Numerical Simulation Analysis Of Post-seismic Afterslip And Viscoelastic Relaxation Following The Wenchuan Earthquake

On 12 May 2008,the Wenchuan earthquake of Ms8.0 magnitude occurred in the Longmenshan Fault Zone,causing great damage to the area.This earthquake started with a retrograde rupture in the southwest segment,which later developed into a rightlateral slip rupture to the northeast,with a total rupture length of 300 km.The postearthquake deformation of the Wenchuan earthquake contains information on the rheology of the regional media in the Longmenshan fault zone and information on the crustal and deep tectonic movements in the region.This dissertation presents a joint study on the post-earthquake deformation of the Wenchuan earthquake.Therefore,the combined study of early and long-term post-earthquake deformation of the Wenchuan earthquake is of unique significance not only for understanding the post-earthquake mechanism of this earthquake,but also for the study of regional rheological dynamics in the eastern Tibetan Plateau.In this paper,a three-dimensional viscoelastic model based on the finite element method is developed to simulate and calculate the post-earthquake viscoelastic relaxation effects of the Wenchuan earthquake,followed by the inversion of the postearthquake afterslip using kinematic methods to obtain the surface displacement deformation fields separately to investigate the physical mechanisms of the two postseismic deformation mechanisms.The main work accomplished is as follows.1.GNSS data processing and deformation analysis.Using the GAMIT/GLOBK software,the observed data from 52 GNSS stations were processed and the required cumulative displacement of deformation for 1 and 7 years after the earthquake were obtained using the logarithmic fitting method.From the displacement distribution results: the displacement direction of the stations distributed on the eastern region of the Tibetan Plateau from southwest to northeast has a gradual trend towards north;the post-earthquake deformation values on both sides of the fault are very different and have great asymmetry,indicating that the deep rheological properties on both sides of the fault are very different.2.post-earthquake viscoelastic relaxation simulation analysis.In this section,four three-dimensional viscoelastic models with elastic layer thicknesses of 25 km,30 km,33 km and 35 km are established for the eastern region of the Tibetan Plateau.The finite element method is used for the normalization,and then the best elastic layer thickness and the best viscous coefficient of the middle and lower crust of the Longmenshan fault zone are obtained by the grid search method.According to the results,the minimum fitting error is RMS=10.61 mm for an elastic layer thickness of 25 km,and the maximum deformation is 8.47 cm and the minimum is 7 mm,which is close to the observed deformation value.By comparing the simulated values with the displacement field distribution of the deformation observations,it can be concluded that the fitting error is large in the mid-near-field region and better in the far-field region.The viscous time evolution simulations were also carried out,and the viscoelastic relaxation effects were orthogonalized from 1 to 7 years after the earthquake to obtain the viscous coefficients of the upper and lower crust of the Longmenshan Fracture Zone from 1 to7 years after the earthquake,and the results of previous studies were compared and found that the viscous coefficients of the upper and lower crust of the Longmenshan Fracture Zone generally increase with the increase of the post-earthquake time scale.3.Simulation analysis of the single afterslip mechanism.In this section,the kinematic inversion method is used to calculate the afterslip distribution on the rupture fault surface 1 year after the earthquake and obtain the surface deformation field using the GNSS deformation observation data 1 year after the earthquake as input values and the inversion function in the paper.The smoothing factor was set to 0.008,and the upper and lower boundaries of sliding in the Qingchuan-Beichuan-Yingxiu section were constrained to be 0 m and 1.5 m,respectively,and the upper and lower boundaries of sliding in the Penggu section were constrained to be 0 m and 0.5 m,respectively.The inversion results showed that the residual slip was mainly distributed in the area where no sliding or less sliding occurred during the same earthquake,with a peak value of about 0.597 m,which was located in the deep slip zone of the Hongkou-Yingxiu section.The best fitting effect was found in the near-field area,and the worst fitting effect was found in the far-field area,showing a gradual decrease of the fitting value from near to far.4.Joint mechanism simulation analysis.Based on the best rheological parameters obtained from the viscoelastic relaxation simulation,the viscous hysteresis coefficient of the lower crust in the Longmenshan rupture zone was assumed to be constant at3.0×1018 Pa-s for 7 years after the Wenchuan earthquake,and the thickness of the elastic layer was 25 km,and the viscoelastic relaxation normalization was calculated.The residual slip distribution and surface deformation field of the rupture fault surface are obtained by using the above inversion procedure,and the surface deformation results obtained from the forward viscoelastic relaxation and the inversion are added together to form the joint results of the two mechanisms.The results show that the closer the rupture area is,the closer the two methods are to each other,and the results tend to overlap near the fault surface,indicating that the closer the site is,the more the deformation is influenced by afterslip and the less the influence of viscoelastic relaxation.The combined viscoelastic relaxation and after-slip mechanism is more effective in explaining the post-earthquake deformation characteristics.In this paper,we simulated the viscoelastic relaxation effect within 7 years after the earthquake by finite element method,and obtained the optimum thickness of the elastic layer in the eastern part of the Qinghai-Tibet Plateau of 25 km,the viscosity coefficient of the middle and lower crust of 3.0×1018 Pa-s,and the viscosity coefficients of the upper mantle on both sides of the fault of 1.0×1019 Pa-s and1.0×1020 Pa-s,respectively.The kinematic inversion was used to calculate the afterslip distribution of the fault surface within 1 year of the Wenchuan earthquake and to simulate the effects of both viscoelastic relaxation and afterslip mechanisms.The distribution characteristics of the post-earthquake deformation are better explained by the dual mechanism than by the single mechanism.