## 3 Case Analysis
### Case Analysis of the Main Powerhouse  
On 7 January 2019, two slight rockburst events were monitored at the main powerhouse. The on-site construction progress had slowed down. The HHT spectrum was used to analyze the MS activity laws of the whole rockburst process inside surrounding rocks. The time-frequency information of the rockburst precursor was extracted to provide a scientific basis for efficient construction. Figure 9a shows an MS signal captured at 00:06:18 on 7 January 2019. The apparent stress value of the event was 16.186 KPa, which was relatively high. The energy released was 170 J. The Es/Ep was 2.72, which indicated that the rock damage type was mainly tensile damage. The FD range of the event was about 200-600 Hz and had a significant downward shift phenomenon over time. The frequency value was relatively low during 0.06-0.08 s. There was some energy concentration in the low-frequency band (200-300 Hz), which indicated there were relatively large-scale cracks inside the rock. The signal lasted approximately 0.08 s.  
Figure 8: Photos of rock (granite) damage: (**a**) main powerhouse; (**b**) access tunnel in the main powerhouse. Figure 9: _Cont._  
Figure 9b shows an MS signal captured at 00:08:31 on 7 January 2019. The apparent stress value of the event was 13.394 KPa, which was relatively high. The energy released was 8 J. The Es/Ep was 1.53, which indicated that the rock damage type was mainly tensile damage. The FD range of the event was about 300-700 Hz during 0.01-0.05 s, and was about 300-450 Hz during 0.05-0.065 s. The FD of the event had a downward shift phenomenon over time. The signal lasted approximately 0.055 s.  
Figure 9c shows an MS signal captured at 00:12:54 on 7 January 2019. The apparent stress value of the event was 25.308 KPa, which was relatively high. The energy released was 263 J. The Es/Ep was 4.17, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 200-600 Hz during 0.025-0.07 s, and was about 100-300 Hz during 0.07-0.085 s. The FD of the event had a significant downward shift phenomenon over time. The FD range was obviously low, and there was some energy concentration in the low-frequency band. It indicated that the scale of the crack was relatively large. The signal lasted approximately 0.06 s.  
Figure 9d shows an MS signal captured at 00:15:04 on 7 January 2019. The apparent stress value of the event was 19.756 KPa, which was relatively high. The energy released was 103 J. The Es/Ep was 3.98, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 300-500 Hz during 0.025-0.07 s, and  
Figure 9: HHT spectrum of MS events: **(a)** 00:06, 7 January 2019; **(b)** 00:08, 7 January 2019; **(c)** 00:12, 7 January 2019; **(d)** 00:15, 7 January 2019; **(e)** 01:07, 7 January 2019; **(f)** 03:02, 7 January 2019; **(g)** 03:05, 7 January 2019; **(h)** 03:14, 7 January 2019; **(i)** 03:17, 7 January 2019; **(j)** 03:22, 7 January 2019; **(k)** 03:25, 7 January 2019.  
was about 150-300 Hz during 0.07-0.1 s. The FD of the event had a significant downward shift phenomenon over time. The frequency points had good continuity, and the FD range narrowed. The energy distributions of the event were mainly concentrated within 200-300 Hz. The signal lasted approximately 0.075 s. The frequency spectrum characteristics of the event were typical precursory information of rockburst, indicating that rockburst was likely to occur. Figure 9e shows a rockburst signal captured at 01:07:33 on 7 January 2019. The apparent stress value of the event was 66.482 kPa, which was high. The energy released was 3060 J. The Es/Ep was 5.67, which indicated that the rock damage type was mainly mixed damage. The FD range of the event was about 150-400 Hz during 0.025-0.09 s. The frequency energy distribution of the signal had no downward trend over time. The overall FD range of the event was relatively low. The frequency points had good continuity, and the FD range was relatively narrow.