Toward an SCEC Broadband Platform with 3D Low-Frequency Synthetics (BBP 3D)

ABSTRACT The Statewide California Earthquake Center Broadband Platform (BBP; Maechling et al., 2015; Silva et al., 2025) is an important tool for structural and geotechnical earthquake engineering applications and probabilistic seismic hazard analysis. Several BBP modules calculate scenario earthquakes by a combination of physics-based ground-motion simulations at low frequencies (typically ≤1 Hz) and stochastic signals at higher frequencies. However, a notable limitation is the use of 1D optimized, region-specific velocity models for calculating the low-frequency (LF) component of ground motion that do not capture the complexities introduced by 3D sedimentary basins and complex topography, potentially leading to inaccuracy in subsequent seismic hazard assessment. We have tested the gain in ground-motion prediction efficacy of replacing the 1D by 3D physics-based LF simulations with support for topography into a new BBP 3D. The BBP 3D improves the 0.2–1 Hz mean Fourier amplitude spectral (FAS) bias by ≤89% for the 1989 Mw 6.9 Loma Prieta, 1996 Mw 6.7 Northridge, and 2019 Mw 7.1 Ridgecrest earthquakes using source ensembles with 10 realizations as compared with the current BBP 1D results, with the largest improvement on the vertical component. The largest reduction in FAS bias from the BBP using horizontally stratified models to the BBP 3D is obtained in the most populated basin areas associated with the largest seismic risk, with topography accounting for ≤50% of the improvement. The median 0.2–1 Hz duration and cumulative absolute velocity show 1D-to-3D increases in goodness of fit of ≤79% and 70%, respectively, with a smaller improvement between 1 and 2 Hz.