Shallow Geomaterial Properties Control Strike-Slip Surface Rupture Expression

Curtis W. Baden, Josie M. Nevitt, & Fernando E. Garcia

Submitted August 30, 2026, SCEC Contribution #15163, 2026 SCEC Annual Meeting Poster #TBD

Observations of strike-slip surface rupture show that rupture-zone width, distributed shear, vertical deformation, and surface slip interpretations can vary dramatically between earthquakes. Natural ruptures illustrate the complexity of surface faulting, but isolating the role of shallow fault-zone mechanics from field observations alone remains difficult. Differences in surface rupture expression are often interpreted in terms of fault geometry, rupture complexity, or near-surface structural heterogeneity, yet the contribution of shallow material properties is less well constrained. Here, we use discrete-element models of strike-slip faulting through unconsolidated near-surface materials to test how frictional strength, cohesion, shear-induced volumetric strain, and shallow fault-zone depth influence the partitioning of deformation between discrete fault slip and distributed off-fault strain.

Our results show that shallow material properties strongly influence first-order rupture behavior. Frictionally stronger materials produce wider shear zones, whereas weaker materials localize deformation into narrower fault zones. Fault zones grow particularly wide when frictionally strong materials dilate during shear. We also observe displacement-dependent deformation, in which volumetric strain dominates early shallow fault-zone development, while shear strain becomes increasingly important as meter-scale displacement accumulates. Direct quantification of shear and volumetric strain shows how deformation partitions between discrete fractures and distributed shear within the evolving fault zone.

These results have important implications for interpreting natural surface ruptures. Even in models with a straight basal fault, material properties alone generate substantial differences in surface faulting style. These simple models reproduce many features observed in natural ruptures and show that slip measured across discrete fractures may not capture the full surface displacement budget when deformation is distributed across a broader shear zone. These findings highlight the importance of shallow fault-zone material properties for interpreting off-fault deformation, surface rupture observations, and hazards to infrastructure and population centers.

Key Words
fault rupture, material properties, strike-slip fault, discrete element modeling

Citation
Baden, C. W., Nevitt, J. M., & Garcia, F. E. (2026, 08). Shallow Geomaterial Properties Control Strike-Slip Surface Rupture Expression. Poster Presentation at 2026 SCEC Annual Meeting.

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