## 3 Materials and Methods
### Soil Cover Characterization  
Most of the observed landslides in the study area involve eluvial-colluvial soil covers. To characterize the presence and distribution of the weathered covers developed on the different lithologies, more than 50 direct measurements of total cover thickness were collected, while the maximum visible thickness in natural and man-made cuts was surveyed at 264 points to constrain minimum soil cover thickness (Figure 3). Available stratigraphic logs performed in the frame of microzonation studies were also analyzed to enrich the DB and distribution of soil cover thickness. To define the spatial distribution of the soil thickness in the study area and constrain the potential volume mobilized by shallow landsl, two empirical models of the soil cover thickness proposed by [PERSON] et al. (1985) were adopted. These simplified models, calculated in a GIS environment, compute the local thickness according to the local slope angle (S-model) and elevation (Z-model) through the linear correlation hereafter reported:  
\[Z_{\text{model}}:\;\text{h}_{\text{i}}=\text{h}_{\text{max}}-\frac{Z_{\text{i }}-\,Z_{\text{min}}}{Z_{\text{max}}-\,Z_{\text{min}}}(\text{h}_{\text{max}}- \text{h}_{\text{min}}), \tag{1}\]  
\[\text{S}_{\text{model}}:\;\text{h}_{\text{i}}=\text{h}_{\text{max}}\bigg{(}1- \frac{\tan\,\theta_{\text{i}}-\,\tan\,\theta_{\text{min}}}{\tan\,\theta_{ \text{max}}-\,\tan\,\theta_{\text{min}}}\bigg{(}1-\frac{\text{h}_{\text{min}}} {\text{h}_{\text{max}}}\bigg{)}\bigg{)}, \tag{2}\]  
Figure 3: (**a**) Map of the soil cover depth developed according to the SGZ model. Estimated landslide depth, inferred through area vs. volume relationship proposed by [PERSON] et al. (2004), is also reported. Minimum (**b**) and maximum (**c**) endmembers of weathered soil cover visible in outcrops are shown. where \(\rm h_{i}\) is the thickness of the soil cover in the i-cell (m); \(\rm h_{max}\) is the maximum thickness of the soil cover in the area (m); \(\rm h_{min}\) is the minimum thickness of the soil cover in the area (m); \(\rm Z_{i}\) is the elevation of the i-cell (m a.s.l. ); \(\rm Z_{max}\) is the maximum elevation of the study area (m a.s.l. ); \(\rm\theta_{i}\) is the slope angle in the i-cell (\({}^{\circ}\)); \(\rm\theta_{max}\) is the maximum slope angle in the study area (\({}^{\circ}\)), and \(\rm\theta_{min}\) is the minimum slope angle in the study area (\({}^{\circ}\)). These two models were merged into one, called the SGZ model (Figure 3), which was created with the aim of merging the weights of the two factors in the development of the alteration covers and incorporating the contribution given by the different lithologies, adopting a simplified relationship between them:  
\[\rm h_{i}(SGZmodel)=\frac{h_{i}\ (Smodel)+\ h_{i}(Zmodel)}{2}\times G_{i,i}, \tag{3}\]  
where the multiplicative factor \(\rm G_{i,i}\) incorporates the contribution of the lithology to the thickness of the soil cover. This parameter is defined as the ratio between the average of the thicknesses referring to each lithology, and the maximum thickness of the lithology outcroppings in the area:  
\[\rm G_{i,i}=\frac{h_{i}}{h_{\rm{max}}}, \tag{4}\]  
where \(\rm G_{i,i}\) is the lithological factor of the l-lithology in the i-cell (-), \(\rm h_{i}\) is the thickness of the l-lithology (m) and \(\rm h_{\rm{max}}\) is the maximum thickness among the lithologies outcropping in the area (m). By using the SGZ model, a soil cover depth map was obtained (Figure 3) which was useful for conducting the slope stability analyses presented below. The reactivated landslides are also reported on this map, whose sliding surface was deduced according to the semiempirical relation with the volume proposed by [PERSON] et al. [40]. Soil covers sampling, in situ permeability, and density tests were conducted directly on the field following standard procedures [41].