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Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of known terrestrial source of tectonic activity has produced 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 a type of dip-slip fault. 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 term megathrust 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. It is mostly American terminology.
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 often generate a significant series of tsunami waves. They are known to produce intense shaking for periods of time that can last for up to a few minutes.
A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-called "flat slab subduction". 
Examples of megathrust earthquakes are listed in the following table.
|Tectonic Plates Involved||Other Details/Notes|
|365 Crete earthquake||8.0+||African Plate subducting beneath the Aegean Sea Plate|
|869 Sanriku earthquake||8.6–9.0||Pacific Plate subducting beneath the Okhotsk Plate||
|1575 Valdivia earthquake||8.5||Nazca Plate subducting beneath the South American Plate|
|1700 Cascadia earthquake||8.7–9.2||Juan de Fuca Plate subducting beneath the North American Plate||
|1707 Hōei earthquake||8.6–9.3||Philippine Sea Plate subducting beneath the Eurasian Plate||
|1730 Valparaíso earthquake||8.7-9.0||Nazca Plate subducting beneath the South American Plate|
|1737 Kamchatka earthquake||8.3–9.0||Pacific Plate subducting beneath the Okhotsk Plate||
|1755 Lisbon earthquake||8.5–9.0 ||Hypothesized to be part of a young subduction zone but origin still debated. Related to the Azores–Gibraltar Transform Fault|
|1868 Arica earthquake||8.5–9.0||Nazca Plate subducting beneath the South American Plate||
|1877 Iquique earthquake||8.5–9.0?||Nazca Plate subducting beneath the South American Plate||
|1906 Ecuador–Colombia earthquake||8.8||Nazca Plate subducting beneath the South American Plate|
|1946 Nankaidō earthquake||8.1||Philippine Sea Plate subducting beneath the Eurasian Plate||
|1952 Kamchatka earthquake||9.0||Pacific Plate subducting beneath the Okhotsk Plate||
|1957 Andreanof Islands earthquake||8.6||Pacific Plate subducting beneath the North American Plate||
|1960 Great Chilean earthquake||9.5||Nazca Plate subducting beneath the South American Plate||
|1964 Alaska earthquake ("Good Friday" earthquake)||9.2||Pacific Plate subducting beneath the North American Plate||
|2001 southern Peru earthquake||8.4||Nazca Plate subducting beneath the South American Plate||
|2004 Sumatra-Andaman earthquake ("Indian Ocean earthquake")||9.1–9.3||India Plate subducting beneath the Burma Plate||
|2010 Chile earthquake||8.8||Nazca Plate subducting beneath the South American Plate||
|2011 Tōhoku earthquake and tsunami||9.1||Pacific Plate subducting beneath the Okhotsk Plate||
|2014 Iquique earthquake||8.2||Nazca Plate subducting beneath the South American Plate||
|2015 Illapel earthquake||8.3||Nazca Plate subducting beneath the South American Plate||
- "Tsunami Terminology". The National Tsunami Hazard Mitigation Program History, 1995–2005. Pacific Marine Environmental Laboratory.
- "Earthquake Glossary - dip slip". Earthquake Hazards Program. U.S. Geological Survey.
- Park, J.; Butler, R.; Anderson, K.; et al. (2005). "Performance Review of the Global Seismographic Network for the Sumatra-Andaman Megathrust Earthquake". Seismological Research Letters. 76 (3): 331–343. doi:10.1785/gssrl.76.3.331. ISSN 0895-0695.
- Fault curvature may control where big quakes occur, Eurekalert 24-NOV-2016
- Ishikawa, Yuzo (February 2012). Re-evaluation of Mw of the 1707 Hoei earthquake (PDF). G-EVER1 Workshop. Tsukuba, Japan: Asia-Pacific Region Global Earthquake and Volcanic Eruption Risk Management (G-EVER1) Consortium.
- Gutscher, M.-A.; Baptista, M.A.; Miranda, J.M. (2006). "The Gibraltar Arc seismogenic zone (part 2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity". Tectonophysics. 426 (1-2): 153–166. doi:10.1016/j.tecto.2006.02.025. ISSN 0040-1951.
- "M 9.1 - near the east coast of Honshu, Japan". Earthquake Hazards Program. USGS. 2016. Retrieved 21 November 2016.
- Kidd, Kenneth (12 March 2011). "How "mega-thrust" earthquake caught forecasters by surprise". Toronto Star. Retrieved 12 March 2011.
- Reilly, Michael (11 March 2011). "1722 GMT, 11 March 2011: Japan's largest ever earthquake". Short Sharp Science. New Scientist. Retrieved 11 March 2011.