The frictional strength of faults controls the stability and the dynamics of slip in diverse natural phenomena including earthquakes, induced seismicity, and landslides. It has been shown that geological faults and rock discontinuities in general are rough in a wide range of scales but the dependency of slip dynamics on surface roughness for a large spectrum of heights has never been measured. Here we show specifically how slip dynamics dramatically varies as a function of surface roughness using direct shear experiments performed on laboratory-generated faults in Diabase. We demonstrate experimentally that under relatively low normal stress of 5 MPa, stick-slip oscillations, commonly referred to as laboratory earthquakes, only occur in a very limited range of roughness values within which a specific level, defined here as the critical roughness, triggers the highest amplitude of oscillations. Sliding across roughness higher or lower than critical is typically stable. Using monitored vertical motions through slip (dilation) coupled with numerical modelling we show that sliding on extremely “smooth” surfaces is typically stable because the very small height of asperities does not allow for the nucleation and motion arrest required for ‘stick’ phases to ensue, whereas sliding on extremely rough “fractured” surfaces is typically characterized by shearing through the tall asperities. Sliding across “sawcut” surfaces, however, is found to be particularly susceptible to stick slip deformation, and the shear motion is shown to be purely dilatant where the dilation and compression of the sliding interface are in phase with the stick slip oscillations. We conclude therefore that dilatant shear across moderately rough interfaces is a prerequisite for stick slip oscillations and consequently, for sliding instability.

Abstract We study how the asymmetric geometry of thrust faults affects the dynamics of supershear ruptures and their associated trailing Rayleigh ruptures as they interact with the free surface, and investigate the resulting near-field ground motions. Earthquakes are mimicked by propagating laboratory ruptures along a frictional interface with a 61° dip angle. Using an experimental technique that combines ultrahigh-speed photography with digital image correlation, we produce sequences of full-field evolving measurements of particle displacements and velocities. Our full-field measurement capability allows us to confirm and quantify the asymmetry between the experimental motions of the hanging and footwalls, with larger velocity magnitudes occurring at the hanging wall. Interestingly, because the motion of the hanging wall is generally near-vertical, while that of the footwall is at dip direction shallower than the dip angle of the fault, the horizontal surface velocity components are found to be larger at the footwall than at the hanging wall. The attenuation in surface velocity with distance from the fault trace is generally larger at the hanging wall than at the footwall and it is more pronounced in the vertical component than in the horizontal one. Measurements of the rotations in surface motions confirm experimentally that the interaction of the rupture with the free surface can be interpreted through a torqueing mechanism that leads to reduction in normal stress near the free surface for thrust earthquakes. Nondimensional analysis shows that the experimental measurements are consistent with larger-scale numerical simulations as well as field observations from thrust earthquakes.

Abstract Fault damage zones dominate the mechanical, hydraulic and seismological properties of faults yet the relative contributions from processes leading to their development and growth is obscure. In this study, we investigate the damage development related to slip on rough faults and passage of earthquake ruptures. We compare the cumulative damage with slip and damage distribution from numerical models against field data from exhumed faults with slip less than 3.5 m within the Atacama Fault Zone in northern Chile. Models are constrained by experimentally determined mechanical properties of the host rock. We perform simulations of damage accumulation during quasistatic slip on rough faults and during sequences of earthquakes on planar and rough faults governed by rate and state friction laws. Both sets of simulations include Drucker–Prager rheology of the bulk to identify off-fault damage where the yield stress is exceeded. Our results indicate that the extent and distribution of damage depend on the characteristics of fault roughness, amount of slip, and, when present, the intensity and variability of dynamic ruptures. When typical values for fault roughness are used, the scaling of damage zone width versus slip during quasistatic slip is comparable to that observed in the field data. Earthquake rupture on smooth faults by itself does not explain the field data. Simulations of earthquake sequences on rough faults leads to significantly larger damage zone widths with slip than that observed in the field data, suggesting the development of damage for small displacement is dominated by quasistatic slip on rough faults.