## 2 Materials and Methods  
### Study Area and Landslides Inventories  
The Imera Settentrionale River basin is 343 km\({}^{2}\) long in northern Sicily (Italy, Figure 1a), between the northern sector of the Madonic Mountains and the Tyrrhenian Sea. The central-western sector of the Sicilian Fold and Thrust Belt, in which the basin falls, is the result of the retreat of the subduction hinge of the Ionian oceanic lithosphere and the post-collisional convergence between Africa and Europe [14; 15; 16]. The intense compressive tectonic activity starting in the Oligocene [17] was followed by a syntectonic relaxing phase, during which the thrust sheets' basins were filled with terrigenous deposits [18]. Due to this tectonic activity, the basin is characterised by a hilly landscape and isolated carbonate reliefs. Rills, gullies, pipes, badlands systems [19; 20; 21; 22; 23; 24] and landslides [25; 26; 27; 28; 29; 30] model the slopes where carbonate, siliceous-carbonate and siliciclastic successions alternate with more ductile pelitic flysch sediments, alluvial clastic sediments and marine pelite outcrops [29; 30]. As statistical models are calibrated on landslide sets for which common factors are expected to control the potential instability conditions, a susceptibility-analysis-oriented re-classification scheme was proposed here. In particular, three types of landslides are defined in the landslide inventory (Figure 1b): \"slides\", \"slope flows\" and \"local flows\". In particular, rotational and translational slides (1608 cases--Figure 2a,c,d) are grouped together as we assumed to be explainable by recalling the role of the same predictors. In fact, they are strongly controlled by the type of outcropping lithology, being typically generated from alternances of pelitic and arenitic layers. In fact, the arenitic caprock allows for the maintenance of steep slopes and for the required infiltration downward to the pelitic (silty to sandy clays) levels, feeding the failure surfaces. At the same time, in this research, small (local) and large multi-storied (slope) flow landslides were split into two classes. Slope flows (5134 cases--Figure 2b,e) are typical in the Sicilian Apennines and correspond to very large, non-rapid earth/debris flows, frequently involving those slopes totally set on pelitic units. Both slides and slope flows are characterised by large extensions and diachronic activity. In the susceptibility analysis, LIPs were extracted along the crown boundary at  
Figure 1: (**a**) Location of the Imera Settentrionale river basin; (**b**) distribution of slide, local-flow and slope-flow LIPs. In square brackets, the count of LIPs is indicated. its highest topographic point. In contrast, local flows (23,055 cases--Figure 2e,f) are small, non-rapid earth flows and densely affect the peiltic slopes, with a very limited extension of the total landslide area. In addition, these surficial slope failures almost seasonally activate, accounting for a wandering-like spatial distribution inside the clayey slopes. In light of their limited extension and ephemeral features, they were directly mapped with a LIP alone. To correctly define the date of the activation of phenomena (and to potentially link the landslide inventory with a singular rainfall season), the landslide inventories were detected using high-resolution (0.25 m) LIDAR (Light Detection and Ranging) images taken in 2012 by ARTA (Assessorato Regionale al Territorio e all'Ambiente) and compared with four different Google Earth satellite images dated 15 July 2011 (DigitalGlobe Catalog ID: 1030050011F8C200), 29 June 2011 (DigitalGlobe Catalog ID: 103001000B21B700), 21 June 2011 (DigitalGlobe Catalog ID: 103001000B544500) and 3 May 2013 (DigitalGlobe Catalog ID: 103001002332 DE00). Figure 2: Some examples of the three types of landslides analysed located in the study area: slides (a,c,), slope flows (b,d) and local flows (e,f). LIPs are reported according to the legend of Figure 1 (brown for slide, green for slope flow and light orange for local flow).