Groove Generation and Coalescence on a Large‐Scale Laboratory Fault

Abstract

Faults are the products of wear processes acting at a range of scales from nanometers to kilometers. Grooves produced by wear are a first‐order observable feature of preserved surfaces. However, their interpretation is limited by the complex geological histories of natural faults. Here we explore wear processes on faults by forensically examining a large‐scale controlled, laboratory fault which has a maximum offset between the sides of 42 mm and has been reset multiple times for a cumulative slip of approximately 140 mm. We find that on both sides of the fault scratches are formed with lengths that are longer than the maximum offset but less than the cumulative slip. The grooves are explained as a result of interaction with detached gouge rather than as toolmarks produced by an intact protrusion on one side of the fault. The density of grooves increases with normal stress. The experiment has a range of stress of 1–20 MPa and shows a density of 10 grooves/m/MPa in this range. This value is consistent with recent inferences of stress‐dependent earthquake fracture energy of 0.2 J/m 2 /MPa. At normal stresses above 20 MPa, the grooves are likely to coalesce into a corrugated surface that more closely resembles mature faults. Groove density therefore appears to be an attractive target for field studies aiming to determine the distribution of normal stress on faults. At low stresses the groove spacing can be measured and contrasted with areas where high stresses produce a corrugated surface. The surfaces of faults have grooves that hold information about the fault's mechanics and history. Interpreting these grooves on a complex fault needs to be guided by models of simpler systems; however, such models must also be sufficiently large to capture the multiscale processes carving the fault surface. Here we perform experiments on a 3‐m‐long artificial fault in a laboratory setting and find that the fault surface develops roughness during slip. First, small particles break off in between the surfaces and excavate grooves on each side. These grooves are created by detached particles rather than protrusions attached to one side or the other. As the normal stress between the fault sides increases, so does the groove density. In a region of the laboratory fault where the normal stress is high, the grooves coalesce to form a corrugated surface that appears more like a natural fault than the other experimentally created surfaces. The work both gives insight into roughness formation and suggests a strategy for future field work that could use roughness to map normal stress variations. Grooves on a laboratory fault are generated by detached particles and gouge clumps (third‐body wear) Groove density increases systematically with normal stress in the experiment At stresses higher than 20 MPa, grooves coalesce into a qualitatively different, corrugated morphology