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Traditionally, probabilistic seismic hazard analysis (PSHA) considers only mainshock events and models their temporal occurrence through a homogeneous Poisson process. Thus, it disregards foreshocks and aftershocks, assuming they have a minor effect on PSHA. However, recent earthquake sequences, such as those in 2016–2017 in Central Italy and 2010–2011 in Christchurch, New Zealand, exposed the shortcomings of such a universally used but unconservative approach. Our efforts to quantify the bias in seismic hazard and risk estimates follow from these considerations. Herein, we investigate the epidemic-type aftershock sequence (ETAS) model’s ability to reproduce the statistical features of long-term historical seismicity in Italy in two different regions. In addition, we calculate and compare the seismic hazard at two sites in Central Italy using different approaches: (1) with seismicity clustering modeled using the ETAS model; (2) with only mainshocks modeled by means of the Poissonian approach; and (3) with seismicity clustering modeled via a combination of Poisson and modified Omori law. We consider two cases: (1) the “unconditional case,” which uses years of varying seismicity as initial conditions and, therefore, can be considered as a tool for predicting the long-term average hazard, and (2) “conditional case,” in which the hazard is estimated after a specific period, in our case higher than average seismicity. We scrutinize the different modeling assumptions during the process and investigate the effect of using different declustering methods in Poisson-based models. As expected, we find that using the mainshock-only seismicity models yields lower hazard estimates compared to those obtained with the Omori and ETAS model. In addition, we show that Omori and ETAS model predict similar results in the unconditional case, but the Omori model considerably underpredicts the hazard in the conditional case, for a site close to the sequences, when temporal variations in seismic hazard are accounted for.

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Structures may be subjected to earthquake sequences after major mainshock (MS) events in seismically active sites within a short time. As a result, they may be susceptible to damage accumulation, which may hinder their performance under consecutive seismic loading. This study evaluates the effects of earthquake sequences on the seismic performance of seismic‐resistant concentrically braced steel frames designed to Eurocode‐8. The frames under investigation have concentric chevron‐type braces with replaceable hourglass‐shaped pins made of duplex stainless steel. The seismic energy is dissipated through inelastic deformations concentrated in the pins while keeping the other members elastic. The stainless‐steel pins provide the frame with high‐post‐yield stiffness to reduce the residual drifts after a seismic event. The seismic behaviour of the frame is assessed using site hazard‐specific mainshock‐consistent‐aftershock (MS‐AS) sequences selected for a site in Terni, Central Italy. Nonlinear back‐to‐back dynamic analyses are performed at multiple intensity levels while adopting detailed numerical nonlinear models created in OpenSees. We show that the implemented behaviour factor satisfies the life safety assurance objective while keeping the maximum residual inter‐storey drifts below 1/300 to permit an easy substitution of the damaged pins after the design‐level earthquake without being curtailed by the potential following events. We then develop prediction models for the damage accumulation in the pins, considering different energy‐based intensity measures and we show that the cumulative absolute velocity‐based model is the most efficient predictor in this particular case. Finally, the damage accumulation in the pins is evaluated, confirming their superior low‐cycle fatigue capacity under earthquake sequences.