New activity on old fault lines: French earthquake no surprise

The relatively powerful earthquake that hit eastern France last Saturday confirms the findings of the postgraduate research currently being conducted by Gideon Lopes Cardozo at the Université Louis Pasteur in Strasbourg and the Faculty of Earth and Life Sciences at the VU Amsterdam. Lopes Cardozo is investigating the causes of earthquakes in the southern part of the Rhine Graben. His research is sponsored by the European Union and has shown that the movements in the earth’s crust in the area around the Rhine occur along fault lines that have a long geological history. Until recently, it was thought that these movements actually took place more often on younger fault lines.

The earthquake, measuring 5.4 on the Richter scale, occurred at a depth of ten kilometres below the town of Saint-Dié in central Vosges. The Vosges are formed as a part of a mountain range that covered a large area of central Europe three hundered million years ago. The ancient fault lines that developed at that time became zones of weakness in the earth’s crust. If stress in the earth’s crust recurs, these are the zones most likely to move. Because of this, they have been used time and again throughout geological history for movements in the earth’s crust and are more often the focus of seismic activity.

In the course of the past millions of years, the existing subterranean fault lines north of the Alps have been re-activated by a collision between the African and European plates. The Alps are the most obvious result of this. Earthquakes in the Vosges and the adjacent Rhine Valley (stretching even as far as Roermond in the Netherlands in 1992) are also caused by the stresses resulting from this collision.