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Evaluation of the quaternary seismic activity in Southern Jura faults

Camille De La Taille has defended her thesis on 10th December 2015 at University of Savoie in Chambéry (France).​

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/DEI/SARG/BERSSIN

Authors > DE LA TAILLE Camille

Publication Date > 10/12/2015

Summary

Inside the Jura Mountains, left-lateral strike-slip faults intersect the geological structures. In its southern part, these faults, from north to south, the Vuache fault, the Culoz fault and the Col du Chat fault, affect a Mesozoic sedimentary coverage distorted of 2 to 3 km thick and may be the basement. This deformation is marked by the existence of a very shallow seismicity.

The first major aim of this thesis is to obtain new information about the kinematic of Quaternary faults of the southern Jura. The second aim is to answer to questions about the geometry and the rooting of the structures.

To answer these questions, I realized a multidisciplinary study combining subsurface geophysical (electrical resistivity tomography), lacustrine high resolution seismic, the deployment of a seismic network dedicated to the detection of very low magnitude earthquakes, analysis of reprocessed industrial seismic profiles, and field geology (the study of the structure and deformation of Quaternary deposits). We were able to establish that these faults are active in the Quaternary.

The Vuache fault is rooted in the basement, has a well-documented historical and instrumental seismicity, and affects in its southern part Quaternary sediments as shown by electrical resistivity profiles and high resolution seismic profiles illustrating the infill of Annecy Lake.

The Culoz fault presents a historical and instrumental seismicity emphasizing its roots in the basement. On land, the electrical resistivity profiles illustrate the quaternary activity, in Lake Le Bourget, the fault Culoz presents at depth a flower structure while to the surface in superficial sediments, we could highlight Riedel fractures. On land, the observation of old faulted Quaternary sediments confirms its activity.

The Col du Chat fault affects post-Würm filling of Lake Bourget, and seems to be associated with a deep seismicity.

The three studied faults, Vuache, Culoz and Col du Chat are faults showing geometric continuity between the crystalline basement and cover. These faults were considered as transfer faults in the Jura, which was itself considered as a chain formed without the involvement of the basement. So, it seems that at present the deformation of Jura involves the basement. We can then follow the hypothesis, that as shown by the analysis of ECORS profile, that the latest strain of the Jura is an out of sequence deformation involving a basement thrust bellow the High Jura chain probably reusing boundary faults of a Carboniferous basin. This basement thrust would be the main potential source of high magnitude earthquakes in the region.

Through these methods, lengths and rooting depth faults were determined through different scenarios. For the three faults studied an estimate of the possible moment magnitude was achieved, an earthquake of Mw 6.2 to 7.2 is geometrically possible on the Vuache fault, of 6.4 to 6.7 along the Culoz fault and of Mw 5.4 to 6.1 is possible along the Col du Chat fault.

The studied strike-slip faults have a sliding rate about tenth of a millimeter per year, probably not stable through the Quaternary period. In view of the in situ stresses measured in the southern Jura, the driving-force of the deformation on these faults cannot be explained only by the rotation of the Adria plate relative to Eurasia. It is therefore possible that present-day deformation is due to this rotation but also to a driving force linked to the chain itself. This driving-force could be the deflexuration of the plate following mass transfer’s deglaciation and a slab break-off inducing the current uplift of the axial part of the Alps and a horizontal stress in the external parts of the chain.

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