a megathrust earthquake occurs at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another. these interplate earthquakes are the planet's most powerful, with moment magnitudes that can exceed 9.0. since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. no other type of known terrestrial source of tectonic activity has produced earthquakes of this scale.