Scale-dependent wear and re-roughening processes in a single fault zone

Abstract

One factor that exerts a major control on earthquake source parameters is the geometry of a fault zone. Observations previously compiled from multiple faults show that fault surface shape evolves with displacement. 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. The geometry of cross sectional traces of contacts of different relative age was analyzed with power spectral density and variogram calculation. 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 and localization of strain onto thinner layers. 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. Continuous wear from processes such as grain plucking and sliced asperities also smoothed surfaces at scales larger than the clast dimension, and also re-roughened surfaces at scales smaller than the clast dimension. Spatially discontinuous wear due to complexities within the fault zone results in periodic fault traces reflecting a characteristic wavelength for the contact. Additionally, periodic variations in fault thickness define the characteristic asperity size and are a signature of wear on the fault surface. Scale dependent smoothing and re-roughening and the interplay between these processes explain the varying geometry of immature and mature faults. The evolution of roughness and asperity size we observe can explain differences in the source characteristics of earthquakes as faults mature.