## 7 Conclusions  
Using the test-retest data of the relative gravity field and the earthquake monitoring catalog of the TGR, this paper systematically analyzed the changes in the gravity field in the head area of the reservoir and the temporal and spatial distribution characteristics of seismic activity during the impoundment process. It also employed the cross-correlation and reshuffling tests to calculate the cross-correlation between the reservoir water level and the seismic activity sequence and discussed the influence of the rising reservoir water level on the gravity field and seismic activity. By constructing a three-dimensional finite-difference model based on the theory of fluid-solid coupling, the forming mechanism of the Three Gorges Reservoir earthquake was discussed from the aspect of direct reservoir water load and reservoir water seepage. The result showed the following:  
1. The rising reservoir water level has had a crucial impact on the gravity field and seismic activity in the reservoir's head area. The cumulative changes in the gravity field from October 2001 to November 2008 show that water impounding has led to a huge banded positive anomaly of gravity that reaches \(450\times 10^{-8}\) ms\({}^{-2}\) along the river near Xiangxi. The seismicity activity dominated by micro-earthquakes rose rapidly after the 135 m water level, and the monthly average earthquake frequency increased from 2.00 before the impoundment to 92.60 after the 175 m stage. The massive amount of water load caused the study area to have a positive anomaly of banded gravity along the river, which is centered on Badong county and Xiangxi Estuary of Zigui county. It also formed a good spatial envelope due to the sudden increase in seismic activity during the corresponding periods, which means that the variation of gravity can reveal the influence range of direct reservoir water load. 2. From the beginning of the impoundment to the experimental impoundment stage of 175 m, the time series correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage to correlated when the time lag was 0 months at a 95% confidence threshold. Further gravity observations and numerical simulation results showed that the seismic activity obviously has a direct relationship with the load pressure produced by the rapid rise of the reservoir water level, which causes the instability of the mines, karst caves, shallow rock strata, and faults within 10 km along the river and near the reservoir bank, and consequently induces earthquakes. 3. As the TGR entered the 175 m high-level operation stage, the cross-correlation test confirmed that the seismic activity and the reservoir water level showed negative correlation characteristics under the time lag of 4 to 5 months, suggesting that the seismic activity has a lagging response to the reservoir water level change. Gravity and numerical simulation results showed the continued infiltration of the reservoir water, followed by the softening of the faults and other actions, triggering the Xiangxi M4.1 earthquake at the center of the four quadrants of gravity anomalies near Xiangxi on 22 November 2008. The Xiangxi segment of the reservoir and its periphery, a triangular geological region where the Xiannvshan faults, the Jiuwanxi fault, and the Yangtze River meet, might be at risk of having reservoir-induced tectonic earthquakes. Conceptualization, methodology, coding, validation, writing, Q.M. and X.D. ; formal analysis, Q.M. and L.Z. ; conceptualization, methodology, review, Y.Y. and W.L. All authors have read and agreed to the published version of the manuscript. This study was funded by the project of China Three Gorges Corporation (0799217) and the National Natural Science Foundation of China (Grant No. observation data for this study. The author also wants to give heartfelt thanks to the anonymous Reviewers, who provided valuable comments on this article. The authors declare no conflict of interest.