Evaluation of proxies of the averaged shear-wave velocity to a depth of 30 m (Vs30) for site characterisation in Italy

Near-surface geological conditions strongly influence seismic ground motion, and the time-averaged shear-wave velocity to a depth of 30 m (Vs30) is a commonly used parameter to describe site effects in ground-motion models applied in shaking scenarios and seismic hazard assessment. However, the limited availability of direct Vs measurements has led to the growing use of morphometric and geological proxies. This study examines the relationship between Vs30 and a comprehensive set of topographic and lithological proxies across Italy, utilising a dataset comprising approximately 15,000 Vs profiles distributed across Italy compiled from seismic microzonation studies, regional databases, and the ITalian ACcelerometric Archive (ITACA). Topographic proxies were derived from the high-resolution (10 m) national digital terrain model, resampled to multiple spatial scales (150–1000 m), while lithological information was obtained from the national maps and complemented by global soil thickness data. This study provides the first nationwide assessment of multi-proxy correlations with Vs30 in Italy and establishes the foundation for the development of a robust, spatially continuous, and lithologically informed Vs30 model.