2021 Chignik earthquake

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

This was the largest earthquake in the United States since the 1965 Rat Islands earthquake, and the 7th largest earthquake in US history. 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.

### 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. 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. Two smaller earthquakes shook near the seismic gap in 1917 and 1948. That proposal also highlighted the potential for a tsunami accompanying an earthquake on the gap.

### 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. 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. 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.

## 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 89 days later on the same year. The United States Geological Survey have since considered the two earthquakes as foreshocks to the Mw  8.2 in 2021.

### July 2020

The earthquake on July 21, 2020, was the result of thrust faulting on the Aleutian subduction zone where the Pacific plate subducts underneath the North American plate, forming the Aleutian Trench and Arc. the July 2020 quake was initially reported as a Mw 7.4, then later upgraded it to a Mw 7.8. 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. 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. 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.

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).

The earthquake appears to have only partially ruptured the Shumagin segment of the subduction zone, at 75%. Estimated fault rupture dimenions estimate 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. Mild damage occurred in Sand Point with reports of damaged docks, cracked roads and cement. It was reportedly felt as far as Vancouver and Victoria, more than 1,500 miles (2,400 km) away.

### October 2020

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

Same with the July 2020, the quake was initially reported as a Mw 7.4, then later upgraded it to a Mw 7.5.

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

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

### Other events

On March 4, 2021, 146 days or nearly 5 months before the Alaska mainshock, an 8.1 magnitude earthquake struck the Kermadec Islands, a sparsely populated territory owned by New Zealand, and the 2021 South Sandwich Islands earthquakes. This is unusual since megathrust earthquakes occur only about once every 5\_10 year on average. There is no evidence of a causal relationship between the three events. Such events make 2021 the first year with a least two 8.0+ earthquakes since 2013.

## 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.

Initially reported as a magnitude 7.2 (some say 8.1), then later received it to an 8.2.

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) It is thought to have ruptured the same subduction zone segment that was also involved in a similar-sized event in 1938.

## 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. 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. 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.