Abstract:In the current study,three characteristic focal depths of mining-induced seismicity in coalmines,i.e. initiation depth,roof upper-bound depth,and floor lower-bound influence depth,are identified from field microseismic monitoring data. These characteristic depths depend on the relative distance between the hypocenter of a seismic event and ground surface or the location of the roof and floor of the coal seam. The initiation depth is the one beyond which rockbursts will happen. The roof upper-bound depth is defined as the depth of the seismic events located above the roof whose magnitudes are greater than certain thresholds;and the floor lower-bound influence depth is defined as the difference between the depth of the observed maximum seismic event below the floor and the depth of the floor. The mechanisms of the characteristic focal depths are also discussed. The initiation depth is explained by a power law model considering coal strength and the stress acting on the coal body,the roof upper-bound depth is explained by a bending model of a thick plate or beam;and the floor lower-bound influence depth is explained by the combined effect of elastic rebound due to unloading and the effect of Poisson¢s ratio as well as the influence of active faults. Finally,the impact of understanding the characteristic focal depths on mining safety is discussed. It is expected that a better understanding of these characteristic depths will facilitate the mitigation and control of mining-induced seismic hazards in underground coalmines.