Friction experiments, where the applied shear force required to produce slip between normally planar rock surfaces, are designed to simulate displacement observed on natural faults. One difficulty that arises in the interpretation of experimental results is the problem of scale: the size of the samples is measured in centimeters, whereas natural faults are many kilometers in length and depth; and the disparity in the scales of time that are characteristic of natural and laboratory processes is as great.
Walsh is studying ways to bridge the gap caused by the disparity in time and length scales by developing theoretical models which simulate behavior observed experimentally. He is now studying slip between rough elastic surfaces where slip, opposed by Coulomb friction, occurs on the microscale at the contacts between asperities. This analysis, which has immediate application to sliding experiments in the laboratory, is the basis for the analysis of earthquake faulting, where 'roughness' of lab samples is equivalent to heterogeneity in the normal stress and shear stress fields at the fault interface. The study can be modified by introducing more complicated frictional behavior, such as the rate-state model, now being studied in several laboratories.