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

海啸对沿海社区和基础设施构成了非常重大的危险，2004年影响印度洋的巨大地震海啸和2011年影响日本的海啸就是一例。最近，汤加火山匈牙利-汤加-匈牙利哈阿派火山于2022年初喷发，也导致了具有局部和远场影响的破坏性海啸。由于海啸的罕见程度和极高的震级，人们对海啸的了解非常复杂，这使得测量海啸的波浪变得格外困难。我们的许多知识仅限于识别古代海啸沉积，主要基于它们的磨合(最大距离内陆)和抬高(达到的最大高度)。来自这类矿床的沉积学信息在很大程度上仅限于其矿床的总体粒度。海啸波性质的大部分证据都锁定在这些沉积物中，但直到最近，我们还缺乏能够阅读这些证据的知识。在这里，我们将采用一种新的方法，利用最近在理解深海流动方面的巨大进步，纯粹基于它们的沉积。深海沉积似乎不太可能是海啸研究的起点，但有许多令人惊讶的相似之处。沙粒大小(或更大)的物质在深海中的沉积主要是一类水流的结果，其中包括浑浊流和泥石流，统称为沉积物重力流(SGF)。深海中的这种流动非常罕见，而且非常强大，因此我们对实际洋流的测量非常少，即使是这些测量也几乎完全限于斜坡，而不是盆地底部。即使对于这些水流，我们对流速分布的了解也比对泥沙浓度、垂直分层和沉积物组成的了解要好得多。因此，我们对这些强大的水下水流的了解主要是从它们的沉积物中获得的。在过去的10-15年里，我们对这些SGF矿床的理解发生了一场革命，并意识到它们不仅仅是低浓度的湍流或高浓度的粘性泥石流。我们现在认识到，就浓度和凝聚力而言，存在一个完整的流动范围，具有很大范围的“过渡流动”，并且个别流动事件可能在纵向和在给定时间点随着时间的推移显示流动性质的巨大变化。最近，我们开发了一些方法来识别这些不同的流动状态，并评估它们的流动演变。监督小组一直处于深水沉积学这些进展的前沿，并了解过渡和高浓度水流的动力学。最近，我们对海啸沉积物进行了一项概念验证研究，以审查这些深水方法的适用性。在这个研究项目中，这名学生将对日本和苏格兰的已知海啸沉积物进行采样。我们还打算整合海洋和陆地岩心，以及来自其他例子的一些浅水多波束数据集。这些数据集将用于记录沉积记录的特征(颗粒大小、形状、矿物学)，使用利兹大学沉积物、土壤和污染物分析实验室的尖端设备，以更好地了解海啸下的形成过程。重新研究海啸沉积物的沉积学，以解决相关的地质灾害，并改善沿海社区和基础设施的复原能力，具有巨大的潜力。拟议的PHD项目的主要目的是利用从深海沉积物中形成的概念得出的新的沉积学理解，提高对与海啸相关的地质灾害的理解。

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.