Recent earthquakes, among which also that of May 20, 2012 in the Ferrara region (northern Italy), highlighted unexpected large vertical ground motions in the near field and specific-related damage, particularly on masonry and precast buildings (Grimaz, 2012). These aspects have caused criticism to the estimates obtained by probabilistic seismic hazard analyses represented by the national seismic hazard maps considered in the building code of various countries [see for a detailed discussion Slejko (2012)], where the main focus is on the horizontal component of seismic ground motion. In the Ferrara area, instead, 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, see, e.g. Papazoglou and Elnashai (1996), Yang and Sato (2000), Elgamal and He (2004), and Aghabarati and Tehranizadeh (2009). The majority of the building codes, in fact, consider simply an empirical scaling factor between horizontal and
vertical accelerations [equal to 0.9 in the case of the Eurocode 8 (CEN, 2004) or 0.7 according to the building code of New Zealand The influence of local geology is considered in general limited for vertical acceleration (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). Perhaps for the above reasons or, more probably, because
the vertical shaking has been considered of secondary importance in engineering seismology, seismic hazard maps in terms of vertical quantities [PGA and spectral accelerations (SA)] are still not very popular, although vertical ground motion prediction equations (GMPEs) are available in the literature (e.g., Ambraseys and Simpson, 1996; Akkar and Bommer, 2010).
