Results of the site-effects study in the village of Claut (N.E. Italy)

A magnitude 4.3 earthquake occurred in the Claut area (N.E. Italy) on April 13, 1996, causing damage to infrastructures which can be quantified as VI-VII on the MCS macroseismic intensity scale. Local geology and topographic features seem to have strongly affected the areal damage distribution. In particular, the buildings in the central area of the village, built on soft alluvial deposits, suffered the most severe damage. We presume that these unconsolidated, shallow deposits are at the origin of amplifications of the seismic ground motion. The sharp velocity contrast between the sedimentary cover and the underlying bedrock traps seismic energy within the incoherent deposits producing ground motion amplifications. The data used in this study consist of waveforms of about 45 earthquakes recorded by, at most, eight 3-component stations installed in the Claut area. All the records have been corrected for instrument response. We determined the response spectra for selected earthquakes. In addition, we present the results obtained from the analysis of spectral ratios computed with respect to a reference site. Moreover, for each site we computed the spectral ratios between the horizontal and the vertical components of motion both for earthquakes (receiver function) and for background noise (Nakamura\'s technique). Our results indicate that: i) the response spectra show a discrete variability of site amplification effects within the investigated area; ii) spectral ratios computed versus a reference site display remarkable differences in the amplifications of the ground motion for the different sites. Finally, we found that our results do not depend on the azimuthal distribution of the earthquakes used.