Analysing the relationship between rainfalls and landslides to define a mosaic of triggering thresholds for regional scale warning systems

We propose an original approach to develop rainfall thresholds to be used in civil protection warning systems for the occurrence of landslides at regional scale (i.e. tens of thousands kilometres). A purposely developed software is used to define statistical intensity-duration rainfall thresholds by means of an automated and standardized analysis of rainfall data. The automation and standardization of the analysis brings several advantages that in turn have a positive impact on the applicability of the thresholds to operational warning systems. Moreover, the possibility of defining a threshold in very short times compared to traditional analyses allowed us subdividing the study area in several alert zones to be analyzed independently with the aim of setting up a specific threshold for each of them. As a consequence, a mosaic of several local rainfall thresholds is set up in place of a single regional threshold. We subsequently analyzed how the physical features of the test area influence the parameters and the equations of the local thresholds, founding a significant correlation with the prevailing lithology. A validation procedure and a quantitative comparison with some literature thresholds showed that the performance of a threshold can be increased if the areal extent of its test area is reduced, as long as a statistically significant landslide sample is present. In particular, we demonstrated that the effectiveness of a warning system can be significantlyenhanced if a mosaic of site specific thresholds is used instead of a single regional threshold.