fzanalyzer

Analysis of fracture zones using crossing profiles

Synopsis

gmt fzanalyzer crossprofiles -Ffzlines [ -Amin/max/inc ] [ -Cmin/max/inc ] [ -Dcorrwidth ] [ -IFZ[/profile] ] [ -S[b|c]] [ -Tprefix ] [ -V[level] ] [ -Wmin/max/inc ] [ -bobinary ] [ -donodata[+ccol] ] [ -iflags ] [ -oflags ] [ --PAR=value ]

Note: No space is allowed between the option flag and the associated arguments.

Description

fzanalyzer is a tool developed as part of the Global Seafloor Fabric and Magnetic Lineation Project [see GSFML for a full description of the project]. It reads processed fracture zone (FZ) traces and cross-profiles as produced by grdtrack. It then analyzes the trace of each FZ by examining cross-sections orthogonal to the FZ trend and modeling these profiles using a blend model of “Atlantic”-style symmetric troughs and “Pacific”-style asymmetric, dipole-like anomalies, modulated with some peripheral bulges (“compression”). We also fit just the symmetric trough model and examine the empirical data minimum and trough width. Estimates are made of the width of the data trough and 1-sigma uncertainties on the best FZ locations given by the various models. We also compute several statistical measures and return all the model parameters as a function of distances along each FZ.

Required Arguments

crossprofiles

This file is a table with cross-profiles as produced by grdtrack -C from an approximate digitized trace (with lon, lat) of one or more FZs. This is an ASCII (or binary, see -bi) file that must contain 7 data columns: lon, lat, dist, azimuth, vgg, age, fzdist.

-Ffzlines

Here, fzlines is a file with resampled track lines obtained by running grdtrack -D. As for crossprofiles the file must contain the same 7 data columns lon, lat, dist, azimuth, vgg, age, fzdist. See Input Files for more details.

Optional Arguments

-Amin/max/inc

Specifies one or three parameters that control how the blending of the model signals will be done. Here, min is the minimum asymmetry value [0, i.e., “Atlantic” symmetric trough], max is the maximum asymmetry [1], i.e., “Pacific” dipole signal], and inc is the increment used for the search [0.05]. To specify just a single asymmetry value (no search), just provide the single desired asymmetry.

-Cmin/max/inc

Specifies one or three parameters that control how the search for the “compression” model signal will be done. Here, min is the minimum compression value [0], max is the maximum compression [1, i.e., “Mexican Hat” end-member], and inc is the increment used for the search [0.05]. To specify just a single compression value (no search), just provide the single desired compression value..

-Dcorrwidth

Specifies a corrwidth (in km) that sets the width of the central corridor [25]. The purpose of this corridor is to constrain how far off center we may seek to relocate the location of the FZ trough.

-IFZ[/profile]

By default, we will analyze the cross-profiles generated for all FZs. However, you can use -I to specify a particular FZ id id (first id is 0). Optionally, you can select that only one profile from that FZ be processed [Default is all]. Note that the output files will still contain all profiles but derived quantities will be zero except for the chosen profiles.

-S[b|c]

Output the parameters set by the command-line options in a format suitable for inclusion in a Bourne/bash shell script. Alternatively, append c for csh/tcsh syntax.

-Tprefix

Sets the file name prefix used for all output files [fztrack].

-V[level]

Select verbosity level [w]. (See full description) (See technical reference).

-Wmin/max/inc

Specifies three parameters that control how the modeling of the cross-FZ signal will be done. Here, min is the minimum FZ signal width (in km) for a nonlinear width search [1], max is the maximum width [50], and inc is the increment used for the width search [1]. It is recommended to determine suitable limits of a particular region and tune -W accordingly. Selecting too wide a range might lead to spurious fits driven by data features not associated with the FZ trough.

-borecord[+b|l] (more …)

Select native binary format for table output.

-donodata[+ccol] (more …)

Replace output columns that equal NaN with nodata.

-icols[+l][+ddivisor][+sscale|d|k][+ooffset][,…][,t[word]] (more …)

Select input columns and transformations (0 is first column, t is trailing text, append word to read one word only).

-ocols[+l][+ddivisor][+sscale|d|k][+ooffset][,…][,t[word]] (more …)

Select output columns and transformations (0 is first column, t is trailing text, append word to write one word only).

-q[i|o][~]rows|limits[+ccol][+a|t|s] (more …)

Select input or output rows or data limit(s) [all].

