Abstract: In order to reveal the deformation and cumulative stress state in Longmenshan and its adjacent faults before Wenchuan earthquake,a 3D viscoelastic finite element model,which includes Longmenshan,Longriba,Minjiang and Huya faults is built in this paper.Using the GPS measurement results of 1999-2004 as the boundary constraints,the deformation and movement of Longmenshan fault zone and its adjacent zones before Wenchuan earthquake are simulated.The conclusions are drawn in this paper as follows:First,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,the compression rate in the northern part of Longmenshan Fault is lower than that in the southern part.Second,extending from SW to NE direction along Longmenshan Fault,the angle between the main compressive stress and the direction of the fault changes gradually from the nearly vertical to 45 degrees. Compressive stress and shear stress accumulation rate is high in southwest segment of Longmenshan Fault and compressive stress is greater;the stress accumulation rate is low and the compressive stress is close to shear stress in the northeast segment of the fault.This is coincident with the fact that small and medium-sized earthquakes occurred frequently and seismic activity is strong in the southwest of the fault,and that there are only occasional small earthquakes and the seismic activity is weak in the northeast of the fault.It is also coincident with the rupture type of thrust and right-lateral strike-slip of the Wenchuan earthquake and thrust of the Lushan earthquake.Third,assuming that the same type and magnitude of earthquake requires the same amount of stress accumulation,the rupture of Minjiang Fault,the southern segment of Longmenshan Fault and the Huya Fault are mainly of thrust movement and the earthquake recurrence period of the three faults increases gradually.In the northern segment of Longriba Fault and Longmenshan Fault,earthquake rupture is of thrusting and right-lateral strike-slip. The earthquake recurrence period of former is shorter than the latter.In the southern segment of Longriba Fault,earthquake rupture is purely of right-lateral strike-slip,it is possible that the earthquake recurrence period on the fault is the shortest in the study region.

Key words: Longmenshan Fault, viscoelasticity, deformation movement, stress accumulation rate, numerical simulation

摘要： 为揭示汶川地震前龙门山及其周缘断裂的形变与应力累积状态，文中构建了包含龙门山、龙日坝、岷江和虎牙4条断裂的三维黏弹性有限元模型，以1999-2004年GPS结果为约束，模拟了龙门山断裂带及其周缘区域的形变运动。得到以下结论：1）平行于龙门山断裂带的速度分量主要被龙日坝断裂吸收，垂直于龙门山断裂带的速度分量主要被其自身吸收；岷江和虎牙断裂对龙门山断裂带北段起到一定的屏障作用，导致其北段压缩量明显低于南段。2）沿龙门山断裂带由SW向NE方向延伸，主压应力与断层走向的夹角由接近垂直逐步转至约45°；断层南段挤压、剪切应力累积速率高，且压应力大于剪应力，北段应力累积速率低，压应力与剪应力接近。这与龙门山断裂带SW段中小地震频发、地震活动强烈，NE段偶有小震、地震活动微弱相吻合；也与汶川M_S8.0地震逆冲兼具右旋走滑、芦山M_S7.0地震逆冲破裂的方式相一致。3）假设发生震级、类型相同的地震所需应力积累量相同，那么研究区内岷江断裂、龙门山断裂南段和虎牙断裂破裂以逆冲运动为主，3条断裂的地震复发周期依次变长；龙日坝断裂北段和龙门山断裂北段以逆冲兼具右旋走滑为主，前者地震复发周期短于后者；龙日坝断裂南段则以纯右旋走滑为主，地震复发周期有可能最短。

关键词: 龙门山断裂带, 黏弹性, 形变运动, 应力累积, 数值模拟