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Gravity instability - Disasters in the Granier
***Summary :
– April-may 2016
– The landslide of January 9, 2016
– The 1248 disaster illustrated in comic strip by Jean-Yves Duhoo (magazine Spirou - 02/2013)
Since April 28, several landslides have occurred on the eastern slope of Mont Granier (report on France 3 and article in the newspaper Le Dauphiné Libéré). After the landslide at the beginning of January on the North West pillar (see below on the page), with a volume of approximately 55,000 m3 (article in Le Dauphiné Libéré), the events of April and May 2016 concern this time the North East pillar.
One of these events occurred while an ISTerre team was on site for a photo survey for a 3D reconstruction. This made it possible to film almost the entire rockfall, from the summit drop to the propagation of the blocks at the foot of the slope. Video below to watch with sound to feel the impression of power of this phenomenon.
film_granier_ISterre from David Amitrano on Vimeo.
The largest landslide in this sequence took place on Saturday, May 7 at 8:32 am without causing any damage. Its volume, estimated at 50000 m3, is comparable to that of the Northwest pillar in January 2016 (article in The Dauphiné Libéré). It was preceded by another smaller landslide at 8:30 am. The document shows photos taken before and after the landslides of May 3 and 7.
These landslides were also recorded by seismological stations in the Sismalp network up to more than 100 km from the Granier. The figure opposite (PDF) shows the seismic signals of each rockfall recorded by the stations of theCliff Observatory of Saint-Eynard, about 27 km from the Granier (ground movement in the 3 directions for the 3 stations FOR, MOL and RES, filtered high pass at 1 Hz). Since the beginning of 2016, 8 landslides in the Granier have been detected :
- 9 january at 3:57, of magnitude 2.2, with a volume estimated by photogrametry at 55 000 m3,
- 28 april at 2:54, of magnitude 1.0
- 28 april at 3:03, of magnitude 1.4
- 29 april at 21:40, of magnitude 1.5,
- 30 april at 07:47, of magnitude 1.5,
- 3 may at 09:11, of magnitude 1.0,
- 7 may at 08:30, of magnitude 1.5,
- 7 may at 08:32, of magnitude 2.1.
The two events of April 28 were not observed in the field, but the similarity of the signals with the other landslides and the location of the source near the Granier suggest that this event is also a landslide. The other signals correspond to field observations. The event of May 3 is the one that was filmed, the signal is very weak, because recorded more than 25 km away, and barely comes out of the noise. The magnitude of the seismic signal allows a first estimate of the collapsed volume. A magnitude difference of 1 corresponds to a factor of 30 on the volume. If the January 9 landslide is used as a reference (magnitude 2.2, volume 55000 m3), the May 7 landslide is estimated at 42000 m3, and the smallest detected landslide (May 3, magnitude 1) would have a volume of about 1000 m3. The seismic signals all show a low impulse start, followed about 5 seconds later by waves of higher amplitude and lower frequency. The first waves probably correspond to the detachment phase of the blocks, and the largest amplitudes to the impact of the blocks at the bottom of the cliff, which would correspond to a free fall height of 150 m.
Two seismological stations have been installed to monitor the activity of the Granier. One station in Chapareillan, another on the plateau, with a sensor on an unstable scale and another on the plateau. Below is the video of the installation.
Installation_7_juin_720p_2_1 from David Amitrano on Vimeo.
It’s in the media :
7 may 2016 - Article in the newspaper Le Dauphiné Libéré : 50000 m3 de roches sont tombés au mont Granier hier matin
4 may 2016 - Report on TV France 3 : L’éboulement du Granier filmé par un chercheur du laboratoire ISTerre de Grenoble
2 may 2016 - Report on TV France 3 : Le Mont Granier se comporte comme un volcan
30 april 2016 - Article in the newspaper Le Dauphiné Libéré : Nouveaux éboulements au Granier
More information : Focus on the Mont Granier landslides by the CNRS Alps delegation
Contacts : Agnès HELMSTETTER et David AMITRANO
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– The landslide of January 9, 2016
At the end of the night, a spectacular rockslide occurred near the West Pillar of Mont Granier (article in Le Dauphiné Libéré. This major event, both in terms of its magnitude and rarity, was recorded by local and regional seismological networks with a magnitude equivalent to M=2.2.
In order to know precisely the geometry of the collapsed volume, Gaëlle Le Roy, David Amitrano and Agnès Helmstetter reconstructed the relief before and after the rockslide. A call for contributions made it possible to find photos of the relief before the event. Recent advances in multi-view photogrammetry have made it possible to use these images of unequal quality to build a 3D digital model without having made specific measurements before. The 3D model after the rockslide was much easier to obtain thanks to the profusion of available shots and the aerial shots taken by Julien Carcaillet and Arnaud Pêcher. Thus the eroded volume is estimated at 120,000 m3 (+/- 5%) i. e. 1.5 times the volume of concrete of the Millau viaduct. This important value explains the amplitude of the seismic signal emitted by this rockfall and makes it one of the most important in the Alps recently recorded.
Thank you to all contributors to this first collaborative data collection experience at ISTerre. This has allowed us to go back in time to better describe the rockfall, in order to better understand this phenomenon.
Landslides in the limestone cliffs surrounding the Grenoble region (Chartreuse and Vercors)
From the analysis of the RTM38 archives, it is reasonable to estimate that landslides with volumes greater than 100,000 m3 (or 0.1 hm3) occurring over the last two centuries have been identified (RTM, 1996 ; Hantz et al., 2003). There are only 4 of them :
- Saint Bernard du Touvet, Grande Roche (june 1826) : 20 hm2 (ou 20 ha) of covered wood, all the tributaries of the Bresson blocked.
- Saint Ismier (17 to 22 may 1867) : obstruction of the torrent of the Ecorchiers on 700mx100mx10-40m, 10 dams covered or destroyed.
- Saint Paul de Varces, Charbonniers (16/04/1889) : estimated at 100,000 m3 (replica of 20,000 m3 in 2007, whose scar is still clearly visible).
- Saint Paul de Varces, Echarina (28/04/1988) : tearing niche still clearly visible (visually and on the IGN map at 1/25000).
We can conclude that such a volume is of centennial occurrence on the scale of a massif such as the Chartreuse or the Vercors. To find larger volumes (in Chartreuse-Vercors), it is necessary to go back several centuries :
- At the end of the 17th century, a landslide destroyed the old village of Saint Paul de Varces (hamlet "Les Ruines"). A scree cone extends over 150 hm2. The volume would therefore be several hm3.
- In 1248, there was the so-called Granier landslide with a volume of 500 hm3 (5,108 m3), consisting mainly of marl materials, but also of about 25 hm3 of limestone from the north face of Mont Granier (Nicoud et al., 1999), 250 times more than the January 2016 rockfall. This landslide is the largest reported in the Alpine arc for the last 2 millennia (larger ones have occurred since the retreat of the Würmian glaciers).
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"The disaster in the Granier is known from chronicles of the time, including that of a Benedictine monk, who described it as divine revenge. The mud and stone flow covered the valley with a layer 10 to 40 metres thick, over a length of 9 kilometres."
In the February 20 issue of Spirou (n°3906), Jean-Yves Duhoo takes an interest in gravitational instabilities and questions ISTerre researchers in "the lab", a section that presents scientific questions in the form of comic strips in a laboratory.
At ISTerre, the draftsman sketched some of the laboratory’s researchers with dexterity... while interesting Spirou readers to our ongoing work on understanding field movements...
See the comic strip (PDF) - in french :
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