Preliminary identification of different nearsurface geology effects in the area of Catania (Italy)

The Catania area is prone to a moderate magnitude local seismicity linked to Mt. Etna volcano, and to a more dangerous seismic activity connected to the regional size tectonic structures that gave origin to the strongest earthquakes of the area, such as the 1693 and the 1908 ones, located in the Malta-Hyblean escarpment and the Messina Straits, respectively. This study investigates about local variations of seismic response due to the different lithotypes overlaying the Quaternary clay bedrock in the area of Catania. The area is characterised by a complex litho-stratigraphic sequence formed by sedimentary deposits overlaid by more or less alterated lava flows and pyroclastics. Geological information available from surface surveys and drillings were gathered. In addition, elastic parameters of various lithotypes were calculated by deploying short seismic refraction profiles and using the available down-hole data. Data collected allowed us to draw a lithological-technical map of the area, showing the peculiar nearsurface geology features, represented by soft sediments interbedded between the clay basement (bedrock) and upper lava infillings. Six broad-band, high-dynamic range seismological stations were used to record ground velocity at different sites. The conventional Nakamura technique was also used processing nearly 200 samples of microtremors. Using the same methodology, the natural period of several public and private buildings, belonging to different typologies, has been evaluated. The results of measurements performed inside the buildings and in their neighbourhoods show that the natural period of buildings does not contaminate significantly the ground response of the surrounding area, therefore suggesting that the soil-structure interaction does not affect, at least in our observations, the H/V measurement in the free field. Preliminary results of Nakamura technique set into evidence that slightly larger H/V amplitudes seem to characterise alterated lava flows and both recent and terraced alluvial overlaying the sedimentary deposits.A comparison of microtremors with earthquake spectral features basically shows a good agreement in the H/V spectral ratios.