Baku, October 29 (AZERTAC). In ad 563, more than a century after the Romans gave up control of what is now Geneva, Switzerland, a deadly tsunami on Lake Geneva poured over the city walls. Originating from a rock fall where the River Rhône enters at the opposite end of the lake to Geneva, the tsunami destroyed surrounding villages, people and livestock, according to two known historical accounts.

Researchers now report the first geological evidence from the lake to support these ancient accounts. The findings, published online in Nature Geoscience, suggest that the region would be wise to evaluate the risk today, with more than one million inhabitants living on the lake's shores, including 200,000 people in Geneva alone.

“It’s certainly happened before and I think we can expect that it will probably happen again sometime,” says geologist Guy Simpson, from the University of Geneva, one of the researchers behind the project.

The team found the evidence while creating high-resolution profiles of the sediment composition of Lake Geneva using seismic reflection from a ship. The technique is similar to sonar, but uses seismic waves that penetrate and reflect off sediment layers.

The seismic profiles showed a “very strange and very different layer” that had been deposited in a single event in the Lake Geneva sediments, says geologist Stéphanie Girardclos, one of the research team. The unusual layer was lens-shaped, averaging five metres thick and covering 50 square kilometres. Girardclos knew of the accounts of the ad 563 rock fall and tsunami, known as the Tauredunum event, and thought that this might be evidence of its existence.

Sediment cores collected from the bottom of the lake showed that the deposition happened some time between ad 381 and ad 612. As the only major natural event described around that time, Tauredunum seemed a logical match.

Tsunami experts say that a more detailed risk assessment would help to make the threat clearer, including a survey of the stability of the surrounding slopes and more refined models for how a tsunami would affect modern Geneva, for example.