What is a Megathrust Earthquake?

A megathrust earthquake is an interplate earthquake where one tectonic plate slips beneath (subducts) another. Due to the size of the tectonic plates and the shallow dip of the plate boundary, these earthquakes are among the world's largest, with moment magnitudes that can exceed 9.0.

For the most part they occur in the Pacific and Indian Oceans and are connected to 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 (magnitude 9.0) — Juan de Fuca Plate subducting under the North American Plate, slip length 1000 km (625 mi).
• 1737 Kamchatka earthquake (magnitude 9-9.3) — Pacific Plate subducting beneath the Okhotsk Plate, duration 15 minutes, depth 40 km.
• 1755 Lisbon earthquake (magnitude ~9) — believed to be part of a young subduction zone.
• 1952 Kamchatka earthquake (magnitude 9.0) — Pacific Plate subducting beneath the Okhotsk Plate, depth 30 km.
• 1957 Andreanof Islands earthquake (magnitude 8.6) — Pacific Plate subducting under the North American Plate.
• 1960 Great Chilean Earthquake (magnitude 9.5) — 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) — 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) — India Plate subducting under the Burma Plate, duration 8 - 10 minutes, slip length 1600 km (994 mi), slip motion 35 m (108 ft).