Observations relevant to seismic driving stress, stress drop, and efficiency

Abstract

Results of previous studies of seismicity and source parameters of tremors in a deep mine in South Africa have been compared with underground observations of the effects of tremors and laboratory analyses of material properties. These comparisons indicate that the ruptures are driven by shear stresses averaging about 400–700 bars in the source region; the driving stress apparently increases with depth. Comparison of these high driving stresses to seismic stress drops, that had previously been found to range from 5 to 50 bars, implies very low seismic efficiencies. Most of the released energy is consumed in creating the fault zone, with less than 1% being radiated seismically. Underground observations of displacement across fault zones associated with mine tremors show that the faulting mechanism is very inhomogeneous. Measured displacements show considerable variation, and typical values are more than a factor of 10 greater than those anticipated on the basis of Brune's seismic source model. A series of measured shear displacements averaging 6 cm in the well‐explored fracture systems argue strongly for high, localized stress drops, perhaps in the neighborhood of 700 bars or greater. Because the fracture system interacts with the much larger mine excavation during its formation, it apparently absorbs strain energy from a region considerably broader than the fracture itself. The source dimensions determined from the seismic radiation probably indicate the sizes of the broad regions from which strain energy is drawn rather than the dimensions of contiguous fracture systems.