Design-elastic displacement spectra for seismic codes and zoning studies of seismic hazard for long-period structures are becoming of utmost importance in modern earthquake engineering. The increase of displacement-based design approaches, together with the availability of good-quality digital strong-motion data, allow the creation of reliable spectral shapes for seismic design of up to 10 s. Nevertheless, to characterize the site amplification effects during large levels of shaking, additional studies are needed, especially in deep sedimentary basins and plains that can be strongly excited by large amplitude long-period waves. Data within deep basins is rare because accelerometric seismic networks are normally deployed on rock or stiff soil, and modeling of waveforms is hence necessary for bridging this gap. Focusing on the numerical modeling of the 3D wavefield in 1D flat layered and 3D structures in the Po and Venetian plains (northern Italy) and on earthquakes with magnitudes M 6.6 and hypocentral distances greater than 50 km, in this study we (1) estimate the displacement response spectra (DRS) from 1 to 10 s in the Po and Venetian plains, (2) compare the DRS from numerical simulations using flat layered and heterogeneous 3D models against the currently available uniform-hazard DRS for Italy, and (3) evaluate the influence of deep sediments on DRS shape and thus on seismic codes. Finally, 1D- and 3D-simulated DRS are compared with DRS obtained from recordings in the Kanto basin where similar long-period displacement is observed.