Current national seismic hazard models neglect time-dependent hazard due to triggered earthquakes, although these can certainly generate damaging ground motions. To understand the relative importance of aftershock hazard and risk in the context of a mega-thrust subduction-zone earthquake, we develop a new simulation framework for spatiotemporal seismic hazard and risk assessment of a mega-thrust earthquake and its aftershocks along the plate boundary and in the onshore continental crust. Tohoku region in the northeast Japan is considered as an example to show how the new simulation framework can be implemented to assess the spatiotemporal hazard and risk of aftershocks triggered by a M9 Tohoku-like earthquake. We generate quasi-3D synthetic aftershock catalogs using the Epidemic Type Aftershock Sequences (ETAS) model, modified to characterize aftershocks of large and anisotropic finite mainshock sources. By including the mainshock source model in the new simulation framework, the uncertainty of generating synthetic aftershock catalog is small in comparison with the observation. Therefore, should the mainshock source model is available right after the mainshock, the new simulation framework can be used for the quasi-real time hazard and risk assessments of aftershocks in different regions. For Tohoku region, we assess the relative importance of subduction-zone versus onshore-crustal aftershocks. The results show that the subduction-zone aftershocks tend to dominate hazard with peak ground velocity (PGV) < 60 cm/s (the boundary between VIII (severe) and IX (violent) of Modified Mercalli Intensity). On the other hand, onshore-crustal aftershocks control extreme hazards exceeding PGV of 60 cm/s. Moreover, on the day of the mainshock, aftershocks contribute about 23% of the onshore hazard with PGV > 60 cm/s, and the aftershock hazards remain relatively high for 4-5 days depending on different sites. From a seismic risk viewpoint, the subduction-zone and onshore-crustal aftershocks in the mega-thrust sequence affect buildings differently; both have similar potential to cause minor damage, whilst the latter tend to cause more severe damage.

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...

Megathrust earthquake sequences, comprising mainshocks and triggered aftershocks along the subduction interface and in the overriding crust, can impact multiple buildings and infrastructure in a city. The time between the mainshocks and aftershocks usually is too short to retrofit the structures; therefore, moderate-size aftershocks can cause additional damage. To have a better understanding of the impact of aftershocks on city-wide seismic risk assessment, a new simulation framework of spatiotemporal seismic hazard and risk assessment of future M9.0 sequences in the Cascadia subduction zone is developed. The simulation framework consists of an epidemic-type aftershock sequence (ETAS) model, ground-motion model, and state-dependent seismic fragility model. The spatiotemporal ETAS model is modified to characterise aftershocks of large and anisotropic M9.0 mainshock ruptures. To account for damage accumulation of wood-frame houses due to aftershocks in Victoria, British Columbia, Canada, state-dependent fragility curves are implemented. The new simulation framework can be used for quasi-real-time aftershock hazard and risk assessments and city-wide post-event risk management.