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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,Mw7.9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.
The Zipingpu Reservoir near the Longmen Shan fault zone in western Sichuan Province began to store water in September 2005. After about 2.7 years, on May 12, 2008, the Mw7.9 Wenchuan earthquake occurred on the Longmenshan fault zone and the two events Which are close to each other in time and space, reveal that they may be related to each other, but many previous studies give different or even opposite results.In this study, based on the completely coupled pore-elastic theory, two-dimensional finite element model (FEM) Based on the evolution of Coulomb stress evolution, the influence of the pore pressure and stress field on the pore water pressure and stress field in the Longmen Shan fault zone is discussed. The simulation results show that the water pressure in the Zipingpu Reservoir breaks the original regional pore pressure balance and forms Pore pressure gradient source to the surrounding crust; which in turn causes the positive range of Coulomb stress on the Longmenshan fault zone to continuously expand, affecting the whole fault from shallow to deep, especially on the shallow layer, reaching up to 100 kPa, Which provided the foundation for the instability of the fault plane.The Coulomb stress of the source region continued to increase, and it increased from about several thousands of Pascals to several tens of Pascals before the Wenchuan Earthquake. Even though the initial Coulomb stress was negative, Coulomb stress may still change from negative to positive at some point in the future, and the Coulomb stress increases with the increase of fault dip by calculating the Coulomb stress corresponding to various probable faults within the Wenchuan earthquake and its vicinity. According to the analysis of small earthquakes around Zipingpu reservoir area, the small earthquakes increased obviously after water storage, and the earthquakes in different periods The distance between the dense area and the reservoir gradually increases, which is consistent with the trend of pore pressure diffusion.The above results show that the water storage in Zipingpu Reservoir increases the risk of the Wenchuan earthquake.