Interpolated crustal strain rates based on the 20091130142023 release
of the PBO GPS velocities, merged with the compilation by Kreemer and
Hammond (Geology, 2007). A high quality selection of data was used and
processed via standard Generic Mapping Tools software (surface,
grdfilter, grdgradient). The procedure follows that used in Platt et
al. (EPSL, 2008), see that paper and Platt and Becker (submitted,
2010; http://geodynamics.usc.edu/~becker/preprints/pb10.pdf) for
details. 

File strain1.dat is the first of Dave's ASCII format's, i.e. 

lon lat exx eyy exy 

where x,y for East and West components, respectively, and eii
strain-rates, with units are 10^{-15} 1/s. The maximum shear stress as
computed from the eigenvalues e1 and e2, (e1-e2)/2, should look like
the map of the included PDF.

Thorsten Becker, March 8, 2010