Regional body-wave attenuation using a coda source normalization method: application to MEDNET records of earthquakes in Italy

We develop a new methodology to determine apparent
attenuation for the regional seismic phases Pn, Pg, Sn, and Lg using coda-derived source spectra. The apparent Q, combining path and site attenuation, is determined from the difference between the geometrical spreading-corrected amplitude and the independently determined source spectra derived from the coda methodology. We apply the technique to 50 earthquakes with magnitudes greater than 4 in central Italy as recorded by MEDNET broadband stations around the Mediterranean at local-to-regional distances. A power law attenuation of the form Q(f) = Qof^gamma fit all the phases quite well over the 0.5 to 8 Hz band. The measured apparent Q values are quite repeatable from event to event. Finding the attenuation function in this manner guarantees a close match between inferred source spectra from direct waves and coda techniques. This is important if coda and direct wave amplitudes are to produce consistent seismic results.