Site-Specific Ground Motion Simulations via Spectral-Scaling Transfer Function Method

Esra Zengin, Monica D. Kohler, Thomas H. Heaton, & Becky Roh

Submitted September 7, 2025, SCEC Contribution #14343, 2025 SCEC Annual Meeting Poster #218

This study presents a spectral scaling method for simulating site-specific ground motions for larger-magnitude earthquakes by transforming the Fourier amplitude spectra (FAS) of smaller events within the same source region. The method builds on Aki’s (1967) theory of universal similarity in earthquake radiation, which assumes that far-field spectral amplitudes scale predictably with event size due to self-similar rupture processes and the characteristic f^-2 decay at high frequencies. Assuming constant stress drop and self-similar rupture geometry, the method provides a transfer function that depends on magnitude and frequency and defines the spectral ratio between two events. At low frequencies, spectral amplitudes scale with slip amplitude for a fixed rupture area, while at high frequencies they scale with rupture area, reflecting the radiated energy. The formulation is most accurate at distances much greater than the rupture dimensions, where waveforms are governed by far-field radiation and scale with the moment-rate function.
The method is evaluated using recordings from the 2019 Ridgecrest and 2010 El Mayor-Cucapah earthquake sequences. Scaled Fourier amplitude spectra are compared with observed data and predictions from the NGA-West2 CB14 ground-motion model (GMM). Results demonstrate that the spectral scaling method shows good agreement with observations and generally outperforms the GMM, particularly for long-period ground motions and sites within sedimentary basins. This highlights the method’s ability to capture spatial and period-dependent variations in ground motion more accurately. The approach provides a physically grounded and computationally efficient framework for generating synthetic ground motions where large-event recordings are limited, with reliable performance for earthquakes up to magnitude 7.2. However, larger events with more complex rupture dynamics may require further model refinement. Continued validation in other tectonic environments will further support its utility in site-specific seismic hazard analysis and regional ground motion characterization.

Key Words
Spectral scaling, self-similarity theorem, ground motion simulations, transfer function, basin amplification

Citation
Zengin, E., Kohler, M. D., Heaton, T. H., & Roh, B. (2025, 09). Site-Specific Ground Motion Simulations via Spectral-Scaling Transfer Function Method. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)