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.
Highlights
Earthquakes are the result of a sudden release of energy during rapid slip (> 1 m s−1) along geologic fault zones in the Earth’s crust or upper mantle, which generates seismic waves that can be highly destructive at the Earth’s surface
Because the dynamics of the CNS model are different from rate-and-state friction (RSF), CNS-based numerical simulations of fault slip may lead to new insights into the physics of fault deformation
We reviewed experimental and microphysical modelling work on the physics of low-velocity fault friction processes carried out at Utrecht University (UU) since the early 2000s
Summary
Earthquakes are the result of a sudden release of energy during rapid slip (> 1 m s−1) along geologic fault zones in the Earth’s crust or upper mantle, which generates seismic waves that can be highly destructive at the Earth’s surface. It is of utmost importance to improve prognoses on the frequency, location, and magnitude of future seismic events This demands sophisticated modelling of earthquake nucleation and dynamic rupture propagation, which in turn requires a fundamental understanding of fault sliding, or the internal fault rock shearing mechanisms that are active under in situ conditions in the Earth. We go on to present key results from experiments on simulated faults composed of halite– phyllosilicate and phyllosilicate–quartz mixtures, as well as of calcite Data from these experiments consistently suggest that low velocity frictional deformation of fault gouge is controlled by competition between rate-sensitive (diffusional or crystal–plastic) deformation and rate-insensitive sliding interactions (dilatant granular flow) – competition which was already suggested on the basis of theoretical considerations by Rutter and Mainprice (1979). We outline the principles of this model and present some applications and implications for reproducing laboratory data and numerical simulations of earthquake nucleation and the full earthquake cycle
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