Towards Phase Consistent Hybrid Broadband Ground Motion Simulations: A Disentangling Formulation of Spectral Acceleration Accounting for the Contributions of Fourier Phase and Amplitude Spectra

ABSTRACT Hybrid method for broadband ground motion simulation combines deterministic low‐frequency (LF) simulation with stochastic high‐frequency (HF) simulation. The stochastic simulation combines empirical Fourier amplitude spectra (FAS) matching with random phase assumptions, thereby avoiding the need for high‐resolution velocity structures and the massive computational resources required for deterministic HF simulation. However, hybrid simulation introduces inconsistencies in both the Fourier amplitude spectra and Fourier phase spectra (FPS) between the LF and HF components. While FAS discrepancies could be addressed by spectral blending, the inconsistency between the deterministic LF and the random HF phase remains a challenge. This inconsistency introduces unphysical distortions into the broadband spectral acceleration (SA), which compromise subsequent structural response analyses. To resolve this challenge, a clear understanding of the effect of FAS and FPS on SA is required, leading to the proposal of a disentangling formulation of SA in this study. The formulation expresses SA as a product of (1) a baseline SA, (2) an FAS effect term, and (3) an FPS effect term. Validation against the NGA‐West2 and KiK‐net databases confirms the formulation's robustness. Crucially, the proposed formulation reveals that the FPS effect is governed predominantly by low‐frequency phase. This insight enables a new hybrid method: while long‐period SA is simulated with the deterministic method, short‐period SA can be reconstructed by combining HF simulated FAS with only the LF phase from deterministic simulations, thereby guaranteeing phase consistency. The efficacy of this strategy is demonstrated via ground motion simulation of the Noto earthquake.