A megathrust earthquake occurs at subduction zones (destructive plate boundaries), where one tectonic plate is forced under (subducts) another. Due to the shallow dip of the plate boundary, these earthquakes are among the world's largest, with moment magnitudes (M_w) that can exceed 9.0.

The major subduction zone is associated with the Pacific and Indian Oceans and are responsible for the volcanic activity associated with the Ring of Fire. Since these earthquakes deform the ocean floor, they almost always generate a significant tsunami.

All five earthquakes since 1900 of magnitude 9 or greater have been megathrust earthquakes.

Some examples of megathrust earthquakes are:

• 1700 Cascadia Earthquake (estimated magnitude 8.7-9.2) — Juan de Fuca Plate subducting under the North American Plate, slip length 1000 km (625 mi).
• 1737 Kamchatka earthquake (estimated magnitude 9.0-9.3) — Pacific Plate subducting beneath the Okhotsk Plate, duration 15 minutes, depth 40 km.
• 1755 Lisbon earthquake (estimated magnitude ~9) — hypothesised to be part of a young subduction zone but origin still debated.
• 1952 Kamchatka earthquake (magnitude 9.0 Richter scale of magnitude) — Pacific Plate subducting beneath the Okhotsk Plate, depth 30 km.
• 1957 Andreanof Islands earthquake (magnitude 8.6-9.1 M_w) — Pacific Plate subducting under the North American Plate.
• 1960 Great Chilean Earthquake (magnitude 9.5 M_w) — Nazca Plate subducting under 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 Good Friday Earthquake (magnitude 9.2 M_w) — Pacific Plate subducting under 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 (magnitude 9.3 M_w) — India Plate subducting under the Burma Plate, duration 8 - 10 minutes, slip length 1600 km (1000 mi), slip motion ~33 m, total vertical displacement measured by sonar survey is about 40 m in the vicinity of the epicentre and occurred as two separate movements which created two large steep almost vertical cliffs one above the other.

External links

• National Resources Canada: Giant Megathrust Earthquakes