## 4 Gravity Field Changes and Seismicity during the Process of Water Impounding
### The 175 m Experimental Stage  
After September 2008, the Three Gorges Project entered the 175 m experimental impoundment stage. The reservoir water level continued to rise and reached the high point of above 172 m for the first time in December 2008. The large-scale positive anomaly of gravity along the river appeared again in the study area, especially around the Xiangxi reservoir segment of the Yangtze River, with a positive anomaly extreme value of about \(150\times 10^{-8}\) ms\({}^{-2}\) (Figure 6h). The seismic activity still continued the transient response characteristics of the 156 m impoundment stage (Figure 6b), further extending to both sides of the reservoir bank with the Yangtze River as the axis. After the reservoir water level rose to 175 m, which is the high-level operation stage, the annual periodic change of the reservoir water level fluctuated between 30 and 40 m (Figure 4). The gravity field and seismic activity of the reservoir area began to show more complex dynamic changes. As shown in Figure 6g, there was a noticeable banded negative anomaly of gravity in the area along the Yangtze River. The negative anomaly extreme values were about \(-40\times 10^{-8}\) ms\({}^{-2}\), appearing near Taipingxi of Badong and Zigui because the direct reservoir water load decreased due to the drop in the water level from November 2008 to October 2009. At the same time, it was clear that there was a positive anomaly of gravity on the north and south sides of the Xiangxi reservoir segment of the Yangtze River. Luoquanhuang of Zigui recorded an extreme positive anomaly value of about \(40\times 10^{-8}\) ms\({}^{-2}\). There is a high gradient characteristic of excessive positive and negative anomalies near Xiangxi, which is in the center area of the four quadrants. The Xiannvshan faults and the Jiuwanxi faults are once again consistent with the gravity field, suggesting that the former is starting to control the latter. Moderate and strong earthquakes are prone to occur in the central area of the four quadrants of changing gravity related to tectonic activities or in the high gradient zone of the transition between positive and negative anomalies [16]. The Xiangxi segment of the reservoir, a triangular area where the Xiannvshan faults and Jiuwanxi faults meet the Yangtze River, may be at risk of having moderate earthquakes on the periphery of this area. When the reservoir water level reached 172 m for the first time, the Zigui M4.1 earthquake erupted in November 2008, which was the first earthquake with a magnitude of M4.0 or more since the impoundment of the TGR. Time Series Correlation Test of Seismic Activity and Reservoir Water Level Based on Reshuffling Tests  
The above analysis of seismic activity and gravity field differentiation in the water impounding process indicates that there is a clear correlation between the impoundment of the Three Gorges Project and seismic activity. To evaluate this correlation and give quantitative evidence, we used the cross-correlation and reshuffling tests to give the time series correlation between the reservoir water level and the monthly frequency of earthquakes. Figure 7 shows the cross-correlation coefficients and 95% confidence threshold under different time lag conditions. Figure 7: Correlation test between monthly frequency of seismicity and reservoir water level in the study area during the impoundment of the Three Gorges Project (January 2001–December 2009). (**a**) Before the impoundment; (**b**) 135 m stage; (**c**) 156 m stage; (**d**) 175 m stage. Since the beginning of water impounding in May 2003, the water level of the TGR had gone through the stages of 135 m, 156, and 175 m. The correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage (Figure 7a) to correlated when the time lag was 0 months at a 95% confidence threshold (Figure 7b-d), indicating that the continuous rise of the reservoir water level has a decisive effect on the seismic activity.