Seismic fragility analysis considering mainshock-aftershock sequences: application to Chilean highway bridges

Seismic assessments of critical infrastructure traditionally focus on isolated mainshock (MS) events, often overlooking the effects of aftershocks (AS). This study proposes a probabilistic framework to evaluate structural fragility under MS–AS sequences. The approach integrates stochastic AS simulation via an epidemic-type aftershock-sequence model, a recurrent neural network to reproduce conditional intensity measure (IM) statistics, and a ground-motion selection strategy that identifies representative MS–AS pairs capturing hazard-consistent demands while reducing computational cost. The framework is applied to a representative Chilean highway bridge, using 12000 nonlinear dynamic analyses. Results show that, despite lower intensity, AS can cause moderate to extensive damage at certain hazard levels (HLs) by acting on a structure already weakened by the MS, where the MS alone would not have produced such damage. At intermediate HLs, AS markedly increased damage probabilities in components such as abutments and, most notably, elastomeric bearings, which are the most AS-sensitive elements across the structural system. Columns remain primarily governed by MS demands. At the system level, AS sequences reduce the median IM required to trigger damage by up to 39%, while increasing response variability. In contrast, complete damage states are primarily governed by MS.