[CIG-SHORT] RELAX Question

[Sylvain Barbot]

Hi Austin,

I often get questions about units. There are no formal choice of units
in Relax, except that S.I. units, or more precisely, their
inter-relationships, is assumed. The expected input of the current
version of Relax is

n x1 x2 length width t3 T phi

where n is the index, x1 and x2 and north and east, length and width
are the dimension of the source patch, t3 is the traction (force per
unit area), and T and phi is the period and phase assuming the
following model

p(t)=t3*sin(2*pi*t/period+phi)

t3 is the load in units of stress. If you prescribed your Lamé
parameters in units of MPa, you need to describe the load in MPa.

I apologise that the examples I sent you correspond to an old version
of Relax. The current version assumes stress units. (The previous
version expected t3/G.) The factor of 1,000 comes in depending on your
choice of length units. If you use km, it introduces a factor of 1,000
in the displacement field. You can compensate this effect by dividing
the rigidity by the same factor.

Cheers,
Sylvain


On Wed, Jan 20, 2016 at 2:03 PM, Austin Madson <amadson at ucla.edu> wrote:
> Prof. Barbot,
>
> Thanks for the response and the example data. I am working out my parameters
> (and units now). I think I have a lock on the elastic and viscoelastic
> params. However, I am having trouble with the load parameters.
>
> I see that on some of your input files the surface load/traction parameters
> are;
> nb x1 x2 length width t3 T phi
>
> and another has;
> nb x1 x2 t3 length width T phi
>
> I assume the former is correct, yes?
>
> Also, I'm trying to wrap my head around the units for force(t3). In the
> GRACE example (the _km.xyz file), the z is used in the following calculation
> ($3*9.8/3e10) in order to get force(t3). What are the initial units in the
> _km.xyz file for the third column, z? And what are the units after the
> calculation ($3*9.8/3e10) in order to derive force(t3)?
>
> Also, it appears that the output vertical (z) displacement units are in mm,
> may I assume that is correct?
>
> Cheers and thanks A LOT for your responses!
> Austin M
>
> On Tue, Jan 19, 2016 at 9:29 AM, Sylvain Barbot <sylbar.vainbot at gmail.com>
> wrote:
>>
>> Hi Austin,
>>
>> The Tibet lakes have a great loading signal. You can use Relax to
>> simulate the deformation, but you need to be careful about how
>> important the elastic stratification is. If your data are close to the
>> source, you're fine. I presume you are using paleo-shoreline data. If
>> so, use the shallow rigidity for the entire domain. If you are
>> modeling far-field data, you should use a code that have a layered
>> elastic structure, i.e., (Farrell, 1972).
>>
>> The example attached models the deformation from the Indian monsoon
>> using Grace data. The example cycleload.sh creates a periodic surface
>> load.
>>
>> Best wishes,
>> Sylvain
>>
>>
>>
>> On Tue, Jan 19, 2016 at 10:51 AM, Austin Madson <amadson at ucla.edu> wrote:
>> > Professor Barbot,
>> >
>> > Thanks for the prompt response. I looked at the documentation and the
>> > man
>> > page (as well as the examples on the PDF on the geodynamics website) and
>> > have made progress.
>> >
>> > As an fyi, we plan on modeling several hundred years of surface loading
>> > from
>> > very large lakes (i.e. time dependent loads). Are there any RELAX
>> > surface
>> > load examples laying around (they can just be single/multiple point and
>> > even
>> > instantaneous (no time function)? The GRACE example on the example pdf
>> > "hides" the data in various dat files (which makes sense if you're using
>> > a
>> > lot of data, but it's hard to see their example inputs).
>> >
>> > I expect to see only millimeters of deformation throughout the run -
>> > will
>> > the outputs be precise enough if I use the correct SI units throughout?
>> >
>> > Cheers,
>> > Austin Madson
>> >
>> > On Sun, Jan 17, 2016 at 4:06 PM, Sylvain Barbot
>> > <sylbar.vainbot at gmail.com>
>> > wrote:
>> >>
>> >> Hi Austin,
>> >>
>> >> The details of the input file for surface loads are in the
>> >> documentation
>> >> and in the man page. There are some pdf files on the geodynamics
>> >> website
>> >> that show examples of surface loads for the Himalayan region if I
>> >> recall
>> >> well.
>> >>
>> >> But the input file simply expects a list of squares with their
>> >> associated
>> >> traction. So depending on your project, this can be a single point, say
>> >> to
>> >> represent the loading of a dam, or a complicated function of space and
>> >> time.
>> >> If your change of load is instantaneous, you need only one event. Of
>> >> your
>> >> load is time dependent, you need as many events as you have time steps.
>> >> I
>> >> can be more specific, but I need more information about your practical
>> >> goals.
>> >>
>> >> Cheers,
>> >> Sylvain
>> >>
>> >>
>> >> On Monday, January 18, 2016, Austin Madson <amadson at ucla.edu> wrote:
>> >>>
>> >>> Professor Barbot,
>> >>>
>> >>> I have a quick question re: RELAX - Is there any more information out
>> >>> there with respect to utilizing RELAX for deformation responses to
>> >>> surface
>> >>> loading? I have looked through all of the examples as well as the
>> >>> slides/tutorials on the geodynamics.org website.
>> >>>
>> >>> Can you provide any further information? Or can you point me in a
>> >>> better
>> >>> direction?
>> >>>
>> >>> Cheers,
>> >>> Austin Madson
>> >
>> >
>
>