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Megathrust earthquakes occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is subducted (forced underneath) by another. Due to the shallow dip of the plate boundary, which causes large sections to get stuck, these earthquakes are among the world's largest, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all six earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of known tectonic activity can produce earthquakes of this scale.


During the rupture, one side of the fault is pushed upwards relative to the other, and it is this type of movement that is known as thrust. They are one type of dip-slip faults. A thrust fault is a reverse fault with a dip of 45° or less. Oblique-slip faults have significant components of different slip styles. The termmegathrust does not have a widely accepted rigorous definition, but is used to refer to an extremely large thrust fault, typically formed at the plate interface along a subduction zone such as the Sunda megathrust.


The major subduction zone is associated with the Pacific and Indian Oceans and is responsible for the volcanic activity associated with the Pacific Ring of Fire. Since these earthquakes deform the ocean floor, they almost always generate a significant series of tsunami waves. They are known to produce intense shaking for long periods, such as several minutes.