2021 Chignik earthquake: Difference between revisions

Coordinates: 55°28′26″N 157°55′01″W / 55.474°N 157.917°W / 55.474; -157.917
From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Filling citations
Added new section on Shumagin gap
Line 49: Line 49:


==Tectonic setting==
==Tectonic setting==
Off the coast of Alaska lies the [[Aleutian subduction zone|Aleutian Subduction Zone]]–a 2,500 mile long [[Convergent boundary|convergent plate boundary]] where the [[Pacific Plate]] subducts under the [[North American Plate]] at a rate of 6–7&nbsp;cm/yr. This [[Megathrust earthquake|megathrust]] fault has been the source of many large earthquakes including the [[1964 Alaska earthquake|1964 Alaskan earthquake]] that registered a magnitude 9.2 and remains the [[Lists of earthquakes|second largest]] earthquake in recorded history.<ref name="USGS_2">{{cite web |title=M 8.6 - 1946 Aleutian Islands (Unimak Island) Earthquake, Alaska |url=https://earthquake.usgs.gov/earthquakes/eventpage/official19460401122901_30/executive |website=earthquake.usgs.gov |publisher=USGS |access-date=9 July 2021}}</ref><ref name="USGS_1">{{cite web |title=M 9.2 - 1964 Prince William Sound Earthquake, Alaska |url=https://earthquake.usgs.gov/earthquakes/eventpage/official19640328033616_30/executive |website=earthquake.usgs.gov |publisher=USGS |access-date=9 July 2021}}</ref>
Off the coast of Alaska lies the [[Aleutian subduction zone]]–a 2,500 mile long [[Convergent boundary|convergent plate boundary]] where the [[Pacific Plate]] subducts under the [[North American Plate]] at a rate of 6–7&nbsp;cm/yr. This [[Megathrust earthquake|megathrust]] fault has been the source of many large earthquakes including the [[1964 Alaska earthquake|1964 Alaskan earthquake]] that registered a magnitude 9.2 and remains the [[Lists of earthquakes|second largest]] earthquake in recorded history.<ref name="USGS_2">{{cite web |title=M 8.6 - 1946 Aleutian Islands (Unimak Island) Earthquake, Alaska |url=https://earthquake.usgs.gov/earthquakes/eventpage/official19460401122901_30/executive |website=earthquake.usgs.gov |publisher=USGS |access-date=9 July 2021}}</ref><ref name="USGS_1">{{cite web |title=M 9.2 - 1964 Prince William Sound Earthquake, Alaska |url=https://earthquake.usgs.gov/earthquakes/eventpage/official19640328033616_30/executive |website=earthquake.usgs.gov |publisher=USGS |access-date=9 July 2021}}</ref>

===Background===
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.<ref name="Huene">{{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 |url=https://pubs.geoscienceworld.org/gsa/geosphere/article/15/2/324/568599/The-Shumagin-seismic-gap-structure-and-associated}}</ref> 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. That proposal also highlighted the potential for a [[tsunami]] accompanying an earthquake on the gap.<ref name="Sykes1971">{{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}}</ref>

===1938 earthquake===
The '''1938 Alaska 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.<ref name="M8.2_1938" /> 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.<ref>{{cite web |title=Tsunami Event Information |url=https://www.ngdc.noaa.gov/hazel/view/hazards/tsunami/event-more-info/1722 |website=ngdc.noaa.gov |publisher=NCEI |access-date=2 August 2021}}</ref> 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 aproximately 300 km. Rupture of this earthquake did not reach into the Shumagin segment.<ref name="Satake & Johnson">{{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 |url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/94754/grl7445.pdf?sequence=1}}</ref>


==Foreshocks==
==Foreshocks==

Revision as of 14:01, 2 August 2021

2021 Chignik earthquake
2021 Chignik earthquake is located in Alaska
Anchorage
Anchorage
Kodiak
Kodiak
Perryville
Perryville
2021 Chignik earthquake
UTC time2021-07-29 06:15:49
ISC event620857937
USGS-ANSSComCat
Local dateJuly 28, 2021
Local time22:15:49
Magnitude8.2 Mw
Depth32.2 km (20.0 mi)
Epicenter55°28′26″N 157°55′01″W / 55.474°N 157.917°W / 55.474; -157.917
FaultAleutian subduction zone
TypeMegathrust
Areas affectedAlaska
Max. intensityMMI VIII (Severe)[1]
Tsunami0.21 m (8 in)
Foreshocks7.8 Mw  and 7.6 Mw  in 2020
AftershocksMultiple. Strongest is 6.1 Mw

