The Lake Serre-Ponçon (Alps, France), the main reservoir in France, is 15 km long and 7 km wide. Its water depth of about 120 m varies by +/- 30 m. These seasonal lake loading / unloading should be responsible for millimeter to centimeter vertical seasonal displacements (subsidence and uplift) distributed over ten's of kilometres around the lake. Because of these characteristics, the Lake Serre- Ponçon study will require challenging methodological developments in InSAR to extract reliable information in this vegetated and mountainous area. Especially, the non-linearity of the displacement with time is an interesting issue to be addressed. Fortunately, the data set is particularly large (~100 ERS scenes, ~30 Envisat scenes on a single track/frame), and will allow to follow the seasonal fluctuations in ground motion associated with lake level changes.

Implementation of time series analyses is required for this study and will be based on WP1, WP3 and WP4 developments. To deal with atmospheric perturbations, approaches developed in WP6 will be used. Finally, within WP7, by optimizing the geophysical parameters estimation, we should be able to constrain the effective elastic moduli of the upper and mid crust, detect possible phase delays (for a poroelastic behavior of the upper crust), and possibly plastic creep.

These parameters are information about the knowledge of the lithosphere rheology, which is fundamental to the understanding of both cyclic, earthquake-related deformation, and long-term tectonic deformation. Studying crustal deformation due to reservoir impoundment offers an interesting and very useful means of probing lithospheric rheology and the time scale associated with stress or slip transfer (Kaufmann and Amelung, 2000 ). This study will benefit from the ENS experience acquired on a similar study on the lake Mead, in US (Cavalié et al., 2007) where the exploitation of ERS archive into time series shows that we can recover subcentimeter subsidence fluctuations associated with lake level changes. That implies a non purely elastic lithosphere behavior on a year time-scale. As for lake Mead, promising InSAR results on Serre Ponçon may help promote the installation of GPS or tiltmeter to obtain ground truth data.