DifSAR and PS InSAR: A Dual Approach to Investigating Continental Deformation on the Eastern Tibetan Plateau
Nockles, Victoria1; Parsons, Barry1; Holley, Rachel2; Wright, Tim3
1University of Oxford, UNITED KINGDOM; 2Fugro NPA Ltd, UNITED KINGDOM; 3University of Leeds, UNITED KINGDOM
DifSAR (Differential Interferometric Synthertic Aperture Radar) uses the difference in radar phase returns from satellite passes acquired at two different times to measure the displacement of the Earth's surface. Measurements of ground displacements across faults can be used in two ways to test models of continental deformation and to assess seismic hazard. The slip rates across faults on the Eastern Tibetan Plateau are not well known and vary significantly for different tectonic models. The two main competing models are: eastward rigid block translation (Molnar and Tapponier 1975) and distributed deformation throughout the lithosphere (Houseman and England 1986). We would expect much lower slip rates for distributed deformation than rigid block translation. Given estimated slip rates across faults we can then approximate the slip rate deficit since the last earthquake and calculate the magnitude of earthquake required to release this build up of strain.
22 interferograms were processed for a 1000km long swath across the Eastern Tibetan plateau covering a series of left-lateral strike-slip faults accommodating strain, including the Kunlun and Yushu-Xianshuihe. A mean line of sight velocity map was constructed for the region. We invert to solve for the best fitting model parameters using the Savage and Burford backslip model (1973) to obtain a slip rate of 7mm/yr, which is in agreement with geological slip rates to within error.
To reduce decorrelation effects DifSAR uses pairs of scenes with relatively short time windows on the order of a few years (small temporal baselines) and acquisitions that follow similar satellite orbits (small perpendicular baselines). This limits the amount of data that can be used to generate interferograms. Persistant Scatterer (PS) InSAR is a later adaptation that uses statistical techniques to identify pixels within a DifSAR image that are dominated by a single back scatterer, producing high amplitude and stable phase returns (Feretti et al. 2001, Hooper et al. 2004). In order to relax the constraint on small temporal and perpendicular baselines and reduce decorrelation effects PS InSAR was applied to the same region. We show that the two techniques are complementary. DifSAR is shown to effectively delineate broad scale long wavelength displacements and PS InSAR provides valuable data in regions that are incoherent in DifSAR. Our measurements of rates of displacement across the left-lateral Kunlun fault are shown to be consistent for both techniques.