Three-dimensional seismic input and structural response in near-fault scenarios

The extensive ground motion data collected in recent decades has significantly advanced the understanding of near-fault seismic phenomena, which encompass both horizontal and vertical components. In the horizontal direction, the most prominent effects are fault-normal forward directivity, where rupture propagates toward the site at a velocity near the shear wave velocity, resulting in long-period velocity pulses, and fault-parallel fling effect, characterized by permanent displacements due to unilateral rupture. In dip-slip faulting scenarios, vertical slip further introduces strong vertical velocity pulses. While these effects have been widely documented in the literature, their implications for structural performance remain insufficiently explored. The long-period nature of horizontal pulses poses substantial displacement demands on flexible systems, such as tall or base-isolated structures, especially when their natural period resonates with the pulse period. Simultaneously, the vertical component can exhibit spectral acceleration ratios exceeding unity at short periods (0.05–0.10 s), particularly in soft-soil conditions, posing a critical threat to vertical-sensitive structures including base-isolated, masonry, and precast systems. This study aims to refine the seismic input definition for near-fault scenarios by employing recently developed three-dimensional ground motion models calibrated on extensive seismic databases. Specifically, near-fault response spectra were derived for varying fault distances (0–30 km) to preliminarily assess structural demand. Subsequently, a ground motion selection methodology was implemented to match both horizontal and vertical target spectra, accounting for differing structural periods in each direction. The proposed approach provides a reliable basis for nonlinear dynamic analyses, enabling more accurate evaluation of structural response to near-fault seismic events.