Abstract
SUMMARY We performed extensive ambient vibration measurements in the basin of Santiago de Chile (Chile), and we look for testing the reliability of the horizontal-to-vertical amplitude spectra ratio method (H/V ) as a tool to provide qualitative and quantitative information of site effects in complex geological media. The interpretation of the H/V data was carried out conformably to the SESAME project consensus criteria and outlines three major patterns: (1) clear peaked H/V curves related to sharp underground velocity contrast; (2) H/V peak of low amplitude and flat curve related to weak contrast and (3) broad H/V peak indicating the presence of strong lateral variations of underground structure. H/V measurements, however, reveal a discrepancy between the computed soil resonance frequencies and the expected building resonance, therefore not leading to a straight interpretation of the intensity distribution derived from observed damage to one storey houses in Santiago after the 1985 Valparaiso earthquake. Indeed, the H/V technique mostly maps the first fundamental frequency; however, it fails to show higher resonance modes. In the case of the city of Santiago, this method works well for assessing the seismic hazard for high-rise buildings, but is questionable for smaller structures as is the case of a great percentage of constructions in the city.
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