Geolithological features and site response in the town of Catania

The evaluation of seismic site response in the urban area of Catania was tackled in two test areas potentially favorable to large local amplifications of ground motion. The two selected areas are located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response was evaluated using spectral ratios of ambient noise and numerical simulations. Such a method is particularly suitable in urban areas where the nature of the outcropping geological units is masked by city growth and anthropic intervention on the surface geology. A moderate amplification is mainly observed, at a frequency of 2 Hz, in recent alluvial deposits. Other soft sediments show amplitude spectral peaks smaller than those usually reported in the literature for such soils. Evidence for amplifications of site effects (frequency range 6-8 Hz) were observed in the sampling sites located on the fault, with a rapid decrease of spectral amplitude just a few tens of metres away from the discontinuity. Numerical simulations evidenced the importance of knowing the geolithological features at depth greater than 20-30 m. Besides this, the results strongly confirm the importance of the subsurface geological conditions in the estimate of seismic hazard at urban scale.