British tsunamis 'happen much more often' than thought

By Ken Macdonald
BBC Scotland Science Correspondent

Image source, AFP/Getty Images

New research has found that devastating tsunamis hit the British Isles much more often than previously thought.

Scientists had known that a huge wave had swamped large areas of what are now the British Isles, Norway, the Faroe Islands, Iceland and Greenland more than 8,000 years ago.

But discoveries on Shetland have found another two hitherto unknown tsunamis have hit more recently.

Now the hunt is on to find what caused them.

Only then will the experts be able to work out what, if anything, we can do about it.

The big one we already knew about is called the Storegga Slide.

It was a series of submarine landslides, probably triggered by an earthquake. It happened on the edge of the continental shelf off the coast of Norway about 8,200 years ago.

The dislodged debris was 300 times the amount of sediment carried each year by all of the world's rivers put together.

It drove billions of tonnes of seawater before it to create a massive tsunami.

By the time it hit Shetland it was at least 20m (65ft) high.

Image source, NERC British Geological Survey
Image caption,
Basta Voe on Yell is one site where researchers have found evidence of the most recent tsunami to hit the British Isles

Some of the debris carried by the wave has also been found far inland on the Scottish mainland.

The fact that it happened so long ago had brought a degree of reassurance that such a catastrophic event was rare, happening perhaps once every 10,000 or even 100,000 years.

Now we know otherwise.

Researchers from the British Geological Survey (BGS) and Dundee University, funded by the Natural Environment Research Council, have discovered that two more tsunamis, each at least 40ft high, hit Shetland 5,000 and 1,500 years ago.

On the geological timescale, those are mere blinks of the eye.

How do they know? Thanks to the debris the tsunamis dumped. It takes the form of thick layers of sand between the Shetlandic peat.

Image source, NERC British Geological Survey
Image caption,
Scientists found evidence of a relatively recent tsunami at Dury Voe on the Shetland mainland

Dr Sue Dawson of Dundee University has been using a high-definition micro-computed tomography (micro CT) scanner in which X-rays build up a 3D picture of core samples.

Each of the hundreds of samples is imaged from 3,000 different angles.

She has found deposits from the two tsunamis from cores extracted from many points around Shetland. They are up to 13m (42ft) above the current sea level.

That is about the same height as the 2011 tsunami that devastated parts of Japan.

Dr Dawson points to a 7cm-thick band of sand, evidence of the most recent of the great waves on Shetland. Above it, 35cm of dark peat which took 1,500 years to lay down. The sandy layer below was deposited almost instantaneously.

3D view

She says the CT scans are creating for the first time a complete 3D view of the cores.

"This detail will show us which direction the wave was travelling in, identify the elements present in the sand and much more," she explains.

These findings are vital because we do not yet know what caused the two relatively recent tsunamis.

We know where the Storegga Slide happened because it has left a huge scar on the seabed. But what happened 5,000 and 1,500 years ago?

The working assumption is that both events were also caused by seabed landslides. But where - were they big slides far from the islands or relatively closer and smaller?

"Think of me as Sherlock Holmes," says Prof Dave Tappin of the BGS. His colleagues call him Dave the Wave.

Image source, NERC British Geological Survey

As Dr Dawson uncovers the secrets of the deposits, it is his job to deduce where on the seabed the tsunamis originated.

He says the BGS research vessel White Ribbon has carried out seismic surveys close to the shores of Shetland but has so far found nothing.

He thinks it is an essential task because it is now clear these events do not just take place every 100,000 years.

'Far more serious risk'

"They're much higher frequency, and 1,500 years ago is very, very recent - it's 500 AD if you want to think about it like that," he says.

"It means that the hazard - the risk - is far more serious than we thought previously.

"And so what we're trying to do now is better define it."

Making sense of the clues may take decades. Without it, the researchers cannot know how much more frequent tsunamis may be - or what if anything we can do about them.

Prof Tappin aims to use the data to create 3D simulations of the Shetland tsunamis, then work backwards to pinpoint their sources.

The work is part of the Landslide-Tsunami project, part of NERC's Arctic research programme. One focus is on what effect climate change may have on seabed stability.

Seabed changes

Dr Dawson says it is important to get the information into the public arena.

He explains: "The work we've been doing allows us to think about the frequency of events.

"We know that something happening in the northern North Atlantic is not pie in the sky.

"It is actually an event - and it's not just maybe one in 10,000 [years].

"There could well be changes on the seabed, changes around the Greenland margin that may well increase the risk of future tsunamis to this part of the world."

A few words of comfort for the good people of Shetland: neither tsunamis nor science are picking on them. It is no-one's intention to cause a house price collapse in Lerwick.

Shetland is in the spotlight because its geology is perfect for spotting tsunami sediments.

But a tsunami will happen again - and it will be science that dictates how much notice we get next time.