# Numerical Simulation of the Influence of the Baihetan Reservoir Impoundment on Regional Seismicity
## 5 Discussion  
According to the seismic records, earthquakes were widely distributed before the reservoir impoundment, with many on each fault (Figure 1). We have drawn the water  
Figure 6: \(\Delta\)CFS and seismic record distribution along the Xiaojiang Fault. The northern end of the fault is the origin point, as shown in Figure 2a (from A to A\({}^{\prime}\) and from B to B\({}^{\prime}\)). The different colors represent earthquakes in different \(\Delta\)CFS regions, and the yellow pentagrams represent \(M\geq 3.0\) earthquakes. I and II are related to the points’ location in Figure 5. (**a**) shows the XJF1, and (**b**) shows the XJF2. level change and earthquakes' M-T (Figure 7a,b), and compared the focal depths of the background earthquakes before the impoundment with those of earthquakes during the first year of impoundment (the seismicity before impoundment is called \"background earthquakes\" in this paper) (Figure 7d). The seismic b-value is one of the important indexes for seismological research and seismic risk assessment. Figure 7: (**a**) Reservoir water level curve and M-T (magnitude-time) distribution. The blue curve denotes reservoir water level data. (**b**) The curve of earthquake number and water level. (**c**) The magnitude–frequency of background earthquakes (26 April 2020–25 April 2021) and the earthquake in one year after the impoundment (26 April 2021–25 April 2022) in the research region. (**d**) The focal depth of background earthquakes and the earthquakes in the research region will be one year after the impoundment. We get the Gutenberg-Richter (G-R) magnitude-frequency relationship in our study area and use Equation (17) to calculate the seismic b-value (Utsu, 1966) (Utsu, 1966) (Figure 7c). Our seismic data show great integrity, and the earthquake frequency increases significantly after the reservoir begins to impound with b-value increases from 0.8179 to 0.8430. It may indicate that the accumulated stress is released due to the infiltration of pore fluid, and mostly small earthquakes with magnitudes smaller than \(M\)3.0 (226 earthquakes smaller than \(M\)2.0), and \(M\geq\) 3.0 earthquakes rarely occur (only 5 times) (Figure 8). For the depth distribution, the earthquake numbers at depths shallower than 10 km have not increased, but the numbers at depths deeper than 10 km have increased significantly, from 84 to 197. Especially at depths between 10 km and 15 km, there were 61 earthquakes before, but the number of earthquakes increased to 139 after impoundment, even more than twice as much as before (Figure 7d). This reflects that the previously unaffected deep faults are gradually opened by pore pressure as the reservoir water level gradually increases, resulting in shear stress on the fault plane exceeding the sum of its friction and cohesion, and earthquakes occur on the faults to release their energy. Because the fault stress is released before it is fully accumulated, there are usually more earthquakes than background earthquakes, with generally smaller magnitudes. There were 258 smaller and 5 bigger-than-\(M\)3.0 earthquakes before, and 390 smaller and 6 bigger-than-\(M\)3.0 earthquakes after impoundment. \[b=\frac{1}{log_{10}\big{(}\overline{M}-(M_{c}-\Delta M/2)\big{)}} \tag{17}\]  
where \(\overline{M}\) is the average magnitude, \(M_{c}=1.0\) is the minimum complete magnitude, and \(\Delta M=0.1\) is the minimum magnitude interval. The \(M_{5}\)5.0 earthquake, which occurred between LFF and ZLF on 18 May 2022, brought a large number of aftershock activities (Figure 8). It may have caused many earthquakes to occur in the NE direction away from the reservoir, and there seems to be no distinct change in seismic records between LFF and ZLF. Compared with background seismic records, the later earthquakes at all depths are concentrated in the area with the largest pore pressure increase below the reservoir center, and even a \(M_{5}\)3.0 earthquake occurs at a depth of 5 km (Figure 8a).