Historical Records Suggest Lake Geneva Is At Risk For Future Deadly Tsunami
October 30, 2012

Historical Records Suggest Lake Geneva Is At Risk For Future Deadly Tsunami

Lawrence LeBlond for redOrbit.com - Your Universe Online

Scientists have found evidence of a deadly tsunami that inundated coastal Switzerland in AD 563 after a rockfall near the River Rhone swept Lake Geneva. A wall of water 42 feet high crashed the shore, destroying villages along the coast and killing people and livestock, according to historical accounts of the incident.

Scientists now say that Geneva and Lausanne remain vulnerable to similar events today, as do other cities on the edge of mountain lakes and high-sided fjords.

Reporting in the journal Nature Geoscience, scientists suggest the region would be wise to evaluate the risk of such a catastrophe today, with more than a million people living on Lake Geneva´s shores, including 200,000 in Geneva alone.

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

“It´s certainly happened before and I think we can expect that it will probably happen again sometime,” added geologist Guy Simpson, also from the University of Geneva.

After investigating the historical accounts of the “Tauredunum Event,” as the AD 563 event is known, Kremer´s team decided to delve deeper into the mysterious event.

The catastrophe was first described by a French Bishop, Gregory of Tours, who called it bewildering and terrifying. The giant wave raced across the lake, destroying everything in its path, crashed over Geneva´s city walls and drowned all those who stood in its way, according to the Bishop's account.

The research team found real historic evidence of the event while creating high-resolution profiles of the sediment composition of Lake Geneva using seismic reflection from a ship. Similar to sonar, this technique uses seismic waves that penetrate and reflect off sediment layers.

During their investigation they uncovered a huge, oval-shaped pile of sediment, more than six miles long, three miles wide, 16 feet thick, with the thickest point occurring at the southeast, indicating an origin in the Rhone River delta. They took four cores of the sediment and carbon-dated vegetal remains embedded in it. Through their analysis they determined the age of the debris dated it to between AD 381 and AD 612.

Stephanie Girardclos, one of the researchers on the team, said the analysis indicates the sediment field was deposited in a single event. She knew of the accounts of the Tauredunum event and believed this could be the evidence of its existence.

“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,” said the research team.

Kremer believes that on that fateful day, the shock of the rockfall caused the nearby delta´s slopes to collapse. The resulting displacement of water generated a series of waves, one measuring 42 feet high on the lake´s northern shore. A wave 26 feet high hit Geneva 70 minutes after the initial sediment collapse. Long, narrow and crescent-shaped, Lake Geneva created the ideal conditions for amplifying the waves generated from the rockfall.

A reconstruction of the event showed the researchers how the wave would have breached Geneva´s city walls, as reported in the historical records.

“Today, a wave of this height would completely inundate large parts of the inner city of Geneva,” the researchers wrote. They added that this event was “by no means unique.”

Seismic records showed that large mass movements of sediment had been generated on the lake several times in the past 12,000 years, any of which could have triggered a giant tsunami.

“Given that riverine sediment input is still loading on the slopes of the Rhone delta, tsunamis may well occur in Lake Geneva in the future, whether they are triggered by rockfall, earthquakes, or simply large storms, resulting in slope failure,” noted Kremer. “Such tsunamis ... pose a direct and hitherto largely ignored threat to at least a million people living along the lake shores.”

“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,” wrote the team. “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.”

“The risk posed by lake tsunamis has been underestimated in the past,”  tsunami sedimentologist Max Engel of the University of Cologne in Germany, told Nature´s Jessica Marshall. Lakes with similar geological characteristics, such as Switzerland's Lake Lucerne, are at risk of similar events, he added.

Tsunamis have almost always been associated with near-coast sea floor earthquakes, displacing water when the seabed lowers or rises, forming waves. But research since 2004´s Indian Ocean tsunami suggest that amplified waves can be created by a range of other scenarios, including from volcanic eruptions and dam breaks.