Editing Cascadia subduction zone (section)

==Megathrust earthquakes==
[[Image:Cascadia earthquake sources.png|thumb|upright=1.4|3D bloc of Cascadia subduction zone with earthquake sources]]

===Earthquake effects===
[[Megathrust earthquake]]s are the most powerful earthquakes known to occur, and can exceed [[Moment magnitude scale|magnitude]] 9.0, which releases 1,000 times more energy than magnitude 7.0 and 1 million times more energy than a magnitude 5.0.<ref name=Nedimovic/><ref>{{Cite web|last=Haas|first=Ryan|date=2015-03-09|title=What Is A 9.0 Earthquake?|url=https://www.opb.org/news/series/unprepared/what-is-a-90-earthquake-/|url-status=live|access-date=2021-06-21|website=Oregon Public Broadcasting|language=en|archive-url=https://web.archive.org/web/20150126173645/http://www.opb.org:80/news/series/unprepared/what-is-a-90-earthquake-/ |archive-date=2015-01-26 }}</ref><ref>{{Cite web|title="How Much Bigger…?" Calculator|url=https://earthquake.usgs.gov/education/calculator.php|url-status=live|access-date=2021-06-21|publisher=United States Geological Survey|department= Earthquake Hazards Program|archive-url=https://web.archive.org/web/20190928152746/https://earthquake.usgs.gov/education/calculator.php |archive-date=2019-09-28 }}</ref> They occur when enough energy (stress) has accumulated in the "locked" zone of the fault to cause a rupture. The magnitude of a megathrust earthquake is proportional to length of the rupture along the fault. The Cascadia subduction zone, which forms the boundary between the Juan de Fuca and North American plates, is a very long sloping fault that stretches from mid-Vancouver Island to Northern California.<ref name="Nedimovic" />

Because of the great length of the fault, the Cascadia subduction zone is capable of producing very large earthquakes if rupture occurs along its entire length. Thermal and deformation studies indicate that the region 60 kilometers (about 40 miles) [[Strike and dip|downdip]] (east) of the deformation front (where plate deformation begins) is fully locked (the plates do not move past each other). Further downdip, there is a transition from fully locked to [[Aseismic creep|aseismic sliding]].<ref name=Nedimovic>{{cite journal | last1 = Nedimović | first1 = Mladen R. | last2 = Hyndman | first2 = Roy D. | last3 = Ramachandran | first3 = Kumar | last4 = Spence | first4 = George D. | title=Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface | journal=[[Nature (journal)|Nature]] | volume=424 | issue=6947 | date = 24 July 2003 | pages=416–420 | pmid=12879067 | doi=10.1038/nature01840|bibcode = 2003Natur.424..416N | s2cid = 4383885 }}</ref>

In 1999, a group of Continuous [[Global Positioning System]] sites registered a brief reversal of motion of approximately 2 centimeters (0.8&nbsp;inches) over a 50 kilometer by 300 kilometer (about 30 mile by 200 mile) area. The movement was the equivalent of a 6.7 magnitude earthquake.<ref name=Dragert2001>{{cite journal | last1 = Dragert | first1 = Herb | last2 = Wang | first2 = Kelin | last3 = James | first3 = Thomas S. | title=A silent slip event on the deeper Cascadia subduction interface | journal=[[Science (journal)|Science]] | volume=292 | issue=5521 | date = 25 May 2001 | pages=1525–1528 | pmid=11313500 | doi=10.1126/science.1060152|bibcode = 2001Sci...292.1525D | s2cid = 10928887 | doi-access = free }}</ref> The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures.<ref name=rogers2003>{{cite journal | last1 = Rogers | first1 = Garry | last2 = Dragert | first2 = Herb | title=Episodic tremor and slip on the Cascadia subduction zone: the chatter of silent slip | journal=[[Science (journal)|Science]] | volume=300 | issue=5627 | date = 20 June 2003 | pages = 1942–1943 | pmid=12738870 | doi=10.1126/science.1084783|bibcode = 2003Sci...300.1942R | s2cid = 2672381 | doi-access = free }}</ref>

In 2004, a study conducted by the Geological Society of America analyzed the potential for land subsidence along the Cascadia subduction zone. It postulated that several towns and cities on the west coast of Vancouver Island, such as [[Tofino]] and [[Ucluelet]], are at risk for a sudden, earthquake initiated, 1–2 m subsidence.<ref>{{cite journal |title=Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models| first1 = Lucinda J. | last1 = Leonard | first2 = Roy D. | last2 = Hyndman | first3 = Stéphane | last3 = Mazzotti | s2cid = 62833386 | doi = 10.1130/B25369.1 | journal = [[GSA Bulletin]] | volume = 116 | issue = 5–6 | pages = 655–670 | bibcode = 2004GSAB..116..655L |year=2004}}</ref>

