Qβ, Qc, Qi, Qs of the Gargano Promontory (Southern Italy)

Abstract

We have provided the first estimate of scattering and intrinsic attenuation for the Gargano Promontory (Southern Italy) analyzing 190 local earthquakes with M-L ranging from 1.0 to 2.8. To separate the intrinsic Q(i) and scattering Q(s) quality factors with the Wennerberg approach (1993), we have measured the direct S waves and coda quality factors ( Q(beta), Q(c)) in the same volume of crust. Q(beta) parameter is derived with the coda normalization method (Aki 1980) and Q(c) factor is derived with the coda envelope decay method (Sato 1977). We selected the coda envelope by performing an automatic picking procedure from T-start = 1.5T(S) up to 30 s after origin time (lapse time T-L). All the obtained quality factors clearly increase with frequency. The Q(c) values correspond to those recently obtained for the area. The estimated Q(i) are comparable to the Q(c) at all frequencies and range between 100 and 1000. The Q(s) parameter shows higher values than Q(i), except for 8 Hz, where the two estimates are closer. This implies a predominance of intrinsic attenuation over the scattering attenuation. Furthermore, the similarity between Q(i) and Q(c) allows us to interpret the high Q(c) anomaly previously found in the northern Gargano Promontory up to a depth of 24 km, as a volume of crust characterized by very low seismic dumping produced by conversion of seismic energy into heat. Moreover, most of the earthquake foci fall in high Q(i) areas, indicating lower level of anelastic dumping and a brittle behavior of rocks.