Underwater landslides can be far larger than any landslide seen on land. For example, the Storegga Slide that occurred 8,200 years ago offshore Norway is larger than Scotland. It contained over 3,000 cubic kilometres of material (300 times the amount of sediment carried each year by all of the world's rivers combined) and ran out for 800 kilometres into the deep ocean. This truly prodigious mega-landslide generated a tsunami that ran up to heights of three to six metres along northern parts of the UK coastline. A modern day event of a similar scale to the Storegga Slide would be likely to lead to significant loss of life and devastating damage to key infrastructure, and there are few other natural events that would have such a disastrous impact on the UK.
A team of scientists is embarking on a four–year investigation to assess the hazard that landslide-tsunamis in the Arctic could pose to the UK over the next 100 to 200 years. This team is led by the National Oceanography Centre and involves seven other UK institutions, together with international project partners. The other UK institutions include NERC's British Geological Survey, Imperial College London, and the Universities of Aberdeen, Cambridge, Dundee, Manchester, Southampton, and Ulster. The team will work alongside representatives from government bodies and the reinsurance industry, including the Willis Research Network. They will look at the likely impact on human society and infrastructure, the degree to which existing sea defences are effective, and how the threat of tsunamis can be incorporated into the UK's multi-hazard flood risk management.
Worldwide, most tsunamis - such as the 2004 Indian Ocean tsunami, and 2011 tsunami offshore Japan - are triggered by large earthquakes near plate boundaries. Tsunamis triggered by mega-landslides are far less frequent than those caused by earthquakes. However, landslide-tsunamis may represent a greater threat to the UK, which is located away from the plate boundaries that create large earthquakes.
In the past, submarine mega-landslides near to the UK have been very rare, and considerable uncertainty still surrounds the frequency of mega-landslides. The Storegga Slide was initially thought to be several events, some occurring more than 30,000 years ago. More detailed research was necessary to show it was a single and more recent event. The available sea floor mapping suggests that at least six mega-slides have occurred beneath the Norwegian and Greenland Seas during the last 20,000 years. It is not yet clear whether all of these mega-landslides generated large tsunamis, or whether they produced two recent tsunami deposits found in the Shetland Islands.
Importantly, it has been proposed that mega-landslides will become more frequent due to future ocean warming that causes melting of gas hydrate (crystalline solids resembling ice that contain methane) which weakens sea floor sediment. It has also been proposed that melting ice sheets will cause an increase in the frequency of large earthquakes, as the Earth's crust adjusts to the removal of the ice sheet's weight. Such earthquakes could potentially generate more frequent mega-slides and tsunamis. The project will test these hypotheses rigorously, to determine whether there is credible evidence that the frequency of mega-landslides and tsunamis will increase significantly in the near future.
A key point for hazard assessment is that submarine mega-landslides are much more poorly understood than almost all other types of natural hazard, such as river floods or storm surges. Submarine mega-landslides occur on sea floor slopes of just one or two degrees -similar to the gradient of a Premiership soccer pitch. Hillsides with such remarkably low gradients are almost always stable on land, and it is not yet clear why submarine landslides occur on such low gradients. A second major uncertainty is how submarine mega-landslides are set in motion - do they occur in one or multiple stages?
Scientists have so far been unable to monitor a mega-landslide in action. This is important because the way in which the landslide moves determines the size of the tsunami it produces. Landslides that occur in a series of stages produce much smaller tsunamis. The project will analyse sediment flow deposits generated by mega-landslides to help understand how the landslides moved.
The specific aims of the research project are four-fold: to clarify the frequency and timing of major Arctic submarine slides; to better understand trigger factors and assess whether the frequency of the slides is likely to increase as climate changes and oceans warm, and to assess the magnitude necessary for landslide-tsunamis to flood parts of the UK coast.
The fourth strand of the consortium's research will be an attempt to quantify the likely cost to the UK of different types of inundation triggered by different types of landslide occurring in different locations. The science team will work closely with stakeholders, including government bodies – notably the Scottish Government and Defra (the Department for Environment, Food and Rural Affairs) - the Environment Agency and the reinsurance industry and will try to quantify the risk to key UK infrastructure (including nuclear power stations) the likely costs of tsunami inundation, as well as measures that can be taken to offset its impact, such as improved flood defences.
The project will use a range of techniques, including shipboard expeditions that will map the Arctic seafloor and extract sediment cores from the seabed, fieldwork on land to identify and date coastal tsunami deposits, slope stability modelling, laboratory experiments showing how hydrate dissociation affects sediment strength, and modelling of future trends in seismicity. It will also include modelling of landslide motion, tsunami wave generation and propagation, and how tsunami waves would interact with existing UK coastal defence structures. A sensitivity analysis will aim to capture uncertainties, and the key factors that determine societal cost.
Project leader, Dr Peter Talling of the National Oceanography Centre, said: "This is the first extensive study to assess the probability and likely impact of a landslide-tsunami on the United Kingdom. It is timely as it has been proposed that climate change may be a factor that increases tsunami frequency significantly. This hypothesis is in need of careful testing. We have assembled a broad range of expertise to look at this issue and to produce findings which will have a significant influence on future decision-making on flood protection and resilience. We hope that the project will produce a step-change in scientific understanding about some of the most remarkable and largest natural events that occur on our planet."
Dr Phil Newton, the Natural Environment Research Council's Director of Science said: "NERC plays a leading role in the UK's research into natural hazards, such as tsunamis which, although extremely rare, would have a serious impact on our communities and economy. Working with government and a range of partners and stakeholders this project will add an important dimension to the assessment of flood risk by building tsunamis into the framework, while leaving a legacy of expertise in this area."
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