One of the long-standing controversies over the past several decades has been focused on
how strain is distributed across the North America-Pacific plate boundary in California. While the
major boundary, the San Andreas Fault, has been long recognized, deformation models using
geologic data disagree with each other [ Bird and Rosenstock, 1984; Weldon and Humphreys,
1986]. Unfortunately, geodetic measurements from the mid-1980's did not clarify the issue. The
framework in which both geodetic and geologic measurements were compared
was the RM2 (Relative Motion 2) plate
model [ Minster and Jordan, 1978]. This model predicted relative
motion between the North American and Pacific plates of 
mm/yr at an azimuth of 
(model computed for Central California). Even taking into account Basin and Range extension,
measured crustal deformation could not accommodate 56 mm/yr of relative motion. When GPS
measurements began in 1985, the question of this ``missing motion'' was being investigated
through geodetic means. One test of the global plate models was to estimate geodetic spreading
rates across the Gulf of California. Using GPS observations spanning 3.5 years, Dixon et al.
[1991] estimate a plate rate of 47
7 mm/yr at an azimuth of 
. Their contemporary
measurements agree within a standard deviation with the NUVEL1-A global plate prediction of
mm/yr at 
[ DeMets et al., 1990]. For
this crustal deformation experiment, all geodetic
sites were 50-200 km from the plate boundary.