Tsunami hit Geneva in AD 563: scientists

29 Oct 2012, 5:45 am   -   Source: AAP

Scientists have warned that cities on the edge of mountain lakes are vulnerable to a tsunami like one that swept Lake Geneva nearly 1500 years ago.

Nearly 1500 years ago a tsunami triggered by a rockfall swept Lake Geneva, engulfing its shores with a wall of water up to 13 metres high, Swiss scientists report.

The incident suggests Geneva and Lausanne remain vulnerable today, as do other cities on the edge of mountain lakes and high-sided fjords, they say.

"The risk is underestimated because most of the people just do not know that tsunamis can happen in lakes," Katrina Kremer, an Earth scientist at the University of Geneva, told AFP.

In a letter to the journal, Nature Geoscience, Kremer's team said they delved into the "Tauredunum Event", an episode that occurred in AD 563.

A contemporary account by a French bishop, Gregory of Tours, described a catastrophe that was as bewildering as it was terrifying.

A giant wave charged down the lake, destroying villages and herds of animals, and then passed over the city walls of Geneva, on the western tip, where it drowned several people.

Was this a "lake tsunami"? Some experts have argued so.

They point to evidence that part of a mountain slipped into the River Rhone about five kilometres from where it flows into Lake Geneva at the lake's eastern point.

Keen to find out more, Kremer's team swept the deepest part of lake with high-resolution radar.

They uncovered a huge, oval-shaped pile of sediment, more than 10 kilometres long, 5km wide and five metres thick.

The thickest point was in the southeast, indicating an origin in the "delta" where the River Rhone flows into the 73km-long lake.

The team then took four cores of the sediment and carbon-dated tiny vegetal remains embedded in it. The estimated age of this debris is between AD 381 and AD 612.

"Since the AD 563 event is the only significant natural event recorded in historical accounts within our calculated age interval, we consider our dating results to be a strong indication that the deposit is linked to the AD 563 rockfall and tsunami," the letter said.

On that fateful day, believes Kremer, the shock of the rockfall caused the nearby delta's built-up slopes to collapse.

Long, narrow and crescent-shaped, Lake Geneva created ideal conditions for funnelling the maelstrom of mud and water into coherent, amplified waves.

Over the past 10,000 years, several big slips have occurred, and they could happen again as sediment is building up on the delta slopes - posing a threat to more than a million people living on the lake shores, said the letter.

"Our study highlights that not only cities located on sea coasts and fjords are at risk from destructive tsunamis, but so are densely-populated lake shores," it said.

Tsunamis are usually associated with an earthquake on the sea floor that occurs close to a coast. The quake raises or lowers the sea bed, and the displaced water forms waves.

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