Spatial Variability of Ground Motions due to Rupture Directivity in Seismic Hazard Assessments

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

Submitted September 7, 2025, SCEC Contribution #14581, 2025 SCEC Annual Meeting Poster #TBD

Rupture directivity produces systematic variations in ground motions from the interaction between rupture propagation and the seismic radiation pattern. It can cause significant azimuthal variation in near-fault ground motions at a given source-to-site distance, affecting both median values and variability. Directivity effects have previously been incorporated into ground-motion models (GMMs) by adjusting the median ground motions and aleatory variability. Only recently have these effects begun to be implemented in large-scale probabilistic seismic hazard assessments (PSHA), such as those used in the USGS National Seismic Hazard Maps (NSHM). This emerging focus introduces new complexities that must be considered in hazard calculations. For instance, accounting for azimuthal variations due to directivity depends on the rupture's hypocenter location and the direction of rupture propagation. One method for incorporating this effect involves adding a nested loop over possible hypocenter locations for every rupture in the hazard calculation, which introduces additional variability in both the range of ground motions and the associated uncertainty. This approach also necessitates the assumption of a hypocenter probability distribution along the fault, a quantity that is generally poorly constrained.
In this work, we evaluate the effects of rupture complexity from the UCERF3 fault database, specifically focusing on how assumptions about hypocenter distributions influence rupture directivity patterns. We explore several hypocenter distributions along fault strike and dip, including uniform, Gaussian, and exponential distributions, and assess their impact on hazard calculations. We compare hazard difference and ratio maps computed with and without directivity effects, progressing from simple to more complex rupture geometries. We also consider asymmetric hypocenter distributions along strike, simulating scenarios where ruptures preferentially initiate and propagate in a particular direction, a behavior observed in some historical ruptures. The resulting spatial variability is isolated and evaluated to understand its influence on seismic hazard. These findings help to quantify how assumptions about rupture propagation affect both median hazard estimates and epistemic uncertainty. Ultimately, this analysis informs how rupture directivity contributes to the spatial variability in hazard, providing critical insights for improving future hazard models.

Key Words
rupture directivity, ground motion, seismic hazard

Citation
Withers, K. B., Moschetti, M. P., & Bayless, J. (2025, 09). Spatial Variability of Ground Motions due to Rupture Directivity in Seismic Hazard Assessments. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)