Small-Scale Factors Controlling Onset of the Debris Avalanche of 4 March 2005 at Nocera Inferiore (Southern Italy)

The debris avalanche occurred on 4 March 2005 in Nocera Inferiore (SA) was a further case of deadly landslides in pyroclastic soil mantled slopes of Campania (southern Italy), which caused the loss of three human lives. The landslide was triggered by a heavy rainstorm occurred in the preceding hours and hydrologically predisposed by snowfall of the antecedent days. The landslide took place on a slope with approximately linear longitudinal and transversal profiles, with a slope angle varying between 30° and 40°. It involved ash-fall pyroclastic soils, mainly derived from the eruptive activity of the Somma-Vesuvius volcano, deposited on bedrock constituted of Mesozoic fractured limestone. As typical of this type of shallow landslides, the mass movement initiated with a small debris slide (about 400 m3) that activated a subsequent debris avalanche. Owing to the apparently equivalent influence of recognizable landslide susceptibility factors such as slope angle, slope shape and man-made morphological alterations, specific field surveys were carried out to understand features controlling landslide initiation and its precise localization. A total thickness of the pyroclastic mantle, generally varying from 1.5 to 3.5 m, and a spatially variable stratigraphic setting were found. The matching between the landslide initiation zone and the local increase of pyroclastic soil thickness is the most important outcome. This finding extends the understanding of the landslide susceptibility in pyroclastic soil mantled slopes of the Campania region (southern Italy), also associating it to the spatially inhomogeneous distribution of pyroclastic soils along slopes.