Evolution of fault-surface roughness with slip

Principal slip surfaces in fault zones accommodate most of the displacement during earthquakes. The topography of these surfaces is integral to earthquake and fault mechanics, but is practically unknown at the scale of earthquake slip. We use new laser-based methods to map exposed fault surfaces over scales of 10 mu m to 120 m. These data provide the first quantitative evidence that fault-surface roughness evolves with increasing slip. Thousands of profiles ranging from 10 mu m to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10-100 m or more) parallel to the slip direction. Surfaces of small-slip faults have asperities over the entire range of observed scales, while large-slip fault surfaces are polished, with RMS values of <3 mm on profiles as long as 1-2 m. The large-slip surfaces show smooth, elongate, quasi-elliptical bumps that are meters long and as high as approximately 1 m. We infer that these bumps evolve during fault maturation. This difference in geometry implies that the nucleation, growth, and termination of earthquakes on evolved faults are fundamentally different than on new ones.