[CIG-SHORT] A Question About Pylith

[zeynep yılmaz]

Thank you very much for taking time to respond my questions and for your
detailed explanation. I am new at Pylith, your response helped me so much.

I have implemented a model with the dimensions of 150 km for a 20x10 km
fault plane as you have suggested. I also increased the resolution of the
mesh. However, the linear solve converged after 1700 iterations. I would
also like to ask you how I can choose the number of iterations after which
Gram-Schmidt orthogonalization is restarted (ksp_gmres_restart).

Thanks a lot again.

Best wishes and kind regards,
Zeynep

On Mon, Nov 14, 2016 at 11:27 PM, Charles Williams <willic3 at gmail.com>
wrote:

> Dear Zeynep,
>
> I am also sending my response to the cig-short mailing list.  For future
> requests, could you please make use of the mailing list?  This helps both
> by reducing the amount of e-mails we need to reply to, and it also helps
> everyone on the mailing list by answering questions that others might
> have.  You can join this mailing list at:  https://geodynamics.org/cig/
> about/mailing-lists/.
>
> In response to your question, there are several issues involved when
> comparing Okada’s analytical solution to finite element results:
>
> 1.  The finite element solution and the analytical solution are modeling
> two different things, so the solutions will never agree completely.  The
> Okada solution represents displacements due to slip on rectangular patches
> with constant slip on each one, while the finite element solution
> represents slip at vertices that decreases either linearly or bilinearly
> (depending on element type) to zero at the surrounding vertices.
> Obviously, the finite element solution is not capable of representing slip
> that varies over an infinitesimal distance (and this is actually not
> realistic in any case).
>
> 2.  If you want to accurately represent an analytical solution, you must
> make sure that you have sufficient mesh resolution, particularly in regions
> where the solution is changing most rapidly.  Looking at the meshes you
> provided, this does not seem likely to be the case.  Your meshes are
> somewhat graded, but do not appear to have sufficient resolution in the
> vicinity of the fault.  Furthermore, in relation to point #1 above, if you
> want to accurately represent a constant-slip rectangular patch, you would
> need extremely high resolution around the fault edges.
>
> 3.  Another issue when representing an analytical solution is insuring
> that the mesh boundaries are sufficiently far from the deformation source
> that they are not influencing the solution.  Again, this does not seem to
> be the case for your mesh.  As a general rule of thumb, the distance to the
> mesh boundaries should be at least a factor of 3 larger than the dimensions
> of the deformation source.
>
> A final question is to determine why it is important to compare the
> results against the analytical solution.  If it is important to do this,
> then you probably want to evaluate both the analytical solution and
> numerical solution at an identical set of points and integrate the error
> over the volume.  Some of these questions were explored quite some time
> ago.  You can find some old benchmarks here:
>
> https://geodynamics.org/cig/working-groups/short-term-
> crustal-dynamics/benchmarks/
>
> Unfortunately, these benchmarks are extremely old and have not been
> updated for current versions of PyLith.
>
> Good luck with your modeling, and please contact the cig-short mailing
> list if you need more help.
>
> Cheers,
> Charles
>
>
>
> On 15/11/2016, at 1:55 AM, zeynep yılmaz <ylmz.zeynep at gmail.com> wrote:
>
> Dear Dr. Charles Williams,
>
> I am working on calculation of static displacements for a reverse buried
> fault (with a dip of 70 degrees) by using Pylith Software. I would like to
> ask you how I can compare the accuracy of the FEM models (different size
> models attached below) with the Okada's analytical solution. Is there an
> easy way to compare the solutions by using Pylith? I have used Coulomb 3
> software for Okada's solution, however it takes time to adapt. I have
> also downloaded Cigma package to calculate the erors. However I did not
> understand well if it is comparing the results with Okada's solution.
>
> If you could have time to answer my question, I would be so glad. Thanks
> in advance.
>
> Best Regards,
> Zeynep Yilmaz
>
> <model1_40x40x50.vtk><model2_80x80x100.vtk>
>
>
>
> *Charles Williams I Geodynamic ModelerGNS Science **I** Te Pῡ Ao*
> 1 Fairway Drive, Avalon 5010, PO Box 30368, Lower Hutt 5040, New Zealand
> *Ph* 0064-4-570-4566 I *Mob* 0064-22-350-7326 I *Fax* 0064-4-570-4600
> *http://www.gns.cri.nz/* <http://www.gns.cri.nz/> *I* *Email: *
> *C.Williams at gns.cri.nz* <your.email at gns.cri.nz>
>
>
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