## 5 Conclusions  
1. The \"square effect\" exists in longwall coal mining workfaces that have hard-thick main roofs near the coal seam; a simplified plate model was established to explain this phenomenon, which indicates that when the workforce length equals the advancing distance, where the planar shape of the gob forms a square, the normal stress of the plate reaches its maximum and is prone to rupture and cave, causing violent strata behaviors;
2. The joint active-passive seismic CT technology and equipment were introduced at a typical kilometer coal burst longwall face LW23908 with hard-thick main roof to investigate the stress distribution in the coal seam at different periods during the mining. The inversed P-wave velocity indicates that at the beginning stage of mining, the front and side abutment pressures of LW23908 are about 100 m and 30 m,  
Figure 12: Total energy and frequency of mining tremors before and after “square effect”.  
Figure 13: Average and maximum energy of mining tremors before and after “square effect”.  
respectively, and the \"square effect\" appears as early as the workface ahead of the theoretical position at 50 m with a 100 m significant influencing zone, which provides a strong guarantee for accurate risk evaluation of coal burst;
3. Based on the detection of the high-stress areas, supplementary targeted destress projects can be designed, and for LW23908 coal seam, large-diameter borehole destress and deep-hole blasting of hard-thick roof were carried out immediately in the high-stress area. The microseismic system was used to monitor and verify the effect of destress by analyzing the total energy, average energy, and maximum energy as well as frequency of mining tremors, precursor information before the \"square effect\" could be identified based on total energy features, which had period peaks in this case. The total everyday energy was always less than 105 J before and at the \"square effect,\" which has proved the effectiveness of the destress program;
4. The average energy indicates that the seismic type during the \"square effect\" stage at LW23908 belonged to a foreshock-mainshock pattern; therefore, the precursor information can be used to provide an early warning of coal burst, and active-passive combined seismic CT technology can delineate the scope of the high stress. The combination of these two, seismic events and CT, can improve the accuracy of coal burst danger, distinguishing notably;
5. The fracturing could change the break and movement of the hard-thick roof, overall reducing the cave intensity and impact on the coal seam; after fracturing, the energy resale mode during the break is changed from centralized to decentralized, manifested as massive low-energy microseismic events dominating even at the \"square effect\" stage, thus avoiding the accumulation of excessive elastic energy and releasing in an extremely short time.  
Conceptualization and methodology, H.H.; investigation, J.Z. and R.C.; data curation, Z.M. ([PERSON]); software, Z.M. ([PERSON]). All authors have read and agreed to the published version of the manuscript.  
This research was funded by the State Key Research Development Program of China (Grant No. 2022 YFC3004605), the National Natural Science Foundation of China (No. 51974302; 52274147), and the Fundamental Research Funds for the Central Universities (No. 2013 QNB30).  
Not applicable.  
The data used for conducting classifications are available from the corresponding author upon request.  
The authors would like to thank the engineers [PERSON], [PERSON], [PERSON], and [PERSON] from Xukuang Energy Co., Ltd. (Xuzhou, Jiangsu Province, China) for their valuable contributions to the paper.  
The authors declare no conflict of interest.