An earthquake occurred off the coast of the Alaska Peninsula on July 28, 2021, at 10:15 p.m. local time.[2] The large megathrust earthquake had a moment magnitude (Mw) of 8.2 according to the United States Geological Survey (USGS).[1] A tsunami warning was issued by the National Oceanic and Atmospheric Administration (NOAA) but later cancelled.[3] The mainshock was followed by a number of aftershocks, including two that were of magnitude 5.9 and 6.1, respectively.[4][5]

This was the largest earthquake in the United States since the 1965 Rat Islands earthquake, and the 7th largest earthquake in US history.[6] It was also the strongest earthquake in the world since the 2018 Fiji earthquake.

Tectonic setting

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.[7][8]

Background

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.[9] 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. That proposal also highlighted the potential for a tsunami accompanying an earthquake on the gap.[10]

1938 earthquake

The 1938 Alaska 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.[11] 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.[12] 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 aproximately 300 km. Rupture of this earthquake did not reach into the Shumagin segment.[13]

Foreshocks

In July 2020, the Aleutian Subduction Zone was the source of an Mw  7.8 earthquake which struck the same region south of the Alaska Peninsula. It was followed by another aftershock of Mw  7.6 in October the same year.[14][15][1] The United States Geological Survey have since considered the two earthquakes as foreshocks to the Mw  8.2 in 2021.

July 2020

July 2020 foreshock event
2021 Chignik earthquake is located in Alaska
2021 Chignik earthquake
UTC time2020-07-22 06:12:44
ISC event618688608
USGS-ANSSComCat
Local dateJuly 22, 2020 (2020-07-22)
Local time20:12:44
Magnitude7.8 Mw
Depth28.0 km (17.4 mi)
Epicenter55°04′19″N 158°35′46″W / 55.072°N 158.596°W / 55.072; -158.596
Areas affected United States
Max. intensityMMI VII (Very strong)
Tsunami0.24 m (0.79 ft)
CasualtiesNone

The earthquake on July 21, 2020, was the result of thrust faulting on the Aleutian subduction zone[16] 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 Mw9.2 1964 Alaska earthquake; the largest recorded in North America, and the second largest in the world.[17] On average, the rate at which these plates converge is about 64 mm/year. The earthquake struck east of the Shumagin Gap, a 125 mile-wide (200 km) seismic gap in the subduction zone which hasn't ruptured with a large quake for at least 100 years.[18] 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.[19]

The earthquake ruptured an area of about 120 mi × 60 mi (193 km × 97 km), equal to about 7,200 sq mi (19,000 km2). The estimated maximum slip along this fault surface was about 3 to 4 m (9.8 to 13.1 ft).[20] It was reportedly felt as far as Vancouver and Victoria, more than 1,500 miles (2,400 km) away.[21][22]

No severe damage or injuries were reported in the aftermath of the earthquake.[23][24] Mild damage occurred in Sand Point with reports of damaged docks, cracked roads and cement.[25]

October 2020

October 2020 foreshock event
2021 Chignik earthquake is located in Alaska
2021 Chignik earthquake
UTC time2020-10-19 20:54:38
ISC event619498906
USGS-ANSSComCat
Local dateOctober 20, 2020 (2020-10-20)
Local time12:54:38
Magnitude7.6 Mw
Depth28.4 km (17.6 mi)
Epicenter54°36′07″N 159°37′34″W / 54.602°N 159.626°W / 54.602; -159.626
Areas affected United States
Max. intensityMMI VII (Very strong)
Tsunami0.76 m (2.5 ft)
CasualtiesNone

This earthquake occurred along a north northwest striking strike-slip fault rather than the thrust mechanism seen in the July event.[26][27] 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.[26]

The earthquake was described as "weird" and "the wrong type" by researchers at Pennsylvania State University.[28] 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 and is likely a remnant of a fault structure formed at a 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.[29]

A tsunami warning was issued but was later downgraded to an advisory.[30] A wave was measured at 2.3 ft. (70.1 cm) at Sand Point.[31] At King Cove, the tsunami had a wave height of 2.1 ft. (64.0 cm) and was 2.5 ft. at Chignik Bay.[32][33] In the state of Hawaii, alert level Advisory was issued at 5:55 pm.[34] Wave heights ranging between 0.1-1.2 ft (2–38 cm) were detected along the islands.[35] Tsunami warning was later cancelled at 05:05:15 (UTC).[36]

