Evaluating Variation in Rupture Directivity from Hypocenter Distributions in PSHA

Kyle B. Withers, Morgan P. Moschetti, & Jeff Bayless

Submitted August 30, 2026, SCEC Contribution #15403, 2026 SCEC Annual Meeting Poster #240

Rupture directivity models modify the median and variability of ground-motion models (GMMs) but alternatives remain for how to properly incorporate them into probabilistic seismic hazard analyses (PSHAs). Directivity model modifications depend on either specific hypocenter locations or the cumulative influence of a hypocenter distribution, and hence hypocenter modeling decisions have an impact on hazard calculations. Here, we evaluate key modeling parameters used in representing directivity within PSHA. We vary hypocenter probability distributions along strike and down dip, altering how the mean hazard accumulates directivity effects from each rupture. Exponential and Gaussian probability distributions of hypocenters are examined and compared with a baseline uniform distribution that assigns equal weight across the rupture hypocenters. We also explore asymmetric strike-parallel distributions to simulate cases in which faults preferentially rupture in one direction. The rupture representation is progressively expanded from simple geometries to more complex configurations, including those in the Uniform California Earthquake Rupture Forecast version 3 (UCERF3) fault database. Comparing hazard maps, we find that symmetric variations in hypocenter distributions have relatively limited influence on mean hazard. However, asymmetric distributions strongly affect the spatial distribution of amplification that arises from the superposition of multiple directivity adjustments, leading to localized changes off the ends of the fault. Finally, we evaluate the contribution of hypocenter variability to aleatory uncertainty by examining differences in the hazard curves among individual hypocenter realizations. Both this aleatory and the aforementioned epistemic uncertainties might be important to capture when assessing the range of possible hazard outcomes.

Citation
Withers, K. B., Moschetti, M. P., & Bayless, J. (2026, 08). Evaluating Variation in Rupture Directivity from Hypocenter Distributions in PSHA. Poster Presentation at 2026 SCEC Annual Meeting.

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