Flash Heating and Local Fluid Pressurization Lead to Rapid Weakening in Water-Saturated Fault Gouges

Coseismic fault displacement is quite large at shallow depths in some earthquakes, and it implies that fault gouges and sediments have extremely low dynamic friction during seismic slip. However, the dynamic weakening mechanisms of gouges under wet conditions are still not well constrained. Here we present direct evidence for the occurrence of flash heating and local fluid pressurization in water-saturated gouges, by performing low- to high-velocity (V = 10 μm/s to 1 m/s) friction experiments in a pressure vessel, under conditions specially designed to suppress weakening effects of bulk thermal and compaction-induced pressurization. The tested gouges exhibit transition from velocity strengthening to drastic velocity weakening as slip rates increase. Strong dynamic weakening starts to occur at V ≥ 0.04 m/s at the initiation of sliding (<~0.1 m), which is much more efficient than previously reported in terms of the weakening velocity and distance. Furthermore, the onset of weakening is always accompanied by an instantaneous dilatancy (10–25 μm), which is much larger than that observed in dry tests and in contrast with gradual changes displayed in the wet tests without dynamic weakening. Numerical modeling integrated with microstructural observation reveals that bulk thermal pressurization cannot explain the experimental results, while flash weakening triggered by vaporization of water layers on/around asperity contacts and the resultant local fluid pressurization may be responsible for the observed rapid weakening concomitant with instantaneous dilatancy. Given high efficiency of such weakening process, fluid-infiltrated faults could be weakened more rapidly than previously recognized during seismic slip.