A “landslide” is the movement of a mass of rock, debris, or earth down a slope, under the influence of gravity (Nemčok et al., 1972; Varnes, 1978; Hutchinson, 1988; WP/WLI, 1990; Cruden, 1991; Cruden and Varnes, 1996). Different phenomena cause landslides, including intense or prolonged rainfall, earthquakes, rapid snow melting, and a variety of human activities. Landslides can involve flowing, sliding, toppling or falling movements, and many landslides exhibit a combination of two or more types of movements (Varnes, 1978; Crozier, 1986; Hutchinson, 1988; Cruden and Varnes, 1996; Dikau et al., 1996). ... The range of landslide phenomena is extremely large, making mass movements one of the most diversified and complex natural hazard (Figure 1.1). Landslides have been recognized in all continents, in the seas and in the oceans. On Earth, the area of a landslide spans nine orders of magnitude, from a small soil slide involving a few square meters to large submarine landslides covering several hundreds of square kilometres of land and sea floor. The volume of mass movements spans sixteen orders of magnitude, from a single cobble falling from a rock cliff to gigantic submarine slides. Landslide velocity extends at least over fourteen orders of magnitude, from creeping failures moving at millimetres per year (or even less) to rock avalanches travelling at hundreds of kilometres per hour. Mass movements can occur singularly or in groups of up to several thousands. Multiple landslides occur almost simultaneously when slopes are shaken by an earthquake or over a period of hours or days when failures are triggered by intense or prolonged rainfall. Rapid snow melting can trigger slope failures several days after the onset of the triggering meteorological event. An individual landslide-triggering event (e.g., intense or prolonged rainfall, earthquake, snow melting) can involve a single slope or a group of slopes extending for a few hectares, or can affect thousands of square kilometres spanning major physiographic and climatic regions. Total landslide area produced by an individual triggering event ranges from a few tens of square meters to hundreds of square kilometres. The lifetime of a single mass movement ranges from a few seconds in the case of individual rock falls, to several hundreds and possibly thousands of years in the case of large dormant landslides.

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