Discrimination of quarry blasts from tectonic microearthquakes in the Hyblean Plateau (Southeastern Sicily)

The seismic network set up in the Hyblean Plateau (Southeastern Sicily) in the framework of the POSEIDON project is aimed at the seismic surveillance of the zone, and in particular the identification of faults with enhanced activity. The seismic activity as inferred from the records of the years 1994-1998 showed an apparent concentration of events in the zone between Augusta and Syracuse where important petrochemical facilities are present, with a resulting elevated secondary seismic risk. However, the heterogeneity in the distribution of events with respect to the time of day made us suspect that these seismicity maps are severely biased by artificial events, such as quarry explosions. We distinguished between tectonic earthquakes and quarry blasts by the inspection of waveforms of certain key stations, and by spectral analysis. As a general rule we found that the local tectonic microearthquakes are richer in high frequencies than the quarry blasts. All events which were identified as quarry blasts occurred during the daytime between 08:00 a.m. and 03:00 p.m. GMT and on weekdays from Monday to Friday. The aforementioned concentration of seismicity between Augusta and Syracuse disappeared when filtering out these events. Automatic discrimination was carried out in a straightforward way using Artificial Neural Networks (ANN) in a supervised classification. The application of the ANN to various test data sets gave a success of about 95%. This showed that our results obtained with a visual discrimination are mathematically reproducible and not arbitrary