Ground Motion Directionality in Simulation-Based Probabilistic Seismic Hazard Analysis
Alan Poulos, Evan T. Hirakawa, Grace A. Parker, & Annemarie S. BaltaySubmitted September 7, 2025, SCEC Contribution #14603, 2025 SCEC Annual Meeting Poster #TBD
We characterize earthquake ground motion polarization in simulation-based PSHA using the CyberShake Southern California Study 22.12. The intensity of earthquake ground motion can vary significantly depending on horizontal orientation, a phenomenon known as ground motion directionality. However, current ground motion models (GMMs) do not capture these variations, as they use a single intensity value (typically the median) to represent all horizontal orientations. Consequently, probabilistic seismic hazard analyses (PSHA) which rely on GMMs inherit this simplification. One alternative to GMMs is the use of physics-based simulations, which provide full ground motion waveforms from which intensities can be computed for all horizontal orientations. In this work, we use the latest CyberShake study of Southern California (Study 22.12) to conduct simulation-based PSHA at the 335 sites included in the study. We characterize ground motion intensity using pseudo-spectral accelerations (PSAs) at periods ranging from 1 to 10 s. At most sites, PSAs with return periods of 2,475 years, a value typically used in earthquake-resistant design, vary significantly with horizontal orientation and can differ substantially from the commonly used median intensity. For example, for PSA at a period of 3 s, the minimum and maximum hazards across all horizontal orientations are, on average, 10% lower and 15% higher than the median hazard, respectively. These average differences increase with period, with PSA at 10 s having minimum and maximum hazards across all orientations that are 19% lower and 22% higher than the median hazard, respectively. In most cases, these variations are driven by physical mechanisms that polarize seismic waves, such as the radiation pattern of strike-slip earthquakes, co-seismic static offsets, and the influence of subsurface structures (e.g., sedimentary basins). This latter effect is clearly observed in both the Los Angeles Basin and the Ventura Basin. Producing PSHA estimates at specific horizontal orientations allows for a more comprehensive description of seismic hazard, which could support improvements in future seismic risk analyses and earthquake-resistant design procedures.
Key Words
seismic hazard, ground motion directionality
Citation
Poulos, A., Hirakawa, E. T., Parker, G. A., & Baltay, A. S. (2025, 09). Ground Motion Directionality in Simulation-Based Probabilistic Seismic Hazard Analysis. Poster Presentation at 2025 SCEC Annual Meeting.
Related Projects & Working Groups
Ground Motions (GM)
