| On May 12,2008,the Wenchuan Ms8.0 earthquake occurred on Pengguan and Yingxiu-Beichuan fault along Longmenshan fault belt between Sichuan Basin and Qinghai Tibet Plateau.The earthquake has brought enormous losses to people's lives and property.In this paper,the deformation of fault and stress loading process of Longmenshan thrust nappe system is further understood by studying the afterslip of the Wenchuan earthquake.Up to now,most of the researches on the afterslip of the Wenchuan earthquake adopted kinematic inversion method,which can best fit the observation data,but only consider the simulative effect of afterslipand lack of the research on the physical mechanism of afterslip.Some researches on the afterslip of other earthquakes have shown that the afterslip model driven by stress can explain the dynamics mechanism of afterslip reasonably and simulate the distribution of afterslip effectively.Therefore,we study the physical mechanism of afterslip of the Wenchuan earthquake and simulate the afterslip dynamically based on the afterslip model driven by stress.The main contents in this paper are as follows:1.GNSS data processing and deformation analysis.The observation data of 22 GNSS stations around the fault of the Wenchuan earthquake is processed by Bernese 5.2,and the time series of postseismic deformation are extracted.The time series of postseismic deformation are simulated by logarithmic function model,and the deformation fitting curves of east and north direction of all GNSS stations in one year after the earthquake are obtained.The results show that the deformation of stations increases rapidly in the early stage after the earthquake,and the growth trend decreases rapidly over time,besides,the movement direction of GNSS stations on the hanging wall of the fault changes from east-southward to east and to east-northward from the southwest to the northeast of the fault.2.Calculation and analysis of Coulomb stress change caused by the Wenchuan earthquake.The Coulomb stress change is calculated based on the layered viscoelastic half-space dislocation theory by PSGRN/PSCMP.1)Taking the distribution model of coseismic slip of Wang Qi?2011?as the source fault model,the Coulomb stress change at 10 km depth around the fault of the Wenchuan earthquake is calculated,and the influence of strike,dip and rake angle of the receiver fault,friction coefficient and Skempton coefficient on the reliability of Coulomb stress change is evaluated by coefficient of variation?CV,the ratio of the standard deviation and the absolute mean of the calculated Coulomb stress change at each location?.The results show that the unreliable Coulomb stress change is mainly distributed near the fault and the boundary between positive and negative Coulomb stress change,and the dip angle of the receiver fault has the greatest effect on the reliability of Coulomb stress change.2)A new distribution model of coseismic slip is obtained by redividing sub-fault blocks according to the fault geometry of Tan Kai?2012?model.Based on this new model,the coseismic Coulomb stress change of the fault of the Wenchuan earthquake is calculated.The results show that the positive of coseismic Coulomb stress change is mainly distributed around coseismic slip,and the Coulomb stress change is large near the transition area between the fault slope and the detachment layer in consequence of the large change of coseismic slip gradient,and the maximum of Coulomb stress change is about 6MPa.3.dynamics simulation of afterslip of the Wenchuan earthquake.Based on the velocity-strengthening friction model,the afterslip of the Wenchuan earthquake is simulated by the way of uniform,subsection and subsection-layered considering the uniformity,transverse and longitudinal differences of friction attributes of the fault,and the distribution characteristics of dynamics afterslip of these simulations are analyzed.1)Taking the observation data of GNSS stations after the earthquake as the constraint and the coseismic Coulomb stress change of the fault as the driving force of afterslip,the optimal friction parameters are obtained by simulated annealing algorithm based on the Chang?2013?dynamics numerical simulation.The results show that the optimal friction parameters are as follows:In uniform simulation,it is estimated to be 6.0×10-3.In subsection simulation,they are estimated to be 4.4×10-33 on Beichuan-Qingchuan section,2.7×10-33 on Yingxiu-Beichuan section and 24.2×10-33 on Pengguan section,in which the velocity-strengthening effect is most weak on Yingxiu-Beichuan section.In subsection-layered simulation,they are estimated to be 4.9×10-33 at 0-8 km depth and 2.5×10-33 at 8-17 km depth on Beichuan-Qingchuan section,to be 2.5×10-33 at 0-8 km depth,2.2×10-33 at 8-17 km depth and0.7×10-33 at 17-29 km depth on Yingxiu-Beichuan section,and to be 25.6×10-33 at 0-8 km depth and 5.2×10-33 at 8-17 km depth on Pengguan section.It is obvious that the velocity-strengthening effect is weaken with the increase of depth.2)The distribution of afterslip of the Wenchuan earthquake is calculated with the fault friction parameters.The results show that the afterslip is mainly distributed around coseismic slip,which is similar to the distribution of the positive of coseismic Coulomb stress change.The peak value of afterslip is 0.851m in the uniform simulation,1.160m in the subsection simulation and 1.373m in the subsection-layered simulation,and the root-mean-square?RMS?of the misfit between the observed and modeled surface displacement of all GNSS stations is 14.7mm in the uniform simulation,13.7mm in the subsection simulation and 13.0mm in the subsection-layered simulation.3)The friction parameters decrease with the increase of depth,and the proportion of aftershock decreases with the increase of depth,therefore,the relationship between the friction parameters and the proportion of aftershock may be positive.4.Kinematic inversion and comparative analysis of afterslip of the Wenchuan earthquake.Based on Okada's?1985?elastic half-space dislocation theory,the distribution of afterslip of the Wenchuan earthquake is inversed by the constrained least square algorithm.The smoothing factor is 0.008.The constrained boundary is set to be 0 m and 1.5 m on Yingxiu-Beichuan-Qingchuan section,and 0 m and 0.5 m on Pengguan section.The results show that the afterslip is mainly distributed around coseismic slip,and the peak value of afterslip is about 1 m,which distributes on the deep detachment layer of Hanwang and Caopo sections.Comparing the above results with the dynamics simulation results,the distribution of afterslip is similar that is mainly distributed in the area where there is no slip or little slip during coseismic.In the kinematic inversion results,the distribution area of afterslip is larger and smoother.In the dynamics simulation results,the distribution area of afterslip is smaller,the peak value of slip is larger,and the simulated movement trend of GNSS stations is basically in line with the observed trend,but there is quite difference in amount of deformation.In this paper,the distribution of friction parameters of fault of the Wenchuan earthquake is obtained by dynamics numerical simulation.According to the distribution of friction parameters,the friction properties of fault of the Wenchuan earthquake and the correlation with the distribution of aftershock are analyzed.The distribution and variation characteristics of afterslip of the Wenchuan earthquake are analyzed from dynamics point of view,which provides reliable physical parameters for dynamics simulation of deformation and stress evolution of strong earthquakes.To simulate the distribution of afterslip of the Wenchuan earthquake with kinematics and dynamics methods and carry out comparative analysis between these two results,so as to improve the research of simulation theory of afterslip. |
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