Microphysical Model Predictions of Fault Restrengthening Under Room‐Humidity and Hydrothermal Conditions: From Logarithmic to Power‐Law Healing

Abstract

The maximum fault strength and rate of interseismic fault strengthening ("healing") are of great interest to earthquake hazard assessment studies, as they directly relate to event magnitude and recurrence time. Previous laboratory studies have revealed two distinct frictional healing behaviors, referred to as Dieterich-type and non-Dieterich-type healing. These are characterized by, respectively, log-linear and power-law increase in the strength change with time. To date, there is no physical explanation for the frictional behavior of fault gouges that unifies these seemingly inconsistent observations. Using a microphysical friction model previously developed for granular fault gouges, we investigate fault strengthening analytically and numerically under boundary conditions corresponding to laboratory slide-hold-slide tests. We find that both types of healing can be explained by considering the difference in grain contact creep rheology at short and long time scales. Under hydrothermal conditions favorable for pressure solution creep, healing exhibits a power-law evolution with hold time, with an exponent of~1/3, and an "apparent" cutoff time (α) of hundreds of seconds. Under room-humidity conditions, where grain contact deformation exhibits only a weak strain-rate dependence, the predicted healing also exhibits a power-law dependence on hold time, but it can be approximated by a log-linear relation with α of a few seconds. We derive analytical expressions for frictional healing parameters (i.e., healing rate, cutoff time, and maximum healing), of which the predictions are consistent with numerical implementation of the model. Finally, we apply the microphysical model to small fault patches on a natural carbonate fault and interpret the restrengthening during seismic cycles.

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

“…where ) is the instantaneous porosity at the peak strength.The application of this relation here is verified by our steady-state simulation of Figure 5. Those results show that the gouge compacted by ~10% while the friction coefficient increased by 0.17 to attain the peak strength -which is roughly consistent with the estimate ∆> 4<h ≤ 0.199 made from equation (17).…”

“…In a recent study, J. Chen et al. (2020) incorporated granular flow plus a general grain‐scale creep law into the CNS model to predict the steady‐state frictional behavior of dry calcite gouge at low temperatures and slip rates from 0.1 μm/s to 1 mm/s. The V ‐strengthening friction results obtained are similar to the experimental data obtained at low velocities (e.g., Figure 1), supporting the mechanisms that we infer here.…”

“…. This portion of the model has recently been successfully applied to explain the strength profile of a calcite gouge sheared at 550°C and across slip rates spanning almost 6 orders of magnitude(Chen et al, 2020b).…”

Flow‐to‐Friction Transition in Simulated Calcite Gouge: Experiments and Microphysical Modeling

“…Combining all of the above, our interpretation is that shear strain accommodation at v < 0.1 μm/s in our experiments on crushed calcite occurred by a combination of diffusion and dislocation creep (hereafter referred to the flow regime). However, around the critical velocity v cr , shear strain accommodation is characterized by a "brittle" component, as indicated by the large stress exponent (n~87, Figure 7a) for v = 0.03-0.1 μm/s (Figure 4; Brantut et al, 2013;Chen et al, 2020) and by the "friction-like" transient response to a step in v (Figure 2b versus equivalent stress (σ) in the logarithmic scale from the two experiments, which were sheared with downward v-stepping sequence from 0.1 to 0.001 μm. Assuming a general creep law of a power law form ( _ ε ∝ σ n ), the n-value can be obtained using the relation n = dlog _ ε ð Þ=dlog σ ð Þ for all the steps, as indicated by the slopes.…”

Frictional properties of simulated fault gouges from the Groningen gas field and implications for induced seismicity.

“…Considering only porosity reduction via thermally-activated compaction mechanisms such as pressure solution, the CNS model suggests a maximum absolute increase in friction coefficient of the order of 0.3-0.4, depending on the starting porosity (Chen et al, 2020). Cohesion development alongside porosity reduction will increase this limit, but to what values is unknown.…”