An Experimental Approach for Estimating Seismic Amplification Effects at the Top of a Ridge, and the Implication for Ground-Motion Predictions: The Case of Narni, Central Italy

From March to September 2009, a velocimetric network was installed in Narni, central Italy, a village on the top of a limestone ridge. The aim was to investigate local site effects due to the 220-m-high ridge, which is characterized by slopes ranging from 22 degrees to 35 degrees. To investigate amplification without and with a reference site, three stations were installed at the base of the hill and seven at the crest. The network recorded 702 earthquakes, many of them from the 2009 L'Aquila sequence. To determine the dependence of amplification on the morphological features, the spectra were computed for horizontal components rotated into a range of azimuths. Both the ratio of the horizontal-to-vertical-component spectra and the ratio of the spectra at the ridge crest with respect to a reference station at the base of the ridge showed amplification by a factor of circa 4.5 for frequencies between 4 Hz and 5 Hz. The highest amplifications were seen for the directions of the ground motion perpendicular to the main elongation of the ridge. Finally, considering events with an epicentral distance less than 30 km, empirical ground-motion models were calibrated for maximum horizontal peak ground acceleration (PGA), velocity, and acceleration response spectra (5% damping) up to 1 s, to estimate the site-corrective coefficients for topographic amplification. The data show corrective coefficients between 0.35 and 0.48 (log 10 scale; amplification, 2.2-3.0) for the spectral ordinates between 0.2 s and 0.3 s.