Hydrogeological, kinematic and stability characterisation of the 1993 Senerchia landslide (Southern Italy)

Abstract

The study area is located in southern Italy, south east of Naples. Slope instability phenomena and floods have long been a constant feature of this portion of the Apennines, with earthquakes and precipitation representing the main triggering factors. Landslides have often resulted in dreadful economic and human losses. The 23 November 1980 earthquake (M=6.9), which struck a large area in Southern Italy particularly harshly, caused the reactivation of many previously dormant landslides along the upper valley of the Sele River. The village of Senerchia, located on the right-hand side of the Sele valley, was seriously damaged by the collapse of various buildings as well as by a large mass movement (SE1 landslide) on the south eastern side of the village. In the study area the outcropping lithofacies can be divided into four types. 1) Limestones and dolomitic limestones (Triassic-Cretaceous) outcrop widely, mainly from 650-700 to 1800 m asl. 2) Clayey-marly and clayey-marly-arenaceous flysch, shales, marls, chert limestones, sandstones and varicoloured clays (Upper Cretaceous - Paleocene) outcrop widely, mainly from 150-200 to 650-700 m asl. 3) Detrital and breccia deposits of rockfall or scree (Quaternary) have carbonate or pyroclastic nature and outcrop at the foot of the carbonate relief. 4) Alluvial deposits (Pleistocene-Holocene) are present along the course of the Sele River and its main tributaries and outcrop from 150-200 m asl to the valley bottom. In the area surrounding Senerchia the detectable hydrogeologic units are made of carbonate rocks that exhibit the highest relative permeability. These carbonates establish contact with the remaining two significant hydrogeological lithofacies, the detrital and breccia deposits and the flysch. The main springs of this hydrogeological unit are found at the lowest point of this contact. The detrital and breccia deposits constitute a slab which is in contact with the carbonate rocks in the west and with the SE1 landslide in the south east. The relative permeability of the slab is medium to high; thus, it can be considered a secondary aquifer. The flysch lies at the base of this slab. Wherever the flysch does not outcrop, it underlies the lithofacies, with which it is in contact. This complex acts as an impervious boundary for the carbonate rocks and the detrital deposits. The role of the impervious limit played by this complex does not improve the stability of the slope where the Senerchia settlement occurs. Water losses from the two hydrogeological water-bearing lithotypes toward the area under study have to be assumed. Following the main shock of 23 November, the SE1 landslide was gradually mobilised over a period of a couple of weeks. The aftershocks activated a 2,500-m-long, 500-m-wide mudslide, and mobilised a soil mass of about 28x106 m3. Several minor mass movements affected the whole slope, particularly along the flanks of the main landslide body. More specifically, a subsidiary landslide took place on the middle-upper portion of the left bank of the main landslide. The SE2 landslide originated from this in 1993 and continue to be still active today.