When a severe earthquake shook Central Switzerland in 1601, the water in the Lake of Lucerne began to splash backwards and forwards for several hours. This destructive tidal wave, which reached a height of up to four metres, was triggered by underwater mudslides in the lake sediments. In the context of a major palaeoseismic study, researchers from the Geological Institute of ETH Zurich (1) and the Swiss Seismological Service at ETH have shown that the 1601 earthquake was not the only one that left traces in the sediment layers of the Lake of Lucerne. The scientists were able to locate five other events that must have happened in the last 15,000 years. (2)During a further study, the geologists have now carried out a similar investigation in the Lake of Zurich. As they report in the latest issue of the scientific journal “Geology” (3), it is now apparent that three of the five large prehistoric earthquakes also caused sediment movements not just in the Lake of Lucerne but in the Lake of Zurich as well.

Good agreement

“Mudslides after earthquakes are recognised by the disturbed layers in the lake sediments,” explains Michael Strasser from the Geological Institute, who studied the sediment deposits in the Lake of Zurich as a doctoral student. Using reflection-seismic investigations, the researchers were able to determine exactly where the sediments were set in motion. To enable the events to be sequenced accurately in time, the researchers took sediment samples at the critical points, after which they dated the corresponding layers using the C-14 method. According to these analyses, the three events in the Lake of Zurich occurred 2,200, 11,500 and 13,800 years ago. Strasser explains that “These figures agree very well with the values of three events in the Lake of Lucerne. Severe earthquakes do not happen very often in Switzerland, and we are therefore reasonably certain that the mudslides in the two lakes were triggered by the same earthquakes.” Based on historically recorded earthquakes, the researchers reconstructed the strength of these three prehistoric events. “When mudslides in a lake are triggered by an earthquake, the macroseismic intensity reaches Level VII,” is how Strasser explains the reconstruction. “If such a quake happened today, clearly visible damage to buildings would be observed and roofs would even collapse occasionally.”

Seismic focus centre in Central Switzerland?

Assuming as the simplest hypothesis that the focus centres of the three prehistoric quakes were situated somewhere between the two lakes, the resulting minimum magnitude is 6.5 – which is at any rate a value similar to that of the big Basel earthquake in 1356, the largest historically documented incident in Central Europe, which reached a magnitude of 6.9. According to Strasser, “It was not known that such severe earthquakes happened in the region of the Lake of Zurich. Previously all that was known was one earthquake near Thalwil in 1674, which reached a magnitude of 5.4.” Exactly where the three prehistoric earthquakes were triggered has still not been conclusively explained.

This is the platform from which the ETH geologists took sediment samples to date the events. large

Michael Strasser (l.) and his guide Flavio Anselmetti prepare to take the samples. large

Strasser thinks that "From the geological point of view it is rather unlikely that the quakes were actually initiated between the two lakes. It is more probable that they originated from a focus centre to the east of the Lake of Lucerne.” However, in this case the quakes would have reached a magnitude of distinctly greater than 6.5 since otherwise they would not have triggered any mudslides in the Lake of Zurich. Strasser presumes that the quakes originated at the base of the Alpine rock strata. “It is entirely possible that earthquakes reaching the required magnitude of 6.5 to 7 could have occurred there.”

A real residual risk

The new knowledge is very important in assessing the earthquake risk to the Swiss plateau (4). Insurers base their calculations on events with a recurrence rate of 500 years. Events with a longer recurrence period, which includes the three earthquakes that have now been discovered, are classified as a residual (marginal) risk. The new study shows that for the Lake of Zurich region this residual risk is a real magnitude. Moreover, the results also affect the assessment of the longer-term risk for critical infrastructure installations.