Megathrust earthquake

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Megathrust earthquakes are interplate earthquakes that occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is subducted (forced underneath) by another. These type of earthquakes are the planet's most powerful, with moment magnitudes (M_w) that can exceed 9.0. Since 1900, all six 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[edit]

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. 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. 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.[3][4]

Areas[edit]

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 almost always 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.

The Big One[edit]

"The Big One" is a term often used in casual conversation by residents of California, Oregon, Washington, and British Columbia to describe the megathrust earthquake anticipated as inevitably striking the Cascadia Subduction Zone. Much of the United States infrastructure has been hardened specifically for the anticipation of "The Big One". The name has also been applied to a megathrust earthquake expected to happen in Tokyo, Kantō region, Japan with the epicenter in Sagami Bay where the Philippine Sea Plate and North American Plate movements cause large earthquakes regularly, with an interval of approximately 70 years.[citation needed]

Examples[edit]

Examples of megathrust earthquakes are listed in the following table.

Event Estimated Magnitude
(M_W)
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.[5]
869 Jogan 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[6] 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 [7] 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 from ten to a hundred thousand deaths.
  • The grave of Nuno Álvares Pereira was forever lost 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
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 Maule earthquake ("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[8][9][10]
  • Duration: 6 minutes
  • Depth: 30 km
  • Slip length: 500 km (310 mi)
  • Slip width: 200 km (125 mi)
  • Slip motion: 20 m (60 ft)

References[edit]

  1. ^ Tsunami Terms
  2. ^ Earthquake Glossary - dip slip
  3. ^ Geophysical literature search showing almost 200 papers with the word "megathrust" in the title University of Strasbourg
  4. ^ Park; et al. (2005). "Performance Review of the Global Seismographic Network for the Sumatra-Andaman Megathrust Earthquake". Seismological Research Letters 76 (3): 331–343. doi:10.1785/gssrl.76.3.331. 
  5. ^ [dead link]http://ssn.dgf.uchile.cl/home/terrem.html
  6. ^ Yuzo Ishikawa.(2012).Re-evaluation of Mw of the 1707 Hoei earthquake. PDF
  7. ^ Gutscher, M.-A.; Baptista M.A. & Miranda J.M. (2006). "The Gibraltar Arc seismogenic zone (part 2): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by tsunami modeling and seismic intensity". Tectonophysics 426: 153–166. Bibcode:2006Tectp.426..153G. doi:10.1016/j.tecto.2006.02.025. 
  8. ^ "Magnitude 8.9 – NEAR THE EAST COAST OF HONSHU, JAPAN 2011 March 11 05:46:23 UTC". USGS. 11 March 2011. Retrieved 11 March 2011. 
  9. ^ Kenneth Kidd (12 March 2011). "How ‘mega-thrust" earthquake caught forecasters by surprise". Toronto Star. Retrieved 12 March 2011. 
  10. ^ Reilly, Michael (11 March 2011). "Japan's quake updated to magnitude 9.0". New Scientist (Short Sharp Science ed.). Retrieved 11 March 2011. 

External links[edit]