Research On The Influence Of The Three Gorges Dam On Seismicity In Western Sichuan

Since 2003, the Three Gorges Dam was begun filling and officially completed until 2006. It is a huge and remarkable project. As the Three Gorges Dam has huge reservoir capacity, it may induce earthquakes. Many scholars have explored to solve this problem. However, these studies were in connection with the reservoir area in view of the geological and seismographic point. In this paper, the author started from the seismic data themselves using the mathematical methods and studied outside the reservoir area’ laws from the perspective of statistical. As for this paper the author selected Western Sichuan to study the earthquake laws and the main work included the following aspects:1. This paper researched, analyzed, concluded the characteristics of Western Sichuan earthquakes, and there were the followings:(1) 18 years after the construction of the Three Gorges Dam over the previous 18 years in Western Sichuan earthquake frequency has increased significantly.(2) Most of Western Sichuan earthquakes dispersed distribution before the impoundment of the Three Gorges Dam. In the period of the low water impoundment earthquakes were mainly distributed around the Anning River fault zone, and a small amount of earthquakes were distributed near the Xianshui River fault zone. As the water level continued to rise, earthquakes were concentrated in the vicinity of the Longmen Shan fault zone, and presented the trend towards Anning River fault zone.(3) With the water level rising, earthquakes of Yangtze River gathered to Western Sichuan region.(4) In each of the Three Gorges Dam impoundment period the average earthquake focal depth was smaller than before impounding, and the change was more obvious during high water level.(5) In the high water level of the Three Gorges Dam energy released by earthquakes was higher than in the low water level.2. Using Fourier transform techniques verified the similarity on the shape of the two curves between Three Gorges Dam water level and frequency of earthquakes in Western Sichuan from the perspective of the frequency domain and using the correlation coefficient of both verified whether there was a correlation from the statistical point. Conclusions indicated that some degree of correlation existed between both.3. From the perspective of the gravity field variation in Western Sichuan region, the paper provided a corresponding explanation on the correlation and the law of earthquake accordingly. With the Three Gorges Dam impounding and draining, the gravity field in Western Sichuan region showed positive changes and negative changes. At the beginning of the water level rising, gravity field variation was not obvious and the gravity field change gradually increased with the extension of time. So the dam water level change affected Western Sichuan regional gravity field. However dynamic changes of gravity field can reflect material change and deformation process in the interior of the earth and there are close ties between dynamic changes of gravity field and bred earthquake in the depths of the earth’s crust. So the Three Gorges Dam would indirectly affect Western Sichuan seismic activity by influencing the change of gravity field in Western Sichuan region.4. Finally, in combination with the Three Gorges Dam itself, the paper analyzed the causes of the Three Gorges Dam impacting on Western Sichuan seismic activity. The paper drew a conclusion—the construction of the Three Gorges Dam influenced on Western Sichuan seismic activity to some extent.