GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 1.1 61 al. , 2003, 2015). Average recurrence of seismoturbidites, deduced considering chronology and number of turbidites, is about 500 yrs which is in good agreement with the average frequency of major earthquakes in Calabria during historical time. However, age modelling performed on the entire Holocene sequence suggests that recurrence time is not constant, but rather it varies between 100 and 700 yrs with periods of higher frequency of seismic events. The AD 1908 Messina earthquake and tsunamis. The AD 1908 seismo-turbidite consists of three different stacked turbidites, suggesting that seismic shaking triggered at least three slope failures. The first failure, that sources the basal stack, occurred in a deeper environment because of more marine geochemical proxies and the absence of inner shelf foraminifera. This deeper slope failure was followed by two additional failures, with sources apparently located closer to the coast, as indicated by more entrained material from shallow water. The Straits area is where the first cable break occurred and where the Ionian fault was described (Polonia et al. , 2012, 2014). The 1908 earthquake seafloor rupture displacement, which is reported to be on the order of 1 m (Loperfido, 1909; Valensise and Pantosti, 1982), can drive a runup of the same amplitude or a bit more (Okal and Synolakis, 2004). It follows that part of the 11 m 1908 tsunami runup must have been triggered by landslides. This is in agreement with the observation of the stacked seismo-turbidite, three submarine cables breaks downslope along the margin (Ryan and Heezen, 1965) and models that include a composite source (e.g. earthquake and landslide) (Tinti et al. , 2008; Tappin et al. , 2008; Favalli et al. , 2009). The location of slope failures, however, is highly debated (see Billi et al. , 2009 and references therein). The AD 1908-stacked seismo-turbidite that we observed in our cores suggest that more than one slope failure occurred after the earthquake. These sediment failures can be considered the likely source, together with seismic shaking, for the generation of the AD 1908 tsunami. The AD 365 Crete earthquake. Destructive earthquakes/tsunamis have affected repeatedly the circum Mediterranean highly populated coastal regions. A record of these past events can be provided by large-volume turbidites or megaturbidites, detected in the marine sedimentary record. Megaturbidites have been identified in the Ionian basin (central Mediterranean) that is located between two tectonically active subduction zones (i.e. the Calabrin Arc to the north and the Hellenic Arc to the east). The uppermost megabed, has been named “Homogenite” (Kastens and Cita, 1981) or “Augias turbidite” (Hieke, 1984). Its well defined stratigraphic position, above the regional marker sapropel bed S1, has been interpreted as evidence that it was deposited in a single, basin-wide event capable to put into suspension simultaneously sediment at a basin-wide scale. Absence of absolute dating of the megabed and of a detailed chronostratigraphy of the deposits above and below the turbidite, have allowed a number of different correlations of this megaturbidite with the 3500 yr BP Minoan eruption of Santorini and related tsunamis in the Aegean Sea (Kastens and Cita, 1981), with the 7.600 ± 130 yr B.P. collapse of a flank of the Etna Volcano (Pareschi et al. , 2006) or with a major earthquake in the Mediterranean Sea (Vigliotti, 2008). Based on studies of sediment cores we collected from the Ionian seafloor (mineralogy, micropaleontology, elemental and isotopic geochemistry and radiocarbon dating), we show that the Homogenite/Augias turbidite (HAT), up to 20-25 m thick, was related to multi-source turbidity flows triggered by theAD 365 earthquake and tsunami (Polonia et al. , 2013b). We were able to reconstruct the different units deposited in response to the 365 AD Cretan earthquake/ tsunami and the results confirm that the HAT is a unique instance of deep sea tsunami deposit. Backwash flows and related gravity-driven processes are the primary means of downslope sediment transport. An older similar deep sea megaturbidite was deposited in the Ionian Sea about 15.000 years B.P., implying a large recurrence time of such extreme sedimentary events in the Mediterranean Sea. Conclusions. The Ionian Sea is a landlocked basin where convergence between Africa and Eurasia produced the emplacement of two opposite verging subduction/rollback systems