On average, the Swiss Seismological Service (SED) registers two earthquakes per day in Switzerland and its neighbouring countries, making a total of between 500 and 800 earthquakes a year. Approximately 10 to 15 of these events are strong enough to be felt by the population (i.e. have a magnitude of approximately 2.5 or above).
Earthquakes with a magnitude of at least 5 occur every 8 to 15 years. Larger earthquakes, with a magnitude of 6 or more, occur only every 50 to 150 years. Though rare, they can occur anywhere and at any time in Switzerland. The strongest historically documented earthquake in Switzerland, with a magnitude of approximately 6.6, occurred in Basel in 1356. Such events should be expected to happen roughly once every 1,500 years.
At present, earthquakes cannot be predicted, though it is possible to calculate the probability with which a certain ground acceleration will be exceeded within a given period at any location in Switzerland. Consequently, the country can be divided into lower- and higher-hazard regions. Valais is the region subject to the highest hazard, followed by Basel, then Grisons, the St. Gallen Rhine Valley and Central Switzerland. All regions of Switzerland are exposed to some degree of seismic hazard.