Discriminating Between Large Mine Collapses and Explosions Using Teleseismic P Waves |
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| Authors: | D Bowers W R Walter |
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| Institution: | AWE Blacknest, Brimpton, Reading, Berkshire. RG7 4RS. UK, E-mail: bowers@blacknest.gov.uk, GB
Lawrence Livermore National Laboratory, P.O. Box 808, L-205, Livermore, CA 94551-0808. USA. E-mail: bwalter@llnl.gov, US
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| Abstract: | —?Some of the most suspicious seismic disturbances under the Comprehensive Nuclear-Test-Ban Treaty (CTBT) are likely to be those associated with mining, as they are shallow, and at least some have an explosion-like m b :M s signature. Previous research highlighted the potential of broadband teleseismic P waves as a way of identifying large mine tremors. Broadband teleseismic P from two suspected large mine collapses, one in Germany (1302 UT, 13 March 1989, 5.4?m b ) and another in Wyoming (1526 UT, 3 February 1995, 5.3?m b ), show differences in character despite the similarity of the reported ground failure and mine types. We apply a full moment-tensor analysis to the teleseismic P waves and show that the data are inconsistent with either a shallow explosion or an earthquake (double-couple) at depth, but this method is unable to distinguish between a shallow dip-slip source and a closing-crack moment tensor. However, three-component surface-wave seismograms recorded at regional distances fit the shallow closing-crack model, but are inconsistent with a shallow earthquake source, because strong Love waves, expected from a double-couple source, are not observed at a number of stations well distributed in azimuth. Here, we restate the equivalence for shallow sources of the closing-crack model and a gravitational collapse model. We use the latter to model the broadband P waves from these mine tremors and show that, while non-unique, the differences in the observed broadband P waves from the two tremors can be attributed to the area, amount of collapse, depth, and rate of collapse. The collapse model predicts negative first-motion for all P waves in contrast to the positive polarity expected from explosions. Thus, the broadband teleseismic P waves have the potential to discriminate between large collapses and explosions. |
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