水库诱发地震时空演化与库水加卸载及渗透过程的关系——以紫坪铺水库为例

Relationship between the evolution of reservoir-induced seismicity in space-time and the process of reservoir water body load-unloading and water infiltration-a case study of Zipingpu reservoir

为了解水库蓄水过程中,水库诱发地震活动的动态响应机制,本文建立了针对水库诱发地震(Reservoir-induced seismicity,RIS)定量化研究的数理模型,并以紫坪铺水库为例,对库区地质构造及水文地质结构条件、水库蓄水后库区小震活动时空演化特征进行了详细的研究.在此基础上,利用有限元方法计算了水库蓄水过程中弹性附加应力场、有效附加应力场、孔隙压力和断层稳定性的动态变化,讨论了RIS时空演化与库水加卸载及渗透过程的动态响应关系.结果表明:(1)RIS诱发机制的定量化模型可分为2个层次:一是以孔隙介质为载体的流体渗流对岩体变形和稳定性的影响,由流-固耦合形式的岩体变形与孔隙渗流模型进行描述;二是对断层相关的RIS定量研究可将水库附加水头压力沿断层面(区)的扩散与断层库仑应力变化联系起来.两种形式模型方法的结合能为RIS定量研究提供一个相对宏观的力学框架;(2)断裂渗透结构对孔隙压力变化下断裂的力学响应具有重要的影响,研究区主干断裂可能属于一种上盘破碎带导水、下盘地层及断层核阻水的"下阻上导型"的渗透结构类型,不同程度的具有使地表水体向深部渗流的通道性.库区深部岩体渗透稳定性的差异在很大程度上导致了诱发地震活动对岩性条件的依赖.(3)紫坪铺水库蓄水后,小震活动在空间分布上呈现出条带状分布、丛集分布和地震迁移的特点,小震震源深度优势分布在地下4～10km范围内,在通济场断裂与安县—灌县断裂的深部汇聚区域震源分布最为密集.同时,小震活动主要集中发生在脆性程度高、渗透稳定性低的碳酸盐岩地层中,而在岩性较软弱、渗透稳定性高的三叠系须家河组砂泥岩和煤系地层中很少有地震发生.在水库蓄水后地震活动的时间响应特征上,水库西南侧和东北侧两个丛集区的小震活动可能属于"快速响应型"RIS,而都江堰小震群活动可能属于"滞后响应型"诱发地震活动;(4)RIS的发生与库水加卸载及渗透过程中库底岩体有效应力的变化密切相关.在以挤压为主的构造应力环境中,库体荷载作用的结果一般会使库底断层更趋向稳定,而水库附加水头压力扩散的效应则是促使断层趋向失稳,正是这个矛盾双方相互制约与平衡的动态过程,控制了断层库仑应力变化的取向,从而决定了RIS时空演化的规律.

In order to know the dynamic response mechanism of reservoir-induced seismicity （RIS） during the process of reservoir storage impounding, this paper established a quantitative mathematical model for RIS study, and took Zipingpu reservoir as an example, detailedly studied the geological structures and hydrogeologic conditions in the reservoir area, analyzed RIS evolution characteristics in space-time. On the basis of those works, we calculated the dynamic changes of elastic additional stress field, effective additional stress field, pore pressure and fault stability based on finite element method; the relationship between the evolution of RIS in space- time and the process of reservoir water body loading-unloading and water infiltration was discussed. Our study shows that： （1） The mathematical model for RIS is divided into two levels in this paper： One is the rock-mass distortion and stability influenced by liquid seepage in porous rock media, which is described in solid-liquid coupling model; the other is a fault-related quantitative model for RIS, which is described by the linkage between additional reservoir water pressure diffusion along the faults and the change of Coulomb failure stress on fault plane. The combination of two forms of modeling approach will provide a comparatively macro-mechanical framework for RIS quantitative study; （2） Fault permeability structure has an important influence on mechanical response of faults under a changing pore pressure. At deep depths of these faults, there may be a new type of permeability structure for the main faults in the study area, which is water-conductive in hanging wall strata and water-resisting in footwall strata. The difference of infiltration stability of the deep rock-masses brings on the RIS depending on [ithology condition; （3） After Zipingpu reservoir storage impounding, the small earthquakes showed the characteristics of banded distribution, cluster distribution, and seismic migration; the source depth of small earthquakes preponderantly distributed in the depth range of 4~ 10 kin, and the activity in the deep pooling area of Tongjichang fault and Anxian-Guanxian fault was most intensive. Meanwhile, the small earthquakes mainly occurred in the carbonate rock-masses with high brittle lithology and low infiltration stability, but there were few earthquakes in the Triassic Xujiahe sand mudstone and coal measure strata which was relatively weak in lithology and high in infihration stability. In terms of response speed, the small earthquakes in the southwest and northeast seismic cluster area of the reservoir may belong to ＂rapid response type＂, and the Dujiangyan small seismic swarm may fall into ＂delayed response＂ induced seismicity; （4） RIS was closely related to changes of effective additional stress in the rock-mass under the reservoir body. In the mainly compressional tectonic stress environment, the result of reservoir water body loading commonly trends to enhance fault stability, while the effect of additional water pressure diffusion tends to promote fault instability. It is a dynamic process, in which two contradicting effects restrict and balance each other, that controls the orientation of Coulomb failure stress change, and determines the eharacteristies of RIS evolution.