Measuring the evolution of slip surface roughness with LiDAR

Roughness of faults plays an important role in controlling the resistance of faults to slip, yet all faults do not have the same roughness. A previous study used ground-based LIDAR to analyze 12 fault surfaces from 8 localities in the Western United States, and inferred that small slip faults (slip <1 m) are rougher in the slip parallel direction than large slip faults (slip 10 m - 300 m) [Sagy et al., 2006]. Here we more than double the dataset and expand the types of faults studied in order to test this hypothesis. We have applied the same methods to 13 Italian faults. The new faults are primarily normal faults with intermediate slip distances (slip 1 m - 150 m). The data includes strike-slip faults exhumed from depth, a fault type not included in the previous study. The new data set also controls for lithology by focusing on carbonate rocks. We measure roughness using mean power spectra of the topography in both the slip perpendicular and slip parallel directions. The roughness in the slip perpendicular direction is identical to the Western US faults. Slip perpendicular roughness is insensitive to slip and rock type. The slip parallel direction is smoother for all faults than the slip perpendicular direction. Preliminary results suggest that roughness is intermediate between the small slip (<1 m) and large slip (>10 m) faults studied previously. Roughness is also sensitive to time of exposure as revealed by a fault surface that has been progressively exposed during roadwork. The large-scale data analysis is being enabled by Slugview, a program written by the UC Santa Cruz Seismology Group. Slugview is a 3D point cloud visualizer used to manipulate raw data. The beta version of SlugView 2.0 is available to anyone interested, and can be downloaded at www.pmc.ucsc.edu/~msteffec/SlugView/.