Ambient noise measurements for preliminary site-effects characterization in the urban area of Florence, Italy

Environmental noise measurements and borehole data were used for a preliminary seismic characterization of the Plio-Quaternary sedimentary basin underlying the city of Florence (Italy). An extensive survey of ambient vibrations was conducted by using the seismic noise horizontal-to-vertical (H/V) spectral ratio technique (HVSR) to map the fundamental resonance frequency of the sedimentary cover. In order to use this information to infer main features of the basin subsurface geometry, an estimate of the S-wave velocity profile in the sediment layer was obtained from array noise recordings along with information on resonance frequency and direct measurements of the sediment thickness available at 23 borehole sites. A first-order reconstruction of the seismic bedrock topography in the area was provided, which shows a good consistency with available geological/log data. From these pieces of information, a preliminary soil classification of the whole area was attempted in order to identify situations where possible seismic amplification effects are expected during future earthquakes and where more accurate investigations are needed to parameterize the local seismic response.