British tsunamis "occur much more often"

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AFP / Getty Images

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

Scientists knew that a huge wave had submerged vast areas of what is today the British Isles, Norway, the Faroe Islands, Iceland and Greenland, there are more than 8 000 years.

But discoveries on the Shetland Islands have revealed that two other yet unknown tsunamis have hit more recently.

Now the hunt is looking for what caused them.

This is only while the experts will be able to determine what we can do, if any.

The big one we already know is called the Slide Store.

It was a series of underwater landslides, probably triggered by an earthquake. About 8,200 years ago, he arrived at the edge of the continental shelf off Norway.

The dislodged debris represented 300 times the amount of sediment transported each year by all the rivers in the world combined.

He drove billions of tons of seawater before creating a massive tsunami.

By the time he hit Shetland, he was at least 20 feet tall.

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British Geological Survey NERC

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

The fact that this happened so long ago had provided some assurance that such a catastrophic event was rare, perhaps occurring once every 10,000 or even 100,000 years.

We now know differently.

Researchers at the British Geological Survey (BGS) and Dundee University, funded by the Natural Environment Research Council, have found that two other tsunamis, each at least 40 feet in height, struck the Shetland Islands 5,000 and 1,500 years old.

On the geologic time scale, these are just winks.

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

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British Geological Survey NERC

Sue Dawson, of the University of Dundee, uses a high-definition micro-computing scanner (micro CT) in which X-rays constitute a 3D image of core samples.

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

She found deposits of both tsunamis on carrots extracted from many points around the Shetland Islands. They are up to 13 meters above the current sea level.

It's about the same height as the 2011 tsunami that devastated parts of Japan.

Dr. Dawson shows a 7 cm thick strip of sand, evidence of the most recent large waves on the Shetland Islands. Above, 35 cm of dark peat, it took 1500 years to lie down. The sandy layer below was deposited almost instantly.

3D view

She says that CT scans create for the first time a complete 3D view of hearts.

"This detail will tell us in which direction the wave was moving, identify the elements present in the sand and much more," she says.

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

We know where the Slide Storegga arrived because it left a huge scar on the seabed. But what happened 5000 and 1500 years ago?

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

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

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British Geological Survey NERC

As Dr. Dawson discovers the secrets of the deposits, it is up to him to deduce where the tsunamis come from on the seabed.

He says that the BGS White Ribbon research vessel has carried out seismic surveys near the shoreline of the Shetland Islands but has so far found nothing.

He thinks it's an essential task as it is now clear that these events do not happen every 100,000 years.

"Much more serious risk"

"They are much more common, and 1,500 years ago, it's very, very recent – it's 500 AD if you want to think about it like that," he says.

"It means that the danger – the risk – is much more serious than we thought before.

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

Becoming aware of clues can take decades. Without this, researchers can not know how much more frequent tsunamis can be – or if we can do anything about them.

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

The work is part of the Landslide-Tsunami project, which is part of the NERC Arctic Research Program. One of the objectives is to know what effect climate change may have on the stability of the seabed.

Changes of seabed

Dr. Dawson says that it is important to disseminate information in the public arena.

He explains: "The work we have done allows us to reflect on the frequency of events.

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

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

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

A few words of comfort to the good people of the Shetland Islands: neither the tsunamis nor the sciences are targeting them. Nobody has the intention of causing a collapse in the price of real estate in Lerwick.

The Shetland are in the spotlight because their geology is perfect for spotting tsunami sediments.

But a tsunami will happen again – and that will be the science that dictates how long we wait.