Geomechanical conditions of causes of high-energy rock mass tremors determined based on the analysis of parameters of focal mechanisms

The aim of the research was to determine the cause of high-energy rock mass tremors (energy E ≥ 105 J) in the area of longwall H-2a located in seam 409/3 in Borynia-Zofiówka-Jastrzębie colliery, basing on the analysis of geological and mining conditions, seismic activity, focal mechanism and local stress field. The research employed the method of seismic moment tensor inversion which provides parameters of focal mechanism (percentage share of its components: isotropic, uniaxial compression or tension, shear component; trend and dip of nodal planes, directions of tension axes and compression stress). The parameters describe processes occurring in focuses of tremors and they are clearly linked with stress conditions in a given area. The conducted tests showed that the cause of occurrence of high-energy tremors and three rockbursts in lot H while mining seam 409/3 with longwall H-2a, was dynamic destruction of roof rocks which could displace towards the cavity created after mining the seam. An additional factor significantly magnifying the process was the share of stresses, which originate from the faults in the area, existing in the rock mass. Results of the research provided additional information to determine the degree of rockburst hazard in the area. Because of very dangerous work conditions (stress parameters reflect the rock mass of high shear strength, where dynamic influence of tremors is stronger) mining activity in longwall H-2a was terminated a few dozen metres earlier than it had been originally planned.