An attenuation study using earthquakes from the 1997 Umbria-Marche sequence

We studied spatial and temporal characteristics of seismic attenuation in Central Italy using S- and coda- waves recorded by the Marchesan Seismograph Network from earthquakes located in the epicentral area of the 1997 Umbria-Marche sequence. The amplitude decay of the S waves with distance was defined calculating empirical attenuation functions at 15 frequencies between 1 and 25 Hz. We analyzed separately foreshocks and aftershocks and we found the same attenuation functions, suggesting that the possible temporal variations could be confined in a small area. The frequency dependence of Q(s) was approximated by the equation Q(s) = 18 . f(2.0) between 1 and 10 Hz. At higher frequencies (10-25 Hz), the frequency dependence of Q(s) weakens, having an average value of (Q) over bars = 990. We also estimated Qs from coda waves (Q(c)) using the single-scattering models of Aki and Chouet (1975) and Sato (1977). We found that Q(c) = 77 . f(0.6), (between 2 and 20 Hz) at the western side of the mountain chain, using either foreshocks or aftershocks. This relation is consistent with previous estimates of Q(c) reported for the Central Apennines. For a volume sampling the Colfiorito basin, the Apennines and the Marche region we found that Q(c) = 55 . f(0.8), indicating high attenuation below the mountain belt. To detect small temporal changes of Q, we calculated spectral ratios of 5 temporal doublets located in the epicentral area and recorded at the closest station. We found temporal changes of Q that vary from 27% to 56%, depending on the location of the doublets. This variability suggests that the temporal change of attenuation may depend on the spatial variation of Q and perhaps on the spatial distribution of tectonic stress in the epicentral area.