An earthquake is a vibration of the solid earth triggered by tectonic processes in the earth’s interior.
The risk of earthquakes in Switzerland is classified as moderate to average.
The risk is higher in Canton Valais and the Basel region and slightly higher in Central Switzerland, the Engadin and the St. Gallen Rhine valley. It has been possible since the beginning of the 20th century to measure the strength of earthquakes using special sensors known as seismographs.
The strength of an earthquake is expressed as magnitude. Earthquakes that move furniture and cause slight damage to buildings, for example, measure around magnitude 5 on the Richter scale. These are likely to occur once in ten years in Switzerland. Earthquakes that cause serious damage to buildings in the affected area or even cause them to collapse measure around magnitude 6. Such an earthquake is expected once in 100 years.
Knowledge of the earthquake situation in a region is important for assessing the long-term safety of a deep geological repository. Analysing earthquakes allows active fault zones to be located; these have to be avoided when selecting a site for a repository.
Investigations worldwide in shafts, tunnels and caverns in earthquake zones show that damage to underground structures is rare and decreases rapidly with increasing depth. Geological repositories located at a depth of several hundred metres away from fault zones are therefore not at risk from strong earthquakes.
The fact that earthquakes have different impacts at the surface and underground can be seen from the example of the quake in the Chinese city of Tang-Shan on 28th July 1976 (magnitude 7.8 on the Richter scale). Buildings at the surface were completely destroyed, but the damage to an underground structure in the same area was small.
|Destruction of buildings at the surface after the Tang-Shan earthquake of 1976. Image: USGS Earthquake Information Bulletin 305.|
|Underground structure with only slight damage after the Tang-Shan earthquake. Image: China Academy of Building Research (1986): “The mammoth Tangshan earthquake of 1976”.|