Numerical Simulation Of Dynamic Rupture Propagation In The Middle Section Of Longmenshan Fault And Seismic Hazard Estiamtion

The 2008 Wenchuan earthquake has caused extremely serious disasters to human society along the northwest of Sichuan Basion,indicating high seismic risk in the middle section of the Longmenshan thrust belt.Numerical simulations of dynamic rupture processe can help us understand the mechanics of rupture nucleation,propagation,and arrest,as well as the characteristics of near-field strong ground motions,which should hopefully lead to the formulation of improved strategies for seismic hazard assessment.Here we simulated the dynamic rupture propagation process of multiple faults in the middle section of Longmenshan,to explore the mechanism behind the complex rupture pattern of the 2008 Wenchuan earthquake,and at the same time to predict the future seismic risk in this area.We first introduce two methods:the finite element method(FEM)and boundary integral equation method(BIEM).Then the effects of different friction laws on the dynamic rupture have been explored.We found that different friction laws show generally consistent rupture behavior on the macroscopic scale,and the rupture velocity and slip rate,boundary healing,as well as the stress evolution in the cohesive zone can be explained by different friction models.Compared with the slip-weakening friction law,the rate-state friction law contains the behaviors of slip-strengthening and slip-weakening.In addition,the rate-statetemperature-dependent frictional law can provide a reasonable explanation for the universal observation of self-healing impulse-like ruptures.We also explore the free-surface-induced supershear ruptures on oblique faults,and find that oblique thrust faults are more likely to produce free-surface-induced supershear transition than oblique normal faults and significant asymmetry arises between the two opposite directions along fault strike.Our results help explain the lack of universal observation of supershear earthquakes,as well as absence of supershear transition in the northern part of Beichuan Fault.We use FEM to simulate spontaneous dynamic rupture propagation of Wenchuan earthquake on the geometrically complex multifault system loaded by heterogeneous stress.Our model reproduce many observed features including the multifault ruptures,locations of the maximum slip,Interferometric Synthetic Aperture Radar(InSAR)observations,source time functions and peak ground velocities.Our results suggest that the maximum horizontal principal stress in the northern part of Beichuan Fault(BCF)is rotated approximately 6.5°counterclockwise compared to that in the southern part of BCF,yet this rotation is not sufficient to produce a supershear rupture.We infer that the fault core zone in the southwestern BCF may have been more severely damaged than the northeastern counterpart,which provides an explanation that the rupture in the northern BCF propagates faster than in the southern portion.Besides,we find that the mainshock may push WMF to approach the edge of the new rupture to some extent.However,due to the aftershock slip,the shallow affected area of ΔCFS,and the unknown absolute stress on WMF,this speculation has a large uncertainty.We perform 3D dynamic earthquake rupture simulations on the Wenchuan-Maoxian Fault(WMF)and Lixian Fault(LXF),to explore the possible size of earthquakes and distribution of high seismic risk in the future.We firstly invert 391 focal mechanism solutions(FMS)of earthquakes that occurred between 2009 and 2016 in the Longmen Shan,to get heterogeneous tectonic stress field as initial stress of simulation.Then we develop a new method to set up fault geometry through inverting long-term slip rates,which is based on the Wallace-Bott hypothesis.Several fault nucleation points,friction coefficients,and initial stress states are tested,and the general rupture patterns for these earthquake scenarios are evaluated and could fall into three groups.The dynamic rupture may start in the LXF,leading to an around magnitude-7.0 earthquakes;or,start in the WMF,then cascades through the LXF,leading to magnitude-7.5 earthquakes;or,both start and arrest in the WMF,leading to around magnitude-6.5 or 7.0 earthquakes.We find that the cascade rupture tends to jump from oblique fault to strike-slip fault,but reverse process is more difficult.The rupture propagating eastward causes larger coseismic displacements than the westward propagation.In addition,the surface displacements derived from simulation show the relatively high peak ground velocities and displacements near the southwestern or northeastern end of WMF,which suggests higher seismic risk in the future.Overall,although WMF is more stable than BCF and PGF,the high seismic risks of WMF and LXF should still needs attention.