# Numerical Simulation of the Influence of the Baihetan Reservoir Impoundment on Regional Seismicity
## 5 Discussion
The seismicity with the focal depths of 5-10 km below the reservoir center increased, from 164 to 171, and the \(M\geq\) 3.0 earthquakes gradually migrated to the reservoir center after impoundment (Figure 8b). The number of earthquakes with focal depths between 10 and 15 km has significantly increased after impoundment, mostly distributed in the areas near the reservoir center, where the pore pressure increases the most (about 40 kPa). The seismic rates in the other areas far from the reservoir are constant or even decreased compared with the pre-impoundment seismicity level (Figure 8c). This may be because the shallow pore pressure has increased and the accumulated stress has been released due to river seepage before the impoundment. In contrast, deep pore pressure also increases due to growth of the reservoir water level, and the fault has a stimulation effect on the earthquake. The pore pressure below the reservoir center exceeds the typical static triggering threshold (10 kPa) after impoundment. These earthquakes can be considered induced earthquakes. Nevertheless, the impoundment does not induce earthquakes immediately in the areas far from the reservoir center. For example, the Qiaojia \(M_{5}\)4.3 earthquake on 30 March 2022, following another Qiaojia \(M_{5}\)5.0 earthquake on 18 May 2020 (Figure 8b) might be considered a triggered earthquake resulting from the Baihetan Reservoir impoundment. However, previous studies have shown that a large earthquake can be triggered when the pore pressure increases by 0.1 bar ([PERSON], 1966). We find that the pore pressure at the epicenter of this \(M_{5}\)4.3 earthquake increases by \(\sim\)1180 Pa according to the calculation results, and the stress caused by reservoir gravity is also on the order of kilopascals, which is much smaller than the typical static triggering threshold (10 kPa); thus, the reservoir may promote the occurrence of earthquakes in advance but the impact is limited. Therefore, the \(M_{5}\)4.3 Qiaojia earthquake can be considered a natural event. According to a previous study [33], the viscosity of the upper mantle is \(-2.8\times 10^{22}\) Pa\(\cdot\)s, and the viscosity of the crust is less than \(1.0\times 10^{25}\) Pa\(\cdot\)s in the Sichuan-Yunnan region. Reservoirs are usually designed to serve as power supplies for hundreds of years. To study whether the long-term existence of the reservoir has an impact on regional seismicity, we have calculated the 200 years after the completion of the reservoir impoundment and calculated the fault stress changes resulting from the viscosity characteristics of the crust and mantle (Figure 9). The results indicate that the long-term existence of the reservoir gravity load results in the relaxation effect in the surroundings, and the stress on the fault plane, will change constantly. However, the normal stress of the fault has changed slightly within 200 years. Even for the XJF1 below the reservoir center, the normal stress change caused by viscosity is no more than 20 Pa, which is far less than the stress changes imparted by tectonic loading. Consequently, the reservoir seismicity will be stabilized after the  
Figure 8: Pore pressure results and seismicity statistics at different depths. The grey points are the regional background earthquakes for one year before the reservoir impoundment from 26 April 2018, to 25 September 2019. The other color points are the earthquakes in different periods, and the five-pointed stars denote the \(M\geq\) 3.0 earthquakes. The changes in pore pressure after reservoir impoundment with different depths have been shown in the figures. (**a**) The depth is 5 km, and the focal depth is 1–5 km. (**b**) The depth is 10 km, and the focal depth is 5–10 km. (**c**) The depth is 15 km, and the focal depth is 10–15 km. (**d**) The depth is 20 km, and the focal depth is 15–20 km. equilibrium of pore pressure and fluid field, and the long-term existence of the reservoir has little impact on seismicity.