Using geological and geophysical data, we set up a 3-D digital description of the underground structure in the central part of the Po alluvial plain. By means of computer-simulated propagation of seismic waves, we were able to identify the structural features that caused the unexpected elongation and amplification of the earthquake ground motion that was observed in the area during the 2012 seismic crisis. The study permits a deeper understanding of the seismic hazard in alluvial basins.