摘要
本文以龙滩水库为例,根据库区地质构造、深部速度结构及数字地面高程,建立了库区三维有限元模型,基于孔隙弹性理论计算了水库蓄水过程中库底断层和围岩体孔隙压力、有效附加正应力、剪应力和库伦应力的动态变化,并结合水库蓄水后库区地震活动时空分布的特征,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及其可能的成因机制.结果表明：（1）龙滩水库蓄水后地震活动呈现出明显的丛集性,主要分布在罗妥（丛Ⅰ）、八茂（丛Ⅱ）、拉浪（丛Ⅲ）、坝首（丛Ⅳ）和布柳河（丛Ⅴ）5个水库蓄水后淹没的深水区,这些区域也恰恰是库水加卸载及渗透过程中△CFS增加最明显的区域,而△CFS的影区几乎没有地震发生,表明水库蓄水后库区地震活动与△CFS的变化密切相关.（2）在水库蓄水过程中,与水库有直接水力联系且渗透性较好的断裂成为地表水体附加水头压力向深部扩散的优势通道,沿此通道附加水头压力扩散的最大深度达13 km左右,震旦系一古生界以碳酸盐岩为主的地层成为附加水头压力扩散的主体层位,这与蓄水后库区中、小地震震源深度均小于13 km,且优势分布在5~10 km的特征相吻合,表明由于孔隙压力的存在降低了岩石的抗剪强度,同时部分抵消了围压的影响,致使该层位的岩体易于产生脆性破坏从而诱发地震活动.（3）无论是深部还是浅部,各丛地震密集发生的时段绝大部分与相应深度ACFS加速升高或阶段性高值时段相重叠,可能说明在库水位快速抬升或阶段性高值时段,受外部荷载加载速率快速升高的影响,库底岩体和断层、裂隙等结构面更容易实现失稳扩展;深、浅部地震响应时间、活动频度和强度的差异可能与不同层位岩体力学性质及渗透性能的不均匀性有关.（4）各丛地震诱发的物理力学机制有所不同.丛Ⅰ、丛Ⅱ、丛Ⅲ地震的诱发可能与库体重力荷载、孔隙压力扩散和库水浸润弱化3种作用都有关;丛Ⅳ地震的诱发主要受控于库体重力荷载作用,孔隙压力扩散和库水浸润弱化不起主导作用;丛V地震的诱发主要受孔隙压力扩散和库水浸润弱化作用的影响,库体重力荷载作用一定程度上抑制了地震的发生.
In order to know the dynamic response mechanism of reservoir-induced seismicity（RIS）,this paper has established the quantitative mathematical model and taken the Longtan reservoir as an example,established a 3D-poreelastic finite element model according to geologic structure,deep velocity structure and digital elevation of the reservoir area,calculated the dynamic changes of pore pressure,effective additional normal stress,shear stress and Coulomb failure stress in the faults and rocks at bottom of the reservoir during the process of reservoir storage impounding.Combined with the characteristics of seismic activity in the temporal and spatial distribution of the reservoir area after impoundment,the relationship between the evolution of RIS in space-time and the process of reservoir water body load-unloading and water infiltration,and their possible mechanism have been discussed.Our study shows that：（1）After impoundment of the reservoir in September 30,2006,seismic activity level around the Longtan reservoir obviously enhanced,and earthquakes mainly concentrated in 5 flooded deepwater areas,that are Luotuo（Cluster Ⅰ）,Bamao（Cluster Ⅱ）,Lalang（Cluster Ⅲ）,Dam head（Cluster Ⅳ）and Buliu River（Cluster V）earthquake clusters.The above areas were also the most obviously increasing areas of △CFS during the process of reservoir water body load-unloading and water infiltration,but there were seldom earthquakes in the shadow area of △CFS.It indicates that there exists closely relationship between variation of △CFS and reservoir seismic activities after the impoundment of the Longtan reservoir.（2）Numerical simulation results show that those faults,which have better permeability and directly hydraulic connection with the reservoir,are the superior channels for additional water head pressure diffusion,through those channels,the affection of additional head pressure diffusion can almost reaches deep into 13 km under the ground,and those strata dominated by carbonate from Sinian to Paleozoic become main layer to receive the head pressure.The above results are consistent with the characteristics that focal depths around the Longtan reservoir have been totally less than 13 km and dominantly distributed in 5~10 km since impoundment.It indicates that pore pressure reduces the shear strength of rocks and partially offsets the effect of confining pressure,so rock mass at those layer are prone to induce seismicity.（3）Whether it is deep or shallow,there exists the vast majority of time overlaps between the intervals of earthquake occrence frequently and the intervals of △CFS rising acceleratedly or in high value stage.This maybe indicate that,being influenced by the rapid increase of external loading rate,rocks and faults at the bottom of the reservoir are more prone to become unstable.Meanwhile,the difference of seismic responding time,frequency and magnitude between the deep and the shallow may be related to inhomogeneity of mechanical properties and permeability in different strata.（4）There exists different physical and mechanical mechanism of RIS in the Longtan reservoir：Cluster Ⅰ,Ⅱ,Ⅲ maybe have something to do with the effect of reservoir body gravity loading,pore pressure diffusing and water infiltration weakening;ClusterⅣ are mainly controlled by the action of reservoir body gravity loading,and the pore pressure diffusion and water infiltration weakening action does not play a leading role;Cluster Ⅴ is mainly affected by the pore pressure diffusion and water infiltration weakening effect,the body gravity loading effect maybe hold down the seismicity in a certain extent.
出处
《地球物理学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第9期2846-2868,共23页
Chinese Journal of Geophysics
基金
地震科技星火计划(XH12035)
广西科技攻关计划(1298005-2
1377002
12426001)项目资助
关键词
龙滩水库
水库诱发地震
孔隙弹性
有限元
数值模拟
Longtan reservoir
Reservoir-induced seismicity
Pore elastic
Finite element
Numerical simulation
