Ground-motion scaling in the Apennines (Italy)

Regressions over a data set of broadband seismograms are performed to quant ify the attenuation of the ground motion in the Apennines (Italy), in the 0 .25-5.0 Hz frequency band. The data set used in this article consists of ov er 6000 horizontal-component seismo,srams from 436 events, with magnitude r anging from M-w similar or equal to 2 to M-w = 6.0. Waveforms were collecte d during recent field experiments along the Apennines. Data from two MedNet broadband stations, located in central and southern Apennines, were also u sed, Seismograms are bandpass-filtered around a set of sampling frequencies, and the logarithms of their peak values are written as AMP(f, r(ref)) = EXC(f, r(ref)) + SITE(f) + D(r, r(ref,) f). EXC(f, r(ref)) is the excitation term for the ground motion at the hypocent ral distance r(ref). SITE(f) represents the distortion of the seismic spect ra induced by the shallow geology at the recording site. D(r, r(ref), f) in cludes the effects of the geometrical spreading, g(r), and of a frequency-d ependent crustal attenuation Q. It is determined as a piecewise linear func tion, allowing to consider complex behavior of the regional attenuation. A first estimate of D(r, r(ref), f) is obtained using a coda normalization technique (Aki, 1980; Frankel et al., 1990) and used as a starting model in the inversion of the peak values. Then, by trial and error, the empirical D(r,r(ref),f) is fitted using a trilinear geometrical spreading, with cross over distances at 30 and 80 km, and the crustal parameter Q(f) = 130(f/f(ref))(0.10); f(ref) = 1.0 Hz These results suggest a low-e crust in the entire Apennines in the 0.25-5.0 Hz range, implying that the seismic hazard in the region may be dominated by the local seismicity. The final section is devoted to highlight the limitations of the formula pr oposed by Console ct nl, (1988) to estimate duration magnitudes M-d, in Ita ly.