Step 17: Choose a velocity reference frame

Since the 1960's, geologists have understood that absolute motion is meaningless, and only relative motion is measurable. It makes no physical difference* which velocity reference frame is chosen, as long as the choice is explicit, and is applied consistently across all programs. The Shells package of programs recognizes this by requiring you to choose a two-letter plate code for the plate which will be fixed in your dynamic calculations and your graphics.  (Within Shells this 2-character name is referred to as the variable named pltRef.)  Select one of the codes from the map below, and enter this at the beginning of line 13 of the parameter input file (sample parameter input file: iEarth5-049.in).

[*One exception: If you use type-3 boundary conditions (explained on the next page) to describe subduction zones, results will depend on the choice of pltRef.  I recommend that you either avoid type-3 boundary conditions, or else choose a plate like pltRef = “AF” which approximates the hot-spot reference frame.]