Strong earthquakes in Switzerland – where and when they occur

Switzerland is not a high earthquake risk country. On Wednesday, however, the ground in the Jura again shook above average. With a magnitude of 4.3 on the Richter scale, the earthquake was one of the 24 strongest earthquakes since 1975 in Switzerland. The epicenter was about 13 kilometers southwest of Pruntrut. The earthquake occurred at 3:50 p.m. at a depth of 5.7 kilometers, according to the Swiss Seismological Service of ETH Zurich (SED).

The quake was felt in large parts of the Swiss Mittelland. The SED received reports from the canton of Freiburg as far as the Muotha valley in Schwyz, 146 kilometers away. The earthquake was also felt in France, from Belfort to Mulhouse. However, no reports of damage have been received by either the Swiss or the French police.

In Switzerland, smaller earthquakes usually occur again and again. Since the beginning of 2023, the SED has recorded 228 earthquakes in Germany. Some 1,000 to 2,500 earthquakes occur in Switzerland each year, but the population only feels 10 to 20. These usually have a magnitude of 2.5 or more.

4757 such earthquakes (magnitude ≥ 2.5) can be found in the SED database for Switzerland and neighboring countries. They go back to the year 250, when the earth shook in Augusta Raurica near Basel. However, the registrations are only reliable from 1975 and the construction of a network with approximately 200 measuring stations. Previously, the magnitude of the earthquake was estimated using historical records, natural records (lake deposits, broken stalactites in caves, etc.) and a statistical procedure.

The earthquakes in Switzerland are mainly the result of the collision of the European and African lithospheric plates. Valais is the Swiss region with the highest risk, followed by Basel, Graubünden, the Rhine Valley of St. Gallen and Central Switzerland. There are no regions in Switzerland where earthquakes are not possible.

Earthquakes as strong as the one in Porrentruy on Wednesday are quite rare in Switzerland and neighboring countries. The SED database lists 374 earthquakes with a magnitude of 4.0 or greater, up from 13 so far in the new millennium.

The strongest earthquake in Switzerland to date is the Basel earthquake on October 18, 1356. It had a magnitude of about 6.6 and destroyed large parts of the city of Basel. There are different reports on the number of deaths. Some historical sources speak of as many as 2,000 victims, but others only speak of about 100 victims, as many Basel residents fled the city after the afternoon earthquake.

If a similar earthquake were to happen in Basel today, several thousand deaths, tens of thousands of people with serious or minor injuries and material damage of up to 140 billion Swiss francs would be expected.

The probability of a catastrophic earthquake of magnitude 6 or more is about one percent per year. On average, such an event can be expected every 50 to 150 years. The last time an earthquake of this magnitude occurred was in 1946 near Sierre in Valais. So it could be that time again soon. According to the SED, a devastating earthquake can happen anywhere, anytime in Switzerland.