# Numerical Simulation of the Influence of the Baihetan Reservoir Impoundment on Regional Seismicity
## 4 Results  
The computed model predicts deformation, stress, and pore pressure for each time step. The results of surface deformation are shown in Figure 3. The deformation is affected by both water gravity load and fluid pore pressure, and the continuous rise of the reservoir water level gradually increases the vertical compression, directly impacting the strata under the reservoir. In addition, the gradually increasing water load and the continuous downward diffusion of pore pressure cause the compressive stress under the reservoir to increase (Figure 4). During the impoundment period of the reservoir, the water gradually squeezes towards both sides of the Jinsha River. This causes the principal stress in the reservoir center to be characterized by NW-SE compression and NE-SW tension, with near NS compression in the periphery. As the depth increases, the stress variation decreases. The stress begins to decrease due to the relaxation effect after the impoundment is completed. The stress changes share the same characteristics as the displacement changes. Reservoir impoundment has different effects on the seismicity of different faults. Faults are affected by the pressure of the reservoir water gravity in the crust, and the stress imparted by the water gravity depends on the fault geometries. The closer to the reservoir center, the more significantly the compressive stress of the fault increases. According to the Coulomb failure stress change theory ([PERSON] and [PERSON], 2013; [PERSON], 2014; [PERSON], 2014), an increase in compressive stress will reduce Coulomb failure stress (CFS) and the seismic risk of the fault, while an increase in pore pressure will enhance the CFS and seismic hazard. Based on the fault geometries of Table 1, we selected the reasonable values (Table 3), and combined with the stress state (Figure 4), we calculated the CFS changes (\(\Delta\)CFS) of all the faults at different times via Equation (16),  
\[\Delta\text{CFS}=\Delta\tau+\mu(\Delta\sigma_{n}+\Delta p) \tag{16}\]  
where \(\Delta\sigma_{n}\) and \(\Delta\tau\) are the normal stress change and the shear stress change in the rupture direction, which are calculated based on fault geometries, respectively, and \(\Delta p\) is the pore pressure change. \(\mu\) is the friction coefficient, and a small value represents the easily slipping fault. This study set used \(\mu\) = 0.5 in the calculation because all fault seismicity in the study area is active. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Layer**} & **Thickness** & **K** & **G** & **\(\eta\)** & & & & **\(\kappa\)** \\  & **(km)** & **(10\({}^{10}\) Pa)** & **(10\({}^{10}\) Pa)** & **(N\(\cdot\)s/m\({}^{2}\))** & **\(\
u\)** & **\(\
u_{u}\)** & **B** & **(10\({}^{-6}\)** \\  & & & & & & & & **m\(\cdot\)s/kg)** \\ \hline
1 & 1 & 2.68 & 1.84 & \(1\times 10^{25}\) & 0.22 & 0.24 & 0.8 & 5 \\
2 & 9 & 2.98 & 2.05 & \(1\times 10^{25}\) & 0.22 & 0.24 & 0.8 & 5 \\
3 & 10 & 4.66 & 2.8 & \(2.8\times 10^{22}\) & 0.25 & 0.3 & 0.8 & 5 \\
4 & 10 & 4.66 & 2.8 & \(2.8\times 10^{22}\) & 0.25 & 0.3 & 0.9 & 5 \\ Faults & - & 2.38 & 1.64 & \(1\times 10^{25}\) & 0.22 & 0.24 & 0.8 & 5 \\ \hline \hline \end{tabular} (K is the bulk modulus, G is the shear modulus, \(\eta\) is the viscosity coefficient, \(\
u\) is the Poisson ratio, \(\
u_{u}\) is the undrained Poisson’s ratio, B is the Skempton coefficient, and \(\kappa\) is the permeability.)  
\end{table} 
Table 2: The material information of different layers and faults. Figure 3: Displacement changes with time on the surface. The subgraphs illustrate the displacement change over time of the upper surface after the water load is applied to the model in Figure 2. Subfigures (**a1**–**a3**) are horizontal displacement u (EW direction; east is positive). Subfigures (**b1**–**b3**) are horizontal displacement v (NS direction; north is positive). Subfigures (**c1**–**c3**) are vertical displacement w (up is positive).