The strength and sliding behaviour of faults in the upper crust are largely controlled by friction and effective stress, which is modulated by the fluid pressure. 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. Our study aims to explore the complex interactions between long-term tectonic loading, fluid pressure, rapid fault slip and elastic stress transfer that occur during earthquakes on plane faults in two dimensions. Our models incorporate rate- and state-dependent friction and dramatic changes in the fault permeability during sliding. In some models, we have also incorporated shear heating and thermal pressurisation.
Our most basic models consider slow fluid pressure generation during the interseismic period. Conversely, during an earthquakes, the permeability increase enables fluid overpressures to rapidly dissipate. This is commonly known as fault valve behaviour. In these modes, earthquakes are nucleated where fluid pressures are locally high. Ruptures are then propagated as slip pulses onto stronger parts of the fault. Our model produces a wide range of sliding velocities from rapid to slow earthquakes, which occur due to high pore pressures prior to rupture. The models also show evidence of aftershocks driven by fluid transfer along the fault plane after the main shock. Overall, we find that relatively modest fluid overpressures tend to reduce coseismic slip, stress drop, maximum sliding velocity, rupture velocity, and the earthquake recurrence time relative to ruptures in a dry crust. We show that fluids can exert an important influence on earthquakes in the crust, mostly due to modulation of the effective stress and variations in permeability and, to a lesser extent, to poroelastic coupling. For models exhibiting 'valve' behaviour, we observe complex ruptures and swarm activity associated with rising fluid pressure.
In addition to 'valve' behaviour, faults are sometimes considered to actively pump fluids into or out of fault zones over the earthquake cycle. We have studied one aspect of this pumping through the incorporation of thermal pressurisation arising from shear heating. Our models show that thermal pressurisation has the effect of increasing the stress drop, coseismic slip and sliding velocity relative to pure valve models. Both pump and valve models can explain anomalously weak faults, though they are each associated with distinctly different fluid pressure and strength evolution over the seismic cycle and during rupture.
Finally, in addition to studying slip dynamics we have also examined the fluid pressure fields in the Earth's crust following earthquakes. Much research has focused on post-seismic surface deformation that results from earthquake-induced fluid flow, commonly known as 'poroelastic rebound'. These studies have focused exclusively on fluid flow induced by coseismic volumetric changes in the crust. In our investigation we have studied surface deformation linked to breaching of a pressurised fault zone during rupture. Our study shows that regardless of the fault mechanism (i.e., reverse versus normal), fluid drainage induced by rupture tends to cause surface subsidence above the rupture zone after an earthquake. These observations allow us to interpret better the diversity of post-seismic deformation.
