[IMRight]

when a plate moves there is more than one side

Sure... but that doesn't mean the whole plate moves. If it did... you would notice more than one earthquake.

Also, I think it was the Indian Ocean side of the fault that slipped - different plate.

[WildTurkey]

Sure... but that doesn't mean the whole plate moves. If it did... you would notice more than one earthquake.

The underlying plate (compression) moves under the upper VERY slowly building up forces on the upper plate.

Also, I think it was the Indian Ocean side of the fault that slipped - different plate.

I think you are correct from my memory. When it 'slipped', the mass 'fell' causing a water displacement.