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Accueil du site > Les thèmes scientifiques > Couplages rhéologiques > Lac Mead, Nevada

Contacts : Marie-Pierre Doin, Cécile Lasserre, Pierre Briole & Olivier Cavalié

Lac Mead, Nevada

Interférométrie radar

In this study, ground motion around the lake Mead (Nevada, USA) was measured using InSAR techniques. This artificial lake has been filled with water in 1935. An earlier study, based on leveling measurements, has shown that the load associated with lake impoundment induced a subsidence of 17 centimeters. This relaxation process has been argued as analogous to the postglacial rebound, but at a smaller spatial scale and with a much lower viscous relaxation scale.

GIF - 84.3 ko
Exemples d’interférogrammes
Comparison between the stacks (a) of interferograms and the predicted motion using a viscoelastic (b) or elastic (c) model. The modelled ground motions in (b) and (c) represent a stack of modelled displacements between the same dates as in (a). (d) corresponds to the residue, (b)-(a), showing the poroelastic response of few areas close to the lake, and the motion due to the aquifer system deformation near Las Vegas.

To quantify the deformation and thus constrain the crust and mantle rheological parameters in the lake area, we analyzed multiple interferograms (241) based on 43 ERS images acquired between 1992 and 2001. With baselines smaller than 300 m, all interferograms have a very good coherence due to the desert region. Most of interferograms show strong atmospheric delays that are partly due to the variation of water vapor vertical stratification between two satellite passes. This tropospheric delay is computed for each interferogram and then inverted for each date of SAR images before interferogram correction. These corrections are validated using data from a global atmospheric model (ERA40). Corrected interferograms are then inverted to solve for the time series of ground motion in the lake Mead area. The linear inversion treats each pixel independently from its neighbors and uses the data redundancy to reduce errors such as local decorrelations. Additional constraints such as temporal smoothing allow to reduce the local atmospheric delays. We obtain a time series of the deformation in the lake Mead area with a millimetric accuracy. The deformation is non linear in time and spreads over a large spatial scale. In particular, we observe a subsidence of up to 1.6 cm between 1995 and 1998 due to a 10 meters water level increase, followed by an uplift due to the drop of the water level after 2000. The deformation model allows to constrain the rheology in the lake Mead area. The model takes into account the loading history of the lake since 1935. We show that a simple elastic response with parameters constrained by seismic wave propagation does not explain the amplitude of the observed motion. To fit the data, a low viscosity (around 1018 Pa.s) in the lithospheric mantle below a 30 km thick elastic layer is required.

JPG - 787.2 ko
Image satellite, Google Earth
GIF - 71.7 ko
Sismicité du Lac Mead
JPG - 141.9 ko
Lac Mead, Névada
Affleurements le long des berges du lac