Site response evaluation in the urban area of Catania (Italy): effects of natural and artificial cavities

The nearsurface geology effects in the seismic response evaluation have been investigated in the area of Catania, paying particular attention to the influence due to the existence of underground cavities. The litho-stratigraphic features of the area consist in a Quaternary clay bedrock overlaid by a complex sedimentary sequence formed by sands and gravels, which is widely covered by more or less alterated lava flows and pyroclastics. The peculiarity of the area is therefore represented, besides the soft sediments interbedded between the clay basement and the upper lava infillings, by the possible presence of cavities inside the lava formation. The grottoes, formed during the flow and cooling of lavas, have often been enlarged by people digging building materials. A map of the main shallow natural and artificial cavities existing in the area was obtained gathering information available from literature and local spelaeological associations. The characterisation of different seismic response due to the existence of underground cavities was obtained comparing the results of noise measurement, performed both inside and around the identified cavity, and processing the data using the conventional Nakamura technique. The theoretical fundamental period obtained by modelling the grottoes as a resonant cavity was also compared with the natural period of buildings erected nearby, and sometimes over, the investigated cavities. The preliminary results show not very pronounced increment in the peaks observed in the horizontal-to-vertical spectral ratios measurements performed either over or outside the grottoes. The influence of the cavity geometry seems however to play an important role.