Results of microtremor measurements in the urban area of Catania, Italy

More than 200 time histories of ambient noise have been recorded at 158 sites in the urban area of Catania. Among them, 144 sites are aligned along 15 profiles crossing the most representative lithologies outcropping in the study area. The standard Nakamura (1989) technique has been applied to compute the H/V spectral ratio along these profiles, where the upper-layer structure has been reconstructed in detail using surface geology surveys as well as data from available wells. The geological peculiarity of Catania is the presence of an extended, high- velocity lava cover of varying thickness that fills a large part of the urban area; lower-velocity sedimentary layers outcrop only in small windows in the northern part of the town, however, they predominate in the southern part. In such a complex geological setting, the application of the Nakamura technique provides results that do not correspond strictly to the expectation for usual hard and soft-site spectral shapes. Measurement results have indicated that, in general, the H/V amplitudes do not attain large values in the study area: only at 15 sites are the spectral peaks greater than 3 units, and this occurs predominantly on lava outcrops, where the maximum amplification occurs between 7 and 10 Hz. This frequency band is consistent with weathering processes of the lava flows. At soft sites the observation of significant amplitude (>3) spectral peaks is limited to a few cases. The recordings of six broad-band stations laying on or near the selected profiles have been used for a preliminary comparison between microtremor results and amplifications observed during individual earthquakes. The H/V spectral ratios are generally similar for microtremor and earthquake data, microtremor tending to underestimate the amplitude of horizontal ground motions of earthquakes. But amplifications at sedimentary outcrops (with reference to a massive lava site) can be significant during individual earthquakes, and in some cases include frequency bands where no tendency to amplify was inferred from the microtremor H/V spectral ratios. Even though this comparison needs more data before reaching a stable conclusion, a preliminary analysis of earthquake data confirms that caution is required in using ambient noise for engineering purposes in complex and laterally sharply varying nearsurface geological structures such as those presented by the urban area of Catania.