[CIG-SHORT] slip orientation for a nonplanar fault surface

[Brad Aagaard]

On 10/31/2014 02:53 PM, Jiangzhi Chen wrote:
> Thanks for the clarification. If in global coordinates, the surface
> normal at the point on a subduction fault interface is n=[nx,ny,nz], is
> the lateral motion direction ns parallel to n×[0,0,1] and the reverse
> motion direction parallel to n×ns? And the fault opening is zero if all
> the points move tangentially?

Yes, if the fault normal is [nx,ny,nz], and "up" is [0,0,1], then the 
lateral direction is up x normal (cross product) and the reverse 
direction is normal x lateral. If the fault opening is zero, then the 
slip is tangential to the fault surface.

Regards,
Brad

> On 10/31/2014 02:19 PM, Brad Aagaard wrote:
>> Jiangzhi,
>>
>> All fault slip and tractions are specified (and output) in the local
>> fault coordinate system (left-lateral, reverse, and opening). If you
>> know the slip in a global XYZ coordinate frame, you will need to
>> compute the fault slip in the local coordinate system. If you have the
>> fault slip in the fault slip coordinate system (left-lateral, reverse,
>> opening) no conversion is necessary.
>>
>> If you need to convert from a global XYZ coordinate system, then I
>> suggest running a simulation that only initializes
>>
>> [pylithapp]
>> initialize_only = True
>>
>> and request output of the fault orientation information
>>
>> [pylithapp.timedependent.interfaces.fault.output]
>> vertex_info_fields = [strike_dir,dip_dir,normal_dir]
>>
>> so that PyLith will give you the local fault coordinate orientation to
>> use in the conversion.
>>
>> Regards,
>> Brad
>>
>>
>>
>> On 10/31/2014 02:11 PM, Jiangzhi Chen wrote:
>>> Hi Brad,
>>>
>>>      Just one small question. When specifying the fault parameters in
>>> terms of lateral motion and reverse motion for a 3d nonplanar fault, are
>>> the motions evaluated along the fault surface locally? To prescribe
>>> left-lateral-slip and reverse-slip for a specific point on the surface,
>>> I still need to calculate the strike, dip and rake angle at that point,
>>> is that correct?
>>>
>>> Jiangzhi
>>>
>>> On 10/20/2014 03:57 PM, Brad Aagaard wrote:
>>>> On 10/20/2014 03:55 PM, Jiangzhi Chen wrote:
>>>>> Oh I understand what you mean, since the strain is not finite, the
>>>>> deformation should not change the slip direction great enough, so the
>>>>> prescribed slip should work.
>>>>
>>>> Correct.
>>>>
>>>> Regards,
>>>> Brad
>>>>
>>>>> On 10/20/2014 03:50 PM, Brad Aagaard wrote:
>>>>>> Jiangzhi,
>>>>>>
>>>>>> I don't understand why you think prescribed slip won't work for a
>>>>>> kinematic fault interface?
>>>>>>
>>>>>> Regards,
>>>>>> Brad
>>>>>>
>>>>>>
>>>>>> On 10/20/2014 03:44 PM, Jiangzhi Chen wrote:
>>>>>>> The time scale is several hundreds of years, about one earthquake
>>>>>>> cycle.
>>>>>>>
>>>>>>> On 10/20/2014 03:34 PM, Brad Aagaard wrote:
>>>>>>>> Jiangzhi,
>>>>>>>>
>>>>>>>> What time scale are you modeling? PyLith should be appropriate
>>>>>>>> if the
>>>>>>>> problem can be solved with infinitesimal or small strains.
>>>>>>>> Additionally, the fault formulation in PyLith breaks down when the
>>>>>>>> offset become large enough so that the slip vector would need to be
>>>>>>>> changing. We expect that a finite-strain formulation would be
>>>>>>>> needed
>>>>>>>> as well for those types of problems.
>>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Brad
>>>>>>>>
>>>>>>>> On 10/20/2014 03:24 PM, Jiangzhi Chen wrote:
>>>>>>>>> Thanks for the quick response. The region is part of the Cascadia
>>>>>>>>> subduction zone, where the Juan de Fuca plate moves northeastward
>>>>>>>>> and
>>>>>>>>> subducts. The aim is to understand how the subduction
>>>>>>>>> influences the
>>>>>>>>> surface displacement field, and explains the geodetic
>>>>>>>>> observation. The
>>>>>>>>> upper slab interface is supposed to be a dynamic fault surface,
>>>>>>>>> and
>>>>>>>>> the
>>>>>>>>> lower interface is a kinematic surface. For now, I am using two
>>>>>>>>> kinematic surface just to make the slab go under.
>>>>>>>>>
>>>>>>>>> Jiangzhi
>>>>>>>>>
>>>>>>>>> On 10/20/2014 03:08 PM, Brad Aagaard wrote:
>>>>>>>>>> Jiangzhi,
>>>>>>>>>>
>>>>>>>>>> I don't understand what you are trying to do. What research
>>>>>>>>>> question
>>>>>>>>>> are you trying to answer? What do you want to learn from the
>>>>>>>>>> simulation?
>>>>>>>>>> What are your observations/constraints?
>>>>>>>>>>
>>>>>>>>>> Regards,
>>>>>>>>>> Brad
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> On 10/20/2014 03:02 PM, Jiangzhi Chen wrote:
>>>>>>>>>>> Hi everyone,
>>>>>>>>>>>
>>>>>>>>>>>      I am working on a subduction simulation, where the fault
>>>>>>>>>>> interface
>>>>>>>>>>> is nonplanar, and the subduction slab moves in northeast
>>>>>>>>>>> direction
>>>>>>>>>>> at 4
>>>>>>>>>>> cm/year before it meets the overriding plate. My question is how
>>>>>>>>>>> I can
>>>>>>>>>>> specify the kinematic fault interface. It is not appropriate to
>>>>>>>>>>> use
>>>>>>>>>>> predefined slip functions, because the slip rates depend on the
>>>>>>>>>>> location
>>>>>>>>>>> which is changing with time. Is there a way to make this
>>>>>>>>>>> work? Any
>>>>>>>>>>> help
>>>>>>>>>>> is appreciated.
>>>>>>>>>>>
>>>>>>>>>>> cheers,
>>>>>>>>>>> Jiangzhi
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
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