Linking near-surface material behavior to strike-slip surface rupture patterns and shear zone width with discrete element modeling

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

Submitted September 7, 2025, SCEC Contribution #14369, 2025 SCEC Annual Meeting Poster #TBD

Surface-rupturing strike-slip earthquakes often produce complex deformation zones that host through-going shear bands, echelon fractures, and extensional or contractional structures near the fault trace. These inelastic features influence the distribution of displacement, introducing uncertainty in estimates of ground rupture used for fault displacement hazard analyses. Laboratory and field studies have documented these effects, but many observations are limited to 2D perspectives, and continuum-based models often cannot resolve localized shear band formation.

Here, we use 3D discrete element method (DEM) models to explore how inelastic properties of shallow geomaterials influence shear zone widths during strike-slip faulting. We first performed DEM-based direct shear tests on a parametric suite of geomaterials to quantify emergent internal friction angle, cohesion, and dilatancy angle. Because DEM input parameters do not directly correspond to bulk material behavior, these shear tests allowed us to characterize the macroscopic response of each material configuration. We then used the calibrated material behaviors in 3D strike-slip simulations to investigate how bulk properties influence the development and localization of near-surface shear zones.

Our results show that differences in internal friction and dilatancy angles influence near-surface shear zone widths. Materials with reduced internal friction and minimal dilatancy produce narrow, localized zones, while more dilatant, frictionally strong materials promote broader, distributed deformation. These trends may help explain differences in surface rupture observed in earthquakes of similar magnitude. For example, the 2019 M7.1 Ridgecrest earthquake produced narrow shear zones as rupture propagated through silty clays interpreted to be frictionally weak and minimally dilatant, whereas the 2010 M7.1 Darfield rupture occurred in gravels likely exhibiting higher friction and greater dilatancy. Our findings suggest that site-specific differences in material behavior may contribute to the contrasting deformation styles observed, highlighting the value of material-specific, physics-based models in surface rupture hazard assessments, including probabilistic fault displacement hazard analysis.

Key Words
surface rupture, rheology, deformation, discrete element models

Citation
Baden, C. W., Nevitt, J. M., & Garcia, F. E. (2025, 09). Linking near-surface material behavior to strike-slip surface rupture patterns and shear zone width with discrete element modeling. Poster Presentation at 2025 SCEC Annual Meeting.

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