Tidal Waves in Europe? Study Sees North Sea Tsunami Risk
The last known tsunami to hit Europe was over 8,000 years ago. But new research reveals that there have been a number of deep-sea earthquakes since then, and that a landslide along the continental slopes could pose a serious risk to the cities and towns on the North Sea coast.
It was a catastrophe of apocalyptic proportions. An earthquake shook Norway's coast between Bergen and Trondheim about 8,150 years ago. The tremors ripped pieces of land the size of Iceland from shallow water and sent them crashing into the deep sea. Like a stone thrown into a pond, the landslide produced ripples of waves that spread at the speed of a train -- powerful tsunamis racing across the North Sea. Along the beaches of Scotland the waves were up to six meters (20 feet) high. Geologists have discovered a ravaged Stone-Age site there.
Could something like this happen again? The environmental conditions in those days were different: 10,000 years ago the three-kilometer ice crust that had covered northern Europe during the Ice Age was beginning to melt. This released the earth's crust, which sometimes raised itself jerkily, quaking the earth. Since then the ground has calmed down, experts have believed until now. Strong shakes along the earth's tectonic plates in the seabed are rare, and these tend not to generate giant waves. For example on Jan. 24, 1927, there was a quake in the sea between Norway and Great Britain. On June 7, 1931, there was one between Denmark and Great Britain. And on Nov. 18, 1929, one in North West Scotland. A quake in the streets of Dover in 1580 was mentioned in Shakespeare's Romeo and Juliet. Otherwise, everything's been calm. Or so it seemed.
Chronicles Reveal 16th Century Earthquakes
Geologist Roger Musson of the British Geological Survey in Edinburgh has unearthed documents that paint a different picture. Several sources from the 16th century mention an earthquake on Sept. 19, 1508. "It was a great earthquake, not only in Scotland, but also, indeed, even the whole of England, which shook the churches especially, which was interpreted as an omen of the overthrowing of religion," wrote the Scottish Bishop of Ross, John Leslie.
Obviously this was not one of the local tremors which often occur in Scotland and England, both then and now. This at least was the thesis put forward by Musson recently at the European Geosciences Union (EGU) conference in Vienna. He argues that the fact that churches "especially" affected points to a significant quake. Heavy quakes that occur far away tend to be more likely to cause high buildings such as church towers to move. The fact that no damage was reported would indicate that the source of the shock was deep in the seabed.
Many years of research in libraries, church archives and old chronicles led Musson to further disturbing reports. Entries suggest that in 1089, 1508, 1607, 1686 and 1847 the seabed near Great Britain suffered severe quakes, the scientist writes in a study to be published soon.
It seems that earthquakes not only occurred more frequently than had previously been thought, but that they were also stronger, Musson told SPIEGEL ONLINE. It is, however, difficult to prove -- as earthquakes along Europe's coasts have only been recorded with measuring instruments since the 1970s.
Unstable Continental Slope
Musson's archival discoveries have focused attention on northwestern Europe's continental slope. The underwater cliffs between shallow water and the deep sea off Norway's coast are 3,800 meters deep. Could a quake cause the sediment lying on the slope to slide -- just like 8,150 years ago?
Norwegian scientists who have carried out years of research are certain that there is no danger of a landslide off the coast of Storegga. Most of the volcanic tephra had slid off during the giant landslide 8,150 years ago, Tore Kvalstad of Norway's Geotechnical Institute told SPIEGEL ONLINE.
However, there has not been the same extensive research into the coasts north and south of the Storegga slope. A group led by Petter Bryn from energy company Norsk Hydro and Anders Solheim of the International Center for Geohazards in Oslo have researched Norway's west coast. The result: There were landslides in many places along the coast over the past million years.
Most were significantly smaller than the one at Storegga. In 1999 researchers discovered traces of a violent landslide on Norway's north coast that is being surveyed now for the first time. A group led by Daniel Winkelmann and Wilfried Jokat of the Alfred Wagner Institute (AWI) in Bremerhaven reported in Geochemistry Geophysics Geosystems that the sand masses are of a similar strength to those in the Storegga landslide. The so-called Yermak Slide (also Hinlopen Slide) is assumed to have fallen into the deep sea around 30,000 years ago and caused tsunamis, according to the researchers.
Dangerous Gaps in Knowledge
Whether this could occur again depends upon the mix of sediment on the continental slope. There has been hardly any research into this deposit -- a gap in knowledge with possibly fatal consequences. The sediment could hide extensive layers of clay, which could act as a slick slope for a landslide. Steep layers of sand would be another cause for alarm, because even a light ground motion could cause them to move.
Possibly the greatest elements of uncertainty are the so-called methanhydrates: gas-containing ice caps which keep the sand attached to the slopes like a kind of weak sticker. In the scientific journal Eos, Angus Best of the University of Southampton warns that if the water level or the temperature were to change, this "cement" could dissolve. An earthquake could also cause the volatile architecture to slide, says AWI researcher Wolfram Geissler.
A computer model designed by Norwegian scientists shows the possible consequences of a mega-landslide. They have forecast the progression of a disaster: Minutes after the landslide 14-meter-high waves would hit Norway's coast, with fatal results, as many cities lie at sea level or in bays with sharply canted floors, where the waves would rise even higher. After three hours 20-meter-high breakers would crash onto the Shetland Islands. Two hours later the Faeroe Islands would be covered in waves of up to 14 meters high. After six hours, the tsunamis would still be six meters high, tearing along Scotland's beaches toward the coastal cities of Edinburgh, Aberdeen and Dundee. As they head southwards the waves would become smaller, the oscillating North Sea acting as a break.
The model predicts that Germany's North Sea coast would only see light flooding -- but even elaborate computer models can be wrong.
© SPIEGEL ONLINE 2006
All Rights Reserved
Reproduction only allowed with the permission of SPIEGELnet GmbH