Interseismic Strain Accumulation Across The Western Altyn Tagh Fault From Wide-swath InSAR Observations

Normal and major strike-slip faults system dominate the current deformation pattern of active structures within the interior of the Tibetan Plateau. Understanding the present-day kinematics of the Indo-Asian collision requires identifying individual faults and quantifying their rates of motion. The Altyn Tagh Fault (ATF) is prominent in the physiography and tectonics of the Northern Tibetan Plateau. It is of great geologic and geodynamic significance to determine its current deformation mechanisms by using space geodetic and remote sensing observationsNowadays space geodesy provides quantitative maps of the surface velocity field within tectonically active regions, supplying constraints on the spatial distribution of deformation, the forces that derive it, and the brittle and ductile properties of continental lithosphere. As a natural laboratory of earthquake cycle and continental tectonics study, it still remains controversial that how the convergence of India with Eurasia has been accommodated by continental lithosphere. A key observation in constraining the two competing models, namely continuum model with crustal thickening and crustal lateral extrusion model, is the measurement of ATF fault slip rates and the distribution of crustal strain.It is still challenging to determine the spatial and temporal variations of interseismic deformation with InSAR. By comparison with traditional strip-map mode, ScanSAR/wide-swath mode of SAR data has four times large swaths and is more suitable for large-scale crustal deformation monitoring. Therefore, small-baseline time series analysis with wide-swath interferograms has exceptional advantages in observing interseismic strain accummulation. Complete processing flow of WS InSAR technique includes estimation of Doppler centroid frequency, calculation of burst synchronization and unsynchronized signal removal, subswath mosaicing and filtering, compensation for non-integer PRI part of burst gaps. Deformation uncertainty in wide-swath InSAR was investigated in this study, which mainly relates to all kinds of spatial-temporal decorrelation, topographic and phase unwrapping errors, phase delay in troposhpere and ionosphere, long-wavelength orbital planar ramp, as well as decorrelation effect from azimuth scan pattern synchronization.Observations in coseismic and volcanic deformation events were derived from wide-swath InSAR method, including2003Mw6.6Iran Bam earthquake,2008Mw7.9China wenchuan earthquake,2008Mw6.4and mw5.9Tibet Gerze earthquake and2008East Africa Afar volocano eruption. It has demonstrated that WS InSAR has large potential in constraining Mw7.0earthquake and other seismic hazards.Detailed validations of current global Digital Elevation Models, e.g. ASTER GDEM version1, version2and CSI SRTM version4.1were conducted in this study. In five study areas in China, absolute vertical accuracy was derived with comparison to high precision GPS measurements, while correlation analysis between ASTER GDEM and SRTM by raster-based comparison. Spatial pattern of elevation differences was provided by exploring functional relationships with slope, aspect and land cover categories. Reasons and relative improvements were discussed in geolocation correction, in-land water-body masking and systematic negative offsets. The results will make a contribution to those applications requiring accuracy constraints.An ellipsoidal height datum (e.g. WGS84) is required in SAR Interferometric processing, whilst DEMs are often referenced to a geoid datum (e.g. EMG96). Therefore, geoid height error is introduced due to the inconsistency of height datums in InSAR derived displacement maps. In this paper, the relationship between geoid heights and uncertainties in InSAR displacements is investigated using27scenes of Envisat ASAR ScanSAR images and44scenes of Image-mode images covering the western Altyn Tagh Fault, northern Tibetan Plateau. Atypical error of8.8m in SRTM DEM leads to3mm uncertainty in an InSAR interferogram with a perpendicular baseline of100m, whilst a geoid height of20m can result in10mm uncertainty. Geoid height ranges from one place to another, and are highly correlated with topography. The impacts of geoid height on InSAR displacements increase with perpendicular baselines, but SAR images with a small perpendicular baseline may not be available in some cases. In addition, it may also not always be feasible to use a best-fitting plane to remove the impacts of geoid height. It is thus highly recommended in this paper that DEMs with an ellipsoidal height datum should be employed in interferometric processing, particularly when long-wavelength crustal deformation is targeted.In order to reduce the coseismic effects from2008Mw7.1Yutian earthquake, its coseismic slip distribution was first derived from WS interferograms. With detailed comparison with published results, constraints can be well constrained in the far-field motion in this study. Moreover, it will provide important implications to other studies about Mw>7continental earthquakes all over the world. Interseimic slip rates were observed with Envisat ASAR wide-swath SAR acquisitions in the western ATF and Long-mu Gozha Co Fault for the first time. Comparison with GPS and strip-map mode InSAR results indicates that WS InSAR time series analysis technique can provide effect constraint in lithosphere tectonics and rheology. Therefore, the low slip rates(0?6mm/yr) estimated for the western ATF do support crustal shortening and thickening models, instead of the end-member micro-plate models requiring fast slip on a few faults accommodating convergence of India with Eruasia. In other words, large-scale deformation is continuous with the strength of the ductile lower crust and upper mantle dominating deformation as viscous flow and the brittle upper crustal faults are relatively minor with small offsets and rates, and most of the convergence is accommodated by crustal thickening.