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

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