Numerical Simulation Of Deformation Movement And Stress Accumulation In Longmenshan And Its Adjacent Zone Before The Wenchuan Earthquake

As the boundary fault of the Bayan Har block and Sichuan Basin, Longmenshan fault occurred Wenchuan Ms8.0 and Lushan Ms7.0 earthquake in 2008 and 2013.Longriba, Minjiang and Huya faults, which are located in the eastern part of the Bayan Har block. The results of field geological survey and earthquake sequence research show that the right lateral strike slip rate of the Longriba fault is(5.4±2.0)mm/a, Minjiang fault occurred MS7.0 and MS7.5 earthquake in 1713 and 1933, Huya fault continuous occurred MS6.5, MS7.2, MS6.7, MS7.2 earthquake in 1973-1976. It reflects that these faults still have strong activity. Therefore, simulated the deformation motion of the Longmenshan fault and its adjacent zone in consider of the active faults which are located in the eastern of Bayan Har block. Then, analysis seismic area stress field, strain field and fault stress accumulation rate before the Wenchuan earthquake,which can help us reveal the correlation between different faults, rupture mechanism of the Wenchuan and Lushan earthquake, earthquake recurrence period of different faults and other related issues. It has certain practical significance.In the paper, according to geological, geophysical data and flow movement research results. We build a 3D viscoelastic finite element model, which included Longmenshan, Longriba, Minjiang and Huya faults, by using ANSYS finite element.Using peripheral GPS measurement results around the model in 1999-2004 as theconstraint, simulate the deformation movement of Longmenshan fault and its adjacent zone before the Wenchuan earthquake. Meanwhile, we discuss the deformation movement on the effect of Longriba Minjiang and Huya faults, characteristics of the regional stress field and strain field, the changes of principal compressive stress along the Longmenshan fault, compressive stress and shear stress accumulation rate of the faults with relationship between the earthquake activities and fault rupture, the earthquake recurrence period of different faults and different segments of the same fault. The conclusions are drawn in this paper as follows:1) The eastern part of the Bayan Har block present pattern of strong deformation and high stress state is relatively uniform characteristics under dynamic action.However, different faults have different effect in the regional deformation movement.Velocity component parallel to longmenshan fault is mainly absorbed by Longriba fault and velocity component perpendicular to the Longmenshan fault is mainly absorbed by itself. Because of the barrier effect of Minjiang and Huya faults on the north section of Longmenshan fault, northern part of Longmenshan fault compression rate is lower than the southern.2) Extending from SW to NE direction along Longmenshan fault, the angle between the main compressive stress and the direction of the fault is gradually transferred from the nearly vertical to 45 degrees. Compressive stress and shear stress accumulation rate in high in southwest segment of Longmenshan fault and compressive stress is greater. In northeast segment of the fault, stress accumulation rate is low and compressive stress close to shear stress. This is coincident with small and medium-sized earthquakes occurred frequently in the southwest of the fault,seismic activity strong, only some small earthquakes occurred occasionally in northeast of the fault and seismic activity weak. It is also coincident with the rupture of Wenchuan earthquake with thrust and right lateral strike slip and Lushan earthquake with thrust type.3) Assume that the same type of earthquake fault rupture strength is the same.Then, Minjiang fault, southern segment of Longmenshan fault and Huya fault rupture mainly thrust movement and earthquake recurrence period of the three faultsgradually increase. In northern segment of Longriba fault and Longmenshan fault,earthquake rupture is thrusting and right lateral strike slip. The former earthquake recurrence period is shorter than the latter. In southern segment of Longriba fault,earthquake rupture is pure right lateral strike slip motion, earthquake recurrence period is the shortest in the simulation region.