## 4 Discussion
The penetration depth ranges between 19 and 77 km (Table 1). The frequency-dependence of \(Q_{c}(f_{c})\) in Equation (4) is interpreted as a tectonic parameter. Regions of high tectonic activity are characterized by low \(Q_{c}\) when compared to stable regions. Many measurements of \(Q_{0}\) and \(\alpha\), in different tectonic and geological settings, have indicated a trend of higher \(Q_{0}\) (roughly 100 to 1000) and lower \(\alpha\) (about 0.3 to 0.7) for stable continental regions, while a lower \(Q_{0}\) (from 40 to 200) and a higher \(\alpha\) (from 0.7 to 1.1) are found in tectonically active areas (e.g., [68; 69; 70]). In Table 2, we reported the results of the \(Q_{0}\) and \(\alpha\) measurements in several regions of active tectonics worldwide, for comparable values of \(t_{L}\). In this study, \(Q_{0}\) varied between 26 and 66 (Table 1), indicating that the upper lithosphere is seismically active. The frequency dependence exponent, \(\alpha\), varies between 0.79 and 1 (Table 1); the lower value of \(\alpha\) occurs for \(t_{L}\) = 15 s, corresponding to a penetration depth of 30 km, in the lower crust, suggesting that the higher degree of heterogeneities corresponds to the depth where the seismicity of the GP is concentrated. \(Q_{c}\), computed for each station, shows a low-quality factor and great variability at low \(t_{L}\), while at high \(t_{L}\), \(Q_{c}\) estimates appear to be regular and higher (Figure 6). This result may indicate, as observed worldwide, that the upper crust is more heterogeneous than the lower crust, with the presence of high-frequency scattering. Stations of the OT network have very similar values and regular trends of \(Q_{c}\) at all the lapse time windows \(t_{L}\) (Figure 6). Regarding the IV stations, \(Q_{c}\) estimates are always higher for stations that are inside the GP (SGRT, MSAG, APRC) than those outside the GP (AMUR, CAPA, MOCO, MRVN, SGTA, TREM) at all \(t_{L}\), reflecting the heterogeneous characteristics of the crust in this area (Figure 6). It is worth noting that \(Q_{c}(f_{c})\) for the GP follows a trend very similar to that of the Central and Southern Apennines, Italy [39; 62]. The Italian Apennines are the site of both historical and recent destructive earthquakes [71], so the similarity in \(Q_{c}(f_{c})\) with that of the GP may have evident and important implications regarding the seismic hazard. In fact, although for several decades, the GP has been affected by earthquakes of small or very small magnitude, it has historically been the site of very destructive earthquakes (Figure 1). Differences among \(Q_{c}(f_{c})\) estimated in this study and that estimated in other areas in Italy and worldwide (Figure 7a,b) can be ascribed to the different geological-structural settings derived from distinct geodynamic and paleogeographic domains. Considering that \(Q_{c}\) is representative of the intrinsic attenuation and that this similarity increases at high frequencies, as already discussed, we can interpret the tomographic images in Figure 8. All \(Q_{c}\) images are coupled with the standard deviation \(\sigma\), which indicates the high quality and robustness of the results. Seismic attenuation is homogeneous and high at all depths, at both low and medium frequencies (Figure 8a,b). A high \(Q_{c}\) anomaly (marked with A in Figure 8a) is present in the southwestern GP sector at all depths but, since it corresponds to an increased \(\sigma\), it is poorly resolved. Some heterogeneities can be observed at high frequencies (Figure 8c). In the shallowest map sections (at 0 km, 8 km and 16 km) \(Q_{c}\) shows a well-localized high \(Q_{c}\) anomaly (labeled B in Figure 8c) and a north-eastward decreasing trend (toward label C in Figure 8c). The high \(Q_{c}\) anomaly (zone labeled B, in Figure 8c) is observed at depth down to about 16 km in the northernmost part of the GP. This \(Q_{c}\) anomaly is remarkable, since it corresponds with the area studied by [PERSON] et al.