Transient and Steady‐State Friction in Non‐Isobaric Conditions

Abstract

The frictional properties of faults control the initiation and propagation of earthquakes and the associated hazards. Although the ambient temperature and instantaneous slip velocity controls on friction in isobaric conditions are increasingly well understood, the role of normal stress on steady‐state and transient frictional behaviors remains elusive. The friction coefficient of rocks exhibits a strong dependence on normal stress at typical crustal depths. Furthermore, rapid changes in normal stress cause a direct effect on friction followed by an evolutionary response. Here, we derive a constitutive friction law that consistently explains the yield strength of rocks from atmospheric pressure to gigapascals while capturing the transient behavior following perturbations in normal stress. The model explains the frictional strength of a variety of sedimentary, metamorphic, and igneous rocks and the slip‐dependent response upon normal stress steps of Westerly granite bare contact and synthetic gouges made of quartz and a mixture of quartz and smectite. The nonlinear normal stress dependence of the frictional resistance may originate from the distribution of asperities that control the real area of contact. The direct and transient effects may be important for induced seismicity by hydraulic fracturing or for naturally occurring normal stress perturbations within fault zones in the brittle crust.

Velocity and Temperature Dependence of Steady‐State Friction of Natural Gouge Controlled by Competing Healing Mechanisms

“…Furthermore, phyllosilicates typically exhibit a strong increase of the effective friction coefficient with normal stress (Moore & Lockner, 2008). Generalizing the constitutive equations to incorporate the effect of normal stress (Barbot, 2024) explains this behavior (Figure S8 in Supporting Information S1).…”

“…Capturing the brittle-to-flow transition requires a constitutive model dependent on multiple deformation mechanisms (Barbot, 2023;Barbot & Zhang, 2023). Given the isobaric experimental conditions considered in this study, we also ignore the nonlinear direct and evolutionary effects of normal stress (Barbot, 2024).…”