Evaluation of seismic site response nearby underground cavities using earthquake and ambient noise recordings: A case study in Catania area, Italy

The influence of underground cavities in assessing local seismic response was studied in some test sites in the urban area of Catania. The investigations were performed both in a natural cavity and in artificial tunnels, taking into account the near-surface geological features. Earthquake and ambient noise records were processed through spectral ratio techniques, investigating also on the possible directional effects. A set of 70 local and regional earthquakes was recorded by seismic stations deployed nearby a grotto located in the northern part of the town as well as in a reference site on the bedrock. As regards ambient noise measurements, about 30 recording sites were selected, locating the mobile stations inside the cavities as well as along short profiles crossing them. Spectral ratios coming out from performed measurements show that spectral peaks in the frequency range 3.0–7.0 Hz appear stable nearby the cavity, and show a tendency to be less pronounced at increasing distance from it. Moreover, the natural period of buildings erected nearby and over the considered cavity was evaluated to investigate on possible soil-structure interactions. The results so far obtained point out that the height of the cavities plays an important role. It appears evident that only cavities having height greater than about 4 m show significant H/V amplification peaks. Moreover, spectral ratios deriving from measurements performed inside the cavities show less pronounced spectral peaks than those obtained from measurements over and outside the cavities. Investigations about the interaction with nearby edifices show slight increments in the amplitude of spectral peaks representing the fundamental period of buildings located over the investigated grotto. Finally, our observations set into evidence a tendency toward a pronounced polarization of the ground motion that is parallel to the main axis of the grotto.

► Results coming out from microtremor data are in good agreement with those obtained from earthquake records in a grotto.
► Significant ground motion amplifications take place when the cavities height is not less than about 4 meters.
► Evidences of pronounced polarization of the ground motion, parallel to the main axis of the grotto, are observed.
► Amplitude increment of spectral peaks representing the fundamental period of edifices located over the grotto are observed.