Italian Map of Design Earthquakes from Multimodal Disaggregation Distributions: Preliminary Results

Probabilistic seismic hazard analysis (PSHA) allows to calculate the mean annual rate of exceedance of a ground-motion intensity measures (IM) given the seismic sources the considered site is subjected to. Moreover, through disaggregation of seismic hazard, it is possible to identify one or more dominant earthquakes; i.e. the values of magnitude (M), source to site distance (R) and e (number of standard deviations that the ground-motion parameter is away from its median value estimated by the assumed attenuation relationship) providing the largest contributions to the hazard related to a specified IM value. These events may be referred to as the design earthquakes. Building codes worldwide require the identification of the design earthquakes to define seismic input for seismic analysis of structures. For example, the Italian building code explicitly asks the accelerograms to be used for non-linear dynamic analysis to be selected reflecting the earthquakes dominating the hazard at the site. Although INGV (Istituto Nazionale di Geofisica e Vulcanologia) provides disaggregation data for all Italian territory in terms of Peak Ground Acceleration (PGA) values, it should be emphasized that the disaggregation results change with the spectral ordinate (which should reflect the dynamic features of the structure of interest) in addition to return period. This is especially important when multiple design earthquakes exist; i.e., more than a single event dominate the hazard. In this work disaggregation of all Italian sites for structural periods equal to 0 sec and equal to 1.0 sec is presented. Four different return period are considered (975, 475, 50 and 30 years), they correspond to four limit states for ordinary structures. It will be discussed how seismic source geometry and attenuation factors can influence the number and features of dominant earthquakes, and in what cases a close small earthquake can still be the most important for long return periods also if a large and distant design earthquake exist.