Megathrust earthquake

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Megathrust earthquakes occur at subduction zones at destructive plate boundaries (convergent boundaries), where one tectonic plate is forced under (subducts) 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 (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 tectonic activity can produce 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.^[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

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 long periods, such as several minutes.

Examples

Examples of megathrust earthquakes are listed in the following table.

Event	Estimated Moment Magnitude (Mw)	Tectonic Plates Involved	Other Details/Notes
365 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]
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)
1737 Kamchatka earthquake	9.0–9.3	Pacific Plate subducting beneath the Okhotsk Plate	Duration: 15 minutes Depth: 40 km Slip length: maybe 900 km over (560 mi over)
1755 Lisbon earthquake	9.0	Hypothesized to be part of a young subduction zone but origin still debated
1868 Arica earthquake	9.0	Nazca Plate subducting beneath the South American Plate	Slip length: maybe 600 km over (370 mi over)
1877 Iquique earthquake	8.8	Nazca Plate subducting beneath the South American Plate
1946 Nankaidō earthquake	8.1	Philippine Sea Plate subducting beneath the Eurasian Plate
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	8.6–9.1	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	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 width: 210 km (130 mi) Slip motion: 23 m (69 ft)
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: 1600 km (994 mi) Slip width: 180 km (112 mi) Slip motion: 33 m (108 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[6][7][8]	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

1. ^ Tsunami Terms
2. ^ Earthquake Glossary - dip slip
3. ^ http://cdsads.u-strasbg.fr/cgi-bin/nph-abs_connect?db_key=AST&db_key=PHY&db_key=PRE&qform=AST&arxiv_sel=astro-ph&arxiv_sel=cond-mat&arxiv_sel=cs&arxiv_sel=gr-qc&arxiv_sel=hep-ex&arxiv_sel=hep-lat&arxiv_sel=hep-ph&arxiv_sel=hep-th&arxiv_sel=math&arxiv_sel=math-ph&arxiv_sel=nlin&arxiv_sel=nucl-ex&arxiv_sel=nucl-th&arxiv_sel=physics&arxiv_sel=quant-ph&arxiv_sel=q-bio&sim_query=YES&ned_query=YES&adsobj_query=YES&aut_logic=OR&obj_logic=OR&author=&object=&start_mon=&start_year=&end_mon=&end_year=&ttl_logic=OR&title=megathrust&txt_logic=OR&text=&nr_to_return=200&start_nr=1&jou_pick=ALL&ref_stems=&data_and=ALL&group_and=ALL&start_entry_day=&start_entry_mon=&start_entry_year=&end_entry_day=&end_entry_mon=&end_entry_year=&min_score=&sort=SCORE&data_type=SHORT&aut_syn=YES&ttl_syn=YES&txt_syn=YES&aut_wt=1.0&obj_wt=1.0&ttl_wt=0.3&txt_wt=3.0&aut_wgt=YES&obj_wgt=YES&ttl_wgt=YES&txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1 Geophysical literature search showing almost 200 papers with the word "megathrust" in the title
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. ^ http://ssn.dgf.uchile.cl/home/terrem.html
6. ^ "Magnitude 8.9 – NEAR THE EAST COAST OF HONSHU, JAPAN 2011 March 11 05:46:23 UTC". USGS. 11 March 2011. http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/usc0001xgp.php. Retrieved 11 March 2011. 
7. ^ Kenneth Kidd (12 March 2011). "How ‘mega-thrust” earthquake caught forecasters by surprise". Toronto Star. http://www.thestar.com/news/world/article/952957--how-mega-thrust-earthquake-caught-forecasters-by-surprise. Retrieved 12 March 2011. 
8. ^ Reilly, Michael (11 March 2011<!- – 17:22 GMT -->). "Japan's quake updated to magnitude 9.0". New Scientist. http://www.newscientist.com/blogs/shortsharpscience/2011/03/powerful-japan-quake-sparks-ts.html. Retrieved 11 March 2011. 

External links

• Natural Resources Canada: Giant Megathrust Earthquakes