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Location:
39 km NE of Adak, Alaska
Period:
25 May 2021 01:40:59 - 26 May 2021 05:01:06 (1 day 3 hours 20 minutes)
Volcanoes in 100km radius:
Great Sitkin(8km), Moffett(39km), Kasatochi(50km), Kanaga(67km), Koniuji(76km), Bobrof(85km)
Earthquakes:
165
34 swarms found nearby.
2002
S20020318.1(13.9km)
17 Mar
2 days 8 hours
75 earthquakes
2013
PS20130831.1(111.8km)
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(18.2km)
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
VS20200226.1(10.2km)
25 Feb
20 days 20 hours
580 earthquakes
20 Mar
31 days 4 hours
579 earthquakes
23 Apr
15 days 9 hours
279 earthquakes
VS20200515.1(10.7km)
14 May
5 days 18 hours
76 earthquakes
30 May
10 days 21 hours
210 earthquakes
VS20200615.1(11.1km)
14 Jun
4 days 1 hours
69 earthquakes
5 Jul
11 days 1 hours
252 earthquakes
18 Jul
3 days 5 hours
57 earthquakes
S20201108.1(13.1km)
7 Nov
1 day 14 hours
31 earthquakes
2021
22 Jun
2 days 8 hours
34 earthquakes
VS20210712.1(10.8km)
11 Jul
5 days 5 hours
120 earthquakes
23 Jul
5 days 4 hours
83 earthquakes
VS20210729.1(13.5km)
28 Jul
8 days 13 hours
161 earthquakes
11 Aug
2 days 19 hours
49 earthquakes
2023
S20230102.2(15.2km)
2 Jan
23 hours
30 earthquakes
2025
PS20250320.1(111.2km)
19 Mar
22 hours
5 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20210525.1: Analysis of Activity Near Adak, Alaska

A seismic swarm designated S20210525.1 was recorded 39 km northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 01:40 UTC on 25 May 2021 and concluded at 05:01 UTC on 26 May 2021, spanning 27 hours and 20 minutes. During this interval, 165 earthquakes were detected.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.1 to 1.5, with the majority falling between 0.4 and 1.2. Depths clustered between 4 km and 12 km, though several events registered at shallower levels near 2–3 km or deeper at 11 km. The sequence showed no dominant temporal clustering beyond a steady rate of occurrence, consistent with swarm behavior rather than a classic mainshock-aftershock pattern.

Adak lies within the Aleutian volcanic arc, a tectonically active segment of the Pacific Ring of Fire. The region experiences frequent seismicity due to the subduction of the Pacific plate beneath the North American plate along the Aleutian Trench. This convergent margin produces both shallow crustal events and deeper Wadati-Benioff zone activity, alongside volcanic systems that contribute to localized stress changes.

Historical records indicate that swarm activity is not uncommon in this sector. Since 1 January 2000, 27 swarms have been documented in the broader Adak vicinity. These occurred in the following years with the listed event counts: 2002 (1), 2013 (1), 2018 (10), 2019 (2), and 2020 (13). Such recurrent swarms reflect ongoing plate-boundary deformation and fluid migration within the subduction interface.

The 2021 swarm’s shallow focal depths and modest magnitudes align with typical background seismicity observed along the central Aleutians. Continued monitoring by regional networks remains essential for distinguishing swarm episodes from potential precursors to larger events.

References

USGS Earthquake Hazards Program – Aleutian Arc tectonics overview (2023 update)
Alaska Earthquake Center – Regional seismicity summaries
SeismoSight internal swarm classification records