The Influence of Fluids on Earthquakes: Insights from Mechanical Modelling

The strength and sliding behavior of faults in the upper crust are largely controlled by friction and effective stress, which is itself modulated by the fluid pressure. However, while many studies have investigated the role of friction on the earthquake cycle, relatively little effort has gone into understanding the effects linked to dynamic changes in fluid pressure. Here, we explore coupled interactions between slow tectonic loading and fluid pressure generation during the interseismic period with rapid sliding and elastic stress transfer during earthquakes on a plane strain thrust fault in two dimensions. Our models incorporate rate- and state-dependent friction along with dramatic changes in the fault permeability during sliding. In these modes, earthquakes are nucleated where fluid pressures are locally high and then propagated as slip pulses onto stronger parts of the fault. For the model without overpressure, the ruptures are more crack-like. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to the presence of high pore pressures prior to rupture. The models also show evidence for aftershocks that are driven by fluid transfer along the fault plane after the mainshock. Overall, we find that the presence of relatively modest fluid overpressures tends to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity, and the earthquake recurrence time relative to ruptures in a dry crust. This study shows that fluids can exert an important influence on earthquakes in the crust, which is mostly due to modulation of the effective stress and variations in permeability, and to a lesser extent to poroelastic coupling.