Three-dimensional numberical modeling of activity of the Longmenshan fault zone driven by the India Plate

The effect of the continuous northward indentation of the India Plate along the Longmenshan fault zone and its surrounding areas is modeled using finite element analysis. Comparing the simulation result with GPS surveys, seismological data and geological analysis,we quantitatively analyze the slip rate and the distribution characteristics of major principal stress of the Longmenshan fault zone.And then,the triggering mechanics of the Wenchuan earthquake is discussed accordingly.The model consists of an elastic upper lithosphere and a vicoelastic lower lithosphere.The upper lithosphere includes the Tibetan Plateau,the Sichuan Basin,the South China Block,and the Indochina Block.The faults in the block are introduced as Coulomb-type frictional zones in the modeling.The results are as follows:(1) By adjusting the faults friction,we find that with friction coefficient increasing,the slip rate of the fault zone tends to decrease.When the friction coefficient touches 0.95,the slip rate is close to the result of GPS survey.Therefore,we assume that the Longmenshan fault zone should be under the condition with intensive friction. (2) Compared with other regions of the Songpan-Garze block,the Longmenshan fault zone is in a strong friction and low stress environment,which favors significant energy accumulation to trigger great earthquakes.(3) According to the distribution characteristics of major principal stress,we can divide the Longmenshan fault zone into four short segments from SW to NE as the Luding-Dayi segment,the Dayi-Maoxian segment,the Maoxian-Beichuan segment and the Beichuan-Qingchuan segment,respectively,and the maximum differential principal stress (σ_1) between hangingwalls and footwalls of faults appears in the Dayi-Beichuan segment,and the minimum differential principal stress(σ_1) appears in the Luding-Dayi segment.In the Maoxian-Beichuan segment,the principal stress(σ_1) of hanging walls of fault is almost equal to footwalls of the fault.Such distribution of major principal stresses is similar to the spatial distribution of the Wenchuan earthquake and its aftershocks.