  Dilatancy associated with fault slip produces a transient pore pressure drop
which increases frictional strength. This effect is analysed in a rupture model
that includes frictional weakening, slip-dependent fault dilation and fluid
flow. Dilatancy is shown to increase the stress intensity factor required to
propagate the rupture tip. With increasing rupture speed, an undrained
(strengthened) region develops near the tip and extends beyond the frictionally
weakened zone. Away from the undrained region, pore fluid diffusion gradually
recharges the fault and strength returns to the drained, weakened value. For
sufficiently large rupture dimensions, the dilation-induced strength increase
near the tip is equivalent to an increase in toughness that is proportional to
the square root of the rupture speed. In general, dilation has the effect of
increasing the stress required for rupture growth by decreasing the stress drop
along the crack. Thermal pressurisation has the potential to compensate for the
dilatant strengthening effect, at the expense of an increased heating rate,
which might lead to premature frictional melting. Using reasonable laboratory
parameters, the dilatancy-toughening effect leads to rupture dynamics that is
quantitatively consistent with the dynamics of observed slow slip events in
subduction zones.
