2021 Chignik earthquake#October 2020

Off the coast of Alaska lies the Aleutian subduction zone–a 2,500 mile long convergent plate boundary where the Pacific plate subducts under the North American plate at a rate of 6–7 cm/yr. This megathrust fault has been the source of many large earthquakes including the 1964 Alaskan earthquake that registered a magnitude 9.2 and remains the second largest earthquake in recorded history.{{cite web|title=M 8.6 – 1946 Aleutian Islands (Unimak Island) Earthquake, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/official19460401122901_30/executive|publisher=United States Geological Survey|access-date=9 July 2021}}{{cite web|title=M 9.2 – 1964 Prince William Sound Earthquake, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/official19640328033616_30/executive|publisher=United States Geological Survey|access-date=9 July 2021}}

South of the Alaska Peninsula is a segment of the Aleutian subduction zone known as the Shumagin segment, named after the nearby Shumagin Islands. This segment lies between the Unimak and Semidi segments which had ruptured in earthquakes in 1946 and 1938 respectively.{{cite journal|author1=Roland von Huene|author2=John J. Miller|author3=Anne Krabbenhoeft|title=The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin|journal=Geosphere|date=2019|volume=15|issue=2|pages=324–341|doi=10.1130/GES01657.1|bibcode=2019Geosp..15..324V|s2cid=134488756|doi-access=free}} In 1971, the Shumagin segment drew the attention of seismologists as it was proposed as a seismic gap because no major seismic activity have occurred in recent times. The most recent events is thought to have occurred in 1788, which were a pair of large events, and in 1854.{{cite journal|author1=Chengli Liu|author2=Thorne Lay|author3=Xiong Xiong|author4=Yangmao Wen|title=Rupture of the 2020 MW 7.8 Earthquake in the Shumagin Gap Inferred From Seismic and Geodetic Observations|journal=Geophysical Research Letters|date=2020|volume=17|issue=22|doi=10.1029/2020GL090806|bibcode=2020GeoRL..4790806L|s2cid=228935781|url=https://websites.pmc.ucsc.edu/~thorne/TL.pdfs/LLXW_Shumagin_GRL2020.pdf}} Two smaller earthquakes shook near the seismic gap in 1917 and 1948.{{cite web|title=M 7.4 – 81 km SE of Sand Point, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/iscgem913517/executive#executive|publisher=United States Geological Survey|access-date=2 August 2021}}{{cite web|title=M 7.1 – 88 km SE of King Cove, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/iscgem897327/executive|publisher=United States Geological Survey|access-date=2 August 2021}} That proposal also highlighted the potential for a tsunami accompanying an earthquake on the gap.{{cite journal|author1=Lynn Ray Sykes|title=Aftershock zones of great earthquakes, seismicity gaps, and earthquake prediction for Alaska and the Aleutians|journal=Journal of Geophysical Research|date=1971|volume=76|issue=32|pages=8021–8041|doi=10.1029/JB076i032p08021|bibcode=1971JGR....76.8021S}}

The 1938 Semidi earthquake occurred on November 10 at 20:18 UTC. It had an estimated magnitude of 8.2 on the moment magnitude scale, and had an epicenter 40 km west of the epicenter of the 2021 earthquake. It was felt with a maximum Modified Mercalli intensity of VI (Strong), causing limited damage in the relatively unpopulated region of the Alaska Peninsula. While the earthquake generated an ocean-wide tsunami, the maximum wave height was only measured at 0.3 meters.{{cite web|title=Tsunami Event Information|url=https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/event-more-info/1722|publisher=National Geophysical Data Center|access-date=2 August 2021}} An unusually weak tsunami was generated because the earthquake occurred at a deep depth of 35 km.

