Systematic measurements of rupture directivity for M3+ earthquakes in California

Anne Patton, & Daniel T. Trugman

Submitted August 30, 2026, SCEC Contribution #15323, 2026 SCEC Annual Meeting Poster #TBD

Rupture directivity is an important but poorly characterized earthquake source property that can directly affect the azimuthal distribution of ground motions and, consequently, earthquake hazard. Directivity has been measured for many moderate-to-large earthquakes in California, but systematic measurements for smaller earthquakes are needed to obtain a more complete picture of the underlying rupture and fault mechanisms that control directivity and amplified ground motion. Developed by Ross & Ben-Zion (2016) and expanded by Calderoni et al. (2023), the directivity index is a measure of azimuthal difference in the spectral ratio between a target event and a nearby empirical Green’s function (EGF) event, averaged over a frequency range spanning the target and EGF corner frequencies. Here, we generalize this approach by calculating the azimuthal distribution of the directivity index, which allows for inference of dominant rupture direction in cases where the rupture plane is unknown a priori. We apply this technique to assess spatiotemporal patterns in rupture directivity characteristics for over 600 M3+ earthquakes throughout California. We find many small clusters of earthquakes with consistent unilateral directivity, and other cases where the preferred rupture direction is temporally correlated throughout the sequence. We focus on the 2018 Cahuilla swarm and the 2021 Calipatria sequence as two such examples, where fluid movement likely drives the generation and migration of seismicity, as well as the preferred rupture direction. The comprehensive, statewide assessment of rupture directivity patterns produced in this study provides useful information for our understanding of earthquake physics and its effects on strong ground motion.

Key Words
seismology, small earthquakes, rupture directivity, earthquake source

Citation
Patton, A., & Trugman, D. T. (2026, 08). Systematic measurements of rupture directivity for M3+ earthquakes in California. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology