The seismic performance of geotechnical structures placed close to active faults is influenced by the peculiarities of the near-source seismic wave propagation. This issue is of paramount importance for large and massive structures, such as earth dams, for which the strong variability of the ground motion at the foundation level and the contribution of the vertical component of the motion could induce high differential settlements in the embankment with an increased risk for the structure. When modelling a structure in near-source conditions, the domain of analysis should be considerably wider than that typically adopted in the engineering field, since the seismic source and the propagation medium up to the reference site should be added to the structure and to the soil volume interacting with it. The paper illustrates the procedure developed to define the input motion at the Conza Dam site (Italy) during the 1980 Irpinia earthquake (November 23, 1980; Mw = 6.9). Due to the short distance of the site from the fault area, the Conza Dam is representative of several large dams placed in near-source areas in Italy and worldwide. The paper focuses on (1) the reconstruction of the regional structure of the Campania–Lucania region on the basis of former literature studies and on a new calibration of the 1D velocity model at regional scale; (2) the definition of the reference input motion at the site of Conza Dam with highlights on the spatial variability of the ground motion below the dam foundation and related effects on the structure. © 2017, Springer Science+Business Media Dordrecht.