[CIG-SHORT] Fault direction in 3D fault

[Charles Williams]

Until the problem from the cig-short list, I had always assumed a horizontal fault would be a sill, and in that case we usually only care about fault-normal movement.  Having a fault that actually goes horizontal is a case I hadn’t considered.  The symptom of nearly-aligned vectors is a flipping in the horizontal directions for strike_dir and dip_dir, which can be easily seen when we plot those up as vectors.  I think we can try the tolerance for now, and see how it works.  Will we have a default value that could be overridden by the user?

Charles


On 6/05/2014, at 8:24 am, Brad Aagaard <baagaard at usgs.gov> wrote:

> Matt,
> 
> In 3-D the long-used convention for defining the two shear directions (along-strike, and up-dip) is based on the fault normal and the
> "up" direction. The convention breaks down for a horizontal fault (vertical normal direction). The underlying assumption is that faults in nature are not horizontal, and this holds up quite well. I have never seen a convention for horizontal faults, so we don't have an easy fall back.
> 
> We can try a tolerance of 1.0e-6 and see how well it works.
> 
> Brad
> 
> 
> On 05/05/2014 01:15 PM, Matthew Knepley wrote:
>> On Mon, May 5, 2014 at 2:59 PM, Brad Aagaard <baagaard at usgs.gov> wrote:
>> 
>>> Charles,
>>> 
>>> We use the same orientation function for Neumann BC and fault BC. The
>>> issue is whether we use a strict mag == 0.0 check for parallel directions
>>> or a mag <= tolerance check. Currently, we use mag == 0.0. If we use mag <=
>>> tolerance, then we need a robust value for tolerance. That is, we want it
>>> to catch cases that are problematic without triggering errors in cases that
>>> are okay.
>> 
>> 
>> Since faults can now be pretty twisty, does it make sense to have just one
>> coordinate system? Could we have
>> one for each element if we wanted?
>> 
>> I think its fair to warn the user about a very ill-conditioned coordinate
>> transform. I think there is no problem with
>> a fairly large tolerance, 1e-6, since then you really should not be doing
>> that transform.
>> 
>>    Matt
>> 
>> 
>>> 
>>> Brad
>>> 
>>> 
>>> 
>>> On 05/05/2014 12:34 PM, Charles Williams wrote:
>>> 
>>>> If I’m not mistaken, when we’re applying Neumann BC, we actually exit
>>>> with an error when up_dir and normal_dir are aligned.  This obviously
>>>> doesn’t happen on faults, though.
>>>> 
>>>> Charles
>>>> 
>>>> 
>>>> On 6/05/2014, at 3:22 am, Brad Aagaard <baagaard at usgs.gov> wrote:
>>>> 
>>>>  On 05/05/2014 05:30 AM, Matthew Knepley wrote:
>>>>> 
>>>>>> On Sun, May 4, 2014 at 8:58 PM, Charles Williams <willic3 at gmail.com>
>>>>>> wrote:
>>>>>> 
>>>>>>  It appears that the problem is that your fault at the bottom of the
>>>>>>> slab
>>>>>>> is completely horizontal over much of its extent.  This causes
>>>>>>> problems for
>>>>>>> PyLith when the default up_dir [0,0,1] is used.  The way to diagnose
>>>>>>> this
>>>>>>> problem is to have PyLith output the orientation information for the
>>>>>>> faults, as Brad suggested previously.  When you plot the strike_dir as
>>>>>>> vectors, you will see it varies quite a bit for the horizontal portion
>>>>>>> of
>>>>>>> the fault, because the direction is undefined.  The solution is to
>>>>>>> choose a
>>>>>>> vector that does not align with the fault normal vector.  I chose a
>>>>>>> vector
>>>>>>> that points slightly to the west.  When this is done, the various fault
>>>>>>> slip directions are well-defined.
>>>>>>> 
>>>>>>> 
>>>>>> Great find, Charles. Is there a way we can automate this check?
>>>>>> 
>>>>> 
>>>>> The difficulty is defining a good tolerance. Right now, we compute the
>>>>> cross product of the up direction and the fault normal to differentiate
>>>>> between along-strike shear and up-dip shear. If the magnitude of the cross
>>>>> product is strictly zero, we give an error. We could make it epsilon, but
>>>>> we may have to play around with the order of magnitude so that it is not
>>>>> overly aggressive.
>>>>> 
>>>>> Brad
>>>>> 
>>>>> _______________________________________________
>>>>> CIG-SHORT mailing list
>>>>> CIG-SHORT at geodynamics.org
>>>>> http://lists.geodynamics.org/cgi-bin/mailman/listinfo/cig-short
>>>>> 
>>>> 
>>>> 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
>>>> 
>>>> _______________________________________________
>>>> CIG-SHORT mailing list
>>>> CIG-SHORT at geodynamics.org
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>>>> 
>>>> 
>>> 
>> 
>> 
> 
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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