Seismological and energy balance constraints on the mechanism of a catastrophic bump in the Book Cliffs coal mining district, Utah, U.S.A.

Abstract

On 21 January 1993, an underground mine in the Book Cliffs coal mining district of eastern Utah experienced a ML 3.6 seismic event. The event coincided with the crushing out of 24 18.2 m × 18.2 m × 2.7 m (seam height) pillars encompassing an area of nearly 15,000 m2. Seismic stations of the University of Utah array, Bureau of Reclamation arrays, and other western seismic stations recorded the event. Digital recordings from the University of Utah provided 41 waveforms for first-motion polarity determinations. All but three reliable polarity readings were dilatations. Source parameter and radiated seismic energy determinations were made using four of the University of Utah three-component stations. Attenuation-corrected P-wave and S-wave radiated energies were determined. The sum of P- and S-wave radiated energies is 1.5–7.1 GJ, depending on the attenuation model. For comparison with the radiated energy, the maximum possible strain energy in the crushed pillars is estimated as 4.1 GJ. Given other energy-absorbing processes involved in the damage, additional sources of energy beyond the pillar strain energy are required. Six scenarios for the event mechanism are examined for consistency with the seismic analysis and on-site damage observations. This study suggests that this catastrophic coal bump was precipitated by a subsidence-related fault slip, with normal slip on a fault in close proximity to the mine workings.