Microseismicity induced by a controlled, mine collapse at White Pine, Michigan

Abstract

Abstract We recorded an, explosively induced, 320 m deep, mine collapse and subsequent aftershocks, at White Pine, Michigan, using an array of 12 seismic stations. The collapse resulted from the rubblization of a 3×10 4 m 2 panel of a room-and-pillar copper mine, performed to facilitate leaching operations. The explosions produced little seismic energy. However, the collapse generated strong tension-crack, free-fall and slap-down phases. Regional data indicate a magnitude (m bLg ) of 3.1, leading to estimates of displaced mass, 5.6×10 8 kg and thickness, 11 m. Peak acceleration was 300 cm/s 2 at ground zero and dropped to 20 cm/s 2 at 1.1 km. Most of the locatable affershocks, (90%) occured in the first 2 hours following collapse. At ground zero, the occurrence rate followed the modified Omori, law: Rate=560·(time-0.01) −1.3 , with time in hours. The largest aftershock generated a moment magnitude of 1.0. We obtained locations of 135 aftershocks. The aftershock zone was less than 100 m thick, situated on top of the collapsed panel. The shallowest aftershocks occurred at depths of 200 m, giving no indication of collapse-related deformation extending to the surface. Aftershocks concentrated along the only edge of the collapsed panel open to the room-and-pillar mine. If the seismically active area reflects the extent of the de-stressed zone used in modeling stress redistribution, the asymmetrical distribution with respect to the collapsed panel was consistent with lower-than-predicted stresses measured in the first row of intact pillars.