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megathrust earthquake

 
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A very large earthquake that occurs when stress accumulates in the bedrock between two locked tectonic plates. Megathrust earthquakes occur in subduction zones in which one of the earth's plates is thrust under the other. When there is a sudden slip along a fault between a subducting and an overriding plate, the result is a megathrust earthquake.

The most powerful of all earthquakes, megathrusts often register 9 or above on the Richter scale. They can cause underwater landslides off a continental shelf into deep oceans. Megathrust earthquakes can also cause tsunamis if they cause vertical movements on the sea floor that displace a large volume of water.

Last updated: December 28, 2004.

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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 (Mw) 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 earthquake can produce earthquakes of this scale.

Contents

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.[1] They are one type of dip-slip faults, also dip slip. A thrust fault is a reverse fault with a dip of 45° or less.[2] Oblique-slip faults have significant components of different slip styles. Though megathrust or mega-thrust has no exact definition, the term is widely used[3] 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 include the following:

Event Estimated Moment Magnitude (Mw) 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.[4]
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
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[5]
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: 1600 km (1000 mi)
  • Slip motion: 33 m

References

  1. ^ http://nthmp-history.pmel.noaa.gov/terms.html
  2. ^ http://earthquake.usgs.gov/learn/glossary/?termID=59
  3. ^ [1]
  4. ^ http://ssn.dgf.uchile.cl/home/terrem.html
  5. ^ Megathrust earthquakes, coastal uplift, and emergent marine terraces of Costa Rica's Nicoya Peninsula, Jeff Marshall Cal Poly Pomona University

External links


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