Quaternary tsunami events in the eastern Ionian Sea - reconstructing and modelling extreme events based on interdisciplinary geo-scientific investigations
Final Report Abstract
Geo-scientific investigations within the framework of the Eastern Ionian Sea Tsunami (EIST) research project have shown – for the first time – that palaeotsunami events are a common feature in coastal evolution during the Holocene and traces of (pre-)historic impacts can be found in any coastal setting in the wider study area. However, our research made clear that the spatial variability of palaeotsunami deposits is very high. Such deposits and the way they are conserved in the coastal landscape are strongly dependent on the local relief controlling tsunami inflow and backflow dynamics as well as on the intensity of post-depositional processes which may lead to considerable alterations. Our results thus proved that palaeotsunami flow reconstructions have to be based on detailed, high-resolution investigations of appropriate geo-archives; singular evidence must not be taken as stand-alone argument for or against tsunami impact on coastal landscapes. As another major outcome of the project, it was shown that the overall coastal evolution in the individual study areas has been much more influenced by tsunami events than previously thought. In contrast, the contribution of gradual coastal processes such as longshore drifts and others seem to have been overestimated in their significance for the general evolution. We therefore suggest, that within the scale of several centuries, coastal geomorphologies and both littoral and near-coast sedimentary archives are controlled by extreme events and not by daily dynamics. Based on detailed geomorphological, sedimentological and geoarchaeological studies, it was possible to reconstruct the general flow dynamics of palaeotsunami landfalls. Numerical simulations of tsunami inundation for selected sections of the western coast of Greece were found to be in good accordance with field evidence. Within the EIST project, detailed comparison of computer modelling results with field data, for example for the Gulf of Kyparissia, turned out to be essential for the evaluation and calibration of simulated data. So far, computer models of tsunami inundation have been mostly published as singular and stand-alone piece of evidence. Our studies thus document the strong reliability and significance of geomorphological data in efforts to assess the modern tsunami risk for a specific coastal area. Significant methodological progress was also made using terrestrial laser scanning techniques for measuring and modelling coarse-clast tsunami deposits as a realistic base for calculating hydraulic tsunami wave parameters. We could show that traditional approaches resulted in considerable overestimations of tsunami wave parameters which consequently reduced the acceptance against tsunami-related coastal processes. A major surprise in the project development was the discovery of high-energy flood deposits in ancient Olympia, located some 19 km distant from the present shoreline. Results of multi-proxy sediment core analyses, macro- and microfaunal studies, radiometric dating and numerical simulation, are best explained by the Olympia Tsunami Hypothesis (OTH) saying that strong tsunami inundation of the lower Alpheios River valley led to overflow of tsunami waters into the smaller Kladeos River catchment directly heading towards Olympia. Geomorphological studies along a transect from the present coast of the Gulf of Kyparissia to Olympia clearly showed that palaeotsunami fingerprints found in near-coast geological archives are identical with the fingerprints of high-energy flood deposits around Olympia. These news were launched by press release on June 30th, 2011 and picked up by regional radio stations (DRadio Kultur, SWR), newspapers (Frankfurter Rundschau, Handelsblatt, The Times) and scientific media (Archäologie online, Bild der Wissenschaft, Spektrum der Wissenschaft – epoc). In the form of a large varnish profile recovered from south of the Olympic Stadium, the OTH also found its way into the exhibition “Mythos Olympia – Kult und Spiele” which was shown in the Martin-Gropius-Bau Berlin from 31 August 2012 to 07 January 2013. In a summary view, research within the EIST project made clear that the tsunami hazard in the eastern Mediterranean is highly underestimated. We found a recurrence interval for mega-tsunamis of 500-1000 years with some indications that the next event is overdue. Modern tsunami risk assessment is rare in Greece and is strongly recommended to take palaeotsunami findings into account on local, regional and supra-regional scales.
Publications
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(2010): Beachrock-type calcarenitic tsunamites along the shores of the eastern Ionian Sea (western Greece) – case studies from Akarnania, the Ionian Islands and the western Peloponnese. – In: Zeitschrift für Geomorphologie N.F., Suppl. Issue 54 (3): 1-50
Vött, A., Bareth, G., Brückner, H., Curdt, C., Fountoulis, I., Grapmayer, R., Hadler, H., Hoffmeister, D., Klasen, N., Lang, F., Masberg, P., May, S.M., Ntageretzis, K., Sakellariou, D., Willershäuser, T.
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(2011): Olympia’s harbour site Pheia (Elis, western Peloponnese, Greece) destroyed by tsunami impact. – In: Die Erde 142 (3): 259-288
Vött, A., Bareth, G., Brückner, H., Lang, F., Sakellariou, D., Hadler, H., Ntageretzis, K., Willershäuser, T.
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(2011): Sedimentary burial of ancient Olympia (Peloponnese, Greece) by high-energy flood deposits – the Olympia Tsunami Hypothesis. – In: Grützner, C., Pérez-Lopez, R., Fernández Steeger, T., Papanikolaou, I., Reicherter, K., Silva, P.G., Vött, A. (eds.): Earthquake Geology and Archaeology: Science, Society and Critical facilities. Proceedings of the 2 nd INQUA-IGCP 567 International Workshop on Active Tectonics, Earthquake Geology, Archaeology and Engineering, 19-24 September 2011, Corinth (Greece): 259-262
Vött, A., Fischer, P., Hadler, H., Handl, M., Lang, F., Ntageretzis, K., Willershäuser, T.
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(2011): Sedimentological and geoarchaeological evidence of multiple tsunamigenic imprint on the Bay of Palairos-Pogonia (Akarnania, NW Greece). – In: Quaternary International 242: 213- 239
Vött, A., Lang, F., Brückner, H., Gaki-Papanastassiou, K., Maroukian, H., Papanastassiou, D., Giannikos, A., Hadler, H., Handl, M., Ntageretzis, K., Willershäuser, T., Zander, A.
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(2013): 3D model-based estimations of volume and mass of high-energy dislocated boulders in coastal areas of Greece by terrestrial laser scanning. – In: Zeitschrift für Geomorphologie N.F., Suppl. Issue
Hoffmeister, D., Ntageretzis, K., Aasen, H., Curdt, C., Hadler, H., Willershäuser, T., Bareth, G., Brückner, G., Vött, A.