The 1938 earthquake ruptured east of the Shumagin segment for a length of approximately 300 km. Rupture of this earthquake did not reach into the Shumagin segment.{{cite journal|author1=Kenji Satake|author2=Jean M. Johnson|title=Rupture extent of the 1938 Alaskan earthquake as inferred from tsunami waveforms|journal=Geophysical Research Letters|date=1994|volume=21|issue=8|pages=733–736|doi=10.1029/94GL00333|bibcode=1994GeoRL..21..733J|url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/94754/grl7445.pdf?sequence=1|hdl=2027.42/94754|hdl-access=free}}

In July 2020, the Aleutian Subduction Zone was the source of an {{M|w}} 7.8 earthquake which struck the same region south of the Alaska Peninsula. It was followed by another aftershock of {{M|w}} 7.6 in October the same year.{{cite web|title=M 7.8 – 99 km SSE of Perryville, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/us7000asvb/executive|publisher=United States Geological Survey|access-date=July 29, 2021}}{{cite web|title=M 7.6 – 99 km SE of Sand Point, Alaska|url=https://earthquake.usgs.gov/earthquakes/eventpage/us6000c9hg/executive|publisher=United States Geological Survey|access-date=July 29, 2021}} The United States Geological Survey have since considered the two earthquakes as foreshocks to the {{M|w}} 8.2 in 2021.

{{Infobox earthquake

| title = July 2020 foreshock event

| timestamp = 2020-07-22 06:12:44

| isc-event = 618688608

| anss-url = us7000asvb

| local-date = {{Start date|2020|07|21}}

| local-time = 20:12:44

| map2 = {{Location map many | Alaska | relief=1

| label=

| lat=55.072

| long=-158.596

| mark=Bullseye1.png

| marksize=50

| position=top

| width= 250

| float=right

| caption=}}

| magnitude = 7.8 {{M|w|link=y}}

| depth = {{convert|28.0|km|abbr=on}}

| type = Thrust

| fault = Aleutian subduction zone

| location = {{coord|55.072|N|158.596|W}}

| countries affected = United States

| tsunami = {{convert|0.24|m|ft|abbr=on}}

| intensity = {{MMI|VII}}

| casualties = None

}}

The earthquake on July 21, 2020, was the result of thrust faulting on the Aleutian subduction zone{{Cite web|title=Event Page {{!}} Alaska Earthquake Center|url=https://earthquake.alaska.edu/event/0209dha24u/detail|access-date=2020-08-14|website=earthquake.alaska.edu}} where the Pacific plate subducts underneath the North American plate, forming the Aleutian Trench and Arc. This convergent boundary is one of the most active in the world, and was the location of the {{M|w|9.2|link=yes}} 1964 Alaska earthquake; the largest recorded in North America, and the second largest in the world.{{Cite journal|last1=Carver|first1=Gary|last2=Plafker|first2=George|date=2008|title=Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview|url=https://ui.adsabs.harvard.edu/abs/2008GMS...179...43C/abstract|journal=Washington DC American Geophysical Union Geophysical Monograph Series|series=Geophysical Monograph Series|volume=179|pages=43–63|doi=10.1029/179GM03|bibcode=2008GMS...179...43C|isbn=9781118666395}} On average, the rate at which these plates converge is about 64 mm/year. The earthquake struck east of the Shumagin Gap, a {{convert|125|mi|km|adj=mid|-wide}} seismic gap in the subduction zone which hasn't ruptured with a large quake for at least 100 years.{{Cite web|last=Temblor|date=2020-08-01|title=Does Alaska's magnitude-7.8 Simeonof earthquake finally close a seismic gap?|url=https://temblor.net/earthquake-insights/does-alaskas-magnitude-7-8-simeonof-earthquake-finally-close-a-seismic-gap-11496/|access-date=2020-12-09|website=Temblor.net}} Previously, it was thought that the subducting plate in the Shumagin Gap was poorly coupled to the overriding crust, quietly slipping and preventing large quakes. However, research suggests that the seismic gap, contrary to speculation that it might be a relatively "safe" seismic zone in the arc, may pose a threat similar to that of the rest of the Aleutian subduction zone.{{Cite web|title=A New Perspective on Seismic Hazards in the Shumagin Gap {{!}} Alaska Earthquake Center|url=https://earthquake.alaska.edu/new-perspective-seismic-hazards-shumagin-gap|access-date=2020-12-09|website=earthquake.alaska.edu}} It was also the largest earthquake in 2020 by magnitude.

