Three-Dimensional Distinct Element Simulations of Dynamic Process of Fault Gouge Evolution

William Marsh Rice University, Houston TX

Investigators

Abstract

Brittle fault zones undergo continual wear of fault surfaces during slip, which leads to a continuous variation in fault surface roughness and a progressive increase in the thickness of the fault gouge zone with net slip. The dynamic changes in gouge zone features may affect fault frictional strength, stability, and earthquake characteristics. Although previous field, laboratory, and numerical studies have made significant progress in investigating the geometrical and physical properties of fault gouge zones and their effects on fault zone frictional and mechanical behavior, these results still provide limited understanding of the complete process of gouge zone development under a wide range of geological settings, and how these materials might affect the earthquake cycle. Recent modifications to the Distinct Element Method (DEM), including addition of interparticle bonding and breakage, allows us to model the fracture process of cohesive rocks, and makes it possible to directly observe gouge accumulation and modification, mimicking natural fault zone deformation. 2-D DEM simulations of gouge evolution have successfully reproduced many observations in the field and laboratory, yielding interesting findings that have not been reported in previous gouge zone studies. These include stages of gouge zone development and their dependence on shear displacement, normal stress, and rock strength. These encouraging results suggest the feasibility, necessity, and importance of improving numerical models and extending the current studies into the future. This proposal describes a program to simulate the dynamic processes of gouge zone evolution, focused on constraining the relationships between gouge thickness, grain size distribution, shear zone strength, and surface roughness. We will use much larger assemblages that will allow a wider range of surface and particles sizes and geometries, and conduct these simulations in parallel on the new Rice Cray XD1 supercomputer. Results of these numerical experiments will help in interpreting observations of more complex natural faults, leading to an improved understanding of fault zone processes. The work will support training of a postdoctural associate, undergraduates, and provide enhanced 3D codes of fault gouge evolution to the community. Understanding how fault zones deform can help improve understanding of how faults slip and thus earthquake hazards.