Broadband Seismic Recordings of Mining Explosions and Earthquakes in South America

Abstract : S/P ratios have shown great promise as discriminants for explosions and earthquakes recorded at regional distances. Spectrograms, or the frequency-time displays of seismograms, have been used to separate explosion populations into those which are single-shot or multiple-hole instantaneous explosions and those which are ripple-fired explosions. However, despite the promise of these discriminants, there are problems of transportability. The character of regional phases is extremely sensitive to differences in propagation paths, and it is essential to calibrate regions of monitoring interest. We have investigated the seismicity (both natural and man-made) in central Chile recorded on a high-frequency seismic network. We are developing a data base to quantify distance-dependent spectral ratios and other potential discriminants. Within the seismic network is a large open-pit copper mine which has occasional multiple-shot explosions with magnitudes as large as 3.7. We find at close epicentral distances (< approx. 60 km) that mining explosions show spectral scalloping indicative of 'ripple-fire' sources and that S-P amplitude ratios indicate enhanced P wave radiation. However, at epicentral distances of approx. 100 km the frequency content has decreased, with little energy above 5 Hz. Preliminary analysis indicates that spectral discriminants for earthquakes and ripple-fire explosions at close epicentral distances operate as other investigators have reported for other regions in the world.