Location:

Andreanof Islands, Aleutian Islands, Alaska

Period:

28 Oct 2022 21:33:36 - 31 Oct 2022 02:07:20 (2 days 4 hours 33 minutes)

Volcanoes in 100km radius:

None

Earthquakes:

12 swarms found nearby.

2008

S20081031.1(1.9km)

31 Oct

3 days 18 hours

94 earthquakes

PS20081031.1(27.0km)

31 Oct

1 day 9 hours

6 earthquakes

2013

PS20130831.1(57.2km)

31 Aug

15 hours

5 earthquakes

PS20130904.2(36.5km)

4 Sep

1 day 7 hours

11 earthquakes

S20130905.1(20.5km)

4 Sep

6 days 6 hours

96 earthquakes

S20130906.1(4.5km)

5 Sep

2 days 8 hours

36 earthquakes

2015

PS20150905.1(17.4km)

4 Sep

18 hours

5 earthquakes

S20150905.1(20.2km)

5 Sep

2 days 11 hours

103 earthquakes

PS20151102.1(75.9km)

2 Nov

15 minutes

5 earthquakes

2016

S20160312.2(20.3km)

12 Mar

2 days 4 hours

52 earthquakes

S20160319.1(13.2km)

18 Mar

3 days 15 hours

59 earthquakes

PS20160319.1(32.2km)

18 Mar

15 hours

5 earthquakes

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Earthquake Swarm in the Andreanof Islands, Aleutian Arc: October 2022

The Andreanof Islands lie within the central Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismic activity along the megathrust and within the overlying crust and subducting slab. The region has long been recognized for both large-magnitude thrust events and episodic earthquake swarms.

Between 21:33 UTC on 28 October 2022 and 02:07 UTC on 31 October 2022, a swarm comprising 39 earthquakes was recorded in the Andreanof Islands. The sequence lasted 52 hours and 33 minutes. Magnitudes ranged from 2.1 to 4.7, with the two largest events (both M4.7) occurring on 29 October at depths of 40 km and 14 km. Most hypocenters clustered between 10 km and 45 km depth, consistent with activity along the plate interface and within the overriding crust.

The swarm displayed a typical pattern of clustered, moderate-magnitude events without a single dominant mainshock. Early activity on 28 October included events of M4.2 and M4.1, followed by a burst of M4.0–M4.7 earthquakes on 29 October. Subsequent days saw lower-magnitude events tapering off by the early hours of 31 October. Depths remained distributed across the seismogenic zone, indicating distributed slip rather than rupture on a single fault plane.

Seismic swarms are recurrent in this segment of the Aleutian arc. Since 2000, twelve such swarms have been documented in the Andreanof Islands region. Earlier episodes occurred in 2008 (two swarms), 2013 (four swarms), 2015 (three swarms), and 2016 (three swarms). These sequences commonly last from one to several days and rarely exceed magnitude 5.0, distinguishing them from the great subduction earthquakes that characterize the arc’s long-term seismic cycle.

The October 2022 swarm occurred within a portion of the arc that last experienced a major thrust earthquake in 1957 (M8.6). Post-1957 aftershock decay has been punctuated by occasional swarm-like clusters, reflecting ongoing stress adjustment along the plate boundary. Current monitoring networks operated by the Alaska Earthquake Center and the U.S. Geological Survey continue to record background seismicity at rates consistent with the long-term average for this tectonic environment.

No damage or felt reports were widely documented for the 2022 swarm, as expected given the moderate magnitudes and offshore location. Such sequences nevertheless provide valuable data for refining models of stress transfer and fault interaction within the Aleutian subduction system.

References

SeismoSight internal swarm catalog (S20221029.1)
Alaska Earthquake Center regional seismicity reports
U.S. Geological Survey Earthquake Hazards Program
Tectonic framework summaries from the Alaska Division of Geological & Geophysical Surveys