Probabilistic assessment of the earthquake-induced soil liquefaction hazard at national scale: macrozonation of the Italian territory

Seismic soil liquefaction is one of the most relevant phenomena of ground failure that may induce disastrous consequences on structures, infrastructures and the environment. This article presents the first probabilistic zonation for liquefaction hazard at national scale, carried out with reference to Italy. Macrozonation is the geospatial identification of areas in a national territory that, in case of an earthquake, may be affected by phenomena associated to soil liquefaction by using a probabilistic approach. Zonation of a large territory for earthquake-induced liquefaction hazard seems to be, at least at a first glance, an unachievable goal, since liquefaction occurs at a very local scale. In this study, the strategy for macrozoning consists in combining and processing geospatial predictors, which represent both ground susceptibility to liquefaction and expected seismic loading. A database was built for the Italian territory including the explanatory variables adopted as proxies for soil density, degree of saturation and ground motion intensity. This database represents the starting point for the application of a geospatial methodology based on logistic regression for assessing the liquefaction hazard in Italy. The outcomes are macrozonation charts computed for three return periods (i.e. 475, 975 and 2475 years) with a spatial resolution on the order of 500 m. The mapping was validated by superimposing historical liquefaction and then compared with the coarser charts recently delivered for Europe. Despite their intrinsic limitations, national scale maps of liquefaction hazard may support decision-makers, civil protection agencies, insurance and re-insurance companies to fund zonation projects at regional or even at urban/suburban scale.