“Evaluation of the near field effects trough numerical modeling: the case of Norcia (Italy)”

The near field condition in seismic events is characterized by its immediate proximity to the seismic source, and is widely proven that ground motion near a causative fault (Near field) can differ significantly from typical ground motion observed at greater distances (far field). Features of near-fault ground motion are high vertical accelerations and the occurrence of high-amplitude, long-duration (2–5s) pulses observed in velocity–time and displacement–time histories aligned with the fault's normal direction. These features and the other effects linked to this condition are critical factors in causing potential damage to structures as the seismic motion in the near-field can subject structures to seismic demands that differ from the design criteria, primarily in terms of intensity and the nature of ground motion. As the seismic hazard quantifies the ground motion expected at a given site, understanding and predicting near-field effects are vital for seismic hazard assessment, structural design, and risk mitigation in the areas where near-field conditions occur. This study aims to investigate the near-field effects in seismic events by employing two-dimension numerical simulations carried out with FLAC 2D Finite Difference Code, to reproduce the features observed during a real earthquake occurred. The selected area is the Norcia plain, one of the intermountain basins widely present in Central Italy—a context of significant interest due to its association with high seismic hazard and high exposure in urban agglomerations. Several active seismic stations have recorded the last important seismic sequence (Central Italy 2017-2018) in particular the third and largest event on 30th October (6.5 Mw), whose epicenter was located close to Norcia (4 km). The validity of near-field conditions for this event has already been established by previous studies. Other scientific studies have been carried out in this direction in similar geological contexts with other software and the advancing here proposed is performing simulations using a non-horizontal interface geometry to apply the seismic input, with both horizontal and vertical components. The simulations consider a geological and tectonic model with few variables changing to provide a comprehensive understanding of how results may be affected by the knowledge of the geological and geotechnical setting, gained from basic studies. This study could have important implications to suggest an updating of the seismic code and the general approach in the seismic design of structures located in the near field domain, for a careful and reliable assessment of seismic risk.