## 3 Case Analysis
### Case Analysis of the Main Powerhouse
The signal lasted approximately 0.065 s. There was a relatively loud \"explosion\" sound. The time interval between the occurrence of the rockburst event and the previous event was 52 min. It indicated that real-time monitoring had sufficient time to ensure safe construction on site. Figure 9f shows an MS signal captured at 03:02:48 on 7 January 2019. The apparent stress value of the event was 17.421 kPa, which was relatively high. The energy released was 77 J. The Es/Ep was 5.16, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 200-600 Hz during 0.02-0.065 s, and was about 100-300 Hz during 0.065-0.085 s. The FD range had a downward shift phenomenon over time. The energy concentration was in the low-frequency band. It indicated that after a rockburst occurred, large-scale cracks continued to occur inside surrounding rocks. The signal lasted approximately 0.065 s.  
Figure 9g shows an MS signal captured at 03:05:10 on 7 January 2019. The apparent stress value of the event was 11.448 kPa, which was relatively high. The energy released was 52 J. The Es/Ep was 3.45, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 300-600 Hz during 0.025-0.07 s, and was about 100-300 Hz during 0.07-0.085 s. The FD range had a significant downward shift phenomenon over time. There was some energy concentration in the low-frequency band. The signal lasted approximately 0.06 s.  
Figure 9h shows an MS signal captured at 03:14:51 on 7 January 2019. The apparent stress value of the event was 5.914 kPa. The energy released was 2 J. The Es/Ep was 11.23, which indicated that the rock damage type was mainly shear damage. The FD range of the event was about 400-700 Hz during 0.025-0.06 s, and was about 100-300 Hz during 0.06-0.07 s. The FD range had a downward shift phenomenon over time. The signal lasted approximately 0.045 s.  
Figure 9i shows an MS signal captured at 03:17:13 on 7 January 2019. The apparent stress value of the event was 7.224 kPa. The energy released was 10 J. The Es/Ep was 5.78, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 400-700 Hz during 0.015-0.06 s, and was about 100-300 Hz during 0.06-0.07 s. The FD range had a downward shift phenomenon over time. Some energy concentration was in the low-frequency band. The signal lasted approximately 0.055 s.  
Figure 9j shows an MS signal captured at 03:22:21 on 7 January 2019. The apparent stress value of the event was 14.008 kPa, which was relatively high. The energy released was 136 J. The Es/Ep was 1.44, which indicated that the rock damage type was mainly tensile damage. The FD range of the event was about 200-500 Hz during 0.025-0.07 s, and was about 100-400 Hz during 0.07-0.085 s. The FD range had a downward shift phenomenon over time. The overall FD of the event was relatively low. The signal lasted approximately 0.06 s.  
Figure 9k shows a slight rockburst signal captured at 03:25:17 on 7 January 2019. The apparent stress value of the event was 44.024 kPa, which was high. The energy released was 1172 J. The Es/Ep was 3.54, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 200-600 Hz during 0.02-0.06 s, and  
was about 100-300 Hz during 0.06-0.085 s. The FD range had a significant downward shift phenomenon over time. The FD range narrowed and the coda was developed. The signal lasted approximately 0.065 s. There was a relatively loud \"explosion\" sound. Before a rockburst, for some events, the FD range had a downward shift phenomenon over time; some energy concentration was in the low-frequency band. The frequency points had good continuity. We need to analyze the HHT spectrum characteristics of every event promptly and in detail. If some events contain rockburst precursor information, it indicates an increased risk of rockburst.