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Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another, caused by slip along the thrust fault that forms the contact between them. 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.
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.
Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the Pacific and Indian Oceans. These subduction zones are not only responsible for megathrust earthquakes but are also largely responsible for the volcanic activity associated with the Pacific Ring of Fire.
Since the earthquakes associated with these subduction zones deform the ocean floor, they often generate a significant series of tsunami waves. Subduction zone earthquakes are also known to produce intense shaking and ground movements for significant periods of time that can last for up to 5–6 minutes.
In the Indian Ocean region, the Sunda megathrust is located where the Indo-Australian Plate is subducting under the Eurasian Plate and extends 5,500 kilometres (3,400 mi) off the coasts of Myanmar, Sumatra, Java and Bali before terminating off the northwestern coast of Australia. This subduction zone was responsible for the 2004 Indian Ocean earthquake and tsunami.
In North America, the Juan de Fuca Plate is subducting under the North American Plate creating the Cascadia subduction zone which stretches from Middle Vancouver, British Columbia to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake. The Aleutian Trench, of the southern coast of Alaska, and the Aleutian Islands, where the North American Plate overrides the Pacific Plate, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis, including the 1964 Alaska earthquake; at magnitude 9.2, it remains the largest recorded earthquake in North America, and the second-largest earthquake instrumentally recorded in the world.
The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated magnitude 9.4–9.6, centered off the coast of Chile along the Peru-Chile trench, where the Nazca Plate is subducting under the South American Plate. This megathrust region has regularly generated extremely large earthquakes historically, the largest megathrust event within the last 20 years being the magnitude 8.8 2010 Chile earthquake.
Notable examples of megathrust earthquakes, which have caused many deaths and extensive damage, 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||9.1–9.3||Nazca Plate subducting beneath the South American Plate||
|1746 Lima-Callao earthquake||8.6–8.8||Pacific Plate subducting beneath the Nazca 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?||
|1906 Ecuador–Colombia earthquake||8.8|
|1923 Great Kantō earthquake||8.2||Philippine Sea Plate subducting beneath the Okhotsk Plate||
|1932 Jalisco earthquakes||8.2||Rivera Plate and Cocos Plate subducting beneath the North American Plate||
|1940 Lima earthquake||8.2||Nazca Plate subducting beneath the South American Plate along the Peru-Chile trench.|
|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||
|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||
|1985 Mexico City earthquake||8.0||Cocos Plate subducting beneath the North American Plate|
|2004 Sumatra-Andaman earthquake ("Indian Ocean earthquake")||9.1–9.3||India Plate subducting beneath the Burma Plate||
|2005 Nias–Simeulue earthquake||8.6||Indo-Australian Plate subducting beneath the Eurasian 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||
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