-^ or just -

Print a short message about the syntax of the command, then exit (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 exit.

-? or no arguments

Print a complete usage (help) message, including the explanation of all options, then exit.

--PAR=value

Temporarily override a GMT default setting; repeatable. See gmt.conf for parameters.

Input Files

The two input files are themselves generated by running grdtrack first. This step requires a set of digitized FZ tracks (lon, lat) and three data grids: (a) a VGG vertical gravity gradient file, (b) a 2 minute crustal age grid, and (c) a grid with distance to the nearest FZ in km, listed in that order. The critical file is the VGG grid. If you don’t have or care about ages and distances you can make dummy grids that are all NaNs. You design your cross-profile layout and resampled FZ trackes using grdtrack options -C and -D..

Nearest Fracture Zone Distances

You can use grdmath to create the nearest fracture zone distance grid (in km) required to prepare the profiles. A typical command suitable for the Nazca plate area might be:

gmt grdmath -R-120/-65/-50/5 -I5m -fg digitize.txt LDIST DEG2KM = dist2fz.nc

Since this is a very slow calculation for numerous FZs it is not necessary to use a very high resolution in -I since the distances change smoothly and interpolation will be approximately correct. Consider making a global grid but do it in quadrants (or smaller region chunks) and run concurrently on a multi-core computer. For example, to make a global grid from quadrants, one may run:

gmt grdmath -R0/180/0/90 -I2m -fg -V3 global_FZ.txt LDIST DEG2KM = WN.nc
gmt grdmath -R180/360/0/90 -I2m -fg -V3 FZ_KM.txt LDIST DEG2KM = EN.nc
gmt grdmath -R0/180/-90/0 -I2m -fg -V3 FZ_KM.txt LDIST DEG2KM = WS.nc
gmt grdmath -R180/360/-90/0 -I2m -fg -V3 FZ_KM.txt LDIST DEG2KM = ES.nc

then blend these together into a global grid with:

gmt grdblend -Rg -I2 -fg EN.nc WN.nc ES.nc WS.nc -Gdist2FZ.nc -V
rm -f WN.nc EN.nc WS.nc ES.nc

To make NaN grids for ages and/or distances for the Nazca area, use:

gmt grdmath -R-120/-65/-50/5 -I5m -fg 0 0 NaN = ages.nc

Output Files

fzanalyzer can produce up to three output files; these are described below:

  1. File prefix_analysis.txt contains the results of the analysis for each cross profile. There 61 output columns containing the fitted or observed values (see Determined Parameters). This file is used by fzblender to produce a smooth and optimal fit to the fracture zone.

  2. File prefix_cross.txt contains both observed and predicted best-fitting models for each cross profile. It can be used for plotting and visual analysis of the results on a profile-by-profile basis.

  3. The prefix_par.[c]sh is either a Bourne (-S) or cshell (-Sc) script that contains all parameters specified by the command line as shell variables. You can include this script in custom mapping or analysis scripts and use the variables as you see fit.

  4. Finally, while not an output file from fzanalyzer, you should use the name prefix_resampled.txt for the output of grdtrack -D as that is what the scripts for plotting expect.

Examples

To analyze digitized FZs we use the Sandwell/Smith VGG (1 minute vertical gravity gradient @earth_vgg_01m), a 1-minute age grid (@earth_ages_01m), and a nearest-FZ distance grid (in km). Given the potential FZ locations in the multi-segment file fz_digitized.txt, we specify a 40 km cross-profile length, with profiles spaced every 5 km, and use an along-cross-profile sampling interval of 2 km, by running:

gmt grdtrack fz_digitized.txt -C40k/2k/5k -G@earth_vgg_01m -G@earth_ages_01m -Gdist.nc -Dtraces_resampled.txt -fg --FORMAT_GEO_OUT=ddd.xxxx --FORMAT_FLOAT_OUT=%.1f > xprofiles.txt

These two data tables can now be used with fzanalyzer to analyze the traces. Here, we specify a 20 km central corridor and accept default values for most settings:

gmt fzanalyzer xprofiles.txt -D20 -Ftraces_resampled.txt -Ttraces -S --FORMAT_GEO_OUT=ddd.xxxx --FORMAT_FLOAT_OUT=%.1f

You can then make plots of these cross-profiles with best-fitting curves and parameters indicated by using:

fzprofiler traces -W6i -H2i -N2

which will plot all cross-profiles in separate 6x2 inch cross-sections stacked in one vertical panel. You can show this information in map view via:

fzmapper traces -W9i -L1 -Ffz_digitized.txt

See the fzprofiler and fzmapper documentation for further details.

