Source parameters estimated from the aftershocks of the 1997 Umbria-Marche (Italy) seismic sequence

We derived the source parameters of 563 aftershocks (1.4 less than or equal to M-L less than or equal to 4.5) of the 1997 Umbria-Marche seismic sequence by analyzing three-component digital recordings of a temporary network and developed a local magnitude scale using the whole available data set (more than 25,000 waveforms) from synthetic Wood-Anderson seismograms. Then, considering a subnetwork composed of nine portable stations, more than 12,000 earthquake spectra were corrected for the near-surface attenuation parameter k and the path attenuation. The corrected spectra were used to compute the seismic moment, the source radius, and stress drop assuming an omega (-2) Brune source model. We calculate the relationship log M-0 = 17.46 + 1.12 M-L between seismic moment and local magnitude for the studied area. The source dimension versus seismic moment relationship did not show any breakdown in the similarity of the rupture process, even when events of magnitude less than 2.2 (the threshold value for the completeness of the considered catalog.) were considered. The average stress drop obtained analyzing the S-wave spectra was 38 +/- 10 bar, and the source dimension ranged between about 40 and 500 m.