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

[Brad Aagaard]

Lucas,

I finally found some time to start looking at this. The first thing I 
have noticed is that your length scale for nondimensionalizing is too 
big. It looks like your discretization size is 10 m but your length 
scale for nondimensionalization is 1 km. You chose to use the dynamic 
nondimensionzalizer so you provided a shear wave speed and a period. It 
looks like your shear wave speed for nondimensionalizing is close to 
your physical properties, so decreasing your period should give you an 
appropriate length scale.

I am not sure if this fixes everything, but it is a place to start. I 
may have some additional minutes later this afternoon to dig deeper.

Brad


On 04/24/2013 01:44 PM, Lucas Abraham Willemsen wrote:
> Hello CIG,
> I still haven't quite figured out what is causing this strange behavior. Could someone please shed some light on what I am doing wrong?
>
> with kind regards,
> Lucas
> ________________________________
> From: Lucas Abraham Willemsen
> Sent: Monday, April 22, 2013 11:28
> To: cig-short at geodynamics.org
> Subject: problem getting fault rupture (slip weakening)
>
> 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
>
>
>
>
>
>
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