Introduction
Megathrust earthquakes are the largest class of interplate seismic events that originate where one lithospheric plate is forced beneath another at a subduction zone. They result from abrupt rupture on the shallow thrust fault that separates the overriding and subducting plates, most commonly located at the base of oceanic trenches. When such rupture occurs offshore it can displace the seafloor over very large areas, producing both vertical and horizontal motion that couples efficiently into the overlying water column.
On the moment‑magnitude scale (Mw) megathrusts attain the greatest magnitudes on Earth and can exceed Mw 9.0; historically, every earthquake of Mw 9.0 or greater has been a megathrust event. The attendant tsunamis—generated by the sudden, broad uplift or subsidence of the seabed—often cause more widespread and long‑range destruction than the local ground shaking, with long‑wavelength waves capable of crossing entire ocean basins.
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Understanding megathrust hazards draws on the broader taxonomy of seismic phenomena (e.g., mainshocks, foreshocks, aftershocks, doublets, blind thrusts, intraplate and remotely triggered events, slow earthquakes, supershear ruptures, and earthquake swarms) and recognizes multiple drivers of seismicity including tectonic convergence (the primary driver for megathrusts), volcanic processes, and human‑induced stresses. Key observational parameters—epicenter and epicentral distance, hypocentral depth, P‑ and S‑wave arrivals, and shadow zones—are measured with seismometers and summarized using magnitude (notably Mw) and intensity scales. Applied efforts in forecasting, regional cataloguing (for example using Flinn–Engdahl regions), and institutional coordination underpin hazard assessment. Complementary geophysical and engineering approaches—such as shear‑wave splitting analyses, density and elastic‑structure constraints from the Adams–Williamson relation, sedimentary seismites, and earthquake engineering—are integral to characterizing earthquake processes and mitigating megathrust risk.
Terminology and mechanism
A megathrust is an exceptionally large thrust fault that develops at the plate boundary of a subduction zone, where the upper surface of the descending slab contacts the overriding plate. These structures can extend for on the order of a thousand kilometers (≈600 mi); the Sunda megathrust is a canonical example, and the designation is sometimes extended to comparably extensive thrusts in continental collision settings (e.g., the Himalayan megathrust).
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Thrust faults are a subtype of reverse fault characterized by shallowly inclined fault planes and substantial crustal shortening. In a thrust, the block above the plane (the hanging wall) is transported up and over the block below (the footwall), producing net horizontal convergence; typical thrust dips are less than 45°, in contrast to normal faults (downward motion of the hanging wall in extensional regimes) and strike‑slip faults (predominantly lateral displacement).
In subduction settings the megathrust marks the coupled interface where the oceanic plate descends into the mantle. Portions of this interface may become frictionally locked during the interseismic interval, preventing steady slip and allowing elastic strain to accumulate in both the subducting slab and the overriding plate. When a locked patch fails, rapid rupture along the megathrust releases the stored strain as seismic energy, producing a megathrust earthquake and often significant vertical motion of the overriding plate.
Occurrence and characteristics
Megathrust earthquakes are essentially confined to subduction-zone interfaces and are most prevalent around the margins of the Pacific and Indian Oceans, where they are integral to the volcanism of the Pacific Ring of Fire. Because ruptures on these plate-boundary thrusts commonly displace the seafloor, they are a major source of ocean-wide tsunamis; they also generate prolonged and severe ground shaking, with slip and shaking durations that can last on the order of minutes (commonly up to 3–5 minutes in the largest events).
Global examples illustrate both the geographic concentration and the range of magnitudes possible. Long, active faults such as the Sunda megathrust (≈5,500 km off Myanmar, Sumatra, Java, Bali to northwest Australia) produced the 2004 Indian Ocean earthquake–tsunami; the adjacent Java Trench is segmented, with western and eastern segments capable of ~Mw 8.9 and Mw 8.8 respectively and a combined rupture approaching Mw 9.1. The Manila Trench in the South China Sea has been assessed capable of at least Mw 9.0 (some estimates up to Mw 9.2 or higher). Japan contains multiple hazardous megathrust systems—the Nankai trough produces recurrent large earthquakes and tsunamis, while the Japan Trench generated the 2011 Tōhoku event (Mw 9.0–9.1). On North America’s Pacific margin, the Cascadia subduction zone (Juan de Fuca beneath North America) produced the 1700 megathrust earthquake; the Aleutian Trench has produced numerous major events including the 1964 Alaska earthquake (Mw 9.1–9.2). The Peru–Chile Trench produced the largest instrumentally and historically recorded megathrust earthquake, the 1960 Valdivia event (Mw ~9.4–9.6). Other regions assessed for very large ruptures include the Himalayan front (1950 Assam–Tibet, Mw 8.7; theoretical models allow Mw ≥9.0 at long recurrence intervals and an extreme single-arc rupture scenario up to ~Mw 9.7), and the Lesser Antilles, where assessments range to Mw 9.3 and, under extreme assumptions, much larger values.
Physical and structural controls help explain why some subduction zones produce the very largest earthquakes. Observational and geophysical studies link the greatest megathrusts to flat or shallowly dipping downgoing slabs and to subduction zones with thick sedimentary prisms; both conditions favor long, uninterrupted rupture propagation and therefore higher magnitudes. Compared with other earthquakes of similar magnitude, megathrust ruptures tend to propagate more slowly and to sustain slip for longer durations. In extreme, multi-segment or combined-rupture scenarios, theoretical upper limits for global megathrust magnitude have been proposed in the Mw 10–11 range, most plausibly arising from very extensive ruptures across adjacent trench systems (for example, combined Japan–Kuril or very large Aleutian or Peru–Chile ruptures).