Megathrust earthquake

A very large earthquake caused by sudden slip on a megathrust fault, usually where one tectonic plate subducts beneath another. Its shallow rupture can displace the seafloor and generate a tsunami.

Subduction zone: A region where one tectonic plate moves beneath another into the mantle. Megathrust faults form along the plate boundary in these regions.
1960 Valdivia earthquake: A magnitude 9.5 earthquake off southern Chile, the largest instrumentally recorded earthquake. Its extensive rupture exemplifies the extreme scale of a subduction megathrust event.
Plate tectonics: The theory that Earth’s lithosphere is divided into moving plates. Converging plates supply the long-term motion that loads megathrust faults.
Tsunami warning system: A network that detects tsunamis and communicates warnings to threatened coastlines. Rapid warnings can reduce exposure after a tsunamigenic megathrust rupture.
Slow slip event: A transient episode of fault slip that lasts days to years without producing ordinary strong earthquake shaking. Its relationship to stress transfer and megathrust rupture remains an active research question.
Megathrust fault: A large, shallowly dipping fault at the boundary between a subducting plate and the overriding plate. Sudden slip on this interface produces a megathrust earthquake.
1964 Alaska earthquake: A magnitude 9.2 earthquake that ruptured the megathrust beneath southern Alaska. It demonstrated how a single rupture can produce strong shaking, landslides, and tsunamis.
Tectonic plate: A moving section of Earth’s rigid outer shell, comprising crust and uppermost mantle. The relative motion of the two plates determines strain accumulation at their boundary.
Seismic hazard: The probability and expected severity of earthquake shaking at a place over time. Megathrust activity is a major source of long-term hazard for subduction-zone regions.
Seismic cycle: The sequence of strain accumulation, earthquake rupture, and postseismic adjustment on a fault. Its irregularity makes the timing and size of future megathrust earthquakes difficult to forecast.

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