## 3 Case Analysis
### Case Analysis of the Access Tunnel in the Main Powerhouse  
On 13 January 2018, two rockburst events were monitored at the tunnel. There was a relatively loud \"explosion\" sound on site. The HHT instantaneous frequency analysis technology was used to analyze the rockburst process. Figure 10a shows an MS signal captured at 07:05:26 on 10 January 2018. The apparent stress value of the event was 77.642 KPa, which was relatively high. The energy released was 154 J. The Es/Ep was 11.19, which indicated that the rock damage type was mainly shear damage. The FD range of the event was about 200-500 Hz during 0.02-0.11 s. The FD range had no downward shift phenomenon over time. The FD range was relatively stable. The energy distribution was relatively dispersed. The frequency points had good continuity. The signal lasted approximately 0.09 s.  
Figure 10: _Cont._  
Figure 10b shows an MS signal captured at 07:05:36 on 10 January 2018. The apparent stress value of the event was 30.660 KPa, which was relatively high. The energy released was 13 J. The Es/Ep was 6.84, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 300-450 Hz during 0.03-0.12 s. The FD range had no downward shift phenomenon over time. The frequency points had good continuity. The FD range of the event was relatively stable and relatively narrow. The coda was developed, and the energy distribution of the event was relatively dispersed. The signal lasted approximately 0.09 s.  
Figure 10c shows an MS signal captured at 08:17:40 on 10 January 2018. The apparent stress value of the event was 59.019 KPa, which was relatively high. The energy released was 102 J. The Es/Ep was 8.06, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 200-600 Hz during 0.025-0.08 s. The frequency points had good continuity. The signal lasted approximately 0.055 s.  
Figure 10d shows an MS signal captured at 22:20:58 on 11 January 2018. The apparent stress value of the event was 75.663 KPa, which was relatively high. The energy released was 261 J. The Es/Ep was 5.81, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was mainly mixed damage. The FD range of the event was about 250-500 Hz during 0.03-0.09 s. The FD range had no downward shift phenomenon over time. The FD range of the event was relatively stable, and the coda was developed. The signal lasted approximately 0.06 s.  
Figure 10e shows an MS signal captured at 02:14:37 on 12 January 2018. The apparent stress value of the event was 62.786 KPa, which was relatively high. The energy released was 261 J. The Es/Ep was 5.81, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 250-500 Hz during 0.03-0.09 s. The FD range had no downward shift phenomenon over time. The FD range of the event was relatively stable, and the coda was developed. The signal lasted approximately 0.06 s.  
Figure 10: HHT spectrum of MS events: (**a**) 07:05, 10 January 2018; (**b**) 07:05, 10 January 2018; (**c**) 08:17, 10 January 2018; (**d**) 22:20, 11 January 2018; (**e**) 02:14, 12 January 2018; (**f**) 02:13, 13 January 2018; (**g**) 03:27, 13 January 2018; (**h**) 03:38, 13 January 2018; (**i**) 04:07, 13 January 2018; (**j**) 04:19, 13 January 2018.  
was 79 J. The Es/Ep was 18.25, which indicated that the rock damage type was mainly shear damage. The FD range of the event was about 250-600 Hz during 0.03-0.1 s. The FD range had a downward shift phenomenon over time. The frequency points had good continuity. The FD range of the event narrowed. The coda was developed, and the energy distribution was relatively dispersed. The signal lasted approximately 0.07 s.  
Figure 10f shows an MS signal captured at 02:13:49 on 13 January 2018. The apparent stress value of the event was 34.294 KPa, which was relatively high. The energy released was 167 J. The Es/Ep was 4.38, which indicated that the rock damage type was mainly mixed damage.