ABSTRACT
Mode of fracturing of the rock mass is the key point in rock mechanics applied both to natural earthquakes foci and to sources of induced or triggered seismic events. The parameter providing the answer is the source mechanism. The moment tensor, currently used as a universal tool for descriptions of the mechanism, involves general balanced dipole sources. However, in case of small-scale seismic events, the moment tensor need not be always reliably determined. In an effort to fit the data, there may be notable non-shear components caused by the low quality of input data. It means that while the orientation of the fracture is usually estimated well, the mode of fracturing itself may be dubious. Constraining the source model to directly determine a simpler one is convenient for describing the physical phenomena expected for a particular focus. An opening of new fractures can be described, to a first approximation, by a tensile crack, optionally combined with a shear slip. Such an alternative model is called a shear-tensile crack (STC) source model. The combination is practical, and can be used to both identify events that reflect purely mode-I (tensile) failure and to determine the dilation angle of the fracture undergoing shear. The advantage of the STC is even enhanced in application to mining tremor foci, as implosion (i.e., tensile process with a negative sign) can be reasonably expected there as the consequence of collapsing mined-out cavities. From the technical point of view, the STC inversion is more robust than the MT one thanks to smaller number of the model parameters (5 vs. 6), which is crucial in cases of a poor monitoring configuration.
We demonstrate the dominance of the STC model over the traditional MT in resolving the mode of fracturing having occurred in foci of several mining associated seismic events recorded at Driefontein now Sibanye gold mine in South Africa. Mechanisms of five events with magnitudes ranging from 2.5 to 2.8 are non-shear, comprising notable share of implosion in addition to a shear-slip, in accordance with the anticipation of collapsing into void – mined-out – cavities. The error analysis in terms of confidence regions for fracture orientation and the fracture mode parameter reveals better resolution of the STC compared to the MT, i.e. higher chance for fracture mode identification even in cases of sparse configurations of the monitoring.
