# Question on example/debugging/step3

**URL:** <https://community.geodynamics.org/t/question-on-example-debugging-step3/1831>\
**Category:** PyLith\
**Created:** [May 11, 2021, 2:18pm UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831 "2021-05-11T14:18:06Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![leonqing](https://avatars.discourse-cdn.com/v4/letter/l/bc79bd/32.png) [@leonqing](https://community.geodynamics.org/u/leonqing)\
**Post date:** [May 11, 2021, 2:18pm UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831/1 "2021-05-11T14:18:07Z")

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Hi there,  
I am trying to follow the example/debugging/step3.cfg to test dynamic spontaneous simulation of a buried fault with static friction. The step3.cfg works well and I change the mesh cell to tet andc  
check slip/traction with other birichlet boundary conditions, like shear and extension.  
case 1 step3.cfg original setting

 ![step3](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/9863a9d73330129eb2fb80873abf6593db95f35a.jpeg)  
case 2 step3.cfg with bc of extensional displacement on xpos and xneg  
 ![step3_extension](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/9bdffe9bcc8c3e6f8828f25e20682e6882d07ec6.png)  
case 3 step3.cfg with bc of shear displacement on xpos and xneg  
 ![step3_shear](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/f63553831b9a38cffa220ee212a112843fb33339.jpeg)

The slip solution looks good, but the traction for case2 and case3 is ~0. Any ideas?

 ![ft_shear](https://canada1.discourse-cdn.com/flex036/uploads/geodynamics/original/1X/047237c5bcaf9a9b4137eccdb3afaf85c48c1e80.png)

Here is solver parameter. Other setting is the same with example/debugging/step3  
`zero_tolerance = 1.0e-8 ksp_atol = 1.0e-9 snes_atol = 1.0e-7`

Could you please let me know what you think about it ? mesh and pylithapp attached  
Thank you very much in advance,  
Leon  
[Archive.zip](https://community.geodynamics.org/uploads/short-url/rAfgFduDPCjt4AlDl35L52Jcz6a.zip) (1.1 MB)

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<div class="post-metadata">

**Author:** ![baagaard](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/baagaard/32/615_2.png) [@baagaard](https://community.geodynamics.org/u/baagaard)\
**Post date:** [May 13, 2021, 2:41am UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831/2 "2021-05-13T02:41:19Z")

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For the case of extension, the fault opens so there are no tractions on the fault (it is a free surface).

When you apply shear with zero normal tractions, the fault will also open, so there are no tractions on the fault (it is a free surface).

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<div class="post-metadata">

**Author:** ![leonqing](https://avatars.discourse-cdn.com/v4/letter/l/bc79bd/32.png) [@leonqing](https://community.geodynamics.org/u/leonqing)\
**Post date:** [May 13, 2021, 3:40pm UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831/3 "2021-05-13T15:40:53Z")

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Thank you for the reply, Brad. Is there any way imposing the traction on the fault surface for these two cases? Set the open free surface to false and add prescribed traction on it, or other solutions?

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<div class="post-metadata">

**Author:** ![baagaard](https://yyz2.discourse-cdn.com/flex036/user_avatar/community.geodynamics.org/baagaard/32/615_2.png) [@baagaard](https://community.geodynamics.org/u/baagaard)\
**Post date:** [May 13, 2021, 4:49pm UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831/4 "2021-05-13T16:49:47Z")

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For zero displacement normal to the fault, you can set the initial normal tractions (negative for compression) using the traction perturbation to prevent tensile tractions and fault opening. Note that with static friction this will affect when the traction value at which the fault starts to slip.

For the case in which you impose extension, you might be able to apply a very large initial fault compressive traction to keep the fault under compression, but this isn’t physically realistic.

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<div class="post-metadata">

**Author:** ![leonqing](https://avatars.discourse-cdn.com/v4/letter/l/bc79bd/32.png) [@leonqing](https://community.geodynamics.org/u/leonqing)\
**Post date:** [May 13, 2021, 4:58pm UTC](https://community.geodynamics.org/t/question-on-example-debugging-step3/1831/5 "2021-05-13T16:58:10Z")

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Thank you for the solution and quick reply.