Local seismic response evaluation in natural and artificial cavities

The influence of underground cavities in evaluating the local seismic response has been studied. Test sites were selected in Eastern Sicily, in some cities of the Hyblean region and in the Catania urban area, performing investigations in several natural and artificial cavities whose origin is related to local near-surface geological features. The local seismic response was evaluated using both earthquake and ambient noise records,processing the signals through spectralratio techniques. The existence of possible directional resonance effects and ground motion polarization was investigated as well. Local and regional earthquakes were recorded by seismic stations deployed nearby a grotto located in the northern part of Catania. The stations were settled inside, over the vault, in the neighbourhood of the grotto and in a reference site on the bedrock. As regards ambient noise measurements, about 400 time histories were recorded in 90 measurements sites that were located along short profiles crossing each grotto as well as inside it. Experimental results were compared with simple numerical modelling as well. The results so far obtained set into evidence that the size of the vertical section of the cavities plays an important role, since only cavities having height greater than 3-4 metres show H/V amplification peaks. Our observations also point out a tendency towards directional resonances parallel to the main axis of the grotto. Moreover, the natural period of buildings erected nearby, and sometimes over the investigated cavities, was evaluated. Slight increments in the amplitude of spectral peaks representing the fundamental period of buildings located over the investigated cavities were observed.