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Three dimensional attenuation images of Mt Etna volcano obtained by the analysis of Q -coda from local volcano-tectonic earthquakes are presented in this work. Seismic sources are confined inside the Etna structure with a maximum focal depth of 35 km below the sea level. The space distribution of the attenuation values was calculated by using 3-D weighting functions derived by the sensitivity kernels of Pacheco & Snieders and approximated by a polynomial interpolation, represented in the maps by using a backprojection method. Data were analyzed in four bands with central frequency placed at 1.5, 3, 6 and 12 Hz, respectively. We observed a frequency dependence of Q -coda with values that range from 55 at 1.5 Hz to 218 at 12 Hz. Q -coda space distribution in the Etna area shows almost uniformity in the average attenuation in the first 35 km below the surface. The images were derived with a resolution of 5 km. We observe as one of our main conclusions that Q -coda attenuation space anomalies are correlated with the areas of highest structural heterogeneities and are distributed along the well-known tectonic structures which characterize the crust in Mt Etna region. Previous and numerous velocity and attenuation images describing the structure of Mt Etna support our main conclusion: high Q -coda volumes almost coincide with the zones marked by high velocity and relative low total attenuation for direct waves. [ABSTRACT FROM AUTHOR]

The mechanism of seismic attenuation for shear waves was studied through the separate measure of the intrinsic dissipation and scattering coefficients. A modified version of the Multiple Lapse Time Window Analysis (MLTWA) was applied to about 5000 local earthquakes with magnitude 1.5 ≤ Ml ≤ 4.8 occurred in the period 2006–2017 in both the tectonic (Peloritani Mountains and Hyblean Plateau) and volcanic (Aeolian Islands and Mt Etna) areas of eastern Sicily. Observed data, processed via direct Fourier Transform instead of the filtering and averaging procedures utilized in the ordinary MLTWA technique, were fit to the correspondent Paasschens solution of the Energy Transport model in 3D. Model parameters B 0, the seismic albedo, and Le , the extinction length, in turn associated with the intrinsic (Qi) and scattering (Qs) quality factors for S waves, are thus obtained from the fit in the frequency bands centred at 1.5, 3, 6 and 12 Hz. Finally, the corrections to these estimates in case of an opaque layer (the crust) overlying a uniform and transparent mantle, were applied in order to obtain more realistic estimates of the seismic attenuation in the areas investigated. Obtained results are first interpreted in terms of the characteristics of each domain and then compared by the same point of view with a huge set of measurements of the same parameters carried out in other tectonic and volcanic domains through the world. Results show how the rock heterogeneity characterizes the tectonic domain and determines the value of total attenuation coefficient, which, on the other hand, is a crucial parameter in the estimation of the seismic hazard. [ABSTRACT FROM AUTHOR]

In this study, we derived the first 3-D P-wave seismic attenuation images (QP) as well as new 3-D VP and VP/VS models for the crust in southeastern Sicily. We used a large data set of local seismic events occurring in the time span 1994-2013. The results of this tomographic study have important implications on the seismic behaviour of the region. Based on velocity and attenuation images, we identified distinct volumes characterized by different fluid content, which correlate well with seismicity distribution. Moreover, the obtained velocity and attenuation tomographies help us to provide a more complete picture of the crustal structure of the area. High VP, high QP and high VP/VS values have been obtained in the crustal basement, below a depth of 8 km, and may be interpreted as due to the presence of serpentinized peridotites. Accordingly, the new model for the degree of serpentinization, retrieved from VP values, shows that the basement has an average serpentinization value of 96 ± 3 vol.% at 8 km, decreasing to 44 ± 5 vol.% at about 18-20 km. Our joint interpretation of geophysical and petrophysical evidence suggests that the nature and composition of the Hyblean upper lithosphere may differ from accepted and longestablished geological models, which consider this lithospheric block a continuation of the Africa continental plate. [ABSTRACT FROM AUTHOR]

Seismic wave attenuation in southeastern Sicily was investigated by using a data set of about 180 local earthquakes recorded in the period 1994–2003. We first estimated the quality factor of S waves and clarified its frequency dependence by means of the coda-normalization method, applied in the frequency range 1.5–15 Hz. The average as function of frequency is given by . A detailed separation of scattering attenuation from intrinsic absorption was also attempted by applying the multiple lapse time window analysis (MLTWA), under the hypothesis of multiple isotropic scattering with uniform distribution of scatterers. Intrinsic absorption dominates over scattering in the attenuation process at high frequencies (above 3 Hz). Below 3 Hz scattering is the predominant attenuation effect in the region, at the scale length of these frequencies. However, some discrepancies have been observed between the theoretical model and the observations. This indicates that models with depth-dependent velocity structure and/or non isotropic scattering should be taken into account in order to remove ambiguities in the interpretation of the results. [ABSTRACT FROM AUTHOR]