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Location:
43 km ENE of Adak, Alaska
Period:
7 Nov 2020 06:51:51 - 8 Nov 2020 20:51:58 (1 day 14 hours)
Volcanoes in 100km radius:
Great Sitkin(5km), Kasatochi(40km), Moffett(48km), Koniuji(66km), Kanaga(77km), Bobrof(95km)
Earthquakes:
31
34 swarms found nearby.
2002
S20020318.1(21.3km)
17 Mar
2 days 8 hours
75 earthquakes
2013
PS20130831.1(99.0km)
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
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
2021
S20210525.1(13.1km)
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
11 Aug
2 days 19 hours
49 earthquakes
2023
2 Jan
23 hours
30 earthquakes
2025
PS20250320.1(105.2km)
19 Mar
22 hours
5 earthquakes
AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20201108.1: Activity Near Adak, Alaska

Seismic swarm S20201108.1 occurred 43 km east-northeast of Adak, Alaska, in the central Aleutian Islands. The sequence began at 06:51 on 7 November 2020 and concluded at 20:51 on 8 November 2020, spanning 38 hours and producing 31 earthquakes.

Event magnitudes ranged from −0.7 to 1.5, with the majority falling below 1.0. Depths clustered between 3 km and 12 km, indicating shallow crustal processes. The largest event reached magnitude 1.5 at 06:32 on 8 November. Activity showed two clusters: an initial phase on 7 November followed by a more energetic period extending into the early hours of 8 November before tapering off.

This swarm fits within a broader pattern of seismic activity documented in the region since 2000. Twenty-six swarms have been recorded, with notable concentrations in 2018 (10 events) and 2020 (12 events). Earlier occurrences were limited, appearing in 2002 and 2013.

The Adak area lies along the Aleutian subduction zone, where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting generates frequent earthquakes across a wide range of magnitudes and depths. The central Aleutians host both volcanic and non-volcanic seismicity, with swarms often linked to fluid migration or stress adjustments along the megathrust interface and overlying crust.

Historical records confirm persistent seismic productivity. The region experienced great earthquakes in 1957 and 1965, underscoring long-term strain accumulation. Modern monitoring by the Alaska Earthquake Center and USGS networks provides high-resolution detection of microseismicity, revealing that swarms like S20201108.1 represent common, low-magnitude expressions of the same subduction dynamics.

Such sequences typically do not produce significant ground shaking at Adak due to their small magnitudes and moderate depths. Continued surveillance remains essential for distinguishing swarm behavior from potential foreshock sequences preceding larger events.

References

  • Alaska Earthquake Center, University of Alaska Fairbanks
  • U.S. Geological Survey Earthquake Hazards Program
  • Tectonic summary of the Aleutian arc, USGS Professional Paper series