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The Unique Source Mechanism of an Explosively Induced Mine Collapse

Xiaoning Yang, Brian W. Stump, W. Scott Phillips

Geophysics Group (EES-11), Los Alamos National Laboratory


Introduction

Underground mine collapses are of interest because of their similar source characteristics to underground nuclear explosions. Because these similarities include a large isotropic component, existing regional discriminants may identify these collapses as explosions. Their magnitudes can be greater than 4 which is above the triggering threshold of the International Monitoring System at regional distances. Many of these mine collapses are spontaneous collapses that occur unexpectedly. Studies show that most of the shallow, spontaneous collapses have implosional source mechanisms like a closing void. This implosional source mechanism might provide a physical basis to discriminate these collapses from explosions. In this study, we report the analysis results of a planned, explosively induced mine collapse. The collapse occurred during a controlled experiment at White Pine Copper Mine in Michigan. In the experiment, the supporting pillars of an underground mine opening at 320 m depth were destroyed with explosives and the roof collapsed upon the sudden loading of its unsupported weight. The mbLg magnitude of the collapse is 2.8. We analyzed the first motions of the collapse recorded both close-in and at regional distances and recovered the time dependent source moment tensor of the collapse from near-source observations. The results indicate that, in contrast to spontaneous collapses, the explosively induced collapse initiated as an opening crack. This observation may have important implications to the seismic discrimination problem. Based on the results, we propose an explanation for the source processes of the explosively induced mine collapse. The conclusion is supported by the analysis of a second explosively induced collapse at the same mine.

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Experiment and Data
 
Analysis Results
 
Interpretation and Discussion
 
Conclusion