[CIG-SHORT] RELAX Question

[Sylvain Barbot]

Hi Austin,

With your input file, you compute the displacement field at the
surface with units of meters, due to a 4x4 km patch where a load of 20
MPa is applied. Your spatial sampling is 4 km. The value of gamma
should be 8.33e-4 in units of 1/km.

Cheers,
Sylvain

On Fri, Jan 22, 2016 at 4:24 PM, Austin Madson <amadson at ucla.edu> wrote:
> Prof. Barbot,
>
> Thanks again for the response - you've been  very helpful thusfar.
>
> Just to confirm:
> Trying to get at displacement units (let's call them km). If my inputs are
> the following, and I divide out the 1e3 from the displacement field - the
> units are just that, km. Is that correct? Or, if I keep the Lamé parameters
> (along with t3)  in GPa instead of kPa - does the Gamma value then = 8.33e2
> [gamma=8.33e2 = (1-0.25)*3400*9.8/30 or gamma=8.33e4 =
> (1-0.25)*3400*9.8/30000000 ],  and we still divide out the 1e3 from the
> displacement to get the units in "km"?
>
>
>
> Cheers and thanks!
>
> Austin M
>
>
>
>
> # grid dimension (sx1,sx2,sx3)
> 512 512 512
> # sampling (dx1,dx2,dx3), smoothing (beta, nyquist)
> 0.9765625 0.9765625 0.9765625 0.2 0
> # origin position (x0,y0) and rotation
> 0 0 0
> # observation depth (displacement and stress)
> 0 0
> # output directory
> $WDIR
> # lambda, mu, gamma (gamma = (1 - nu) rho g / mu)
> 30 30 8.33e-4
> # time interval, (positive time step) or (negative skip, scaling)
> 1 1 1
> # number of observation planes
> 0
> # number of observation points
> 0
> # number of stress observation segments
> 0
> # number of prestress interfaces
> 0
> # number of linear viscous interfaces
> 0
> # number of nonlinear viscous interfaces
> 0
> # number of fault creep interfaces
> 0
> # number of inter-seismic strike-slip segments
> 0
> # number of inter-seismic tensile segments
> 0
> # number of events
> 1
> # number of coseismic strike-slip segments
> 0
> # number of coseismic tensile segments
> 0
> # number of coseismic dilatation point sources
> 0
> # number of surface loads
> 1
> # nb x1 x2 length width t3 T phi
> 1 0 0 3.937 3.937 20 0 0
> EOF
>
>
> On Tue, Jan 19, 2016 at 11:20 PM, Sylvain Barbot <sylbar.vainbot at gmail.com>
> wrote:
>>
>> 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
>> >> >
>> >> >
>> >
>> >
>
>