Fig. 4: Schematic diagram showing the weakening mechanisms operating during fault sliding in the presence of water in liquid and supercritical states.

a At the beginning of the experiment, the slip zone includes hot, pressurized water and two fault surfaces have microscopic asperities. The viscosity of the fluids (η), the height of the asperities (H) and the characteristic length for pressure changes (L) are the parameters of the Sommerfeld number. b During sliding, the rubbing of the surfaces produces rock powders; water-rock interaction results in the formation of clay minerals and phyllosilicates (Fig. 2d). This assemblage of slip zone traps water which has a vibrational mode (Fig. 2e) typical of H–O–H bending. This bending may result in a weakening of the intermolecular hydrogen–bonding O∙∙∙H. The water in liquid and supercritical states mixed with rock powders exhibits a significant increase in viscosity, potentially facilitating the activation of EHD mixed lubrication (Fig. 3).