Fault-surface geometry controlled by faulting mechanisms. Experimental observations in limestone faults

Surface geometry is an essential component in faulting and earthquake dynamics, yet its evolution and interrelationship with friction are poorly understood. The geometric characteristics of smooth fault surfaces are herein investigated by combining direct shear experiment results with statistical analysis of the surface topography. Ground limestone surfaces, 20 × 20 cm, were sheared for 5 cm under constant normal load of 200 kN. After shear, the surfaces were structurally analyzed and scanned by laser profilometers. Elongated islands of shear zones are observed, characterized by grooves ploughed into the limestone surfaces and by layers of fine grain wear. Two distinctive types of topographic end members are recognized: rough wavy ridged zones that are the surface expression of penetrative cracking and fragmentation, and smooth slip surfaces reflecting localization of shear cataclastic flow and plasticity. The roughness of both end members is geometrically characterized using a power-law relationship between profile length parallel to the slip and its standard deviation from planarity. The power-law exponents are distinctively larger in fractured zones, >0.7, and lower, down to 0.35, in slip surfaces. The topographic variations in the deformed surfaces reflect fracturing and ductile processes, which occur simultaneously under shear. Their relative importance controls fault roughness evolution. We show that geometrical analysis can quantitatively distinguish between products of these different mechanisms.