Strain localization in quartz-rich fault gouge at subseismic slip rates

Understanding strain localization and development of shear fabrics within brittle fault zones at subseismic slip rates is crucial as they have critical implications for the mechanical strength and stability of faults and for earthquake physics. We performed direct shear experiments on ~1 mm thick layers of simulated quartz-rich fault gouge at an effective normal stress of 40 MPa, pore fluid pressure of 15 MPa, and temperature of 100°C. Microstructures were analyzed from strain hardening state (~1.3 mm displacement) to strain softening (~3.3 mm displacement) to steady-state (~5.6 mm) at different imposed shearing velocities of 1 µm/s, 30 µm/s, and 1 mm/s. We performed X-ray Computed Tomography (XCT) on sheared samples with a strain marker to analyze slip partitioning. To analyze and quantify localization from few hundreds to thousands of cross-section images, we used machine learning and developed an automatic boundary detection method to identify the type of shear fabrics and quantify the amount of them. Our results reveal that R₁ and Y (or boundary) shears are the two major localization features that developed in a repeatable manner. Slip on R₁ shears shows little dependency on both shear displacement and slip velocity and amounts to ~5 to ~30% of slip through the entire frictional sliding. On the other hand, Y and boundary shears show a strong correlation with displacement and velocity where more than 40% of strain was accommodated at steady-state for all velocities. However, Y and boundary shears become less prominent with increasing velocity, suggesting that velocity-weakening and the associated nucleation of unstable sliding are less likely to occur at higher slip rates as the overall friction behavior would be controlled by a thicker gouge layer. In other words, this suggests that Y shear development by grain size reduction is less efficient at high slip velocities which has important implications for the amount of heat generated during accelerating slip.