Statistical Methods for the Assessment of Rainfall Thresholds for Triggering Shallow Landslides: A Case Study

La Spezia Province (880 km(2); Liguria, northwestern Italy) is frequently hit by intense rainfalls, which often cause shallow landslides and damage to population and environment. In this regard, the Provincial Administration of La Spezia and the Earth Sciences Department, University of Pisa, promoted a study to define the rainfall thresholds for shallow landslides occurrence. In fact, on 25 October 2011 a very intense rainfall hit two parts of the provincial territory (Cinque Terre-Riviera area and Vara Valley) causing at least 3500 shallow landslides. This event was analyzed together with other 134 shallow landslide events occurred from 2008 to 2014. The rainfall conditions of these events were determined using an algorithm implemented by the CNR-IRPI of Perugia. The rainfall thresholds at different exceedance probability levels of landslide were defined using two statistical techniques: least-squares linear fit (LSF) and Quantile Regression (QR). The results highlight that the LSF thresholds seems to be the best performing from a statistical point of view and, consequently, the "best" for the study area.