NOTE: As this blog is now inactive, the data & this post have been migrated to https://wiki.paleoearthlabs.org/scientificdata/platetectonics/birdplateboundaries
GMT5’s psxy offers a nice feature which lets the user designate fronts on a line segment basis – for example if you have a number of normal faults and subduction zone line segments in a multi-segment file, you can give each line a different symbol, depending on to which side the fault dips (left or right in line direction).
I have taken Peter Bird’s plate boundary file and added these plotting instructions to the indivdual plate boundary line segment headers, for example:
> JF\NA by Peter Bird 1999
for the Juan de Fuca plate subducting under NorthAmerica. With the new header, psxy will now plot a triangle (+t) with a 3pt size (3p) to the right side (+r) of the front, fronts being separated by 0.45cm. And this is how it looks on a Robinson map centered on the dateline, generated with:
pscoast -Rg -JN180/22 -Dl -A5000 -V -G200 -K -Y4 > Bird_PlateBoundaries.ps
psxy -J -R PB2002_boundaries.gmt -W0.5p,red -O -Sf0.25/3p -Gred -Ba30:."Bird's 2003 Plate Boundaries" >> Bird_PlateBoundaries.ps
Here’s the relevant snippet from the psxy man page for the use of -Sf in segment headers:
-Sf front. -Sfgap/size[+l|+r][+b+c+f+s+t][+ooffset]. Supply distance gap between symbols and symbol size.
If gap is negative, it is interpreted to mean the number of symbols along the front instead. Append +l
or BD+r) to plot symbols on the left or right side of the front [Default is centered]. Append +type to
specify which symbol to plot: box, circle, fault, slip, or triangle. [Default is fault]. Slip means
left-lateral or right-lateral strike-slip arrows (centered is not an option). Append +ooffset to offset
the first symbol from the beginning of the front by that amount . Note: By placing -Sf options in the
segment header you can change the front types on a segment-by-segment basis.