| Reservoir-induced seismicity (RIS), which is generated in the process of reservoir storage or drainage, is a special kind of earthquakes. Because these shocks generally occur near important water conservancy establishments and commonly with low focal depth and high seismic intensity, sometimes they can cause very big destructiveness, and may cause personnel casualties and damage of the dams and buildings nearby. Therefore RIS is not only an important study of the water conservancy and hydropower project field, but also one of important contents in seismology, regional geological stability and environmental engineering geology.After several reservoir-induced earthquakes with M≥6 occurred one after another in the world in the 1960s, RIS had quickly drawn the society’s widespread attention, especially from the 1980s, it has gradually become one of the focused and forward-looking topics in the geoscience field. Through half a century’s exploratory study, many a progress have been achieved in such aspects as seismological characteristics, conditions of geological structure being prone to induce seismic activities, mechanism, forecasting and assessing methods for RIS, and so on. While RIS is so complex that there are still a lot of difficult problems at present. For instance, previous studies mostly focused on summing up commonness and rules of RIS from statistical data. Although this method is practical, it still can not explain the origin mechanism of RIS fundamentally. Furthermore, several representative reservoirs studied in relatively high degree were all in extensional or shearing tectonic environments, there has still been short of the detailedly studied example in the compressional tectonic stress environment. Therefore, it needs to break the limitation of statistical analysis, and choose some typical reservoirs which are relatively rich in basic data and have relatively perfect reservoir earthquake monitoring station networks, utilizing multi-disciplinary methods to conduct synthetic study in multi-aspects, to explore the generation mechanism and main controlling factors of RIS, to enrich RIS system info. Presently, although there have been some qualitative understandings of RIS mechanism, it is still not clear what is the dynamic response mechanism of the RIS activities in space-time that responds the process of reservoir water body load-unloading and additional water pressure diffusion. If some revelations can be obtained from qualitative analysis to quantitative study in this aspect, certainly, it will be an innovative work for RIS mechanism’s exploration. In the quantitative study, mainly based on the semi-infinitely elastic medium supposition, the previous work has discussed the static response problems for fault Coulomb stress changes responding to reservoir water body loading and additional water pressure diffusion by using the Green’s function and the pore pressure diffusion equation to solve the stress field and fluid pressure field. Although this method is very convenient, it has a very big limitation when processing complex media condition. At the same time, because they mostly used simplified geological models, and neglected the important influence of those, including complexities of geologic structure and hydrogeology structure, asymmetries of rock-mass’s mechanical properties and permeability, they can hardly quantitatively analyze the small-medium RIS occurring in surrounding rock-mass.According to the above problems, this thesis focuses on the study of evolution characteristics in space-time and dynamic response mechanism of RIS, systematically collects the newest data of global RIS cases, and analyzes the basic features of them in spatial distribution and time response. Taking the Zipingpu reservoir for an example, which is in the compressional tectonic stress environment, this work has studied the geological structure and hydrogeological conditions of the reservoir area, characteristics of swarm earthquake evolution in space-time after reservoir storage impounding, and the relationship between the distribution of swarm earthquakes and the local geological structure. According to the advanced theory of rock-mass deformation and fluid flow coupling and fault stability and using the finite elemental method, the elastic additional stress field, effective additional stress field, pore pressure and fault stability during the process of reservoir storage impounding were calculated. The major induced factors of RIS and the relationship between the evolvement of RIS in space-time and the process of reservoir water body load-unloading and water infiltration were studied. Combined with previous study, the possible relationship between Zipingpu reservoir storage impounding and the Wenchuan earthquake was attempted to be discussed. This study will be not only helpful for enriching the theoretical system of structure fluid dynamics and broadening thinkings of RIS mechanism research, but also have an important reference value for regional stability evaluation and RIS forecast.1.Main research contents and results(1) Analysis on basic characteristics of spatial distribution and time response of global RISⅰ. In the spatial distribution, RIS mainly concentrated in the range of 10 km away from the reservoir margin and confined to a particular segment of the reservoir. When a certain condition of geological structure and hydrogeological conditions are met in the reservoir region, the activities of RIS will migrate with the changes of water level during water storage and drainage. The types of migration have been summed up in this thesis.ⅱ. The time responses of RIS are different from region to region and from reservoir to reservoir. In general, RIS’s initial response is dominant when the reservoir is filled less than 1 year, while most of the main earthquakes of RIS occur in more than 1 year after the reservoir initial storage. The“mixture response”is the main form of RIS activities for the long process of reservoir storage, while the“rapid response”and“delayed response”are only a periodic phenomenon. RIS response has very complex and diverse relationships with the reservoir water level changes, which can be positive correlation or negative correlation, and higher magnitude of RIS shows certain dependence on the rate of water level changes.