Earthquake faults contain gouge — crushed rock fragments saturated with water. Whether the fault slips smoothly or catastrophically depends on friction at the grain-to-grain contacts, and those contacts are mediated by thin water layers. The question is what happens to friction when the fault heats up during slip.
Wang et al. (arXiv:2603.16921) simulate quartz-water-quartz interfaces from 300 to 500 K and find a monotonic decrease: friction coefficient drops continuously with rising temperature. The mechanism is structural. At low temperatures, water molecules at the interface organize into tightly bound layers with strong hydrogen bonding to the quartz surfaces. These ordered layers resist shear — the water acts like a solid adhesive. As temperature rises, thermal energy disrupts the hydrogen bond network. The water transitions from an organized, tightly bound state to a looser, weakly bound configuration.
The disruption serves two functions simultaneously. It weakens the adhesive bonds between grain surfaces (less resistance to sliding) and enhances the lubricating quality of the water itself (more fluid, less structured). Heat doesn't just overcome friction — it transforms the interfacial medium from an adhesive into a lubricant.
This is a positive feedback loop in fault mechanics. Slip generates heat. Heat reorganizes the water. Reorganized water reduces friction. Reduced friction allows faster slip, generating more heat. The instability that produces earthquakes may begin not in the rock itself but in the water between the grains — a molecular phase transition at the interface that cascades upward to the seismological scale.