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| CRUSTAL DEFORMATION IN CHINA ASSOCIATED WITH THE SEISMIC CYCLE OF MAJOR FAULTS OR RELATED TO LAKES LOADING ON THE LITHOSPHERE : MEASUREMENT BY SAR INTERFEROMETRY |
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Measuring and modeling surface deformation related to earthquake cycles and loading of the lithosphere are one of the major challenges in solid-earth geophysics today. Great advances have been made in the past years with the advent of space-based geodesy (GPS, InSAR), allowing in particular the detection of processes (post-seismic deformation, silent earthquakes, deep ductile flow) which do not release seismic energy but contribute a significant fraction of the long-term deformation of the lithosphere and play an important role in the distribution of stress within the seismogenic part of the crust, therefore influencing the generation of earthquakes.To characterize the temporal evolution of stress on a fault during the seismic cycle, and strain localization within and below the seismogenic zone, one must constrain both the deformation through time near the fault and the rheology of the layered lithosphere, from its deep ductile part to its upper brittle section. Indeed, the stress applied to the fault at various depths varies during the seismic cycle, concentrating either in the shallow part of the lithosphere or at greater depth. This results in changes in the elastic strain rate distribution and therefore in surface velocities at short and intermediate distances from the fault, which could potentially be observed by densely spaced geodetic measurements.
In this project, we propose to use Envisat image mode and wide-swath InSAR to (A) study the seismic cycle of major continental faults and (B) place constraints on the ductile behavior of the lower crust and upper mantle from their mechanical response to water level changes of high elevation lakes in Tibet. In part (A) of the project, we will routinely survey major faults across the Tibetan Plateau and analyze along-strike variations of their behavior. Some sections of these faults are quiescent since several 100 yrs (along the Haiyuan and Altyn Tagh faults for example) and could be the loci of large earthquakes in the next decades. We will measure the interseismic strain accumulation across them and compare with observations from continuous or campaign GPS and seismological stations when available. The Altyn Tagh Fault runs along a zone of major relief at the northern edge of Tibet, which makes InSAR range change measurements prone to large errors associated with the radar phase propagation delays. It will be a test site to validate atmospheric corrections of InSAR observations, a critical point to achieve millimeter accuracy in fault rate estimates. Along faults segments which have recently ruptured in major earthquakes (Kunlun fault for example), we will investigate post-seismic processes. We will also undertake coseismic and post-seismic studies of any large earthquake that may occur in China during the lifetime of the project. In part (B) of the project, we will study the visco-elastic response of the lithosphere to loading/unloading of lakes in the Qinghai province and central Tibet, to characterize the lithosphere transient and long-term viscosity structure. InSAR and elevation data along paleo-shorelines will be combined with age estimates of surface samples to quantify the rebound through time.
Where the targeted deformation is of small amplitude (in the case of interseismic deformation, a few mm/yr) with large spatial scale patterns, a few millimeters of accuracy is required for InSAR measurements. This will be achieved using repeated radar data acquisitions in order to build time series of the deformation and cover large sections of the study areas. At the end of the project, which follows a Dragon 1 project and widens its scope, we expect to provide state of the art surface velocity maps at the local faults scale and at a regional scale to help quantifying the lithosphere response to stress, to improve seismic hazard assessment and test large scale deformation models.
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