The strength and sliding behavior of faults in the crust is largely
controlled by friction and effective stress, which is itself modulated
by fluid pressure. Most earthquake models assume a fixed pore fluid
pressure despite widespread evidence that is varies strongly in time due
to changes in permeability. Here we explore how dynamic changes in pore
pressure influence the properties of earthquakes in the upper crust. To
study this problem we develop a two dimensional model that incorporates
slow tectonic loading and fluid pressure generation during the
interseismic period with frictional sliding on a thrust fault whose
permeability evolves with slip. 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 compared to models without fluids. 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, 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 poro-elastic coupling.