Numerical Simulation Of Spontaneous Rupture And Strong Ground Motion In 2008 Wenchuan Earthquake

The Ms 8.0 Wenchuan earthquake occurred in the Wenchuan area of Sichuan Province on May 12,2008,with a location of 103.4°E and 31.06°N,which have caused severe building damages on Wenchuan,Yingxiu and their surrounding areas and heavy casualties.It has ranked as one of the most devastating natural disasters ever occurred in modern Chinese history.Longmen-Shan locats at the intersection of Tibet Plateau and Sichuan Basin.After the earthquake,a large number of geological,geophysical surveying and deformation observation have been carried out in this area,providing rich data to study the rupture mechanism of the Wenchuan earthquake and deformation mechanism of Tibet Plateau.The Longmenshan fault(LMSF)zone is the seismogenic source structure,which consists of three sub-parallel faults,i.e.,the Guanxian-Jiangyou Fault(GJF)in the frontal,the Yingxiu-Beichuan Fault(YBF)in the central fault and the Wenchuan-Maowen Fault(WMF)in the back of the LMSF.In this study,geological survey and seismic profiles are used to constrain the faults geometry and medium parameters.Three visco-elastic finite element models of the LMSF with different main faults are established.From the phase of interseismic stress accumulation to coseismic stress release and postseismic adjustment,the Wenchuan earthquake is simulated using Continuous-Discrete Element Method(CDEM).Modeling results show that before the 2008 Wenchuan earthquake,the GJF becomes unstable due to the interaction between its unique fault geometry and the tectonic stress loading.In the fault geometry model,the GJF is the most gently dipped fault among the three faults,which in return makes it having the smallest normal stress and the greatest shear stress.The continuous shear stress loading finally meets the fault failure criteria and the Wenchuan earthquake starts to initiate on the GJF at the depth of 15-20 km.The earthquake rupture then propagated to the YBF.At the same time,due to the GJF and YBF rupture,the interseismic stress accumulation has been greatly reduced,causing the WMF failed to rupture.Although the stress accumulation in the WMF has been reduced significantly after the earthquake,yet it has not been released completely,which means that the WMF is likely to be with high seismic risk after the 2008 Wenchuan earthquake.We also find that the stress perturbation caused by gently dipping segment of the fault can promote the passive rupture in the steeply dipping segment,making the upper limit of dip angles larger than traditional assumption.Additionally,according to kinematics inversion of source rupture,we incorporate earthquake source rupture model with asperity and barrier models.We also implement the spatialtemporal variations of fault segmentation,inclination and slip rake angles in the simulations.Based on the dynamic simulation of the ground motion,numerical calculation of seismic wave propagation and post-processing analysis,we simulate the ground motion induced by earthquake.Because of the disturbance of mountainous terrain complex area,timely rescue is difficult.In order to better understand the influence of topographic factors on the strong ground numerical simulation,two kinds of models are established,one with complex topography effect and the other without.The results show that: 1)the distance between fault and strong motion station makes an amplification or inhibitory effect on topographic effect.The closer distance is,the more obvious the topographic effect is,vice versa.2)Due to the influence of topography,the peak of ground motion may occur outside the epicenter.3)In model I,Peak Ground Velocity(PGV)distributions resulted from current simulation show much higher PGV values in the areas of the Wenchuan,Beichuan than other places.In model II,higher PGV area located in the second half of the Guanxian-Jiangyou fault,near areas of Qingping,Anxian.The numerical modeling developed in this study has potential application in the ground motion prediction for the earthquake engineering.In addition,the numerical simulation could also be used to generate near-real-time shaking map in the implementation level.