5 Largest Earthquakes in 2021

The year 2021 was a very active year for global seismicity and was even the most seismically active since 2007. Reported were 44 earthquakes with a magnitude above 6.5, which occurred throughout the whole year. Three of them had a magnitude above 8.0, with more than 2,000 fatalities¹.

The previous year, which was the second consecutive year, when earthquake activity was below average, there were only 31 earthquakes above magnitude 6.5, from which not even one was larger than magnitude 8.0. Moreover, there were only 207 casualties¹ reported, which made the year 2020 the least deadly year in the 21^st century for earthquakes.

Picture 1: Magnitude 6.5+ earthquakes in 2020.

Picture 2: Magnitude 6.5+ earthquakes in 2021.

The biggest earthquakes in 2021 were:
- M8.2 Chignik Earthquake (U.S., Alaska offshore, Aleutian Islands) on July 29;
- M8.1 Kermadec Islands Earthquakes (NZ, Kermadec Islands offshore) on March 4;
- M8.1 South Sandwich Islands Earthquakes (South Georgia and the South Sandwich Islands offshore) on August 12;
- M7.7 Loyalty Islands Earthquake (New Caledonia offshore, southeast of the Loyalty Islands) on February 10;
- M7.5 Northern Peru Earthquake (Peru, Loreto) on November 28,
- M7.5 South Sandwich Islands Earthquakes (South Georgia and the South Sandwich Islands offshore) on August 12.

M8.2 Chignik Earthquake (U.S., Alaska offshore, Aleutian Islands) on July 29, 2021

The biggest earthquake in 2021, and the biggest earthquake globally since the 2018 Fiji earthquake – Chignik Earthquake - occurred on July 29, 2021, at 10:15 p. m. local time off the coast of the Alaska Peninsula, on the Aleutian Islands. With a magnitude of 8.2, it got recorded as the 7^th largest earthquake in U.S. history. It had a depth of 43 km.

The earthquake occurred on 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 fault area is often the source of many strong earthquakes and is one of the most active in the world.

The mainshock was followed by several aftershocks, including three that were of magnitude 5.9, 6.1, and 6.9 respectively.

The most powerful U.S. earthquake in half a century left Alaska mostly unscathed, with no major damage or injury reported, thanks to the remote location and depth of the epicenter.

M8.1 Kermadec Islands Earthquakes (NZ, Kermadec Islands offshore) on March 4, 2021

The second biggest earthquake in 2021 occurred on March 5, 2021, at 8:28 p. m. local time on the Kermadec Islands offshore. The main M8.1 earthquake was preceded by an M7.4 foreshock and followed by a magnitude 6.1 aftershock. The earthquake had a depth of 10 km. The epicenter was about 600 miles (950 km) northeast of New Zealand, with shaking reported in some parts of the country and nearby islands. There were no reports of serious damage or injuries.

It occurred on the Kermadec-Tonga subduction zone, which marks the boundary between the denser Pacific Plate and the less dense Australian Plate. The former subducts or dives beneath the latter. It occurred as the result of reverse faulting in the subduction zone at a depth of ~22 km.

This subduction zone spans from north of the East Cape, around 2600km to the north-east in an almost straight line to the south of Samoa, and is the longest and deepest such system on Earth.

At the location of the earthquake, the Pacific plate moves westward relative to the Australia plate at a velocity of about 60 mm/yr, subducting to the west beneath the Australia plate at the Tonga and Kermadec Trenches.

Earthquakes triggered tsunami warnings and evacuations along the east coast of New Zealand.

M8.1 South Sandwich Islands Earthquakes (South Georgia and the South Sandwich Islands offshore) on August 12, 2021

One more earthquake of M8.1 occurred in 2021. It was a part of the so-called South Sandwich Islands Earthquakes, which were a pair of strong earthquakes, followed by around a thousand strong aftershocks which struck along the South Sandwich Islands in South Georgia on August 12, 2021, at 6:32 p. m. local time. It was the largest to strike the country since the 1842 M8.1 Cap-Haïtien earthquake. A lot of damage was reported, with more than 138,000 buildings completely destroyed and with more than 2000 fatalities, making this earthquake the deadliest quake of 2021.

