Fault Slip Inversion Of Wenchuan Earthquake Using InSAR Coseismic Surface Deformation Observations

A magnitude Mw=7.9 earthquake struck Yingxiu town (N31.01°, E103.42°) in Wenchuan county, Sichuan Province, China on May 12,2008. The fault rupture occurred over a length of ～320 km along the northeast strike of Yingxiu-Beichuan fault in Longmenshan area, and ～70 km along the PengGuan fault. The strong ground motion caused huge casualties and property loss. The satellite geodesy can be an efficient space observation technique for obtaining coseismic surface deformation field. The measured coseismic displacements can be greatly helpful to infer the seismic mechanism, inverse the fault slip, estimate the fault stress change, and assess the fault rupture risk.Interferometric synthetic aperture radar (InSAR) is a newly developed space geodetic technique with the significant characteristics of high measurement accuracy, wide spatial coverage and high ground resolution. In this thesis, the coseismic deformation fields of Wenchuan earthquake was observed from PALSAR images. The GPS observations were used to remove the residual flat-earth phase, and interferometric fringes between adjacent orbits were smoothened with the consistency constraints of high quality surface deformation data. The corrected coseismic deformation shows that the accuracy and smoothness were significantly improved, and the RMSE of InSAR at high coherent area reaches 3.6 cm with a decrease by 60%. The smoothness factors of interferomrams for the adjacent orbits are improved by 33%.In order to detect the listric fault geometry of Wenchuan earthquake, a new method to inverse the varying fault dip angles along the depth was proposed in this thesis. The fault is devided into a series of sub-faults along the down-dip direction. Geodetic observations were considered as the constraint in the inversion, and the non-linear iteration algorithm was used to minimize the objective function to derive the dip angle of sub-faults. The proposed method was applied to the Wenchuan earthquake. The inverse results show that the Yingxiu-Beichuan fault has a steep dipping angle of 73° in the upper crust, and progressively becomes less steep along the depth but the dip angle greater than 65°in the upper 6 km. A significant decrease of dip angle occurs at the depth of 6-14 km with 45°at 14 km depth, then rapid decrease to 2°at the depth of 20 km. The inversion results show the Yingxiu-Biehcuan fault has a listric surface and finally merges with the PengGuan fault, rooting into the middle crust at the depth of 20 km.The fault slip model of Wenchuan earthquake was jointly constrained by InSAR and GPS deformation observations. The trade-off relation among the model misfit, model misfit gradient and seismic moment as a function of average solution roughness was explored for choosing the preferred smoothing factor. The slip inversion results show that the misfit of InSAR data is 7.1 cm with a decrease by 50%. The faulting pattern varies along the strike direction. First, the fault rupture occurred with mostly thrusting in the Dujiangyan segment near the source, then it progressively transformed to a mixed mode of dextral slip and thrust motion in the middle segment, finally, became a dominant right-lateral slip in the northeast segment. Moreover, five high-slip concentrations in Dujiangyan, Gaochuan, Beichuan, Fangshi and hongguang have been found along the strike of Yingxiu-Beichuan fault with the maximum slip up to 10.03m.For overcoming the limited capability of revolving deep coseismic slip only based on geodetic data, a joint inversion approach incorporating stress changes and GPS surface displacements is applied to characterize the fault slip of Wenchuan earthquake. The spatial distribution of fault slip from the joint inversion suggests that the coseismic slip occurs not only above the hypocenter, but also with a significant thrusting motion of a mean slip of 8.5 m and maxima of 9.7 m along a depth of 10-16km. A significant high-slip concentration is found in this study, which suggests that the coseismic faulting extends toward ～16 km southwest of the Yingxiu-Beichuan fault with a dextral strike-slip of mean 4.8 m at a depth of 7-19 km. The joint inversion model misfits with 1.7 cm of GPS and 0.02 Mpa of stress change have a good compatibility between the two kinds of datasets. And the moment magnitude inferred from the preferred slip model is Mw 7.96 for the quake event.