Developing a Geologically Based VS30 Site-Condition Model for Portugal: Methodology and Assessment of the Performance of Proxies

Abstract

The inclusion of site-specific conditions is essential to adequately represent the seismic hazard and the seismic risk for a region. We acquired, gathered, and organized a near-surface shear-wave velocity database for Portugal and applied a three-step methodological approach for developing a VS30 site-condition map using extrapolation based on surface geology. The methodology includes (1) defining a pre- liminary set of geologically defined units, (2) calculating the probability distribution of log VS30 for each unit, and (3) merging the units according to the results of statistical tests. The final model comprises three geologically defined units characterized by logVS30 distributions that are statistically significantly different from each other: F1, igneous, metamorphic, and old sedimentary rocks; F2, Neogene and Pleistocene formations; and F3, Holocene formations. The site conditions for the F3 unit may be further refined using correlations with topographic slope based on the Shuttle Radar Topography Mission at 3 arcsec resolution (SRTM3) dataset. We analyzed the perfor- mance site-condition models based on correlations with exogenous data (topographic slope and surface-geology analogs). The results show that the residual distributions between log VS30 values measured and estimated from those proxies are strongly biased for some geological units, emphasizing the need for acquiring regional VS data.