Stress Inversion In The Longmenshan Fault Zone And Dynamic Rupture Simulation Of The 2008 Wenchuan Earthquake

Knowing the state of stress in the Earth’s crust and studying the dynamic rupture process of strong and shallow earthquakes and their control factors help not only to promote the process of numerical earthquake prediction,but also to evaluate the potential rupture extent and behavior of faults in seismic gaps where large earthquakes are believed to occur in the future.The 2008₍（2）7.9 Wenchuan earthquake and the 2013₍（2）6.6 Lushan earthquake occurred successively in the Longmenshan fault zone,causing great casualties and economic losses.This highlights the importance of investigating the stress state and the dynamic rupture mechanism of such strong earthquakes in this tectonically active area.Stress inversion of earthquake focal mechanisms is a powerful tool for obtaining the crustal stress state.Such an inversion generally requires identifying which of the two nodal planes in a focal mechanism more likely represents the active fault plane that actually failed during the earthquake.Focal mechanisms of the aftershock sequence of a large earthquake are often inverted for regional stress,but existing methods for stress inversion lack consideration of mainshock-aftershock triggering mechanisms.To solve this problem,we propose a new method（D-value-based method,DVBM）that uses specifically stress changes imparted by the mainshock to identify the likely active fault plane in an aftershock focal mechanism.We then incorporate DVBM into a linear stress inversion method.To validate this method,we comprehensively consider the location of aftershocks with respect to the mainshock fault,focal mechanism error and coefficient of friction,and conduct multiple synthetic tests related to reverse,normal and strike-slip faulting stress regimes respectively.Four general conclusions are drawn from the results.（1）The simulation results are well consistent with the observations that most aftershocks occur at fault ends and in low-slip areas associated with the mainshock fault,but still,a few aftershocks occur in high-slip areas.This indirectly suggests the correctness of assumptions that DVBM is based on.（2）Although not all fault planes of aftershock focal mechanisms can be identified by DVBM,the identified fault planes are expected to be sufficient for stress inversion because it needs only some（≥20）rather than all aftershocks.（3）For aftershocks occurring in the unslipped area of the mainshock fault,once DVBM can distinguish the two nodal planes from each other,the corresponding accuracy rates are nearly 100%;In the low-slip area with a high aftershock triggering rate,the accuracy rate is coincidently high;In the high-slip area with a high aftershock triggering rate,the accuracy rate is close to 100%as well.Note that the above spatial relations hardly change with the value of μ,suggesting the effectiveness of DVBM.（4）In terms of noisy focal mechanisms of aftershocks,DVBM is better than or comparable to the commonly-used fault-instability-based method（FIBM）in some areas surrounding the mainshock fault.To address the controversy on the crustal stress field in the Longmenshan fault zone—how it changes along the strike of the Beichuan fault（sometimes called the Yingxiu–Beichuan fault）,we employ five representative fault slip models of the Wenchuan earthquake and a large number of focal mechanisms of its aftershock sequence and estimate a high-resolution one-dimensional stress field along the Beichuan fault using DVBM and a damped linear stress inversion method.The results show that the stress field in the Longmenshan fault zone is highly heterogeneous.Along the SW–NE direction,the maximum principal stress₁ is consistently subhorizontal and generally oriented ESE–E without a significant change,while the orientation of the minimum principal stress₃ generally follows the trend of“subvertical–subhorizontal–subvertical–subhorizontal”.Compared with other studies on the stress state in the Longmenshan fault zone,we use the focal mechanisms of aftershocks for stress inversion,but the orientation of regional stress estimated in this study is expected to be not distinct from that before the Wenchuan earthquake.In other words,the Wenchuan earthquake did not significantly change the direction of regional stress.Kinematic source models and field investigations of the surface rupture zone reveal some spatiotemporal faulting characteristics of the Wenchuan earthquake,yet they can not explain the reasons behind them.Physics-based dynamic rupture simulation is a powerful tool for solving these problems.However,existing dynamic rupture models of the Wenchuan earthquake are in poor agreement with the coseismic surface displacements observed by GPS（Global Positioning System）and In SAR（Interferometric Synthetic Aperture Radar）and the kinematic source models.Besides,it is difficult for them to explain why the rupture of both the Beichuan and Pengguan faults during the earthquake stop naturally at their southwestern ends.This may be because the previous dynamic rupture simulations either adopt homogeneous regional stress or consider a simple rotation of regional stress along fault strike.To solve the above problems,we use the high-resolution and heterogeneous regional stress obtained in this study to construct a dynamic rupture model of the Wenchuan earthquake.Additionally,we discuss the influence of regional stress on the dynamic process and analyze the role of the Pengguan fault in this earthquake.Three main conclusions are drawn from the results.（1）Our preferred dynamic model not only is well consistent with the coseismic surface displacements observed by GPS and In SAR and slip measurements of coseismic fault scarps but also reproduces many characteristics revealed by the kinematic source models and the field investigations of the surface rupture zone,including the concurrent rupture of the Beichuan and Pengguan faults and their natural termination at the southwestern end,multi-peak slip distribution,sub-shear rupture,etc.（2）Compared with the other three dynamic models obtained under different conditions,we find that the transition of reverse faulting stress regime to strike-slip faulting stress regime within～10 km along the fault strike（near Hongkou）strongly affects the dynamic process of this earthquake.For example,it controls the natural rupture termination of the Pengguan fault at its southwestern end.（3）Without the participation of the Pengguan fault in the Wenchuan earthquake,the slip distribution of the Beichuan fault would not show multi-peak characteristics,even if the initial fault stress itself is unevenly distributed.