Published by the American Geophysical Union as part of the Geodynamics Series, Volume 30.

An important change in ideas about plate tectonics has been the recognition that the boundaries between plates are often broad zones of deformation, rather than the narrow boundaries originally assumed in plate tectonic theory. Hence, although it used to be common to straddle the San Andreas fault and view one leg as being on the Pacific plate and the other in North America, it is now recognized that such a straddle would require a 1500-km leg span across the plate boundary zone extending across much of western North America.