We analyze recordings of seismic events induced by underground mining operations at the Moonee Colliery, located in Australia, and at the Springfield Pike Limestone Quarry, located in the United States. The data were recorded underground in the vicinity of active mining operations and were initially used by the monitoring systems at the two mines to evaluate the potential for roof failure based on the temporal and spatial distribution of the seismic activity. In an earlier study, the roof failures at the two underground mine locations were found to be the result of two distinct failure processes, both of which were correlated with escalated seismic activity before the roof collapsed. In this study, we reexamine the recordings of these seismic emissions for a further assessment of the state of instability in the roof.

We estimate the static seismic moment and radiated seismic energy for each recorded seismic event induced by mining operations at the two underground mine locations. These seismic source parameters are estimated from source spectra have been corrected for the instrument response, propagation effects and bandwidth limitations. The apparent stress, which provides an estimate of the stress drop (or stress release) associated with a seismic event, is then determined from the product between the modulus of rigidity and the ratio between the radiated seismic energy and static seismic moment. The validity of constant stress drop scaling for the seismic events at the two underground mine locations is tested. Estimation of the seismic source parameters indicate that the stress drop of the mining induced events increases over three orders of magnitude of increasing seismic moment (106 N·m ≤ M0 ≤ 109 N·m) and indicate a divergence from constant stress drop scaling. When these results are compiled with the results from seven other independent studies, which analyzed the seismicity associated with a variety of seismogenic environments, this trend is found to span over ten orders of magnitude of seismic moment (106 N·m ≤ M0 ≤ 1016 N·m).

The observation that the mining induced events do not conform to constant stress drop scaling may assist in gaining a better understanding of the evolution of the roof failure process. We have found that the stress drop at one of the studied mines appears to increase through time prior to a roof collapse. More data are necessary to test this hypothesis. If this hypothesis is validated, it would have important implications for monitoring roof stability. Incorporation of near-real-time estimates of the stress drop into the existing seismic monitoring protocol may provide improved warning of imminent roof collapse hazards.