Over a century after the Romans gave up control of what’s today Geneva, Switzerland, in 563 A.D., a deadly tsunami on Lake Geneva poured over the city’s walls. It originated from a rock fall, near the place where the River Rhône enters the opposite side of the lake. The ensuing tsunami destroyed villages, livestock and killed many people, according to historical accounts.
The scientists published their findings in the journal Nature Geoscience. This could indicate that the region is at risk today since there are about one million inhabitants living on the lake’s shores. This happened before and it will probably happen again, states geologist Guy Simpson, from the University of Geneva, one of the researchers involved in this project.
The team used high-resolution profiles of the sediment composition of the lake, which they gathered using seismic reflection from a ship. The technique is similar to sonar, but uses seismic waves that penetrate and reflect off sediment layers. These profiles showed a layer that had been deposited in a single event. This unusual layer was lens-shaped, averaged 5 meters in thickness, and covered 50 square kilometers.
The 563 A.D. rock fall and tsunami are known as the Tauredunum event, and this layer could be evidence of its actual existence. The deposition happened between 381 and 612 A.D. Since the Tauredunum is the only major natural event described around that time, it seems like a match. The scientists cannot say what exactly created the tsunami, but it’s been proposed that a falling rock caused an accumulated heap of sediment in the Rhône delta to collapse. This could have launched the wave and carried the sediment from the delta to the center of the lake.
Using predictive models, it was possible to estimate that a 13-meter wave hit Lausanne 15 minutes after the rock fall. An 8-meter wave reached Geneva after 70 minutes. Further analysis of the deeper cores will allow the team to learn more about the size and frequency of such events, and whether they were all big enough to generate tsunamis. This could help assess the risk to modern day Geneva and other lakeside communities.
A survey of the stability of the surrounding slopes and a more refined tsunami model could also help with the risk assessment.