GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 estimates of the maximum seismic ground motion from a wide set of possible earthquake scenarios (including the maximum considered earthquakes), the flexibility of NDSHA allows for the generation of ground shaking maps at specified long-term occurrence rates (Magrin et al., 2017), as well as for the definition of routinely updated time-dependent prognostic seismic hazard maps through formally defined earthquake forecasting (Peresan, 2018 and references therein). Where the data permit, a comparative analysis versus the documented seismic activity observed in reality is carried out, showing how available observations about past earthquakes can contribute to assessing the performances of the different methods. Specifically, a systematic comparison over the territory of Italy is carried out exploiting the uniqueness of the Italian earthquake catalogue, a data set covering more than a millennium (a time interval about ten times longer than that available in most of the regions worldwide) with a satisfactory completeness level for M>5, which warrants the results of analysis. The final Global Seismic Hazard Assessment Program (GSHAP) results and the 2013 version of Seismic Hazard Harmonization in Europe (SHARE) project maps, along with other national scale probabilistic maps, all obtained by PSHA, are considered for this comparative analysis (Nekrasova et al., 2015). A first comparative analysis carried out by Zuccolo et al. (2011), made considering the current PSHA map (MPS04, Stucchi et al., 2011), evidenced that NDSHA provides values larger than those given by the PSHA in high-seismicity areas and in areas identified as prone to large earthquakes on the basis of morphostructural analysis (i.e. seismogenic nodes, Gorshkov et al., 2002). Comparatively smaller values are obtained in low-seismicity areas as a natural consequence of the smoothing property of PSHA. Moreover, it turned out that the highest values of NDSHA compare well with PSHA estimates for the longest considered return period (i.e. 2475 years). The updated comparative analysis performed by Nekrasova et al. (2015) was expanded to include the past and newly available NDSHA and PSHA maps for the territory of Italy, as well as documented seismic activity observed in reality. This quantitative comparison evidenced that probabilistic maps have a higher tendency to overestimate the hazard, with respect to the corresponding neo-deterministic maps and reality (Nekrasova et al., 2015). In particular, the probabilistic seismic hazard map SHARE, published in 2013, did not provide a better fit to observed seismicity for the Italian territory than existing ones, including GSHAP. This observation suggests the importance for a progressively updated comparative analysis, extended to the most recent seismic hazard maps (including latest European SHARE maps), which may help understanding how the models are developing to the most recent one. Case studies from recent earthquakes As reported in Peresan (2018), in Italy the time-dependent neo-deterministic ground motion scenarios associated with the alarmed areas have been regularly updated every 2 months since 2006. The NDSHA time-dependent ground-shaking scenarios associated with CN predictions, defined for the time interval 1 March 2012 to 1 May 2012, correctly predicted the ground shaking recorded for the Emilia earthquake (Mw = 6.1, 20 May 2012), as reported by Peresan et al. (2012). Remarkably, the ground shaking for this earthquake systematically exceeded the values expected at the bedrock in the area according to the current Italian seismic regulation, which is based on