Editing Megathrust earthquake (section)

==Terminology and mechanism==
[[Image:Subduction-en.svg|right|thumb|400px|Diagram of a [[subduction]] zone. The megathrust fault lies on the top of the subducting slab where it is in contact with the overriding plate.]]

The term ''megathrust'' refers to an extremely large [[thrust fault]], typically formed at the plate interface along a subduction zone, such as the [[Sunda megathrust]].<ref name="ParkButler2005">{{cite journal | first1=J. | first10=G. | first11=K. | first12=R. | last1=Park | last10=Ekstrom | last11=Anderson | last12=Aster | title=Performance Review of the Global Seismographic Network for the Sumatra-Andaman Megathrust Earthquake | last2=Butler | first2=R. | last3=Anderson | first3=K. | last4=Berger | first4=J. | last5=Davis | first5=P. | last6=Benz | first6=H. | last7=Hutt | first7=C. R. | last8=McCreery | first8=C. S. | last9=Ahern | first9=T. | journal=Seismological Research Letters | year=2005 | volume=76 | issue=3 | pages=331–343 | issn=0895-0695 | doi=10.1785/gssrl.76.3.331 | bibcode=2005SeiRL..76..331P | display-authors=3}}</ref><ref name=BilekLay2018>{{cite journal |last1=Bilek |first1=Susan L. |last2=Lay |first2=Thorne |title=Subduction zone megathrust earthquakes |journal=Geosphere |date=1 August 2018 |volume=14 |issue=4 |pages=1468–1500 |doi=10.1130/GES01608.1|bibcode=2018Geosp..14.1468B |s2cid=133629102 |doi-access=free }}</ref> However, the term is also occasionally applied to large thrust faults in continental collision zones, such as the [[Himalayas|Himalayan]] megathrust.<ref>{{cite journal |last1=Elliott |first1=J.R.|last2=Jolivet |first2=R. |last3=González |first3=P. J. |last4=Avouac |first4=J.-P. |last5=Hollingsworth |first5=J. |last6=Searle |first6=M. P. |last7=Stevens |first7=V.L.|title=Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake |journal=Nature Geoscience |date=February 2016 |volume=9 |issue=2 |pages=174–180 |doi=10.1038/ngeo2623|bibcode=2016NatGe...9..174E|url=http://eprints.whiterose.ac.uk/95920/8/GonzalezHimalayan%20Megathrust%20Geometry.pdf}}</ref> A megathrust fault can be {{convert|1000|km|sigfig=1|sp=us}} long.<ref name="PNSN"/>

[[File:Nor rev.png|thumb|upright=1.25|right|Cross-sectional illustration of normal and reverse faults]]
A thrust fault is a type of [[reverse fault]], in which the rock above the fault is displaced upwards relative to the rock below the fault. This distinguishes reverse faults from [[normal fault]]s, where the rock above the fault is displaced downwards, or [[strike-slip fault]]s, where the rock on one side of the fault is displaced horizontally with respect to the other side. Thrust faults are distinguished from other reverse faults because they dip at a relatively shallow angle, typically less than 45°,<ref name="DipSlipDefinition">{{cite web | url=https://earthquake.usgs.gov/learn/glossary/?termID=59 | title=Earthquake Glossary – dip slip | department=Earthquake Hazards Program| publisher=United States Geological Survey }}</ref> and show large displacements.<ref>{{cite book |last1=Fossen |first1=Haakon |title=Structural geology |date=2016 |publisher=Cambridge University Press |location=Cambridge, United Kingdom |isbn=9781107057647 |pages=485, 488, 491 |edition=Second}}</ref><ref name="ThrustDefinition">{{cite web | url=http://nthmp-history.pmel.noaa.gov/terms.html | title=Tsunami Terminology | work=The National Tsunami Hazard Mitigation Program History, 1995–2005 | publisher=Pacific Marine Environmental Laboratory | url-status=dead | archive-url=https://web.archive.org/web/20110225143835/http://nthmp-history.pmel.noaa.gov/terms.html | archive-date=2011-02-25 }}</ref> In effect, the rocks above the fault have been thrust over the rocks below the fault. Thrust faults are characteristic of areas where the [[Earth's crust]] is being compressed by tectonic forces.{{sfn|Fossen|2016|p=356}}

Megathrust faults occur where two [[tectonic plates]] collide. When one of the plates is composed of [[oceanic lithosphere]], it dives beneath the other plate (called the ''overriding plate'') and sinks into the [[Earth's mantle]] as a ''[[slab (geology)|slab]]''. The contact between the colliding plates is the megathrust fault, where the rock of the overriding plate is displaced upwards relative to the rock of the descending slab.<ref name=BilekLay2018/> Friction along the megathrust fault can lock the plates together, and the subduction forces then build up strain in the two plates. A megathrust earthquake takes place when the fault ruptures, allowing the plates to abruptly move past each other to release the accumulated strain energy.<ref name="PNSN">{{cite web |title=Cascadia Subduction Zone |url=https://www.pnsn.org/outreach/earthquakesources/csz |publisher=Pacific Northwest Seismic Network |access-date=7 October 2021}}</ref>