Fault core and slip zone geometry, wear and evolution

Abstract:

The static strength and dynamic shear resistance of seismic faults depend on composition, shape, spatial variability, and distribution of asperities on fault surfaces. To characterize these qualities in a paleoseismic fault, we studied the Boyd Fault, a Laramide thrust exposed in crystalline rocks south of Palm Desert, CA. High-resolution digital elevation models of outcrops were rectified with fault strike and dip and used to map 62 m of exposed fault core in the field at three exposures over 175 m along fault strike. The fault core exhibits stratified layers of fault gouge of varying thicknesses with crosscutting relationships and discontinuous layers. One layer is interpreted as the most recent slip event because it crosscuts all the others and has injection veins branching into both the footwall and hanging wall. Microstructures and the presence of gouge injections indicate fluidization of the gouge throughout the fault slip zone during seismic slip. We compared the geometry of the most recent slip event to the fault core as a whole to constrain how the characteristics of the fault evolve with displacement. We found four “thresholds”: a. the length scales at which the variance of 1. total fault core thickness and 2. the most recent gouge layer thickness do not fluctuate and remain stable, via experimental semivariograms; b. the length scale at which wear is scale dependent, via power spectral density (PSD) calculations from cross sections through the most recent layer and the fault core; and c. the scale at which fault wear transitions from inelastic to elastic, via the maximum length of hanging wall asperity clasts in the fault gouge. By comparing the most recent event with the core as a whole, these results indicate that faults smooth with displacement, but clast compositions provide evidence of preferential wear due to wall rock composition differences. In addition, slip zone thickness decreases and becomes less variable. We suggest the correlation length scale in the slip zone thickness defines the length scale of an asperity on the fault surface. Overall, our results indicate an evolution of the size and distribution of asperities, implying dynamic shear strength and the static strength of the fault also vary with increasing displacement.