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Stability of terrain

The terrain of the Apennines (as well as that of the Alps) is to a large degree unstable due to various types of landslides, including falls and slides of rocks and debris, flows of earth and mud, and sink holes. The Institute for Environmental Protection and Research (Istituto Superiore per la Protezione e la Ricerca Ambientale), a government agency founded in 2008 by combining three older agencies, published in that year a special report, Landslides in Italy, summarizing the results of the IFFI Project (Il Progetto IFFI), the Italian Landslide Inventory (Inventario dei Fenomeni Franosi in Italia), an extensive survey of historical landslides in Italy undertaken by the government starting in 1997. On December 31, 2007, it had studied and mapped 482,272 landslides over 20,500 km (7,900 sq mi). Its major statistics are the Landslide Index (LI here), the ratio of the landslide area to the total area of a region, the Landslide Index in Mountainous-Hilly Areas (here LIMH) and the Density of Landslides, which is the number per 100 km (39 sq mi).

Italy as a whole has a LI of 6.8, a LIMH of 9.1 and a density of 160. Lombardia (LI of 13.9), Emilia-Romagna (11.4), Marches (19.4), Molise (14.0), Valle d'Aosta (16.0) and Piemonte (9.1) are significantly higher. The most unstable terrain in the Apennines when the landslide sites are plotted on the map are in order from most unstable the eastern flanks of the Tuscan-Emilian Apennines, the Central Apennines and the eastern flank of the southern Apennines. Instability there is comparable to the Alps bordering the Po Valley. The most stable terrain is on the western side: Liguria, Tuscany, Umbria and Lazio. The Apennines are slumping away to the northeast into the Po Valley and the Adriatic foredeep; that is, the zone where the Adriatic floor is being subducted under Italy. Slides with large translational or rotational surface movements are most common; e.g., a whole slope slumps into its valley, placing the population there at risk.