Observations of shear-wave polarizations from rockbursts in a South African gold field: an analysis of acceleration and velocity recordings

Rockbursts in the deep gold mines of South Africa have seismic magnitudes ranging from ML less than zero to ML more than 5. They occur in very confined volumes surrounding the working face of the mining excavations. An examination of three-component acceleration and velocity seismograms shows that the polarizations of shear waves recorded within the shear-wave window above an active mining area have a nearly uniform alignment. The polarization alignment and the measured time delays are consistent with shear waves propagating through the effective anisotropy of parallel vertical microcracks throughout the rockmass. Polarizations measured from velocity transducers were compared with polarizations measured from strong-motion acceleration recordings to show the amount of scatter in the data. We conclude that the dry fractures caused by the high stresses during normal mining processes have negligible effect at the wavelengths at which shear waves are recorded at the surface. The anisotropy observed at the surface appears to be due to microcracks aligned by the regional stress regime rather than local disturbances to the stress regime due to mining operations.