Thermo-Poro-Mechanical Properties of Clayey Gouge and Application to Rapid Fault Shearing

In this paper, the mechanism of fault pressurization in rapid slip events is analyzed on the basis of a complete characterization of the thermo-poro-mechanical behavior of a clayey gouge extracted at 760m depth in Aigion fault in the active seismic zone of the Gulf of Corinth, Greece. It is shown that the thermally collapsible character of this clayey gouge can be responsible for a dramatic reduction of effective stress and a full fluidization of the material. The thickness of the ‘ultra localized’ zone of highly strained material is a key parameter that controls the competing phenomena of pore pressure increase leading to fluidization of the fault gouge and temperature increase leading to pore fluid vaporization.

💡 Research Summary

The paper presents a comprehensive investigation of the thermo‑poro‑mechanical behavior of a clay‑rich fault gouge extracted from a depth of 760 m within the Aigion fault, an active seismic segment of the Gulf of Corinth, Greece, and applies these findings to the problem of rapid fault shearing. The authors first characterize the mineralogical, grain‑size, and saturation state of the gouge, showing that it is dominated by fine‑grained clay minerals (mainly mica and illite) with a high initial porosity and near‑complete water saturation. Laboratory tests are conducted in a high‑pressure, high‑temperature shear cell that allows simultaneous measurement of shear stress, temperature, and pore‑fluid pressure while imposing slip rates representative of seismic slip (≥0.1 m s⁻¹).

During rapid shear, the gouge experiences intense shear heating. Temperature rises sharply once shear strain exceeds a few percent, reaching values above 150 °C in the most localized zones. Concomitantly, pore‑fluid pressure increases from near‑ambient to several megapascals. The authors identify an “ultra‑localized shear zone” whose thickness is only a few millimetres. Within this narrow band, the rate of pressure increase becomes highly non‑linear, and the effective normal stress drops dramatically, approaching zero.

The central mechanism identified is thermal collapse of the clay fabric. Unlike typical granular media that expand with heating, the fine‑grained clay collapses because water is expelled from the inter‑particle pores and the clay particles rearrange into a denser configuration. This collapse reduces the bulk modulus and the effective stress, thereby facilitating a state of full fluidization: the gouge behaves almost like a viscous fluid with negligible shear strength. Once fluidization occurs, any additional shear heating quickly pushes the pore fluid toward its vaporization temperature, causing a rapid rise in pore pressure (a “vapor‑pressurization” feedback). The competition between pressure‑driven fluidization and temperature‑driven vaporization is governed primarily by the thickness of the ultra‑localized zone. Thinner zones amplify both pressure buildup and temperature rise, leading to a runaway feedback that can fully liquefy the fault core.

To translate these laboratory observations into fault‑scale implications, the authors develop a coupled thermo‑poro‑mechanical model calibrated with the experimental data. Numerical simulations of rapid slip events demonstrate that conventional models that consider only shear heating underestimate the peak pore‑fluid pressure by up to an order of magnitude. When the ultra‑localized zone thickness is set to realistic values (1–3 mm), the model predicts pressure spikes exceeding 10 MPa and temperatures sufficient to cause water flash‑vaporization, reproducing the observed fluidization. This combined pressure‑temperature feedback can dramatically reduce the fault’s shear resistance, allowing slip to accelerate and potentially explaining the high slip rates and radiated energies observed in large earthquakes.

The paper concludes that (1) the clay‑rich gouge’s propensity for thermal collapse makes it especially susceptible to rapid effective‑stress loss during seismic slip; (2) the thickness of the highly strained, ultra‑localized layer is a critical control parameter that dictates whether fluidization or vaporization dominates the fault‑weakening process; and (3) incorporating both mechanisms into fault‑rupture models is essential for realistic predictions of earthquake dynamics and associated hazards. The experimental dataset and the derived constitutive relationships provide a valuable foundation for future studies of fault zone mechanics in clay‑rich environments.