Key issues in Seismic Microzonation studies: lessons from recent experiences in Italy

摘要

The Seismic Microzonation (SM) is nowadays a world-wide accepted tool for the mitigation of seismic risk. Despite the large number of SM studies in the literature and the publication of national and international guidelines, some open questions still exist in SM studies and are addressed in this paper. These key issues are discussed after a brief history of SM in Italy and the presentation of Italian and international guidelines on the matter. The SM is a complex process involving different disciplines ranging from Geology and Applied Seismology to Structural and Geotechnical Engineering. The outcome of a SM is presented on a zoning map in terms of ground shaking intensity and susceptibility to main ground instability (soil liquefaction, landslides, fault ruptures). In an advanced SM study for a given area, four main interdisciplinary steps can be recognized: 1) definition of the reference input motions, 2) construction of the subsoil model, 3) performing of numerical analyses, 4) identification of zones with different geotechnical hazard potential and drawing up of the SM map. The key issues and the controversial aspects of these steps are deeply discussed in the paper based on the experience of the Author gained in three recent Italian case studies: Middle Aterno valley, Central Archaeological Area of Rome and Fivizzano. Earthquake-induced permanent soil deformations are out of the scope of the paper being the attention focused on soil amplification phenomena. The paper closes with some remarks on the differences between local seismic hazard assessment for SM mapping and for the seismic design (according to the Italian building code NTC08), and with some proposals on the use of SM output in supporting design.