S. Peruccacci (1), M.T. Brunetti (1), S. Luciani (1,2), C. Vennari (2), F. Guzzetti (1), 2011, The role of lithology and season on rainfall thresholds for the initiation of landslides in central Italy, AGU Fall Meeting 2011, 2011,
URL: http://www.cnr.it/prodotto/i/273393

We compiled a catalogue of 442 rainfall events with landslides in the Abruzzo, Marche, and Umbria regions, central Italy, in the period 2002-2010. The landslide information was obtained through the analysis of national, regional, and local newspapers, and reports of landslide events compiled by fire brigades. For each rainfall event that has resulted in one or more landslides, we calculated the cumulated rainfall E (mm) and the duration D (h) of the rainfall event, using a dense network of 150 rain gauges. Landslides were mapped as single points using Google Earth®. We exploited the catalogue to determine new cumulated event rainfall - rainfall duration (ED) thresholds. To determine the ED thresholds, we modified a Frequentist probabilistic method first proposed to determine ID thresholds. The method assumes that the threshold curve is a power law E = ? × D^?, where E is the cumulated rainfall (mm), D is the duration of the rainfall event (h), ? is a scaling constant (the intercept), and ? is the slope of the power law curve. We improved the method adopting a bootstrapping statistical technique to evaluate the uncertainties in the parameters that define the threshold curve model, and the sensitivity (robustness) of the thresholds to the number of (D,E) data points in the empirical data set. We determined ED thresholds for 1% and 5% exceedance probability levels for the entire study area, and for the three individual regions that comprise the study area (Abruzzo, Marche, and Umbria). To investigate the role of lithology and season on the occurrence of rainfall-induced landslides, we segmented the rainfall ED data by the main lithological domains in the study area (i.e., Post-Orogenic sediments, Flysch deposits, and Carbonate rocks), and season of the events (May - September, October - April). Comparison of the ED thresholds indicates that the cumulated amount of rainfall necessary to trigger landslides in Flysch deposits is larger than in Post-Orogenic sediments and Carbonate rocks, and that the thresholds for Post-Orogenic sediments and Carbonate rocks were statistically indistinguishable in the study area. The seasonal thresholds are statistically different for D < 12 h, and for D > 100 h. In particular, for short rainfall periods (D < 12 h) the cumulated rainfall required to initiate landslides is higher in the summer period May - September than in the period October - April. The results obtained are applicable for landslide forecasting based on empirical rainfall thresholds, and have implications for landslide hazard and risk assessment.

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