The slow breakup of a continent
Beneath the city of Strasbourg and the plains of the Rhine River, a 350-kilometer-long and 50-kilometer-wide chasm is actively, if slowly, splitting the European continent. This feature is the Upper Rhine Graben, the prominent segment of the European Cenozoic Rift System. Its formation began in the Eocene epoch, around 40 million years ago, as a direct consequence of the immense compressive forces that were building the Alps to the south. As Africa pushed into Europe, the crust to the north stretched and thinned, causing a massive block of land to drop down.
This sunken block, or "graben," created the wide, flat valley floor where the Rhine now flows. On either side, the edges of the rift were pushed upwards, creating the Vosges mountains in France and the Black Forest in Germany. These parallel ranges are the rift's shoulders, uplifted by more than 2,500 meters before erosion shaped them into their current forms. The total horizontal extension, or the amount the crust has pulled apart, is estimated to be between 8 and 8.5 kilometers. The process continues today, with the rift widening at a rate of less than 1 millimeter per year, accompanied by a slow, ongoing subsidence of the valley floor. The crust beneath the graben is significantly thinner than in adjacent areas, with the Earth's mantle located at a depth of about 24-26 kilometers, compared to 30 kilometers or more elsewhere.
A shaky foundation
The constant, slow stretching of the crust builds stress along faults deep underground. This stress is regularly released in the form of earthquakes, making the Upper Rhine Graben one of the most seismically active regions in intraplate Europe. While most tremors are minor, the potential for powerful earthquakes is a feature of life in the region.
The most destructive earthquake in central Europe's recorded history occurred here on October 18, 1356. The Basel earthquake had an estimated moment magnitude between 6.5 and 7.1 and its effects were catastrophic. It destroyed the city of Basel and flattened buildings in a radius extending up to 30 kilometers. The shaking was felt as far away as Paris and Prague. Historical records document a series of powerful foreshocks and aftershocks that continued for months. Today, a dense network of seismometers across the region monitors the near-constant small-scale seismic activity, showing geologists that the forces that caused the 1356 event are still at work. The thick layers of sediment filling the graben, up to 4 kilometers deep in some places, can amplify the shaking from these quakes.