Site effect evaluation in the basin of Santiago de Chile using ambient noise measurements

Abstract

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.