Where Earthquakes Occur in Switzerland
Since 1975, the SED has run a network of seismic stations which record and pinpoint the location of between 500 and 600 earthquakes a year in Switzerland and neighbouring countries, on average. However, very few of these quakes are strong enough to be felt by the local population. The vast majority of earthquakes recorded in Switzerland occur in the Swiss Alps, especially in the cantons of Valais and Grisons, but the northern Alpine foothills, Central Switzerland, the Jura mountains and the Basel region are also particularly seismically active.
The depths at which earthquakes occur in the Alps differ markedly from the values recorded in Nothern Switzerland or the Alpine foothills. In the northern Alpine foothills, earthquakes occur at all levels of the Earth's crust, down to the so-called Moho, the boundary between the Earth’s crust and its mantle at a depth of between 30 and 50 km. By contrast, seismic activity beneath the Alps is limited to the upper part of the Earth's crust, with earthquakes only occurring at depths of roughly 15 to 20 km.
Links Between the Alps and Earthquakes, Or Why the Ground Shakes in Switzerland
The Alps are a product of the complex geological history of two large lithospheric plates: Europe and Africa. The lithosphere is the solid outer section of the Earth's crust. It is approximately 100 km thick and consists of two layers, the Earth's crust on top of the mantle lithosphere. Our planet's lithosphere is split into numerous tectonic plates of various sizes, which move in different directions on the viscous mantle and, in so doing, rub against each other or collide. These processes are described by the theory of plate tectonics.
As a first approximation, the earthquakes observed in Switzerland can be said to result from collisions between the European and African lithospheric plates, reflecting the underlying mechanics of this process. Seismic waves penetrate underground, affording us a view of the plate parts deep beneath the Earth's surface, including underneath the Alps. Recent years have seen the discovery of a key detail: following the complete subduction of the oceanic lithosphere and the subsequent collision of both continents, today a residual part (so-called 'slab') of the original mantle lithosphere remains stuck to the European plate. In the northern Alpine foothills this slab bends the lithosphere downwards, thus indirectly causing seismicity spread over a wide area of Switzerland's central plateau. Since the motions described by the theory of plate tectonics take place on a geological timescale, spanning eras, it can be assumed that the present seismicity in the Alpine region is likely to continue for millions of years yet.