Microseismic Analysis and Source Mechanism Characterization of Surface Hydraulic Fracturing in a Low-Permeability Coal Seam

This study systematically analyzes microseismic (MS) monitoring data collected during staged hydraulic fracturing in low-permeability coal seams from surface wells. Results show that during the fracture initiation stage, pump pressure rose rapidly and triggered initial MS events. In the fracture propagation stage, the pressure remained high, accompanied by intensive MS activity. Scattered MS events were still observed during the pressure release stage, indicating a delayed response of the coal seam to fracturing disturbances. A total of 349 MS events were identified based on spectral differences and the oscillation characteristics of noise event power spectra. Energy analysis revealed that Stage 1 exhibited the widest energy distribution, suggesting the most effective fracturing. Stages 3 and 5 followed, showing good vertical propagation. In contrast, energy in Stage 7 was concentrated near the perforation zone, implying limited fracture growth. The strike and dip rose diagrams indicate that fractures mainly developed along the direction of the maximum horizontal principal stress (SHmax), demonstrating strong directional consistency. In this study, an improved inversion method based on multifeature fusion of seismic mechanisms is proposed. Seismic source mechanism analysis revealed that fractures were predominantly shear-type and generally aligned with the SHmax direction. Among the 349 events, 206 (59.0%) were strike-slip (SS), 81 (23.2%) were thrust-fault, and 59 (16.9%) were normal-fault type. Further analysis of source mechanism components showed that most events were dominated by double-couple (DC) components, typically ranging from 50 to 95%, significantly higher than the Isotropic (ISO) and Compensated Linear Vector Dipole (CLVD) components.