Wear and Re-roughening Processes in a Single Fault Zone Characterized by Structure from Motion

Abstract:

The geometry of a fault zone exerts a major control on earthquake source parameters. Observations previously compiled from multiple faults show that fault surface shape evolves with displacement but the specific processes driving fault evolution within a single fault zone and their influence on fault geometry are not well known. Here, we characterize the deformation history and geometry of an extraordinarily well-exposed fault using maps constructed with the Structure from Motion photogrammetric method. Crosscutting relations between multiple gouge-filled slip zones within the fault core define the evolution of the structures. The last slip zone to form in the fault is smoothest of any surface and is distinct from other surfaces, recording significant smoothing on a single structure. We infer that smoothing occurred abruptly due to localization of slip onto a new slip zone rather than as a result of progressive wear of a fault surface. However, when cross-sectional traces of contacts between different slip zones were partitioned based on the age of the slip event that last directly affected the contact, the evolution is scale dependent. At wavelengths less than 10 centimeters, the oldest layers are the smoothest indicating that re-roughening processes occur at short wavelengths during later slip events. Our observations show that scale-dependent wear is caused by processes such as grain plucking and asperity failure that re-roughened surfaces at scales smaller than the clast dimension and smoothed surfaces at scales larger than the clast dimension. The resulting clast aspect ratios record scale-dependent wear as longer clasts are more elongate. Furthermore, clasts do not exist above one meter in length, suggesting a transition in the rheology or processes affecting the surface. Together, these results demonstrate that net fault wear from immature and mature faults is the result of competing smoothing and re-roughening processes. The evolution of roughness and asperity size we observe can explain differences in the source characteristics of earthquakes as faults mature.