Pore Fluid Pressure Effects on Friction and Fracture

Slip instabilities leading to earthquakes, landslides, and glacier surges may be triggered by high fluid pressures. On the other hand, high fluid pressures 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 pore pressure induced frictional instabilities: (1) the length scale over which pore fluid overpressure is maintained, and (2) a time scale defined by relative rates of deformation propagation and pore fluid transport. These scales are also dependent on rheological regime, and we find three end-member regimes: (1) shallow and/or low temperature deformation where ambient stress is low, faults are close to frictional failure, and shear is easily delocalised; (2) deformation in a brittle and frictionally unstable regime (such as the crustal seismogenic zone), where planes are close to frictional failure and slip tends to localise; and (3) environments where viscous deformation is preferred over frictional, and hence bulk stress is low, frictional strength is high, and delocalisation dominant. In regimes 1 and 3, fluid-driven instabilities tend to be confined to local areas of overpressure, because deformation delocalises in the bulk and dilatant hardening prevents further propagation. In regime 2, however, slip tends to localise and it is potentially favourable for fluid-induced instabilities to grow, provided slip surfaces are sufficiently close to failure. These regimes also apply to glaciers, where viscous flow of ice competes with frictional sliding on the glacier base - here, interconnected overpressured water at the glacier base is a commonly invoked mechanism that promotes frictional instability. These concepts imply that fluid-driven frictional instabilities are only as large as the areas where fluid overpressured patches can be interconnected, and therefore highlight the key role of fluid pressure heterogeneity in determining whether fluid-induced instabilities can propagate.