Source mechanisms of microearthquakes associated with underground mines in eastern Utah

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Abstract
A detailed microearthquake survey, with acoustic monitoring, was conducted in the Sunnyside coal mining district of eastern Utah to investigate the possible relationship between earthquakes and mining. Acoustic emissions from events of magnitude less than −1 showed an average rate of 180/hr and decreased following the latest mining activity. These small events appear to have originated as shear failures near the mine walls. Rates of earthquake activity averaged hundreds per day with magnitudes from −0.5 to +2.8. The zone of greatest seismic activity was located 1 km beneath a portion of the mine which exhibited floor and roof failures. A composite fault-plane solution defines reverse faulting with nodal planes striking N15°E. The P axis strikes S75°E and dips 25°W, and the T axis strikes N75°W and dips 65°E. These directions are in general agreement with the stress pattern attributed to the tectonic development of the nearby San Rafael swell and suggests the possibility that the main earthquake energy may be derived from regional tectonic stress. Spectral analyses of direct shear waves from the submine earthquakes yielded estimates of source parameters at hypocentral distances from 1.3 to 3.0 km for events of magnitude +1.2 to +2.8. Stress drops range from 0.2 bar for a moment of 2.4 × 1017 dyne-cm to 9.6 bar for a moment of 63 × 1017 dyne-cm with corner frequencies from 10 to 14 Hz. These data document the range of measured source spectra to hypocentral distances less than 2 km.