[CIG-SHORT] fault traction orientation

[Brad Aagaard]

Birendra,

The sign convention in Pylith for tractions/slip is:

1. Positive normal slip/traction is always opening/tension.

2. In 2-D, positive shear slip/traction is always left-lateral.

3. In 3-D, positive horiz shear slip/traction is always left-lateral. If 
the fault normal is up (positive z), positive up-dip shear slip/traction 
is reverse.

The sign conventions for the fault orientation directions:

1. normal_dir is the fault normal direction. The normal direction points 
from the "negative" side of the fault to the "positive" side of the fault.

2. strike_dir and dip_dir are the directions of positive slip/traction 
on the "positive" side of the fault.

The normal direction is set when we insert the cohesive cells into the 
mesh. The strike_dir and dip_dir are set from the normal_dir and up_dir, 
(0,0,1) by default, and PyLith will sometimes flip the dip_dir to give 
the correct sense of motion according to the conventions listed above.

For nonplanar faults with changing dips (for example dipping east and 
then dipping west), we choose the strike_dir and dip_dir to be 
consistent, so these cases require checking the orientation vectors to 
insure the fault moves in the desired directions.

Regards,
Brad


On 06/17/2015 05:12 PM, Birendra jha wrote:
> Hi Charles
>
> I agree with positive tractionZ denoting tension. But isn't the updip
> direction determined by the "dip_dir" vector? So for the fault in
> picture, which has the given dip_dir vector, isn't positive tractionY
> pointing downdip? What is the purpose of dip_dir vector?
>
> Thanks Birendra -------------------------------------------- On Wed,
> 6/17/15, Charles Williams <willic3 at gmail.com> wrote:
>
> Subject: Re: [CIG-SHORT] fault traction orientation To:
> cig-short at geodynamics.org Date: Wednesday, June 17, 2015, 3:58 PM
>
> Hi Birendra,
>
> I’m fairly sure that positive traction y should mean updip shear, and
> positive traction z should be tension.  Similarly, positive traction
> x should be left-lateral shear.  Unfortunately, I don’t think we have
> a 3D example that demonstrates this, but you could put one together
> by putting in a dipping fault with no slip.  You can then apply BC
> that should give updip shear, and another set of BC to produce
> tension.  Look at the resulting fault stresses and see if this agrees
> with what I said.  Let us know if this works for you.
>
> Cheers, Charles
>
>
>>
> On 17/06/2015, at 2:29 am, Birendra jha <bjha7333 at yahoo.com> wrote:
>>
>> Hi
>>
>> I have questions on
> pylith fault traction orientations. Please open the attached figure.
> I loaded the fault info.vtk file in paraview to find out normal_dir
> and dip_dir directions. These vectors are in global coord system as
> per manual. For the fault in the picture, can you please answer those
> questions?
>>
>> Thanks a
> lot
>>
> Birendra<fault_tractions.png>_______________________________________________
>
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> Charles A. Williams Scientist GNS Science 1 Fairway Drive, Avalon PO
> Box 30368 Lower Hutt  5040 New Zealand ph (office): 0064-4570-4566
> fax (office): 0064-4570-4600 C.Williams at gns.cri.nz
>
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