Large boulder deposits by tsunami waves along the Ionian coast of south-eastern Sicily (Italy)

The Ionian coast of south-eastern Sicily, between the towns of Augusta and Siracusa, is characterized by the occurrence ofanomalous calcareous boulders. They are mostly scattered along large terraces located 2–5 m above sea level, gently slopingtowards the sea. Boulders are up to 182 t in weight and are arranged either in isolated elements or small groups composed of a fewstacked elements. Several boulders show biogenic encrustations (serpulids, balanids, lithophaga) all over their surface whichsuggest that they were dragged from the mid-sublittoral zone. Other boulders are partially covered by biogenic encrustations andshow morphological features (karstic pools, exposed fracture surfaces) suggesting that they were detached and scattered from themid-supralittoral zone. Direct observations on each boulder (distance from the shoreline, size and weight), together with statisticalanalysis of the storm regime of the area, allowed to operate hydrodynamic estimations useful to verify if tsunami or storm waveswere responsible for their detachment and transport, while radiocarbon age determinations on marine organisms constrained thetiming. Collected data, compared to historical catalogues, suggest that in the last 1000 years three seismic events with local sourcescould have triggered tsunami waves associated with the boulder deposits occurring in the area. The first two were probablytriggered by the earthquakes of February 4, 1169 and January 11, 1693 which destroyed south-eastern Sicily. According togeological data and numerical modelling, the seismogenic source could be located in the Ionian offshore between Catania andSiracusa. The third tsunami was generated by the strong earthquake which took place in the Strait of Messina on December 28,1908.