[CIG-SHORT] pylith install error

[Brad Aagaard]

On 05/27/2014 05:28 PM, Birendra jha wrote:
> Hi Charles
>
> Few things I want to share with the community:
>
> 1. Please correct me on this, but I found that it is not necessary to
> set zero load at the top boundary, which is at a depth of 500 m. I
> can represent stresses at depth without including all the overburden
> in my domain. So I can keep the original initialstress, topboundary,
> and rightboundary spatialdb files which had overburden at 500 m.

As long as the stresses balance, you should be able to use whatever 
combination of Neumann (traction) boundary conditions and initial 
stresses you want.

> 2. It was also not necessary to increase my critical slip parameter
> from 5 mm to 5 m. However, I understand its importance once the fault
> starts to slip.

Correct, the slip weakening parameter does not affect the solution at 
all when there is zero slip.

> 3. The only crucial part in getting the nonlinear solver converged to
> the zero solution was to decrease the ksp tolerances to the order of
> 1e-20.

Yes, we have tried to make it clear that for quasistatic spontaneous 
rupture simulations you should be controlling the solve with the 
absolute tolerance values. This means the KSP and SNES relative 
tolerances should be set to extremely small values (for example, 1e-20) 
so that the KSMP and SNES absolute tolerances control the convergence 
criteria. The KSP absolute tolerance need to be set to a value about one 
order of magnitude smaller than the zero slip and slip rate tolerance 
for sliding (friction zero tolerance). Note that the convergence 
tolerance is compared against the norm of the preconditioned residual, 
so you are not guaranteed that all residual values are less than the 
absolute tolerance. This is why one order of magnitude smaller is about 
right.

This can be a subtle issue for people unfamiliar with the PETSc 
iterative solvers. I will add some additional notes on this to the 
manual. Thanks for pointing this out.

Regards,
Brad