Other events

On March 4, 2021, nearly 5 months before the Alaska mainshock, an 8.1 magnitude earthquake struck the Kermadec Islands,[37] a sparsely populated territory owned by New Zealand. This is unusual since earthquakes of magnitude 8 or higher occur only about once every year on average.[38] There is no evidence of a causal relationship between the two events. Such events make 2021 the first year with at least two 8.0+ earthquakes since 2013.[39]

Earthquake

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 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 200 km × 100 km (124 mi × 62 mi) equal to 20,000 km2 (7,700 sq mi) with an estimated maximum slip of 2.5–3 m (8.2–9.8 ft)[40] It is thought to have ruptured the same subduction zone segment that was also involved in a similar-sized event in 1938.[41][11]

Tsunami

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.[42] A tsunami height of 0.7 feet (21.3 cm) was recorded in the city of Old Harbour in the Kodiak Island Borough on Kodiak Island.[43] In Port San Luis (Avila Beach) on the Central Coast of California, a 1.3-feet tsunami surge was measured by the National Weather Service in a tweet.[44][45]

Tsunami height observations
Location Height Source
Avila Beach, San Luis Obispo on San Luis Obispo County 1.3 feet (39.6 cm) [46]
Old Harbour, Kodiak Island Borough on Kodiak Island 0.7 feet (21.3 cm) [43]
Sand Point, Aleutians East Borough on Popof Island 0.5 feet (15.2 cm) [43]
Kodiak, Kodiak Island Borough on Kodiak Island 0.5 feet (15.2 cm) [47]
King Cove, Aleutians East Borough on Deer Island (Aleutian Islands) 0.4 feet (12.2 cm) [47]
Cape Alitak, southern Kodiak Island 0.4 feet (12.2 cm) [46]
Unalaska, Aleutians West Census Area on Unalaska Island 0.3 feet (9.14 cm) [46]

