Provided by: gmt-common_5.4.3+dfsg-1_all 
NAME
grdflexure - Compute flexural deformation of 3-D surfaces for various rheologies
SYNOPSIS
grdflexure topogrd -Drm/rl[/ri]/rw -ETe[u] -Goutgrid [ -ANx/Ny/Nxy ] [ -Cppoisson ] [ -CyYoung ] [
-Fnu_a[/h_a/nu_m] ] [ -Llist ] [ -N[f|q|s|nx/ny][+a|d|h|l][+e|n|m][+twidth][+w[suffix]][+z[p]] [
-Sbeta ] [ -Tt0[u][/t1[u]/dt[u]|file] |n][+l] ] [ -V[level] ] [ -Wwd] [ -Zzm] [ -fg ]
Note: No space is allowed between the option flag and the associated arguments.
DESCRIPTION
grdflexure computes the flexural response to loads using a range of user-selectable rheologies. User may
select from elastic, viscoelastic, or firmoviscous (with one or two viscous layers). Temporal evolution
can also be modeled by providing incremental load grids and specifying a range of model output times.
REQUIRED ARGUMENTS
topogrd
2-D binary grid file with the topography of the load (in meters); See GRID FILE FORMATS below. If
-T is used, topogrd may be a filename template with a floating point format (C syntax) and a
different load file name will be set and loaded for each time step. The load times thus coincide
with the times given via -T (but not all times need to have a corresponding file). Alternatively,
give topogrd as =flist, where flist is an ASCII table with one topogrd filename and load time per
record. These load times can be different from the evaluation times given via -T. For load time
format, see -T.
-Drm/rl[/ri]/rw
Sets density for mantle, load, infill (optional, otherwise it is assumed to equal the load
density), and water or air. If ri differs from rl then an approximate solution will be found. If
ri is not given then it defaults to rl.
-ETe Sets the elastic plate thickness (in meter); append k for km. If the elastic thickness exceeds
1e10 it will be interpreted as a flexural rigidity D (by default D is computed from Te, Young’s
modulus, and Poisson’s ratio; see -C to change these values).
-Goutfile
If -T is set then grdfile must be a filename template that contains a floating point format (C
syntax). If the filename template also contains either %s (for unit name) or %c (for unit letter)
then we use the corresponding time (in units specified in -T) to generate the individual file
names, otherwise we use time in years with no unit.
OPTIONAL ARGUMENTS
-ANx/Ny/Nxy
Specify in-plane compressional or extensional forces in the x- and y-directions, as well as any
shear force [no in-plane forces]. Compression is indicated by negative values, while extensional
forces are specified using positive values.
-Cppoisson
Change the current value of Poisson’s ratio [0.25].
-CyYoung
Change the current value of Young’s modulus [7.0e10 N/m^2].
-Fnu_a[/h_a/nu_m]
Specify a firmoviscous model in conjunction with an elastic plate thickness specified via -E.
Just give one viscosity (nu_a) for an elastic plate over a viscous half-space, or also append the
thickness of the asthenosphere (h_a) and the lower mantle viscosity (nu_m), with the first
viscosity now being that of the asthenosphere. Give viscosities in Pa*s. If used, give the
thickness of the asthenosphere in meter; append k for km.
-N[a|f|m|r|s|nx/ny][+a|[+d|h|l][+e|n|m][+twidth][+v][+w[suffix]][+z[p]]
Choose or inquire about suitable grid dimensions for FFT and set optional parameters. Control the
FFT dimension:
-Na lets the FFT select dimensions yielding the most accurate result.
-Nf will force the FFT to use the actual dimensions of the data.
-Nm lets the FFT select dimensions using the least work memory.
-Nr lets the FFT select dimensions yielding the most rapid calculation.
-Ns will present a list of optional dimensions, then exit.
-Nnx/ny will do FFT on array size nx/ny (must be >= grid file size). Default chooses dimensions
>= data which optimize speed and accuracy of FFT. If FFT dimensions > grid file dimensions,
data are extended and tapered to zero.
Control detrending of data: Append modifiers for removing a linear trend:
+d: Detrend data, i.e. remove best-fitting linear trend [Default].
+a: Only remove mean value.
+h: Only remove mid value, i.e. 0.5 * (max + min).
+l: Leave data alone.
Control extension and tapering of data: Use modifiers to control how the extension and tapering
are to be performed:
+e extends the grid by imposing edge-point symmetry [Default],
+m extends the grid by imposing edge mirror symmetry
+n turns off data extension.
Tapering is performed from the data edge to the FFT grid edge [100%]. Change this percentage
via +twidth. When +n is in effect, the tapering is applied instead to the data margins as no
extension is available [0%].
Control messages being reported: +v will report suitable dimensions during processing.
