Holocene tsunamis in coastal areas of northern Greece: sedimentological and geoarchaeological evidences

Shallow drill cores in flat and southerly exposed coastal areas around the Thermaikos Gulf (Thessalonica and Katerini areas, and west coast of Kassandra, the western “finger” of Chalkidiki) and the coast in northern Greece near the cities of Lagos and Alexandropoulis provided evidence for past high-energy sedimentary events, which are interpreted as tsunamites. As a result, several Holocene coarse clastic layers have been found intercalated in sandy beach, clayey or gypsiferous lagoonal deposits. These layers have erosive bases, show fining-up and thinning-up sequences, and include shell debris, foraminifera and rip-up clasts of lagoonal sediments. Widely observed significant feature of these layers involve mud-coated beach clasts and rip-up clasts that rework the high-plasticity clays of lagoons. Such features that indicate highly disturbed sedimentological conditions (hyperpyncal flows) are rarely described elsewhere. Repeated intercalations of these layers with all the mentioned indicative features downhole are interpreted paleotsunami deposits from tsunamis generated by earthquakes or earthquake-triggered submarine landslides resulted by seismic shaking in the Thermaikos Gulf or the North Aegan Basin. However, we have to distinguish individual events (the one layer case) and packages of fining-up deposits, which are deposited during one event, but during several waves (usually 3-4 subsequent fining-up layers). Another important observation is that open beach conditions end immediately with a tsunamigenic event, and later lagoons form. Hence, both the coastal parallel currents, which are currently promoting spit deposits and lagoons, and tsunami events are shaping the coastlines of northern Greece.
   A major tsunamigenic source is located along the western tip of the North Anatolian Fault Zone (NAFZ) in the North Aegean Basin, where water depths ranging between 1.200 and 1.650 m are sufficiently deep to generate tsunamis. Historic tsunamis have also been observed, e.g. the 1893 Samothraki event. However, the event layers up to now cannot be assigned to individual seismic or landslide sources, but the potential of a tsunami threat in the Thermaikos Gulf area can now be tested, following both sedimentological and modelling processes. Such potential threat regarding the Thermaikos Gulf has only recently been notified but never tested and studied in depth. Modelling of the tsunami potential of the basin bounding fault southwards of the Thermaikos Gulf provides an example for possible tsunami generation at only one segment of NAFZ along an approx. 55 km normal fault at the southern fault-bound margin of the North Aegean Basin.
   The Herodotus Histories report on inundations and sea withdrawals occurring during the Greek-Persian war, which occurred near Potidea on Kassandra. In the ancient Greek village Mende we found evidence for a tsunamigenic layer, dated with shells to 2500 BP, which may tentatively be interpreted as the sedimentary remains of the “Herodotus tsunami” in 479 BC. Other tsunamigenic events, e.g. near Sozopoli village, occurred c. 5000 BP.