## 3 Overview and Mechanism of Mining-Induced Earthquake Events at the 3308 Working Face
### Analysis of Microseismic Activity Patterns Before and After High-Energy Mining-Induced Earthquakes
#### 3.3.2 Calculation of Mining-Induced Earthquake Source Mechanism
The isotropic index (_ISO_) is commonly used to measure whether a rupture contains an isotropic component [36; 37]:  
\[ISO=\frac{M_{xx}+M_{yy}+M_{zz}}{3M_{0}} \tag{9}\]  
When the _ISO_ value is close to 0, it indicates that the rupture is purely shear. A value greater than 0 suggests the presence of an isotropic component, and when _ISO_ approaches 1, it signifies that most of the seismic moment is generated by tensile rupture. The obtained microseismic localization and source mechanism results are shown in the figures below. The coordinates for the high-energy mining-induced earthquake event on 10 September are (11,629.29, 10,343.37, \(-\)833.27), located within the fault-protected coal pillar behind the 3308 working face, near the fault, approximately 20-40 m above the coal seam, 20-30 m from the cutout of the 3308 working face, and 30-40 m from the track roadway. The fracture type was mixed (primarily shear), with an energy of 1.43 \(\times\) 10\({}^{6}\) J, a moment magnitude of 0.904, and a fracture volume of 2.81 m\({}^{3}\). During the 10 September high-energy tremor, although the mining face had only advanced 0.8 m and the gob area was relatively small with no large-scale roof sag, it is unlikely that roof failure alone would cause such a large seismic event. The microseismic location revealed that the tremor occurred in the roof of the 3306 fault protection pillar behind the 3308 panel, near the DF60 fault. This indicates that the mining disturbance from the 3308 panel affected the incompletely collapsed roof of the 3306 gob area, the DF60 fault, and the protection pillar, leading to roof instability and fault slippage. Figure 12: Microseismic data and source mechanism of the 10 September mining-induced earthquake event: (**a**) original microseismic waveform; (**b**) source mechanism. The coordinates for the high-energy mining-induced earthquake event on 20 December are (11,598.03, 10,310.52, \(-\)845.87), located 20-30 m above the track roadway of the 3308 working face in the roof area. This event was a tensile fracture, with an energy of 1.22 \(\times\) 10\({}^{5}\) J, a moment magnitude of 0.19, and a fracture volume of 0.21 m\({}^{3}\). For the 3308 working face, rock mass failure occurring in the roof is relatively common. Regarding the two major mining-induced earthquakes, the high-energy mining-induced earthquake on 20 September occurred near the roof of the fault protection pillar of the 3306 panel, close to the DF60 fault. The high-energy mining-induced earthquake on 20 December occurred in the high-stress areas behind the 3308 longwall face. Therefore, the location of the microseismic events is closely related to the mining activities in the 3308 panel and the expansion of the gob area. The tremors were primarily triggered by roof instability and fault slippage caused by mining-induced disturbances and stress concentrations. As the gob area behind the 3308 panel continues to expand, there remains a risk of high-energy tremors, mainly triggered by roof fractures under high static and dynamic loads, influenced by high geological stress, especially tectonic stress, and incompletely collapsed roofs. The influence of the DF56 and DF34 faults will also affect the scale of future seismic events. Moreover, the side support pressure from the shallow gob area and the uncollapsed roof will further impact the safe mining of the 3308 longwall face. Figure 13: Microseismic data and source mechanism of the 20 December mining-induced earthquake event: (**a**) original microseismic waveform; (**b**) source mechanism.