GNGTS 2013 - Atti del 32° Convegno Nazionale

The 2012 Emilia sequence: application of an automatic procedure to determine moment magnitude A. Gallo 1,2 , G. Costa 2 , P. Suhadolc 2 1 AREA Science Park – Trieste, Italy 2 Seismological Research and Monitoring Group, Department of Mathematics and Geosciences, University of Trieste, Italy On May 2012 a seismic sequence took place in the Emilia Romagna region, northern Italy. The two main-shocks on May 20 and May 29 with M L =5.9 and M L =5.8, respectively, were followed by several relevant aftershocks with M L >4. Using a procedure implemented by the SeisRaM group of the Department of Mathematics and Geosciences of the University of Trieste, the seismic moment is estimated, as well as the moment magnitude and the corner frequency of the events recorded by strong motion instruments. The goals of this procedure are: the rapid determination of earthquake parameters and an interface to obtain a fast and reliable communication of the parameters related to the seismic events to the Civil Defense. We analyze a strong-motion dataset consisting of high-quality records (among which the two main events and several aftershocks with magnitudes ranging from 3 to 5) obtained by the National Strong Motion Network (RAN). The RAN is distributed on the Italian territory to record earthquakes of medium and high intensity. It is managed by the Seismic Monitoring Service of the Territory within the Seismic and Volcanic Risks Office of the Civil Protection Department (DPC) in Rome (Gorini et al. , 2010; Zambonelli et al. , 2011). RAN has more than 500 digital stations equipped with a GSM modem or GPRS, connected to the RAN data capture Centre of Rome ( last update: 20 May 2011 ). The Antelope ® software (BRTT, Boulder) that collects and archives data, and the SeisRaM procedure to determine moment magnitude and all seismic source parameters in near real-time (Gallo et al. , 2013), is now installed also at DPC for managing and analyzing recorded data. The SeisRaM procedure is extensively described in Gallo et al. (2013), but we recall here only the main aspects. Using spectral analysis, the seismic source parameters are calculated following Andrews (1986). The source model used is a simple ω 2 model proposed by Brune (1970, 1971). For the attenuation we use the Q frequency-depend attenuation factor (Console and Rovelli, 1981) and we assume a body-waves theory for the geometrical spreading. From the corrected amplitude spectra the corner frequency also the Brune low-frequency spec- trum amplitude and the seismic moment are computed. Finally, the moment magnitude is determined according to the Kanamori (1979) formula. The procedure starts by taking the event location, Richter magnitude (Richter, 1935), P and S phases, signals and instrumental response from Antelope databases. We remove the average, trend, spike and instrumental re- sponse. We limit ourselves to events with epicentral distances up to the maximum of 70 km, in order to respect the assumption of the spherical geometrical decay. It is essential to de- termine the frequency range over which the observed spectral levels are significantly higher than noise. We select band pass corner frequencies using SNR. The minimum frequency corresponding to the first value for which SNR > 2.5, the maximum frequency the last value for which SNR > 5. Following Ottemoller and Havskov (2003), in the selected frequency window the SNR average must be everywhere larger than 1.5. This additional requirement that the average ratio between signal and noise spectral amplitude in the selected frequency range be above a threshold value, allows to choose the final frequency window and to avoid processing only noise. We apply a Butterworth band pass filter, and then we obtain accel- erations, velocities and displacements from the derivative or the integral of the signal. We apply the Fast Fourier Transform (FFT) to obtain the signal spectra. Then we correct them for geometrical spreading and intrinsic attenuation to retrieve the source spectra. At the end we calculate seismic moment and moment magnitude and strong motion parameters like as PGA, PGV Arial and Housner intensity, and store these results in a database table. A report is also generated within 10 min from the event. 59 GNGTS 2013 S essione 1.1