The influence of fracture propagation mode on surface rupture morphology during the 2019 M7.1 Ridgecrest, California earthquake

Josie M. Nevitt, Benjamin A. Brooks, Todd L. Ericksen, Nima Ekhtari, Craig L. Glennie, Kenneth W. Hudnut, & Simone Yeager

Submitted August 30, 2026, SCEC Contribution #15384, 2026 SCEC Annual Meeting Poster #TBD

Earthquake surface rupture results from the propagation and interaction of fractures in the shallow crust but is rarely investigated using fracture mechanics concepts. In this study, we use mobile laser scanning, low-altitude aerial imagery, and previously published optical image correlation displacement data to investigate the formation of surface rupture fractures within the maximum slip zone of the 2019 M7.1 Ridgecrest, California, earthquake. The study area is characterized by ~1-5 m of right-lateral slip and 0-1 m of dip slip, constrained by piercing points adjacent to the rupture, and includes a ~600-m-long releasing bend along the principal rupture trace. Fracture length tends to be greatest within the bend where right-lateral slip is relatively low and dip slip is relatively high. Outside the bend, the rupture is characterized by segmented echelon fractures up to ~5 m long, whereas the bend interior includes more continuous fractures reaching ~25 m in length. We hypothesize that variations in fracture length reflect differences in fracture propagation mode, which depends on the local loading conditions at the rupture tip. To test this, we estimate fault-local 3D displacement gradient tensors from the surface displacement fields, and infer fracture mode with the assumption that the parent fault was vertical, initially buried, and propagated upward from depth. We find that Mode III dominates outside the bend, transitioning to Modes I, II, and mixed mode loading within the bend. This transition provides a possible mechanical explanation for the observed variations in fracture length. Prior studies have shown that Mode III propagation causes fractures to twist away from the parent tipline and break into shorter segments, whereas the tipline remains intact during Mode I and II propagation. Linking fracture mode to rupture morphology provides a mechanical framework for understanding patterns of surface rupture relevant to fault displacement hazard.

Citation
Nevitt, J. M., Brooks, B. A., Ericksen, T. L., Ekhtari, N., Glennie, C. L., Hudnut, K. W., & Yeager, S. (2026, 08). The influence of fracture propagation mode on surface rupture morphology during the 2019 M7.1 Ridgecrest, California earthquake. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)