North of Mendocino and far offshore is the spreading center between the Pacific and Juan de Fuca plates. A spreading center is where volcanic eruptions/volcanic rock fill in the space problem from 2 plates moving apart.

Not sure what you mean by "contribute to the pressure", but I think I know: if you mean the M4 or 5 quakes within the Juna de Fuca plate; no, these would not have any significant effect on motion of the Pacific plate. As for the spreading center, I am told by people who should know (Tanya Atwater for example) that it is the subducting slab that drag the plate with it. The spreading center in that interpretation does not push very hard on the plate, if at all.
Chris