# Numerical and Field Investigations of Tremors Induced by Thick-Hard Strata Fracture
## 3 Field Observations and Verification  
The 73\({}_{\rm upper}\)07 working face of the Baodian coal mine is located in the No. 7 mining area, with an average burial depth of 591.8 m. The working face is close to the boundary coal pillar between the Baodian and the Dongtan coal mines, and is adjacent to the goal of the 73\({}_{\rm upper}\)05 working face and the 73\({}_{\rm upper}\)09 working face in the south and north directions, respectively. The mining seam of the working face is the 3\({}_{\rm upper}\) coal seam, with an buried depth of \(-\)536 m--565 m and a thickness of 3.00-5.55 m (the average is 5.38 m). The immediate roof of the 73\({}_{\rm upper}\)07 working face is mudstone, with an average thickness of 1.20 m, which has low strength and can easily fall. The primary roof #1 is sandstone with an average thickness of 18.60 m. The primary roof #2 is mudstone with an average thickness of 40.28 m, and upper statum is the red bed with a thickness of 250-300 m. In addition, the red bed has high strength, huge thickness, and does not easily deform or break. A MS monitoring system was installed in the Baodian coal mine, which could accurately locate and record MS events by receiving the elastic waves radiated by coal and rock ruptures. Figure 10 shows the plane and spatial distributions of mining-induced tremors in the coal and rock during the mining process of the 73\({}_{\rm upper}\)07 working face. According to the borehole column of rock strata in the 73\({}_{\rm upper}\)07 working face, the immediate roof and the primary roof 2\(\#\) were mudstone, and the primary roof 1\(\#\) was relatively fractured. Therefore, the low-level roof strata could break and fall quickly after goat formation. The red bed stratum was mainly composed of fine sandstone, with higher strength and thickness, which is not easy to fracture synchronously with the low-level rock. Based on the previous simulation results, the high tensile stress and compressive stress may be easily concentrated in the middle and at two ends of the red bed, respectively. When the concentrated stress reaches the strength limit of the rock, the rock would fracture, and a strong tremor would occur. As seen in Figure 10a, high-energy events were generally concentrated in the primary roof 2\(\#\) and the red bed. In addition, the 73\({}_{\rm upper}\)07 working face is adjacent to the 73\({}_{\rm upper}\)05 goat. Under the action of lateral abutment pressure, the headentry of the 73\({}_{\rm upper}\)07 working face was in a state of high static stress. When the superimposed stress of static load and dynamic load by strong tremor exceeded the strength limit of the surrounding rock, the risk of rockburst increased sharply. As seen in Figure 10b, the high-energy events at the headentry near the 73\({}_{\rm upper}\) 05 goaf were significantly more than that at the tailentry, indicating that the area was in a state of high stress concentration. Previous studies have shown that the dominant frequency of seismic wave has a significant difference with different breaking scales of the source. The lower the dominant frequency  
Figure 9: Kinetic energy curves of roof fracture with different shear strength. of the source was, the larger the rupture scale would be. To further study the fracture characteristics of the red bed, the event 1# with the maximum energy in the red bed was selected to conduct spectrum analysis, and the results are shown in Figure 11.  
As seen in Figure 11, the duration of this strong tremor was long, and the energy attenuation was slow. As seen in Figure 11, the spectrum distribution characteristics of the event received by different geophones were similar and the dominant frequency was lower. The dominant frequencies of the waveforms received by #3, #4, #5, and #6 geophones were 2.9 Hz, 5.3 Hz, 3.0 Hz and 4.3 Hz, respectively. In addition, the energy was mainly concentrated in the frequency range of 0-10 Hz, indicating that large-scale rupture occurred in the red bed, due to local stress concentration.