Self-excitation process due to local seismic amplification and earthquake-induced reactivations of large landslides

During the year of 2002 two large landslides were triggered in Italy by the 06/09/2002 Palermo earthquake (MI = 5.6) and the 31/10/2002 Molise earthquake (MI = 5.4), respectively close to Cerda (Sicily) and Salcito (Molise) villages. These landslides showed similar features: the involved volumes were in the order of 20-40 x 10(6) m(3) of varicoloured scaly clays; the extensions were about 1.5 km; the landslides showed translational mechanisms with failure zones up to 50 m below ground level (b.g.l.); average S-wave velocity values up to 400 m/s were recorded within the landslide masses; the epicentral distance was about 50 km. The Nakamura analysis of ambient noise records pointed out significant amplification effects within the landslide masses at about I Hz; these effects can be related to both the pre-existing landslide masses and the geological setting. Given the observed amplification effects, a self-excitation process can be hypothesised for the analysed seismically induced reactivations.