Depth-dependent intrinsic and scattering seismic attenuation in north central Italy

In this study the attenuation mechanism of seismic wave energy in north central Italy is
estimated using low-magnitude earthquake local data recorded at six stations managed by
INGV. Most of the analysed events are located along the Alpine chain in the zone of Iseo and
Garda lakes, while a minor part in the Po valley. The zone investigated is characterized by the
occurrence of significantly intense earthquakes (magnitude up to 6.6) the most recent occurred
in 2004 close to the city of Sal`o on the coast of the Garda lake (Mw = 5.0). Due to the high
population density and presence of industrial activity the investigated area is characterized
by a high seismic risk. First, the ordinary Multiple Lapse Time Window Analysis (MLTWA)
method is applied in the assumption of uniformvelocity and scattering and the couple of B0, the
seismic albedo and Le−1, the extinction length inverse (corresponding to the total attenuation
coefficient) is calculated in the frequency bands of 1.5, 3, 6 and 12 Hz. To retrieve more realistic
estimates, the obtained values of B0 and Le−1 are corrected taking into account the effects of
a depth-dependent earth model, consisting of an earth structure characterized by a transparent
upper mantle and a heterogeneous crust. We find that the corrected intrinsic and scattering
attenuation parameters (which are proportional to the inverse of the intrinsic/scattering quality
factors, QI−1 and Qs−1) are strongly frequency dependent, with a prevalence of scattering
attenuation over the intrinsic dissipation. The corrected and uncorrected values of total Q are
in agreement with the total Q values obtained with different approaches for the same area.