Using a Ground-Motion Modeling Framework to Fit Stress Drops for the SCEC Community Stress Drop Validation TAG Synthetic Exercise

Grace A. Parker

Submitted August 30, 2026, SCEC Contribution #15282, 2026 SCEC Annual Meeting Poster #122

I fit spectral stress drop, linear site amplification, and kappa (the high frequency attenuation parameter), to synthetically generated earthquake ground motion Fourier amplitude spectra (FAS) as part of the SCEC Community Stress Drop Validation Technical Activity Group (TAG) synthetic exercise. The goal of the synthetic exercise is to understand how various fitting methods influence estimates of earthquake stress drop. As part of the exercise, TAG leadership generated synthetic earthquake ground motion spectra using simple convolution approaches that prescribe frequency-dependent source, path and site components and include synthetic seismic noise. Earthquakes are modeled as co-located point-sources, with moment magnitudes between 1.5 and 6, and are recorded on 25 stations at 1 to 66 km epicentral distance. I use an empirical ground-motion model (GMM) for FAS recently developed as part of the NGA-West3 as the framework to fit spectral parameters. I compare the distance attenuation in the synthetics to the path component of the GMM, which includes near-source saturation, geometrical spreading, and frequency-dependent Q (i.e., anelastic attenuation), and then adjust each synthetic spectrum to 0 distance. Keeping the seismic moment fixed, I perform a generalized inversion over all synthetics simultaneously to fit values of earthquake corner frequency, site kappa, and a frequency-independent site amplification. I used a nonlinear mixed-effects regression where earthquakes and sites are taken as grouping factors. I explore the influence of fitting velocity versus acceleration spectra, varying distance attenuation slopes, the effect of source model complexity, and effects of site amplification. I compare my best-fit values to the true values used to generate the synthetic spectra and assess possible reasons for discrepancies.

Citation
Parker, G. A. (2026, 08). Using a Ground-Motion Modeling Framework to Fit Stress Drops for the SCEC Community Stress Drop Validation TAG Synthetic Exercise. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology