Editing Megathrust earthquake (section)

==Occurrence and characteristics==
{{See also|List of megathrust earthquakes}}
Megathrust earthquakes are almost exclusive to tectonic subduction zones and are often associated with the [[Pacific Ocean|Pacific]] and [[Indian Ocean]]s.<ref name=BilekLay2018/> These subduction zones are also largely responsible for the [[volcanic]] activity associated with the Pacific [[Ring of Fire]].<ref name="NOAO">{{cite web |title=What is the Ring of Fire? |url=https://oceanexplorer.noaa.gov/facts/rof.html |website=Ocean exploration |publisher=National Ocean and Atmospheric Administration |access-date=7 October 2021}}</ref>

Since these earthquakes deform the [[ocean floor]], they often generate strong [[tsunami]] waves.<ref>{{cite journal |last1=Maksymowicz |first1=A. |last2=Chadwell |first2=C. D. |last3=Ruiz |first3=J. |last4=Tréhu |first4=A. M. |last5=Contreras-Reyes |first5=E. |last6=Weinrebe |first6=W. |last7=Díaz-Naveas |first7=J. |last8=Gibson |first8=J. C. |last9=Lonsdale |first9=P. |last10=Tryon |first10=M. D. |title=Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake |journal=Scientific Reports |date=April 2017 |volume=7 |issue=1 |pages=45918 |doi=10.1038/srep45918|pmid=28378757 |pmc=5381107 |bibcode=2017NatSR...745918M }}</ref> Subduction zone earthquakes are also known to produce intense shaking and ground movements that can last for up to 3–5 minutes.<ref>{{cite journal |last1=Megawati |first1=K. |last2=Pan |first2=T.-C. |title=Regional Seismic Hazard Posed by the Mentawai Segment of the Sumatran Megathrust |journal=Bulletin of the Seismological Society of America |date=1 April 2009 |volume=99 |issue=2A |pages=566–584 |doi=10.1785/0120080109|bibcode=2009BuSSA..99..566M }}</ref>

In the [[Indian Ocean]] region, the [[Sunda megathrust]] is located where the [[Indo-Australian plate]] subducts under the [[Eurasian plate]] along a {{convert|5500|km|}} fault off the coasts of [[Myanmar]], [[Sumatra]], [[Java]] and [[Bali]], terminating off the northwestern coast of [[Australia]]. This subduction zone was responsible for the [[2004 Indian Ocean earthquake and tsunami]].<ref name="Sieh">{{cite journal |last1=Sieh |first1=Kerry |journal=Journal of Earthquake and Tsunami |date=March 2007 |volume=01 |issue=1 |pages=1–19 |doi=10.1142/S179343110700002X | title = The Sunda megathrust: past, present and future}}</ref> In parts of the megathrust south of [[Java]], referred to as the [[Sunda Trench|Java Trench]], for the western part, {{M|w}} 8.9 is possible, while in the eastern Java segment, {{M|w}} 8.8 is possible, while if both were to rupture at the same time, the magnitude would be {{M|w}} 9.1.<ref>{{Cite journal |last1=Widiyantoro |first1=S. |last2=Gunawan |first2=E. |last3=Muhari |first3=A. |last4=Rawlinson |first4=N. |last5=Mori |first5=J. |last6=Hanifa |first6=N. R. |last7=Susilo |first7=S. |last8=Supendi |first8=P. |last9=Shiddiqi |first9=H. A. |last10=Nugraha |first10=A. D. |last11=Putra |first11=H. E. |date=2020-09-17 |title=Implications for megathrust earthquakes and tsunamis from seismic gaps south of Java Indonesia |journal=Scientific Reports |language=en |volume=10 |issue=1 |pages=15274 |doi=10.1038/s41598-020-72142-z |pmid=32943680 |pmc=7499206 |bibcode=2020NatSR..1015274W |issn=2045-2322}}</ref>

In the [[South China Sea]] lies the [[Manila Trench]], which is capable of producing {{M|w}} 9.0 or larger earthquakes,<ref>{{Cite journal |last1=Megawati |first1=Kusnowidjaja |last2=Shaw |first2=Felicia |last3=Sieh |first3=Kerry |last4=Huang |first4=Zhenhua |last5=Wu |first5=Tso-Ren |last6=Lin |first6=Yunung |last7=Tan |first7=Soon Keat |last8=Pan |first8=Tso-Chien |date=2009-09-04 |title=Tsunami hazard from the subduction megathrust of the South China Sea: Part I. Source characterization and the resulting tsunami |url=https://www.sciencedirect.com/science/article/pii/S1367912008001922 |journal=Journal of Asian Earth Sciences |series=Tsunamis in Asia |volume=36 |issue=1 |pages=13–20 |doi=10.1016/j.jseaes.2008.11.012 |bibcode=2009JAESc..36...13M |hdl=10220/8672 |issn=1367-9120|hdl-access=free }}</ref> with the maximum magnitude at Mw 9.2 or higher.<ref>{{Cite journal |last1=Zhao |first1=Guangsheng |last2=Niu |first2=Xiaojing |date=2024-01-26 |title=Tsunami Hazard Assessment in the South China Sea Based on Geodetic Locking of the Manila Subduction Zone |url=https://nhess.copernicus.org/preprints/nhess-2023-227/ |journal=Natural Hazards and Earth System Sciences Discussions |volume=24 |issue=7 |language=English |pages=2303–2313 |doi=10.5194/nhess-2023-227|doi-access=free }}</ref>

