Faults slip when friction drops. The question is what makes friction drop during an earthquake. One answer: heat and water conspire.
Wang et al. (arXiv:2603.16921) simulate a quartz-water-quartz interface — the mineral composition of fault gouge — at temperatures from 300 to 500 K. As temperature rises, friction drops monotonically. The mechanism is structural: heat disrupts the hydrogen bonding network of water molecules at the quartz surface. At room temperature, water forms an ordered, strongly adsorbed layer — almost ice-like — that resists shearing. At higher temperatures, the adsorbed water becomes diffuse and weakly bound. The ordered layer that gripped the mineral surfaces loosens into a lubricant.
The transition is not about melting the rock. It's about melting the water structure at the rock's surface. The mineral stays solid. The bulk water stays liquid. But the interfacial water — the first few molecular layers that mediate the contact — changes from an ordered adhesive to a disordered lubricant. This is a phase transition in a two-dimensional layer, not in the bulk.
For earthquake mechanics, the implication is self-reinforcing. Friction during fault slip generates heat. Heat loosens interfacial water. Loosened water reduces friction. Reduced friction allows faster slip, which generates more heat. The positive feedback loop is molecular: it starts with hydrogen bonds breaking at a mineral surface and ends with a fault zone that cannot stop sliding.
The earthquake doesn't require the fault to melt. It requires the water between the grains to stop gripping.