Oh, so you meant those locations ON the Pacific plate are moving relative to the Pacific plate as a whole. Gotcha.

Yes, I can see that in the GPS data. All three stations are moving, but relatively to each other, if we assume Honolulu as the fixed point, the other stations are moving south-east. Or looked at another way, Honolulu is moving FASTER and leaving the others behind.

But I don't see the plates as rigid. Of course they are deforming. After all, look at how much they deform adjacent to the plate boundaries. For example, the subducting slab bends quite a bit before it dives. OK, it bends, but it should also be able to stretch and compress. (I'm thinking long range elastic deformity, drawing from elastic rebound theory.)

I think the GPS data shows exactly that by the relative motions of stations across any plate. It's very apparent in the western USA with it's dense GPS network.

As such, I think it's perfectly possible for there to be 20mm of relative motion between Honolulu and San Nicholas. After all, we're not talking 20mm across a fault line, but across a good chuck of the plate - about 4100km.

As for the reference point, there's no one physical location. They use the IGS08 reference frame, which appears to be tied to the International Terrestrial Reference Frame. I couldn't find exact details quickly, but if it's anything like other reference frames I'm a little familiar with in astronomy, it's probably fixed to astronomical phenomenon and not a point on the Earth, other than being centered on the center of the Earth. In that way, every location on or near the surface of the Earth can move and there's still a reference.

Brian