Fluid pressure effects on friction and fracture

Slip instabilities leading to earthquakes, landslides, and glacier surges may be triggered by high fluid pressures at the slipping interface. High fluid pressures may, however, also suppress instability because of large nucleation length-scales in overpressured systems. We review geological, glaciological, and rock mechanical observations and highlight two key scales that control fluid pressure-enabled frictional instabilities: (1) the length scale over which fluid overpressure is maintained, and (2) a time scale defined by relative rates of deformation propagation and fluid transport. These scales are also dependent on rheological regime, and we suggest three end-member regimes: (1) brittle-localising (BL) regimes where deformation and fluid flow localise to discrete interconnected structures; (2) brittle-delocalising (BD) regimes where deformation and fluid flow are distributed over a broad region, and instability typically stabilised by fluid diffusion; and (3) transiently brittle (TB) regimes where in dominantly viscous rock or ice, fluid pressure may induce a transient frictional rheology that is typically delocalised and stabilised by dilatant hardening. In the BL regime, fluid overpressure can be sustained over a sufficient length scale to enable the nucleation and propagation of frictional instabilities. In the BD and TB regimes, however, fluid-driven instabilities tend to be suppressed, confined to local areas of overpressure, or rate-limited by fluid diffusion, because deformation delocalises in the bulk and/or dilatant hardening limits slip propagation. The three regimes can also be applied to glaciers, where viscous flow of ice competes with frictional sliding on the glacier base—here, sustained and interconnected high water pressure at the glacier base is commonly invoked as promoting frictional instability. In both rock and ice, these concepts imply that fluid-driven frictional instabilities are limited to areas where fluid overpressured patches can be interconnected, and therefore highlight the key role of fluid pressure heterogeneity in development of slip instabilities.