Location:

Period:

18 May 2022 06:27:54 - 19 May 2022 16:24:20 (1 day 9 hours 56 minutes)

Volcanoes in 100km radius:

Little Sitkin(87km), Davidof(93km), Semisopochnoi(97km)

Earthquakes:

M 7.0+:

M 7.8; Rat Islands, Aleutian Islands, Alaska; (17 Nov 2003) (13km from the swarm center)

5 swarms found nearby.

2003

PS20030317.1(68.3km)

17 Mar

5 hours

7 earthquakes

PS20031117.1(29.1km)

17 Nov

1 day 5 hours

18 earthquakes

2005

PS20050614.1(47.7km)

14 Jun

14 hours

7 earthquakes

2014

PS20140623.2(84.2km)

23 Jun

1 day 3 hours

11 earthquakes

2020

PS20200126.1(88.3km)

26 Jan

18 hours

8 earthquakes

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Seismic Swarm S20220518.1 in the Rat Islands, Aleutian Islands, Alaska

A seismic swarm designated S20220518.1 occurred in the Rat Islands region of the Aleutian Islands, Alaska, from 06:27 on 18 May 2022 to 16:24 on 19 May 2022. Over 33 hours and 56 minutes, the sequence registered 28 earthquakes. The largest event reached magnitude 5.7 at a depth of 18 km, occurring at the swarm's onset. Subsequent events ranged from magnitude 1.1 to 3.8, with depths between 2 km and 35 km. Most activity concentrated in the first 12 hours, featuring a mix of shallow and intermediate-depth shocks that gradually diminished.

The Rat Islands lie within the western segment of the Aleutian arc, where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin produces intense seismicity, including megathrust earthquakes and volcanic activity along the arc. The swarm's location aligns with the subduction interface and overlying crustal faults, consistent with stress release in a highly active tectonic setting.

Historical records since 2000 indicate five prior swarms in the region: two events in 2003, and single swarms in 2005, 2014, and 2020. A magnitude 7.8 earthquake struck the Rat Islands on 17 November 2003, centered 13 km from the 2022 swarm epicenter. Such sequences highlight recurring clustered seismicity amid the broader pattern of subduction-driven deformation.

Analysis of the May 2022 swarm shows a rapid initial phase dominated by the magnitude 5.7 mainshock, followed by smaller aftershocks clustered at depths of 13–23 km. Later events included isolated deeper shocks up to 35 km, suggesting migration along the plate interface. The overall low-magnitude character and short duration distinguish this swarm from larger mainshock-aftershock sequences typical of the arc.

The Aleutian subduction zone remains one of the most seismically productive regions globally, with ongoing monitoring essential for understanding precursory patterns in swarm activity.

References

SeismoSight internal swarm classification records for S20220518.1.
USGS Earthquake Catalog for regional tectonics and historical events.