Engineering-geology model of the seismically-induced Cerda landslide (Sicily, Italy)

On September 6, 2002, a MS=5.4 earthquake caused a large landslide close to the village of Cerda (Sicily, Italy) . The epicentre of the mainshock was located offshore, in the Tyrrhenian Sea, about 45 km NE of the city of Palermo and 50 km from the landslide site. The landslide involved an area of about 1.5 km2 and caused significant damage to farmhouses, roads and aqueducts. The observed ground crack pattern pointed to a translational mechanism for the landslide, consistent with a sliding surface of up to 60 m b.g.l.. The sliding surface was recognised by two boreholes within the landslide mass, respectively at 10 and 30 m b.g.l.. The resulting stratigraphy showed a shallow 13-m-thick layer of weathered and remoulded clays and the landslide mass s.s., which involved softened scaly clays. The landslide mass was characterised by a large basal shear zone, composed by a 7-m-thick oxidised layer of scaly clays, including a 2-m-thick layer of completely remoulded and oxidised clays. Cross-hole seismic prospecting as well as seismometric measurements were carried out in order to better define the engineering-geology model and the elastic properties of the landslide mass. The S-wave profile inferred from cross-hole measurements showed a sharp velocity increase just below the sliding surface where high consistency and undisturbed scaly-clays were sampled. Amplification in the range 0.5-1 Hz comes from the ambient noise in a wide area within the landslide mass, corresponding to the upper portion of the slope; this effect could be related to the structural setting of the geological bedrock in the landslide area. On the other hand, amplification in the range 2.5-4.5 Hz can be referred to the local thickness of the landslide mass. The records of two weak motions confirm the evidence given by the ambient noise analysis.