A Method for Consistent Decomposition of Regional Near-Fault Ground Motions into Dynamic and Fault-Slip-Induced Components

Reliable separation of dynamic and fault-slip-induced components in near-fault ground motions remains a critical challenge for regional seismic analysis because existing approaches have been primarily developed for individual records and lack spatial consistency. This study proposes a unified frequency-based framework in which a single optimal cutoff frequency is determined through a global objective function evaluated over all spatial locations. The results indicate that an optimal cutoff frequency of 0.29 Hz provides the best overall decomposition, corresponding to a mean objective function value of 0.208 and a standard deviation of 0.078, reflecting both good decomposition performance and limited spatial variability. The proposed framework enables consistent separation of acceleration-dominated dynamic motion and displacement-dominated fault-slip-induced deformation across the regional ground-motion field. Based on the decomposed components, spatial distributions of peak ground-motion parameters and response spectra are analyzed to quantify their respective contributions. The results show that dynamic components control acceleration-related responses, whereas fault-slip-induced components dominate permanent displacement and long-period spectral behavior. Directional spectral analysis further reveals a clear difference between the fault-parallel and fault-normal directions. In the fault-parallel direction, the fault-slip-induced component becomes dominant at long periods, leading to a pronounced transition from dynamic to fault-slip-controlled response. In contrast, in the fault-normal direction, the dynamic component remains significant even at long periods, and the contribution of the fault-slip-induced component is comparatively limited. These results indicate that the influence of fault slip on seismic response is strongly direction dependent, with a much greater impact in the fault-parallel direction than in the fault-normal direction. These findings demonstrate that a unified cutoff frequency can effectively ensure consistent decomposition across a regional ground-motion field while revealing the distinct and direction-dependent roles of dynamic motion and fault-slip-induced deformation in near-fault seismic response.