Submarine earthquake geology: potential, limits and results in the Mediterranean Sea

Submarine geohazards, are exceptional events capable of producing large-volume turbidites that result from catastrophic slope failures and the associated downslope mass transport of enormous quantities of sediment from continental shelves and slopes to the deep sea. We are investigating the effects of italian earthquakes and tsunamis on the sedimentary record of the Ionian Sea, through a multi-proxy analysis of turbidite deposits. A comparison between radiometric dating and historical earthquake catalogs suggests that turbidite emplacement triggered by major earthquakes (i.e. AD 1908, 1693 and 1169) represents over 90% of the deposits during the last millennia (Polonia et al., 2013a; 2013b). Although the average recurrence time of single events is about 500 yrs, age modelling indicates that their occurrence is not periodic, but rather varies between 100 and 700 yrs, with clusters of higher frequency (Polonia et al., 2015). Seismic reflection images show that some turbidite beds are very thick and marked by acoustic transparent homogenite mud layers at their top. Based on a high-resolution study of the most recent of such megabeds, the Homogenite/Augias turbidite (Kastens and Cita, 1981; Cita and Aloisi, 2000; Hieke and Werner, 2000), we show that it was not triggered by the Santorini caldera collapse but by giant turbidity flows triggered by the AD 365 earthquake and tsunami (Polonia et al., 2013b; 2016). Backwash flows and related gravity-driven processes are the primary means of downslope sediment transport, confirming that the HAT is a unique instance of thick deep sea tsunami deposit. An older similar deep sea megaturbidite was deposited about 15,000 years BP, implying a large recurrence time of such extreme sedimentary events in the Mediterranean Sea. Marine sediments may thus be considered as seabed archives of paleo-earthquakes capable of reconstructing seismicity back in time, during several earthquake cycles.