The Influence of Particle Size Distributions on Flash Heating and Thermally Induced Weakening in Fault Gouges

Flash heating is a critical phenomenon in fault gouges, occurring during rapid slip events that generate temperature surges at the grains or asperity contacts. These abrupt temperature increases trigger mechanisms that weaken the gouge material, reducing its shear strength. This process plays a key role in modulating fault dynamics and can significantly impact the propagation of earthquakes [1, 2]. Although many studies have considered microscale friction of uniform or narrowly graded grain sizes, natural fault zones commonly consist of a wide range of particle sizes [3].

In this work, we used a three-dimensional discrete element framework to simulate shear in a granular fault gouge to replicate the rotary-shear experiment. A local temperature-dependent friction law was implemented in which the friction angle of a grain decreases sharply once its temperature exceeds a specified cut-off threshold [4], quantifying how grain size ranges influence the onset of thermally-activated weakening by varying the PSD of our simulated fault gouge—from monodisperse (all grains have the same size) to highly polydisperse (very broad size ranges)—and determining how this range modulates the onset of flash heat weakening. Our numerical results demonstrate that monodisperse samples distribute contact forces more evenly among grains, and that heating and subsequent weakening occur more synchronously across the sample. This uniform distribution of contact forces and thermal effects results in a drop in the macroscopic friction coefficient in a relatively abrupt way. In contrast, samples with polydisperse particles have a wide range of grain sizes, which can give rise to a heterogeneous stress state and thereby strengthen local frictional heating in some areas of the sample, thus allowing a gradual and disordered decline in the macroscopic friction coefficient. Thus, larger grains in polydisperse assemblies can be considered as stress bridges, sustaining higher values of contact forces; on the other side, small grains have lower thermal capacities (via small masses), and they heat up and soften quicker than larger ones with the same frictional work rate. Such a mixture of these aspects explains a larger percentage of weakened particles in polydisperse assemblies and its lower residual friction compared to samples with narrower PSDs. These results emphasize the importance of considering the natural heterogeneity of grain sizes in models of, and observations of temperature-dependent weakening in fault gouges. The polydispersity degree can have a dramatic impact on the rate and extent to which flash heating can induce a transition from a strong (frictional) resisting to a weakened state. Knowledge of these PSD-dependent processes leads to a more realistic representation of dynamic fault weakening and enhances our understanding of earthquake rupture dynamics at naturally heterogeneous fault zones.

 

References

[1] J.R. Rice. Journal of Geophysical Research: Solid Earth, 111(5), 2006.

[2] B.P. Proctor, T.M. Mitchell, G. Hirth, D. Goldsby, F. Zorzi, J.D. Platt, and G. Di Toro. Journal of Geophysical Research, Solid Earth, 119(iv):3076–3095, 2014.

[3] C. Marone and C.H. Scholz. Journal of Structural Geology, 11(7):799–814, 1989.

[4] A. Taboada and M. Renouf. Geophysical Journal International, 233(2):1492–1514, 2023.