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Discriminating quarry blasts from earthquakes using coda waves

DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF SOUTHERN CALIFORNIA, LOS ANGELES, CALIFORNIA 90089-0740
GEOPHYSICAL INSTITUTE UNIVERSITY OF ALASKA, FAIRBANKS, ALASKA 99701

Abstract

A new quarry blast-earthquake discrimination method is presented based on the analysis of seismic coda waves. Quarry blasts and local earthquakes in the area encompassing the south-central Mojave Desert and Eastern Transverse Ranges were used to test this method. We found that the coda decay rate Q_c^–1 is significantly higher for quarry blasts than earthquakes for lower frequencies (1.5 and 3 Hz) for lapse time up to about 30 sec. This result is attributed to the greater contribution of surface waves to quarry blasts due to the shallowness of their source depth. The difference in Q_c^–1, however, disappears for lapse time greater than 30 sec for the same frequency range as well as for higher frequencies (6 and 12 Hz) for lapse time greater than 20 sec. This suggests that the coda waves, at the lapse time greater than these critical values, are dominated by the same type of body waves, probably S waves, for both quarry blasts and earthquakes. The power spectrum P₀() obtained after the correction for attenuation was compared between earthquakes and quarry blasts at the same stations, and a significant difference was found in the spectral shape between these two data sets. The curves of power spectrum P₀() versus frequency for quarry blasts decrease more sharply than for earthquakes at high frequencies, indicating a lack of energy in high frequencies for quarry blasts as compared to earthquakes. The different frequency dependence of power spectrum P₀() between quarry blasts and earthquakes is attributed to their different source properties and can be used for seismic discrimination of quarry blasts from earthquakes.

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