# Numerical Simulation of the Influence of the Baihetan Reservoir Impoundment on Regional Seismicity
## 4 Results
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **Fault** & **Strike** & **Dip** & **Rake** & \(\mathbf{\mu}\) &
\begin{tabular}{c} **Dip** \\ **Direction** \\ \end{tabular} \\ \hline F1 & ZMHF & 330\({}^{\circ}\) & 60\({}^{\circ}\) & 0\({}^{\circ}\) & 0.5 & NE \\ F6 & XJF1 & 340\({}^{\circ}\) & 70\({}^{\circ}\) & 60\({}^{\circ}\) & 0.5 & E/W \\ F7 & XJF2 & 345\({}^{\circ}\) & 70\({}^{\circ}\) & 0\({}^{\circ}\) & 0.5 & E/W \\ F8 & ZLF & 40\({}^{\circ}\) & 35\({}^{\circ}\) & 60\({}^{\circ}\) & 0.5 & NW \\ \hline \end{tabular}
\end{table} 
Table 3: Fault geometry for \(\Delta\)CFS calculation. The XJF1 and XJF2 are the two closest faults to the reservoir center, so pore pressure grows fast. PDHF and ZLF are far from the reservoir; thus, the pore pressure increases relatively slowly (Figure 5a). The \(\Delta\)CFS on XJF1 and ZLF increases with time, indicating increasing seismic risks on the faults. However, the combined effects lead to the decrease of \(\Delta\)CFS on XJF2 and PDHF, which suggests a lower possibility of earthquake occurrence (Figure 5b). The pore pressure and \(\Delta\)CFS on each fault become stable when the reservoir impoundment is completed, which indicates that the influence of reservoir impoundment on the underground pore pressure is stabilized. Regional stress also gradually reaches a new equilibrium state, which may lead to the stabilization of seismicity. Since other faults were far away from the reservoir and the seismicity changes were not obvious after impoundment, the relationship between the \(\Delta\)CFS and seismicity on XJF1 and  
Figure 4: Stress and pore pressure change with depth. The red arrows indicate the magnitude and direction of horizontal principal stress change. The gray maps show the changes in pore pressure at different depths. Subgraphs (**a-c**) show the results at depths of 5 km, 10 km, and 15 km, respectively. Figure 5: Pore pressure and \(\Delta\)CFS evolution of major faults. (**a**) The pore pressure changes with time in different faults. (**b**) The \(\Delta\)CFS changes with time in different faults. XJF2 was primarily studied and discussed. The \(\Delta\)CFS on the fault plane of XJF1 and XJF2 along the Jinsha River and the earthquake distribution within 10 km were drawn to explore the relation between the \(\Delta\)CFS and seismicity (Figure 6). Compared with the background seismicity, the earthquake almost occurred near the area or below where the \(\Delta\)CFS was positive after the impoundment. One hundred fifty-two earthquakes occurred near the XJF1 after the impoundment, all of which occurred close to the area where the CFS increases, and there were 62 earthquake epicenters with \(\Delta\)CFS exceeding 10 kPa, compared to the data of 11 and 5, respectively, before impoundment. The XJF2 seismicity was more complex. A total of 112 earthquakes occurred near the XJF2 after the impoundment; 39 earthquakes were located near the negative area of \(\Delta\)CFS and 73 earthquakes near the positive area, where the \(\Delta\)CFS at the epicenter of 27 earthquakes exceeded 10 kPa, with a ratio of about 1.87. However, there were 20 earthquakes on the XJF2 before impoundment, and the ratio of the positive and negative areas of \(\Delta\)CFS was 12.8 = 1.5. Therefore, \(\Delta\)CFS does have an impact on the seismicity. In particular, three \(M>3.0\) earthquakes occurred near the fault plane with larger \(\Delta\)CFS after the impoundment, which reflects the impoundment having a certain role in promoting the frequency and magnitude of earthquakes. The \(\Delta\)CFS was negative on the southern side of the XJF2, which corresponded to a decrease in seismicity in the area. According to the collected seismic records, there was less seismicity in this area, and a seismic gap was formed at the southern end, which further validated our calculation results.