THE EVOLUTION OF THE PRINCIPAL SLIP SURFACE AND THE FAULT ZONE CYCLE

Geometry and roughness of fault surfaces play central roles in faulting dynamics and kinematics. Faults are expected to smooth with increasing slip, but the degree of smoothing is not well-constrained for natural faults. Earthquake rupture modeling commonly uses the geometry (often assumed to be planar) and rheology of the fault interface as given conditions. Here I combine ground-based LiDAR data of exposed slip surfaces with observations of fault zone architecture to suggest that these two aspects of the fault are intimately linked and evolve with increasing offset.

For faults with small total offsets, the roughness at the scale of centimeters to meters is approximately self-similar with a ratio of asperity height to length ~0.01. As offset accumulates, faults gradually smooth in the slip parallel direction while remaining rough in the slip perpendicular direction. For instance, for slip parallel segments of length 0.5 m, H≈2x10^-3 D^-0.1 where H is the RMS roughness and D is the cumulative displacement with both quantities measured in meters. This is a weaker function than the linear decrease in H with D previously suggested based on theoretical and laboratory work.

The relatively gradual evolution of roughness supports a scenario where the accumulated gouge helps to partially mitigate the abrasional smoothing with increasing slip through re-roughening processes including: (1) evolution of splays in the weak material as the original fault surface heals and strengthens and (2) distortion of deformable layers abutting the principal slip surface. Both re-roughening processes are supported by observations of fault zone architecture. For example, at the Flower Pit Fault, OR, the correlation between fault gouge thickness and slip surface topography is consistent with lensing of gouge distorting the slip surface.

The coupling between gouge deformation and localized slip results in the following picture of the fault zone cycle: Slip at an interface abrades material generating gouge. The strained gouge deforms into thick and thin areas. Future slip localizes at the gouge edge due to the competence contrast. Since the gouge edge follows the variable thickness of the layers, the resultant slip surface is wavy. This dynamically generated fault surface topography then governs stress accumulation and the locus of future slip.