[CIG-SHORT] Fault direction in 3D fault

[Brad Aagaard]

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