How do megathrust earthquakes generate a tsunami?
In this animation, we are showing an ocean/continent convergent boundary at the leading edge of the plate. We see the denser oceanic plate diving beneath the continental plate. The down-going oceanic plate eventually warms up to the temperature of the surrounding mantle. Such destruction (recycling) of oceanic plates occurs along convergent boundaries where plates collide and an oceanic plate is subducted. (This animation does not address volcanoes formed inboard of the boundary where water released from the oceanic plate facilities magma production in the mantle wedge beneath the continent.)
Denser oceanic plate dives beneath the continental plate. Plates are locked by high friction. Over time, friction is overcome in process called elastic rebound Sudden movement of overlying plate generates a tsunami