Integrated use of ambient vibrations and geological methods for seismic microzonation

Ambient vibrations spectral ratios at forty-five sites and shear wave velocity profiles obtained by inverting surface waves dispersion curves were used to assess seismic site response properties in the Floridia village located in southeastern Sicily. The results of the horizontal-to-vertical spectral ratios analysis often indicate fundamental in the frequency range 1.5-4.0 Hz. Conversely, where the bedrock outcrop, spectral ratios are flat. The calcarenites outcropping in the north-west sector of the village and the soft sediments in the south-east are characterized by shear wave velocity of about 700 m/s and ranging between 200 and 600 m/s, respectively. The geological substratum has instead shear wave velocity of about 1500 m/s. The achieved geophysical and geological data help to build a subsoil model of the studied area, classifying the main lithotypes above the bedrock and determining the hidden morphology. Moreover, amplification functions were computed for zones with homogenous geological and geophysical properties. Therefore, these site amplification functions were used to simulate ground motion scenarios corroborating the efficiency of the obtained results. The used strategy, involving geophysical and geological methods, represents an important step to derive ground motion scenarios and seismic risk maps. Moreover, the characterization of amplification due to local site effects could help to understand the pattern and the damage distribution of historical earthquakes.