Study On Reservoir-induced Seismicity In The Subsequent Project Of Danjiangkou Reservoir

The reservoir-induced seismicity (RIS) is formed due to impoundment, which changes the frequency and intensity of earthquakes at the reservoir area. Though the research of RIS has achieved a lot by the efforts of more than sixty years, there are problems to resolve. For example, now there is not a mature theory to explain the mechanism of RIS, and many researches belong to the qualitative analysis. The Danjiangkou reservoir is one of the built large-scale reservoirs at home, its subsequent project is the headstream for south-to-north water diversion project. The seismic activity in the reservoir area enhanced and the largest event with magnitude 4.7 occurred in Songwan Gorge on November 29, 1973. The intensity of the earthquake is 7, which heavily threatened the safeties of dam and inhabitant. Whether the new seimicity will occur is regarded after the water level rises in the subsequent project. So the research of Danjiangkou Reservoir induced seismicity is profound. The Paper aims at exploring the mechanism of RIS basd on Danjiankou reservoir, and evaluates the seismic activity of its subsequent project. Therefore, the researches include several factors as follows:1. Collect and analyze some materials on geology, and hydrology, and research the influences of geological and hydrological conditions on RIS. The study shows that the hydrological condition of Danjiang reservoir is very complex, which is helpful for water to 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.2. Analyze and compare the seismic characteristic after impoundment in the initial project of Danjiangkou reservoir. The data shows that the seismic activity in Danjiang reservoir became strong, while that in Hanjiang reservoir remained unchangeable. The water levels are correlated to seismicity. Hypocentrum move to the bank of reservoir, and form two seismic concentrated place-Linmaoshan 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.3. The visual database of Danjiankou reservoir induced seismicity is developed based on the Geography information system-Mapinfo. It is an important reference to the evaluation of earthquake, quakeproof and reducing disaster of reservoir area. The system can be installed in any computer whose operational system is windowa95/98/200, which can be expediently expanded by adding and modifying basic data. It has simple interactive interface that is help to operate.4. The study firstly sets up 3-D definite element models in terms of the seismotectonics of Danjiangkou reservoir area to calculate tectonic stress field, earth stress field and stress field of water pressure basing on the real imformation. In lights of the calculation of stress field of water pressure, water acts as triggering roles in RIS, and the roles include several respects as follows: change the stability of faults; form main tense stress regime in the bank of reservoir, which leads to the open of original cracks and makes it easy for water to permeate deeper places; decrease the strength of rock; Pore pressure decreases effective stress of fault panels, which correspondingly decreases the strength of fault.5. The tectonic stress field is researched in detail, the data shows that faults have important influence on distribute and direction of stress field. The stress of faults is lower than that of adjacent area. Stress in those places which are the ends of faults and where faults are about to intersect is concentrated. While it is lower in the point of intersection of faults.6. Completely analyze the stress field, strain field and strain energy at Danjiangkou reservoir area before and after impound. On the same time consider the stress field of water pressure to come up with dangerous places in the in subsequent project in terms of intensity. The results are valuable to the quakeproof in reservoir area.7. The geodynamic method is firstly used to evaluate the seismicity in the subsequent project of Danjiangkou reservoir. The results show that the possibility of RIS is very great after the water levels of reservoir rise. The biggest earthquake magnitude is 4.5.On the basis of the summary of the whole article, the study comes up with some problems to resolve and suggestions.