[CIG-SHORT] problem getting fault rupture (slip weakening)

[Lucas Abraham Willemsen]

Hello,

I'm sometimes encountering issues where the shear stress on the fault is larger than the shear strength, yet no slip occurs. I know several of the SCEC benchmarks have successfully been implemented for slip-weakening, so my problems are probably caused by misconfiguration. I've tried changing several settings, but have so far not found the one that is the cause of my problems.

Therefore I created a minimal working example illustrating the difficulties I'm having. This example does the following:

-Block of material: roller BC on left and bottom.
-Initial normal stress on the top boundary is -10 MPa.
-Initial normal stress on the right boundary is -25 MPa.
-Gravity is turned off. This way the initial stress field is uniform.
-Normal traction rate of -1MPa per day is applied on a part of the right boundary. The reason for applying the traction rate to only a section of the right boundary is that this way the entire fault will probably not rupture simultaneously.
-Static friction coefficient is 0.6, dynamic is 0.2 and slip weakening distance is 5mm. 0 Cohesion
-Fault has a dip of 50 degrees.
-Implicit simulation

With these settings, the initial normal traction on the fault should be -18.8 MPa and the shear traction should be -7.4 MPa. This is correctly calculated by Pylith.

-The initial ratio of shear traction to normal traction is 0.39, which is below the static friction coefficient of 0.6. No rupture should occur, and it doesn't. So far so good..
-The figure attached to this email shows evolution of the shear traction [traction (0)] and evolution of normal traction [traction (1)] as function of each of the 20 10-year timesteps for the fault node displayed as a purple dot. (total simulation time 200 years). The figure shows that the ratio of shear traction / normal traction quickly exceeds the static friction coefficient of 0.6. After 200 years the shear traction is even larger than the normal traction. Yet there is 0 slip.

As I mentioned before, this phenomenon probably indicates that I have incorrectly implemented a certain setting. I'd be grateful if someone could point me in the right direction. I have attached the code in the zip file. Running it should take a couple of minutes at most.

cheers,
Lucas



-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://geodynamics.org/pipermail/cig-short/attachments/20130422/a2e8f460/attachment-0001.htm 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: prq-2D-utractnoslip.zip
Type: application/x-zip-compressed
Size: 587337 bytes
Desc: prq-2D-utractnoslip.zip
Url : http://geodynamics.org/pipermail/cig-short/attachments/20130422/a2e8f460/attachment-0001.bin 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: whynoslip.png
Type: image/png
Size: 53893 bytes
Desc: whynoslip.png
Url : http://geodynamics.org/pipermail/cig-short/attachments/20130422/a2e8f460/attachment-0001.png