Megathrust earthquakes occur at convergent plate boundaries, where one

tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

is forced underneath another. The earthquakes are caused by slip along the

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. I ...

that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (''M_w'') that can exceed 9.0. Since 1900, all

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

s of magnitude 9.0 or greater have been megathrust earthquakes. The thrust faults responsible for megathrust earthquakes often lie at the bottom of

oceanic trench Oceanic trenches are prominent, long, narrow topography, topographic depression (geology), depressions of the seabed, ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers ...

es; in such cases, the earthquakes can abruptly displace the sea floor over a large area. As a result, megathrust earthquakes often generate

tsunami A tsunami ( ; from , ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and underwater explosions (including detonations, ...

s that are considerably more destructive than the earthquakes themselves. Teletsunamis can cross ocean basins to devastate areas far from the original earthquake.

Terminology and mechanism

The term ''megathrust'' refers to an extremely large

, typically formed at the plate interface along a subduction zone, such as the Sunda megathrust. However, the term is also occasionally applied to large thrust faults in continental collision zones, such as the Himalayan megathrust. A megathrust fault can be long. Nor rev

A thrust fault is a type of reverse fault, in which the rock above the fault is displaced upwards relative to the rock below the fault. This distinguishes reverse faults from

normal fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s, where the rock above the fault is displaced downwards, or

strike-slip fault In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

s, where the rock on one side of the fault is displaced horizontally with respect to the other side. Thrust faults are distinguished from other reverse faults because they dip at a relatively shallow angle, typically less than 45°, and show large displacements. In effect, the rocks above the fault have been thrust over the rocks below the fault. Thrust faults are characteristic of areas where the

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

is being compressed by tectonic forces. Megathrust faults occur where two

tectonic plates Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...

collide. When one of the plates is composed of

oceanic lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time sc ...

, it dives beneath the other plate (called the ''overriding plate'') and sinks into the Earth's mantle as a '' slab''. The contact between the colliding plates is the megathrust fault, where the rock of the overriding plate is displaced upwards relative to the rock of the descending slab. Friction along the megathrust fault can lock the plates together, and the subduction forces then build up strain in the two plates. A megathrust earthquake takes place when the fault ruptures, allowing the plates to abruptly move past each other to release the accumulated strain energy.

Occurrence and characteristics

Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...

and

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

s. These subduction zones are also largely responsible for the

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

activity associated with the Pacific

Ring of Fire The Ring of Fire (also known as the Pacific Ring of Fire, the Rim of Fire, the Girdle of Fire or the Circum-Pacific belt) is a tectonic belt of volcanoes and earthquakes. It is about long and up to about wide, and surrounds most of the Pa ...

. Since these earthquakes deform the

ocean floor The seabed (also known as the seafloor, sea floor, ocean floor, and ocean bottom) is the bottom of the ocean. All floors of the ocean are known as seabeds. The structure of the seabed of the global ocean is governed by plate tectonics. Most of ...

, they often generate strong

waves. Subduction zone earthquakes are also known to produce intense shaking and ground movements that can last for up to 3–5 minutes. In the

region, the Sunda megathrust is located where the Indo-Australian plate subducts under the Eurasian plate along a fault off the coasts of

Myanmar Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and has ...

Sumatra Sumatra () is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the list of islands by area, sixth-largest island in the world at 482,286.55 km2 (182,812 mi. ...

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

and

Bali Bali (English:; Balinese language, Balinese: ) is a Provinces of Indonesia, province of Indonesia and the westernmost of the Lesser Sunda Islands. East of Java and west of Lombok, the province includes the island of Bali and a few smaller o ...

, terminating off the northwestern coast of

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. This subduction zone was responsible for the

2004 Indian Ocean earthquake and tsunami On 26 December 2004, at 07:58:53 local time ( UTC+7), a major earthquake with a magnitude of 9.2–9.3 struck with an epicentre off the west coast of Aceh in northern Sumatra, Indonesia. The undersea megathrust earthquake, known in the sci ...

