Why do earthquakes follow one another in the south of Alsace and the Jura?

One again ! A few days ago, the sector of Mulhouse, in the south of Alsace, recorded a new earthquake. Of magnitude 3.1, this small seismic event is the latest in a series that began at Christmas 2021. Although the region is known to present a moderate seismic hazard, it is however not common to record such a sequence of earthquakes, several of which exceeded magnitude 4 and were, each time, rather well felt by the population. Apart from a few small scares, no real damage is to be reported, however.

On closer inspection, these are actually two areas that are currently being shaken by this seismic crisis. One is centered on the Basel-Mulhouse sector, at the southern end of the Rhine ditch, the other is located a little further south, on the front of the Jura mountains, at the Franco-Swiss border between Montbéliard and Porrentruy.

A region shaken regularly for 18 months

It all started on December 24, 2021 with a shallow earthquake of magnitude 4.1 whose epicenter was located a few kilometers from the Swiss town of Porrentruy. Perceived as far as the Vosges, this earthquake would have been created by the rupture of a reverse fault, as explained in the report produced by the BCSF (Bureau central sismologique français).

In September 2022, it was the turn of the city of Mulhouse to suffer a magnitude 4.8 earthquake, followed by a magnitude 3.2 aftershock the next day. An event of moderate magnitude but which turns out to be the strongest for 20 years in the region. The mechanisms at the focus indicate that it is here a strike-slip fault which would be at the origin. The BCSF report also points out that it is a fault located only about twenty kilometers from the Basel-Reinach fault, which is supposed to be the cause of the historic earthquake of 1356. Of record magnitude (for the region) of 6.5 (or more), this earthquake would have destroyed the city of Basel at the time and caused the death of 1,000 to 2,000 people.

Rebelote in March 2023, where an earthquake of magnitude 4.3, which was followed by a little felt aftershock of magnitude 3.1, again shook the Franco-Swiss border and in particular the Territory of Belfort and the country of Montbéliard in Franche-Comte.

We put the cover back on May 29 with a new earthquake of magnitude 4 in the same sector.

Finally, the 1^er June, it is again Mulhouse which is affected by an earthquake of magnitude 3.9.

The thrust of the Alps in question

If this seismic sequence which lasts over time may surprise, it is however not abnormal for the region which is put “under pressure” by the continuous thrust of the Alps, a mountain range resulting from the collision between two tectonic plates. Despite their proximity, the two zones affected by the recent earthquakes nevertheless undergo slightly different stresses, as evidenced by the different focal mechanisms observed on the activated faults (inverse in the case of the Jura, and stalling in the case of the Rhine ditch).

The Jura massif is indeed a thrust front which results directly from the Alpine thrust. It is, in a way, the outermost edge of the Alpine system. In comparison, the Rhine ditch in which the city of Mulhouse is located results from a thinning (and therefore a collapse) of the crust which is nothing other than a “side effect” linked to the formation of the Alps. , a phenomenon that produces an asthenospheric flow in the peripheral regions.

Anyway, these earthquakes indicate, in case we forget, that the region is still very active, with several faults that break regularly in order to release the accumulated tectonic stresses. The events are closely monitored by the Strasbourg national seismic monitoring network (RéNaSS), which also carries out information and prevention operations with the population.