| The key water control project for Danjiangkou is a large irrigation works to develop Hanjiang. The government allows developing it in two stages. The first stage project had been finished in 1973, and the seismic activities have become stronger since the complement of the project. The second stage project is about to be carried out recently which is the resource project of dispatching south water to the north. In the project, the water level will lift 17 meters, which will maybe make it possible to trigger bigger earthquake. The goal of researches which is the main content of the project of the educational ministry "the plan of aiding financially core teachers in institutes", is to use the three-dimension finite element models to quantitatively research stress field in Danjiangkou reservoir area , and to make some conclusions about the mechanisms of RIS, and to predict the possibility, space and intensity of RIS in the second stage project. Therefore, the researches include several factors as follows:1. Collect and sort some materials on geology, hydrology and geophysics. The study shows that geological and hydrological conditions have important influences on RIS. The hydrological condition of Danjiang reservoir is very complex, where carbonate rock distribute in the canyon areas, and a series of funnels are formed. At the same time watertight rock surround the carbonate rock, which is helpful to water percolate deeply and trigger earthquake. However, Hanjiang reservoir consists of watertight rock and has simple geological condition, so it is difficult for water to percolate deeply and to induce reservoir seicmicity. Though there are large fault both in Danjiang segment and Hanjiang segment, yet the seismic activity is very different in the two areas for different geological and hydrological conditions. Therefore, the seismic activity concentrates on Danjiang reservoir segment while Hanjiang reservoir segment becomes more stable after storing water.2. Analyze and compare the changes of time, space and intensity of seismic activities in reservoir area before and after storing water. The data show that the seismic activity in Danjiang reservoir became strong , while that in Hanjiang reservoir remained unchangeable. The water levels is correlated to seismicity. Hypocentrum move to the top of reservoir, and form two seismic concentrated place-LinmAl2O3han and Songwan. However, the correlated property gradually disappear and seismic activity decrease when water levels come to the highest levels of the fist stage project and correspondingly trigger earthquake with 4.7 magnitude. Reservoir impoundment do not change collecting energy level of reservoir, while make the strain energy release ahead of time according to the calculation. So the largest reservoir seismicity will not exceedthe historical levels.3. Former many mathematic models is based on panel, so the result cannot completely response the properties of stress field and strain field. The study sets up 3-D definite element models in terms of the seismotectonics of Danjiangkou reservoir area. The models use Drucker-Prager yielded criterion to stimulate material property, which consider the influences of average stress and the second main stress on material strength. It use three border conditions to calculate tectonic stress field, earth stress field and stress field of water pressure, which will be helpful to quantitatively research RIS.4. The data show that faults destruct uniform distribute of stress and strain, and faults are concentrated places of stress according to results of stress field, earth stress field and stress field of water pressure. In lights of the calculation of stress field of water pressure , the stress caused by water is utterly small in contrast to earth stress field, so it cannot directly induce large reservoir. Water acts as triggering roles in RIS, and the roles include several respects as follows: (1) change the stability of faults. According to calculation, Danjiang fault takes place relative s... |
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