D. Place, P. Mora, and S. Abe QUAKES The University of Queensland

1. 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

2. Contents • LSM principles • Effect of friction on fracture • Fault strength with different numerical approaches (LSM & DEM) • Fault gouge strength • Summary & future work • Conclusions

3. LSM Principles Friction Fracture

4. Fracture Experiment

5. Fracture Experiment • Bi-axial experiment • Constant confining pressure • Random particle size

6. Coefficient of friction ranging from 0 to 0.8 Fracture Experiment

7. 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.

8. Effect of the Numerical Method on Fault Strength

9. Experimental Setup

10. Shear Elasticity • DEM approach vs. LSM approach

11. Fault Strength LSM approach DEM approach

12. Slip Dynamic LSM approach DEM approach

13. Fault Strength • The method/approach does not affect the fault strength • However, the type model used has an effect on the dynamic of slip

14. Effect of Friction on Fault Gouge Strength

15. Numerical Experiment

16. Fault Gouge Strength (for μ=1.0)

17. Fault Gouge Strength (for μ=0.6)

18. Fault Gouge Compaction

19. 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.

20. Numerical Experiment

21. Experiment With Bare Surfaces

22. Fault With Gouge

23. Grain Shape

24. 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

25. 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.

26. 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)

27. 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 0.025 to 1.0 • Particles cannot fracture

28. 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

29. 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

30. Thank You