*3.1. Geomorphological Approach*

The information and the references related to the geological setting of the study area (already included in the text) are included in the caption of Figure 1b.

The detailed study of the morphodynamic processes active in the study area, with particular regard to landslides, was initially carried out following the classical principles of a detailed field geomorphological survey. In the first phase, all the available geological information sources (maps, profiles, stratigraphic sections, stratigraphic logs, etc.) were acquired. The base cartography was the geological map at 1:10,000 scale, which presents an almost complete coverage even at a national level; nevertheless, it is not uniform in symbolism and the legend as it is the product of autonomous regional projects. Furthermore, since this is a relatively old document (about 20 years old), the perimeters and the state of activity of the gravitational processes were updated through a detailed survey, through which soil samples were taken for subsequent geotechnical tests. In this regard, it should be emphasized that specific geotechnical data for outcropping formations are quite rare, and in some cases, it was necessary to use the results of tests conducted on samples taken from the same formations but in different (albeit neighboring) locations.

The synthesis of these surveys, mainly addressing the characterization of the type and evolutionary mechanisms of landslides, is shown in Figure 4a.

From a lithological point of view, the area can be divided into two main sectors, one to the west and one to the east of the relief of Mt. Falcone, consisting of arenaceous and arenaceous-conglomeratic lithotypes associated with a coastal transition environment. In the western sector of the relief, as previously mentioned, the Messinian arenaceous-pelitic and pelitic-arenaceous members of the Laga Formation outcrop with a counter-dip-slope attitude, ranging between 30◦ and 40◦ (Figure 4b). Pliocene arenaceous-pelitic and peliticarenaceous lithotypes also emerge east of the Mount Falcone relief; the frequency and consistency of the arenaceous levels is, however, less marked as along with the strata inclination (generally dip-slope and between 15–25◦). A common feature of some clayey lithotypes is the presence of more or less thick and frequent weathered (weak) levels; these layers, characterized by poor geotechnical properties, are observed mostly in the western sector, within the pelitic-arenaceous member of the Laga formation.

The bedrock is often masked by powerful thicknesses (up to 30 m) of fine colluvial deposits; the greater thicknesses are concentrated corresponding to the numerous minor valleys, which originate from the arenaceous-conglomeratic plate.

The hydrographic network is fairly developed, and because of the low permeability of bedrock and covering soils, the major gullies and streams radially develop from the top of the relief, although there is important tectonic conditioning especially in the lower hierarchical reaches. Concerning water circulation, runoff is limited to a few days/weeks after considerable meteoric events, while the groundwater circulation, due to the predominantly clayey nature of bedrock, is generally limited. However, the presence of widespread perched aquifers within the colluvial deposits, with the water table close to the surface, is a crucial element for the stability of the slopes.

Landslides, as mentioned, are widespread over the area, both for the lithological nature of bedrock and the morphological–structural setting of the slopes, characterized by discrete slope angle and by strata dip often favorable to the activation of gravitational phenomena. The typology of movement is also very highly variable, with falls, topples, flows and slides with different styles and states of activity being present (Figures 4a and 5). Although the stratigraphic setting is favorable to the activation of slides (rotational/planar) particularly on the eastern side, these are less frequent, while flows are dominant; this apparently contradictory aspect can be associated, as described in the following, with the good strength and deformation properties of bedrock and with the presence, on the other hand, of important thicknesses of unconsolidated continental deposits often hosting a perched water table. It is not uncommon, however, to observe complex phenomena

characterized by rotational slides in the uppermost portion and flows in the medianterminal one.

**Figure 4.** (**a**). Geological and geomorphological map of the study area: 1—slope and colluvial deposits (Holocene); 2—rotational and planar slides (Holocene); 3—flows (Holocene); 4—old and recent fluvial deposits (Pleistocene–Holocene); 5—arenaceous-conglomeratic bedrock (Mount Falcone body—Middle Pliocene); 6—mainly arenaceous-pelitic bedrock (Argille Azzure Formation—Middle Pliocene); 7—mainly clayey bedrock (Argille Azzure Formation—Middle Pliocene); 8—mainly pelitic-arenaceous bedrock (Laga formation—Late Messinian); 9—mainly arenaceous-pelitic bedrock (Laga formation—Late Messinian); 10—main faults; 11—gravitational trench; 12—strata attitude; 13—trace of cross-section used for the numerical simulation; 14—trace of geological cross-section described in (**b**).

The type of movement was mainly attributed on the basis of geomorphological considerations, taking into account size, morphology, typology of material involved, strata dip and damage eventually observed along roads or to infrastructures (Figure 5).

**Figure 5.** Typical landslides in the study area. (**a**) Rotational slide affecting clayey bedrock and (**b**) consequences on the road conditions; (**c**) example of earthflow involving fine slope deposits and (**d**) secondary road deformed by a landslide.
