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

Mon Apr 23 11:46:35 PDT 2012

On Mon, Apr 23, 2012 at 2:34 PM, bhhager <bhhager at mit.edu> wrote:

> Brad-
>
> 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.
>

It is due to the formulation+solver. Brad can correct me if my explanation
is deficient. The formulation has friction in compression
and nothing intension. Your dike example would have some viscous BC in
tension. This formulation is a variational inequality, but
no one in tectonics solves it this way, including us, because it is hard
(we hope to change this eventually since people in contact
mechanics like Dan Negrut at Madison solve 100M unknown VIs now). What the
solver currently does is us just the fault itself to
get an approximation of the slip, and then feeds this to the complete
problem, and iterates to self-consistency using Newton. If the
approximation from looking at just the fault is bad, as it is in the case
of tension or through-going faults, this does not work well at all.

    Matt

> If this is correct, you can shorten your TODO list.
>
> What am I missing?
>
> Thanks,
> Brad
>
> On Apr 23, 2012, at 12:48 PM, Brad Aagaard wrote:
>
> > Ruel Jerry,
> >
> > Attached are two examples that I created that illustrate imposing shear
> dislocations via either prescribed slip (kinematic fault) or frictional
> sliding (dynamic fault). I examined the stresses and they look correct with
> symmetry or asymmetry in the appropriate components. You can run these
> examples by extracting the tarball in the examples/3d/hex8 directory of
> your pylith distribution and running:
> >
> > pylith testkin.cfg
> > pylith testdyn.cfg
> >
> > The tricky part for the frictional sliding case is making sure the fault
> remains in compression. As described in the next paragraph, the fault
> implementation in the current release forces zero tractions when the fault
> opens. If you have trouble replicating this behavior in your simulation,
> make sure the solution is converging and check the deformation field to
> make sure it looks correct.
> >
> > I constructed a similar example with fault opening but I had to modify
> PyLith to get the desired behavior. In the current release, as soon as the
> fault goes into tension we enforce zero traction (free surface) boundary
> conditions on the fault. In order to get the fault to open with initial
> tensile tractions on the fault, we will need to add a switch to turn on/off
> enforcing zero tractions for fault opening. Clearly, there are classes of
> problems involving dikes for which people would like to impose initial
> fault tractions that result in fault opening. This feature only requires
> adding a few lines of code, so I will put it on the TODO list for the
> version 1.7 release scheduled for June.
> >
> > Regards,
> > Brad
> >
> >
> >
> > On 04/23/2012 08:03 AM, Ruel Jerry wrote:
> >> I'm using FaultCohesiveDyn and trying to get the same result that I
> >> would get if I imposed slip. I am getting equal but opposite
> >> tractions on the two sides of the fault, but that's different from
> >> the FaultCohesiveKin results which have the same sign on either
> >> side.
> >>
> >>
> >> On Apr 23, 2012, at 10:58 AM, Brad Aagaard wrote:
> >>
> >>> Ruel Jerry,
> >>>
> >>> Please describe how you are applying "dynamic conditions" on the
> >>> fault. Are you using a Neumann or FaultCohesiveDyn object to apply
> >>> the tractions? The Neumann boundary condition is intended for
> >>> external boundaries, not interior interfaces. The FaultCohesiveDyn
> >>> object is intended for frictional interfaces. The
> >>> db_initial_tractions property can be used to impose initial
> >>> tractions that will be equal and opposite on the two sides of the
> >>> fault. We have verified that it works for mode II and mode III
> >>> (shear) behavior. We have also verified that it works for fault
> >>> opening in cases where the opening is driven by Dirichlet BC, but I
> >>> will have to investigate what happens if the opening is driven by
> >>> initial tractions.
> >>>
> >>> Regards, Brad
> >>>
> >>>
> >>>
> >>> On 04/22/2012 06:45 AM, Ruel Jerry wrote:
> >>>> Hi, I have what seems like a basic problem, but I can't seem to
> >>>> figure it out. The model that I'm running includes faults at
> >>>> depth that I would like to impose normal stresses on. When I use
> >>>> a kinematic fault where I impose fault opening displacements I
> >>>> get the surface displacements and the stress fields that I would
> >>>> expect. When I try to use dynamic conditions on the same fault
> >>>> and I impose tractions instead of displacement I expect to get
> >>>> similar results. For some reason I get the correct stress on one
> >>>> side of the fault, but the other side seems to have the sign of
> >>>> the stress reversed, and this happens for modes 1,2 and 3. For
> >>>> example in mode 1 if I impose slip, if I draw a line through the
> >>>> middle of the fault an look at the XX stress, the stress values
> >>>> from left to right decrease until I reach the fault then they
> >>>> increase symmetrically on the other side back to 0 eventually. If
> >>>> I impose tractions the same line shows stress values from left to
> >>>> right decrease until I reach the fault and then at the interface
> >>>> I get a dislocation, then a positive value with the same
> >>>> magnitude, then the stress decreases.
> >>>>
> >>>> Is this supposed to be different or am I doing something wrong?
> >>>> It seems like a sign problem somewhere, but I only used one value
> >>>> for initial tractions  and one vale for fault opening.
> >>>>
> >>>> Thanks, Ruel Jerry
> >>>> _______________________________________________ CIG-SHORT mailing
> >>>> list CIG-SHORT at geodynamics.org
> >>>> http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
> >>>>
> >>>
> >>> _______________________________________________ CIG-SHORT mailing
> >>> list CIG-SHORT at geodynamics.org
> >>> http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
> >>
> >>
> >
> > <testkindyn.tgz>_______________________________________________
> > CIG-SHORT mailing list
> > CIG-SHORT at geodynamics.org
> > http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>
> _______________________________________________
> CIG-SHORT mailing list
> CIG-SHORT at geodynamics.org
> http://geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>

-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://geodynamics.org/pipermail/cig-short/attachments/20120423/33fd2121/attachment.htm 