The Alps are the result of a complex geological history involving two large lithospheric plates: Europe and Africa. The lithosphere is the outermost solid shell of the Earth. It is about 100 km thick and consists of two layers, the crust on top and the mantle lithosphere below. The global lithosphere is fractured into numerous large and small plates that move in different directions on the viscous mantle, rubbing against or colliding with one another. The entity of all of these processes is called plate tectonics.
Due to their composition, we differentiate between two types of lithosphere: continental and oceanic. The oceanic lithosphere is denser than the viscous mantle below it and can therefore sink. The continental lithosphere is less dense than the mantle and therefore remains floating on top of it. When oceanic lithosphere meets continental lithosphere during the collision of two plates, the oceanic lithosphere dives into the viscous mantle below it – a process that is called subduction and that is usually associated with large earthquakes and volcanic activity. If two continental lithosphere plates collide, a mountain range develops along the plate boundary.
Not only the European and African plates participated in the formation of the Alps, but also several smaller lithospheric plates, which are known as micro plates. Particularly important is the Adriatic micro plate. Following the complete subduction of the oceanic lithosphere, and thus the closing of the original ocean (called Alpine Tethys by scientists) between Europe and Africa about 35 million years ago, the continental parts of the European and the Adriatic/African plates collided, initiating the formation of the Alps. Similarly to a floating iceberg, the weight of the mountains is supported by the buoyancy of a large crustal root. With increasing height of the mountains, erosion of rocks at the Earth’s surface plays an ever more important role. With the removal of rock material at the surface the weight of the mountains decreases and the crust beneath the Alps rises again to retain the isostatic equilibrium. The massive erosional debries of the mountains has accumulated on both sides of the Alps over the last 30 million years: as the Molasse sediments in the north and as sediments filling the basin beneath the Po plain in the south. The Alps as we know them today have thus been shaped by forces from the Earth’s interior and by erosion. Today, the Alps are rising by approximately 1 millimeter per year and are simultaneously eroding.
Generally speaking, the earthquakes that we observe in Switzerland are the results of the collision between the European and the African lithospheric plates as they reflect the underlying mechanics of these processes. Seismic waves penetrate the subsurfaces thus illuminating the deep parts of the lithosphere beneath the Alps. One important detail has been discovered in the last few years: after the complete subduction of the oceanic lithosphere and the subsequent collision of the two continents, a piece of the original mantle lithosphere is still attached to the European plate (known as the mantle slab). This slab is bending the lithosphere in the northern foothills of the Alps downward, thus indirectly causing the widely distributed seismicity on the Swiss plateau. Since plate tectonic processes take place over geological time scales, it can be assumed that the current seismicity in the region of the Alps will remain the same for millions of years to come.