Megathrust earthquake

Megathrust earthquakes occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is forced (or subducts) under another. Due to the shallow dip of the plate boundary, which causes large sections to get stuck, these earthquakes are among the world's largest, with moment magnitudes ($M_{w}$) that can exceed 9.0. Since 1900, all five earthquakes of magnitude 9.0 or greater have been megathrust earthquakes; in fact, no other type of known tectonic activity can produce earthquakes of this scale.

Terminology

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 one type of dip-slip faults. A thrust fault is a reverse fault with a dip of 45° or less. Oblique-slip faults have significant components of different slip styles. Though megathrust or mega-thrust has no official definition, the term is widely used and generally considered to mean an extremely powerful thrust fault.

Areas

The major subduction zone is associated with the Pacific and Indian Oceans and are responsible for the volcanic activity associated with the Pacific Ring of Fire. Since these earthquakes deform the ocean floor, they almost always generate a significant series of tsunami waves. They are known to produce intense shaking for long periods, such as several minutes.

Examples

Examples of megathrust earthquakes are listed in the following table.

Event	Estimated Moment Magnitude ($M_{w}$)	Tectonic Plates Involved	Other Details/Notes
365 Crete earthquake	8.0+	African Plate subducting beneath the Eurasian Plate	The quake generated a large tsunami in the eastern Mediterranean Sea and caused significant vertical displacement in the island of Crete.
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	Slip length: 1000 km (625 mi)
1737 Kamchatka earthquake	9.0–9.3	Pacific Plate subducting beneath the Okhotsk Plate	Duration: 15 minutes Depth: 40 km
1755 Lisbon earthquake	9.0	Hypothesized to be part of a young subduction zone but origin still debated
1868 Arica earthquake	9.0	Nazca Plate subducting beneath the South American Plate
1877 Antofagasta (Northern Chile) earthquake	8.8	Nazca Plate subducting beneath the South American Plate
1946 Nankaidō earthquake	8.1	Philippine Sea Plate subducting beneath the Eurasian Plate
1950 Nicoya Peninsula (Costa Rica) earthquake	7.7	Cocos Plate subducting beneath the Caribbean Plate
1952 Kamchatka earthquake	9.0	Pacific Plate subducting beneath the Okhotsk Plate	Depth: 30 km
1957 Andreanof Islands earthquake	8.6–9.1	Pacific Plate subducting beneath the North American Plate
1960 Great Chilean Earthquake	9.5	Nazca Plate subducting beneath the South American Plate	Depth: 33 km Slip length: 1000 km (625 mi) Slip width: 200 km (125 mi) Slip motion: 20 m (60 ft)
1964 Alaska earthquake ("Good Friday" earthquake)	9.2	Pacific Plate subducting beneath the North American Plate	Duration: 4–5 minutes Depth: 25 km Slip length: 800 km (500 mi) Slip motion: 23 m (69 ft)
2004 Indian Ocean earthquake	9.3	India Plate subducting beneath the Burma Plate	The total vertical displacement measured by sonar survey is about 40 m in the vicinity of the epicenter and occurred as two separate movements which created two large, steep, almost vertical cliffs, one above the other. Duration: 8–10 minutes Depth: 30 km Slip length: 1300 km (1000 mi) Slip motion: 33 m
2010 Chile earthquake	8.8	Nazca Plate subducting beneath the South American Plate

External links

• Natural Resources Canada: Giant Megathrust Earthquakes