The earthquake ruptured an area of about {{convert|120|x|60|mi|abbr=on}}, equal to about {{convert|7200|sqmi|abbr=on}}. The estimated maximum slip along this fault surface was about {{convert|3|to|4|m|abbr=on}}.{{Cite web|title=Event Page {{!}} Alaska Earthquake Center|url=https://earthquake.alaska.edu/event/0209dha24u/fault|access-date=2020-08-14|website=earthquake.alaska.edu}}

The earthquake appears to have only partially ruptured the Shumagin segment of the subduction zone, at 75%. Estimated fault rupture dimensions are a width of 140 km, and length of 250 km, with a depth ranging from 11 to 49 km along the subduction zone. The rupture of this earthquake did not propagate towards the shallow, near trench section of the subduction zone. The lack of seismic activity on the shallow section may suggest that section is locked and accumulating elastic energy, raising the issue on the potential for tsunami earthquakes occurring.

No severe damage or injuries were reported in the aftermath of the earthquake.{{Cite web|agency=Associated Press|date=2020-07-22|title=7.8-magnitude quake rocks Alaska, tsunami warning canceled|url=https://abc7.com/6328442/|access-date=2020-07-22|website=ABC7 Los Angeles|archive-date=July 24, 2020|archive-url=https://web.archive.org/web/20200724193919/https://abc7.com/6328442/|url-status=dead}}{{Cite web|date=2020-07-22|title=Powerful 7.8 quake hits Alaska isles; tsunami threat over|url=https://www.wyff4.com/article/preliminary-magnitude-78-earthquake-strikes-off-coast-of-alaska/33387916|access-date=2020-07-22|website=WYFF}} Mild damage occurred in Sand Point with reports of damaged docks, cracked roads and cement.{{Cite web|last=McKenney|first=Hope|title=Sand Point's Docks, Road To Harbor Damaged in Magnitude-7.8 Earthquake|url=https://www.kucb.org/post/sand-points-docks-road-harbor-damaged-magnitude-78-earthquake|access-date=2020-08-11|website=kucb.org|date=July 23, 2020}} It was reportedly felt as far as Vancouver and Victoria, more than {{convert|1500|mi||abbr=}} away.{{Cite web|title=DYFI felt reports, USGS|url=https://earthquake.usgs.gov/earthquakes/eventpage/us7000asvb/map|access-date=2020-07-22|publisher=United States Geological Survey}}{{Cite web|title=Impacts {{!}} Alaska Earthquake Center|url=https://earthquake.alaska.edu/event/0209dha24u/impacts|url-status=live|archive-url=https://web.archive.org/web/20200726125437/http://earthquake.alaska.edu:80/event/0209dha24u/impacts|archive-date=July 26, 2020|access-date=2020-09-26|website=earthquake.alaska.edu}}

{{Infobox earthquake

|title = October 2020 foreshock event

|timestamp = 2020-10-19 20:54:38

|isc-event = 619498906

|anss-url = us6000c9hg

|local-date = {{Start date|2020|10|19}}

|local-time = 12:54:38

|map2 = {{Location map many | Alaska | relief=1

| label=

| lat=54.602

| long=-159.626

| mark=Bullseye1.png

| marksize=40

| position=top

| width= 250

| float=right

| caption=}}

|magnitude = 7.6 {{M|w|link=y}}

|depth= {{convert|28.4|km|abbr=on}}

|location={{coord|54.602|-159.626}}

|countries affected = United States

|tsunami = {{convert|0.76|m|ft|abbr=on}}

|intensity = {{MMI|VII}}

|casualties = None

}}

This earthquake occurred along a north northwest striking strike-slip fault rather than the thrust mechanism seen in the July event.{{Cite web|title=Finite Fault|url=https://earthquake.usgs.gov/earthquakes/eventpage/us6000c9hg/finite-fault|publisher=United States Geological Survey}}{{Cite web|title=Significant Earthquake Information ALASKA|url=https://www.ngdc.noaa.gov/hazel/view/hazards/earthquake/event-more-info/10520|access-date=21 December 2020|website=NGDC}} It did not occur along the megathrust boundary but within one of the two interacting plates. Maximum slip along the fault is estimated at 3.4 meters.

