Effects of heterogeneous gouge segments on the slip behavior of experimental faults at dm scale

Abstract

Frictional heterogeneity within fault zones is one of the factors proposed to explain the spectrum of slow, intermediate, and fast slip behaviors exhibited by faults in nature. Numerical modeling shows how even a simplified model setup incorporating sliding on a velocity-weakening (VW) patch surrounded by velocity-strengthening (VS) material can reproduce a rich variety of slip behaviors resembling nature. However, experimental investigations of sliding on heterogeneous faults are few. In this study, the slip behavior of three, 347 mm long × 50 mm wide, heterogeneous experimental faults, constructed using diagonally sawcut PMMA forcing blocks, was investigated at low normal stresses (<10.5 MPa) and room temperature. Fault friction was controlled by including an 80 mm long, 2 mm thick, central segment of VW gypsum “gouge” flanked by two VS segments composed of calcite, quartz, or kaolinite. The length of the VW segment was of the same order or just below the critical nucleation length for gypsum gouge. Strain gauges and multi-rate digital imaging were used to map stress and displacement along the fault zone. At the highest normal stress the data showed confined ruptures, whereby rupture nucleated in the VW gypsum and was arrested or strongly decelerated in the VS segments. Slip rates on the VW segment were close to dynamic slip rates, but significant slow slip was also observed in the VW segment between rapid events. Lowering the normal stress on the fault, from 10.5 to 1 MPa resulted in slow slip events occurring uniformly over the whole fault (calcite–gypsum fault), near-stable sliding on the whole fault (quartz–gypsum fault), or persistent stick-slip behavior on the VW segment as at higher normal stress (kaolinite–gypsum fault). The spectrum of behaviors observed is consistent with that predicted by previous numerical modeling of frictionally heterogeneous faults with a similar geometry as the experiment. The experiments also showed how stress concentrations influence slip behavior. Specifically, normal stress concentrations, formed due to heterogeneous compaction of the kaolinite and VW gypsum gouges, promoted unstable sliding in the latter. Shear stress concentrations at the extremities of the VW segment caused significant slow slip to occur at the extremities of the VW segments, and in the quartz–gypsum experiment seemed to promote local slip events at low stresses. The observed fault slip behavior was thus controlled by gouge friction and stress distribution, and is consistent with numerical models and theory as applied to both natural and induced seismicity.