A geolithological approach to seismic site classification: an application to the Molise Region (Italy)

Seismic site classification is a critical issue in order to reliably predict seismic ground motion parameters for the development of microzonation maps and site-specific hazard studies, addressed to territorial planning, real-time generation of shaking maps, and seismic design of engineering structures. The equivalent shear wave velocity through the topmost 30 m (VS30) is commonly assumed as the reference parameter to provide practical site classifications. Different approaches aimed at expressing local site conditions in terms of VS30 exist. Overall geological and morphological characteristics are often suggested in the technical literature as proxy to infer VS30 at a regional scale. In this study, the reliability of two commonly accepted approaches based either on the topographic slope proxy or on a basic geological classification was assessed with reference to a well-documented case study, the Molise Region (Italy). A comprehensive database of Down-Hole tests was collected, validated and processed in order to compare measured VS30 values with those inferred from proxy-based approaches. Furthermore, an up-to-date review of the geology of the area was carried out, and a novel methodology based on a geolithological approach was proposed for site classification at a regional scale. The methodology is based on the definition of a set of homogeneous geolithological complexes, and on their classification according to a statistical analysis of the measured VS30 and of the bedrock depth. Original maps of the seismic ground types were developed following classification criteria based upon the European building code for seismic design (EC8) and relevant modifications proposed in the literature. The comparison of the results obtained by the geolithological approach versus those deriving from the application of the above mentioned proxies pointed out an enhanced capability of the proposed method to fit the distribution of ground types, as assessed on the basis of the VS30 values measured in the reference area.