A unique 4000 year long geological record of multiple tsunami inundations in the Augusta Bay (eastern Sicily, Italy)

We present the geological evidence for a 4000 year long record of multiple tsunami inundations along the coast of the Augusta Bay (eastern Sicily)and discuss its implications. The research was carried out through a multi-theme approach which benefited from an extraordinarily long historical record that we used to guide detailed geomorphologic and geologic surveys, coring campaigns and laboratory analyses. Two sites, named the Augusta Hospital and Priolo Reserve, were selected and investigated in detail along the 25 km-long coastline of Augusta Bay. We found evidence for six (possibly seven) tsunami deposits; three of them may be tentatively associated with the 1693 and 365 AD Ionian Sea historical tsunamis and the ~3600 BP Santorini event. The other three (possibly four) deposits are evidence for unknown paleo-inundations dated at about 650–770 AD, 600–400 BC and 975–800 BC (at Augusta Hospital site), and 800–600 BC (at Priolo Reserve site). We use these ages to extend further back the historical record of tsunamis available for this coastal area. The exceptional number of tsunami deposits found with this study allowed us to derive an average geologic tsunami recurrence interval in the Augusta Bay of about 600 years for the past 4 ka. Conversely, the historical tsunami data for the past millennium suggest an average tsunami recurrence interval of about 250 years. This difference in the average recurrence intervals suggests that only the strongest inundations may leave recognizable geological signatures at the investigated sites (i.e. the evidence for the 1908 and 1169 tsunamis is missing) but also that the geomorphological setting of the site and its erosional/depositional history are critical aspects for the data recording. Thus, an average recurrence interval derived from the geological record should be considered as a minimum figure. The identification and age estimation of tsunami deposits represent a new and independent contribution to tsunami scenarios and modeling for coastal hazard assessment in Civil Protection applications. Furthermore, our study cases provide new elements on tsunami deposit recognition related to exceptionally large events that occurred in the Aegean Sea