Ground-motion variability in SCEC CyberShake

Morgan P. Moschetti, Scott Callaghan, Kyle B. Withers, Xiaofeng Meng, & Robert W. Graves

Submitted August 30, 2026, SCEC Contribution #15551, 2026 SCEC Annual Meeting Poster #TBD

We analyze CyberShake version 22.12 to identify the controlling features of variability from three-dimensional (3D) ground-motion simulations. Low-frequency results (T≥2 s) consist of pseudospectral accelerations from seismic sources in the Uniform California Earthquake Rupture Forecast, version 2, simulated at 335 stations in southern California. This dataset comprises one of the most complete sets of simulated ground motions available for earthquakes, 6.15≤M_W≤8.45. Ground-motion variability, which is used directly to compute probabilistic seismic hazard curves in CyberShake, is governed by variations in slip and hypocenter location, along with minor, stochastic variations in moment, rupture speed, and rise-time parameters. Unlike empirical ground-motion models, CyberShake allows variability to be directly estimated at each site from the standard deviation of the (natural logarithm) ground motions for each seismic source. We extract the subset of 2,680 seismic sources that are simulated at all stations and compute standard deviations for all seismic sources and stations. Standard deviations are comparable to “single-station sigma” values, as they exclude site-to-site variability (ϕ_S2S).

Our results show that rupture directivity strongly influences variability for most individual sources. We fit the azimuthal variations in standard deviation for each source, then remove modeled directivity effects to produce directivity-corrected standard deviations. Mean trends in the fitted directivity parameters and in the directivity-corrected standard deviations are presented. We find evidence of period-dependent trends in the corrected standard deviations with distance, showing lower standard deviations at greater distances. Additionally, we observe positive correlations between corrected standard deviations and earthquake magnitude, with larger effects at longer periods (T≥5 s). These magnitude-dependent trends may result from increased rise-time values in larger-magnitude earthquakes. Work is ongoing to better understand the effects of kinematic rupture parameters on ground-motion variability.

Citation
Moschetti, M. P., Callaghan, S., Withers, K. B., Meng, X., & Graves, R. W. (2026, 08). Ground-motion variability in SCEC CyberShake. Poster Presentation at 2026 SCEC Annual Meeting.

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Community Capability Building (CCB)