Sub-surface characterization of the amphiteatrum flavium area (Rome, Italy) through single-station ambient vibration measurements

The Amphiteatrum Flavium in Rome (Italy) is one of the most known monument in the world. With the aim of understanding the seismic response of the area where the Amphiteatrum Flavium is built and investigating possible soil-structure interactions, we performed a seismic experiment in 2014, based on ambient vibration (AMV) recordings. The measurements were performed at the original ground level, on the foundation and at different floors of the monument. Data were analyzed in terms of standard Fourier analysis (FAS) and horizontal-to-vertical spectral ratio technique (H/V). oreover, we performed an active P-wave seismic tomography analysis of the foundation materials to better understand their influence on the recorded signals. Our results point out that there is a strong temporal and spatial stability of the H/V curves, suggesting a uniform seismic response at the monument site. Conversely, spectral amplitudes of AMV show relevant temporal and spatial variability at the investigated site, due to the daily variations of AMV levels and to the low-pass filtering effect of the stiff Amphiteatrum Flavium foundation that strongly attenuates the signals for frequencies above 4 Hz, i.e. those mostly originated by traffic vibrations. Moreover, we observe that the main vibration frequencies of the super-structure are not present as energetic peaks in the spectra of the ground-motion recorded at its base.