===San Andreas Fault connection===
Studies of past earthquake traces on both the northern [[San Andreas Fault]] and the southern Cascadia subduction zone indicate a correlation in time which may be evidence that quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas during at least the past 3,000 years or so.  The evidence also shows the rupture direction going from north to south in each of these time-correlated events.  The [[1906 San Francisco earthquake]] seems to have been a major exception to this correlation, however, as it was not preceded by a major Cascadia quake.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2008/04/080403131923.htm|title=Earthquakes Along The Cascadia And San Andreas Faults May Be Linked, Affecting Risk To San Francisco Bay Region|website=ScienceDaily}}</ref>

=== Earthquake timing ===

{| class="wikitable sortable" align="right" style="text-align:center;"
|+ Great earthquakes
! colspan="2" | estimated year !! interval
|-
! 2005 source<ref name="Atwater-et-al-2005">
{{cite book | author=Brian F Atwater | author2=Musumi-Rokkaku Satoko | author3=Satake Kenji | author4=Tsuji Yoshinobu | author5=Ueda Kazue | author6=David K Yamaguchi | year=2005 | title=The Orphan Tsunami of 1700 – Japanese Clues to a Parent Earthquake in North America | edition=U.S. Geological Survey Professional Paper 1707 | publisher=University of Washington Press | location=Seattle and London | isbn=978-0-295-98535-0 | url=https://archive.org/details/orphantsunamiof100atwa/page/100 | page=[https://archive.org/details/orphantsunamiof100atwa/page/100 100 (timeline diagram)] | url-access=registration }}
</ref> !! 2003 source<ref name="Atwater-et-al-2003">
{{citation
 |author=Brian F Atwater
 |author2=Martitia P Tuttle
 |author3=Eugene S Schweig
 |author4=Charles M Rubin
 |author5=David K Yamaguchi
 |author6=Eileen Hemphill-Haley
 |title=Earthquake Recurrence Inferred from Paleoseismology
 |journal=Developments in Quaternary Science
 |publisher=Elsevier BV
 |issn=1571-0866
 |year=2003
 |volume=1
 |doi=10.1016/S1571-0866(03)01015-7
 |url=http://www.gps.caltech.edu/~simons/TectonicObservatory/AtwaterQuantSciece1.pdf
 |access-date=2011-03-15
 |at=Figures 10 and 11 (pp 341, 342); article pp 331–350
 |url-status=dead
 |archive-url=https://web.archive.org/web/20120319210743/http://www.gps.caltech.edu/~simons/TectonicObservatory/AtwaterQuantSciece1.pdf
 |archive-date=2012-03-19
|series=Developments in Quaternary Sciences
 |isbn=9780444514707
 }}
</ref> !! (years)
<!-- See this article's talk page for more about this table. -->
|-
| colspan="2" | <span style="display:none">Y</span> About 9 p.m., January 26, 1700 ([[Old Style and New Style dates|NS]]) || 780
|-
| <span style="display:none">W</span> 780–1190 [[Common Era|CE]] || 880–960 CE || 210
|-
| <span style="display:none">U</span> 690–730 CE || 550–750 CE || 330
|-
| <span style="display:none">S</span> 350–420 CE || 250–320 CE || 910
|-
| <span style="display:none">N</span> 660-440 [[Common Era|BCE]] || 610–450 BCE || 400
|-
| <span style="display:none">L</span> 980–890 BCE || 910–780 BCE || 250
|-
| <span style="display:none">J</span> 1440–1340 BCE || 1150–1220 BCE || unknown
|}

The last known great earthquake in the northwest was the [[1700 Cascadia earthquake]], {{age in years|26 JAN 1700}} years ago. [[Geology|Geological]] evidence indicates that great earthquakes (> magnitude 8.0) may have occurred sporadically at least seven times in the last 3,500 years, suggesting a return time of about 500 years.<ref name=Discover/><ref name = NatGeo/><ref name = AMNH/> Seafloor core evidence indicates that there have been forty-one subduction zone earthquakes on the Cascadia subduction zone in the past 10,000 years, suggesting a general average earthquake recurrence interval of only 243 years.<ref name="Schulz2015" /> Of these 41, nineteen have produced a "full margin rupture", wherein the entire fault opens up.<ref name="Discover"/> By comparison, similar [[subduction]] zones in the world usually have such earthquakes every 100 to 200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.<ref name=PNSN>{{cite web|url=http://pnsn.org/outreach/earthquakesources/csz|title=Cascadia Subduction Zone|publisher=Pacific Northwest Seismic Network}}</ref>

