Heating and weakening of faults during earthquake slip

TLDR

Field observations indicate that crustal fault slip occurs in a thin (<1–5 mm) shear zone, and that thermal processes—thermal pressurization of pore fluid, flash heating at microcontacts, and possibly macroscopic melting or gel formation—are likely the main weakening mechanisms during large earthquakes. The study aims to test whether the shear fracture energies implied by these thermal weakening mechanisms match those observed in real earthquake ruptures. The authors model thermal pressurization and flash heating using laboratory‑derived hydrologic, poroelastic, and friction properties, and compile seismic fracture‑energy data to compare with the model predictions. The models predict that strength loss is nearly complete at large slip and that melting is unlikely over most of the seismogenic zone, and the predicted fracture energies agree with observations within uncertainty, supporting the dominance of thermal weakening.

Abstract

Field observations of mature crustal faults suggest that slip in individual events occurs primarily within a thin shear zone, &lt;1–5 mm, within a finely granulated, ultracataclastic fault core. Relevant weakening processes in large crustal events are therefore suggested to be thermal, and to involve the following: (1) thermal pressurization of pore fluid within and adjacent to the deforming fault core, which reduces the effective normal stress and hence also the shear strength for a given friction coefficient and (2) flash heating at highly stressed frictional microcontacts during rapid slip, which reduces the friction coefficient. (Macroscopic melting, or possibly gel formation in silica‐rich lithologies, may become important too at large enough slip.) Theoretical modeling of mechanisms 1 and 2 is constrained with lab‐determined hydrologic and poroelastic properties of fault core materials and lab friction studies at high slip rates. Predictions are that strength drop should often be nearly complete at large slip and that the onset of melting should be precluded over much (and, for small enough slip, all) of the seismogenic zone. A testable prediction is of the shear fracture energies that would be implied if actual earthquake ruptures were controlled by those thermal mechanisms. Seismic data have been compiled on the fracture energy of crustal events, including its variation with slip in an event. It is plausibly described by theoretical predictions based on the above mechanisms, within a considerable range of uncertainty of parameter choices, thus allowing the possibility that such thermal weakening prevails in the Earth.