[CIG-SHORT] Kinematic faults vs dynamic faults and fault opening

[Brad Aagaard]

On 04/23/2012 11:34 AM, bhhager wrote:
> I am confused by your description.  I think that dike intrusion
> should result when the magma pressure is large relative to the
> background (compressive) stress, thereby forcing the dike to open by
> exerting a large compressive normal traction on the medium bounding
> the dike.  (Imagine a flat jack wedging the medium apart.)
>
> In other words, crack opening is aided by high fluid pressures (that
> is, highly-compressive) that push the fault open.  (Ahead of the
> fault, the medium might go into tension, but that's another story.)
>
> Following this "logic,"  I don't see why putting the compressive
> tractions on a fault that are needed to push the adjacent planes
> apart should be inconsistent with the frictional sliding case
> requiring either compressive or zero normal tractions.
>
> If this is correct, you can shorten your TODO list.
>
> What am I missing?

In the current version of PyLith, the FaultCohesiveDyn object implements 
what I would describe as a frictional contact:

   T_f = C - mu_f*Tn if Tn <= 0

   T_f = 0 if Tn > 0

T_f = shear traction (friction) on fault
C = Cohesion
mu_f = coefficient of friction
T_n = normal fault traction

In implementing this contact condition, if the fault opens then we 
enforce zero tractions (shear and normal) on the fault surface so that 
it is a free surface. This corresponds to frictional contact behavior.

If the deformation causing the fault opening is part of the elasticity 
solution, then the frictional contact implementation allows the fault to 
open (with the fault surface becoming traction free). On the other hand, 
if one wants to simulate a dike intrusion via imposed tractions on an 
interior surface (a flat jack wedging the medium apart) to approximate 
intrusion of a fluid, this is incompatible with enforcing a free surface 
when fault opening occurs. One can simulate dike intrusion via 
prescribed slip, but it is more naturally done via tractions.

In the past we have discussed modeling dike intrusions by adding a 
FaultCohesiveTract object. I think we can add in the functionality that 
most people would want for dike intrusions by simply adding a switch 
that allows initial tractions to be imposed even when the fault is open. 
This alone would not allow for transient intrusions but when combined 
with the planned spatial and temporal perturbations in fault tractions 
(intended for earthquake rupture nucleation), it could extend the 
behavior to cover some simple transient intrusions as well.

Brad