Source, attenuation, and site parameters of the 1997 Umbria-Marche seismic sequence from the inversion of P wave spectra: A comparison between constant QP and frequency-dependent QP models

The attenuation of P waves, the site responses, and the source parameters (seismic moment, corner frequency, source dimension) of 490 seismic events that occurred during the 1997 Colfiorito, Umbria-Marche seismic sequence have been inferred from the inversion of P wave velocity spectra. The Boatwright source model has been assumed to model the source spectra. A global nonlinear inversion scheme was developed to avoid any a priori selection of the initial Q(P) and the corner frequency. To establish if a frequency-dependent Q(P) model fits the data better than a constant Q(P) model, two inversion results have been compared. Application of the Akaike information criterion indicates that the constant Q(P) model represents the best compromise between model simplicity and data misfit. The station Q(P) values are small: in the range of 50 to 200. A one-dimensional Q(P) model is obtained by back projecting the inverted t*. Our results indicate both well-defined near-site attenuation effects at some sites and heterogeneity in the inelastic properties of the crust. With the exception of the amplification response at five seismic stations, most of the recording sites did not show amplification peaks at particular frequencies. The stress drop clearly increases as a function of the seismic moment, which indicates a deviation from self-similarity, whereas it does not show an increase with depth, probably owing to the effects of fluid pressurization in the crust. A stress drop of about 39 MPa is inferred. The relationship between the seismic moment and the local magnitude for P waves has been calibrated.