INVESTIGATING THE EVOLUTION OF COMPLEX SHEAR ZONES ARISING FROM SHALLOW STRIKE-SLIP FAULTING WITH DISCRETE ELEMENT ANALYSES (Invited Presentation)

Shallow strike-slip faulting generates complex distributions of deformation that often include through-going shear zones, networks of en-echelon fractures, and contractional and extensional structures expressed at Earth’s surface. This distributed inelastic deformation may reduce the amount of fault slip that reaches Earth’s surface, potentially biasing slip estimates used for probabilistic seismic hazard and fault displacement hazard analyses. These deformation features have been studied in laboratory, field, and numerical analyses, but many field interpretations arise from 2D perspectives, and numerical investigations often cannot resolve discrete, localized shear structures.

In this investigation, we use 3D discrete element method (DEM) models to examine how changes in shallow fault zone structure, material properties, and strike-slip loading conditions affect the predicted distributions of fault slip and strain. We use the unique advantages of DEM models (e.g., improved resolving power in capturing discrete shear-zone structure development and evolution) to understand how material properties of the shallow crust, such as the internal angle of friction, cohesion, and dilatancy, affect the shallow fault zone fracture patterns in strike-slip faulting regimes. Our models suggest that both material dilatancy and cohesion strongly influence cumulative shear zone widths, fracture patterns, and shear strain accommodation within shallow fault zones. Fault-induced shear within dilatant materials (e.g., dense sands and gravels) produces wide, branching shear structures that contrast with narrower, subvertical shear structures in non-dilatant materials (e.g., loose silts and clays) under identical loading conditions. Increases in material cohesion also produce wider shear zones, but concentrate shear strain along narrow, localized fractures. We find that fractures and shear zones coevolve with porosity and cohesive strength as strike-slip displacement increases within modeled fault zones. We compare our results with field data from recent earthquake ruptures to understand how these models may inform surface displacement and deformation distribution predictions surrounding strike-slip faults.