ⅲ. RIS spatial and temporal distribution is controlled by the existing internal causes including local terrain features, geological structure, lithologic and hydrogeological conditions in the reservoir area. In fact, all these causes that influenc the spatial and temporal distribution of RIS are interdependent and dialectical united and can not be completely separated.(2) Establishment of the mathematical model for the quantitative study of RIS and improvement of the finite element solving procedureⅰ.The incidence of RIS is not only in connection with pre-existing large-scale faults under the reservoir area, but also controlled by the combination form of faults and their surrounding rock-mass, rock-mass composition, and heterogeneity of mechanical properties and penetrating quality reflected by the lithologic changes. Based on this understanding, a 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 by a solid-liquid coupling modal. This model can reflect the heterogeneity of mechanical properties and penetrating quality in surrounding rock-masses and their deformation differences in response to the process of reservoir water body loading and unloading. 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 change of Coulomb failure stress on fault plane. The combination of two forms of modeling approach provides a comparatively macro-mechanical framework for RIS quantitative study.ⅱ. According to Galerkin’s finite element theory, the“weak integral”form of finite element formulation has been deduced, and a calculation program has been compiled utilizing the finite element program automatically generating software FEPG. An attempt is made to solve the difficult problem of dealing with complex media conditions in quantitative studies.(3) Construction of a two-dimensional geological model based on analysis of the geological structure and hydrogeological conditions in the Zipingpu reservoir and adjacent regionsⅰ. The Zipingpu reservoir is located in the middle of the Longmenshan orogenic belt in the eastern edge of the Qinghai-Tibet plateau. Five major faults including the Maoxian-Wenchuan, Beichuan-Yingxiu, Tongjichang, Anxian-Guanxian and Guangyuan-Dayi controls the basic structure frame of the study area., The main faults near the reservoir area can provide a leak path for the surface water flowing into the deep crust to various degrees. At deep depths of these faults, there may be a new permeability structure type, which is water-conductive in hanging wall strata and water-resisting in footwall strata. This fault permeability structure has an important influence on the mechanical response under a changing pore pressure.ⅱ. In the geological tectonic units, the Zipingpu reservoir lies in the frontal detachment zone of the Longmenshan system. In the vertical direction, the detachment zone is separated by the sedimentary formation of the Leikoupo group and Jialingjiang group of Triassic as a slide plane. The formation composition, tectonic deformation degrees and styles are significantly different between the upper part and lower part.ⅲThe cause of induced seismic activities depending on the lithologic conditions is mainly influenced by the difference of permeability stability of rock-mass. According to Gu Dezhen’s classification standard, the rock-mass structure in the studied area can be divided into three types, which are integral structure, layered structure and unconsolidated structure, and the permeability stability of rock-mass can be divided into 3 types which are high, medium and low.ⅳ. Based on previous research and combined with the theory and methods of comparative tectonics and analytical tectonics, a two-dimensional geological model has been built up by analyzing the structural characteristics in upper and lower formations in the study area.(4) Evolution characteristics of RIS in space-time during the process of reservoir storage impounding and the relationship between small earthquake distribution and geological structure and permeability stability of rock-mass in the Zipingpu reservoir areaⅰ. From 1970, when small earthquakes could be relatively reliably monitored in the study area, to the time of Zipingpu reservoir storage impounding, the middle-small earthquakes taking place in Zipingpu reservoir area and its adjacent regions were scattered in spatial distribution, and also did not show obviously cyclical variation in time distribution.ⅱ. Making use of the accurate seismic data which were relocated by the double difference location method from August 16, 2004 to May 10, 2008 recorded by the Zipingpu reservoir network, the characteristics of swarm earthquake evolution in space-time after the reservoir storage impounding have been analyzed and found that: (ⅰ) After Zipingpu reservoir storage impounding, the activity of small earthquakes in the spatial distribution exhibited banded or cluster distribution characteristics. Most epicenters were concentrated within 10 km range away from the reservoir margin, and predominantly distributed in NE direction parallel to the main structure line. The small earthquakes were clustered in the northeast and southwest of the reservoir, and also in 9~18 km downstream of the dam site which is adjacent to Xingfu, Zhongxing and Juyuan town of Dujiangyan city. While in the reservoir water body covered area, there was little earthquakes. The small earthquakes also shows characteristics of seismic migration accompanying with water level changes with reservoir’s being loaded or unloaded.(ⅱ) After Zipingpu reservoir storage impounding, the source depths of small earthquakes were preponderantly distributed in the range of 4~10 km under the ground, and the deep pooling area of the 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 little earthquakes occurred in the Triassic Xujiahe sand mudstone and coal measure strata which were relatively weak in lithology and high in infiltration stability.