Earthquake

An earthquake is the sudden movement of the earth's surface from the release of energy in the earth's crust.

What causes earthquakes?

According to its origin three types of earthquakes are distinguished:

• Volcanic tremors
• Tremors caused by underground collapse
• Tectonic tremors

Three percent of all earthquakes have a volcanic origin. The reach of the seismic waves is limited. About 7 % of the earthquakes are triggered by the collapse of cavities in the geological underground.

The most destructive earthquakes (about 90 % of all tremors) are of tectonic origin. Such earthquakes occur when the earth crust (plate) moves along an active fault. Strain energy is building up along this fault line resulting in huge elastic tension. As soon as a certain threshold is exceeded the energy is released immediately. The shock waves (seismic waves) propagate from the focus, called hypo centre. The epicentre is located on the earth surface right above the focus.

What damage may occur?

In Switzerland about 200 earthquakes occur every year. However, people recognize only about 10 % of them. The last major earthquakes causing damage occurred in 1946 in the Sion area and 1855 in the Visp area. A very large event destroyed large parts of Basel City in 1356. Earthquakes cause major damage to buildings, bridges, or dams, depending on the event's intensity, the underground conditions and the type of structure.

Earthquakes can trigger secondary events like rock avalanches, rock fall, landslides or liquefaction phenomena (cf. picture). Of particular importance are secondary phenomena along the coastline: sub-aquatic tremors can trigger tsunamis. These large waves can completely destroy coastal areas.