Seismo-acoustic measurements made before and after major mine subsidence on March 27, 1968, show significant changes in the level of activity and in the b value associated with the magnitude-frequency relationship. At one mine level (−400 ft), the following observations were recorded.
Both the changes in the number of events and the b value can be correlated with stress changes in the mine as a result of the subsidence. The use of high-gain instrumentation with significant reponse in the acoustic range allows the recording of large numbers of events. These measurements provide information on the link between the laboratory measurements on rock specimens and the ultramicroearthquake field observations.