, Volume 68, Issue 4, pp 567-577
Date: 28 Jul 2009

Néotectonique et grands mouvements de versant. Le cas de Séchilienne (Isère, France)

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Résumé

Les mouvements de versant observés à Séchilienne, dont les vitesses sont globalement dépendantes de la pluviométrie, sont essentiellement dus à la décompression du massif rocheux responsable de l’ouverture de discontinuités héritées d’une histoire tectonique régionale complexe. Il arrive cependant que des accélérations se produisent indépendamment du taux de précipitations mesuré. L’analyse détaillée d’une de ces anomalies de comportement, enregistrée en février 1999, a permis de soupçonner l’implication du séisme de Laffrey du 11 janvier 1999. La sollicitation mécanique provoquée par ce séisme, dont l’épicentre n’était distant du massif instable que de quatre kilomètres, pourrait également être à l’origine du changement pérenne du régime du mouvement à partir de cette date. Les indices de néotectonique dans la vallée de la Romanche, tels que le pull-apart de Séchilienne, à remplissage quaternaire, et les éboulements récurrents sur la faille de la Romanche, qui passe en pied de versant, avaient déjà permis d’envisager un rôle partiel de l’activité sismique dans le déclenchement ou l’accentuation des instabilités observées.

Abstract

In a homogeneous fracture massif, the release of stress related to the last glacial melting induces the opening of fractures inherited from the tectonic history of the rock mass. In some cases subsidence, similar to sagging from uphill facing scarps, occurs. The mechanism can be briefly described as bending of huge rock blades towards the valley, known as cambering. Nowadays the movements are due to climatic phenomena (rains, snow melting, frost, thaw, huge thermal difference) or mechanical ones such as seismic vibrations, the influence of which is generally more difficult to identify. The Séchilienne slope movement is located on the right bank of the river Romanche, about 20 km upstream of Grenoble, in the district of Isère, south east France, and is representative of this kind of phenomenon. This unstable slope has been affected by recurrent rock falls which, according to old records, were already mentioned in the eighteenth century. The rock mass is made of micaschist and psamites of early Palaeozoic age, belong to the external Belledonne alpine range which is still moving up at a rate of a few millimeters per year. As the terrain has had a long and complex geological history, characterized by five tectonic periods, the hazard geometry and slope movement directions are mainly controlled by faults associated with this succession of structural events. Of particular importance is a regional fault called the “faille de la Romanche” which is some 20 km long and extends from Séchilienne westwards to the Vercors limestone ridge and eastwards through the Taillefer massif to reach the Liassic terrain of the Oisans massif near the village of Ornon. This fault is located at the foot of the unstable slope. There are several arguments in favour of a recent movement of the Romanche fault. The first is linked to some regional geomorphological features such as the lozenger shape of the Romanche valley in the Séchilienne area. This feature resembles a pull-apart basin, bordered on the north and south by the Romanche fault and filled with more than 200 m of Quaternary alluvial sediments. In addition, since 1960 eleven earthquakes have affected the villages of Vizille and Séchilienne. One of these, the Laffrey earthquake (11 January 1999), had a magnitude of 3.5 with an epicenter located only 4 km south of the Séchilienne site. The monitoring devices installed on the slope allow an accurate analysis of the widespread displacements which occur in the area. The analysis has led to the establishment of the influence of the local seismicity on the observed movements. Slope displacements measured since 1985 have shown important speed variations between winter and summer and a progressive long term acceleration. A detailed analysis of the daily measurements shows an increase in the displacements from 29 January to 11 February 1999 without any associated rainfall. Moreover, a systematic examination of the successive slope accelerations related to meteoric water supply has shown that, since the 1999 Laffrey earthquake, the acceleration occurred with only a short delay of less than 24 h after the beginning of heavy rainfall compared with the delay of several days which was common before the 1999 earthquake.