On Saturday, an earthquake with an epicenter south of Mulhouse shocked large parts of Switzerland. Seismic activity is not at all unusual in this country. There have been much stronger tremors in the past. Answers to common questions.
The painter Ernst Stückelberg captured the aftermath of the Basel earthquake of 1356 in a painting (1886, oil on canvas).
How often does the earth tremble in Switzerland?
The Swiss Seismological Service (SED) at ETH Zurich records earthquakes in this country. According to the SED, earthquakes like the one on September 10, 2022 occur in the affected region every ten to twenty years. The 200 sensors of the measuring network record an average of three to four earthquakes in Switzerland or in neighboring countries every day. That’s about 1,000 to 1,500 earthquakes a year. But people only notice 10 to 20 of them. All others are too weak and go unnoticed.
How strong can tremors get in this country?
The strongest tremors in historical times occurred in 1356 near Basel. The strength of this quake can only be estimated based on the effects. A magnitude of 6.6 is assumed – the magnitude provides information about the energy that is released during the tremors. At that time several houses collapsed; there were also fires. However, it is quite possible that earthquakes of even greater magnitude will occur in Switzerland. This is indicated, for example, by undersea landslides whose deposits have been preserved. Scientists believe that earthquakes with a magnitude of 7 are possible in Switzerland. However, they should only occur every few thousand years.
Why does the information on the strength of earthquakes in Switzerland differ depending on the seismic monitoring service?
The earthquake near Basel on September 10, 2022 according to the SED a magnitude of 4.7. The US Geological Survey, on the other hand, came up a magnitude of 4.3. Such differences occur for a variety of reasons. In principle, researchers cannot measure the strength of an earthquake directly; they have to estimate it using computer models from the observed seismic waves. In addition, there are different types of magnitudes: In this specific case, the SED has given a magnitude type based on very precise seismic measurements in the vicinity of the earthquake; the US Geological Survey calculated a magnitude for earthquakes further away from their own measuring network – hence the difference.
Which regions in Switzerland are particularly at risk?
The risk is highest in Valais. The region around Basel follows in second place. But Graubünden, the Rhine Valley in the canton of St. Gallen and central Switzerland are also among the endangered regions, and there is no area without any risk of earthquakes. However, the threat situation in Switzerland is not comparable to Italy or Greece.
Why are there earthquakes in Switzerland at all?
The main reason is the collision of two tectonic plates – European and African. This is also responsible for the formation of the Alps. The movement of the earth’s plates not only deforms the earth’s crust, but also creates additional stresses that are discharged at irregular intervals in the form of earthquakes. Human activities can also trigger tremors, for example geothermal projects in Switzerland.
Are houses in Switzerland earthquake-proof?
Certainly not all buildings would be able to withstand very strong vibrations. Since Switzerland rarely experiences strong earthquakes, earthquake-proof construction has never been such an important issue as it is in California or Japan, for example. According to the Federal Office for the Environment there is currently no precise overview of the building stock. What we do know, however, is that the majority of buildings and facilities in Switzerland are either not built with sufficient stability or their robustness is not yet known. In any case, in the past, seismic safety was not or not sufficiently taken into account in building standards. The supporting structure of the building often had to be reinforced afterwards.
How big can the damage from earthquakes be here?
In an emergency, such as an earthquake with a magnitude of 6.5, many buildings in this country would begin to crumble. The risk of damage is particularly high in the two cities of Basel and Geneva. This is partly due to the value of the potentially affected buildings and partly to the unfavorable subsoil. According to the Swiss Insurance Association, the economic damage could amount to more than 100 billion francs in the worst case.
Do earthquakes pose a threat to the planned nuclear repository?
Earthquakes are mainly noticeable on the earth’s surface. There, the tremors can cause loose rock to vibrate. This happens much less at depth. The potential earthquake hazard to a repository was taken into account by Nagra when deciding on the Nördlich Lägern site. This is far removed from tectonic faults. In addition, according to Nagra, the rock there tends to crack the least compared to the other sites.