紫坪铺水库造成孔隙弹性耦合变化及其对2008年汶川地震触发作用

位于川西地区龙门山断裂带附近的紫坪铺水库于2005年9月开始蓄水.约2.7年后,2008年5月12日,M_w7.9级汶川地震在龙门山断裂带上发生,两事件在时间和空间上的接近,揭示其可能相互关联,但前人的诸多研究给出了不同甚至是相反的结果.本研究基于完全耦合孔隙弹性理论,利用二维有限元模型(FEM),模拟水库蓄水造成的区域孔隙压力场和应力场的演化过程,基于库仑应力演化探讨其对龙门山断裂带活动的影响.模拟结果显示紫坪铺水库蓄水打破了原来的区域孔隙压力平衡,形成孔隙压力梯度源,向周围地壳传播;进而造成龙门山断裂带上库仑应力正值范围不断扩大,由浅入深影响到整条断层,尤其对浅层范围的加载作用明显,达上百千帕,为整个断层面的失稳提供了基础.震源区域库仑应力呈持续增长趋势,汶川地震发震前,增长了约数千帕~数十千帕,即使初期库仑应力为负,在未来某时刻库仑应力仍可能由负转正,并不断增大.通过计算汶川地震震源及其附近区域内相对于多种可能断层倾角的库仑应力,发现库仑应力随断层倾角增大而增加.因此整体来看,紫坪铺水库蓄水对龙门山断裂带起加载作用,有可能触发地震.对紫坪铺库区周围的小震分析也显示,蓄水以来小震明显增多、且不同时段小震发生密集区与水库距离逐渐增大,与孔隙压力扩散趋势一致.以上结果表明紫坪铺水库的蓄水增加了汶川地震的危险性.

Triggering effect of the Zipingpu Reservoir on the 2008 Mw7.9 Wenchuan,China, Earthquake due to poroelastic coupling

Impoundment of the Zipingpu Reservoir (ZR), China, began in September 2005 and culminated at an average water depth of 100 meters. This impoundment was followed 2.7 years later by the 2008 M_w7.9 Wenchuan Earthquake (WE), which ruptured the Longmen Shan Fault (LSF). The close proximity of reservoir impoundment and the WE in both space and time suggests that the events could be coupled. Previous studies, however, obtained split or even opposite results on the problem. Based on the fully-coupled poroelastic theory, we employ two-dimensional Finite Element Models (FEMs) to simulate the evolution of pore pressure and stress fields due to reservoir impoundment, and its effect on the Coulomb stress on the LSF. The results indicate that reservoir impoundment broke the balance of pore pressure in the region, and formed a pore-pressure front that slowly propagated through the crust with fluid diffusion. Due to diffusion of the pore pressure, the effective normal stress was steadily increasing, and the Coulomb stress change on fault could be reversed from negative to positive. The FEMs predict the positive Coulomb stress changes on the LSF growing from the upper crust to lower crust along with the growth of pore pressure. The Coulomb stresses on reported hypocenters increase by the range of a few to tens of kPa. The reservoir loading had increased the Coulomb stress on the shallow part of the LSF up to hundreds of kPa, significantly in favor of the failure of the LSF. It is still quite uncertain about the geometry and extent of the LSF at depth, we therefore calculate the Coulomb stress changes under a wide range of scenarios of hypocenter locations and fault dip angles. Our result shows that the Coulomb stress increases along with the dip angle. We conclude that the impoundment of ZR increased Coulomb stress loading of the LSF on the whole, and was capable of triggering earthquakes. The micro-seismicity around the ZR also showed an expanding pattern from the ZR since its impoundment, which could be associated with diffusion of positive pore pressure. These results suggest a poroelastic triggering effect of the WE due to the impoundment of the ZR.