Source mechanism of an explosively induced mine collapse

Abstract

Abstract Mining explosions and collapses, in addition to earthquakes, may trigger the future Comprehensive Test Ban Treaty (CTBT) monitoring system. Many shallow, spontaneously occurring mine collapses have implosional source mechanisms that might provide a physical basis to discriminate them from explosions. In this study, an explosively induced mine collapse was investigated. The collapse occurred immediately after the support pillars of a 320-m-deep underground mine opening were destroyed by explosives. It had an Lg body-wave magnitude (mbLg) of 2.8. We analyzed free-surface ground-motion data (within 1200 m) from the collapse by waveform forward modeling and time-dependent source moment-tensor inversion. The results indicate that the source mechanism of the collapse can be represented by a horizontal opening and closing crack. The time functions of the diagonal source moment-tensor components are similar to that of a spall source accompanying an underground explosion. A unique source characteristic of the induced collapse is that, unlike spontaneous collapses, the induced collapse initiated as a tensile crack. Because of the initially expansive source characteristic, this kind of induced mine collapses may pose some difficulties to the seismic discrimination problem. Despite the similarities between the induced mine collapse and underground explosions, the collapse has a more band-limited source spectrum and seems to be more efficient in shear and surface-wave generation.