Generation of Orientation-Independent Response Spectrum Matched Records Satisfying Minimum Fourier Amplitude and Power Spectral Density Requirements

Luis A Montejo 

The seismic design and assessment of critical infrastructure, particularly within the nuclear industry, relies heavily on acceleration time histories compatible with a Design Response Spectrum (DRS). While current spectral matching algorithms effectively tune ground motions to match a target DRS, this process can inadvertently deplete the signal’s energy at specific frequencies, leading to unstable and unconservative In-Structure Response Spectra (ISRS). To mitigate this, the U.S. Nuclear Regulatory Commission (US-NRC) Standard Review Plan (SRP) 3.7.1 mandates a secondary check ensuring the motion satisfies a minimum Power Spectral Density (PSD) requirement. However, while modern seismic hazard definitions have shifted toward orientation-independent metrics (RotDnn), existing matching tools address PSD compliance only for single components. This article presents a unified algorithm that modifies bi-directional seed records to simultaneously match a target RotDnn response spectrum and satisfy minimum RotDnn Fourier Amplitude Spectrum (FAS) and PSD requirements. The methodology integrates the Continuous Wavelet Transform (CWT) with an iterative multi-stage scaling process—combining global frequency adjustments for FAS compliance with localized time-domain gains for PSD sufficiency. This approach allows for the generation of records that meet strict regulatory energy criteria while preserving most of the non-stationary characteristics and directionality of the seed motions.