Role of Quartz Grain Size and Saturation on Fault Strength and Frictional Healing

Rocks near the Earth's surface (less than 5 kilometers deep) often break and grind against one another to create crushed rock materials known as fault gouge. These fault gouges are filled with fragmented particles and broken rocks, which vary in grain size over time and space due to repeated earthquakes. This variation in grain size distribution along fault gouge can affect the strength of the fault, how it slips during an earthquake and how it 'heals' or recovers strength between earthquakes. Further, fault gouges often contain water from underground sources, which can influence how they slip. Previous studies have examined the influence of grain size, along with other factors such as pressure, stress, and surface roughness; however, isolating the role of grain size in controlling frictional stability and healing remains insufficiently explored. This study focuses on how the size of grains in fault gouge and the presence of water affect the fault strength and the way faults regain strength after a period of no movement. We performed a series of friction experiments on quartz material with four different grain sizes and studied how they responded to stress slipping under dry and water-saturated conditions at the same pressure found in the shallow parts of Earth's crust. The results suggest that gouges with smaller grains show sudden slip, a pattern similar to small earthquakes, where stress builds up and is then quickly released, whereas those with larger grains show increasingly stable sliding like creep. We also found that water makes faults more unstable and increases the rate at which they regain strength after slipping. These findings help us understand how faults behave and how earthquakes initiate, which can improve hazard assessments and earthquake prediction.