The minimum scale of grooving on faults

Abstract:

The roughness of fault surfaces is the fingerprint of past slip events and a major parameter controlling the resistance to slip. The most obvious slip indicator and record of tractions are the grooves and striations with elongate axes in the direction of slip. We focus on this roughness feature by analyzing the micro-roughness of slip surfaces from natural and experimental fault zones at scales of several millimeters down to one micron. For each topographic map acquired by White Light Interferometry, an average Fourier spectrum is computed in the slip parallel and slip perpendicular direction seeking to define the scale dependence of the roughness anisotropy. We show that natural and experimental fault surfaces have a minimum scale of grooving at 4-500 micrometers. Below this scale, fault surfaces are isotropic. We have systematically measured this minimum scale of grooving on 42 topographic maps of eight different natural fault zones and 25 topographic maps of nine experimental fault zones. Our results are interpreted in terms of the aspect ratio H/L with H the average asperity height and L the observation scale. This aspect ratio is proportional to the strain necessary to completely flatten the asperities. H/L systematically increases with the decreasing of L. The transition between anisotropic and isotropic is well predicted by a critical aspect ratio. With the scale of observation decreasing the grooves become steeper and once they reach a critical aspect ratio they fail. At all scales, evidence of failure of the slip surfaces are observed and we interpret the minimum scale of grooving as a manifestation of the change in deformation mode from brittle- to plastic-dominated. As the scale of observation decreases, the aspect ratio of the grooves increases and the resulting higher stress concentrations at micro-asperities favor plasticity. The transition is dependent on the rock properties and faulting history, and for each fault one unique critical aspect ratio (between 0.1-8%) maps the transition scale. This transition in deformation mode will control the asperity distribution and therefore be an important factor in controlling the frictional strength. The observations underline the crucial role that plasticity might play at the micrometer scale in controlling sudden large-scale brittle failures along faults.