The physics of fault friction: insights from experiments on simulated gouges at low shearing velocities

Abstract

Abstract. The strength properties of fault rocks at shearing rates spanning the transition from crystal–plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in tectonically active regions. We review experimental and microphysical modelling work, which is aimed at elucidating the processes that control the transition from pervasive ductile flow of fault rock to rate-and-state-dependent frictional (RSF) slip and to runaway rupture, carried out at Utrecht University in the past 2 decades or so. We address shear experiments on simulated gouges composed of calcite, halite–phyllosilicate mixtures, and phyllosilicate–quartz mixtures performed under laboratory conditions spanning the brittle–ductile transition. With increasing shear rate (or decreasing temperature), the results consistently show transitions from (1) stable velocity-strengthening (v-strengthening) behaviour, to potentially unstable v-weakening behaviour, and (2) back to v strengthening. Sample microstructures show that the first transition seen at low shear rates and/or high temperatures represents a switch from pervasive, fully ductile deformation to frictional sliding involving dilatant granular flow in localized shear bands where intergranular slip is incompletely accommodated by creep of individual mineral grains. A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive (diffusional or crystal–plastic) deformation of individual grains and rate-insensitive sliding interactions between grains (granular flow), predicts both transitions well. Unlike classical RSF approaches, this model quantitatively reproduces a wide range of (transient) frictional behaviours using input parameters with direct physical meaning, with the latest progress focusing on incorporation of dynamic weakening processes characterizing co-seismic fault rupture. When implemented in numerical codes for crustal fault slip, the model offers a single unified framework for understanding slip patch nucleation and growth to critical (seismogenic) dimensions, as well as for simulating the entire seismic cycle.

Global physics-based database of injection-induced seismicity

“…In this section, we primarily verify the approximate depth of the crystalline basement where the crust is widely accepted to be critically stressed (Townend and Zoback, 2000) and the strength properties of faults render them more susceptible to seismic slip (seismogenic faults, Verberne et al, 2020). These characteristics of the crystalline basement are consistent with observations of the nucleation of the vast majority of large earthquakes in the basement (Horton, 2012;Goebel et al, 2017;Buijze et al, 2019;Williams-Stroud et al, 2020).…”

Semi‐Brittle Deformation of Carrara Marble: Hardening and Twinning Induced Plasticity

“…Furthermore, the semi‐brittle regime may be the source location of slow slip events and tremors (e.g., Fagereng et al., 2014; Reber et al., 2015). In calcite rocks, the slip stability is affected by pressure and temperature dependent processes (Aubry et al., 2020; Carpenter et al., 2016; Passelègue et al., 2019; Verberne et al., 2017; Verberne et al., 2015) and the rate dependence of the empirical rate‐and‐state friction law may be physically explained by the competition of brittle and plastic processes at contact asperities (e.g., Chen et al., 2021; Chen & Spiers, 2016; Verberne et al., 2017; Verberne et al., 2020). It is also conceivable that the state variable is affected by the hardening coefficient that evolves with strain and depends on the P C – T – $$\dot{\varepsilon }$$ conditions in the semi‐brittle regime.…”

Cooling-induced reactivation of distant faults during long-term geothermal energy production in hot sedimentary aquifers

“…( 1 ) in “ Methods ”), so they will be larger in deeper reservoirs of higher temperature and stiffness. Deep formations are also characterized by a higher seismogenic potential, meaning that they are more prone to seismic rupture 16 . The cooled region can reach several hundreds of meters after a few decades of injection 13 , cooling down fractures and faults present in the target formation.…”