in order for unstable slip to be able to nucleate under sufficiently reduced
stiffness [Segall and Rice, JGR, 1995]. Here f is
the friction coefficient, assumed to decrease in steady state at rate b
- a with logarithm of slip-velocity, 
is the rate of increase of inelastic porosity with log slip-velocity,
and b is a compressibility parameter. Such stabilization can occur
when the characteristic time for equilibration of fault pore pressure with
that of the surroundings, Tp, is long compared to the
rupture time scale; Tpcontrols the amount that dilatancy
reduces the pore pressure, and thus increases the effective clamping stress,
to mitigate against frictional weakening.
We address dilatancy here as a factor controlling rupture in the shallow,
certainly fluid-infiltrated, portion of a subduction fault zone. This is
done using a simple 2D plane-strain model in which slip varies with down-dip
distance and time. The governing equations, solved quasi-dynamically, incorporate
the temperature (and hence depth) dependence of b - a, represent
inelastic porosity changes as above, and treat equilibration of pore pressure
between the fault and its surroundings by a lumped reservoir model with
characteristic diffusion time Tp. We present results
for Tp= 10-8 yr and 10-1 yr, in
which cases the fault responds as if were, respectively, fully drained
and undrained on the dynamic rupture propagation time-scale.
There are corresponding nucleation sizes 
and 
,
the latter existing only at sufficiently great depths that the effective
stress exceeds ,
30 MPa in our simulations. Both cases exhibit periodic large events with
characteristics that are representative for subduction zones, and ruptures
nucleate at similar depths in the two cases. However, slip propagating
up-dip extends all the way to the trench for the drained fault, but the
rupture front slows and comes to a halt at shallow depths in the undrained
case.
Dilatant effects like those modeled may explain the typically aseismic
response of the shallow thrust zone, and could be a primary factor controlling
the magnitude of tsunami generation, since coupling of slip to wave generation
is strongest for slip extending to near the trench.