Mainshock‐aftershock state‐dependent fragility curves: A case of wood‐frame houses in British Columbia, Canada

Abstract

During a mainshock-aftershock (MSAS) sequence, there is no time to retrofit structures that are damaged by a mainshock; therefore, aftershocks could cause additional damage. This study proposes a new approach to develop state-dependent fragility curves using real MSAS records. Specifically, structural responses before and after each event of MSAS sequences are used to obtain statistical relationships among the engineering demand parameter prior to the seismic event (pre-EDP), the intensity measure of the seismic event (IM), and the engineering demand parameter after the seismic event (post-EDP). The developed fragility curves account for damage accumulation, providing the exceeding probability of damage state (DS) given the IM of the event and the DS of the structure prior to the seismic excitation. The UBC-SAWS model, which was developed for wood-frame houses in British Columbia, Canada, is considered as a case study application. Results indicate that for the examined structural typology, state-dependent fragility curves based on residual interstorey drift ratio (pre-EDP), peak ground velocity (IM), and maximum inter-storey drift ratio (post-EDP) are the best choice to characterise the cumulative damage effect. An illustration of the developed fragility curves is provided by considering a hypothetical MSAS scenario of a M w 9.0 Cascadia mainshock triggering a M w 6.0 crustal event in the Leech River fault, affecting wooden houses in Victoria, Canada. The MSAS scenario increases Yellow tags (restricted access) by 12.3% and Red tags (no access) by 4.8%. K E Y W O R D Scloud analysis, multinomial distribution, nonlinear dynamic analysis of wood-frame houses, real mainshock-aftershock sequences, state-dependent aftershock fragility curves /journal/eqe of-freedom system and used the maximum inter-storey drift ratio (MaxISDR) as EDP. Ebrahimian et al (2014) developed a performance-based framework for aftershock risk forecasting, which consists of an epidemic-type aftershock sequence model and event-based aftershock fragility curves. 7 The ground motion records that were constructed for developing event-dependent aftershock fragility curves were selected from the pool of observed aftershock events.The procedure by Luco et al facilitates various postearthquake decision-making, such as building-tagging and seismic loss estimation. However, there are four aspects that can be improved. (1) Since the aftershock records from the back-to-back application are constructed from mainshock records, the link between the prestructural response by the mainshock and the poststructural response by the aftershock is eliminated. Thus, real MSAS records are desirable.(2) The post-EDP may be overestimated, when the back-to-back application of mainshock records is used for aftershock records with IDA. 13 (3) The computational cost of the back-to-back approach with IDA is high. 12 (4) An appropriate set of IMs and EDPs needs to be selected to represent the intensity of ground motions and structural responses, respectively. The spectral accel...

Effects of ground-motion sequences on fragility and vulnerability of case-study reinforced concrete frames

“…Choosing either of them might influence the analysis results due, for instance, to the differences in ground-motion features (e.g., frequency content, spectral shape, and duration) between real and artificial aftershocks within a sequence (Song et al 2014). Although some past studies adopted real MS-AS sequences (e.g., Ruiz-García 2012; Tesfamariam et al 2015;Zhang et al 2020), the available records with high aftershock intensities are generally scarce, which might limit the observations of significant damage increase due to sequential excitations. This can prevent in turn from developing statistically robust models for the derivation of aftershock fragility/vulnerability relationships conditioned on initial mainshock DS.…”

“…Other studies focused on different structural configurations and materials (e.g., Li et al 2014;Ruiz-García and Aguilar 2015;Di Trapani and Malavisi 2019;Zhang et al 2020). For example, Ruiz-García and Aguilar (2015) analyzed a ductile seismically designed steel moment-resisting frame (MRF) via IDA and adopted real MS-AS sequences considering the influence of mainshock-induced residual drift.…”

“…The study resulted in similar observations to those highlighted above in terms of collapse capacity reduction. Zhang et al (2020) developed state-dependent fragility curves for woodframe houses subjected to real MS-AS sequences scaled with moderate scaling factors. They used residual drift to measure mainshock DS and maximum first-storey drift for aftershock DS.…”

Hysteretic energy‐based state‐dependent fragility for ground‐motion sequences

“…This can, in turn, result in fragility curve crossings between various DSs. This is mostly avoided (rather than solved) by (a) using appropriately scaled ground motion sequences, thus modifying the input signals; (b) disregarding the analysis cases for which the AS drift is smaller than the MS one, thus biasing the statistical result (for example, Zhang et al 6 …”

Spatiotemporal seismic hazard and risk assessment of M9.0 megathrust earthquake sequences of wood‐frame houses in Victoria, British Columbia, Canada

“…This is because (1) a seismicity model to describe the mainshock‐aftershock sequences in space and time is not available in the CSZ and (2) state‐dependent aftershock fragility curves are not available for various building typologies, and only mainshock fragility curves that do not account for the cumulative damage effect of aftershocks have been used. Recently, Zhang et al 23 have developed state‐dependent fragility curves of wood‐frame houses in BC to estimate the damage state (DS) of wood‐frame houses after each event during an earthquake sequence. The new state‐dependent fragility curves can be combined with the quasi‐real‐time aftershock forecasting hazard assessment to build a simulation framework of city‐wide spatiotemporal seismic hazard and risk due to megathrust subduction earthquake sequences in the CSZ.…”