Two questions have puzzled seismologists in the past: if vertical peak accelerations could exceed the horizontal ones and if vertical peaks exceeding 1 g were feasible in the near field. Recordings of vertical accelerations larger than 1 g are very rare, only 7 so far (according to the Strong Motion Virtual Data Center, see Table 1). This is mainly due to the scarce availability of recorders located in the near field (d<15 km). In addition, there are some pieces of evidence testifying of boulders upthrown by earthquakes (see again Table 1). Recent earthquakes, especially that of May 22, 2012 in the Ferrara region (northern Italy), have clearly pinpointed the effects of the vertical ground motions on specific structures, especially in the near field. In fact, the most severe damage suffered by the industrial buildings was caused by a vertical peak ground acceleration (PGA) around 1 g, not expected in that area according to the present Italian seismic code. More examples can be found worldwide, The majority of the building codes presently applied in the European countries consider simply an empirical scaling factor between horizontal and vertical accelerations (0.9 in the case of the Eurocode 8). The influence of local geology is considered in general limited (Ambraseys and Simpson, 1996) although moderate effects are observed for stiff sites at shorter periods (<0.25 s) and soft sites at longer periods (>0.7 s).
