Uncertainties in Performance Based Design Methodologies for Seismic Microzonation of Ground Motion and Site Effects: State of Development and Applications for Italy

Performance-Based Design (PBD) is a more rational and general design approach, particularly for seismic regions, in which the design criteria are expressed in terms of achieving stated performance objectives. In this approach it is relevant the performance required to structures and to geotechnical works subjected to stated levels of seismic hazard, as well as the geotechnical constitutive model used to predict the performance. The parameters of the constitutive models are related in turn to soil properties. Earthquake hazard zonation in urban areas is the first and most important step towards a seismic risk analysis in densely populated Regions. The Seismic Microzonation is nowadays a world-wide accepted tool for the mitigation of seismic risk. It is a complex process involving different disciplines ranging from Geology and Applied Seismology to Geotechnical and Structural Engineering. The aim achieved in seismic hazard microzonation studies throughout the last 20 years performed at the presented typical case histories in Italy was to quantify the spatial variability of the site response on some typical historical scenario earthquakes that would be expected in the area. In order to quantify the expected ground motion, the manner in which the seismic signal is propagating through the subsurface was defined. Propagation was particularly affected by the local geology and by the geotechnical dynamic ground conditions. Large amplification of the seismic signals generally occurs in areas where layers of low seismic shear wave velocity overlie material with high seismic wave velocity, i.e. where soft sediments cover bedrock or more stiff soils. Therefore, essential key issue here is to obtain a good understanding of the local subsurface conditions. The study builds on the recent experience of seismic microzonation studies in Sicily (Italy), after the effects of the 2018 seismic sequence. Examples of ground response analysis are presented by using some 1-D and 2-D codes, including methodologies taking into account soil uncertainties for site characterisation.