Probabilistic seismic hazard analysis (PSHA) is typically used to estimate a ground motion intensity measure representing a central tendency within the horizontal plane, such as the median intensity across all horizontal orientations. However, in many cases, ground motion intensity is needed in specific horizontal orientations, particularly along the principal response axes of structures. Depending on the earthquake source, path effects, and site conditions, a site may exhibit significant levels of ground motion directionality, that is, a variation in ground motion intensity within the horizontal plane. If such variations are systematic across the earthquakes that affect the site, they can lead to significant differences in ground motion intensity between horizontal orientations. This paper introduces directional PSHA, an extension of conventional PSHA that quantifies seismic hazard for a specified horizontal orientation. The method primarily involves adapting the outputs of existing ground motion models to be orientation specific. The approach is applied to strike‐slip earthquakes, which tend to produce above‐average ground motion intensity in the transverse orientation. Two simple case studies are used to illustrate the method: a line‐segment fault and the San Francisco Bay Area, limited to the San Andreas and Hayward Faults. Results show strong orientation dependence in seismic hazard at sites located very close to a strike‐slip fault. Similar levels of orientation dependence are found at sites located farther from the fault relative to its length. The proposed method offers a more detailed description of the seismic hazard at a site and can improve seismic risk assessment methods for both new and existing structures.
Poulos et al. (Sun,) studied this question.