GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.1 GNGTS 2023 Off-fault coseismic surface faulting during dip-slip earthquakes: implications for fault displacement hazard analysis P. Boncio 1 , F. Visini 2 , S. Baize 3 , F. Nurminen 4 , B. Pace 1 , O. Scotti 3 1 Department of Engineering and Geology, Università “G. D’Annunzio” Chieti-Pescara, Chieti, Italy 2 INGV – Istituto Nazionale di Geofisica e Vulcanologia, Italy 3 IRSN—Institut de Radioprotection et de Sûreté Nucléaire, Fontenay-aux-Roses, France 4 RINA Consulting S.P.A., Milan, Italy. INTRODUCTION During surface faulting earthquakes, displacement on secondary faults or fractures may occur off the trace of the principal fault (PF), in the vicinity or up to many kilometres away from the PF trace. This is called distributed faulting or distributed rupturing, also known as secondary faulting/rupturing. Though distributed ruptures (DRs) are discontinuous in nature and characterized by lesser amount of displacement compared to the PF, their occurrence may threaten structures the safety and functionality of which are sensible to low levels of permanent ground displacement, such as critical infrastructure. Often DRs occur in unpredictable locations, without previous geologic evidence, making the assessment of fault displacement hazard from DRs challenging. In Probabilistic Fault Displacement Hazard Analysis (PFDHA), the distinction between principal and distributed faulting was first introduced by Youngs et al. (2003). This distinction is important because different equations are used in assessing the probability of occurrence and displacement of principal or distributed ruptures. In general, distributed faulting includes several different types of ground ruptures, from surface faulting along secondary splays and shears connected or unconnected to the PF, to ruptures triggered on remote pre-existing faults distant several kilometres from the PF. In Youngs et al. (2003) and in more recent updates of PFDHA models (Petersen et al., 2011; Takao et al., 2013; Ferrario and Livio, 2021) regressions for distributed faulting do not account of these different typologies, and all the DRs are considered equally. A first attempt of distinguishing regressions for different types of DRs was by Nurminen et al. (2020) for reverse faulting earthquakes. They propose regressions for DRs that are not related to pre-existing fault or fold structures, called ‘simple’ DRs (i.e., non-predictable DRs that may take place anywhere along the strike of the PF). Compared to other probability models, the Nurminen et al.’s regressions are characterized by significantly higher probability of having DRs is the vicinity of the PF (near-fault) and significantly stronger attenuation with distance (Valentini et al., 2021). A major