Differentiating between boulders deposited by tsunamis and storm waves along the south-eastern Ionian coast of Sicily (Italy)

Boulders in three areas (Capo Campolato, Vendicari, and San Lorenzo) along the Ionian coast of Sicily were analyzed, in order to distinguish if they were deposited by storm waves or tsunamis. On the whole, the size, shape, position and long-axis orientation of 328 boulders were accurately evaluated. These mega-clasts were deposited from the sea or moved from the gently sloping rocky coast and distributed within 80 m from the shoreline. Most boulders are rectangular, with sharp, broken edges. They are calcarenite fragments, estimated up to 71 t in mass. Some of the boulders (up to 32 m from shore and up to about 6 t) were observed to have moved after strong winter storms occurred from December 2008 to January 2009, whereas the positions of many other boulders were unchanged. We used hydrodynamic equations jointly with statistical analysis of boulders in order to determine the extreme events (geological or meteorological) responsible for these singular accumulations. Using the wave height and period of maximum observed storms and of historical tsunamis (1693 and 1908 events) along the Ionian coast of Sicily, we estimated the approximate transport distance at which the waves are capable of depositing boulders inland. Results show that the largest storm waves were probably responsible for the current distribution of most boulders, whereas the boulders at a distance > 40 m are likely deposited by tsunamis.