Megathrust earthquake

A megathrust earthquake occurs on the shallow, gently dipping boundary between a subducting plate and an overriding plate. Friction can lock parts of this interface while plate convergence continues, storing elastic strain until the fault ruptures and releases it as seismic energy.

These faults occur at subduction zones, including the margins around the Pacific Ocean and parts of the Indian Ocean. Their broad contact surfaces allow ruptures to extend for hundreds or more than a thousand kilometres, making them capable of unusually large magnitudes. The earthquake’s size depends on the rupture area, the amount of slip, and the rocks’ rigidity; magnitude therefore describes released energy rather than simply shaking intensity at one location.¹

The principal hazard begins when a locked section of the plate boundary suddenly slips, sending seismic waves through the crust and producing prolonged ground motion. Strong shaking can damage buildings, trigger landslides, break lifelines, and generate many aftershocks.

A megathrust rupture becomes especially tsunamigenic when it vertically displaces the seabed and the overlying ocean water. The resulting tsunami may reach nearby shores before a warning arrives, while waves crossing an ocean can affect distant coastlines.² Not every large subduction earthquake produces a major tsunami: rupture depth, seafloor geometry, direction of motion, and coastal form all matter. Conversely, a relatively modest earthquake can generate an unusually large local tsunami if it causes submarine landslides or very shallow deformation.

Megathrust earthquakes are compared chiefly by moment magnitude, which incorporates fault area, average slip, and rock rigidity and remains useful for events too large for older magnitude scales.¹ Historical records, marine sediments, coral growth, drowned forests, written accounts, and modern geodesy help scientists estimate recurrence on faults with short instrumental histories.

Examples include the 1960 Valdivia earthquake in Chile, the 1964 Alaska earthquake, the 2004 Indian Ocean earthquake, and the 2011 Tōhoku earthquake in Japan. The 2011 event demonstrated that a rupture could produce exceptionally large slip close to the trench, intensifying tsunami impacts beyond some earlier hazard models.³ Recurrence is irregular: a long quiet interval is not a precise countdown, and a fault may rupture in segments or through several connected fault patches.

Slow slip events and episodic tremor reveal that some parts of a subduction interface release strain without the sudden shaking of a conventional earthquake. These episodes can alter stress on neighbouring locked areas, although their relationship to great earthquakes varies among subduction zones.

Some megathrust earthquakes are “tsunami earthquakes”: their seismic waves appear weaker than expected for the tsunami they create because rupture is slow and concentrated near the shallow trench. Paleotsunami research has identified otherwise undocumented events, including evidence for a great earthquake and tsunami on the Cascadia margin in 1700.⁴ Forecasting can estimate probabilities and warning agencies can detect an event quickly, but no established method predicts the exact time, place, and size of a future megathrust earthquake.

Subduction zone

A convergent plate boundary where one tectonic plate descends beneath another into the mantle.

Seismic moment

A physical measure of earthquake size calculated from fault area, average slip, and rock rigidity.

Tsunami earthquake

An earthquake whose tsunami is unusually large relative to the strength of its seismic shaking, often because rupture is shallow and slow.