## 4 Gravity Field Changes and Seismicity during the Process of Water Impounding
### The 135 m Stage  
From 25 May 2003 to 19 September 2006, the period was the water impounding stage of 135 m. As the reservoir water level rose rapidly, the direct water load obviously had immediate gravity effects. Two regions in the study area, Badong and Zigui, both had a peaking positive anomaly of banded gravity along the river, covering a huge area. The latter had a positive anomaly reaching \(135\times 10^{-8}\) ms\({}^{-2}\) (Figure 6b). The visible expansion on both sides of the reservoir bank not only demonstrated the scope of the reservoir water's impact, but also showed the strengthening of the indirect gravity effect. As the reservoir water level stabilized near 135 m, a positive gravity anomaly of around \(30\times 10^{-8}\) ms\({}^{-2}\) appeared near Xiangxi, indicating an infiltration of the reservoir water in the area (Figure 6c). At the same time, the spatial consistency between the gravity field and the active fault worsened compared to before the impoundment, implying that the control effect of the active faults weakened in the short term.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Impounding Stages**} & \multirow{2}{*}{**Beginning and End Dates**} & \multicolumn{8}{c}{**Average Monthly Earthquake Frequency**} \\ \cline{3-10}  & & **0.0-0.9** & **1.0-1.9** & **2.0-2.9** & **3.0-3.9** & **4.0-4.9** & **5.0-5.9** & **All** \\ \hline Before the impoundment & 1 January 2001–24 May 2003 & 0.97 & 0.90 & 0.07 & 0.07 & 0.00 & 0.00 & 2.00 \\
135 m & 25 May 2003–19 September 2006 & 16.43 & 2.20 & 0.25 & 0.03 & 0.00 & 0.00 & 18.90 \\
156 m & 20 September 2006–27 September 2008 & 65.08 & 8.58 & 0.63 & 0.00 & 0.00 & 0.00 & 74.29 \\
175 m & 28 September 2008–31 December 2009 & 82.67 & 8.73 & 1.13 & 0.00 & 0.07 & 0.00 & 92.60 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Average monthly earthquake frequency in different magnitude grades of the study area and in different impoundment stages of TGR.  
Figure 6: Gravity field differential dynamic variation images in the study area during the impoundment of the Three Gorges Project (October 2001–October 2009). **(a)** October 2001–October 2002; **(b)** October 2002–October 2003; **(c)** October 2003–November 2004; **(d)** November 2004–October 2005; **(e)** October 2005–November 2006; **(f)** November 2006–November 2007; **(g)** November 2007–November 2008; **(h)** November 2008–October 2009.  
As shown in Figure 4 and Table 2, the time series of the reservoir water level and monthly earthquake frequency indicated that the seismic activity after the 135 m water level rose rapidly, and the monthly average earthquake frequency increased from 2 before the impoundment to 18.9 after. However, the added ones were mainly micro-earthquakes of magnitude 0.0-0.9. In terms of spatial distribution, the seismic activities during this period were predominately distributed in the areas within 10 km on both sides of the reservoir bank, such as Badong and Zigui (Figure 5b). The largest seismic event was the Badong earthquake of magnitude 3.5 on 22 September 2005. The focal depth of seismic activity at this stage was usually shallow, most of them within a range of 5 km. Some earthquakes that coincided with the distribution of mines and karsts along the river were even close to the surface [26]. The seismic activity b value at this stage was about 0.91 [10], which is significantly greater than 0.68, the value before the impoundment. It had obvious characteristics of reservoir-induced earthquakes. It is worth noting that the positive gravity anomaly area along the river forms a good spatial envelope for the seismic activity during the corresponding period, indicating that the direct reservoir water load has an important impact on the seismic activity.