Local seismic response in Catania (Italy): A test area in the northern part of the town

Site effects were studied in an area located in the northern part of Catania town using ambient noise measurements. The study zone is particularly suitable for investigating local seismic response both because most of the terrains characterizing the urban area are here outcropping and a considerable amount of field measurements of their elastic properties is available.The recorded ambient noise was processed through the Nakamura H/V spectral ratios technique. The results set into evidence significant differences in the amplification of the horizontal component versus the vertical one in the litho-types outcropping in the study area. No amplifications were observed in the H/V spectral ratios of sites located on the clayey basement, as well as in those located on massive lavas. By contrast, a tendency towards significant H/V spectral peaks, in the frequency band 2.5–7.0 Hz, was observed in terrains formed by sandy clays, gravelly sands, detritus and altered lavas.Results of ambient noise spectral ratios were compared with preliminary findings from earthquake recordings at two stations located in the investigated area. Finally we computed the theoretical transfer functions using 1D numerical modelling. The results pointed out that the observed H/V peaks can be interpreted as the resonance effect due to the impedance contrast between the soft sediments and the basement quaternary clays, found out at a depth ranging from 20 to 50 m.