Ⓔ Far-Source Long-Period Displacement Response Spectra in the Po and Venetian Plains (Italy) from 3D Wavefield Simulations by Alessandro Vuan, Peter Klin, Giovanna Laurenzano, and Enrico Priolo Abstract 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 shak- ing, 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. Online Material: Comparison between waveforms and response spectra for synthetics and observations and movie of simulated wave propagation. Introduction Because of the increase in the numbers of large-scale structures, such as tall buildings, long-span bridges, and large storage tanks, there is growing interest in the civil engineering community about the characteristics of the strong ground motion for periods longer than 1 s (Koketsu and Miyake, 2008). The importance of long-period strong motion was recognized during some recent earthquakes (e.g., 1985 Michoacan M w 8.1, 1999 Hector Mine M w 7.1, 2002 Denali M w 7.9, and 2003 Tokachi-oki M w 8.0). Heavy damage to large-scale structures at large hypocentral dis- tances (> 200 km) due to long-period surface waves focused the attention of seismologists on the effect of amplification and prolonged duration within deep sedimentary basins and plains. Earthquake-resistant design based on displacement demand is becoming increasingly common in the engineer- ing seismology community (e.g., Faccioli et al., 2004; Crowley et al., 2004). There is the need for more detailed spectral displacement prediction equations in the long-period band in large basins, where thick and soft sediments are expected to amplify long-period ground motion. Pioneering studies in Japan and California (e.g., Aki, 1968; Hanks, 1975) suggested the important role of surface waves to long-period strong motion. Numerical simulations based on modal superposition of surface waves were demon- strated to be successful in simulating long-period waves entrapped in soft surface layers (e.g., Luzon et al., 1995). Recent studies of ground motion in deep sedimentary basins (e.g., Los Angeles basin, Osaka and Kanto basins), mainly based on velocity or acceleration response spectra (e.g., Joy- ner, 2000; Olsen et al., 2003; Miyake and Koketsu, 2005; Koketsu and Miyake, 2008), proved that (1) long-period am- plifications are due to surface waves; (2) for surface waves to be well developed, they need to be entrapped and to travel some distance from the seismic source; and (3) that the dis- tant-source, long-period ground motions have displacement response spectra (DRS) that are, in terms of amplitude and shape, comparable to those of near-fault, long-period ground 1055 Bulletin of the Seismological Society of America, Vol. 101, No. 3, pp. 1055–1072, June 2011, doi: 10.1785/0120090371