Does the direct effect of friction increase continuously with absolute temperature?

Constitutive models of fault friction form the basis of physics-based simulations of seismic activity. A generally accepted framework for the slip-rate and state dependence of friction involves a thermally activated process, whereby the probability of slip along microasperities adheres to an Arrhenius law. This model, which has become widely adopted among experimentalists and theoreticians, predicts a continuous increase of the direct effect with absolute temperature, but is it observed experimentally? Leveraging comprehensive laboratory data across diverse hydrothermal, barometric, and lithological conditions, we demonstrate that, contrary to the classical view, the direct effect for a given deformation mechanism remains largely temperature-independent. Instead, the incremental shifts in the direct effect often coincide with the brittle to semi-brittle transition, across which distinct deformation mechanisms operate. These considerations challenge the validity of the classical model. Realistic constitutive laws for rock failure within the lithosphere must incorporate the contributions of multiple deformation mechanisms within active fault zones.

Microphysical Modeling of Carbonate Fault Friction at Slip Rates Spanning the Full Seismic Cycle

Laboratory studies suggest that seismogenic rupture on faults in carbonate terrains can be explained by a transition from high friction, at low sliding velocities (V), to low friction due to rapid dynamic weakening as seismic slip velocities are approached. However, consensus on the controlling physical processes is lacking. We previously proposed a microphysically based model (the "Chen-Niemeijer-Spiers" [CNS] model) that accounts for the (rate-and-state) frictional behavior of carbonate fault gouges seen at low velocities characteristic of rupture nucleation. In the present study, we extend the CNS model to high velocities (1 mm/s ≤ V ≤ 10 m/s) by introducing multiple grain-scale deformation mechanisms activated by frictional heating. As velocity and hence temperature increase, the model predicts a continuous transition in dominant deformation mechanisms, from frictional granular flow with partial accommodation by plasticity at low velocities and temperatures, to grain boundary sliding with increasing accommodation by solid-state diffusion at high velocities and temperatures. Assuming that slip occurs in a localized shear band, within which grain size decreases with increasing velocity, the model results capture the main mechanical trends seen in high-velocity friction experiments on room-dry calcite-rich rocks, including steady-state and transient aspects, with reasonable quantitative agreement and without the need to invoke thermal decomposition or fluid pressurization effects. The extended CNS model covers the full spectrum of slip velocities from earthquake nucleation to seismic slip rates. Since it is based on realistic fault structure, measurable microstructural state variables, and established deformation mechanisms, it may offer an improved basis for extrapolating lab-derived friction data to natural fault conditions.

Propagation of large earthquakes as self-healing pulses or mild cracks

Observations suggest that mature faults host large earthquakes at much lower levels of stress than their expected static strength^1-11. Potential explanations are that the faults are quasi-statically strong but experience considerable weakening during earthquakes, or that the faults are persistently weak, for example, because of fluid overpressure. Here we use numerical modelling to examine these competing theories for simulated earthquake ruptures that satisfy the well known observations of 1-10 megapascal stress drops and limited heat production. In that regime, quasi-statically strong but dynamically weak faults mainly host relatively sharp, self-healing pulse-like ruptures, with only a small portion of the fault slipping at a given time, whereas persistently weak faults host milder ruptures with more spread-out slip, which are called crack-like ruptures. We find that the sharper self-healing pulses, which exhibit larger dynamic stress changes compared to their static stress changes, result in much larger radiated energy than that inferred teleseismically for megathrust events¹². By contrast, milder crack-like ruptures on persistently weak faults, which produce comparable static and dynamic stress changes, are consistent with the seismological observations. The larger radiated energy of self-healing pulses is similar to the limited regional inferences available for crustal strike-slip faults. Our findings suggest that either large earthquakes rarely propagate as self-healing pulses, with potential differences between tectonic settings, or their radiated energy is substantially underestimated, raising questions about earthquake physics and the expected shaking from large earthquakes.

Dynamic weakening during earthquakes controlled by fluid thermodynamics

Earthquakes result from weakening of faults (transient decrease in friction) during co-seismic slip. Dry faults weaken due to degradation of fault asperities by frictional heating (e.g. flash heating). In the presence of fluids, theoretical models predict faults to weaken by thermal pressurization of fault fluid. However, experimental evidence of rock/fluid interactions during dynamic rupture under realistic stress conditions remains poorly documented. Here we demonstrate that the relative contribution of thermal pressurization and flash heating to fault weakening depends on fluid thermodynamic properties. Our dynamic records of laboratory earthquakes demonstrate that flash heating drives strength loss under dry and low (1 MPa) fluid pressure conditions. Conversely, flash heating is inhibited at high fluid pressure (25 MPa) because water's liquid-supercritical phase transition buffers frictional heat. Our results are supported by flash-heating theory modified for pressurized fluids and by numerical modelling of thermal pressurization. The heat buffer effect has maximum efficiency at mid-crustal depths (~2-5 km), where many anthropogenic earthquakes nucleate.

Slow-slip events in semi-brittle serpentinite fault zones

Slow-slip events are earthquake-like events only with much lower slip rates. While peak coseismic velocities can reach tens of meters per second, slow-slip is on the order of 10^−7±2 m/s and may last for days to weeks. Under the rate-and-state model of fault friction, slow-slip is produced only when the asperity size is commensurate with the critical nucleation size, a function of frictional properties. However, it is unlikely that all subduction zones embody the same frictional properties. In addition to friction, plastic flow of antigorite-rich serpentinite may significantly influence the dynamics of fault slip near the mantle wedge corner. Here, we show that the range of frictional parameters that generate slow slip is widened in the presence of a serpentinized layer along the subduction plate interface. We observe increased stability and damping of fast ruptures in a semi-brittle fault zone governed by both brittle and viscoelastic constitutive response. The rate of viscous serpentinite flow, governed by dislocation creep, is enhanced by high ambient temperatures. When effective viscosity is taken to be dynamic, long-term slow slip events spontaneously emerge. Integration of rheology, thermal effects, and other microphysical processes with rate-and-state friction may yield further insight into the phenomenology of slow slip.