SOURCE MECHANISM OF AN EXPLOSIVELY INDUCED MINE COLLAPSE

Mining explosions and collapses, in addition to earthquakes, may trigg er 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 colla pse was investigated, The collapse occurred immediately after the supp ort pillars of a 320-m-deep underground mine opening were destroyed by explosives. It had an Lg body-wave magnitude (m(bLg)) of 2.8, We anal yzed free-surface ground-motion data (within 1200 m) from the collapse by waveform forward modeling and time-dependent source moment-tenser inversion. The results indicate that the source mechanism of the colla pse can be represented by a horizontal opening and closing crack, The time functions of the diagonal source moment-tenser components are sim ilar 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 cr ack. Because of the initially expansive source characteristic, this ki nd 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 s urface-wave generation.