Step 25: Choose a velocity reference frame

The primary quantities that NeoKinema will calculate are the long-term-average horizontal velocity vectors for all the nodes in your .feg file.
Many other interesting variables will also be derived from these (fault offset rates, strain-rates, stress-directions, interseismic velocities and strain-rates, …)
but it is the horizontal components of long-term-average velocity which are primary.

Horizontal velocities on the surface of a planet are not even meaningful unless they are accompanied by a definition of some rigid framework which is treated, by convention, as “not moving.”
All geodesists and tectonophysicists understand that this choice is somewhat arbitrary; still it cannot be avoided!
The name of the rigid framework is known (generically) as the “velocity reference frame”.
Specifically, NeoKinema requires that your choice of velocity reference frame must be one of the 52 plates of the PB2002 model of Bird [2003],
which are mapped in the color figure below.

If you created a GPS-velocity (.gps) file in Step 16, then probably your choice is already made;
the velocity reference frame for your NeoKinema model should be the same as the velocity reference frame for your GPS-velocity data.

If you are NOT using GPS data in your NeoKinema model, then the choice of reference frame is free.
However, most readers will find your results easier to understand if the rigid plate defining your velocity reference frame is either inside your model area
(defined by your .feg file), or close to it.  Velocity frames located on the other side of the planet are mathematically valid, but confusing.

In the end, choose one of the 2-letter plate abbreviations from the map below (or from Bird [2003]),
and enter it in line #22 of your NeoKinema-parameter file
(which we began to build in Step 8, based on model file parameters_for_NeoKinema.nki.txt).