Effects of underground cavities in the seismic site response evaluation

The influence of underground cavities in assessing local seismic response has been studied. Test sites were selected in some cities of the Hyblean region and in the Catania urban area. The investigations were performed in several natural and artificial cavities taking into account the peculiar near-surface geological features. In the Hyblean area, the local geology is mainly characterized by the presence of limestone formations and different calcarenitic facies, alternating with clayey sediments and Tertiary volcanics. The presence of cavities is therefore connected with karstic phenomena sometimes combined with further enlargement due to anthropic activity. A sensibly different genetic process has to be invoked for grottoes investigated in the Catania area. Here the litho-stratigraphic features consist in a complex sedimentary sequence which is widely covered by more or less altered lavas and pyroclastics. Such geological setting gives rise to the possible presence of cavities inside the lava formation. In this framework, cavities have an origin connected to the flow and cooling of historical and pre-historical lavas (Calvari & Pinkerton, 1998). In the origin of cavities, the effect of anthropic actions has to be taken into account since some of them can be the result of enlargements made on pre-existing grottoes, connected to the location of ancient underground cemeteries, digging of building materials or water supply tunnels (Balsamo et al, 1994; Politano, 1994). The local seismic response was evaluated using both earthquake and ambient noise records, processing the signals through spectral ratio techniques. About 30 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 of microtremors were recorded in 90 measurements sites that were located both inside and over the vault of each grotto, as well as in its neighbourhood, along short profiles. The gathered data have been analysed with H/V spectral ratio techniques and possible directional effects in the observed spectral amplification peaks have been also investigated. Experimental results were compared with simple numerical modelling as well. At present, only preliminary considerations can be inferred. The results so far obtained set into evidence that the size of the vertical section of the cavities plays an important role. According to our data, it appears evident that only cavities having height greater than 3-4 metres show H/V amplification peaks. Such amplification effects seem more evident in the spectral ratios obtained from measurements performed on the vault of the cavity. Our observations also point out a tendency towards directional resonances parallel to the main axis of the grotto. Preliminary results of the numerical analyses seem to corroborate the presence of higher amplification phenomena in correspondence of the vault of the cavities. 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.