2021 Chignik earthquake

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2021 Chignik earthquake
2021 Chignik earthquake shakemap.jpg
Relief map of USA Alaska.png
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Anchorage
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Kodiak
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Perryville
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Interactive map
UTC  time2021-07-29 06:15:49
ISC  event 620857937
USGS-ANSS ComCat
Local dateJuly 28, 2021
Local time22:15:49 AKDT
Magnitude8.2 Mw
Depth32.2 km (20.0 mi)
Epicenter 55°28′26″N157°55′01″W / 55.474°N 157.917°W / 55.474; -157.917
Fault Aleutian subduction zone
Type Megathrust
Areas affected Alaska
Max. intensity VII (Very strong)
Peak acceleration0.26 g [1]
Tsunami2.62 m (8.6 ft) [2]
Foreshocks 7.8 Mw  and 7.6 Mw  in 2020
Aftershocks1,629 (As of 12 July 2022) [3]
Largest 6.9 Mw  [4]
CasualtiesNone

An earthquake occurred off the coast of the Alaska Peninsula on July 28, 2021, at 10:15 p.m. local time. [5] The large megathrust earthquake had a moment magnitude (Mw ) of 8.2 according to the United States Geological Survey (USGS). [6] A tsunami warning was issued by the National Oceanic and Atmospheric Administration (NOAA) but later cancelled. [7] The mainshock was followed by a number of aftershocks, including three that were of magnitude 5.9, 6.1 and 6.9 respectively. [8] [9] [10]

Contents

This was the largest earthquake in the United States since the 1965 Rat Islands earthquake, and the 7th largest earthquake in US history. [11] It was also the strongest earthquake globally since the 2015 Illapel earthquake, tying the 2017 Chiapas earthquake and 2018 Fiji earthquake.

No casualties or serious damage were reported in the aftermath of the mainshock, as well as some of the strong aftershocks. In Perryville, the closest populated area to the quake, cracks appeared in drywalls and on the ground. Some homes were also shifted. [12] The limited impact from this earthquake was attributed to it occurring offshore from the sparsely-populated Alaska Peninsula. A tsunami warning was rescinded and no large waves were observed because the earthquake took place 32 km beneath the seafloor; deep enough that vertical uplift of the seafloor was limited therefore displacing smaller amounts of water. [13]

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. [14] [15]

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. [16] 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. [17] Two smaller earthquakes shook near the seismic gap in 1917 and 1948. [18] [19] That proposal also highlighted the potential for a tsunami accompanying an earthquake on the gap. [20]

1938 earthquake

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. [21] 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. [22] 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. [23]

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. [24] [25] [6] 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
Relief map of USA Alaska.png
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UTC  time2020-07-22 06:12:44
ISC  event 618688608
USGS-ANSS ComCat
Local dateJuly 22, 2020 (2020-07-22)
Local time20:12:44
Magnitude7.8 Mw
Depth28.0 km (17.4 mi)
Epicenter 55°04′19″N158°35′46″W / 55.072°N 158.596°W / 55.072; -158.596
Fault Aleutian subduction zone
Type Thrust
Areas affectedUnited States
Max. intensity VII (Very strong)
Tsunami0.24 m (0.79 ft)
CasualtiesNone

The earthquake on July 22, 2020, was the result of thrust faulting on the Aleutian subduction zone [26] 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 Mw 9.2 1964 Alaska earthquake; the largest recorded in North America, and the second largest in the world. [27] 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. [28] 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. [29] It was also the largest earthquake in 2020 by magnitude.

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). [30]

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. [17]

No severe damage or injuries were reported in the aftermath of the earthquake. [31] [32] Mild damage occurred in Sand Point with reports of damaged docks, cracked roads and cement. [33] It was reportedly felt as far as Vancouver and Victoria, more than 1,500 miles (2,400 km) away. [34] [35]

October 2020

October 2020 foreshock event
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UTC  time2020-10-19 20:54:38
ISC  event 619498906
USGS-ANSS ComCat
Local dateOctober 20, 2020 (2020-10-20)
Local time12:54:38
Magnitude7.6 Mw
Depth28.4 km (17.6 mi)
Epicenter 54°36′07″N159°37′34″W / 54.602°N 159.626°W / 54.602; -159.626
Areas affectedUnited States
Max. intensity 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. [36] [37] 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. [36]

