The roles of time and displacement in the evolution effect in rock friction

TLDR

Room‑temperature friction experiments on quartz‑feldspathic rocks reveal a velocity‑dependent strength with two opposing effects, one of which has a negative velocity dependence that evolves over a characteristic displacement and has been modeled either by a displacement‑based slip law or a time‑dependent slowness law, but existing empirical laws may not capture both steady‑state and non‑steady‑state behavior. The study aims to identify a more accurate time‑dependent evolution law that could improve seismic‑cycle friction models by reducing inter‑seismic slip. The authors measure the time‑dependent strengthening coefficient b in slide‑hold‑slide tests to evaluate the slowness law. The results demonstrate that the evolution effect depends on time rather than slip, as revealed by slide‑hold‑slide tests performed at varying machine stiffnesses.

Abstract

Room temperature friction experiments on quartzo‐feldspathic rocks obey a velocity dependence of strength which consists of two opposite‐sensed effects. The second of these effects has a negative velocity dependence and evolves over a characteristic displacement. This evolution effect was originally attributed by Dieterich [1978; 1979] to an underlying time‐dependent process but is often described by either of two empirical evolution laws. One depends explicitly on displacement (slip law) and the other retains time dependence (slowness law). The slip law is favored in representing behavior around steady‐state as seen in velocity stepping experiments. However, in this study slide‐hold‐slide tests conducted at different machine stiffnesses show that the evolution effect depends on time, not slip. For the slowness law the coefficient of time‐dependent strengthening b is measured directly in slide‐hold‐slide tests. Existing empirical evolution laws may not be sufficient to describe both near steady‐state and non steady‐state behavior. Provided a more correct form can be found, time‐dependent evolution may improve frictional models of the seismic cycle by reducing the amount of inter‐seismic slip.