A digital seismic network was installed at Atomic Energy of Canada Limited's Underground Research Laboratory in Manitoba to monitor seismicity induced by the excavation of a shaft in granite between 324 and 443 m of depth. The extension of the shaft, with a design diameter of 4.6 m, was monitored from January to August 1988 by several sensors installed in four boreholes to ensure good spatial coverage of the mining faces. Source parameters of 155 tremors, selected from over 1500 located events, with moment magnitude from −3.6 to −1.9 are studied.
The corner frequencies, ranging from 1100 to 4200 Hz for P waves and from 900 to 3500 Hz for S waves, are heavily dependent on the angle of observation, and this fact must be taken into account when corner frequencies are converted into the source radii. The scaling relation between seismic moment and source radius indicates a decrease in stress drop with decreasing seismic moment. Characteristic tremors with a fault length of about 1 m seem to be generated in the area around the excavated shaft.
The ratio of S-wave to P-wave energy ranges from about 1 to 90, and for about 40 percent of the events this ratio is smaller than 10. These S-wave-energy-deficient tremors are also characterized by low values of stress release estimates, which range in general from 0.1 to 25 bars. The high P-wave-energy events are most probably the events with non-double-couple focal mechanisms, implying that tensile failures, or at least shear failures with tensile components, are generated by the excavation of the shaft.