A tsunami warning was issued but was later downgraded to an advisory. [38] A wave was measured at 2.3 ft. (70.1 cm) at Sand Point. [39] At King Cove, the tsunami had a wave height of 2.1 ft. (64.0 cm) and was 2.5 ft. at Chignik Bay. [40] [41] In the state of Hawaii, alert level Advisory was issued at 5:55 pm. [42] Wave heights ranging between 0.1-1.2 ft (2–38 cm) were detected along the islands. [43] Tsunami warning was later cancelled at 05:05:15 (UTC). [44]

The earthquake was described as "weird" and "the wrong type" by researchers at Pennsylvania State University. [45] 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. [46]

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 Mw  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. [47]

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 earthquake was initially reported as having a preliminary magnitude of 7.3 before being upgraded to 8.2. [48]

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 5.5 m (18 ft). [6] [49] It is thought to have ruptured the same subduction zone segment that was also involved in a similar-sized event in 1938. [50] [21]

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, [51] 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. [52] This is unusual since earthquakes of magnitude 8 or higher occur only about once every year on average. [53] 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. [54]

Tsunami

2021 Chignik earthquake Tsunami map 2021 Chignik earthquake Tsunami map.png
2021 Chignik earthquake Tsunami map

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. [55] A tsunami height of 1.4 feet (42.7 cm) was recorded in the city of Old Harbour in the Kodiak Island Borough on Kodiak Island. [56] In Port San Luis (Avila Beach) on the Central Coast of California, a 1.3-foot (40 cm) tsunami surge was measured by the National Weather Service in a tweet. [57] [58] Near the epicentre, a 8.6-foot (2.62 m) was observed. [2]

Tsunami height observations
LocationHeightSource
Near the epicentre8.6 feet (2.62 m) [2]
Old Harbour, Kodiak Island Borough on Kodiak Island 1.4 feet (42.7 cm) [59] [56]
Avila Beach, San Luis Obispo on San Luis Obispo County 1.3 feet (39.6 cm) [60]
Sand Point, Aleutians East Borough on Popof Island 0.5 feet (15.2 cm) [56]
Kodiak, Kodiak Island Borough on Kodiak Island 0.5 feet (15.2 cm) [61]
King Cove, Aleutians East Borough on Deer Island (Aleutian Islands) 0.4 feet (12.2 cm) [61]
Cape Alitak, southern Kodiak Island 0.4 feet (12.2 cm) [60]
Unalaska, Aleutians West Census Area on Unalaska Island 0.3 feet (9.14 cm) [60]

See also

Related Research Articles

<span class="mw-page-title-main">Cascadia subduction zone</span> Convergent plate boundary that stretches from northern Vancouver Island to Northern California

The Cascadia subduction zone is a 960 km fault at a convergent plate boundary, about 112-160 km off the Pacific Shore, that stretches from northern Vancouver Island in Canada to Northern California in the United States. It is capable of producing 9.0+ magnitude earthquakes and tsunamis that could reach 30m. The Oregon Department of Emergency Management estimates shaking would last 5-7 minutes along the coast, with strength and intensity decreasing further from the epicenter. It is a very long, sloping subduction zone where the Explorer, Juan de Fuca, and Gorda plates move to the east and slide below the much larger mostly continental North American Plate. The zone varies in width and lies offshore beginning near Cape Mendocino, Northern California, passing through Oregon and Washington, and terminating at about Vancouver Island in British Columbia.

Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthquakes are the planet's most powerful, with moment magnitudes (Mw) that can exceed 9.0. Since 1900, all earthquakes of magnitude 9.0 or greater have been megathrust earthquakes.

<span class="mw-page-title-main">1946 Aleutian Islands earthquake</span> Earthquake near the Aleutian Islands, Alaska

The 1946 Aleutian Islands earthquake occurred near the Aleutian Islands, Alaska on April 1, 1946. The shock measured 8.6, Mt 9.3 or 7.4. It had a maximum Mercalli intensity of VI (Strong). It resulted in 165–173 casualties and over US $26 million in damage. The seafloor along the fault was elevated, triggering a Pacific-wide tsunami with multiple destructive waves at heights ranging from 45–138 ft (14–42 m). The tsunami obliterated the Scotch Cap Lighthouse on Unimak Island, Alaska among others, and killed all five lighthouse keepers. Despite the destruction to the Aleutian Island Unimak, the tsunami had almost an imperceptible effect on the Alaskan mainland.

<span class="mw-page-title-main">Kamchatka earthquakes</span> Earthquakes in the Kamchatka Peninsula, far eastern Russia

Many major earthquakes have occurred in the region of the Kamchatka Peninsula in far eastern Russia. Events in 1737, 1923 and 1952, were megathrust earthquakes and caused tsunamis. There are many more earthquakes and tsunamis originating from the region.

