geophysical experiment was designed and conducted integrating the re-interpreted geological (Bistacchi et al. , 2013) and structural data (Massironi et al. , 2013). A novel series of borehole stratigraphies made recently available by ENEL were also incorporated in the geophysical modelling. Finally some accurate surface models obtained processing aerial and terrestrial laser data were extremely useful to constrain the inversion of resistivity and seismic data and to properly account for the deformation of the electrical field caused by the rough topography. A geophysical profile collected along the rock wall below Casso on the other side of the Vajont valley was used a reference for the geophysical response of the geological units involved in the landslide. The geophysical images of the landslide body, especially in the near surface, showed a very good correlation with the post-failure geology (Rossi and Semenza, 1965). An initial correlation between lithology and physical parameters has been proposed for the various geological units embedded in the landslide mass. This 3D physical model of the landslide introduces a series of new constrains for an accurate numerical simulation of the landslide kinematics. Geological setting. Geology in the Vajont valley is comprised of a Jurassic-Cretaceous carbonate sequence (Carloni andMazzanti, 1974; Semenza, 1965;Martinis, 1978). The thickness of the various formations, at the landslide scale, could be considered roughly constant. The base of the Jurassic sequence is marked by a massive (Vajont Formation) limestone (350-400 m) overtopped by a layered cherty (10-40 m) limestone (Fonzaso Formation) and followed by the nodular limestone of the Ammonitico Rosso Formation (15 m). The Cretaceous sequence is comprised of the Soccher Limestone (200-250 m) and of the layered marly limestones and marls of the Scaglia Rossa Formation (about 300 m). The upper part of the Fonzaso Formation and the Soccher Limestone Formation were involved in the landslide. Rossi and Semenza (1965), analyzing the post-failure accumulation, mapped six different lithological members indicated with letters from a to f (from the older to the youngest). 190 GNGTS 2013 S essione 3.3