Location:

Period:

10 Aug 2021 22:12:45 - 13 Aug 2021 17:43:59 (2 days 19 hours 31 minutes)

Volcanoes in 100km radius:

Atka Volcanic Complex(2km), Koniuji(69km), Kasatochi(95km)

Earthquakes:

224

6 swarms found nearby.

2008

PS20081031.1(100.3km)

31 Oct

1 day 9 hours

6 earthquakes

2013

PS20130831.1(110.5km)

31 Aug

15 hours

5 earthquakes

PS20130904.2(100.2km)

4 Sep

1 day 7 hours

11 earthquakes

2015

PS20150905.1(109.4km)

4 Sep

18 hours

5 earthquakes

PS20151102.1(105.4km)

2 Nov

15 minutes

5 earthquakes

2016

PS20160319.1(93.8km)

18 Mar

15 hours

5 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20210811.1: Analysis of Activity Near Atka, Alaska

A seismic swarm designated S20210811.1 occurred 13 km north of Atka, Alaska, from 22:12 on 10 August 2021 to 17:43 on 13 August 2021. Over 67 hours and 31 minutes, the swarm produced 224 earthquakes. Analysis of the first 100 events reveals predominantly microseismic activity with magnitudes ranging from -0.7 to 2.1. Depths clustered between 1 km and 5 km, with the majority at 3–4 km. The sequence initiated with low-magnitude events near 3 km depth and showed gradual increases in both magnitude and slight depth variation as activity progressed, culminating in a peak event of magnitude 2.1 at 4 km depth.

This pattern is consistent with fluid-driven processes or minor stress adjustments within the crust. Most events remained below magnitude 1.0, indicating a swarm rather than a mainshock-aftershock sequence. Temporal distribution showed clustering in the initial hours, followed by sustained low-level activity.

The region lies within the Aleutian Islands, part of the circum-Pacific Ring of Fire. Atka Island sits above the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting generates frequent earthquakes and hosts several active volcanoes, including Korovin and Kliuchef near Atka. Shallow crustal seismicity often reflects magma movement, hydrothermal fluid circulation, or faulting induced by the subduction process.

Seismic swarms have occurred in the Atka area since 2000, with six documented episodes: one in 2008, two in 2013, two in 2015, and one in 2016. These prior swarms similarly featured low-magnitude, shallow events, underscoring the area’s recurrent swarm-type behavior linked to its volcanic and tectonic environment.

The 2021 swarm aligns with historical patterns, reinforcing the value of continuous monitoring in this high-hazard subduction zone. Such events provide insights into local stress regimes and potential volcanic unrest, though no surface deformation or eruptive activity was associated with this episode.

References

Alaska Earthquake Center, University of Alaska Fairbanks
U.S. Geological Survey Earthquake Hazards Program
SeismoSight internal swarm classification records