High-frequency ground-motion parameters from weak-motion data in the Sicily Channel and surrounding regions

Abstract

In this paper we characterize the high-frequency (1.0–10 Hz) seismic wave crustal attenuation and the source excitation in the Sicily Channel and surrounding regions using background seismicity from weak-motion database. The data set includes 15 995 waveforms related to earthquakes having local magnitude ranging from 2.0 to 4.5 recorded between 2006 and 2012. The observed and predicted ground motions form the weak-motion data are evaluated in several narrow frequency bands from 0.25 to 20.0 Hz. The filtered observed peaks are regressed to specify a proper functional form for the regional attenuation, excitation and site specific term separately. The results are then used to calibrate effective theoretical attenuation and source excitation models using the random vibration theory. In the log–log domain, the regional seismic wave attenuation and the geometrical spreading coefficient are modelled together. The geometrical spreading coefficient, g(r), modelled with a bilinear piecewise functional form and given as g(r) ∝ r−1.0 for the short distances (r < 50 km) and as g(r) ∝ r−0.8 for the larger distances (r < 50 km). A frequency-dependent quality factor, inverse of the seismic attenuation parameter, Q(f) = 160f/fref0. 35 (where fref = 1.0 Hz), is combined to the geometrical spreading. The source excitation terms are defined at a selected reference distance with a magnitude- independent roll-off spectral parameter, κ 0.04 s and with a Brune stress drop parameter increasing with moment magnitude, from σ = 2 MPa for Mw = 2.0 to σ = 13 MPa for Mw = 4.5. For events M ≤ 4.5 (being Mwmax = 4.5 available in the data set) the stress parameters are obtained by correlating the empirical/excitation source spectra with the Brune spectral model as function of magnitude. For the larger magnitudes (Mw>4.5) outside the range available in the calibration data set where we do not have recorded data, we extrapolate our results through the calibration of the stress parameters of the Brune source spectrum over the Bindi et al. ground-motion prediction equation selected as a reference model (hereafter also ITA10). Finally, the weak-motion-based model parameters are used through a stochastic approach in order to predict a set of region specific spectral ground-motion parameters (peak ground acceleration, peak ground velocity, and 0.3 and 1.0 Hz spectral acceleration) relative to the generic rock site as a function of distance between 10 and 250 km and magnitude between M 2.0 and M 7.0.