<span class="mw-page-title-main">Aleutian Trench</span> An oceanic trench along the southern coastline of Alaska and the Aleutian islands

The Aleutian Trench is an oceanic trench along a convergent plate boundary which runs along the southern coastline of Alaska and the Aleutian islands. The trench extends for 3,400 kilometres (2,100 mi) from a triple junction in the west with the Ulakhan Fault and the northern end of the Kuril–Kamchatka Trench, to a junction with the northern end of the Queen Charlotte Fault system in the east. It is classified as a "marginal trench" in the east as it runs along the margin of the continent. The subduction along the trench gives rise to the Aleutian Arc, a volcanic island arc, where it runs through the open sea west of the Alaska Peninsula. As a convergent plate boundary, the trench forms part of the boundary between two tectonic plates. Here, the Pacific Plate is being subducted under the North American Plate at a dip angle of nearly 45°. The rate of closure is 7.5 centimetres (3 in) per year.

The 1957 Andreanof Islands earthquake occurred at 04:22 local time on March 9 with a moment magnitude estimated between 8.6 and 9.1 and a maximum Modified Mercalli intensity of VIII (Severe). It occurred south of the Andreanof Islands group, which is part of the Aleutian Islands arc. The event occurred along the Aleutian Trench, the convergent plate boundary that separates the Pacific Plate and the North American plates near Alaska. A basin-wide tsunami followed, with effects felt in Alaska and Hawaii, and strong waves recorded across the Pacific rim. Total losses were around $5 million.

The 1995 Antofagasta earthquake occurred on July 30 at 05:11 UTC with a moment magnitude of 8.0 and a maximum Mercalli intensity of VII. The Antofagasta Region in Chile was affected by a moderate tsunami, with three people killed, 58 or 59 injured, and around 600 homeless. Total damage from the earthquake and tsunami amounted to $1.791 million.

The 1906 Ecuador–Colombia earthquake occurred at 10:36:10 (UTC+5) on Wednesday January 31, 1906 off the coast of Ecuador, near Esmeraldas. The earthquake had a moment magnitude of 8.8 and triggered a destructive tsunami that caused at least 500 casualties on the coast of Colombia.

The 2014 Aleutian Islands earthquake occurred on 23 June at 11:53 HDT (UTC-9) with a moment magnitude of 7.9 and a maximum Mercalli intensity of VIII (Severe). The shock occurred in the Aleutian Islands – part of the US state of Alaska – 19 miles (31 km) southeast of Little Sitkin Island.

On July 17 2017, an earthquake struck near the Komandorski Islands, east of the Kamchatka Peninsula in the Bering Sea at. Although there were no casualties from this earthquake, it was notable for a rare characteristic known as supershear, and is one of the few times a large supershear earthquake has been observed. It was preceded by a few foreshocks months earlier, and aftershocks that continued for nearly six months.

<span class="mw-page-title-main">2013 Craig, Alaska earthquake</span> Earthquake in Alaska and British Columbia

The 2013 Craig, Alaska earthquake struck on January 5, at 12:58 am (UTC–7) near the city of Craig and Hydaburg, on Prince of Wales Island. The Mw 7.5 earthquake came nearly three months after an Mw  7.8 quake struck Haida Gwaii on October 28, in 2012. The quake prompted a regional tsunami warning to British Columbia and Alaska, but it was later cancelled. Due to the remote location of the quake, there were no reports of casualties or damage.

<span class="mw-page-title-main">2008 Simeulue earthquake</span> Earthquake in Indonesia

On February 20, 2008, an earthquake with a moment magnitude of 7.4 struck off the coast of Sumatra at a hypocentre depth of 26 km. The earthquake had an epicenter located on the island of Simeulue, northwest from Sinabang, a small town on the island. Three people were killed and an additional 25 seriously injured as a result of the earthquake.

The Nemuro-Oki earthquake in scientific literature, occurred on June 17 at 12:55 local time. It struck with an epicenter just off the Nemuro Peninsula in northern Hokkaidō, Japan. It measured 7.8–7.9 on the moment magnitude scale (Mw ), 8.1 on the tsunami magnitude scale (Mt ) and 7.4 on the Japan Meteorological Agency magnitude scale (MJMA ).