A tsunami warning was issued but was later downgraded to an advisory.{{Cite web|last=Service|first=National Weather|title=National Weather Service – Tsunami Hazards|url=https://www.tsunami.gov/|access-date=2020-10-19|publisher=U.S. Tsunami Warning System}} A wave was measured at {{convert|2.3|ft|cm|abbr=on|sigfig=3}} at Sand Point.{{Cite web|title=Public Tsunami Message Number 6|url=https://www.tsunami.gov/events/PAAQ/2020/10/19/qigvf4/6/WEAK51/WEAK51.txt|website=NOAA}} At King Cove, the tsunami had a wave height of {{convert|2.1|ft|cm|abbr=on|sigfig=3}} and was {{convert|2.5|ft|cm|abbr=on|sigfig=3}} at Chignik Bay.{{Cite web|title=Public Tsunami Message Number 11|url=https://www.tsunami.gov/events/PAAQ/2020/10/19/qigvf4/11/WEAK51/WEAK51.txt|website=NOAA}}{{Cite web|title=Tsunami Event Information SHUMAGIN ISLANDS, AK|url=https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/event-more-info/5755|access-date=21 December 2020|website=NGDC}} In the state of Hawaii, alert level Advisory was issued at 5:55 pm.{{Cite news|last=MARTINEZ|first=PETER|date=20 October 2020|title=7.5 magnitude earthquake strikes near Alaska, triggering tsunami advisory|publisher=CBS News|url=https://www.cbsnews.com/news/alaska-earthquake-7-5-magnitude-tsunami-warning/}} Wave heights ranging between {{convert|0.1|and|1.2|ft|cm|abbr=on}} were detected along the islands.{{Cite web|title=Message: Hawaii Warn/Adv./Watch #3|url=https://www.tsunami.gov/events/PHEB/2020/10/19/20293005/3/WEHW40/WEHW40.txt|website=NOAA}} Tsunami warning was later cancelled at 05:05:15 (UTC).{{Cite web|title=Message: AK/BC/US West Coast Warn/Adv./Watch #13|url=https://www.tsunami.gov/?p=PAAQ/2020/10/19/qigvf4/13/WEAK51|website=NOAA}}

The earthquake was described as "weird" and "the wrong type" by researchers at Pennsylvania State University.{{cite web|last1=A'ndrea Elyse Messer|title=Weird earthquake reveals hidden mechanism|url=https://news.psu.edu/story/651591/2021/03/24/research/weird-earthquake-reveals-hidden-mechanism|website=news.psu.edu|publisher=Penn State News|access-date=6 April 2021|date=24 March 2021}} Adding that the earthquake with its strike-slip mechanism "made no sense" because of the location near a subduction zone. The fault involved with the event is situated within the downgoing Pacific slab. It is likely a remnant of a fault structure formed at a distant mid-oceanic spreading ridge. This steeply-dipping, trench perpendicular fault ruptured up the Pacific slab towards the trench for a length of 70 km and caused slip of up to 5 meters.{{cite journal|author1=Matthew W. Herman and Kevin P. Furlong|author2=Matthew W. Herman and Kevin P. Furlong|title=Triggering an unexpected earthquake in an uncoupled subduction zone|journal=Science Advances|date=24 Mar 2021|volume=7|issue=13|pages=eabf7590|doi=10.1126/sciadv.abf7590|publisher=American Association for the Advancement of Science|pmid=33762346|pmc=7990338|bibcode=2021SciA....7.7590H|issn=2375-2548|doi-access=free}}

