2008年汶川MS8.0地震发生过程的动力学机制研究

2008年5月12日汶川地震突发在现今并不活动的龙门山断裂带上，该地震发生的动力学机制问题引起广泛关注.文中利用黏弹性接触问题的有限元方法，考虑重力作用，对青藏高原东缘的应力场空间分布及其随时间的演化进行了数值模拟，结果显示应力在空间由分散分布逐渐向龙门山及周边地区转移集中.基于前人的研究成果及计算分析，初步认为汶川地震孕育发生的动力学过程如下：青藏高原的物质东流在向东运动过程中由于受到稳定的四川盆地的阻挡，一部分东流物质在川西地区囤积，造成龙门山隆升；高角度(50°~70°)、犁状的龙门山断层面上的正应力随着川西高原向东运动而不断增大，导致该断层的闭锁性逐步加强，并且分布在断层附近的变质杂岩为存贮高密度弹性应变能提供物质保障.但另一方面随着青藏高原较柔软的下地壳物质的不断向东运动，囤积的东流物质对龙门山断裂带上盘的推挤作用会不断加强，从而导致断裂带上剪应力越来越大；当剪应力超过摩擦强度时，断层解锁产生滑动，发生地震.模拟结果还表明龙门山断层面上的摩擦系数较高，断裂带上地震的平均复发周期约为3163年，这与其他资料结果有一致性.

A study on the dynamical mechanisms of the Wenchuan MS8.0 earthquake, 2008

The unexpected Wenchuan earthquake occurred on the Longmen Shan Fault, which is not active recently. It will exert an important influence on the society and scientific community. Especially, the tectonic mechanisms of the Wenchuan earthquake will be paid more and more attention. In the paper, the distribution of the stress field and its evolution with time in the eastern Qinghai-Tibetan plateau is simulated by means of viscoelastic finite element method, with gravity being considered in the model. The result shows that the stresses are transferred to and concentrated in the Longmen Shan area from a state of decentralization in space. Based upon the results from previous studies and the present paper, the dynamical process of the Wenchuan earthquake is assumed as: with the movement of eastward mass flow of the Qinghai-Tibet Plateau, some part of material would accumulate on the west brim of the Sichuan basin since the stable and strong Sichuan basin blocks the eastward flow, resulting in the rise of the Longmenshan Mountains. The value of the normal stresses on the fault plane which is steep (the dip angle of 50°~70°）and of listric shape, increases with the eastward motion of the plateau of the western Sichuan, making the fault plane lock more tightly. At the same time, the accumulated material will exert stronger and stronger forces to the hanging wall of the Longmen Shan fault, therefore the shear stresses on the fault are becoming greater and greater with the movement of eastern mass flow of the Tibet. Once the shear stresses exceed the friction strength, the fault will be unlocked, and it will slip suddenly. Therefore an earthquake occurs. Also, the simulation result suggests that the average recurrence time of earthquakes in the Longmen Shan area is about 3163 years, which is in agreement with the result by other methods.