Coupling onshore and offshore records of extreme events: a sedimentological challenge

Theodore von Kármán Fellow Prof. Dr. Pedro J.M. Costa, Department of Earth Sciences, University of Coimbra, Portugal

Tsunami deposits are the primary source of information on past tsunami events and thereby are crucial for accurate hazard assessments. Study on tsunami deposits has been greatly developed over the last three decades but is still a young geoscientific discipline facing major challenges. Due to their specific physics and sediment transport processes tsunamis tend to leave their onshore sediment imprint in a wide range of low-energy environments. However, some of these environments display a low preservation potential for event deposits and the recognition of tsunami deposits is many times constrained by their poor preservation (or absence) in the stratigraphic record. In contrast, the shallow offshore has potential to be a reliable archive of extreme events, even more so on depths below the storm wave base.

On this presentation, we will discuss the onshore record of the major tsunami events that affected Atlantic Europe during the Holocene and will also present some results from three different research cruises along the coasts of Portugal and the Shetland archipelago (Scotland). The aim of the cruises was to trace the tsunami deposits that are well documented onshore into the offshore realm, study their extent and characteristics in detail, and provide new insights into recurrence intervals for tsunami along the NE Atlantic.

 The results of this combined multiproxy approach strongly suggests the shallow offshore area below storm wave base to host reliable sedimentary archives for tsunami backwash deposits. Furthermore, based in these different study cases we try to prove that coupling onshore and offshore sedimentological records is essential to provide a full reconstruction of past tsunami events and therefore provide more reliable and quantitative information to civil protection authorities and coastal managers.