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This study presents the results of 90 seismic ambient noise measurements in Palermo, the main city of Sicily (Italy). The dataset has been processed using the horizontal-to-vertical spectral ratio (HVNSR) technique and interpreted in terms of local geology, which is characterized by the presence of alluvial sediments of two riverbeds masked by urbanization since the seventeenth century. HVNSRs show significant variations in the study area: when the transition stiff to soft is crossed, a typical spectral peak appears in the HVNSRs, mostly in the frequency band 1-2 Hz, and exceeding a factor of 3 in amplitude. Using available information on subsurface geological structure, we compute theoretical 1D and 2D transfer functions. The resonance frequencies of soft soils obtained by HVNSR are well reproduced by the fundamental frequencies from numerical modeling.The distribution of frequency peaks of HVNSR and their amplitudes are also compared with the local damage caused by historical earthquakes. Previous studies demonstrated that damage variations in Palermo were controlled more by near-surface geology than building vulnerability. A uniform vulnerability is an ideal condition to test statistical methods and their capability in seeking correlation between HVNSR and potential damage due to local geological conditions. We apply two well-established multivariate statistical methodologies (factor analysis and canonical correlation) to the HVNSR dataset and macroseismic data (damage grades of the European macroseismic scale). Through these analyses we quantify the significance of the correlation between the HVNSR peak in the low-medium frequency range (0.5-3 Hz) and the occurrence of the highest damage grades. This approach allows us (1) to estimate the threshold value in the resulting linear combination of the HVNSR amplitudes, which separates zones of light damage from zones of significant damage, and therefore (2) to improve the spatial definition of potentially high hazard zones through a denser grid of microtremor measurements.

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On 6 September 2002 a M w 5.9 earthquake occurred in the southern Tyrrhenian sea, 40 km off the coast of Palermo (Italy). In the days after the mainshock, eight temporary stations were installed in the city to record aftershocks on different geological formations. Seismograms of about 30 earthquakes with magnitude Ն2.8 are analyzed. The data analysis confirms the role of near-surface geology in causing locally significant variations of the ground-shaking level as already inferred by Guidoboni et al. (2003) from historical damage scenario studies. The largest spectral variations estimated through aftershock recordings result in a factor of 10 difference between stiff and soft sites, in frequency bands varying from 1 to 3.5 Hz. The geological structure of the study area is reconstructed by using data from more than 2000 boreholes organized in a Geographic Information System specifically dedicated to the assessment of natural hazards in urban areas. Vertically varying velocity models are used for a comparison with the observed data. In general, 1D transfer functions fit the largest amplification frequency but underestimate amplitudes of observations probably because of 2D and 3D complexity. Because the seismic stations were not installed in free field but at ground or underground level inside buildings, a possible influence of the structure was also investigated. Simultaneous ambient noise measurements were performed on the top, at the base, and outside the buildings where stations were installed. For all but one site, this analysis shows that the estimated ground-motion amplifications do not reflect the building vibration modes and therefore, in these cases, soil-structure interaction does not bias the free-field response of the study sites. Finally, speculations on the effect of the local geology in terms of response spectra of the M w 5.9 mainshock are discussed in the framework of the Eurocode 8 prescriptions.

For the first time, a high-density macroseismic survey has been carried out in the city of Palermo, Italy, after the 6 September 2002, M w 5.9 earthquake. The aim was to investigate the spatial relationships and correlations between intensity data and surface geology. A very dense database has been created to store a large amount of macroseismic, stratigraphic, and geotechnical information. A Geographic Information Systems (GIS) tool, called City-GIS, enables data processing by instruments and research keys devoted to seismic hazard. The results show the feasibility of such procedures, especially for large cities where the surface geology critically influences the ground motion. Our analysis, made on the basis of more than 2000 intensity observations scattered in the Palermo urban area, demonstrates the role of the variable geology in the variability of shaking. Significant variations of intensity are related to the geometry of the geologic formations present in the area. Intensities increase by about one intensity class inside those city sectors where the stratigraphic succession is characterized by a thin layer of calcarenites overlying very thick layers of silty clayey sands. Major amplification of up to two classes occurs in alluvial deposits in the southeastern part of Palermo, as expected. These results extend our knowledge of the seismic response in the outskirts of Palermo, which until now, was limited to the city center.