## 4 Characteristics of Microseismic Events in the Tunnel
### Spatial and Temporal Distribution of Microseismic Events  
The microseismic monitoring system of the diversion tunnel started to run normally on 17 December 2021, and stakes K0+765 to K0+775 of the #2 diversion tunnel were excavated from 3 June to 13 June 2022. During the construction period, a total of 104 valid events were collected after the site interference signals such as mechanical noise were removed, including 92 microseismic events.  
Figure 9 shows the temporal distribution of microseismic events. As can be seen from the figure, the microseismic events gradually increased from 3 June to 8 June, reached the peak on 9 June to 10 June, and maintained at about 20, and the surrounding rock was continuously disturbed. Based on the energy dissipation principle, each micro-fracture of the surrounding rock is accompanied by the release of energy. The cumulative energy released from microseismic events suddenly increased on 9 June and 10 June, indicating that frequent microseismic events intensified the release of energy. Then, the number of microseismic events decreased significantly from 11 June to 13 June and maintained at about 5. The moment magnitude was mostly distributed between \(-1.0\) and \(0.5\), representing small-magnitude events, as shown in Figure 10. The site construction dynamics indicated that the site was under normal construction from 3 June to 8 June, and the construction intensity started to increase on 9 June and 10 June in order to complete the workload, which directly led to the intensification of internal micro-fractures of the rock mass and extremely frequent microseismic events. The site investigation showed that the deformation and failure of the rock mass occurred on the left front of the tunnel face (as shown in Figure 11), and there were risks of rock bursts. For construction safety purposes, the blasting work was stopped on 11 June and 12 June, and support was provided for the surrounding rock. After the support was completed on 13 June, the tunnel excavation was continued, with a much  
Figure 8: Network topology of the microseismic monitoring system.  
Figure 12 shows the spatial distribution of microseismic events, in which the size of the sphere represents the energy level of microseismic events, and different colors represent varying magnitudes of microseismic events. From Figure 12a,b, it can be seen that most of the microseismic events were distributed near the tunnel vault and corners of the side wall, and a few appeared near the side wall of the tunnel, indicating that most of the microseismic events were distributed at the tunnel vault or corners of the side wall. Therefore, more serious failure occurred in this area. The failure area of the surrounding rock of the tunnel can be seen from the spatial distribution of microseismic events. Figure 12c,d are the density cloud maps of the microseismic events, in which the color shades represent varying densities. It can be seen that the energy loss of the rock mass after tunnel excavation was mainly concentrated near the vault and corners of the side wall, which was consistent with the spatial distribution of microseismic events. Therefore, it can be revealed that this area was the main damage zone of the tunnel rock and there were risks of instability failure.