Site seismic response in near-fault conditions: role of vertical input motion

Recently several earthquakes hit Central Italy regions, heavily damaging towns and villages located very close to the source faults. Hence it is important to study the local seismic response of sites in near-fault condition, where actual seismic actions are different from those traditionally accomplished in seismic design codes. Accelerometric recordings show significant values of vertical accelerations, similar frequency content of vertical and horizontal motions, both significantly characterized by P and S waves. Vertical and the horizontal motions are practically simultaneous, giving rise to peculiar near- fault seismic actions and induced damage. Here the results of preliminary analyses of simultaneous propagation of vertical and horizontal input motions for various subsoil conditions are illustrated. The soil frequency response under vertical input motion is identified; motion amplification is investigated and compared to the horizontal one, with the aim of understanding the typical features of surface seismic motions in near-fault areas.