Ground-motion scaling in the region of the 1997 Umbria-Marche earthquake (Italy)

We used broadband waveforms collected at short hypocentral distances (r les s than or equal to 40 km) during the Umbria-Marche (Italy) seismic sequence of September-November. 1997, in order to calculate the scaling relationshi ps for the ground motion within the meizoseismal area, in the 0.5-16.0 Hz f requency band. Data were collected by a 10-station portable seismic network deployed by th e Istituto Nazionale di Geofisica (Rome, Italy) shortly after the occurrenc e of the first mainshock of the sequence, on 26 September 1997. Among the t housands of events recorded, we selected 142 earthquakes characterized by g ood signal-to-noise ratios at all frequencies, and by the absence of multip le shocks within the time window spanned by each recording. The data set of the selected waveforms was made of 2030 horizontal-component seismograms. The logarithm of the peak values of narrow bandpass-filtered versions of th e velocity time histories are modeled at each frequency as AMP(f, r) = EXC(f, r(ref)) + SITE(f) + D(r, r(ref), f). EXC(f,r(ref),=) is the excitation term at an arbitrary reference hypocentra l distance, r(ref); SITE(f) is a site term. The empirical attenuation funct ional, D(r,r(ref),f), represents an estimate of the average crustal respons e for the region, at the hypocentral distance r, at the frequency f. It is modeled by using the following functional form: D(r, r(ref), f) = log g(r) - log g(r(ref)) - pi f(r-r(ref))/beta Q(0)(f/f(r ef))(eta); (f(ref) = 1.0 Hz, r(ref) = 10 km). g(r) = r(-1) is the body-wave geometrical spreading function; beta = 3.5 km /sec is the shear-wave velocity in the crust. Due to the constraints applie d to the system prior to the regressions, the excitation term represents th e expected peak ground motion at the reference distance, as it would be obs erved at a site representative of the average site response of the network. The random vibration theory (RVT) is used to obtain a theoretical predictio n of the attenuation functional. For reproducing D(r,r(ref), f) we use the crustal attenuation parameter Q(f) = 130(f/f(ref))(0.10) obtained by Malagnini et al. (2000) from the analysis of a regional data se t representative of the entire Apennines, in the (0.24-5.0 Hz) band. Two parameters are used to predict shapes and levels of the seismic spectra , the stress drop Delta sigma, and a high-frequency attenuation parameter K O The values used to reproduce the observed velocity spectra are Delta sigma = 200 bars; kappa(0) = 0.04 sec. The indicated stress drop was estimated in this region by Castro et al. (20 00), on recordings of the largest shock of the Umbria-Marche sequence.