PSI - Issue 78

Giulia Giuliani et al. / Procedia Structural Integrity 78 (2026) 929–935

930

Keywords: Near-Fault Seismic Effects; Vertical Ground Motion; Seismic Isolation; Ground Motion Models; Base-Isolated Structures.

1. Introduction The extensive data collected in recent decades has greatly enhanced our understanding of the primary effects of near-fault ground motions, which influence both horizontal and vertical components (Baker (2007) and Ambraseys and Douglas (2003)). In the horizontal direction, the most common near-fault phenomenon is fault-normal forward directivity. This occurs when the fault rupture travels toward a site at a velocity close to the shear wave speed, resulting in a strong, long-period pulse of energy (bilateral velocity pulse). Another key effect is fault-parallel fling, characterized by a unilateral velocity pulse with permanent ground displacement. In dip-slip faults (reverse or oblique), vertical slip also generates a significant vertical velocity component. In Europe, Eurocode 8 mandates consideration of near-source effects for strategic structures (Eurocode 8-1 (2005)) and bridges (Eurocode 8-2 (2005)) located within 10 km of an active seismotectonic fault capable of producing earthquakes with a Moment Magnitude (Mw) greater than 6.5. In such cases, site-specific spectra accounting for near fault effects must be used, as standard design spectra typically underestimate seismic demand near the fault. However, the code lacks clear guidance on how to derive these spectra. This study focuses on central Italy, one of the country’s most seismically active regions. It begins with an analysis of earthquake records from the 2016 seismic sequence, particularly the Mw 6.5 event of October 30 near Norcia, to assess near-fault effects. To address gaps in existing seismic codes, a suitable seismic input for near-fault sites has been developed using a recently proposed three-dimensional Ground Motion Model (GMM) based on a) comprehensive near-fault ground motion database (Lanzano et al. (2019), Sgobba et al. (2021), Ramadan et al. (2021). This approach yields near-fault response spectra at various fault distances (0 – 30 km), providing a preliminary estimate of their structural impact. The study considers different vertical-sensitive structures, including masonry, precast, and base-isolated buildings, the latter of which are especially vulnerable to the combined action of horizontal and vertical ground motions (Micozzi et al. (2022), Cardone et al. (2023), Micozzi et al. (2022)). 2. Motivation: Central Italy Earthquake of 2016 In 2016, central Italy experienced a seismic sequence marked by two mainshocks: an Mw 6.0 event on August 24 and an Mw 6.5 event on October 30. The latter meets the Eurocode 8 threshold for considering near-fault effects within a 10 km radius. For this reason, ground motion records from stations closest to the epicentre were analysed to identify potential near-source effects. Specifically, data from three stations were examined: Castelluccio di Norcia (CLO), Forche Canapine (FCC), and Norcia (NRC), located approximately 6.9 km, 10.9 km, and 5 km from the epicentre, respectively (Fig. 1). Both horizontal and vertical components were assessed using pseudo-acceleration response spectra (Fig. 2a), while displacement spectra were computed for the horizontal components (Fig. 2b). Results indicate that the vertical response spectra (U-D) exhibit very high values in the short-period range, exceeding 1g at CLO and FCC stations. Although NRC is geographically closer to the epicentre, it shows a lower vertical component due to its position relative to the fault plane projection, as discussed in Micozzi et al. (2022). For the horizontal components, the FCC station recorded the highest pseudo-acceleration demand, whereas the largest displacement was observed in the E – W component at CLO. However, displacement values were not excessive, as they did not increase significantly for periods beyond 2 seconds, consistent with the moderate dimensions of the fault plane (Liberatore et al. (2019)). These findings suggest that while horizontal displacement demand is relatively modest compared to other near fault events of higher magnitude (e.g., the February 6, 2023, earthquake in Turkey), the vertical component is substantial and may critically influence structural response. Notably, similar patterns have been observed in past Italian earthquakes, such as L’Aquila (2009) (Savoia et al. (2017)) and Emilia (2012) (Di Sarno et al. (2010)). In contrast, the