Seismic measurements of the site effect in the Mugello basin (Italy)

Shallow sediments can affect dramatically the seismic motion at the surface leading to an increase in the seismic vulnerability and resulting critical for seismic risk assessment. Site effects have been worldwide observed, associated both with destructive large earthquakes and with moderate magnitude earthquakes. The Mugello is a recent fluvio-lacustrine basin located 30 km north from Firenze (Italy) and is interested by strong historical earthquakes, which have produced severe damage in a large area including Firenze. Site effects were analyzed in 85 locations measuring seismic noise for 24 hours. We used three components Lennartz seismometers (LE 3D/5sec), with 5 seconds of eigenperiod, and 16 bits A/D converters. One seismic station was permanently deployed on bedrock assuming no seismic amplification. This station was used as reference both for earthquakes and for noise records. Magnitude of the site effects has been estimated from seismic noise and earthquakes. The amplification factor (Fa) was calculated as the integral of the horizontal-to-vertical spectral ratio (HVRS) of the ground acceleration in the frequency range between 2 and 10 Hz, of engineering interest. Amplitude of the regional earthquakes recorded in the basin varies in agreement with the estimated amplitude of the site effects calculated by the HVRS of the noise, showing the strong relation with the shallow geology. In addition, the HVRS calculated for earthquakes and for noise is consistent, giving a strong reliability to the method used. The site effects of the Mugello basin are represented in maps of the amplification and peak frequency distribution. These maps provide interesting information and appear consistent with the local geomorphology and geology. The estimated amplification factor results quite high near the border of the basin, where shallow sediments cover the bedrock while decreases towards the center in relation with thicker sediments. In the same way, the resonant frequency decreases from the border to the center of the basin, where the bedrock has a depth of ~ 600 meters. In general the amplification distribution seems to be in good agreement with geological and morphological local constraints.