## 5 Discussion
### Occurrence Mechanism of Strong Mining Tremors  
Based on the above analysis, it can be further described that during the goal mining of LW 1208, the roof strata above the coal seam breaks and collapses step by step. The whole process of SMTs induced by the fracture movement of the super-thick strata is shown in Figure 15.  
As can be seen from the Figure:  
1. The Jurassic Yan'an formation strata collapsed before the \"8.20\" SMT, the strata of Jurassic Anding and Zhiluo formation strata formed unstable rock blocks after fracturing, and the mutual movement and rotational extrusion of the blocks induced shear and compression fracture mining tremors, respectively. The super-thick strata did not break, but constantly deformed and accumulated elastic energy, resulting in an increase in surface settlement.
2. As LW 1208 continued to advance, cracks in the super-thick strata continued to expand and develop, and the initial tensile fracture of super-thick strata induced the \"8.20\" SMT.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**SMPs** & **Seismic Moment/N\(\cdot\)m** & **Stress Drop/Pa** & **Apparent Stress/Pa** & **Apparent Volume/m\({}^{3}\)** \\ \hline [PERSON] et al. [40] & 10\({}^{9}\)–10\({}^{12}\) & 10\({}^{4}\)–10\({}^{6}\) & 10\({}^{4}\)–10\({}^{6}\) & / \\ [PERSON] et al. [4] & 10\({}^{10}\)–10\({}^{12}\) & 10\({}^{4}\)–10\({}^{5}\) & 10\({}^{4}\)–10\({}^{6}\) & / \\ [PERSON] et al. [22] & 10\({}^{9}\)–10\({}^{12}\) & 10\({}^{3}\)–10\({}^{5}\) & 10\({}^{4}\)–10\({}^{6}\) & 10\({}^{5}\)–10\({}^{7}\) \\ [PERSON] et al. [41] & 10\({}^{16}\)–10\({}^{18}\) & 10\({}^{6}\)–10\({}^{7}\) & 10\({}^{6}\)–10\({}^{7}\) & / \\ [PERSON] et al. [42] & 10\({}^{19}\)–10\({}^{23}\) & 10\({}^{6}\)–10\({}^{7}\) & 10\({}^{6}\)–10\({}^{7}\) & / \\ [PERSON] et al. [43] & 10\({}^{17}\)–10\({}^{21}\) & 10\({}^{6}\)–10\({}^{8}\) & / & / \\ [PERSON] et al. [44] & 10\({}^{11}\)–10\({}^{16}\) & 10\({}^{4}\)–10\({}^{7}\) & 10\({}^{4}\)–10\({}^{7}\) & / \\ [PERSON] et al. [45] & 10\({}^{12}\)–10\({}^{15}\) & 10\({}^{4}\)–10\({}^{7}\) & 10\({}^{4}\)–10\({}^{7}\) & / \\ Present research & 10\({}^{9}\)–10\({}^{10}\) & 10\({}^{4}\)–10\({}^{5}\) & 10\({}^{4}\)–10\({}^{5}\) & 10\({}^{7}\)–10\({}^{8}\) \\ \hline \end{tabular}
\end{table}
Table 4: Comparison of the size distribution of SMPs.  
After the occurrence of the \"8.20\" SMT, the super-thick strata was in a state of instability. Meanwhile, the disturbance of the SMT made the super-thick strata slip and mismove, which induced the \"8.29\" shear fracture SMT.
4. Mining continued at LW 1208, and cracks developed in the upper thick overburdened area within the mining area of LW 1206A. Further expansion of the cracks led to the tensile failure of the super-thick strata, and eventually induced the \"12.20\" SMT.  
To summarize, the strata under the super-thick strata gradually break upward during goat mining at LW 1208, and the super-thick strata deforms and constantly accumulates elastic energy. When the overlying load carried by the super-thick strata exceeds the bearing limit, the primary and periodic tensile fracture is the fundamental cause of inducing SMT. Meanwhile, the structural adjustment and staggered sliding of the super-thick strata after fracture also easily induces SMT.