megathrust

Megathrust refers to the largest class of fault located at subduction zone plate boundaries, where one tectonic plate is forced beneath another. These faults form along the interface between the subducting slab and the overriding plate and are the primary locus of the most powerful earthquakes on Earth. The fault system generally runs from the trench to the continental or island arc, and is often associated with an accretionary wedge and volcanic arc features.

Seismically, megathrusts are characterized by long rupture zones and large slip during earthquakes. The locked portion of the fault accumulates strain for long periods, then ruptures suddenly, releasing energy over vast areas. Rupture can extend hundreds to thousands of kilometers along strike, and slip magnitudes can reach several meters to tens of meters, producing moment magnitudes typically in the 8 to 9 range. Because the seafloor is often displaced vertically, megathrust earthquakes commonly generate tsunamis.

Hazards: The combination of large magnitude and submarine rupture makes megathrust earthquakes among the most destructive natural hazards, capable of causing widespread ground shaking and devastating tsunamis that affect coastal regions far from the rupture.

Examples and significance: Notable megathrust earthquakes include the 1960 Valdivia earthquake in Chile (Mw about 9.5), the 2004 Sumatra-Andaman earthquake (Mw about 9.1–9.3), and the 2011 Tohoku earthquake in Japan (Mw about 9.0). Megathrust events are a major focus of hazard assessment in subduction zones such as Cascadia, Japan, Chile, Peru, New Zealand, and Indonesia, and are studied through seismic networks, geodetic measurements, and deep-sea observations to understand segmentation and recurrence.