Assessment of P and S wave energy radiated from very small shear-tensile seismic events in a deep South African mine

We discuss requirements for reliable estimates of radiated seismic energy in S and P phases and derive ratios of S-to-P radiated energy (E-S/E-P) of 539 seismic events with local magnitudes -5.23M(L)-2.41 (subdecimeter size) recorded by high-frequency acoustic emission (AE) sensors of the JAGUARS seismic network in the Mponeng deep gold mine, South Africa. The analyzed events are aftershocks of a M(W)1.9 earthquake, and the recording AE sensors are located within about 40m of the events. A shear-tensile model is used to simulate the radiation pattern of P and S phases from a family of rupture models ranging from pure shear to pure tensile failure. The calculations include correction factors for energy estimates associated with given source-receiver geometries and expected focal mechanism with possible tensile component. Synthetic calculations are used to assess the effects of limited observed frequency band and attenuation on the estimated E-S/E-P ratios. The model calculations provide guidelines on when different approximations may be used. The obtained E-S/E-P ratios for the analyzed events are relatively low (median value<5) for the full range of model parameters tested, suggesting that significant number of the events display a tensile component. Events with very small ratios (e.g., <1) may reflect enhanced P radiation associated with rock damage in the source volumes.