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Location:
42 km ENE of Adak, Alaska
Period:
11 Aug 2021 23:44:39 - 14 Aug 2021 19:28:01 (2 days 19 hours 43 minutes)
Volcanoes in 100km radius:
Great Sitkin(0km), Kasatochi(43km), Moffett(44km), Koniuji(70km), Kanaga(73km), Bobrof(91km)
Earthquakes:
49
34 swarms found nearby.
2002
S20020318.1(17.5km)
17 Mar
2 days 8 hours
75 earthquakes
2013
PS20130831.1(103.7km)
31 Aug
15 hours
5 earthquakes
2018
18 Jan
2 days 7 hours
87 earthquakes
5 Jun
8 days 3 hours
430 earthquakes
30 Jun
1 day 22 hours
35 earthquakes
8 Sep
3 days 3 hours
52 earthquakes
28 Sep
1 day 22 hours
33 earthquakes
27 Oct
7 days 6 hours
253 earthquakes
14 Nov
9 days 8 hours
275 earthquakes
26 Nov
6 days 7 hours
191 earthquakes
3 Dec
7 days 18 hours
179 earthquakes
VS20181221.1(10.2km)
20 Dec
2 days 1 hours
30 earthquakes
2019
17 Jun
3 days 10 hours
80 earthquakes
6 Oct
3 hours
37 earthquakes
2020
18 Jan
5 days 5 hours
119 earthquakes
29 Jan
2 days 14 hours
72 earthquakes
2 Feb
1 day 14 hours
37 earthquakes
6 Feb
6 days 21 hours
144 earthquakes
25 Feb
20 days 20 hours
580 earthquakes
20 Mar
31 days 4 hours
579 earthquakes
23 Apr
15 days 9 hours
279 earthquakes
14 May
5 days 18 hours
76 earthquakes
30 May
10 days 21 hours
210 earthquakes
14 Jun
4 days 1 hours
69 earthquakes
5 Jul
11 days 1 hours
252 earthquakes
18 Jul
3 days 5 hours
57 earthquakes
7 Nov
1 day 14 hours
31 earthquakes
2021
25 May
1 day 3 hours
165 earthquakes
22 Jun
2 days 8 hours
34 earthquakes
11 Jul
5 days 5 hours
120 earthquakes
23 Jul
5 days 4 hours
83 earthquakes
28 Jul
8 days 13 hours
161 earthquakes
2023
2 Jan
23 hours
30 earthquakes
2025
PS20250320.1(107.0km)
19 Mar
22 hours
5 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm VS20210812.1 Near Adak, Alaska: Geological Context and Event Analysis

The seismic swarm designated VS20210812.1 occurred in the central Aleutian Islands, approximately 42 km east-northeast of Adak, Alaska. This sequence began at 23:44 UTC on 11 August 2021 and concluded at 19:28 UTC on 14 August 2021, spanning 67 hours and 43 minutes. During this period, 49 earthquakes were recorded, with magnitudes ranging from -0.9 to 1.7 and focal depths between 1 km and 10 km. The events clustered tightly in time and space, characteristic of swarm behavior rather than a mainshock-aftershock sequence.

The Aleutian Islands form a classic volcanic arc resulting from the oblique subduction of the Pacific Plate beneath the North American Plate along the Aleutian Trench. Convergence rates average 6–7 cm per year in this sector, generating frequent seismicity at shallow to intermediate depths. The region hosts active volcanoes such as Mount Adagdak and Mount Moffett on Adak Island, reflecting ongoing magmatic processes linked to the same tectonic regime. Historical records document persistent earthquake activity, including both isolated events and episodic swarms driven by fluid migration or stress perturbations within the overriding plate and along the plate interface.

Key events within the swarm included a magnitude 1.7 earthquake at 5 km depth on 12 August at 19:06 UTC, which represented the largest magnitude recorded. Earlier activity featured multiple events near magnitude 1.0–1.1, while the majority remained below magnitude 0.0. Depths were predominantly in the 4–6 km range, suggesting activity within the upper crust. The swarm exhibited a gradual increase in event rate during the first 24 hours, followed by a steady decline, with the final recorded event being a magnitude 0.0 shock at 4 km depth.

Since 1 January 2000, 32 swarms have been identified in the broader Adak region according to internal classification records. Yearly distribution shows marked variability: one swarm each in 2002 and 2013, ten in 2018, two in 2019, thirteen in 2020, and five in 2021. This pattern indicates episodic clustering influenced by regional tectonics, consistent with the high strain accumulation expected in a subduction-zone setting.

Analysis of swarm parameters reveals predominantly low-magnitude, shallow seismicity consistent with background tectonic noise amplified by transient stress changes. Such sequences contribute to the overall seismic hazard assessment of the central Aleutians, where even small events help delineate active fault structures and potential volcanic unrest. Continued monitoring remains essential given the proximity to populated areas on Adak Island and critical infrastructure.

References

  • Alaska Earthquake Center, University of Alaska Fairbanks (earthquake catalogs and regional tectonics summaries)
  • U.S. Geological Survey Earthquake Hazards Program (Aleutian subduction zone parameters and historical seismicity)