In Japan, the Nankai megathrust under the [[Nankai Trough]] is responsible for [[Nankai megathrust earthquakes]] and associated tsunamis.<ref name="Hirahara">{{cite web |url=http://www.jamstec.go.jp/esc/publication/annual/annual2004/pdf/3project/chapter2/6hirahara.pdf |title=Simulation of Earthquake Generation Process in a Complex System of Faults |last=Hirahara |first=K. |author2=Kato N. |author3=Miyatake T. |author4=Hori T. |author5=Hyodo M. |author6=Inn J. |author7=Mitsui N. |author8=Sasaki T. |author9=Miyamura T. |author10=Nakama Y. |author11=Kanai T. |year=2004 |work=Annual Report of the Earth Simulator Center April 2004 – March 2005 |pages=121–126 |access-date=2009-11-14 |archive-date=2011-09-27 |archive-url=https://web.archive.org/web/20110927124357/http://www.jamstec.go.jp/esc/publication/annual/annual2004/pdf/3project/chapter2/6hirahara.pdf |url-status=dead }}</ref> The largest megathrust event within the last 20 years was the magnitude 9.0–9.1 [[2011 Tōhoku earthquake and tsunami|Tōhoku earthquake]] along the [[Japan Trench]] megathrust.<ref>{{cite web |date=7 November 2016 |title=M 9.1 – 2011 Great Tohoku Earthquake, Japan |url=https://earthquake.usgs.gov/earthquakes/eventpage/official20110311054624120_30/executive#executive |access-date=3 June 2022 |department=Earthquake Hazards Program |publisher=United States Geological Survey}}</ref>

In North America, the [[Juan de Fuca plate]] subducts under the [[North American plate]], creating the [[Cascadia subduction zone]] from mid Vancouver Island, British Columbia down to Northern California. This subduction zone was responsible for the [[1700 Cascadia earthquake]].<ref>{{cite news |title=A Major Earthquake in the Pacific Northwest Looks Even Likelier |url=https://www.theatlantic.com/science/archive/2016/08/a-major-earthquake-in-the-pacific-northwest-just-got-more-likely/495407/ |work=The Atlantic |date=August 16, 2016}}</ref> The [[Aleutian Trench]], of the southern coast of [[Alaska]] and the [[Aleutian Islands]], where the North American plate overrides the [[Pacific plate]], has generated many major earthquakes throughout history, several of which generated Pacific-wide tsunamis,<ref>{{cite journal |last1=Witter |first1=Rob |last2=Briggs |first2=Rich |last3=Engelhart |first3=Simon E. |last4=Gelfenbaum |first4=Guy |last5=Koehler |first5=Rich D. |last6=Nelson |first6=Alan |last7=Selle |first7=SeanPaul La |last8=Corbett |first8=Reide |last9=Wallace |first9=Kristi |title=Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust |journal=GSA Bulletin |date=1 May 2019 |volume=131 |issue=5–6 |pages=707–729 |doi=10.1130/B32031.1|bibcode=2019GSAB..131..707W |s2cid=134362013 }}</ref> including the [[1964 Alaska earthquake]]; at magnitude 9.1–9.2, it remains the largest recorded earthquake in North America, and the third-largest earthquake instrumentally recorded in the world.<ref>{{Cite journal |last1=Ichinose |first1=Gene |last2=Somerville |first2=Paul |last3=Thio |first3=Hong Kie |last4=Graves |first4=Robert |last5=O'Connell |first5=Dan |date=2007 |title=Rupture process of the 1964 Prince William Sound, Alaska, earthquake from the combined inversion of seismic, tsunami, and geodetic data |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2006JB004728 |journal=Journal of Geophysical Research: Solid Earth |language=en |volume=112 |issue=B7 |doi=10.1029/2006JB004728 |bibcode=2007JGRB..112.7306I |issn=0148-0227}}</ref>

In the [[Himalayan region]], where the [[Indian plate]] subducts under the [[Eurasian plate]], the largest recorded earthquake was the [[1950 Assam–Tibet earthquake]], at magnitude 8.7. It is estimated that earthquakes with magnitude 9.0 or larger are expected to occur at an interval of every 800 years, with the highest boundary being a magnitude 10, though this is not considered physically possible. Therefore, the largest possible earthquake in the region is a magnitude 9.7, assuming a single rupture of the whole Himalayan arc and assuming standard scaling law, which implies an average slip of 50 m.<ref>{{Cite journal |last1=Stevens |first1=V. L. |last2=Avouac |first2=J.-P. |date=2016-02-16 |title=Millenary M w > 9.0 earthquakes required by geodetic strain in the Himalaya |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL067336 |journal=Geophysical Research Letters |language=en |volume=43 |issue=3 |pages=1118–1123 |doi=10.1002/2015GL067336 |issn=0094-8276}}</ref>

