Earthquakes are caused by a sudden release of stress along faults in the crust. Due to continuous motion of tectonic plates, stress builds in the rock on both sides of a fault. When the stress exceeds the strength of the rock, it is released in a sudden, jerky movement. The released seismic energy propagates as waves through the earth and along its surface, and these waves cause the shaking that we feel.
The levels of danger refer to the intensity of an earthquake in a specific warning area. The intensity is a measure of the effects of an earthquake. Using a 12-point scale, it describes the consequences of an earthquake for people and buildings in a particular place. An earthquake has one value for magnitude (energy released) and usually several values for intensity. The greatest intensity is usually recorded close to where the earthquake takes place (epicenter) and decreases with distance from the epicenter.
For each warning area, the Swiss Seismological Service estimates the intensity in the center of each region and assigns a particular danger level based on this (see table below).