Design earthquakes and conditional hazard

Disaggregating seismic hazard in terms of some ground motion or source parameters as magnitude and distance to the site of interest, allows to identify the earthquakes giving the largest contribution to the occurrence or exceedance of a specific value of a ground motion intensity measure (IM). If mapped, such infor- mation may be of useful to engineers in better defining the seismic threat for the structure of interest, for example in case of record selection for nonlinear seismic structural analysis. Because disaggregation results change with the spectral ordinate and return period, and more than a single event may dominate the hazard, the issues related to mapping design earthquakes (DE) are discussed in the paper with respect to Italy. Secondly, it is shown how DEs may be useful in rapidly determining conditional hazard of a secondary ground motion intensity measure with respect to a primary IM. As shown by the application for a vector comprised of peak ground acceleration as the primary IM and the Cosenza and Manfredi Index as the secondary IM, conditional hazard may be used for a more refined and consistent definition of design seismic actions on structures. The conditional hazard approach may be applied, in principle, to any other vector of IMs, providing the advantages of vector-valued hazard analysis requiring much less effort, and therefore rendering it ready for implementation in practice.