Forward Modelling of 2008 Wenchuan Earthquake Based on SAR Sub-Pixel Offset Data and Refined Dinsar Data

Differential interferometric synthetic aperture radar (DInSAR) provides an essential constraint for forward modelling of seismic faulting. However, the crucial evidence of deformation along the very near vicinity of a seismic fault is often missing because chaotic ruptures destroy the InSAR signal coherence; such as the case of the Yingxiu-Beichuan fault zone of the 2008 Wenchuan earthquake. We developed an Adaptive Local Kriging (ALK) interpolation method to produce a refined DInSAR dataset which is not interrupted by decoherence gaps. Furthermore, sub-pixel accuracy horizontal displacement maps in both ground range and azimuth directions are derived from SAR amplitude images using the Phase Correlation based Image Analysis System (PCIAS), which partially verify the slant range deformation presented in the ALK refined DInSAR data and meanwhile provide measurements of horizontal motion in radar azimuth direction. Thus the 3D configuration of the seismic faulting can be better constrained for forward modelling using the Poly3D software. Through parameter fine tuning in a sequence of trial and error simulation, the best fitting between the simulated interferogram and the ALK refined DInSAR interferogram is achieved using a structural model of the Yingxiu-Beichuan fault zone which comprises a total of 14 segments with dip angles varying from 20° -60° , in the south-west, to 40° -70° , in the north-east; some major fault segments are composed of multiple planes with varying dip angles. The model reveals that in the much disputed Beichuan-Pengguan area, the Yingxiu-Beichuan fault links with the Pengguan fault (which dips to the NW at ~30°) at a depth of ~13km. These two structures are interpreted as representing the foreland propagation of the Yingxiu-Beichuan fault by footwall collapse, a process that, in the vicinity of the Pengguan fault, involves the transfer of displacement from the older, Yingxiu-Beichuan fault to the younger Pengguan fault.