Aseismic slip may occur during a long preparatory phase preceding earthquakes, and what controls it remains poorly understood. In this study, we explored the role of load point velocity and surface roughness on slow slip during the preparatory stage prior to stick‐slip events. To that end, we conducted displacement‐rate controlled friction experiments by imposing varying load point velocities on sawcut granite samples with different surface roughness at a confining pressure of 35 MPa. We measured the average slip along the fault with the recorded far‐field displacements and strain changes, while acoustic emission sensors and local strain gages were used to capture local slip variations. We found that the average amount of aseismic slip during the preparatory stage increases with roughness, whereas precursory slip duration decreases with increased load point velocity. These results reveal a complex slip pattern on rough faults which leads to dynamic ruptures at high load point velocities.
N2 - Plain Language Summary: Earthquakes occur mostly along preexisting faults in the earth crust. These faults exhibit various geometrical complexities and are subjected to different strain rates. In the laboratory, we produce earthquake analogs by sliding sawcut granite blocks. We vary the geometrical complexity of the faults by roughening their surfaces and modify the strain rate by displacing the blocks at varying velocities. Under these different conditions, we measure how the forces accumulated by friction are released, by measuring stresses and displacements applied on the block's edges, using local strain deformation sensors, and by recording very small earthquakes occurring during sliding along the sawcut faults. We find that smooth sawcut faults tend to release all the energy accumulated very abruptly, after a very small amount of slip, regardless of the load point velocity applied. The processes leading to failure in the case of a rough fault are much more complex, involving a large amount of slip, and numerous small earthquakes which are distributed heterogeneously in space and time. N2 - Key Points:Acoustic emissions highlight the complex preparatory phase prior stick‐slips on rough faults
Preparatory slip increases with roughness and the duration of the preparatory phase decreases with increasing load point velocity
Rough and heterogeneous faults are more stable than smooth faults, but can become unstable with a small increase of load point velocity