Micromechanics of stick-slip deformation across rough faults

We investigate how the initial surface roughness of laboratory faults, generated in stiff diabase rocks, affects the micromechanics of stick-slip oscillations with normal stresses ranging from 2.5 to 15 MPa. The tested surfaces spanned a wide range of roughness: from extremely smooth (Polished; RMS x = 4mm = 0.83–1.33 μm), to intermediate (Saw-cut; RMS x = 4mm = 3.9–9.98 μm), to extremely rough (Fractured; RMS x = 4mm = 154–222 μm). We show that both global shear behavior as well as dilation are strongly influenced by surface roughness and normal stress. By detrending the global dilation signal, we identify a clear relationship between vertical motion, roughness, and normal stress: the amplitude of vertical displacement during stick phases increases with normal stress and is maximized at intermediate (saw-cut) roughness levels. We confirm the ‘critical roughness’ concept previously identified by Morad et al., 2022, that predicts that the intermediate roughness (saw-cut) produces the largest stress drop magnitudes, across a wide range of normal stresses. Both smoother and rougher interfaces exhibit a more stable sliding behavior. Furthermore, a positive correlation between interface shear stiffness during stick-slip cycles (k i ), normal stress (σ n ), and sliding instability as scaled by mean stress drop (Δτ) is firmly established, particularly for rougher interfaces. Finally, we introduce a new energy-based criterion to predict sliding instability potential. This novel approach links the stress drop magnitude during stick-slip cycles to both the intra-cyclic shear stiffness of the interface and the applied normal stress, all measurable quantities, and does not require the critical stiffness (k c ) which cannot be measured directly, as used in rate and state friction laws. • Sliding instability as scaled by stress drop peaks at moderate saw-cut roughness. • Dilation within stick slip cycles also peaks at moderate saw-cut roughness. • This critical roughness remains the same for a wide range of normal stresses. • Dilation within stick slip cycles increases with normal stress. • Elastic energy stored in the loading system is used to predict sliding instability.