Processes beneath the Great Wave: improved understanding of tsunami geohazards using advances in deep-sea sedimentology

Tsunami pose a very significant hazard to coastal communities and infrastructure, as seen in the giant earthquake-generated tsunami that affected the Indian Ocean in 2004, and Japan in 2011. More recently, the eruption of the Tongan volcano Hunga Tonga-Hunga Ha'apai in early 2022 also led to a destructive tsunami with both local and far field effects. Understanding of such tsunami is complicated by their great rarity and very high magnitude, which makes measurements of their waves exceptionally problematic. Much of our knowledge is limited to the recognition of ancient tsunami deposits, based primarily on their run-in (maximum distance inland) and run-up (maximum height reached). Sedimentological information from such deposits is largely restricted to the overall grain-size of their deposits. Much of the evidence for the nature of tsunami waves is locked in these deposits, and yet we have lacked the knowledge to be able to read this until recently. Here we will adopt a new approach that utilises the huge recent advances in understanding of flows in the deep oceans, based purely on their deposits.

Deep-sea deposits may seem an unlikely starting point for the study of tsunami, yet there are many surprising similarities. Sedimentation of sand-sized (and larger) material in the deep-sea is primarily the result of a class of flows that include turbidity currents and debris flows, collectively known as sediment gravity flows (SGFs). Such flows in the deep-sea are highly infrequent and very powerful, and thus we have very few measurements of actual currents, and even these are almost entirely restricted to the slope, rather than the basin floor. Even for these flows, we have a much better idea on velocity distributions than we do sediment concentrations, stratification in the vertical, and sediment composition. Consequently, our knowledge of these powerful underwater flows is dominantly obtained from their deposits. Over the past 10-15 years there has been a revolution in our understanding of these SGF deposits, and a realisation that they are not solely low-concentration, turbulent currents or high-concentration cohesive debris flows. We now recognise that there is a full spectrum of flows in terms of concentration, and cohesive strength, with a large range of 'transitional flows', and that individual flow events can show dramatic changes in flow properties both longitudinally, and over time at a given point. More recently, we have developed methods to be able to identify these different flow states, and to assess their flow evolution.

The supervisory team have been at the forefront of these advances in deepwater sedimentology, and understanding the dynamics of transitional and high concentration flows. More recently we have undertaken a proof-of-concept study on tsunami deposits to examine the applicability of these deepwater approaches. In this research project, the student will sample known tsunami deposits in Japan and Scotland. We also aim to integrate both marine and terrestrial cores, as well as some shallow water multibeam datasets, from additional examples. These datasets will be used to document the character of the sedimentary record (particle size, shape, mineralogy) using the cutting edge equipment in the Sediment, Soil, and Pollutant Analysis Laboratory at the University of Leeds, to better understand the formative processes under the tsunami. There is huge potential to re-examine the sedimentology of tsunami deposits to tackle the associated geohazards, and improve the resilience of coastal communities and infrastructure.

The principal aim of the proposed PhD project is to improve understanding of the geohazards associated with tsunami utilising new sedimentological understanding derived from concepts developed in deep-marine sediments.