ⅲ. For the responding speed of RIS, the small earthquakes in the southwest and northeast seismic cluster area of the reservoir maybe belong to“rapid responsing type”, and the Dujiangyan small seismic swarm maybe fall into“delay responsive”induced seismicity.(5) Major induced factors of RIS and the relationship between the evolution of RIS in space-time and the process of reservoir water body load-unloading and water infiltrationⅰ. RIS is closely related to the change of effective additional stress in rock-masses under the reservoir body. In dominant compressional tectonic stress environment, the result of reservoir water body loading tends to make the faults more stable, while the effect of additional water pressure diffusion tends to promote instability of the faults. It is the dynamic process, in which two contradiction sides restrict and balance each other, that controls the orientation of Coulomb failure stress changes, and determines the characteristics of RIS evolution. The characteristics of small earthquakes after Zipingpu reservoir storage impounding also indicates that the evolution of RIS in space-time is controlled by the dynamic process ofΔCFS.ⅱ. The inducing conditions and factors of RIS are very complicated, some of which are connected with endogenic geological process of the crust, and others closely related to exogenic process. Among those factors which probably affect RIS’s pregnancy and occurrence, the precondition is accumulated elastic strain energy in a seismogenic zone having reached critical state. the principal internal controlling factors are features of fault permeability structure and permeability stability of rock-mass in the reservoir area, and the change scope of water level is the importantly external condition which can directly influence RIS. When the seismic pregnant process having reached critical state, it is the interaction between inducing factors and tectonic dynamic factors that impells the process of RIS’s pregnancy and occurrence.The study has also indicated that because of differences in tectonic stress environment, seismogenic structure scale, and aeolotropy of rock-mass, the critical states of seismogenic structure are different too. According to that, the objective phenomenon that there is relatively high occurrence probability of RIS in a seismic zone of weak-middle activity can be reasonably interpreted.ⅲ. There is a mutually stimulative and restrictive relationship between pore presses diffusion and RIS. The microseisms in the initial period of induced seismic sequence were not only a response to the reservoir storage, but also one kind of feedback factor, and created conditions for the large-scale release of elastic strain energy. (6) Possible correlation between the Zipingpu reservoir storge impounding and the“5.12”Wenchuan earthquakeThe possible correlation between the Zipingpu reservoir storge impounding and the“5.12”Wenchuan M8.0 earthquake has been discussed tentatively based on the analysis, including the lower limit depth of ground surface water’s seepage, the P wave velocity abnormlies in the reservoir area and its vicinities after storage impounding, the differences in characteristics of seismic sequence between the Wenchuan M8.0 earhquake and representative RIS,ΔCFS near the epicenter on the assumption of elastic media condition, and so on. The analysis suggests that there is no direct evidence to prove the obvious relationship between the Zipingpu reservoir storage impounding and the“5.12”Wenchuan earthquake. The nuclear region of this great shock located in the brittle-ductile transition zone about 19 km under the ground, and scientists have realized little about the changes of fault permeability structure, state and transportation characteristics of the liquid at that depth. Therefore, a more comprehensive understanding about this relationship will rely on unprecedented progress in the above research fields.2. Main progresses of this study(1) Multi-disciplinary methods have been synthesized, such as geology, seismology, mechanics, and so on, and integral and forward-looking studying characteristics has also been incarnated. The thesis will provide a typical model for RIS synthetic study in the future.(2) It has been discovered that the small earthquakes mainly occurred in the carbonate rock-masses with high brittle lithology and low infiltration stability, but there were little earthquakes occurred in the Triassic Xujiahe sand mudstone and coal measure strata which are relatively weak in lithology and high in infiltration stability. RIS’s depending on permeable stability of rock-mass has been further proved.(3) The incidence of RIS is not only in connection with pre-existing large-scale faults under the reservoir area, but also controlled by the combination form of faults and their surrounding rock-mass, rock-mass composition, and heterogeneity of mechanical properties and penetrating quality reflected by the lithologic changes. Based on this understanding, a mathematical model for RIS is divided into two levels in this paper, the combination of two forms of modeling approach provides a comparatively macro-mechanical framework for RIS quantitative study. An attempt is made to solve the difficult problem of dealing with complex media conditions in quantitative studies.(4) Based on the coupling theory of solid deformation and liquid seepage and fault stability theory, the dynamic changes of elastic additional stress field, effective additional stress field, pore pressure and fault stability have been calculated and analyzed. Major induced factors and the possibile causes for RIS ocuuring after the Zipingpu resevior impounding have been discovered. It has been approved futher that In dominant compressional tectonic stress environment, the result of reservoir water body loading tends to make the faults more stable, while the effect of additional water pressure diffusion tends to promote instability of the faults. It is the dynamic process, in which two contradiction sides restrict and balance each other, that controls the orientation of Coulomb failure stress changes, and determines the characteristics of RIS evolution. |
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