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Dr. Piero Bellanova

3rd prize of the year 2022

Tracing woody organic tsunami deposits of the 2011 Tohoku oki event in Misawa (Japan)

Institute for Neotectonics and Natural Hazards, RWTH Aachen University

© Piero BellanovaDr. Piero Bellanova (3. Preis): "Tracing woody organic tsunami deposits of the 2011 Tohoku oki event in Misawa (Japan)", Institut für Neotektonik und Georisiken, RWTH Aachen University
Copyright: Piero Bellanova

Description of the work:

Tsunami research is a relatively young scientific field but got much attention after the 2004 and 2011 tsunamis. Most studies feature standard sedimentological methods to analyze tsunami deposits, with this they are limited to the analysis of visually recognizable deposits, mostly sands. The defensive measures, such as breakwaters and dikes, are based on the calculated strength and the inundation of past, often historical, tsunamis. However, the events of the 2011 tsunami in Japan showed that the hazard estimation based on tsunami sand deposits from past tsunamis is not sufficient, as the tsunami hazard was greatly underestimated, leading to drastic destruction and loss of life. In the aftermath of the 2011 tsunami in Japan it become evident, that the visually recognizable sand deposits of tsunamis do not represent the full inundation distance and can reach only as low as 50% inland from the coast. The second half of the inundation or rather deposits within this area are often called ‘invisible’ as they are not visually distinguishable to the background sedimentation before or after a tsunami. These ‘invisible’ deposits are generated as the sediment sources eroded by an inundating tsunami wave change. At first a tsunami erodes mostly coastal and dune sands that are then transported inland, however, the further the water transport the sediment inland the more sediment sources inland are eroded, such as soils, and the more are coarser sediments (sand) with a weakening wave energy deposited. Therefore, at a certain point further inland, no coastal sands are left, and only terrestrial material is transported and redeposited by the tsunami waves. The differentiation between soil and soil, however, proves to be difficult and therefore not many studies have tried to detect these ‘invisible’ deposits.
New methods and approaches are needed to assess these unrecognizable tsunami deposits, thus allowing a better hazard estimation for the future. In the present study, the authors study tsunami deposits at the Misawa harbor in northern Japan. By the application of standard methods (sedimentological) in combination with organic geochemistry as a novel approach, this study achieved new insights into the visible and invisible tsunami deposits. A yet unrecognized woody-organic tsunami layer was discovered further inland than the tsunami sand reached. This deposit was detectable up to 69-72% of the tsunami’s total inundation, exceeding former description of the tsunami sands by c. 10%.
The obtained knowledge and a new method in organic geochemistry allowing the identification of the ‘invisible’ tsunami deposits may improve our understanding of processes during a tsunami and allows a better hazard estimation in the future based on a reassessment of historical tsunamis and the detection of their ‘invisible’ inundation deposits.