Automatic Detection, and P- and S-wave Picking Algorithm: an application to the 2009 L'Aquila (Central Italy) earthquake sequence.

In order to process as quickly as possible the enormous amount of digital waveforms continuously recorded at permanent and temporary seismic stations in Italy, we implemented a semi-automatic procedure in order to identify local earthquakes and to provide consistently-weighted P- and S-wave arrival times. Local earthquake detection is obtained by a STA/LTA ratio-based algorithm applied to 3-component seismograms from individual stations. A minimum of 4 triggered stations are required to declare a seismic event. This setting proves to be extremely effective to detect a large number of very-low magnitude earthquakes (ML>1.5) with a small number of false alarms. The automatic picking system Mannekenpix (Aldersons, 2004), originally working on vertical component data, has been expanded to tackle 3-component data. In order to increase the reliability of P-wave and S-wave picking, the system is now virtually capable of discriminating among noise samples, P-wave samples and S-wave samples. This Identification is performed by a C5 decision tree (Quinlan, 1993) derived from training data. Five groups of predicting variables are included: Energy, Polarization