Control writing of temporary results: For detailed investigation you can write the intermediate
grid being passed to the forward FFT; this is likely to have been detrended, extended by
point-symmetry along all edges, and tapered. Append +w[suffix] from which output file name(s) will
be created (i.e., ingrid_prefix.ext) [tapered], where ext is your file extension. Finally, you may
save the complex grid produced by the forward FFT by appending +z. By default we write the real
and imaginary components to ingrid_real.ext and ingrid_imag.ext. Append p to save instead the
polar form of magnitude and phase to files ingrid_mag.ext and ingrid_phase.ext.
-Llist Write the names and evaluation times of all grids that were created to the text file list.
Requires -T.
-Mtm Specify a viscoelastic model in conjunction with an elastic plate thickness specified via -E.
Append the Maxwell time tm for the viscoelastic model (in ).
-Sbeta Specify a starved moat fraction in the 0-1 range, where 1 means the moat is fully filled with
material of density ri while 0 means it is only filled with material of density rw (i.e., just
water) [1].
-Tt0[u][/t1[u]/dt[u]|file]|n][+l]
Specify t0, t1, and time increment (dt) for sequence of calculations [Default is one step, with no
time dependency]. For a single specific time, just give start time t0. The unit is years; append
k for kyr and M for Myr. For a logarithmic time scale, append +l and specify n steps instead of
dt. Alternatively, give a file with the desired times in the first column (these times may have
individual units appended, otherwise we assume year). We then write a separate model grid file
for each given time step.
-Wwd Set reference depth to the undeformed flexed surface in m [0]. Append k to indicate km.
-Zzm Specify reference depth to flexed surface (e.g., Moho) in m; append k for km. Must be positive.
[0].
-V[level] (more …)
Select verbosity level [c].
-fg Geographic grids (dimensions of longitude, latitude) will be converted to meters via a “Flat
Earth” approximation using the current ellipsoid parameters.
-^ or just -
Print a short message about the syntax of the command, then exits (NOTE: on Windows just use -).
-+ or just +
Print an extensive usage (help) message, including the explanation of any module-specific option
(but not the GMT common options), then exits.
-? or no arguments
Print a complete usage (help) message, including the explanation of all options, then exits.
GRID FILE FORMATS
By default GMT writes out grid as single precision floats in a COARDS-complaint netCDF file format.
However, GMT is able to produce grid files in many other commonly used grid file formats and also
facilitates so called “packing” of grids, writing out floating point data as 1- or 2-byte integers. (more
…)
GRID DISTANCE UNITS
If the grid does not have meter as the horizontal unit, append +uunit to the input file name to convert
from the specified unit to meter. If your grid is geographic, convert distances to meters by supplying
-fg instead.
CONSIDERATIONS
netCDF COARDS grids will automatically be recognized as geographic. For other grids geographical grids
were you want to convert degrees into meters, select -fg. If the data are close to either pole, you
should consider projecting the grid file onto a rectangular coordinate system using grdproject.
PLATE FLEXURE NOTES
The FFT solution to plate flexure requires the infill density to equal the load density. This is
typically only true directly beneath the load; beyond the load the infill tends to be lower-density
sediments or even water (or air). Wessel [2001, 2016] proposed an approximation that allows for the
specification of an infill density different from the load density while still allowing for an FFT
solution. Basically, the plate flexure is solved for using the infill density as the effective load
density but the amplitudes are adjusted by the factor A = sqrt ((rm - ri)/(rm - rl)), which is the
theoretical difference in amplitude due to a point load using the two different load densities. The
approximation is very good but breaks down for large loads on weak plates, a fairy uncommon situation.
EXAMPLES
To compute elastic plate flexure from the load topo.nc, for a 10 km thick plate with typical densities,
try
gmt grdflexure topo.nc -Gflex.nc -E10k -D2700/3300/1035
To compute the firmoviscous response to a series of incremental loads given by file name and load time in
the table l.lis at the single time 1 Ma using the specified rheological values, try
gmt grdflexure -T1M =l.lis -D3300/2800/2800/1000 -E5k -Gflx/smt_fv_%03.1f_%s.nc -F2e20 -Nf+a
REFERENCES
Cathles, L. M., 1975, The viscosity of the earth’s mantle, Princeton University Press.
Wessel. P., 2001, Global distribution of seamounts inferred from gridded Geosat/ERS-1 altimetry, J.
Geophys. Res., 106(B9), 19,431-19,441, http://dx.doi.org/10.1029/2000JB000083.
Wessel, P., 2016, Regional–residual separation of bathymetry and revised estimates of Hawaii plume flux,
Geophys. J. Int., 204(2), 932-947, http://dx.doi.org/10.1093/gji/ggv472.
SEE ALSO
gmt, grdfft, gravfft grdmath, grdproject, grdseamount
COPYRIGHT
2018, P. Wessel, W. H. F. Smith, R. Scharroo, J. Luis, and F. Wobbe