Seismic microzoning: methodology and applications after the 2016-2017 Central Italy seismic sequence

The Seismic Microzonation (SM) is nowadays a world-wide accepted tool for the mitigation of the seismic risk. The SM is a complex process involving different disciplines ranging from Engineering-Geology and Applied Seismology to Structural and Geotechnical Engineering. The outcome of a SM is presented on zoning maps in terms of selected ground shaking intensity parameters and susceptibility to main ground instability (soil liquefaction, settlements, landslides, fault ruptures). In an advanced SM study for a given area, 4 main interdisciplinary steps can be recognized: 1) definition of the reference input motions, 2) construction of the subsoil model, 3) performing of numerical analyses and computing of amplification factors, 4) identification of zones with different geotechnical hazard potential and drawing up of the SM map. After the earthquakes of 2016-2017 that struck a large area of Marche, Umbria, Abruzzo and Lazio Regions in Central Italy, intensive studies of SM were performed, aiming at supporting the reconstruction in these territories. Particularly, the amplification phenomena due to geological, geotechnical and geomorphological conditions have been considered in this context, and the adopted procedure for addressing the main 4 steps that characterize the SM study for soil amplification are here presented. In particular, for the definition of the reference input motions a set of 7 real unscaled accelerograms matching on average the reference spectrum (defined by the Italian building code NTC18 for outcropping rock conditions and the return period of 475 years) in the period range 0.1-1.1 s, was selected. For the construction of the subsoil model, a large number of non-invasive surface tests was performed. To obtain the characterization of nonlinear cyclic behavior of soils some laboratory tests were performed on undisturbed samples. Ground response was assessed using 1D or 2D numerical codes, depending on the complexity of the geological and geomorphological situation. The results were presented both in terms of amplification factors defined as the ratio between the integral of the acceleration elastic response spectrum of the output motion and the corresponding integral of the acceleration elastic response spectrum of the input motion in three selected ranges of periods (0.1-0.5s, 0.4-0.8s and 0.7-1.1s) and in terms of acceleration elastic response spectra. Some critical issues inferring to the abovementioned steps are discussed with reference to paradigmatic examples, as the case of valley characterized by lateral heterogeneities causing significant lateral contrasts in the Vs values. Finally, a synthesis of the results and a proposal to incorporate the output of SM results in seismic design codes for the reconstruction are presented: the SM maps were adopted in land use planning, the results in terms of acceleration response spectra were used in supporting the seismic design of new buildings.