On the Importance of Geolithological Features for the Estimate of the Site Response: The Case of Catania Metropolitan Area (Italy) |
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Authors: | G Lombardo H Langer S Gresta R Rigano C Monaco G De Guidi |
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Institution: | (1) Dipartimento di Scienze Geologiche, Università di Catania, C.so Italia, 57 95129 Catania, Italy;(2) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Catania, Catania, Italy |
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Abstract: | The evaluation of seismic site response in the urban area of Catania was tackled by selecting test areas having peculiar lithological
and structural features, potentially favourable to large local amplifications of ground motion. The two selected areas are
located in the historical downtown and in the northern part of Catania where the presence of a fault is evident. Site response
was evaluated using spectral ratio technique taking the horizontal- to-vertical component ratio of ambient noise. Inferences
from microtremor measurements are compared with results from synthetic accelerograms and response spectra computed at all
drillings available for this area. Such method is particularly suitable in urban areas where the nature of the outcropping
geological units is masked by city growth and anthropic intervention on the surface geology. The microtremor H/V spectral
ratios evaluated at soft sites located within the downtown profile tend to be smaller than that usually reported in the literature
for such soils. A tendency for amplifications to peaks near 2 Hz is observed only in some sites located on recent alluvial
deposits. Evidences for amplifications of site effects (frequency range 4–8 Hz) were observed in the sampling sites located
on the fault, with a rapid decrease of spectral amplitude just a few tenths of metres away from the discontinuity. Numerical
simulations evidenced the importance of geolithological features at depth levels even greater than 20–30 m. Besides this,
the results strongly confirm the importance of the subsurface geological conditions, in the estimate of seismic hazard at
urban scale. |
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Keywords: | local seismic response microtremor H/V spectral ratio Catania downtown fault zone |
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