# Uneven Stiffness Coal Seam: A New Structural Factor Prone to Coal Burst Based on Stiffness Theory
## 6 New Explanation of Engineering Phenomena Based on USCS
### New Explanation of the \"Microseism Deficiency\" Phenomenon before Coal Burst  
During microseismic monitoring of the mining face, occasional observations of a phenomenon known as \"microseism deficiency\" have been made. This phenomenon signifies a significant decrease in both the frequency and energy of microseismic events during specific time periods or in certain areas [39; 40]. The occurrence of a \"microseism deficiency\" phenomenon often indicates a high probability of coal burst, strong mine pressure behavior, or the emergence of high-energy microseismic events. For instance, in 2006, when the 1410 working face of Shandong Huafeng Coal Mine approached a remnant coal pillar in the lower protective layer, the extension of the microseismic event distribution area did not proceed as usual with the advancement of the working face. Instead, it was halted at a distance of 50 m ahead of the remnant pillar, resulting in a continuous reduction in the advancement fracture area. Then a coal burst occurred, characterized by an energy release of 2.2 \(\times\) 10\({}^{7}\) J, once the working face exceeded the coal pillar [41]. Similarly, during a 5 day period from 4-8 October 2011, the frequency and energy of microseismic events in the 8935 fully mechanized caving working face of Yao Coal Mine in Xinzhou remained consistently lower than the average. Subsequently, on October 9 th, a coal burst occurred, releasing an energy of 5.9 \(\times\) 10\({}^{5}\) J [39]. In another case, the 31103 working face of Bayangaole Coal Mine in Inner Mongolia experienced a \"microseism deficiency\" period lasting 2-3 days during monitoring before and after 20 September 2017. This was followed by a high-energy microseismic event with an energy release of 1.1 \(\times\) 10\({}^{5}\) J in the middle of the working face on 22 September, which fortunately did not cause serious damages due to its distance from the roadway [40]. Consequently, scholars have widely acknowledged the \"microseism deficiency\" phenomenon as a precursor feature of coal burst [39; 40]. When the normal stress surpasses a specific critical threshold, weak sections within the coal mass undergo damage and subsequently release energy. If this released energy exceeds the threshold of the microseismic monitoring system, a microseismic event occurs. Generally, as stress levels increase, higher frequencies and energy levels of microseismic events are observed. Conversely, when the stress remains below the critical threshold, the coal sections encounter difficulties in releasing enough energy to reach the microseismic monitoring system's threshold. Consequently, the monitoring results indicate a scarcity of microseismic events. As illustrated by the yellow line in Figure 14, the mining stress field undergoes movement as the working face progresses. Each point within the coal mass ahead of the working face typically undergoes a process wherein the normal stress gradually increases from the average pressure (P) to the peak value (\(\sigma_{\rm{Ymax}}\)), followed by a rapid decrease below P. Therefore, in the absence of notable variations across different positions within the coal mass, the microseismic events in the monitoring results should exhibit an even distribution. The occurrence of the \"microseism deficiency\" phenomenon indicates notable disparities among various positions within the coal seams. As shown in Figure 14, if a fixed USCS is present within the entity coal ahead of the mining face, the artificial USCS generated due to mining operations will overlay with the fixed USCS to varying extents at different positions as the working face progresses. The superposition of two USCSs does not augment the total load, but rather redistributes stress, resulting in an increase of mining stress in HSZ and a decrease in LSZ. Previous analyses and simulation results indicate a positive correlation between the normal stress (\(\sigma_{\rm{YL}}\)) in LSZ and the value of \(K_{\rm{L}}S_{\rm{L}}/E_{\rm{R}}H_{\rm{R}}K_{\rm{H}}S_{\rm{H}}\).