Aftershocks of an explosively induced mine collapse at White Pine, Michigan

Abstract

AbstractWe recorded an explosively induced, 320-m-deep mine collapse and subsequent aftershocks at White Pine, Michigan, using an array of 12 seismic stations, sited within 1 km of surface ground zero. The collapse, which followed the rubblizing of a 2 × 104 m2 panel of a room-and-pillar copper mine, was induced to facilitate leaching operations. The explosions produced little seismic energy; however, fracturing and collapse stages produced large signals that were observed at distances up to 900 km, yielding a magnitude (mbLg) of 2.8. Previous work showed the initial collapse to be an expanding seismic source, interpreted as an opening tensile crack, opposite to the implosional character most often observed for natural mine collapses (Yang et al., 1998). We counted over 4000 aftershocks; their occurrence rate followed the modified Omori law: rate = 560 (time − 0.01)−1.3, with time in hours. Based on P-wave polarities, we identified events of shear-slip, implosional, and tensile character in the aftershock sequence. For shear-slip events, we found stress drops of 1 bar or less, seismic moments of 1015−1017 dyne cm, (Mw−0.8−0.5) and source radii of 10-50 m. Corner frequencies for implosional events were relatively low, an indication that the collapsed cavity played a role in the source process. This caused implosional events to separate from other events in source parameter plots, providing a technique for classifying events of unknown type. We obtained locations of 135 aftershocks using P- and S-wave data. The aftershock zone was less than 100-m thick, situated just above and along the western, mined edge of the collapsed mine panel. Implosional events occurred at the bottom of the active volume, while shear-slip events were distributed throughout. Shear-slip focal mechanisms indicated thrusting along north-striking planes, consistent with the high, eastwest regional compressive stress, coupled with a local decrease in vertical stress. The inferred deficit of vertical stress above the western panel edge following collapse indicated that overburden load shifted preferentially to the surrounding, unmined areas, consistent with lower-than-predicted stresses measured in the first row of intact pillars.