In celebration of the publication of Ashley Curtis' "Why do the Swiss have such great sex?", we're publishing a chapter from the book.

Could a Tsunami Strike Switzerland?

The Tauredenum Event could be the title of a disaster movie. And a disaster it was, but not a movie—yet. Here is a description from the contemporary chronicler Gregory of Tours in his History of the Franks. The year is 563.

A great prodigy appeared in Gaul at the fortress of Tauredenum, 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. The water flooded the higher reaches and submerged and carried everything which was on its banks.

And yet again the inhabitants were taken unawares: as the accumulated water suddenly broke through the blockage, it drowned those who lived lower down, 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. It is told by many that the mass of water was so great that it went over the walls of the city.

In 2012 geophysicists at the University of Geneva published a paper analyzing huge deposits of sediment near where the Rhône enters the lake. Their conclusion was that the Tauredenum event involved a massive landslide that caused a collapse of the Rhône delta and a slippage of sediment at the eastern end of the lake, and this in turn created a tsunami. A 13 meter high wave, traveling at 70 kilometers per hour, would have reached Lausanne 15 minutes after the slippage. Three quarters of an hour after that, its height reduced to 8 meters, it would have inundated Geneva, crashing over the city walls just as Gregory reported.

The Swiss Seismological Service agrees. It catalogues several tsunamis that have crossed Swiss lakes and inflicted widespread devastation. An earthquake near Aigle set off a tsunami in Lake Geneva in 1584. In 1601 an earthquake caused submarine landslides in Lake Luzern, and a 4 meter high wave engulfed the city. Luzern was hit again in 1681, this time with a 5meter tsunami. And in 1806 the Goldau landslide, which destroyed the village of that name and killed 500 of its inhabitants, unleashed a 10meter high wall of water in Lake Laurerz.

Today there are over a million people living on lowlying land around Lake Geneva. And it turns out that the Tauredenum event was not a one-off.

In fact,

The sedimentary record of the deep basin of Lake Geneva, in combination with the historical record, show that during the past 3,695 years, at least six tsunamis were generated by mass movements, indicating that the tsunami hazard in the Lake Geneva region should not be neglected . . . We believe that the risk associated with tsunamis in lakes is currently underestimated, and that these phenomena require greater attention if future catastrophes are to be avoided.

So wrote the Geneva geophysicists, who calculated that we can expect a tsunami on Lake Geneva, on average, once every 625 years. A big one happened in 563. A small one in 1584. Now it’s 2018. Do the math.

Next time you’re in Geneva, don’t just worry about what’s going on in the Large Hadron Collider out by the airport. Keep an eye on that big lake as well—for an only partly unexpected “event.”

(c) Ashley Curtis

PS. You can get a German version of this book as well!