Tsunami Triggering Mechanisms Associated with the 17 th cent. BC Minoan Eruption of Thera Volcano, Greece Dimitris Sakellariou 1 , Grigoris Rousakis 1 , Paraskevi Nomikou 2 , Kathy Croff Bell 3 , Steve Carey 3 , Haraldur Sigurdsson 3 1 Inst. of Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece 2 Dept of Geology, National Kapodistrian University of Athens, Greece 3 Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI, USA ABSTRACT Systematic seismic profiling and swath bathymetry of the seafloor around Santorini volcanic field, including the caldera, shed light to the structure and seismic stratigraphy of the accumulated volcanic deposits and the mechanisms which may have led to the generation of one or multiple tsunamis during the Minoan eruption. We propose that tsunamis may have been triggered by two distinct processes associated with the Minoan eruption: (i) Massive pyroclastic flows entering the ocean around the islands of Santorini. The total volume of the pyroclastic flow deposits on the seafloor around Santorini is about 54.5 km 3 . (ii) The collapse of the Minoan volcanic edifice below the sea-level created new space of minimum volume 4.5-5 km 3 between the sea-level and the 200m depth contour. The total volume created may exceeded 10 km 3 . KEY WORDS: Thera, Minoan eruption, pyroclastic flows, volcano collapse, tsunamigenic mechanisms INTRODUCTION Various scholars from different disciplines refer to the Minoan or Late Bronze Age (LBA) tsunami as a massive, destructive event, which devastated the coastal areas of the entire Eastern Mediterranean Sea, particularly the northern coast of Crete, and led to the decline of the Minoan civilization on the island. Minoan tsunami deposits attributed to the 17 th century BC eruption of Thera volcano have been reported and studied at several sites along the shoreline of the Aegean Sea and the Eastern Mediterranean (Marinatos 1939; Galanopoulos 1960; Ambraseys; 1962; Papadopoulos & Chalkis 1984; McCoy & Heiken 2000; Minoura et al. 2000; McCoy et al. 2001 Guidoboni & Comastri 2005; Bruins et al. 2008; Goodman-Tchernov et al. 2009; Papadopoulos 2009; Papadopoulos et al. 2010; De Martini et al, 2010). Opposite to the general belief of a massive, widely propagated tsunami triggered by the Minoan eruption, Dominey-Howes (2004) has re- examined the original hypothesis as of lacking any substantial archaeological or geological evidence or not complying with the hydrodynamics of tsunami wave propagation. Further evidence for a Minoan tsunami comes from the deep seafloor of the Eastern Mediterranean. Tens of sediment cores collected from the Eastern Mediterranean seafloor include a distinctive, reworked, sedimentary unit called ‘ homogenite’, produced by t he liquefaction of loose sediments (Cita et al., 1980, 1982, 1983, 1984, 1996; Kastens and Cita, 1981; Blechschmidt et al., 1982; Aloisi et al., 1998). Kastens and Cita (1981) suggest homogenite deposition was related to the LBA tsunami. Homogenites occupy the same stratigraphic position as the LBA ash layer, and occur only where the ash layer is absent. This stratigraphic relationship has been confirmed by C14 dates (Hieke 1984, 2000). Based on the occurrence of the homogenites, Cita et al. (1984) proposed that the SW breach of the Santorini caldera may have concentrated the tsunami to the West and WSW rather than to the East and South where no homogenites are reported. Dominey-Howes (2004) suggests that the Minoan tsunami(s) would have attenuated quickly within the Aegean archipelago and may have propagated longer only towards W-WSW, as suggested by the occurrence of homogenites on the seafloor of the Eastern Mediterranean. Modelling performed by Pareschi et al. (2006) demonstrates that Minoan tsunami activity constrained only to the southern Aegean Sea with minimal wave activity beyond this area into the eastern Mediterranean Sea. Further discussion on the relationship of the homogenites and the Minoan tsunami can be found in Papadopoulos (2011). Despite that, calculations of the possible wave heights and modelling of the hypothetical Minoan tsunami(s) have been undertaken by many authors. Minoura et al. (2000), McCoy et al. (2000) and Bruins et al. (2008) estimated wave heights up to 17 m along the northern coastline of Crete. Yokoyama (1978) and Kastens & Cita (1981) used coastal elevations where tsunami deposits were found to assess possible wave heights around Thera and in the eastern Mediterranean and Ionian Seas. Recently, Novikova et al. (2011) simulated the possible tsunamis generated by the Minoan eruption using two tsunamigenic mechanisms. The first involved the entry of pyroclastic flows into the sea, assuming a thick (55 m, 30 km 3 ) flow entering the sea along the south coast of Thera directed towards northern Crete. The second mechanism assumed caldera collapse, of 19 km 3 and 34 km 3 . The calculated nearshore wave amplitudes varied from a few metres to 28m along 61 Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference Rhodes, Greece, June 17–22, 2012 Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set) www.isope.org