Geological record of megatsunamis in Mediterranean deep sea sediments

The Mediterranean Sea hosts two subduction systems along the convergent Africa-Eurasia plate boundary that have produced strong ground shaking and generated tsunamis.  Based on historical descriptions and sedimentary records, one of these events, in 365 CE, impacted a broad geographical area qualifying it as a ‘megatsunami’. Understanding how megatsunamis are produced, and where they are more likely, requires a better understanding of the different secondary processes linked to these events such as massive slope failures, multiple turbidity current generation, and basin seiching.

An extensive collection of cores located in distal and disconnected deep basins, identified turbidites which were analyzed using granulometry, elemental (XRF), micropaleontological, and geochemical data in order to define sedimentary processes during the propagation of the CE 365 Crete megatsunami. The deposit contains a volume of detrital siliciclastic and biogenic components as large as 800 km³. The sediment from the European and African margins was remobilized and transported by tsunamis to abyssal depositional sites and isolated basins during the catastrophic event. The resedimented deposits, when viewed across multiple geomorphological locations of the marine depositional sites (canyon mouth, abyssal, isolated higher basin), demonstrate a complex sequence of processes triggered by the megatsunami wave propagation.

The tsunamis produced multiple far-field slope failures that resulted in stacked basal turbidites. It also caused transport of continent-derived organic carbon and deposition over basal turbidites and into isolated basins of the deep ocean. The composition of sediment in isolated basins suggests their deposition by large-scale sheet like flows similar to what has been caused by the Tohoku earthquake associated tsunamis. A local turbidite bed at the base of the deposit in isolated basins on the accretionary wedge indicates that sediment remobilization from basin walls was possibly related to the passage of the tsunami in deep water. When the tsunami wave hit the continental margins in Italy and Africa, it triggered turbidity currents on the slopes, which resulted in the stacked basal sand and silt units of the resedimented deposit. The analyses of geophysical data shows the presence of additional deposits of a similar nature from older layers with an age of about 14 and 40-50 Ka. This is significant for rectifying and resolving where risk is greatest and how cross-basin tsunamis are generated.