Role of Grain Shape and Inter-Particle Friction on the Strength of Simulated Fault Gouge – Results, Questions, Directions D. Place, P. Mora, and S. Abe QUAKES The University of Queensland

APEC Cooperation for Earthquake Simulation Contents LSM principles Effect of friction on fracture Fault strength with different numerical approaches (LSM & DEM) Fault gouge strength Summary & future work Conclusions

APEC Cooperation for Earthquake Simulation LSM Principles Fracture Friction

Fracture Experiment

APEC Cooperation for Earthquake Simulation Fracture Experiment Bi-axial experiment Constant confining pressure Random particle size

APEC Cooperation for Earthquake Simulation Fracture Experiment Coefficient of friction ranging from 0 to 0.8

APEC Cooperation for Earthquake Simulation Fracture Experiment When using a random lattice –The intrinsic coefficient of friction has no/little effect on the macroscopic characteristics. –Particle size distribution and microscopic strength control the macroscopic behaviour.

Effect of the Numerical Method on Fault Strength

APEC Cooperation for Earthquake Simulation Experimental Setup

APEC Cooperation for Earthquake Simulation Shear Elasticity DEM approach vs. LSM approach

APEC Cooperation for Earthquake Simulation Fault Strength LSM approach DEM approach

APEC Cooperation for Earthquake Simulation Slip Dynamic LSM approach DEM approach

APEC Cooperation for Earthquake Simulation Fault Strength The method/approach does not affect the fault strength However, the type model used has an effect on the dynamic of slip

Effect of Friction on Fault Gouge Strength

APEC Cooperation for Earthquake Simulation Numerical Experiment

APEC Cooperation for Earthquake Simulation Fault Gouge Strength (for μ=1.0)

APEC Cooperation for Earthquake Simulation Fault Gouge Strength (for μ=0.6)

APEC Cooperation for Earthquake Simulation Fault Gouge Compaction

APEC Cooperation for Earthquake Simulation Fault Gouge Strength For both experiments, the fault gouge friction is ~0.6. Both experiments show a drop in the fault strength due to a self- organisation of grains in the gouge.

APEC Cooperation for Earthquake Simulation Numerical Experiment

APEC Cooperation for Earthquake Simulation Experiment With Bare Surfaces

APEC Cooperation for Earthquake Simulation Fault With Gouge

APEC Cooperation for Earthquake Simulation Grain Shape

APEC Cooperation for Earthquake Simulation Friction Law Experiment using rate- and state- dependent friction have shown that when a gouge is present, the inter-particle friction has no/little effect on the fault gouge strength –Limited amount of slip between grain in the gouge

APEC Cooperation for Earthquake Simulation Fault Gouge Strength Microscopic coefficient of friction has no/little effect on the fault gouge strength. Arrangement of grains and grain shape in the gouge control the fault gouge strength.

APEC Cooperation for Earthquake Simulation Summary No/Little effect on fault gouge strength: –The value of the intrinsic coefficient of friction –Algorithm/Method –Friction law Affect fault gouge strength –Grain shape –Grain size –Grain distribution (arrangement)

APEC Cooperation for Earthquake Simulation Limitations of the Model High roughness of grains Limited grain shapes –However by using larger model, broader shapes of grains (group of particles) can be modelled. Limited distribution of particle sizes –Due to computational requirements, particle sizes can range from to 1.0 Particles cannot fracture

APEC Cooperation for Earthquake Simulation Future Work: Grinding When subjected to a high stress a particle can break down into several smaller particles –Would affect localisation phenomena observed in experiment with gouge –Would affect gouge geometry and hence, fault gouge strength and behaviour

APEC Cooperation for Earthquake Simulation Conclusions When a gouge is present (or in a fault system), the microscopic friction has no/little effect on the fault strength Does not affect the fault gouge strength: –the value of the microscopic friction, –the friction law used, –the method used. The geometry controls the fault gouge strength –Grain distribution –Grain size –Grain shape

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