Orlacchio, Mabel (2022) The effects of seismic sequences on seismic hazard and structural vulnerability. [Tesi di dottorato]

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Current practice for seismic risk assessment typically considers that structural damage can only occur in a single seismic event. However, neglecting that earthquakes occur in clusters, both in space and in time, can lead to an underestimation of both seismic hazard and structural vulnerability. Indeed, past research has shown that neglecting aftershocks (seismic events following the mainshock in a sequence), can lead to non-negligible differences in the assessment of seismic hazard. From the point of view of structural vulnerability, the necessity of considering the effects of seismic sequences has also been highlighted by recent seismic events, such as the seismic sequences in Christchurch (2010-2011), in Emilia-Romagna (2012) and in central Italy (2016-2017), to name a few. These events showed that at least part of the seismic damage documented on the building heritage was due to the cumulative effect of multiple shocks clustered closely in time. Therefore, this thesis focuses on two components of seismic risk, i.e., the seismic hazard and structural vulnerability, and on how to treat them in order to account for sequence effects. In the context of seismic hazard, the thesis applies the Sequence-Based Probabilistic Seismic Hazard Analysis procedure to the United Kingdom. This part has been developed in collaboration with the firm Ove Arup and Partners London with the objectives of developing national hazard maps taking into account the effects of seismic sequences and investigating the effects of aftershocks in the assessment of seismic hazard. At the same time, regarding seismic vulnerability, the thesis addresses various issues in the estimation of state-dependent fragility curves, which are fragility models that allow for the possibility that a structure may have already been damaged by previous shocks. Initially, the assessment of state-dependent fragilities is addressed using back-to-back incremental dynamic analysis applied to individual structures belonging to classes of reinforced concrete and masonry buildings. In one case, state-dependent fragilities have been evaluated for a masonry building as part of the collaboration with the firm Arup Italy. On the other hand, the evaluation for building classes has been carried out within the European research project RISE (Real-time earthquake rIsk reduction for a reSilient Europe). These evaluations have allowed touching some critical points that characterize the estimation of state-dependent fragilities such as: the limitation of computational costs, the choice of the intensity measure, the definition of engineering demand parameters, and limit states. Finally, in order to reduce the computational cost of the methodology for state-dependent fragility assessment, a simplified method applicable to first-mode dominated reinforced concrete structures is presented, based on nonlinear static analysis and on a new proposed predictive model, which allows predicting the distribution of capacity curve parameters for already damaged structures.

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