Characterising the spatial distribution, frequency and geomorphic controls on landslide occurrence, Molise, Italy

An 815-km2 area in Molise, central Italy, was used as a natural laboratory to characterise the lithological, topographic, and fluvial controls on the spatial distribution and frequency of active landslides in temperate climates. The study area is part of the central Apennines fold-and-thrust belt and is characterised by the presence of a tectonic mélange of scaly clays, known as Argille Varicolori, as well as more competent limestone and sandstone units. The area has been subjected to a cumulative stream incision of up to 400 m since the mid-Pleistocene. The landslide inventory was developed by combining satellite image interpretation and existing data sets, ground-truthed in the field. Statistical analysis of the inventory data demonstrates that the coupling between landslide occurrence in the area and lithology is strong, with landslides occurring 3–10 times more frequently where the mechanically weak Argille Varicolori (scaly clays) outcrops compared to other lithologies. Analysis of the relationship between landslide occurrence and hillslope gradients shows that 50% of all landslides occur on slopes between 10 and 15°, despite this interval representing only 30% of hillslope gradients in the study area. These results demonstrate the major control that the properties of the regionally significant Argille Varicolori have on landslide occurrence in the area, directly and through their control on hillslope gradients. Additionally, our results show that more than 50% of the landslides occur at a distance of < 260 m from the nearest stream, a value which is 4 times smaller than the median distance (~ 1 km) between the interfluves and the valley floors within the study area. These data therefore demonstrate the important role that ongoing fluvial incision plays in governing the locus and frequency of landslides in an area in which the streams incise at rates of 0.4 to 0.7 mm/y. Finally, an analysis of the landslide size–frequency distribution revealed that the power-law exponent on the area term is 1.56 ± 0.2. This is toward the lower range of values published in the literature and suggests that in this part of the central Apennines active landslide frequency declines more slowly with size than is often modelled. More widely, this study provides new insights into the coupling between fluvial erosion and landsliding in an area of active tectonics and illustrates the fundamental role of lithologies such has the Argille Varicolori in determining landslide susceptibility.