A megathrust earthquake could occur in the [[Lesser Antilles subduction zone]], with a maximum magnitude of 9.3, or potentially even 10.3 through recent evaluations, a value not considered impossible.<ref name=":0">{{Cite journal |last1=Roger |first1=J. |last2=Frère |first2=A. |last3=Hébert |first3=H. |date=2014-07-25 |title=Impact of a tsunami generated at the Lesser Antilles subduction zone on the Northern Atlantic Ocean coastlines |url=https://adgeo.copernicus.org/articles/38/43/2014/ |journal=Advances in Geosciences |language=English |volume=38 |pages=43–53 |doi=10.5194/adgeo-38-43-2014 |doi-access=free |bibcode=2014AdG....38...43R |issn=1680-7340}}</ref>

The largest recorded megathrust earthquake was the [[1960 Valdivia earthquake]], estimated between magnitudes 9.4–9.6, centered off the coast of Chile along the [[Peru-Chile Trench]], where the [[Nazca plate]] subducts under the [[South American plate]].<ref>{{cite journal |last1=Ojeda |first1=Javier |last2=Ruiz |first2=Sergio |last3=del Campo |first3=Francisco |last4=Carvajal |first4=Matías |title=The 21 May 1960 Mw 8.1 Concepción Earthquake: A Deep Megathrust Foreshock That Started the 1960 Central-South Chilean Seismic Sequence |journal=Seismological Research Letters |date=1 May 2020 |volume=91 |issue=3 |pages=1617–1627 |doi=10.1785/0220190143|bibcode=2020SeiRL..91.1617O |s2cid=216347638 }}</ref> This megathrust region has regularly generated extremely large earthquakes.

The largest possible earthquakes are estimated at magnitudes of 10 to 11, most likely caused by a combined rupture of the [[Japan Trench]] and [[Kuril–Kamchatka Trench]], or individually the [[Aleutian Trench]] or Peru–Chile Trench.<ref>{{Cite web |last=Kyodo |date=2012-12-15 |title=Magnitude 10 temblor could happen: study |url=https://www.japantimes.co.jp/news/2012/12/15/national/magnitude-10-temblor-could-happen-study/ |access-date=2023-10-20 |website=The Japan Times |language=en}}</ref><ref>{{Cite journal |last=Matsuzawa |first=Toru |date=2014-06-01 |title=The Largest Earthquakes We Should Prepare for |url=https://www.fujipress.jp/jdr/dr/dsstr000900030248/ |journal=Journal of Disaster Research |volume=9 |issue=3 |pages=248–251 |doi=10.20965/jdr.2014.p0248|doi-access=free }}</ref><ref>{{Cite journal |last1=Hirose |first1=Fuyuki |last2=Maeda |first2=Kenji |last3=Yoshida |first3=Yasuhiro |date=2019-12-01 |title=Maximum magnitude of subduction earthquakes along the Japan-Kuril-Kamchatka trench estimated from seismic moment conservation |url=https://academic.oup.com/gji/article/219/3/1590/5553983 |journal=Geophysical Journal International |language=en |volume=219 |issue=3 |pages=1590–1612 |doi=10.1093/gji/ggz381 |doi-access=free |issn=0956-540X|url-access=subscription }}</ref><ref>{{Cite journal |last1=Yoshida |first1=Masaki |last2=Santosh |first2=M. |date=2020-07-01 |title=Energetics of the Solid Earth: An integrated perspective |journal=Energy Geoscience |volume=1 |issue=1–2 |pages=28–35 |doi=10.1016/j.engeos.2020.04.001 |bibcode=2020EneG....1...28Y |issn=2666-7592|doi-access=free }}</ref> Another possible area could be the Lesser Antilles subduction zone.<ref name=":0" />

A study reported in 2016 found that the largest megathrust quakes are associated with downgoing slabs with the shallowest dip, so-called [[flat slab subduction]].<ref>{{cite journal|url=https://www.eurekalert.org/pub_releases/2016-11/uoo-fcm112216.php |title=Fault curvature may control where big quakes occur, Eurekalert 24-NOV-2016 |journal=Science |volume=354 |issue=6315 |pages=1027–1031 |doi=10.1126/science.aag0482 |pmid=27885027 |date=2016-11-24 |access-date=2018-06-05|last1=Bletery |first1=Quentin |last2=Thomas |first2=Amanda M. |last3=Rempel |first3=Alan W. |last4=Karlstrom |first4=Leif |last5=Sladen |first5=Anthony |last6=De Barros |first6=Louis |doi-access=free |bibcode=2016Sci...354.1027B }}</ref>

Compared with other earthquakes of similar magnitude, megathrust earthquakes have a longer duration and slower rupture velocities. The largest megathrust earthquakes occur in subduction zones with thick sediments, which may allow a fault rupture to propagate for great distances unimpeded.<ref name=BilekLay2018/>