Controls, timing, and characteristics of submarine landslides in the Mediterranean area as an example of science support for preparednessR. Urgeles, A. Camerlenghi, F. Palmer The Mediterranean is one of the most vulnerable areas to offshore geohazards: it has a very densely populated coastline; it is one of the world’s leading holyday destinations and has a high density of seafloor structures. It is also a relatively small Sea, and therefore there is a close proximity between tsunami sources and impact areas. Submarine slope failures of all sizes occur frequently causing damage to property and life. Submarine slopes suffer much more localized erosion and ground water conditions are relatively homogeneous over large areas, which may result in extremely large landslides, at least one order of magnitude larger than those found on land. Therefore, understanding the controls, timing and mechanisms of submarine landslides in this area is key to risk management. Large sedimentary wedges (Ebro, Nile, Rhone) appear to have a high density of large submarine slope failures, while tectonically active margins have numerous but relatively small failures. Most landslides in the Mediterranean Sea originate in water depths exceeding 2000 m on slopes of 2o and most of them arrest only in slightly deeper water depths. This illustrates a) that the continental rise is a place of high slope instability compared to the continental slope and b) that limited energy is available for down-slope sediment transport, with most failures arresting shortly after triggering and/or producing little sediment transport. We know very little on the age of the failure events: of the 524 submarine landslides events catalogued in the Mediterranean Sea, only 44 have somewhat accurate age determinations. The age of 128 events is simply reported in the literature with a geologic epoch. A large amount of these 128 events, are reported as Holocene (68 events), which suggests that climate induced stress changes (sea level and bottom temperature changes and their effect on gas hydrate and gas systems, sedimentary load, ...) have had a major role in triggering slope failures. Physical properties data of marine sediments of Mediterranean continental margins are also scarce and the continental margins' hydrogeological regime is virtually unknown. Back-analysis of the largest submarine landslides suggests that large overpressures must be present at depth. In other instances fluid escape from sub-Messinian formations appears likely to produce significant stress reduction so as to produce failure of the slope when additional environmental factors such as sea level variations are added. |