They occurred in the area where the South American plate subducts westward beneath the Scotia Plate and South Sandwich microplate at a rate of 71 mm/yr. This area is called the Scotia subduction zone. Both earthquakes were the result of intraplate reverse faulting within the lithosphere of the subducting South American plate.

The earthquake’s epicenter was 60 km under the crust.

In the 24 hours following the mainshock, 61 aftershocks of M4.5 or larger were detected, including three aftershocks greater than magnitude 6.0. This series of magnitude 7.5 earthquake is the fifth biggest earthquake in 2021.

M7.7 Loyalty Islands Earthquake (New Caledonia offshore, southeast of the Loyalty Islands) on February 10, 2021

The area southeast of the Loyalty Islands in the Pacific, which is seismically very active, got affected by an M7.7 earthquake on February 10, 2021, at 1:30 p. m. local time. It struck offshore between Vanuatu and New Caledonia and it became one of the five biggest earthquakes in 2021. It is the largest earthquake in New Caledonia since 1995, and the largest in Vanuatu since the 2009 Vanuatu earthquakes. It occurred at depth of 10 km.

Because of their remote locations far from land, these earthquakes do not cause significant shaking in populated areas and are not known to have resulted in damage or fatalities.

It occurred as the result of low angle thrust faulting on or near the plate boundary interface between the Indo-Australian and Pacific plates. At the location of this earthquake, the Indo-Australian plate moves towards the east-northeast with respect to the Pacific at a rate of approximately 75 mm/yr. The region within 250 km of the earthquake has suffered more than a dozen M7+ earthquakes having been recorded since the early decades of the 20th century.

The earthquake was preceded by two foreshocks of M6.1 and M6.0 around an hour before the earthquake. Recordings show that there have been 141 aftershocks above M5.0, the strongest having an M6.3.

M7.5 Northern Peru Earthquake (Peru, Loreto) on November 28, and South Sandwich Islands Earthquakes (South Georgia and the South Sandwich Islands offshore) on August 12, 2021

A major M7.5 earthquake struck northern Peru on November 28, 2021, 05:52 a.m. local time, and thus got recorded as the fifth biggest earthquake in 2021. It occurred at a depth of 131 km. The affected regions experienced four aftershocks – the strongest one with a magnitude of 5.2 the same day.

The earthquake occurred above the convergent plate boundary where the Nazca Plate is subducting beneath the South American Plate, along the line of the Peru–Chile Trench. In northern Peru, the rate of convergence between the plates is about 70 mm/yr. This ongoing subduction affects directly or indirectly most of the seismicity that affects western South America since earthquakes occur on the megathrust interface between the two plates, or within either the subducting or over-riding plates.

Serious damage occurred, in Peru, and some damage was reported in Ecuador. A total of 2,202 people became homeless when the quake damaged many homes. Thousands of homes were damaged, including 772 totally destroyed. The National Institute of Civil Defense of Peru confirmed one fatality and at least 126 injuries. The earthquake has destroyed several roads and caused the Utcubamba River to overflow. As a result, some communities can only be reached by air. The most affected regions were in the Amazon jungle, with very limited access because of the damage caused to the few existing roads.

One more earthquake of magnitude M7.5 occurred as a part of the South Sandwich Islands Earthquakes on August 12, 2021.

Sources:
1) Relief Web. 2021. Earthquake Review 2020. Accessed on 03-Jan-2022. Available at: https://reliefweb.int/sites/reliefweb.int/files/resources/Earthquake%20Review%202020.pdf
2) Relief Web. 2021. Haiti Earthquake Situation Report #2. Accessed on 01-Jan-2022. Available at: https://reliefweb.int/report/haiti/2021-haiti-earthquake-situation-report-2-september-10-2021