Estimating the risk to the UK from landslide tsunamis

Figure 1: tsunami sediment layer in Shetland. Figure 1: tsunami sediment layer in Shetland.
Figure 2: cut core showing a diverse stratigraphy.

The UK is fortunately located well away from any subduction zone where tsunamis are so prevalent, therefore we are not as vulnerable as those around the massively destructive events in the Indian Ocean (2004) and Japan (2011) that have killed over 240 000 people in the past 10 years. However, tsunamis from submarine landslides have struck our shores in the distant past, most notably the Storegga Slide tsunami approximately 8150 years ago. BGS is participating in the NERC Arctic landslides project, which aims to better understand the hazard from these events, particularly as the northern latitudes warm and the seabed may become more unstable as a result of global warming.

2014 expeditions

In May 2014 Dave Tappin, Dave Long and Gareth Carter visited the Shetland Islands along with scientists from Dundee University to examine sediments thought to indicate tsunami events taking place over the past 10 000 years (Figure 1). Determining the frequency is important in risk assessments needed for major coastal infrastructures, particularly with anticipated sea level rise predicted from global warming.

In July 2014 Dave Long and Dave Tappin also joined the NERC funded four week expedition on board the Dutch NIOZ research ship Pelagia, to examine massive submarine slides off the coast of Norway in the Norwegian Sea (see the blog Slides in the deep). The expedition was remarkable for both the success, in gathering 90 piston cores and conducting multibeam surveys over a wide range of seabed features, including some of the largest submarine landslides along the Norwegian margin, and also in experiencing some of the most amazingly calm seas, which resulted in hardly any disruption of the survey programme.

The locations sampled ranged from the small Afen Slide west of Shetland to the largest slide on Earth, the Storegga Slide, which covers an area the size of Scotland. The expedition researched all the way along the Norwegian margin, into the deepest parts of the North Atlantic and north of the Arctic Circle, reaching the Bear Island Fan at 74°N.

Ongoing work

The sediment cores acquired (Figure 2) will be mainly examined by several PhD students at the National Oceanography Centre (NOC) in Southampton: research on the multibeam data will be led by BGS scientists. The results will lead to a major improvement in our understanding of the tsunami hazard in the North Atlantic from submarine landslides via the accurate dating of the age of these events, and an improvement in their mechanisms of failure, particularly as a result of the warming of the high latitude oceans over the coming decades.

Figure 3: the Afen Slide.

At present it is believed that the Storegga landslide, which generated a massive tsunami that inundated Shetland and the north and east coasts of Scotland with waves than ran up to 30 m above the palaeoshoreline, was dominantly a result of deglacial sedimentation and tectonics. The more recent events off Shetland, such as Afen at approximately 2600 years BP (Figure 3) and Trænadjupet at prior to 4000 years BP, are much younger and their triggering mechanism more uncertain.

Various factors can influence mechanisms of landslide triggering and failure style. Deglaciation can load continental margins with thick sequences of rapidly deposited sediments, immediate post-glacial tectonic movements due to the differential uplift of continents through unloading as the massive ice sheets melt, and changes in temperature can destabilise methane hydrate. Each factor can influence mechanisms of landslide triggering and failure style. These different mechanisms are critical as slope failure in stages will have quite a different tsunami potential than a landslide that failed as a single event.

Future work

The Pelagia expedition and the fieldwork on Shetland are only the beginning of research that will now be undertaken on the cores and seabed bathymetry acquired. Over the next three years, scientists from BGS and NOC will piece together by detailed analysis what happened in the North Atlantic over the past 10 000 years, thereby decoding the tsunami history of the UK and better understanding the potential tsunami hazard to the coastlines of the UK.

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