关于紫坪铺水库蓄水是否与汶川地震有关的影响因素综合分析

Comprehensive understanding of the Zipingpu reservoir to the M_S8.0 Wenchuan earthquake

汶川地震后,紫坪铺水库蓄水是否触发了汶川地震在国内外学术界引起了广泛关注.除定性讨论外,许多学者也采用定量分析的方法进行了计算,但因计算结果不同而得出了不同的结论.本文从目前紫坪铺水库蓄水不同研究组定量计算中出现的争议为出发点,通过对水库蓄水定量计算基本原理和可能引起计算结果差异可能因素的分析,找出定量计算中的关键影响因素,了解目前水库蓄水定量计算中存在的不确定性问题所在.初步结果显示：计算方法、模型维数、扩散模型、震源参数和扩散系数等的取值不同是造成计算结果差异的主要因素,特别是裂隙岩体的扩散系数.在紫坪铺水库定量计算中模型维数的差别使得汶川地震震源处的库仑应力变化计算结果相差约3倍;仅考虑断层渗透率（把岩体渗透率视为无穷大）或仅考虑均匀各向同性的岩体渗透率（忽视断层渗透率）,均具有片面性;震源机制解断层走向倾角的差异,会显著影响库仑应力大小计算结果,可到达2~7倍;不同扩散系数下,孔隙压力相差可达几百倍.“紫坪铺水库蓄水是否能够触发汶川大地震的发生？”,鉴于目前的研究成果,库仑应力变化在kPa量级,尚不能排除触发的可能性,但得出的蓄水震源处的库仑应力变化太低,在背景构造应力场不明确的情况下,也不能确定一定有联系.在未来的工作中需有针对性的进行野外考察和室内试验,改进模型,采用高性能模拟分析计算,并在此基础上对中国和世界多个水库地震触发机制进行对比研究,探讨不同机制下水库地震触发机制特点,进一步量化分析水库地震发生的力学机制及水库对构造活动的影响和作用机理.

Since the 12 may 2008 Wenchuan earthquake, many scholars have done a lot of work discussing the impact of Zipingpu reservoir on the Ms8.0 earthquake, which is less than 10 km from the epicenter. Different opinions occurred in the past seven years, not only in the conclusions of qualitative analysis but also the quantitative analysis. In order to comprehensively understand the impact of Zipingpu reservoir on the Ms8.0 earthquake, we analyze the main influence factors by systematically summarizing these researches.Following the impoundment of reservoir, there are three responses which might correspond to two fundamental mechanisms. Firstly, rapid increase of elastic stress may increase the stress promptly upon the fault to critical state, and failure occurs shortly. Secondly, pore pressure beneath the reservoir and its adjacent regions is redistributed by the porous elastic nature of lithosphere-fracture-fault system. Based on the basic principle of quantitative calculation of these responses, we analyze different models and main parameters which may cause the differences. Then we further study the key influence factors of quantitative calculation and know about the existing uncertainty elements during the numerical simulation.The preliminary results show that the calculation methods （analytical method or numerical method）, dimension of models （2-D or 3-D）, diffusion model, diffusion coefficient and focal mechanism are the main factors resulting in the differences, especially the diffusion coefficient of the fractured rock mass. Firstly, without considering the geological environment, the changes of elastic stress due to the impoundment of Zipingpu reservoir can be attained using half space Boussinesq＇s analysis method. In the same way, the pore pressure can be conveniently attained from Biot＇s consolidation equation using analytical method. Though, the geological structures of Longmenshan region are very complex. High performance computation provides a powerful tool to quantitatively evaluate stress, which can conveniently take the geological features into model. In addition, when using the numerical method, we should guarantee sufficient accuracy. The results attained from analytical method can be the calibration of numerical method. Secondly, the 2-D model is a plane strain approximation, which takes infinite linear loads instead of finite loads in limited area. It may exaggerate the loading effect. The change of Coulomb failure stress at the epicenter of Wenchuan earthquake attained from 2-D model is about 3 times greater than that of 3-D model. Thirdly, it is not reasonable only considering the fault permeability （assuming the permeability of rock mass as infinity） or the homogeneous isotropic rock mass permeability （ignoring the fault permeability）. Fourthly, as for the uncertainty of the diffusion coefficient of the deep rock and faults, the difference of Coulomb failure stresses can reach several hundreds times, when selecting different diffusion coefficients. Finally, the different focal mechanisms, especially the focal depth, also could dramatically affect the change of Coulomb failure stress. The differences can research 2~7 times when adopting different focal mechanisms derived from different institutes.Although there are many influence factors affecting the quantitative analysis results, there are still more in common with the work of each group. The impoundment of reservoir makes the thrust fault more stable, whereas leads the normal fault to be more dangerous. And the instantaneous pore pressure and diffusion pressure accelerate the occurrence of the earthquake. At the hypocenter of the earthquake, the values of the Coulomb failure stress change of each group are close to several kPa, we could not rule out the possibility that the Zipingpu Reservoir may trigger the 2008 Wenchuan earthquake. However, for the background stress is not clear and Coulomb failure stress change is too little, we are also not sure there must be a connection between reservoir and earthquake. In future work, we should target on the basis of field survey and indoor experiment, in order to improve the model and develop high performance simulation. Based on this calculation method, we make a comparison with more typical reservoir-triggered earthquakes with different mechanisms both domestic and abroad to discuss the characteristics and further analyze the mechanics of reservoir-triggered earthquake.