## 3 Background of the Event  
### The Geological Background and Geomorphological Setting of the Slope  
The study area (i.e., the investigated slope in Figure 1a,b) is located in the Central-Western Alps, and more in particular in the Lepontime gneiss dome area, where basement nappes alternate with (meta)sedimentary cover series constituting a NW-verging stack of basement and post-Carboniferous cover nappes [29]. The Barrovian metamorphism of Oligocene to Miocene age, affecting the nappes, varies from upper greenschist to amphibolite facies [30]. The Lower Penninic nappes characterise in particular the investigated slope. They are highly metamorphic crustal nappes representing the former rift shoulder and syn-rift European margin [31]. Along the slope, rocks of the following Penninic Units crop out from SE to NW (Figure 2a):  
- Antigorio and Teggiolo Units: they occupy a crucial tectonic position, on the boundary between the Helvetic and Penninic realms [32]. The _Antigorio Unit_ is constituted by monzognaritic to granodioritic orthogneiss representing the basement of the series [29; 31]. It is overlain by the _Teggiolo Unit_, comprised of several sedimentary cycles, separated by erosive surfaces and large stratigraphic gaps, whose age ranges from Triassic to Eocene [30]. The limit between the two units is locally erosive. - Valais Unit: they are composed by calcschists deriving from the sediment of the Valais basin, and they are of Cretaceous-Paleogene age [31]. - Lebendun Unit: it is a paraphesis assemblage [30; 33] constituted of thick clastic deposits that can be defined as polymictic conglomerate and micaschists. Their age is estimated as Cretaceous [29]. They are interpreted as former syn-rift deposits [31]. The area underwent glacial modelling during the Pleistocene Age, when at the Last Glacial Maximum (26.5-19 ka; [34]) the extension of the Toce glacier reached the area down-valley the Maggiore Lake, flowing into the nearby Ticino Glacier, and contributing in building the Lake Maggiore glacial amphitheatre [35]. In correspondence of the Simplon Pass, at the head of the Divedro Valley, a transfluence saddle was present [35], allowing the flow of Swiss Rhone ice dome into the Toce one. The Caiirasca hydrographic basin is characterised, up-valley to the study area, by the glacistructural depression of the Alpe Veglia, bordered by the reliefs where now glaciers are still present [36]. The Alpe Veglia area is then connected with the Caiirasca Valley bottom through a deep gorge, the Caiirasca Gorge, carved into Lebendun paraphesis and carbonate rocks of the Valais Unit, and along which instability processes often occur. The study area, SW exposed, is located along the left hydrographic side of the Caiirasca valley, and it is up-slope characterised by a low steep accumulation area where [37], in correspondence of the Alpe Ciamporino ski area, glacial deposits related probably to a Holocene glacial stage are present. The basin is surrounded by scree slope deposits at the base of the Pizzo del Dosso ridge. Down-slope, the edge of the Alpe Ciamporino terrace, the steepness increases, interrupted by the presence of minor terraces. Signs of the water- and gravity-related processes are evident and dominantly characterise the deglaciated areas. Moreover, a series of NE-SW oriented quite vertical \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Station Name** & **ID** & \begin{tabular}{c} **Latitude N** \\ **(°)** \\ \end{tabular} & \begin{tabular}{c} **Longitude E** \\ **(°)** \\ \end{tabular} & \
\begin{tabular}{c} **Altitude** \\ **(m)** \\ \end{tabular} &
\begin{tabular}{c} **Distance** \\ **(km)** \\ \end{tabular} & **Instruments** \\ \hline San Domenico (Varzo) & A & 46.250 & 8.193 & 1308 & 0.7 & R,T \\ Alpe Veglia & B & 46.275 & 8.146 & 1736 & 4.0 & R,T,S,W,G,P \\ Alpe Devero & C & 46.316 & 8.260 & 1634 & 9.2 & R,T,S,H \\ Domodossola & D & 46.103 & 8.302 & 252 & 19.0 & R,T,W,P,G \\ Formazza & E & 46.433 & 8.358 & 2453 & 24.0 & R,T,S,W \\ \hline \hline \end{tabular}
\end{table}