In this article there's a photo that was taken of the CBD during the quake which shows rising dust plumes due to the "slapdown effect"...also called a "trampoline effect". nzhistory.govt.nz/culture/february-2011-christchurch-earthquake
Convection currents within the earth's mantle. If you've learnt physics, The hotter magma near the core rises up and then cools down, which moves to the side before sinking. Whichever direction the magma goes before cooling will affect the direction of the plate's direction.
Mantle convection currents are only one of three drivers of plate tectonic movement. The other two are plate push at spreading ridge systems and plate pull down at subduction zones.