Observed by scientists was the larger tsunami triggered by the October quake. A possible cause of the larger tsunami was the occurrence of slip on the megathrust boundary as well. Further analysis found that both the strike-slip fault and megathrust rupture had a combined magnitude of {{M|w}} 8.0 but seismic signals from the megathrust rupture was undetected. The October 2020 megathrust rupture occurred at a depth shallower than that of the July 2020 event, but failed to rupture towards the seafloor.{{cite journal|author1=Sean R. Santellanes|author2=Diego Melgar|author3=Brendan W. Crowell|author4=Jiun-Ting Lin|title=Potential megathrust co-seismic slip during the 2020 Sand Point, Alaska strike-slip earthquake|journal=Earth and Space Science Open Archive|doi=10.1002/essoar.10508002.1|s2cid=240543104}}

Occurring southeast of Perryville, Alaska (south of the Alaska Peninsula), the earthquake happened as the result of thrust faulting on or near the subduction zone interface between the Pacific and North America plates. The preliminary focal mechanism solution indicates rupture occurred on a fault dipping either shallowly to the northwest, or steeply to the southeast. The location, mechanism and depth – and the large size of the event – are all consistent with slip occurring on the subduction zone interface between the two plates. At the location of this event, the Pacific plate converges with North America to the northwest at a rate of about 64 mm/yr, subducting at the Alaska-Aleutians trench ~125 km to the southeast of the earthquake.

The earthquake was initially reported as having a preliminary magnitude of 7.3 before being upgraded to 8.2.{{Cite web|url=https://www.kwtx.com/2021/07/29/82-magnitude-quake-aleutian-chain-generates-small-tsunami/|title=8.3 magnitude earthquake in Alaska's Aleutian Islands generates small tsunami|date=July 29, 2021}}

The focal depth of the magnitude 8.2 mainshock at 32.2 km suggest it was deeper than that of the foreshocks. The mainshock ruptured an area of about {{convert|200|x|100|km|abbr=on}} equal to {{convert|20000|sqkm|abbr=on}} with an estimated maximum slip of {{convert|5.5|m|ft|abbr=on}}.{{Cite web|title=Event Page {{!}} Alaska Earthquake Center|url=https://earthquake.alaska.edu/event/0219neiszm/fault|access-date=2021-07-29|website=earthquake.alaska.edu}} It is thought to have ruptured the same subduction zone segment that was also involved in a similar-sized event in 1938.{{cite web|author1=Beth Grassi|title=LARGEST EARTHQUAKE IN 50 YEARS: WHAT WE KNOW SO FAR|url=https://earthquake.alaska.edu/largest-earthquake-50-years-what-we-know-so-far|website=earthquake.alaska.edu|publisher=Alaska Earthquake Center|access-date=29 July 2021|date=29 July 2021}}{{cite web|title=M 8.2 – Alaska Peninsula|url=https://earthquake.usgs.gov/earthquakes/eventpage/official19381110201849000_35/executive|publisher=United States Geological Survey|access-date=29 July 2021}}

It was the second of three earthquakes of magnitude 8 or higher in 2021. On March 4, 2021, nearly 5 months before the Alaska mainshock, an 8.1 magnitude earthquake struck the Kermadec Islands,{{cite web|title=M 8.1 – Kermadec Islands region|url=https://earthquake.usgs.gov/earthquakes/eventpage/pt21063003/executive|publisher=United States Geological Survey-ANSS|access-date=29 July 2021}} a sparsely populated territory owned by New Zealand, and another equally sized event struck the Sub-Antarctic British territory of the South Sandwich Islands on August 12 of that year.{{Cite web|title=M 8.1 – South Sandwich Islands region|url=https://earthquake.usgs.gov/earthquakes/eventpage/us6000f53e/executive|publisher=United States Geological Survey|access-date=12 August 2021}} This is unusual since earthquakes of magnitude 8 or higher occur only about once every year on average.{{cite web|title=How Often Do Earthquakes Occur?|url=https://www.iris.edu/hq/inclass/fact-sheet/how_often_do_earthquakes_occur|publisher=IRIS Consortium|access-date=2 August 2021}} There is no evidence of a causal relationship between the three events. 2021 has had the most 8.0+ earthquakes in a single year since 2007.{{cite web|title=Significant Earthquakes – 2007|url=https://earthquake.usgs.gov/earthquakes/browse/significant.php?year=2007|publisher=United States Geological Survey|access-date=2 August 2021}}

