Time‐ and State‐Dependent Damage Accumulation Due to Aftershocks Under an M9.0 Megathrust Earthquake in the Cascadia Subduction Zone

ABSTRACT This study develops a comprehensive framework for assessing time and state‐dependent aftershock damage accumulation under an M9.0 megathrust interface earthquake in the Cascadia Subduction Zone (CSZ). The framework integrates aftershock probabilistic seismic hazard analysis (APSHA) and state‐dependent fragility analysis (SDFA) within a Markovian model to quantify unit‐time probability transitions to higher damage states under aftershocks. Key advances and novelties include: (1) a full Poisson‐process formulation to deal with high aftershock occurrence rates, (2) CSZ geometry and rupture‐consistent simulations for developing conditional aftershock distance distributions, and (3) the utilization of ordinal regression to deal with the nonlinearity observed in the state‐dependent seismic demand data for efficient and reliable SDFA. The framework is applied to assess the damage accumulation of existing steel moment‐resisting frame buildings located in Vancouver and Victoria, Canada. The multi‐step application consists of site‐specific APSHA, the state‐of‐the‐art subduction ground motion model, a realistic CSZ geometry model, and the selection and pairing of subduction zone mainshock and aftershock motions for nonlinear response history analyses. Results show that the transition to higher damage states under aftershocks increases sharply within the first few days after the mainshock. The damage accumulation is more significant when the building sustains severe mainshock damage. This study provides a detailed multi‐step procedure for aftershock damage accumulation assessment when facing forthcoming megathrust interface earthquakes in the CSZ.