Hydro- and morphodynamic tsunami simulations for the Ambrakian Gulf (Greece) and comparison with geoscientific field traces

Summary In order to derive local tsunami risks for a particular coast, hydro- and morphodynamic numerical models that are calibrated and compared with sedimentary field data of past tsunami impacts have proven very effective. While this approach has widely been used with regard to recent tsunami events, comparable investigations into pre-/historical tsunami impacts hardly exist, which is the objective of this study focusing on the Ambrakian Gulf in northwestern Greece. The Ambrakian Gulf is located in the most active seismotectonic and by this most tsunamigenic area of the Mediterranean. Accordingly, palaeotsunami field studies have revealed repeated tsunami impacts on the gulf during the past 8000 yr. The current study analyses 151 vibracores of the Ambrakian Gulf coast in order to evaluate tsunami signals in the sedimentary record. Based on a hydro- and morphodynamic numerical model of the study area, various tsunami waves are simulated with the aim of finding scenarios that compare favourably with tsunami deposits detected in the field. Both, field data and simulation results suggest a decreasing tsunami influence from the western to the eastern Ambrakian Gulf. Various scenarios are needed to explain tsunami deposits in different parts of the gulf. Whereas shorter period tsunami waves (T = 30 min) from the south and west compare favourably with field data in the western gulf, longer period waves (T = 80 min) from a western direction show the best agreement with tsunami sediments detected in southwestern Aktio Headland and in the more central parts of the Ambrakian Gulf including Lake Voulkaria. Tsunamis from the southwest generally do not accord with field traces. Besides the spatial sediment distribution, the numerical model accurately reflects the sedimentary composition of the detected event deposits and reproduces a number of essential features typical of tsunamites, which were also observed in the field. Such include fining- and thinning-landward and the marine character of the deposits. By contrast, the simulated thickness of tsunami sediments usually lags behind the observed thickness in the field and some event layers cannot be explained by any of the simulated scenarios. Regarding the frequency of past tsunami events and their spatial dimensions indicated by both field data and simulation results, a high tsunami risk has to be derived for the Ambrakian Gulf.