See also

References

  1. ^ a b c "M 8.2 – 91 km ESE of Perryville, Alaska". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  2. ^ "Gulf of Alaska coast under tsunami warning following 8.2 earthquake". Anchorage Daily News. July 29, 2021. Retrieved July 29, 2021.
  3. ^ "U.S. Tsunami Warning System". tsunami.gov. NOAA National Centers for Environmental Information. Retrieved July 29, 2021.
  4. ^ "M 5.9 – 119 km SSE of Chignik, Alaska". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  5. ^ "M 6.1 – 114 km SSE of Perryville, Alaska". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  6. ^ "M 8.7 - Rat Islands, Aleutian Islands, Alaska". USGS. February 4, 1965. Retrieved July 29, 2021.
  7. ^ "M 8.6 - 1946 Aleutian Islands (Unimak Island) Earthquake, Alaska". earthquake.usgs.gov. USGS. Retrieved July 9, 2021.
  8. ^ "M 9.2 - 1964 Prince William Sound Earthquake, Alaska". earthquake.usgs.gov. USGS. Retrieved July 9, 2021.
  9. ^ Roland von Huene; John J. Miller; Anne Krabbenhoeft (2019). "The Shumagin seismic gap structure and associated tsunami hazards, Alaska convergent margin". Geosphere. 15 (2): 324–341. doi:10.1130/GES01657.1.
  10. ^ Lynn Ray Sykes (1971). "Aftershock zones of great earthquakes, seismicity gaps, and earthquake prediction for Alaska and the Aleutians". Journal of Geophysical Research. 76 (32): 8021–8041. doi:10.1029/JB076i032p08021.
  11. ^ a b "M 8.2 - Alaska Peninsula". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  12. ^ "Tsunami Event Information". ngdc.noaa.gov. NCEI. Retrieved August 2, 2021.
  13. ^ Kenji Satake; Jean M. Johnson (1994). "Rupture extent of the 1938 Alaskan earthquake as inferred from tsunami waveforms" (PDF). Geophysical Research Letters. 21 (8): 733–736. doi:10.1029/94GL00333.
  14. ^ "M 7.8 – 99 km SSE of Perryville, Alaska". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  15. ^ "M 7.6 – 99 km SE of Sand Point, Alaska". earthquake.usgs.gov. USGS. Retrieved July 29, 2021.
  16. ^ "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved August 14, 2020.
  17. ^ Carver, Gary; Plafker, George (2008). "Paleoseismicity and neotectonics of the Aleutian Subduction Zone—An overview". Washington DC American Geophysical Union Geophysical Monograph Series. Geophysical Monograph Series. 179: 43–63. Bibcode:2008GMS...179...43C. doi:10.1029/179GM03. ISBN 9781118666395.
  18. ^ Temblor (August 1, 2020). "Does Alaska's magnitude-7.8 Simeonof earthquake finally close a seismic gap?". Temblor.net. Retrieved December 9, 2020.
  19. ^ "A New Perspective on Seismic Hazards in the Shumagin Gap | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved December 9, 2020.
  20. ^ "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved August 14, 2020.
  21. ^ "DYFI felt reports, USGS". earthquake.usgs.gov. Retrieved July 22, 2020.
  22. ^ "Impacts | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved September 26, 2020.{{cite web}}: CS1 maint: url-status (link)
  23. ^ AP (July 22, 2020). "7.8-magnitude quake rocks Alaska, tsunami warning canceled". ABC7 Los Angeles. Retrieved July 22, 2020.
  24. ^ "Powerful 7.8 quake hits Alaska isles; tsunami threat over". WYFF. July 22, 2020. Retrieved July 22, 2020.
  25. ^ McKenney, Hope. "Sand Point's Docks, Road To Harbor Damaged In Magnitude-7.8 Earthquake". www.kucb.org. Retrieved August 11, 2020.
  26. ^ a b "Finite Fault". US Geological Survey.
  27. ^ "Significant Earthquake Information ALASKA". NGDC. Retrieved December 21, 2020.
  28. ^ A'ndrea Elyse Messer (March 24, 2021). "Weird earthquake reveals hidden mechanism". news.psu.edu. Penn State News. Retrieved April 6, 2021.
  29. ^ Matthew W. Herman and Kevin P. Furlong; Matthew W. Herman and Kevin P. Furlong (March 24, 2021). "Triggering an unexpected earthquake in an uncoupled subduction zone". Science Advances. 7 (13). American Association for the Advancement of Science: eabf7590. doi:10.1126/sciadv.abf7590. ISSN 2375-2548. PMC 7990338. PMID 33762346.
  30. ^ Service, National Weather. "National Weather Service - Tsunami Hazards". www.tsunami.gov. Retrieved October 19, 2020.
  31. ^ "Public Tsunami Message Number 6". NOAA.
  32. ^ "Public Tsunami Message Number 11". NOAA.
  33. ^ "Tsunami Event Information SHUMAGIN ISLANDS, AK". NGDC. Retrieved December 21, 2020.
  34. ^ MARTINEZ, PETER (October 20, 2020). "7.5 magnitude earthquake strikes near Alaska, triggering tsunami advisory". CBS News.
  35. ^ "Message: Hawaii Warn/Adv./Watch #3". NOAA.
  36. ^ "Message: AK/BC/US West Coast Warn/Adv./Watch #13". NOAA.
  37. ^ "M 8.1 - Kermadec Islands region". USGS-ANSS. Retrieved July 29, 2021.
  38. ^ "How Often Do Earthquakes Occur?". Incorporated Research Institutions for Seismology. Retrieved August 2, 2021.
  39. ^ "Significant Earthquakes - 2013". earthquake.usgs.gov. USGS. Retrieved August 2, 2021.
  40. ^ "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved July 29, 2021.
  41. ^ Beth Grassi (July 29, 2021). "LARGEST EARTHQUAKE IN 50 YEARS: WHAT WE KNOW SO FAR". earthquake.alaska.edu. Alaska Earthquake Center. Retrieved July 29, 2021.
  42. ^ "Tsunami warning canceled after 8.2 offshore earthquake sent people in many Alaska coastal communities to higher ground". Anchorage Daily News. July 29, 2021. Retrieved July 29, 2021.
  43. ^ a b c "Public Tsunami Message Number 5". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.
  44. ^ "NWS Los Angeles on Twitter". twitter.com. NWS Los Angeles. Retrieved July 31, 2021.
  45. ^ Bethany Reeves (July 29, 2021). "Tsunami surge from Alaska earthquake seen on Central Coast". KSBY. Retrieved July 31, 2021.
  46. ^ a b c "Public Tsunami Message Number 6". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.
  47. ^ a b "Public Tsunami Message Number 6". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.
Retrieved from "https://en.wikipedia.org/w/index.php?title=2021_Chignik_earthquake&oldid=1036753099"