LIDAR MEASUREMENT OF FAULT SURFACE TOPOGRAPHY

Fault zones contain discrete slip surfaces that accommodate most of their relative displacement. The amplitude and wavelength of bumps, or asperities, on fault surfaces affect all aspects of fault and earthquake mechanics (e.g., rupture dynamics, nucleation, and termination, shear strength, fault gouge generation, critical slip distance, and the near-fault stress field), but until recently it was difficult to measure and quantify these parameters in natural faults.

We present systematic measurements of several fault surfaces using ground-based LiDAR and a laboratory profilometer. Thousands of fault profiles ranging from 10 micron to >100 m in length show that small-slip faults (slip <1 m) are rougher than large-slip faults (slip 10 to 100 m or more) on profiles parallel to the slip direction. Surfaces of small-slip faults have asperities on all scales, while large-slip faults surfaces are polished, with RMS roughness (average deviation from a planar surface) values of <3mm up to 1-2 m profile length. Surprisingly, the large-slip surfaces show smooth, elongate, semi-elliptical bumps that are meters long and up to several centimeters high. We infer that these bumps evolve during fault maturation. This roughness characteristic implies that the nucleation, growth and termination of earthquakes on evolved faults are fundamentally different than on new ones.