The possible effect of vertical ground motion on the horizontal seismic response at the surface of a sedimentary structure

Numerical modelling is considered to evaluate the possible effect of vertical ground motion components of input motion on the horizontal seismic response at the surface of a stack of homogenous sedimentary layers. This analysis has been made at four Italian sites where the local Vs profile was available down to the reference bedrock. Computations show that the effect of vertical components on horizontal ground seismic response is frequency dependent and changes as a function of the local Vs profile and accelerometric time history. These outcomes suggest that the common practice of considering only horizontal components of input motion to compute horizontal seismic response may provide biased results, particularly when local hazard is dominated by seismic events that are relatively close to the site of interest. In these cases, vertical component of input motion cannot be neglected and should be considered in seismic response analysis.