Critical roughness controls sliding instability of laboratory earthquakes

The frictional strength of discontinuities in the upper earth crust controls the stability and dynamics of slip in diverse catastrophic phenomena such as earthquakes and landslides. Natural rock surfaces are rough at various scales with significant variability that affects their frictional behavior. Seismological and geophysical observations of large thrust faults suggest that fault geometry affects earthquake characteristics, yet the exact effects are currently being debated. In this study we show, using laboratory direct shear experiments, that a specific surface geometry enhances sliding instability and that the transition from stable to unstable sliding is non-linearly controlled by the magnitude of the initial roughness. In order to isolate the effect of roughness, we generate six levels of surface roughness in split prisms of Diabase rocks, with four orders of RMS magnitude difference between the smoothest and the roughest samples. The experiments are performed under an imposed constant normal stress of 5 MPa and load point (shear piston) velocity of 0.01 mm/s. The sliding target is typically set to 10 - 13 mm as monitored from two horizontal LVDT’s that are attached to the shear box very close to the tested interface. We show that the amplitude of the stick-slip events diminishes towards the two roughness extremes. The roughest sample (RMS = 1300 µm) exhibits a gradual increase of shear stress to a peak value of ~13 MPa, followed by brittle fracture expressed by a large stress drop of 3 MPa and then by transition to a relatively stable sliding. For the midrange roughness (RMS = 7 µm), stick-slip oscillations are obtained with different levels of stress drops and sliding dynamics characteristics. The smooth sample (RMS = 0.85 µm) slide in a relatively stable manner while the smoothest surface (RMS = 0.7) exhibits local peak friction of 0.18, followed by stable sliding with moderate slip hardening. We further demonstrate, both experimentally and numerically, that stick-slip oscillations commonly referred to as laboratory earthquakes, are constrained to a very limited range of surfaces roughness within which a specific level, defined here as the critical roughness, triggers the highest amplitude of oscillations. We therefore suggest that the roughness amplitude strongly affects the frictional stability and slip dynamics of natural faults.