Now you are getting into the nitty-gritty of research problems. These are the kind of
questions that make one think!!
Actually, subduction is not occurring under So. California at this time. A portion of the
Pacific has subducted many years ago. When this happened portions of the
surface (and islands) were actually scraped off the top and make up much
of the Southern California terrain today.
At this time, however, the region of Southern California along the San Andreas
is either deforming vertically and no movement is occurring or it is sliding
horizontally along the San Andreas getting out of the way of the Pacific plate.

Once the East Pacific Rise had subducted, a lot of thing changed in north America.
Along a mid-ocean rise lavas are extruded which form new ocean on either
side of the rise feature. This tends to push the previous material away. Since
the rise is now to the east of So. California, the Pacific in this region (underlying
North America) is actually spreading to the west, not to the east which would
cause subduction.

The motion of the North American plate is the result of motion caused by spreading
at the Mid-Atlantic ridge, subduction of the Pacific along the Central American
subduction zone, slipping along the San Andreas etc. so the net motion of
North America appears to be a rotation around a euler pole in the North Atlantic.

Anyway, as you can see, this can get very complicated very fast. Take a look
at some of the web sites Canie suggested for more details.

N