Seismic amplification effects induced by ancient shallow cavities underneath the urban area of the historical city center of Chieti, Italy

Anthropogenic cavities excavated just beneath the walkable floor (about 1–6 m depth) are present in many ancient urban centres, particularly in Europe and along the Mediterranean basin. The oldest caves are Roman constructions and have been used for different purposes over time (i.e., water storage, secret pathways, shelters against sieges, and wars). For this reason, Seismic Microzonation (SM) studies performed in urban areas must consider the presence of these holes in the subsoil, with their effects predicted through numerical simulations of local seismic response on the surface. In this study, we investigate the seismic effects of ancient anthropogenic cavities in Chieti city centre (Italy) through the cavity amplification factor (CAF). A parametric study was carried out on several different dynamic subsoil models. We define CAF as the ratio between the Acceleration Spectral Intensities on the surface and at the bedrock calculated in the period range of 0.1–0.5 s. The 2-dimensional parametric simulations were performed adopting the soil equivalent linear constitutive law by varying: (1) shear wave velocity values of subsoil, (2) cavity dimensions, (3) cavity depths, (4) cavity lining shear wave velocity values. The CAF plots were calculated on the surface for cavity sizes (width x height) varying from 4 × 5 m to 7 × 9 m. Our study demonstrates that CAF values are relevant only in the period range of 0.1–0.5 s. In particular, results show that: (1) CAF values vary from 1% to 15%; (2) the maximum CAF is felt away from the cavity axis at a distance equal to about 1.5 times the cavity width (L); (3) the maximum difference in CAF values at the surface is equal to 40% within a horizontal distance from the cavity axis ranging from 6 to 14 m. Finally, the case study of the city centre of Chieti (Abruzzo Region) is considered to show how to draw CAF maps for microzonation purposes.