The Friction of Natural Fault Rocks

Heather M. Savage, & Demian Saffer

Submitted September 7, 2025, SCEC Contribution #14901, 2025 SCEC Annual Meeting Poster #TBD

Laboratory friction experiments have advanced our understanding of fault behavior for several decades, however the majority of friction experiments have been conducted on mono-mineralic standards and synthetic mixtures. Natural fault gouges tend to be polymineralic with a mix of stronger and weaker phases that evolve through mechanical and chemical processes over time. Here, we compare friction experiments on natural fault gouges compiled from existing literature to results from synthetic gouges. From the work on synthetic fault gouges, we have a general understanding that the friction coefficient of faults is µ = ~0.6-0.8, with the exception of phyllosilicate-rich faults, which exhibit lower friction of µ = ~0.1-0.4. Furthermore, rate and state friction theory describes how gouges react to changes in sliding velocity, and experiments have generally shown that weak (and phyllosilicate-rich) faults are more likely to fail stably. An emerging body of experimental work on natural fault rocks across a range of geologic and tectonic settings show that natural fault friction is similar to that of synthetic gouges at room temperature, but with more variability as most fault rocks contain some clay. Natural fault gouges are generally stable at velocities greater than ~1µm/s and normal stresses above ~ 15 MPa at room temperature. As seen in previous experiments on synthetic gouges, natural gouges show strong temperature dependence of the frictional rate parameter, with the most unstable values around 300 °C. Unlike synthetic gouges, the velocity-weakening temperature range is not strongly related to lithology. The clearest signal from this dataset is that most experiments on fault rocks are done at room temperature and normal stresses below 50 MPa, whereas the seismogenic zone exists at temperatures from ~150-300 °C and effective stresses ~75-225 MPa. Future work should focus on the P-T space of the seismogenic zone.

Key Words
friction, fault, friction stability, natural gouges

Citation
Savage, H. M., & Saffer, D. (2025, 09). The Friction of Natural Fault Rocks. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)