Landslides along the Lago Maggiore western coast (northern Italy): intense rainfall as trigger or concomitant cause?

The Lago Maggiore catchment is characterized by medium to high altitude (up to 4633 m a.s.l. with a median of 1270 m a.s.l.), high precipitation (~ 1700 mm/yr), and brittle tectonic deformation of impermeable rocks, such as granite and gneiss, that are characterized by a predisposition to slope failure. We analysed daily rainfall data associated with 38 landslides that occurred between 1980 and 2017 from meteorological stations placed into four sub-basins. The purpose was to determine whether or not extreme rainfall events exceeded landslides thresholds reported by previous studies. A statistical analysis using the RClimDex package was done, to verify changes in extreme rainfall over time. A spatial approach using Inverse Distance Weighting (IDW) in QGIS was used to extrapolate rainfall data specific to landslide areas, as well as GIS techniques and processing tools to conduct geomorphic analyses. Finally, a multivariate analysis, (general linear model), was used to understand associations between variables (landslide types, lithology, valley, elevation, slope, land use, rainfall, and the presence of rivers, roads, paths, and buildings), known to affect the generation of landslides. Results show extreme rainfall events to be a secondary factor in the triggering of landslides, whereas the most significant factors are presence of building, proximity to rivers and lithology. It was found that intense rainfall is a concomitant cause to landslides in some instances but does not play a role in others.