Seismic isolation is a widely used strategy in earthquake engineering, aimed at enhancing structural resilience by increasing the natural vibration period and introducing damping to reduce seismic forces. Conventional isolation systems are typically designed to mitigate horizontal ground motions. However, in near-fault regions, typically defined as areas within approximately  15 km of an active fault, the complexity of the seismic input poses additional challenges that demand a more refined design approach. In particular, near-fault ground motions often exhibit distinctive features, such as fault-normal forward directivity, which produces intense, long-period velocity pulses that increases significantly the isolation system demand. At the same time, the vertical ground motions, usually associated with dip-slip fault mechanisms, are often disregarded, despite their potential impact. These vertical components can produce in vertical-to-horizontal (V/H) spectral acceleration ratios greater than unity at short spectral periods (typically between 0.05 and 0.10 s) potentially amplifying vertical demands in the structure and isolation system. These phenomena highlight the need for a more comprehensive design approach for isolation in near-fault contexts.

This study investigates the combined effects of horizontal and vertical near-fault ground motions on seismically isolated structures, focusing on the post-earthquake reconstruction of Castelluccio di Norcia, a historic village severely damaged by the October 30, 2016, Central Italy earthquake. The reconstruction project employs an innovative “Isolated Artificial Ground” system: a stepped reinforced concrete plate conforming to the site’s complex topography, equipped with Concave Curved Surface Sliders (CCSSs) to provide seismic isolation.

To assess the seismic performance of the proposed isolation system, a Finite Element Model (FEM) is developed and subjected to nonlinear time-history analyses under various representative near-fault earthquake scenarios. Results reveal that while vertical ground motion components have limited effect on horizontal displacements, since the devices uplift is generally prevented, they can induce substantial variations in axial loads, leading to significant fluctuations in shear demand within the structural system. Additional considerations are provided regarding the seismic demands on structures above the isolated ground. Overall, the findings underline the importance of including vertical seismic considerations into the design of isolated structures in near-fault zones, contributing to a more robust and comprehensive approach to seismic resilience.