Megathrust earthquake

Megathrust earthquakes occur at subduction zones at destructive convergent plate boundaries, where one tectonic plate is forced underneath another. These interplate earthquakes are the planet's most powerful, with moment magnitudes (M_w) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes. No other type of known terrestrial source of tectonic activity has produced 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. 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.

Areas

The major subduction zone is associated with the Pacific and Indian Oceans and is responsible for the volcanic activity associated with the Pacific Ring of Fire. Since these earthquakes deform the ocean floor, they often generate a significant series of tsunami waves. They are known to produce intense shaking for periods of time that can last for up to a few minutes.

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

Examples

Examples of megathrust earthquakes are listed in the following table.

Event	Estimated Magnitude (Mw)	Tectonic Plates Involved	Other Details/Notes
0365 Crete earthquake	8.0+	African Plate subducting beneath the Aegean Sea Plate	The quake generated a large tsunami in the eastern Mediterranean Sea and caused significant vertical displacement in the island of Crete.
869 Sanriku earthquake	8.6–9.0	Pacific Plate subducting beneath the Okhotsk Plate	Slip length: 200 km over (125 mi over) Slip width: 85 km over (53 mi over)
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) Slip motion: 20 m (60 ft)
1707 Hōei earthquake	8.6–9.3	Philippine Sea Plate subducting beneath the Eurasian Plate	Duration: approximately 10 minutes Slip length: maybe 600 and 700 km (370 and 435 mi)
1737 Kamchatka earthquake	8.3–9.0	Pacific Plate subducting beneath the Okhotsk Plate	Duration: 15 minutes Depth: 40 km Slip length: maybe 700 km over (435 mi over)
1755 Lisbon earthquake	8.5–9.0	Hypothesized to be part of a young subduction zone but origin still debated	Destroyed Lisbon and was followed by a 20 metre high tsunami and many fires. It caused 10,000-100,000 deaths. The grave of Nuno Álvares Pereira was destroyed, along with the historical records of the voyages of Vasco da Gama and Christopher Columbus
1868 Arica earthquake	8.5–9.0	Nazca Plate subducting beneath the South American Plate	Slip length: 600 km (370 mi)
1877 Iquique earthquake	8.5–9.0?	Nazca Plate subducting beneath the South American Plate	Slip length: 420 and 450 km (230 and 245 mi)
1906 Ecuador–Colombia earthquake	8.8	Nazca Plate subducting beneath the South American Plate
1946 Nankaidō earthquake	8.1	Philippine Sea Plate subducting beneath the Eurasian Plate	Slip length: maybe 300 km (190 mi)
1952 Kamchatka earthquake	9.0	Pacific Plate subducting beneath the Okhotsk Plate	Depth: 30 km Slip length: maybe 600 km over (370 mi over)
1957 Andreanof Islands earthquake	8.6	Pacific Plate subducting beneath the North American Plate	Depth: 33 km Slip length: maybe 700 km over (435 mi over)
1960 Great Chilean earthquake	9.5	Nazca Plate subducting beneath the South American Plate	Duration: 5–6 minutes Depth: 33 km Slip length: 850 and 1000 km (530 and 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 and 850 km (500 and 530 mi) Slip width: 250 km (155 mi) Slip motion: 23 m (70 ft)
2001 southern Peru earthquake	8.4	Nazca Plate subducting beneath the South American Plate	Depth: 33 km Slip length: 200 km (125 mi) Slip width: 110 km (70 mi)
2004 Sumatra-Andaman earthquake ("Indian Ocean earthquake")	9.1–9.3	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: 1000 and 1300 km (625 and 810 mi) Slip width: 180 km (110 mi) Slip motion: 33 m (110 ft)
2010 Chile earthquake	8.8	Nazca Plate subducting beneath the South American Plate	Depth: 35 km Slip length: 500 km (310 mi) Slip width: 200 km (125 mi)
2011 Tōhoku earthquake and tsunami	9.0	Pacific Plate subducting beneath the Okhotsk Plate	Duration: 6 minutes Depth: 30 km Slip length: 500 km (310 mi) Slip width: 200 km (125 mi) Slip motion: 20 m (60 ft)
2014 Iquique earthquake	8.2	Nazca Plate subducting beneath the South American Plate	Depth: 20.1 km Slip length: 170 km (105 mi) Slip width: 70 km (45 mi)
2015 Illapel earthquake	8.3	Nazca Plate subducting beneath the South American Plate	Depth: 25 km Slip length: 260 km (160 mi) Slip width: 80 km (50 mi)

External links

• Giant Megathrust Earthquakes – Natural Resources Canada