Strain heterogeneities in laboratory faults driven by roughness and wear

Faults in nature display complex surface characteristics, where fault asperities slip dynamically while other sections are more prone to creep. Fault roughness is critical in determining the contact conditions producing asperities, directly impacting their susceptibility to unstable sliding. Understanding the formation of asperities and how their seismic properties evolve with wear is critical for assessing slip budget and earthquake potential. In this study, a triaxial experiment was conducted on a cylindrical saw-cut sample of Carrara marble, to study its frictional evolution and strain response with wear. Initially the interface produced an audible, high stress drop (∼ 21 MPa) stick-slip event. A fiber-optic based distributed strain sensing (DSS) method was used to study the strain heterogeneities and showed that a central asperity was produced, causing the locking of the interface. The central asperity was explained by both the small curvature ratio ${\rho }_c$ = 0.1% and the locking associated with a roughness drag induced by short wavelength fluctuations in roughness ${\lambda }_{min}$ ∼ 1 μm which should have promoted healing. After the audible stick-slip, the fault produced more stable frictional behavior with low magnitude stress drop events (∼ 2 MPa). We attributed this to (i) changes in the rate- and state-dependent frictional (RSF) parameters, $\left(ii\right)$ decreased roughness-induced stiffness in the central asperity due to smoothing and $\left(iii\right)$ the heterogeneous deposition of gouge formed due to wear. In terms of frictional stability, normal stress increase and smoothing would promote unstable sliding which was not observed. This led us to conclude that small amounts of gouge pushed the fault closer to the frictional stability line and even produced macroscopic velocity-strengthening behavior. Fault-parallel distributed strain measurements confirmed that the gouge allowed strain to be accommodated close to the interface, in contrast to the off-fault matrix that produced the large stress drop stick-slip event. These measurements also suggested that faults could simultaneously produce a range of frictional behaviors, with microscopically small stress drop silent events. These findings, together with the evolution of calcite-rich surfaces, highlight how roughness and its evolution can affect fault stability and drive strain heterogeneities due to gouge development and smoothing of the main slip surface.