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Discrimination of microearthquakes and quarry blasts: a case study in northern Egypt

Badawy A.

Acta Geophysica Polonica

2001

Vol. 49, nr 3

331-341

In northern Egypt, both microearthquakes and quarry blasts are observed in complicated tectonic region. Therefore, reliable event identification, preferably based on a simple method that can be easily used in a routine processing, is essential for accurate and detailed seismotectonic studies. Amplitude spectra of microearthquakes and quarry blasts recorded by stations of the Egyptian National Seismograph Network (ENSN) have been computed. Most quarry blasts in northern Egypt are ripple-fired with time delays ranging from 20 to 25 ms; consequently, we seek an event discrimination method based on the recognition of ripple-firing patterns in the signal. For this end, we compared the spectra of complete seismogram signals of 55 microearthquakes and 150 quarry blasts at the same stations to eliminate wave-path and distance effects. The obtained results clearly indicate that the microearthquakes spectra seem to contain high frequency energy more than thee spectra of quarry blasts. We found a prominent spectral modulation in quarry blasts spectra but not in the spectra computed from microearthquake seismograms. The observed spectral modulation is mainly due to ripple-firing explosion and it is independent of time in the coda. Finally, the quarry blasts can be discriminated from non-ripple-firing events (e.g., earthquakes) on the basis of low frequency (less than 15-20 Hz) content and the spectral modulations.