Near Grenoble: the Belledonne Bordering Fault

For more than twenty years, the Sismalp network has been monitoring seismicity in southeastern France, from Lake Geneva to southern Corsica and from the Massif Central to northern Italy. An average of two earthquakes are detected and located each day by the forty or so seismological stations managed by Sismalp; however, the number of earthquakes felt by the population each year does not exceed thirty.

The Grenoble region, without being particularly exposed, is close to a seismic "alignment" that has been revealed by the locations of earthquakes that occur regularly there. This alignment is called the subalpine seismic arc; its southern part, from Monestier-de-Clermont (Isère) to Albertville (Savoie), has been named "Faille bordière de Belledonne". This fault passes near Uriage-les-Bains, only a dozen kilometres from the city centre of Grenoble. It is not identifiable on the surface because it slides very slowly, most of the time in small earthquakes of magnitude between 0 and 3.5 (for the last twenty years); another reason is that the active zone is at depths of about 5 km.

The largest earthquake that has occurred on the Belledonne Bordering Fault since 1989 (date of installation of the Sismalp network) is the Laffrey earthquake in 1999 (magnitude 3.5) [1]. This earthquake caused very slight damage (mainly plaster cracks) in Saint-Georges-de-Commiers (Isère), but it was also felt throughout the entire Grenoble urban area.

A statistical analysis of the Belledonne Boreal Fault shows that earthquakes of magnitude M=3.5 (such as Laffrey) occur about every 15 years, M=4 every 40 years, M=5 every 300 years and M=6 every 2000 years. But this analysis concerns the entire fault, which is more than 100 km long. Thus, an earthquake of magnitude 5.5, which occurs statistically once every 1000 years, has a much lower probability of occurring over the twenty kilometres of the fault located in the immediate vicinity of Grenoble (order of magnitude: every 5000 years).

Unfortunately, the Belledonne Bordering Fault, even if it is now well known, is not the only fault that can play catastrophically in the Grenoble region. Not much is known about the activity of the "Voreppe Fault", which may extend under the town itself. Above all, we are unaware of the potentially dangerous faults that could exist in the Vercors and Chartreuse massifs. It is perhaps in these two massifs, currently considered as practically "asismic" (without earthquakes), that the next major destructive earthquake could one day occur. The example of Corrençon-en-Vercors, a village affected by a 5.3 magnitude earthquake in 1962 without it being clear what fault line was then in play, should encourage seismologists to be very cautious and modest in their analysis of complex and exceptional phenomena.