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

Abstract

[1] 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 À 5.23 ≤ M L ≤ À 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 40 m 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.Citation: Kwiatek, G., and Y. Ben-Zion (2013), Assessment of P and S wave energy radiated from very small shear-tensile seismic events in a deep South African mine,

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“…Overall, the amplitudes of P-waves relative to S-waves are thus expected to be higher for tensile fracture earthquakes than for shear earthquakes, thereby providing a standard method for identifying the former (or for determining the relative contributions of tensile and shear deformations for 'hybrid' events) (e.g. Walter & Brune 1993;Ramsey & Chester 2004;Šílený et al 2009;Song & Toksöz 2011;Vavryčuk 2011;Kwiatek & Ben-Zion 2013;Eaton et al 2014;Song et al 2014). However, even so, the Appendix shows that the S-wave radiated by a tensile fracture earthquake will usually be stronger than the P-wave, especially in rocks with a low Poisson's ratio.…”

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