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Megathrust earthquakes occur at convergent plate boundaries, where one
tectonic plate Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
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. If ...
that forms the contact between the two plates. These
interplate earthquake An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. Earthquakes of this type account for more than 90 percent of the total seismic energy released around the world. If one plate is trying to move pa ...
s are the planet's most powerful, with moment magnitudes (''Mw'') that can exceed 9.0. Since 1900, all
earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...
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 trenches Oceanic trenches are prominent long, narrow topographic depressions of the ocean floor. They are typically wide and below the level of the surrounding oceanic floor, but can be thousands of kilometers in length. There are about of oceanic tren ...
; in such cases, the earthquakes can abruptly displace the sea floor over a large area. As a result, megathrust earthquakes often generate
tsunamis A tsunami ( ; from ja, 津波, lit=harbour wave, ) 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 other underwater ex ...
that are considerably more destructive than the earthquakes themselves.
Teletsunamis A teletsunami (also called an ocean-wide tsunami, distant tsunami, distant-source tsunami, far-field tsunami, or trans-ocean tsunami) is a tsunami that originates from a distant source, defined as more than away or three hours' travel from the a ...
can cross ocean basins to devastate areas far from the original earthquake.


Terminology and mechanism

The term ''megathrust'' refers to an extremely large
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. If ...
, typically formed at the plate interface along a subduction zone, such as the
Sunda megathrust The Sunda megathrust is a fault that extends approximately 5,500 km (3300 mi) from Myanmar (Burma) in the north, running along the southwestern side of Sumatra, to the south of Java and Bali before terminating near Australia. It is a ...
. 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. A thrust fault is a type of
reverse 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 tectoni ...
, 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 tectoni ...
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 Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...
is being compressed by tectonic forces. Megathrust faults occur where two
tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large ...
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 portion of the upper mantle that behaves elastically on time scales of up to thousands of years or ...
, it dives beneath the other plate (called the ''overriding plate'') and sinks into the
Earth's mantle Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of making up about 84% of Earth's volume. It is predominantly so ...
as a ''
slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate tha ...
''. 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 definition, to Antarctica) in the south, and is bounded by the contine ...
and
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
s. These subduction zones are not only responsible for megathrust earthquakes but are also largely responsible for the
volcanic A volcano is a rupture 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 found where tectonic plat ...
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 region around much of the rim of the Pacific Ocean where many volcanic eruptions and earthquakes occur. The Ring ...
. Since the earthquakes associated with these subduction zones 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 a significant series of
tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) 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 other underwater exp ...
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 3-5 minutes. In the
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
region, the
Sunda megathrust The Sunda megathrust is a fault that extends approximately 5,500 km (3300 mi) from Myanmar (Burma) in the north, running along the southwestern side of Sumatra, to the south of Java and Bali before terminating near Australia. It is a ...
is located where the
Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...
is subducting under the
Eurasian Plate The Eurasian Plate is a tectonic plate that includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent an ...
and extends off the coasts of
Myanmar Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John Wells explai ...
,
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 sixth-largest island in the world at 473,481 km2 (182,812 mi.2), not including adjacent i ...
,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
and
Bali Bali () is a 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 neighbouring islands, notably Nusa Penida, Nusa Lembongan, and ...
before terminating off the northwestern coast of
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
. This subduction zone was responsible for the
2004 Indian Ocean earthquake and tsunami An earthquake and a tsunami, known as the Boxing Day Tsunami and, by the scientific community, the Sumatra–Andaman earthquake, occurred at 07:58:53 local time ( UTC+7) on 26 December 2004, with an epicentre off the west coast of northern ...
. 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, wh ...
is responsible for
Nankai megathrust earthquakes Nankai megathrust earthquakes are great megathrust earthquakes that occur along the ''Nankai megathrust'' – the fault under the Nankai Trough – which forms the plate interface between the subducting Philippine Sea Plate and the overriding ...
and associated tsunamis. In North America, the Juan de Fuca Plate is subducting under the
North American Plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Paci ...
creating the
Cascadia subduction zone The Cascadia subduction zone is a convergent plate boundary that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is a very long, sloping subduction zone where the Explorer, Juan de Fuc ...
which stretches from mid Vancouver Island, British Columbia to Northern California. This subduction zone was responsible for the
1700 Cascadia earthquake The 1700 Cascadia earthquake occurred along the Cascadia subduction zone on January 26, 1700, with an estimated moment magnitude of 8.7–9.2. The megathrust earthquake involved the Juan de Fuca Plate from mid- Vancouver Island, south along th ...
. The
Aleutian Trench The Aleutian Trench (or Aleutian Trough) is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for from a triple junction in the west with the Ulak ...
, of the southern coast of
Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U ...
, and the
Aleutian Islands The Aleutian Islands (; ; ale, Unangam Tanangin,”Land of the Aleuts", possibly from Chukchi ''aliat'', "island"), also called the Aleut Islands or Aleutic Islands and known before 1867 as the Catherine Archipelago, are a chain of 14 large v ...
, where the North American Plate overrides the
Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and I ...
, has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis, including the
1964 Alaska earthquake The 1964 Alaskan earthquake, also known as the Great Alaskan earthquake and Good Friday earthquake, occurred at 5:36 PM AKST on Good Friday, March 27.
; 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 The 1960 Valdivia earthquake and tsunami ( es, link=no, Terremoto de Valdivia) or the Great Chilean earthquake (''Gran terremoto de Chile'') on 22 May 1960 was the most powerful earthquake ever recorded. Various studies have placed it at 9.4– ...
, estimated magnitude 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 tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the N ...
is subducting 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 historically, the largest megathrust event within the last 20 years being the magnitude 9.1 Tōhoku earthquake. 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 Flat slab subduction is characterized by a low subduction angle (<30 degrees to horizontal) beyond the . 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.


See also

*
Lists of earthquakes Earthquakes are caused by movements within the Earth's crust and uppermost mantle. They range from events too weak to be detectable except by sensitive instrumentation, to sudden and violent events lasting many minutes which have caused some of ...


References


Further reading

* {{cite journal , url=https://www.researchgate.net/publication/223469777 , first1=M.-A. , last1=Gutscher , title=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 , last2=Baptista , first2=M.A. , last3=Miranda , first3=J.M. , journal=Tectonophysics , year=2006 , volume=426 , issue=1–2 , pages=153–166 , issn=0040-1951 , doi=10.1016/j.tecto.2006.02.025, bibcode=2006Tectp.426..153G


External links


Giant Megathrust Earthquakes
Natural Resources Canada Natural Resources Canada (NRCan; french: Ressources naturelles Canada; french: RNCan, label=none)Natural Resources Canada is the applied title under the Federal Identity Program; the legal title is Department of Natural Resources (). is the dep ...
Plate tectonics Types of earthquake Tsunami