Energy Dissipation During Shear Along Experimental Rough Faults.

The energy budget and the interplay between stable friction evolution and dynamic stick‐slips are tested here under continuous slip along rough faults. We conducted 34 direct‐shear experiments coupled with precise roughness measurements on diabase and limestone fault samples. The faults broad roughness ranges from highly rough and interlocked fractured interfaces to smooth polished surfaces. The analysis focuses on two slip phases: (a) the evolution of the shear strength of rough sample under stable, cumulative displacement; and (b) the dynamic of unstable stick‐slip sliding. We found that the breakdown work during frictional strength evolution increases with roughness increase across multiple scales. The diabase samples are more sensitive to roughness increase than limestone samples in terms of the breakdown work implied by frictional evolution. We attribute this increased diabase sensitivity with fault roughness to its higher bulk elasticity and not to the fault shear stiffness. The diabase faults displayed multiple periodic system‐size stick‐slips, and the measured stored energy during the preparatory stage were surprisingly independent of the fault roughness. This finding suggests that during the preparatory stage a balance between the intracycle fault stiffness and stress drop govern the stored energy magnitude. Further, this energy balance suggests that some interface conditioning occurs before the spontaneous slip overcomes a sticking barrier. Plain Language Summary: The breakdown energy dissipated as work during friction evolution along laboratory faults can be determined by integration of the shear stress versus shear displacement. In this study, we quantitatively analyze the relationship between the roughness of laboratory faults and the dissipated energy during sliding. The experiments with rough diabase and limestone faults show an increase of breakdown work implied by the frictional strength with increasing fault roughness, and breakdown work increase with increasing host rock rigidity. We also find that the energy dissipated during frictional evolution of rough faults is four orders of magnitude larger than the energy dissipated during system size stick‐slip cycles in the same experimental setting. We attribute this observed discrepancy to the difference in contact area between interlocked, and sheared, surfaces. Surprisingly, our examination of breakdown work during stick‐slip cycles reveals a roughness‐independent behavior. We ultimately show that the energy components of both stick‐slip and friction evolution follow the same power law relationship when scaled to the sliding displacement. Key Points: The breakdown work implied by the evolution of frictional strength scales with roughness (RMS) height and elasticity of intact rockThe stored energy within stick‐slip cycles is four orders of magnitude lower than the energy accommodated during friction evolutionThe stored energy at the onset of breakdown during stick‐slip cycles is generally independent of fault roughness [ABSTRACT FROM AUTHOR]