Landslide Susceptibility Zoning

In the literature, confusion exists between the terms landslide “susceptibility” and landslide “hazard”. Often, the terms are used as synonymous despite the two words expressing different concepts. Landslide susceptibility is the likelihood of a landslide occurring in an area on the basis of local terrain conditions (Brabb, 1984). It is the degree to which a terrain can be affected by future slope movements, i.e., an estimate of “where” landslides are likely to occur. Susceptibility does not consider the temporal probability of failure (i.e., when or how frequently landslides occur), nor the magnitude of the expected landslide (i.e., how large or destructive the failure will be) (Committee on the Review of the National Landslide Hazards Mitigation Strategy, 2004). In mathematical language, landslide susceptibility is the probability of spatial occurrence of slope failures, given a set of geo-environmental conditions. This is called “landslide analysis” by Vandine et al. (2004). Landslide hazard is the probability that a landslide of a given magnitude will occur in a given period and in a given area. Besides predicting “where” a slope failure will occur, landslide hazard forecasts “when” or “how frequently” it will occur, and “how large” it will be (Guzzetti et al., 2005a). Landslide hazard is more difficult to obtain than landslide susceptibility, as susceptibility is a component (the spatial component) of the hazard. More generally, landslide susceptibility consists in the assessment of what has happened in the past, and landslide hazard evaluation consists in the prediction of what will happen in the future. In this Chapter, I discuss landslide susceptibility zoning, whereas landslide hazard modelling will be dealt with in § 7. Here, I review the methods proposed to ascertain landslide susceptibility, including an analysis of the types of mapping units most commonly adopted, and of the relationships between the selected mapping units and the adopted susceptibility methods. I then examine a probabilistic model for landslide susceptibility, including problems and difficulties in its application, and I present an example of a landslide susceptibility model for the Upper Tiber River basin, an area that extends for about 4100 km2 in central Italy. Lastly, I discuss the problem of the verification of the performances of a landslide susceptibility model, including examples for the Collazzone area, in central Umbria. ...