Relationships between ground movements, faults and earthquakes in the Grenoble region

Several active ground movements around Grenoble are located near active faults. (Click on the image to enlarge it) :

**Landslides are frequent in the Trièves clays, near the southern boundary of the Belledonne fault.

This fault could produce an earthquake of magnitude 6 with an estimated average recurrence rate of 10,000 years.
In 1963, an earthquake of magnitude close to 5 occurred in Monteynard, probably due to the filling of the lake, very close to the ground movements of Avignonet and Harmalière. A year before an earthquake of an estimated magnitude of 5.3 had occurred in Corrençon in the Vercors, about 12 km northwest of Avignonet. These earthquakes did not trigger gravitational instability.

**The Séchilienne ground movement is also very close to the Belledonne fault.

Since the installation of the seismological network in May 2007, we have detected
hundreds of small earthquakes located on this fault less than 10 km from Séchilienne, of magnitude ≤ 2.5. In 1999, a 3.5 magnitude earthquake occurred in Laffrey, only 7 km from the Séchilienne ground movement. However, since this movement has been instrumented, we have not detected any significant acceleration of the movement triggered by an earthquake.
Since the installation of the seismological network in 2007, we have also not observed an increase in the number of landslides or micro-seismic events (internal to the slide) after an earthquake. On the other hand, an earthquake of magnitude > 5 near Séchilienne could probably trigger landslides, the size of which could reach several million cubic metres, causing a natural dam in the Romansh.

– Learn more about the Belledonne fault

**Observe and monitor these risk areas.

The objective of the Observatoire Multi-disciplinaire des Instabilités de Versant (OMIV) is to acquire data and make them accessible in order to study the dynamics of gravity movements. However, OMIV does not aim to monitor field movements in real time. As a result, little data is transmitted in real time.

Monitoring of the Séchilienne ground movement and alerting are managed by CEREMA Centre-Est based on continuous displacement measurements transmitted in real time.

Seismological data from OMIV Séchilienne are collected in the field about every two months. They are then analyzed more or less automatically and the detected signals are classified into several categories (earthquake, internal microseismic slip and landslide). The seismological database is updated approximately every 2 months and is accessible on the OMIV website.

In the event of a major earthquake near Séchilienne, or a major crisis (acceleration of movement, intense rainfall, increase in micro-seismic activity, etc.), however, data from one of the 3 stations can be recovered in real time by 3G modem.

The interest of these data would be, on the one hand, to be able to analyse the signals of the earthquake and its aftershocks. If the earthquake were to occur on the southern part of the Belledonne fault, this station would be the closest to the earthquake. In a few hours, we could obtain a list of the earthquakes detected with an estimate of their magnitude. On the other hand, these seismological data, collected and analyzed in real time, would make it possible to study the response of the ground movement to the earthquake. A list of triggered landslides and micro-seismic events and an estimate of their size could be produced.

References:

Grasso, J.-R., F. Guyoton, J. Fréchet, and J.-F. Gamond (1992). Triggered earthquakes as stress gauge: implication for the upper crust behaviour in the Grenoble area. Pageoph 139, 579–605.

Helmstetter, A. and S. Garambois, Seismic monitoring of Séchilienne Rockslide (French Alps): analysis of seismic signals and their correlation with rainfalls, J. Geophys. Res., 115, F03016, doi:10.1029/2009JF001532 (2010)

Thouvenot, F., J. Fréchet, L. Jenatton, and J.-F. Gamond (2003). The Belledonne Border Fault: identification of an active seismic strike-slip fault in the western Alps, Geophys. J. Int. 155, 174–92