. In parts of the megathrust south of

, referred to as the Java Trench, for the western part, 8.9 is possible, while in the eastern Java segment, 8.8 is possible, while if both were to rupture at the same time, the magnitude would be 9.1. In the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan island, Taiwan and northwestern Philippines (mainly Luz ...

lies the Manila Trench, which is capable of producing 9.0 or larger earthquakes, with the maximum magnitude at Mw 9.2 or higher. In Japan, the Nankai megathrust under the

Nankai Trough The is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately offshore. The underlying fault, the ''Nankai megathrust,'' is the source of the devastating Nankai megathrust earthquakes, ...

is responsible for Nankai megathrust earthquakes and associated tsunamis. The largest megathrust event within the last 20 years was the magnitude 9.0–9.1 Tōhoku earthquake along the Japan Trench megathrust. In North America, the Juan de Fuca plate subducts under the North American plate, creating the Cascadia subduction zone from mid Vancouver Island, British Columbia down to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake. The Aleutian Trench, of the southern coast of

Alaska Alaska ( ) is a non-contiguous U.S. state on the northwest extremity of North America. Part of the Western United States region, it is one of the two non-contiguous U.S. states, alongside Hawaii. Alaska is also considered to be the north ...

and the

Aleutian Islands The Aleutian Islands ( ; ; , "land of the Aleuts"; possibly from the Chukchi language, Chukchi ''aliat'', or "island")—also called the Aleut Islands, Aleutic Islands, or, before Alaska Purchase, 1867, the Catherine Archipelago—are a chain ...

, 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 9.1–9.2, it remains the largest recorded earthquake in North America, and the third-largest earthquake instrumentally recorded in the world. In the Himalayan region, where the

Indian plate The Indian plate (or India plate) is or was a minor tectonic plate straddling the equator in the Eastern Hemisphere. Originally a part of the ancient continent of Gondwana, the Indian plate broke away from the other fragments of Gondwana an ...

subducts under the Eurasian plate, the largest recorded earthquake was the 1950 Assam–Tibet earthquake, at magnitude 8.7. It is estimated that earthquakes with magnitude 9.0 or larger are expected to occur at an interval of every 800 years, with the highest boundary being a magnitude 10, though this is not considered physically possible. Therefore, the largest possible earthquake in the region is a magnitude 9.7, assuming a single rupture of the whole Himalayan arc and assuming standard scaling law, which implies an average slip of 50 m. A megathrust earthquake could occur in the Lesser Antilles subduction zone, with a maximum magnitude of 9.3, or potentially even 10.3 through recent evaluations, a value not considered impossible. The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated between magnitudes 9.4–9.6, centered off the coast of Chile along the Peru-Chile Trench, where the

Nazca plate The Nazca plate or Nasca plate, named after the Nazca region of southern Peru, is an oceanic list of tectonic plates, tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru– ...

subducts under the

South American plate The South American plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African plate, with which it forms the southern part of the Mid ...

. This megathrust region has regularly generated extremely large earthquakes. The largest possible earthquakes are estimated at magnitudes of 10 to 11, most likely caused by a combined rupture of the Japan Trench and Kuril–Kamchatka Trench, or individually the Aleutian Trench or Peru–Chile Trench. Another possible area could be the Lesser Antilles subduction zone. 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. Compared with other earthquakes of similar magnitude, megathrust earthquakes have a longer duration and slower rupture velocities. The largest megathrust earthquakes occur in subduction zones with thick sediments, which may allow a fault rupture to propagate for great distances unimpeded.

References

External links

Giant Megathrust Earthquakes
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Natural Resources Canada Natural Resources Canada (NRCan; ; )Natural Resources Canada is the applied title under the Federal Identity Program; the legal title is Department of Natural Resources (). is the department of the Government of Canada responsible for natural r ...

Plate tectonics Types of earthquake Tsunami

Terminology and mechanism

Occurrence and characteristics

See also

References

Further reading

External links