Nanometric flow and earthquake instability

Fault zones are complex geological structures exhibiting deformation localized into high- and low-strain domains. The interplay between these domains determines fault slip modes (earthquakes, slow slip, viscous creep). Localization in high-strain domains triggers weakening mechanisms like thermal pressurization and shear heating, potentially leading to earthquake instability. The localized material undergoes significant transformation, including comminution, metamorphic reactions, microstructural changes, phase transitions, and melting, often resulting in nanocrystalline or amorphous materials. Understanding the rheology of these fine-grained fault rocks is crucial for comprehending fault slip, yet remains poorly constrained. Current models rely on extrapolations from microcrystalline materials, which are inherently unreliable due to the distinct properties of surface-dominated nanocrystalline materials. Nanocrystalline fault rocks form compact zones with crystals or aggregates (1-100s of nm) sometimes embedded in an amorphous matrix. Their small grain size promotes diffusional processes even under conditions where cataclastic flow typically dominates. Evidence suggests these rocks are weaker than coarser-grained material and flow at low temperatures. The challenge lies in isolating the intrinsic properties of nanocrystalline rocks from the signal dominated by coarser-grained material in experiments. High-velocity experiments introduce temperature transients, confounding the effects of heating and nanomaterial formation. This study addresses these problems by creating granitoid nanomaterials via high-energy ball milling and testing their rheological properties under controlled conditions mimicking the base of the seismogenic layer.

Previous research highlights the importance of nanocrystalline fault rocks in controlling fault stability and earthquake nucleation. Studies have shown the formation of nanomaterials during fault slip, often through processes like frictional melting and mechanical amorphization. However, the rheological properties of these nanomaterials have been poorly understood, with current models largely extrapolating from the behavior of microcrystalline materials. Existing experimental work faces challenges in isolating the intrinsic properties of nanocrystalline fault rocks from the mechanical signal of coarser-grained material and temperature transients during high-velocity experiments.

Granitoid nanomaterials were produced by high-energy ball milling of crushed granitoid gouge. The resulting material, characterized by thorough mineral mixing and a uniform chemical composition with a median grain size of ~100 nm, was tested using a solid medium deformation apparatus under controlled pressure (P = 500 MPa) and temperature (T = 200, 300, and 500 °C) conditions. Constant-displacement-rate and stress-stepping experiments were performed to determine the strength, apparent viscosity, friction coefficient, stress exponent (n), and activation energy (Q) of the nanocrystalline fault rocks. Microstructural analysis employed high-resolution scanning electron microscopy (SEM), polarized light microscopy, and transmission electron microscopy (TEM) to study the microstructures of sheared fault rocks at different temperatures. Shear heating calculations were performed to assess the potential for temperature-induced weakening. The experimental flow law for the nanocrystalline fault rocks was determined by combining the constant displacement rate experiments with load-stepping experiments. The values for the stress exponent and activation energy were obtained by fitting the experimental data to an Arrhenius-type flow law and using a linear least-square problem.

The strength of granitoid fault rocks is significantly decreased by reduced grain size. Nanocrystalline fault rocks are approximately an order of magnitude weaker than microcrystalline rocks at identical experimental conditions. At lower temperatures (200-300 °C), the nanocrystalline material shows strain hardening, with Riedel shear fractures and kink bands observed in the microstructure. At higher temperatures (500 °C), the material deforms in a stable manner with continuous flow, exhibiting much less fracturing and smeared out domains in the microstructure. The stress exponent (n) was determined to be 1.3 ± 0.4, and the activation energy (Q) was 16,000 ± 14,000 J/mol. The determined flow law indicates that the nanocrystalline fault rocks deform primarily via diffusion creep, even at low temperatures and fast strain rates. The low activation energy is characteristic of nanomaterials due to their high surface-to-volume ratio. The nanocrystalline material exhibited rate-strengthening behavior (n~1.3), not the rate-weakening behavior typically associated with earthquake instability. Shear heating calculations showed that extremely high stresses would be required for significant temperature increases, indicating that shear-heating instability is unlikely to be the cause of weakening in these materials. Optical anisotropy, surprisingly strong and spatially coherent given the nanoscale grain size, likely arises from sub-wavelength grating effects of nanoparticles interacting with light.

The findings challenge the traditional view of earthquake instability based on velocity weakening. The intrinsic low viscosity of nanocrystalline fault rocks, despite their rate-strengthening behavior, provides a plausible weakening mechanism. Fault weakening occurs due to the generation and coalescence of nanocrystalline material during shearing of coarser-grained material. Once a sufficient volume of nanocrystalline material forms a kinematically favorable failure plane, fault displacement can accelerate without significant temperature increase, potentially triggering an earthquake. This mechanism differs from frictional instability but bears resemblance to models for deep earthquakes involving phase transitions, shear-heating induced viscoelastic deformation, and cavitation. The extremely weak, viscous behavior of the nanocrystalline material suggests that shearing instability due to weak inclusions could operate in the crust, particularly around the brittle-viscous transition zone.

This study demonstrates the significant weakening effect of nanocrystalline materials in fault zones. While the nanocrystalline material exhibits rate strengthening, the formation and coalescence of these weak layers into a kinematically favorable network lead to a distinct type of instability that can trigger earthquake nucleation. Future research should focus on investigating the formation mechanisms of nanocrystalline materials in fault zones and their effect on earthquake dynamics across different geological settings and scales.

The experiments were conducted on homogenized, artificially produced nanomaterials. The actual nanocrystalline structures in natural fault zones may be more complex and heterogeneous. The extrapolation of experimental results to natural conditions, especially regarding shear heating and strain rates, involves uncertainties.