[CIG-SHORT] questions about RELAX

Tue Feb 23 14:16:26 PST 2016

Thank you very much for your answer.

When dealing with a very simple fault, I chose an origin point with coordinates (lon/lat) -90.6648/14.8260. This would be a 300 km fault with a left superior tip at x1 = 0, x2 = -150 km, x3 = 0, and a right superior tip at x1 = 0, x2 =+150 km and x3 = 0. The lon/lat coordinates of these two points obtained with gmt mapproject are  -92.05676406/14.80200819 and -89.27035597/14.84166050. The fault trace in my map was drawn using the geographical coordinates of these three reference points. My input for relax looks like this:

# grid size (sx1,sx2,sx3)
32 32 32
# sampling size (in unit of length), smoothing (0-0.5) & nyquist (dx1,dx2,dx3,beta,nq)
20000 20000 20000 0.2 2
# origin position & rotation
0 0 0
# geographic origin (longitude and latitude), UTM zone and real length unit (usually m or km)
-90.6648 14.8260 16 1
# observation depth for displacement and for stress (stress in only exported in GRD)
0 0
# output directory (all output written here)
$WDIR
# elastic parameters and gamma = (1-nu) rho g / mu = 8.33e-7 /m = 8.33e-4 /km
5.091e10 40e9 4.76e-10
# integration time (in unit of time), step (negative for automatic) and scaling of computed value
40 1
# number of observation planes
0
# no x1 x2 x3 length width strike dip
#   1  0 -3  0      6     6     90  90
# number of observation points
0
# number of Coulomb patches
0
# number of prestress interfaces
0
# number of linear viscous interfaces (where viscosity changes)
1
# no depth gammadot0 cohesion (gammadot0 is shear modulus divided by viscosity)
1   25000   0.126144    0.0
# number of linear ductile zones
0
# number of powerlaw viscous interfaces
0
# number of friction faults
0
# number of interseismic loading strike-slip and opening
0
0
# number of coseismic events (when slip distribution is prescribed)
1
# number of shear dislocations (strike-slip and dip-slip faulting)
1
# no  slip    x1     x2      x3  length width strike dip rake
001   1.59    0 -150000  0  300000 15000 90 90 0
# number of tensile cracks
0
# number of dilatation sources
0
# number of surface traction
0
EOF
___

The results of the coseismic displacements are shown in the ps file. The fault I will work with has a much more complicated shape and I didn't noticed there was a problem with the coordinates until I started paying more attention to the coseismic to look for errors and then I tried to confirm the problem using the fault I'm showing you here. I still can't find where does the shift come from. Thanks again,

Beatriz

_____________________________________
From: Sylvain Barbot <sylbar.vainbot at gmail.com>
Sent: Monday, February 22, 2016 6:28 PM
To: Beatriz COSENZA MURALLES
Cc: cig-short at geodynamics.org
Subject: Re: questions about RELAX

Dera Beatriz,

1. The simplest way to go about coordinates is to locate all the items
relevant to your computation in a local system of coordinates. If you
take the epicenter 15.32°N 89.10°W as a reference coordinate, you can
convert it to UTM as follows

echo -89.10 15.32 | proj +proj=utm +zone=16
274529.84       1694811.97

Then everything that comes your way can be converted UTM relative to
this point. Then, keep in mind that the Relax coordinate system is
x1,x2,x3 where x1 is north, x2 is east and x3 is depth. So a
transformation following (x1-1694811.97), (x2-274529.84) is in order.

2. The simulation for an elastic layer over a viscoelastic substrate
is presumably correct. And your parameterization is presumably fine,
too. So if it doesn't produce the results intended, you are probably
learning that the geometry is not adequate. An elastic layer over a
viscoelastic half space is only relevant for the oceanic lithosphere.
In the backarc of Guatemala, you are in a continental lithosphere,
even though the details may be more complicated than that. So you can
expect a viscoelastic deformation in the lower crust and another one
in the asthenosphere. In this system, afterslip may also play an
important role.

Best,
Sylvain

On Tue, Feb 23, 2016 at 5:04 AM, Beatriz COSENZA MURALLES
<cosenzamural at wisc.edu> wrote:
> Dear Mr. Barbot,
>
>
> I am a first year geophysics grad student at the Department of Geoscience of
> UW Madison. I am using RELAX to obtain the viscoelastic response of the
> Motagua 1976 earthquake (Guatemala) and I have encountering a few problems
> regarding two issues:
>
>
> 1. Coordinates: I am using 9 fault patches. All of them are strike-slip
> vertical faults. I have the location of these patches in geological
> coordinates, so I chose a point of reference to be the origin of the
> coordinate system, which I gave to RELAX, and I obtained the relative
> positions of everything else converting the geological coordinates to utm
> (zone 16) and calculating the distances in km. The direction of the
> coseismic displacements (files 000) seemed odd, so I tried to run RELAX with
> a simpler one-patch 300 km strike-slip fault and I confirmed that the
> displacements are shifted. The place where the fault should be is moved
> upwards and to the right. I wonder if you have some ideas on what could I be
> doing wrong to get this output.
>
>
> 2. The magnitude of the viscoelastic response: I ran the software using a
> viscoelastic layer that starts at a depth of 25 km, with a Maxwell time of
> about 7.9 years (gamma dot of 0.126). I wanted yearly outputs, for the last
> 40 years, so I wrote down for the integration time "40 1". I used the output
> from the 039 and the 040 files to obtain the increment of the last year and
> I'm seeing displacements of more than 2 mm for that last year, which is too
> large and is not being picked up by gps stations, on the contrary, I would
> expect the viscoelastic transient to be negligible by now. I can (and will)
> run for viscoelastic layers with larger viscosities, but it seems odd that
> the model produces such large displacement values after 40 years, so I might
> be misinterpreting something. I am assuming that the output is in meters (as
> is the input) and that the integration time is given in years.
>
>
> Thank you in advance for your help.
>
>
> Sincerely,
>
>
> Beatriz Cosenza
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