The influence of nearsurface geology and particularly of underground cavities in the estimate of site response has been studied. Investigations were performed in several natural and artificial cavities in the urban area of Catania and Syracuse, as well as in other small towns located in the northern boundary of the Hyblean area. The local seismic response was evaluated using the spectral ratio technique taking the horizontal to vertical component ratio of ambient noise. 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. H/V spectral ratios were also evaluated by calculating the ratio of each single horizontal component with respect to the vertical one in order to investigate about possible directional effects in the observed spectral amplification peaks. Comparison with experimental data recorded in a small artificial railway gallery having a simple geometry was also attempted. Further, bidimensional numerical analyses using the BESOIL boundary element computer code (Sanò, 1996) were carried out. Both body and surface waves were assumed as input to investigate the dynamic response of the cavities to different wave fields. Numerical and experimental results were compared in terms of H/V spectral ratios in correspondence of the various points around the cavities where measurements were carried out. The aim of this comparison is to clarify the physical phenomena involved in the site response of the cavities as well as the capability of the numerical code to reproduce experimental measurements. At present, only preliminary considerations can be inferred. Amplification effects appear linked to the size and geometry of the cavities and such effects seem more evident in the spectral ratios from measurements performed on the vault of the cavity. Moreover, our observations 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.
|Titolo:||Effects of natural and artificial cavities in the evaluation of local seismic response|
|Data di pubblicazione:||2006|
|Appare nelle tipologie:||4.2 Abstract in Atti di convegno|