Event #2892: A landslide causing a tsunami, 8 meters high, wipes out the old city of Geneva

Text #965

A great prodigy appeared in Gaul at the fortress of Tauredunum, which was situated on high ground above the River Rhône. Here a curious bellowing sound was heard for more than sixty days: then the whole hillside was split open and separated from the mountain nearest to it, and it fell into the river, carrying with it men, churches, property and houses. The banks of the river were blocked and the water flowed backwards. This place was shut in by mountains on both sides, for the stream flows there through narrow defiles. The water then flooded the higher reaches and submerged and carried away everything which was on its banks.

A second time the inhabitants were taken unawares, and as the accumulated water forced its way through again it drowned those who lived there, just as it had done higher up, destroying their houses, killing their cattle, and carrying away and overwhelming with its violent and unexpected inundation everything which stood on its banks as far as the city of Geneva. Many people maintained that the volume of water was so great that it flowed right over the walls of Geneva: and this is doubtless possible, for, as I have told you, at this spot the Rhône runs through mountainous defiles and, once its course was blocked, there was nowhere for it to turn on either side. It burst through the mountain which had fallen into it and washed everything away.

When all this had happened, thirty monks made their way to the spot where the fortress had collapsed, dug into the earth beneath where the landslide had occurred and found there bronze and iron. While they were busy at their task, they once more heard the bellowing of the mountain. So strong was their lust for gain that they took no notice: and a part of the hillside which had not previously collapsed now fell on top of them. It buried them completely and their dead bodies were never recovered.

Text #966

In this year, the powerful mount of Tauredunum in the territory of Valais collapsed so suddenly that it crushed the fortress which was nearby and the villages along with all their inhabitants. It set in motion the whole lake sixty miles long and twenty miles wide so that the outpouring of water devastated the ancient villages on both banks with their inhabitants and herds and also demolished many holy places with their servants. The force of the water brought down the bridge at Geneva, mills and people, and the flood pouring into the city of Geneva killed a great many people.

Text #3986

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The issue of tsunamis in Alpine lakes grabbed the spotlight two years ago when limn­ogeologist Katrina Kremer, then at the University of Geneva, Switzerland, and her colleagues reported evidence for a major tsunami in Lake Geneva in ad 563 that had wiped out communities living on its shores.¹

Kremer, who has since moved to the Swiss Federal Institute of Technology (ETH) in Zurich, has upped the count to five probable tsunamis in Lake Geneva over the past 4,000 years, including another devastating one during the Bronze Age around 3,400 years ago. Her team relied on cores taken by drilling 30 metres into the lakebed. The cores represent 4,000 years’ worth of sediment deposition, and show telltale structures of lakebed displacement that could have caused a tsunami. Kremer presented the work on 18 August at a meeting of the International Association of Sedimentologists in Geneva.

The ad 563 event, the largest of the five, occurred when part of a mountain fell onto an unstable underwater delta of the Rhône river, which flows into Lake Geneva. The falling rock forced the collapse of sections of the delta, creating a wave that was 8 metres high when it hit and wiped out the old city of Geneva at the other end of the lake.

The Bronze Age tsunami was 6 metres high, and may provide another example of tsunami-related devastation in the Lake Geneva region, because it fits with a possible abandonment by lake-shore dwellers. “We think this could explain an occupation gap in early Bronze Age sites on the northern shore of the lake,” says Kremer. Albert Hafner, an archaeologist at the University of Bern, finds the hypothesis convincing: “They left their sites for many reasons, but tsunamis may have been one of them.”

The episodes add to growing evidence for historical tsunamis in other Alpine lakes, including two events in Lake Como in northern Italy, in the sixth and twelfth centuries ad, and one in Lake Bourget in the French Alps in 1822. A witness of the 1822 tsunami described the water as “boiling like soup”, an effect caused by the release of methane when an earthquake stirs up organic debris in the lake.

In the past decade, a history of tsunamis has also emerged for Lake Lucerne. Research by Flavio Anselmetti, a geologist at the University of Bern, and his colleagues reveals that the lake experienced two tsunamis in the seventeenth century. The first, in 1601, reached 5 metres in height. The amount of sediment on some of the lake’s underwater slopes has since increased, and a strong earthquake could easily dislodge it, triggering a tsunami, says Anselmetti.

1. Kremer, K., Simpson, G. & Girardclos, S. Nature Geosci. 5, 756–757 (2012). [OF] ↩

(© Flavio Anselmetti Group, Univ. Bern)
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