There is also evidence of accompanying [[tsunami]]s with every earthquake. One strong line of evidence for these earthquakes is convergent timings for fossil damage from tsunamis in the [[Pacific Northwest]] and historical Japanese records of tsunamis.<ref name=japan>{{cite web|url=http://pubs.usgs.gov/pp/pp1707/pp1707.pdf|title=The Orphan Tsunami of 1700—Japanese Clues to a Parent Earthquake in North America|access-date=2008-05-06}} USGS Professional Paper 1707</ref>

The next rupture of the Cascadia subduction zone is anticipated to be capable of causing widespread destruction throughout the [[Pacific Northwest]].<ref name=CREW>{{cite web|url=http://www.crew.org/sites/default/files/CREWCascadiaFinal.pdf|title=Cascade Range Earthquake Workgroup – Magnitude 9 scenario|access-date=2012-03-27|archive-url=https://web.archive.org/web/20121024010654/http://crew.org/sites/default/files/CREWCascadiaFinal.pdf|archive-date=2012-10-24|url-status=dead}}</ref>

=== Forecasts of the next major earthquake ===
{{See also|1700 Cascadia earthquake#Future threats}}
Prior to the 1980s, scientists thought that the subduction zone did not generate earthquakes like other subduction zones around the world, but research by [[Brian Atwater]] and [[Kenji Satake]] tied together evidence of a large tsunami on the Washington coast with documentation of an orphan tsunami in Japan (a tsunami without an associated earthquake). The two pieces of the puzzle were linked, and they then realized that the subduction zone was more hazardous than previously suggested.

In 2009, some geologists predicted a 10% to 14% probability that the Cascadia subduction zone will produce an event of magnitude 9.0 or higher in the next 50 years.<ref name="Sooner">{{cite news | url = http://www.oregonlive.com/news/index.ssf/2009/04/big_earthquake_coming_sooner_t.html | newspaper = [[The Oregonian]] | title=Big earthquake coming sooner than we thought, Oregon geologist says | date=April 19, 2009 | first = Lori | last = Tobias }}</ref> In 2010, studies suggested that the risk could be as high as 37% for earthquakes of magnitude 8.0 or higher.<ref>{{cite news | url = http://www.nature.com/news/2010/100531/full/news.2010.270.html | title = Risk of giant quake off American west coast goes up | first = Richard A. | last = Lovett | access-date = 2010-06-08 | doi = 10.1038/news.2010.270 | journal = [[Nature (journal)|Nature]] | date = 31 May 2010}}</ref><ref>{{cite press release | url = https://www.sciencedaily.com/releases/2010/05/100524121250.htm | title = Odds are about 1-in-3 that mega-earthquake will hit Pacific Northwest in next 50 years, scientists say | date = May 25, 2010 | publisher = [[Oregon State University]] | via = [[Science Daily]] }}</ref>

Geologists and civil engineers have broadly determined that the Pacific Northwest region is not well prepared for such a colossal earthquake. The earthquake is expected to be similar to the [[2011 Tōhoku earthquake and tsunami]], because the rupture is expected to be as long as the [[2004 Indian Ocean earthquake and tsunami]]. The resulting [[tsunami]] might reach heights of approximately 30 meters (100&nbsp;ft).<ref name="Sooner"/> [[FEMA]] estimates some 13,000 fatalities from such an event, with another 27,000 injured, which would make it the deadliest natural disaster in American, and North American, history.<ref name="Schulz2015" /><ref>{{Cite web|last=Roos|first=Dave|date=2020-09-18|title=The Deadliest Natural Disasters in U.S. History|url=https://www.history.com/news/deadliest-natural-disasters-us-storm-flood-hurricane-fire|url-status=live|access-date=2021-06-21|website=HISTORY|archive-url=https://web.archive.org/web/20181203045559/https://www.history.com/news/deadliest-natural-disasters-us-storm-flood-hurricane-fire |archive-date=2018-12-03 }}</ref><ref>{{Cite web|last=Powell|first=Kimberly|date=2018-07-19|title=10 Deadliest Natural Disasters in U.S. History|url=https://www.thoughtco.com/top-deadliest-us-natural-disasters-1422019|url-status=live|access-date=2021-06-21|website=ThoughtCo.|archive-url=https://web.archive.org/web/20171014183218/https://www.thoughtco.com/top-deadliest-us-natural-disasters-1422019 |archive-date=2017-10-14 }}</ref> FEMA further predicts that a million people will be displaced, with yet another 2.5 million requiring food and water. An estimated 1/3 of public safety workers will not respond to the disaster due to a collapse in infrastructure and a desire to ensure the safety of themselves and their loved ones.<ref name="Schulz2015" /> Other analyses predict that even a magnitude 6.7 earthquake in Seattle would result in 7,700 dead and injured, $33 billion in damage, 39,000 buildings severely damaged or destroyed, and 130 simultaneous fires.<ref name = Smithsonian/>