An earthquake occurred on 26 August 2012 at 22:37 local time. The earthquake located off the coast of El Salvador measured 7.3 on the moment magnitude scale and had a focal depth of 16.0 kilometres (10 mi). No deaths were reported, however more than 40 people were injured when they were caught in a tsunami generated by the earthquake. Waves from the tsunami were unusually large for an earthquake of this size. The large waves were attributed to the earthquake's unique rupture characteristic. In addition to the absence of fatalities, damage caused by the earthquake and tsunami was minimal as a result of the sparse population around the affected region and the slow rupture characteristic of the event.

<span class="mw-page-title-main">1585 Aleutian Islands earthquake</span> 16th-century seismic event in the North Pacific Ocean

The 1585 Aleutian Islands earthquake is the presumed source of a tsunami along the Sanriku coast of Japan on June 11, 1585, known only from vague historical accounts and oral traditions. The event was initially misdated to 1586, which led to it being associated with the deadly earthquakes in Peru and Japan of that year. The source earthquake was later determined by modern seismological studies to have originated near the Aleutian Islands in the North Pacific Ocean. Paleotsunami evidence from shoreline deposits and coral rocks in Hawaii suggest that the 1585 event was a large megathrust earthquake occurring on the Aleutian subduction zone with a moment magnitude (Mw ) as large as 9.25.

The 2021 Loyalty Islands earthquake was a 7.7 magnitude earthquake that struck offshore between Vanuatu and New Caledonia on February 11, 2021 at 00:19 local time. It is the 4th largest earthquake of 2021.

The 2021 South Sandwich Islands earthquakes were a pair of powerful earthquakes, followed by many strong aftershocks which struck along the South Sandwich Trench in August 2021. The quakes measured 7.5 and 8.1 on the moment magnitude scale, according to the United States Geological Survey. The mainshock is tied with another event in 1929 as the largest earthquake ever recorded in this region, and is tied with the 2021 Kermadec Islands earthquake as the second largest earthquake of 2021.

The 1979 Petatlán earthquake, also known as the IBERO earthquake occurred on March 14 at 05:07 local time in the Mexican state of Guerrero. The earthquake had a surface wave magnitude of Ms  7.6 or moment magnitude of Mw  7.4 and maximum Modified Mercalli intensity of VIII (Severe). The epicenter, onshore, was located 12 km south southeast of Vallecitos de Zaragoza.

The 1979 Saint Elias earthquake occurred near noon local time on the 28th of February. It measured Mw 7.4–7.6. Though the maximum recorded Modified Mercalli intensity was VII, damage was minimal and there were no casualties due to the remoteness of the faulting. The epicenter lies near the Alaskan border between America and Canada.

References

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  35. ↑ "Impacts | Alaska Earthquake Center". earthquake.alaska.edu. Archived from the original on July 26, 2020. Retrieved September 26, 2020.
  36. 1 2 "Finite Fault". US Geological Survey.
  37. ↑ "Significant Earthquake Information ALASKA". NGDC. Retrieved December 21, 2020.
  38. ↑ Service, National Weather. "National Weather Service - Tsunami Hazards". www.tsunami.gov. Retrieved October 19, 2020.
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  49. ↑ "Event Page | Alaska Earthquake Center". earthquake.alaska.edu. Retrieved July 29, 2021.
  50. ↑ 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.
  51. ↑ "M 8.1 - Kermadec Islands region". USGS-ANSS. Retrieved July 29, 2021.
  52. ↑ "M 8.1 - South Sandwich Islands region". United States Geological Survey . Retrieved August 12, 2021.
  53. ↑ "How Often Do Earthquakes Occur?". Incorporated Research Institutions for Seismology . Retrieved August 2, 2021.
  54. ↑ "Significant Earthquakes - 2007". earthquake.usgs.gov. USGS. Retrieved August 2, 2021.
  55. ↑ "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.
  56. 1 2 3 "Public Tsunami Message Number 5". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.
  57. ↑ "NWS Los Angeles on Twitter". twitter.com. NWS Los Angeles. Retrieved July 31, 2021.
  58. ↑ Bethany Reeves (July 29, 2021). "Tsunami surge from Alaska earthquake seen on Central Coast". KSBY . Retrieved July 31, 2021.
  59. ↑ "Tsunami Event (07/29/2021)". ngdc.noaa.gov.
  60. 1 2 3 "Public Tsunami Message Number 6". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.
  61. 1 2 "Public Tsunami Message Number 6". tsunami.gov. Palmer, Alaska: NWS National Tsunami Warning Center. July 29, 2021. Retrieved July 29, 2021.

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