Giant Wave Hit Ancient Scotland

Friday, 7 September, 2001, 18:28 GMT 19:28 UK
Giant wave hit ancient Scotland

By BBC News Online's Helen Briggs
A giant wave flooded Scotland about 7,000 years ago, a
scientist revealed on Friday.

The tsunami left a trail of destruction along what is now the eastern coast of the country.

It looks as if those people were happily sitting in their camp when this wave from the sea hit the camp

Professor David Smith, Coventry University Scientists believe a landslide on the ocean floor off Storegga, south-west Norway, triggered the wave.

Speaking at the British Association Festival of Science in Glasgow, Professor David Smith said a tsunami could strike again in the area but the probability was extremely unlikely.

Radiocarbon dating of sediments taken from the coastline of eastern Scotland put the date of the event at about 5,800 BC.

Settlers at the time would have had little warning of the disaster, scientists believe. But a scattering of tools found in the sand at a hunting camp in Inverness yields some clues.

"It looks as if those people were happily sitting in their camp when this wave from the sea hit the camp," Professor Smith of the department of Geography at Coventry University told BBC News Online.

"We're talking about two, three or four large waves followed by little ones, that would have been 5-10 metres high.

"These waves do strike with such force that they are very destructive," he added. "It's like being hit by an express train."

The research provides an opportunity to assess the hazard of tsunamis in more detail.

They occur frequently in the Pacific Ocean due to underwater earthquakes, landslides and volcanic explosions.

Scientists hope to find more evidence of similar past tsunamis in eastern Scotland to predict the frequency of the destructive waves.

Studies of coastal sediments show that it may be possible to develop a record of past tsunamis extending back several millennia.

Dr Ted Nield, of the Geological Society of London, said: "These events have a long and uncertain time scale.

"While there is no reason for mass panic, the possibility exists that the Storegga slide will go again, and it would be imprudent to ignore that fact."

Tsunami generated by impacts

Although, for a given location on the Earth's surface, the risk of a "direct" hit from an asteroid is slight, researchers realized that an ocean impact had the potential to be much more destructive due to the effects of tsunami. An airburst explosion is a three dimensional event and energy decreases according to the square of the distance but a radiating ocean wave is a two-dimensional phenomenon and, in theory, energy decreases in proportion to distance. Since the early 1990s some advanced computer simulations have been conducted to estimate the effects of asteroid impacts above deep oceans.
The dramatic picture by Don Davis is a little misleading. When an asteroid hits the ocean at 70 000km/h there is a gigantic explosion. The asteroid and water vaporize and leave a huge crater - typically 20 times the diameter of the asteroid (that is, a 100m asteroid will create a 2 kilometre diameter crater). The water rushes back in, overshoots to create a mountain of water at the middle and this spreads out as a massive wave - a tsunami. The centre of the "crater" oscillates up and down several times and a series of waves radiate out. An idea of the mechanism can be obtained by bursting a balloon in a bathtub.

Tsunami! At Lake Tahoe?

Author/s: Kathryn Brown
Issue: June 10, 2000

Surprised tourists could catch the ultimate wave

Postcards from Lake Tahoe all flaunt a peaceful, brilliant-blue stretch of mountain water. But geologists have been snapping a very different picture of the lake lately. Far beneath Lake Tahoe's gentle surface, they say, several hidden earthquake faults snake across the lake's flat bottom. These faults put the lake at a bizarre risk for an inland body of water.

If the researchers are right, Lake Tahoe tourists could one day feel the ground tremble and, just minutes later, face a tsunami. Roiling waves of water would crest to 10 meters at the shore and criss-cross the lake for hours.

Tsunamis typically emerge in oceans, usually after a quake drops or lifts part of the seafloor. Undersea landslides--alone or following a quake--can also trigger these giant waves. In 1998, for instance, a tsunami devastated Papua New Guinea, sweeping away more than 2,000 people living on the country's northern coast (SN: 8/1/98, p. 69). And in the past decade, tsunamis have lashed the coasts of Japan, Nicaragua, and Indonesia, as well. But Lake Tahoe?

While it may seem improbable, Lake Tahoe holds just the right blend of ingredients to brew a tsunami. For one thing, it has plenty of water. As the world's 10th-largest lake, Lake Tahoe stretches 35 kilometers long, 19 km wide, and, in some spots, 500 m deep. What's more, the lake sits smack in the middle of earthquake country, nestled in a fault-riddled basin that straddles California and Nevada. Dozens of minor or moderate quakes erupt along faults in the region every week, and the Lake Tahoe area is no exception. All it would take, scientists say, is a strong quake directly beneath the lake to send the waters spewing, tsunami-style.

To get a better grip on Lake Tahoe's tsunami potential, University of Nevada, Reno geologists have been modeling different quake scenarios. According to their calculations, if a magnitude 7 quake struck either of two major faults under the lake, the bottom could open like a trapdoor, with a chunk of it suddenly dropping as much as 4 m. Just behind it would fall a huge, sinking slosh of water--generating a giant wave that would reach the surface, gather strength, and come barreling to shore as a tsunami.

And that's just the beginning. The scientists think the tsunami, in turn, would create so-called seiche waves, mountainous waves that lurch from shore to shore for hours on end. "Think of the lake like a pan full of water. When you knock one end way down, the water surges and then sloshes back and forth for some time," says Gene A. Ichinose, a geophysics graduate student at Nevada-Reno and lead author of the group's study, which appeared in the April 15 GEOPHYSICAL RESEARCH LETTERS. As in a jostled pan of water, some waves would likely splash past their usual borders--right into the homes and hotels that dot the Lake Tahoe shoreline.

As Ichinose puts it, "If you feel the earth shaking for 5 or 10 seconds, get to high ground."

GIANT IMPACT-WAVE DEPOSIT ALONG U.S. EAST COAST

Along the shore from North Carolina to Maryland and also into Chesapeake Bay, deep-sea drillers have charted the Exmore Boulder Bed. No minor deposit this; it is is over 60 meters thick in places and covers more than 15,000 square kilometers. In the bed are found boulders (up to 2 meters in diameter), cobbles, pebbles, and traces of tektite glass and shocked quartz. The youngest microfossils date from the Eocene, and argon dating of the ejecta yield a date of 35.5 million years, which correlates with the North American tektite strewn field. C.W. Poag et al interpret this boulder bed as follows:

"On the basis of its unusual characteristics and its stratigraphic equivalence to a layer of impact ejecta at Deep Sea Drilling Project (DSDP) Site 612 (New Jersey continental slope), we postulate that this boulder bed was formed by a powerful bolide generated wave train that scoured the ancient inner shelf and coastal plain of southeastern Virginia. The most promising candidate for the bolide impact site (identified on seismic reflection profiles) is 40 km north-northwest of DSDP Site 612 on the New Jersey outer continental shelf."
(Poag, C. Wylie, et al; "Deep Sea Drilling Project Site 612 Bolide Event: New Evidence of a Late Eocene Impact-Wave Deposit and a Possible Impact Site, U.S. East Coast," Geology, 20:771, 1992.)

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