Determined Parameters

Here are the header codes for each of the 61 output columns and what they represent:

  • XR: Longitude of raw digitized trace.

  • YR: Latitude of raw digitized trace.

  • DR: Distance at digitized points along raw digitized trace.

  • AR: Azimuth at digitized points along raw digitized trace.

  • ZR: Data value at digitizing locations

  • TL: Crustal age estimate at left side of FZ (negative distances).

  • TR: Crustal age estimate at right side of FZ (positive distances).

  • SD: Shift of data minimum (in km) from raw line origin.

  • ST: Shift of trough model minimum (in km) from raw line origin.

  • SB: Shift of blend model minimum (in km) from raw line origin.

  • SE: Shift of blend model maximum slope (in km) from raw line origin.

  • BL: Best blend value [0-1].

  • OR: Orientation of blend model profile (-1 means left side is old, +1 means left side is young).

  • WD: Width of data trough.

  • WT: Width of model trough (for trough model).

  • WB: Width of model trough (for blend model).

  • AD: Peak-to-trough amplitude from data.

  • AT: Peak-to-trough amplitude from model (for trough model).

  • AB: Peak-to-trough amplitude from model (blend).

  • UT: Compression indicator (for trough model).

  • UB: Compression indicator (for blend model).

  • VT: Variance reduction (%) from model (for trough model).

  • VB: Variance reduction (%) from model (for blend model).

  • FT: F-statistic (for trough model).

  • FB: F-statistic (for blend model).

  • XDL: Longitude of data minimum left bounds.

  • XD0: Longitude of data minimum.

  • XDR: Longitude of data minimum right bounds.

  • YDL: Latitude of data minimum left bounds.

  • YD0: Latitude of data minimum.

  • YDR: Latitude of data minimum right bounds.

  • ZDL: Value of data minimum left bounds.

  • ZD0: Value of data minimum.

  • ZDR: Value of data minimum right bounds.

  • XTL: Longitude of minimum (for trough model) left bounds.

  • XT0: Longitude of minimum (for trough model).

  • XTR: Longitude of minimum (for trough model) right bounds.

  • YTL: Latitude of minimum (for trough model) left bounds.

  • YT0: Latitude of minimum (for trough model).

  • YTR: Latitude of minimum (for trough model) right bounds.

  • ZTL: Model prediction (for trough model) at left bounds.

  • ZT0: Model prediction minimum (for trough model).

  • ZTR: Model prediction (for trough model) at right bounds.

  • XBL: Longitude of minimum (for blend model) left bounds.

  • XB0: Longitude of minimum (for blend model).

  • XBR: Longitude of minimum (for blend model) right bounds.

  • YBL: Latitude of minimum (for blend model) left bounds.

  • YB0: Latitude of minimum (for blend model).

  • YBR: Latitude of minimum (for blend model) right bounds.

  • ZBL: Model prediction (for blend model) at left bounds.

  • ZB0: Model prediction minimum (for blend model).

  • ZBR: Model prediction (for blend model) at right bounds.

  • XEL: Longitude of maximum slope (for blend model) left bounds.

  • XE0: Longitude of maximum slope (for blend model).

  • XER: Longitude of maximum slope (for blend model) right bounds.

  • YEL: Latitude of maximum slope (for blend model) left bounds.

  • YE0: Latitude of maximum slope (for blend model).

  • YER: Latitude of maximum slope (for blend model) right bounds.

  • ZEL: Model prediction at maximum slope (for blend model) at left bounds.

  • ZE0: Model prediction at maximum slope (for blend model).

  • ZER: Model prediction at maximum slope (for blend model) at right bounds.

See Also

gmt fzblender, fzinformer, fzmapper, fzmodeler, fzprofiler, grdmath, grdtrack, mlconverter

References

Wessel, P., Matthews, K. J., Müller, R. D., Mazzoni, A., Whittaker, J. M., Myhill, R., Chandler, M. T., 2015, “Semiautomatic fracture zone tracking”, Geochem. Geophys. Geosyst., 16 (7), 2462–2472. https://doi.org/10.1002/2015GC005853.