Preliminary Empirical Models for Effective Amplitude Spectra based on the NGA-West3 Dataset

Grace A. Parker, Gail M. Atkinson, Annemarie S. Baltay, David M. Boore, Tristan E. Buckreis, & Jonathan P. Stewart

Submitted September 7, 2025, SCEC Contribution #14660, 2025 SCEC Annual Meeting Poster #TBD

We develop a global earthquake ground motion model (GMM) for effective amplitude spectra (EAS, the smoothed quadratic mean of two horizontal-component Fourier amplitude spectra) as part of the Next Generation Attenuation (NGA) West3 project. The primary input parameters are moment magnitude (M), Joyner-Boore distance (RJB) and the time-averaged shear-wave velocity in the upper 30 m (VS30), with adjustments to account for regional variations in stress drop, depth, anelastic attenuation and kappa. We consider over 91,000 ground motion records from 1091 M≥3 earthquakes in the Western Contiguous United States and Alaska, Mexico, Italy, Greece, Turkey, Japan, Taiwan, and New Zealand. Our model utilizes seismological theory to constrain terms for geometrical spreading, anelastic attenuation (1/Q), high frequency attenuation (κ0), and the spectral shape of the seismic source, and combines them with empirically constrained model terms for the near-source saturation of ground motions, linear site amplification, and basin response. The interplay between the near-source saturation and geometric spreading when describing amplitude decay in RJB is carefully evaluated in model development. We adopt nonlinear site response scaling using recent results of 1-dimensional ground response analysis simulations. We fit models using a sequential approach that utilizes mixed-effects regression to account for the correlation between observations of the same earthquake and at the same station. Additionally, we fit a novel random effect on the attenuation slope (1/Q) for each earthquake to avoid tradeoffs between variations in attenuation and event terms (e.g., Baltay et al., 2020). We explore the performance of alternate earthquake source models, including single and double corner frequency models (Brune, 1976, and Boore et al., 2014, respectively). We observe that Japan has larger than average high-frequency ground motions due to weaker attenuation and stronger high frequency linear site response amplification. By contrast, ground motions in Taiwan are depleted in high frequency energy compared to the global average, due to a large κ0 value. We plan to develop a GMM for the 5%-damped RotD50 component (Boore, 2010) of pseudo-spectral acceleration (PSA) from a compatible dataset, and investigate its consistency with our EAS model converted to PSA.

Citation
Parker, G. A., Atkinson, G. M., Baltay, A. S., Boore, D. M., Buckreis, T. E., & Stewart, J. P. (2025, 09). Preliminary Empirical Models for Effective Amplitude Spectra based on the NGA-West3 Dataset. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)