In a subduction zone the thinner, yet denser
oceanic plate dives beneath the continental plate. The continental plate is locked to
the subducting plate by immense friction along the shallow portion of a vast sloping fault
surface. GPS data from many subduction zones show that
the friction indeed causes the leading edge of the overlying plate to be forced backwards.
During subduction the thin layer on top of the oceanic plate is scraped off and forms
a wedge at the front of the continental plate . The continental plate deforms in response
to the stress. The plates can lock together for hundreds
of years until frictional stress is overcome in a process called elastic rebound. This
can produce M8 to M9 great earthquakes. Tsunamis occur when the ground beneath the
ocean is displaced, as it does in this simplified animation.
This cycle of locking and building stress… followed by catastrophic release… repeats
every few hundred years.