We obtained the first estimation of \({Q}_{\beta },{Q}_{i},{Q}_{s}\) quality factors in the GP and a new estimation of \({Q}_{c}\). We used the micro-seismicity of GP recorded by the OT and the RSN seismic network in the period 2013–2018, recently released in a database of seismic waveforms (Filippucci et al., 2021c,d). The new estimation of \({Q}_{c}\) was achieved by the implementation of an automatic procedure to pick the starting time of envelope decay over coda waves with Eq. (2). Since the slope of the envelope decay can vary over time (in general the slope decreases with time, Filippucci et al., 2021a), the choice of the starting point \({T}_{start}\) could control the \({Q}_{c}\) results. Theoretically, Eq. (2) could be applied to a time window that starts immediately after \({T}_{S}\) but this is not the common choice. Commonly, \({T}_{start}=2{T}_{S}\) (e.g., Jin & Aki, 1988; Ibáñez et al., 1990; Eyidogan & Akinci, 1996; Mamada et al., 1997) or in other cases \({T}_{start}=1.5{T}_{S}\) (e.g., Padhy et al., 2011 and references therein). Mukhopadhyay and Sharma (2010), for the Himalayan region, analyzed the effect of \({T}_{start}\) on attenuation and observed: non-significative differences on \({Q}_{c}\) for different \({T}_{start}\); significative decrease in the total number of available waveforms as \({T}_{start}\) increases from \(1.5{T}_{S}\) to \(2.5{T}_{S}\), especially for lapse times below 50 s. So, \({T}_{start}=1.5{T}_{S}\) is the most conservative choice in terms of data selection. To avoid “a priori” choices on \({T}_{start}\) and to eliminate envelopes with abrupt variations along the coda decay (e.g., signal bumps), some authors prefer to manually pick the decay onset by visually inspecting the envelopes (de Lorenzo et al., 2013; Filippucci et al, 2019a; 2021a). The comparison between \({Q}_{c}\) retrieved from the same dataset but with a manual picking of \({T}_{start}\) (Filippucci et al., 2021a) and \({Q}_{c}\) of this work reveals that \({T}_{start}=1.5{T}_{S}\) systematically falls into a zone of the envelope with a less decay rate respect to the manual picking and produces a small shift between the two estimates, inside the standard deviation. This effect can be observed in Fig. 5, revealing a slightly less attenuating crust for GP than the previously estimated one. Being these differences not dramatic and considering the great costs saved in terms of human resources and time, we considered as acceptable the automatic picking procedure with \({T}_{start}=1.5{T}_{S}\).
\({Q}_{c}\) is confirmed to be frequency dependent and increases with increasing frequency, as already observed worldwide both in tectonic and in volcanic contests.
\({Q}_{\beta }\) was estimated by the coda-normalization method (Aki 1980). This method eliminates source, site and geometrical spreading effects by normalizing S-wave amplitudes with coda waves. It reveals the attenuation on body waves due to the medium anelasticity and to elastic scattering on heterogeneities. Generally, the normalized amplitudes of S-waves decrease with increasing of hypocentral distances, demonstrating that body waves attenuate in amplitude with increasing travel distance and this attenuation depends on both frequency and geometrical spreading. This effect is shown in Fig. 3 where, from the slope of the linear fit line [eq. (1)], \({Q}_{\beta }\) can be computed for different frequency bands. As it can be observed from Table 2 and Fig. 4, \({Q}_{\beta }\) regularly increases in all the frequency range except for \(6\text{H}\text{z}<f<\) 8Hz where it is flat. This observation can be inferred also from plots in Fig. 3. The revealed S-wave attenuation at \({f}_{c}=8\)Hz might be due to the scattering loss at random heterogeneities having a characteristic length or alternatively the 8 Hz frequency might be the predominant frequency band of the anelastic response of the medium, as discussed by Sil et al. (2022) for the Kathmandu region, Nepal. The S-wave attenuation was computed by the assumption of the half-space approximation with uniform \({V}_{\beta }\), that is quite unrealistic especially for the shallower layers in the upper crust. This simplification may produce a bias in the estimation of \({Q}_{\beta }\). Some authors (Akinci et al., 2020; Pisconti et al., 2015) estimated the effect of a depth dependent crustal model on the attenuation for the Central Apennine (Italy). They found out that \({Q}_{s}\) and \({Q}_{i}\) are slightly affected by these assumptions (depth-dependent or homogeneous Earth) in the case of a continental crust, where shallow earthquakes occur, while, with increasing the source-receiver distance, the observed apparent attenuation decreases, suggesting an increasing of propagation efficiency with depth below the Moho. The seismicity of GP occurs at crustal depth above the Moho discontinuity (Miccolis et al., 2021) \({Q}_{\beta }\)attenuation should not be severely affected by the homogeneous half-space approximation.
The separated contribution on coda waves of anelastic attenuation \({Q}_{i}\)and elastic scattering \({Q}_{s}\) was inferred by using the Wennerberg (1993) method. The application of this method requires the availability of independent estimates of \({Q}_{\beta }\) and \({Q}_{c}\) as functions of frequency for the same Earth volume in order to avoid bias in the results. In our case we used the same dataset, representative of the same crust volume of the Gargano Promontory (Southern Italy), for the estimates of the total attenuation of body waves\({Q}_{\beta }\) and of the coda wave attenuation \({Q}_{c}\) in the single-scattering approximation. Results in Fig. 4 indicate a regular descending trend of attenuation as functions of frequency. \({Q}_{s}\) presents an irregular low value at 8 Hz reinforcing the idea that the lower value of \({Q}_{\beta }\) at 8 Hz might be due to a scattering effect. Assuming a uniform half space with constant velocity \({V}_{\beta }=3.86\)km/s as we did, \(f=8\)Hz corresponds to a wavelength \(\approx 0.5\) km, which could represent the average distance of the random scatterers in the Gargano crust. For all the other frequencies, scattering on heterogeneities turns out to be less important than anelasticity in the coda attenuation in agreement with the preceding results in regional studies of Southern Apennine which included marginally the GP area (Bianco et al., 2002).
