Rg waves from rockbursts in South Africa

Most of the recorded seismic activity in South Africa is induced by mining operation. Seismograms of rockbursts show well developed Rg waves, with the fundamental mode propagating at 3.1 km/s. Some records also display clear group velocity dispersion. Recognition of Rg waves on the seismograms indicates that the event has a shallow focal depth (0-5 km), and the large amplitudes and dispersion imply a heterogeneous crustal structure. Studies of the analytical attenuation of Rg waves in South Africa yield a specific quality factor, Q, of 195, which is much lower than the value of 495 obtained using coda waves. This discrepancy can be explained by the presence of a low-velocity upper-crustal layer and, hence, supports previous suggestions of a heterogeneous crust. Analysis of Rg waves therefore gives information on local crustal structure and may also, in some cases, provide a discriminator between rockbursts and earthquakes.