File:2021 Chignik earthquake Tsunami map.png

A tsunami warning was issued by the NOAA about 5 minutes after the earthquake. 1 hour and 45 minutes after, the warning was downgraded to an advisory and later canceled 3 hours after the earthquake.{{Cite web|date=2021-07-29|title=Tsunami warning canceled after 8.2 offshore earthquake sent people in many Alaska coastal communities to higher ground|url=https://www.adn.com/alaska-news/2021/07/28/gulf-of-alaska-coast-under-tsunami-warning-following-strong-earthquake/|access-date=2021-07-29|website=Anchorage Daily News}} A tsunami height of {{convert|1.4|ft|cm|sigfig=3}} was recorded in the city of Old Harbour in the Kodiak Island Borough on Kodiak Island.{{cite web|title=Public Tsunami Message Number 5|url=https://www.tsunami.gov/events/PAAQ/2021/07/29/qwzteb/5/WEAK51/WEAK51.txt|publisher=U.S. Tsunami Warning System|access-date=July 29, 2021|location=Palmer, Alaska|date=July 29, 2021}} In Port San Luis (Avila Beach) on the Central Coast of California, a {{convert|1.3|ft|cm|adj=on}} tsunami surge was measured by the National Weather Service in a tweet.{{cite web|title=NWS Los Angeles on Twitter|url=https://twitter.com/NWSLosAngeles/status/1420845959915872256|via=Twitter|publisher=NWS Los Angeles|access-date=31 July 2021}}{{cite news|author1=Bethany Reeves|title=Tsunami surge from Alaska earthquake seen on Central Coast|url=https://www.ksby.com/news/tsunami-surge-from-alaska-earthquake-seen-on-central-coast|access-date=31 July 2021|agency=KSBY|date=29 July 2021}} Near the epicentre, a {{convert|8.6|ft|m|adj=on}} was observed.{{Cite web|title=M 8.2 Earthquake and Tsunami (2.6 m) in United States on 29 Jul 2021 06:15 UTC|url=https://www.gdacs.org/Tsunamis/report.aspx?eventid=1279046&episodeid=1388004&eventtype=EQ|work=Global Disaster Alert and Coordination System}}

{{Portal|Earth sciences|Alaska}}

• List of earthquakes in 2021
• List of earthquakes in Alaska
• List of earthquakes in the United States
• List of megathrust earthquakes

{{Reflist}}

• {{Cite journal|title=The 29 July 2021 Mw 8.2 Chignik, Alaska Peninsula Earthquake Rupture Inferred from Seismic and Geodetic Observations: Re-rupture of the Western 2/3 of the 1938 Rupture Zone|journal=Geophysical Research Letters|last1=Liu|first1=Chengli|volume=49|last2=Lay|first2=Thorne|publisher=American Geophysical Union|issue=4|doi=10.1029/2021GL096004|year=2022|publication-date=2022-02-28|last3=Xiong|first3=Xiong|bibcode=2022GeoRL..4996004L|s2cid=246885311|doi-access=free}}

• [https://temblor.net/earthquake-insights/alaskas-chignik-earthquake-shows-segmented-subduction-zone-13091/ Alaska's Chignik earthquake shows segmented subduction zone] – Temblor, Inc.
• {{EQ-isc-link|620857937}}

{{Earthquakes in 2021}}

{{Earthquakes in Alaska}}

{{Earthquakes in the United States}}

Category:Earthquakes in Alaska

Category:2021 earthquakes

Category:2021 in Alaska

Category:July 2021 in the United States

Category:Megathrust earthquakes in Alaska

Chignik

Category:Tsunamis in the United States

Category:Lake and Peninsula Borough, Alaska