Megathrust earthquake

Diagram of a subduction zone. The megathrust fault lies on the top of the subducting slab where it is in contact with the overriding plate.

The term megathrust refers to an extremely large thrust fault, typically formed at the plate interface along a subduction zone, such as the Sunda megathrust.[4][5] However, the term is also occasionally applied to large thrust faults in continental collision zones, such as the Himalayan megathrust.[6] A megathrust fault can be 1,000 kilometers (600 mi) long.[7]

Cross-sectional illustration of normal and reverse faults

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 faults, where the rock above the fault is displaced downwards, or strike-slip faults, 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°,[8] and show large displacements.[9][10] 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 is being compressed by tectonic forces.[11]

Megathrust faults occur where two tectonic plates collide. When one of the plates is composed of oceanic lithosphere, 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.[5] 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.[7]

Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the Pacific and Indian Oceans.[5] 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.[12]

Since the earthquakes associated with these subduction zones deform the ocean floor, they often generate a significant series of tsunami waves.[13] 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.[14]

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.[15]

In Japan, the Nankai megathrust under the Nankai Trough is responsible for Nankai megathrust earthquakes and associated tsunamis.[16]

In North America, the Juan de Fuca Plate is subducting under the North American Plate creating the Cascadia subduction zone which stretches from mid Vancouver Island, British Columbia to Northern California. This subduction zone was responsible for the 1700 Cascadia earthquake.[17] 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,[18] 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.[19]

The largest recorded megathrust earthquake was the 1960 Valdivia earthquake, estimated magnitude 9.5, centered off the coast of Chile along the Peru-Chile trench, where the Nazca Plate is subducting under the South American Plate.[20] This megathrust region has regularly generated extremely large earthquakes. The largest megathrust event within the last 20 years was the magnitude 9.1 Tōhoku earthquake.[21]

The largest possible earthquake that is estimated to occur is a magnitude 10, with some scientists even estimating that a magnitude 11 earthquake could occur, though extremely rare. Where they would take place is most likely to be a combined rupture of the Japan Trench and Kuril-Kamchatka Trench.[22][23][24]

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.[25]

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.[5]

• Lists of earthquakes

• [1]

• Giant Megathrust Earthquakes – Natural Resources Canada