Results of \({Q}_{i}\)and \({Q}_{s}\) in tectonic contests worldwide suggest that the attenuation of coda waves is dominated by the intrinsic attenuation (Padhy et al, 2011; Farrokhi et al., 2016; Pujades et al., 1997; Akinci et al., 1995; Aki & Chouet, 1975; Bianco et al., 2005; Tuvè et al., 2006 among the others). The predominance of the anelasticity on coda waves increases by increasing frequency since the percentage ratio \({(Q}_{s}-{Q}_{i})/{Q}_{s}\) moves from 48% at 3Hz to 70% at 12Hz (Table 2). The seismic albedo is \({B}_{0}<0.5\) for all the frequency bands which indicates that anelasticity is the predominant attenuation effect in the region. The only exception is for \(f=8\)Hz where \({B}_{0}\cong 0.5\) indicating a predominance of scattering at the scale length of this frequency. The extinction length \({L}_{e}\) ranges between 18 and 31 km, increases with frequency (with the exception for \(f=8\) Hz where \({L}_{e}\) is lower) and it is comparable with other studies in tectonic domains (Akinci et al., 2020; Londono et al., 2022; Sharma et al., 2015; Shengelia et al., 2020; Talebi et al., 2021 among the others). It is worth noting that the results might be biased by the constant velocity half space assumption (Del Pezzo et al., 2019). The comparison between the outcomes of this study and those previously obtained for other tectonically active regions in Italy and worldwide reveals that the GP values of seismic quality factors \({Q}_{i}\) and \({Q}_{s}\) are within the end members at all frequencies (Fig. 6).
The frequency dependence relationships of \({Q}_{\beta },{Q}_{c},{Q}_{i},{Q}_{s}\) show estimates of \({Q}_{0}\) up to 40 and values of \(\alpha\) greater than 1.25. The paradox consists of both the small values of \({Q}_{0}\) (less than 48) and the high values of \(\alpha\) (greater than 1.1) that should indicate a high tectonic activity and the general absence of high magnitude earthquakes in the GP (Morozov, 2008). The \({M}_{W}=5.7\) Molise earthquake (Southern Italy, 2002 October 31th ) occurred in an adjacent area previously classified as low hazard area, may reinforce the hypothesis that the GP could reasonably located in a tectonically active regime.
At 12 Hz we can observe in Fig. 4 the maximum difference between \({Q}_{s}\) and \({Q}_{i}\), so it might be stated that coda amplitudes at 12 Hz are controlled by anelastic damping and it can be considered predominantly as attenuation of body waves, since attenuation of surface waves is detectable at lower frequencies, less than 10 Hz (Aki & Chouet, 1975). Therefore \({Q}_{c}\approx {Q}_{i}\) at all frequencies and this result has relevant consequences if we examine the recent 3D tomography of coda attenuation in GP (Filippucci et al., 2021a). The 3D images of \({Q}_{c}\) were obtained for three frequencies (3Hz, 6Hz and 12 Hz in Filippucci et al., 2021a) by using the Del Pezzo & Ibanez (2020) approach which consists of computing the polynomial approximation of the analytical sensitivity kernels. The polynomial sensitivity kernels were then used as weighting functions for estimating \({Q}_{c}\) in an Earth volume, which was subdivided in cubic pixels with side of 5 km, producing a horizontal \({Q}_{c}\)image every 8 km of depth for the GP crust. We reported in Fig. 7 the images of the 3D tomography of \({Q}_{c}\) at 12Hz (modified from Filippucci et al., 2021a). Now we can consider \({Q}_{c}\approx {Q}_{i}\) and give a physical interpretation of the observed anomalies by interpreting them as attenuation of body waves. The well-defined high \({Q}_{i}\) anomaly, extending down to 24 km, reveals the presence of a body embedded in a more attenuating one in the northern GP which might agree with the hypothesis of the existence of a high-density and high susceptibility body in the same area (Loddo et al., 1996). This hypothesis agrees also with the observation of some igneous rocks that crop out in GP in a land named Punta delle Pietre Nere (black star in Fig. 1), intruding the upper Triassic sedimentary successions. These magmas are interpreted as derived from an amphibole-bearing lithospheric mantle source at 70–90 km depth (Mazzeo et al, 2018).
Figure 7 shows a \({Q}_{i}\) decreasing trend (attenuation increasing trend) moving toward the Northeast sector of GP. This sector is characterized by an anomalous absence of seismic activity down to 20 km (Miccolis et al., 2021). Therefore, the observed \({Q}_{i}\) decreasing trend well correlates with the hypothesis of a ductile behavior characterizing the upper basement and the sedimentary cover, as confirmed by a recent thermo-rheological model (Lavecchia et al., 2022). At depths greater than 20 km, a seismogenic layer is encountered (Miccolis et al., 2020) and it was modelled as due to the presence of fluids in the deepest part of the basement (Lavecchia et al., 2022). These results agree with the presence of general high \({Q}_{i}\) (low attenuation) values that can be a hint of a brittle and low-strength lower crust in the deepest seismogenic layer ( Fig. 7, 32 km). It is worth to note that at all depths the most part of earthquake foci falls in areas characterized by high \({Q}_{i}\), indicating a quite good agreement between the presence of seismicity and lower levels of anelastic damping both correlated to the brittle behavior of rocks.