## 6 Causing Mechanism of Seismicity during the Impoundment
### The Effect of Reservoir Water Infiltration  
After the 175 m water impounding stage, the Three Gorges Reservoir began the high-level periodical operation stage. Beside the periodical annual changes, the direct water load of the reservoir no longer increased significantly. The difference calculation of relative gravity at this stage mostly reflects the gravitational effect of faults caused by material migration and tectonic deformation. Figure 10 shows that Xiangxi is the tectonic intersection of the Xiannvshan faults and the Jiuwanxi fault. When the reservoir water  
Figure 8: Changes in the gravity field (October 2001–November 2008) and seismicity.  
level stays unchanged, there are still noticeable positive anomalies of gravity, implying that the reservoir has strong bottom water infiltration.  
Figure 10: Changes in the gravity field after November 2008 (November 2008–October 2009) and the focal mechanism of the Zigui M4.1 earthquake.  
Figure 9: The maximum shear–strain field at 500 m below the ground in the study area: (**a**) before the impoundment, (**b**) 135 m stage, (**c**) 156 m stage, and (**d**) 175 m stage.  
From the water pressure distribution of the pores at the A-A\({}^{\prime}\) section shown in Figure 11, it can be seen that, as the reservoir water level rises, the water pressure of the pores in the crust along the river demonstrates an increasing trend. The extreme values at the 135 m, 156 m, and 175 m stages are about 6 Mpa, 9 Mpa, and 12 Mpa, respectively. Significant local extremes of pore water pressure appeared around the structural intersection of the Xiannvshan faults, Jiuwanxi faults, and the Yangtze River (red box in Figure 11). A series of numerical simulations and the results are spatially consistent with the four-quadrant features of gravity anomalies, indicating that the reservoir water is rapidly infiltrating following the structure weakness near the faults' zone. At the same time, the maximum shear-strain field at the intersection of the structure also reached the historical peak value of 7.0, suggesting that the activity of the faults' zone has drastically increased (Figure 9d). It is worth noting that the Zigui M4.1 earthquake on 22 November 2008 occurred right at the center of the four quadrants of gravity anomalies near Xiangxi. It can be seen from the focal mechanism in Figure 10 that a relatively large tectonic earthquake was triggered after the 175 m water impounding stage, indicating that water infiltration and water-rock coupling in the reservoir began to trigger tectonic reservoir earthquakes.  
The mechanism of reservoir water infiltration triggering structural reservoir earthquake is as follows [26]: First, the reservoir water produces water pressure during the infiltration process, diffusing pores and reducing the normal stress, which in turn reduces the friction of the faults and causes earthquakes; second, the physical and chemical effects of the reservoir water on the faults or rocks, such as softening, lubrication, expansion, and stress corrosion, cause rock masses to destabilize and shake.  
Figure 11: Pore pressure of A-A\({}^{\prime}\) profile under different water levels: (**a**) 135 m stage, (**b**) 156 m stage, and (**c**) 175 m stage.