PSI - Issue 78

Iunio Iervolino et al. / Procedia Structural Integrity 78 (2026) 1553–1560

1554

1 Introduction Probabilistic seismic hazard analysis (PSHA; McGuire 2004) provides a rational basis for the definition of seismic actions for structural design or assessment. Two main components of PSHA are a model for earthquake occurrence at the considered seismic source(s), and a model for these earthquakes’ effects at the site of interest, in terms of ground motion. The latter is further split into three component models: a probabilistic magnitude distribution, a probabilistic source-to-site distance distribution, and a ground motion model (GMM) to probabilistically propagate shaking. Classical PSHA, based on large seismic source zones, models the occurrence of earthquakes (i.e., the mainshocks of seismic sequences; see for example Iervolino 2019) via the homogeneous Poisson process (HPP), as it has been shown that this model is often suitable and simple to calibrate. In addition, the magnitude distribution often descends from Gutenberg-Richter type relationships (Gutenberg and Richter 1944). Finally, GMMs generally do not explicitly model narrow-band spatial variations of ground motions due to finite ruptures (Somerville et al. 1997). In the case of PSHA for sites where strategic or critical facilities are located, it is typical to base the analysis on individual faults close to the site of interest, in addition to, or in lieu of, large seismic source zones. When PSHA considers individual faults, one or more of these assumptions can be questioned. For example, mainshock occurrence should not be memoryless , as is the case with HPP (Polidoro et al. 2013). Also, the magnitude distribution should reflect the fact that faults tend to produce large similar ( characteristic ) events (e.g., Convertito et al. 2006). Finally, around finite ruptures there may be propagation effects that lead to peculiar ground motion, such as velocity pulses deriving from forward directivity , that classical PSHA is not able to account for explicitly (e.g., Chioccarelli and Iervolino 2013). These effects can be relevant for structural engineering as they can lead to seismic demand departing from consolidated models such as the equal displacement rule (e.g., Iervolino et al. 2012). The Messina Strait, where the largest-span suspension bridge could be built (Brancaleoni et al. 2009), is one of the cases where PSHA should also be based on individual faults. This is because the construction sites sit on top of the fault deemed responsible of the largest seismic event that occurred in Italy since more than one hundred years, the 1908 M7.1 Messina earthquake (Valensise and Pantosti 1992). In fact, several faults have been mapped in the region around the Messina Strait. Fig. 1 depicts the surface projection of the faults from the Database of Individual Seismogenic Sources (DISS) database (DISS Working Group 2025). The 1908 earthquake fault has ID ITIS013.

N

ITIS044

ITIS011

ITIS041 ITIS040

ITIS045

Messina

ITIS012

ITIS043

ITIS042

ITIS013

0

25

50km

Fig. 1. Individual faults in the Messina Strait area according to DISS. The study presented in the following consists of a preliminary investigation of some of the issues related to near source (NS) PSHA for the Messina strait. More specifically, the shaking accounting for near source effects has been probabilistically assessed for the Messina site (longitude 15.63°E, latitude 38.26°N; see Fig. 1) considering the ITIS013 fault and a characteristic magnitude interval. The forward directivity features of ground motions are accounted for via a probabilistic model for velocity pulse occurrence in conjunction with a narrow-band modification