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Measurement of Interseismic Strain Accumulation Across the Haiyuan Fault by Radar Interferometry

Olivier Cavalie(1), Cécile Lasserre(2), Marie-Pierre Doin(2), Gilles Peltze(3) and Jianbao Sun(4)

(1) ENS, 24 rue Lhomond, 75231 paris cedex, France
(2) ENS, 24 rue Lhomond, 75231 paris cedex <�(, France
(3) UCLA, 595 Charles Young Drive East, Los Angeles, United States
(4) Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China

Abstract

SAR interferometry has shown a great potential in detecting small ground motion. In this study, we measure the interseismic deformation across the Haiyuan fault. This fault is one of the major left-lateral faults at the north-eastern edge of the Tibetan plateau. Our aim is to better constrain the present mechanical behavior of this fault system, at the origin of two M~8 earthquakes in 1920 and 1927, and along which a seismic gap with high potential seismic hazard has been identified. We focus on this 'Tianzhu seismic gap'. We analyze ERS SAR data from two tracks along descending orbits between longitudes 102.6° and 105.3° and latitudes 36°N and 38°N. Along the eastern track (61), we compute 22 interferograms based on 12 ERS images acquired between July 1993 and August 1998. Along the western track (333), we compute 27 interferograms based on 15 ERS images acquired between 1995 and 1998. Baselines are limited to 200 m to maintain phase coherence across the fault and most parts of the scene. The interferometric phase contains informations about the deformation occurring between two satellite passes, as well as satellite orbital errors, and atmospheric delays. Atmospheric delays, in our interferograms, are mainly due to the variation of water vapor vertical stratification in the troposphere between two passes. They result in a clear, mostly linear, correlation between phase and elevation (tropostatic delays). We jointly correct for orbital errors and tropostatic delays, removing a best fitting twisted phase plane and a slope between phase and elevation, respectively. However, other residual atmospheric delays remain important after those corrections. We thus select interferograms showing the best signal to noise ratio (based on the analysis of 2D noise spectra), before stacking them to obtain velocity maps. 5 and 4 interferograms are selected for tracks 333 and 61, respectively, corresponding to a cumulated time of up to 11 years for each track. The results obtained for both independent tracks are remarkably consistent. In the overlapping part of tracks 333 and 61, the 9 selected interferograms can finally be stacked, increasing the cumulated time to 22 years. Obtained fault-parallel velocities, assuming a purely horizontal motion, are best fitted by a fault slipping at 7±2 mm/yr, below a shallow locking depth of 2 km.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry