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Mesures InSAR et modélisation de faibles déformations d'origine anthropique (lac Mead, USA) ou tectonique (faille de Haiyuan, Chine)

Abstract : I used radar interferometry to measure and model the Earth surface deformation of small amplitude ($<$cm). InSAR has proven to be a powerful tool to mapp surface deformation with sub-centimetric accuracy. However, interferograms correction from atmospheric delays is often required to reach such an accuracy. A large part of my work has thus been dedicated to this task. The first study area is the deformation around the Lake Mead (Nevada, USA). 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 impoundement 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 time scale. To quantify the deformation and thus constrain the crust and mantle rheological parameters in the lake area, we analyze multiple interferograms (241) based on 43 ERS images acquired between 1992 and 2001. Corrected interferograms are then inverted to solve for the time series of ground motion in the lake Mead area. 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. 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 and spatial wavelength of the observed motion. To fit the data, a low viscosity (around 10$^{18}$ Pa.s) in the mantle below a 30 km thick elastic layer is required. In a second study, I have studied the interseismic displacement across the Haiyuan fault. This fault is one of the major left-lateral faults that accommodates part of the deformation due to India-Asia collision at the north-eastern edge of the Tibetan plateau. Our objective is to better constrain the present mechanical behavior of this fault system, at the origin of two M$\sim$8 earthquakes in 1920 and 1927 and along which a seismic gap with high potential seismic hazard has been identified. We focus on the Tianzhu seismic gap segment, along which the long-term slip rate has been estimated to 12 (+/-)4 mm/yr from neotectonic studies (Lasserre et al., 1999). We analyze ERS SAR data from two tracks along descending orbits between longitudes 102.6$\degres$E and 105.3$\degres$E and latitudes 36$\degres$N and 38$\degres$N. The results between both independent tracks are very consistent. A screw dislocation model in an elastic half space indicates an average fault-parallel velocity of 6.5 $\pm$mm/yr and a very small apparent locking depth of about 1.7 km. Superficial creep or a compliant zone around the fault could also explain this low value.
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Submitted on : Thursday, January 19, 2012 - 3:46:44 PM
Last modification on : Thursday, October 29, 2020 - 3:01:51 PM
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Olivier Cavalié. Mesures InSAR et modélisation de faibles déformations d'origine anthropique (lac Mead, USA) ou tectonique (faille de Haiyuan, Chine). Géophysique [physics.geo-ph]. Université Paris Sud - Paris XI, 2007. Français. ⟨tel-00661442⟩

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