Location:

Period:

9 Dec 2021 20:06:40 - 12 Dec 2021 08:34:10 (2 days 12 hours 27 minutes)

Volcanoes in 100km radius:

Davidof(6km), Little Sitkin(15km), Segula(15km), Kiska(52km), Semisopochnoi(84km)

Earthquakes:

10 swarms found nearby.

2003

PS20030317.1(98.6km)

17 Mar

5 hours

7 earthquakes

PS20031117.1(100.8km)

17 Nov

1 day 5 hours

18 earthquakes

2006

PS20060614.1(89.6km)

14 Jun

1 hours

5 earthquakes

2014

PS20140623.2(15.2km)

23 Jun

1 day 3 hours

11 earthquakes

S20140623.2(24.3km)

23 Jun

26 days 6 hours

992 earthquakes

S20140624.3(27.0km)

23 Jun

7 days 20 hours

135 earthquakes

2021

S20210622.3(9.2km)

22 Jun

1 day 2 hours

48 earthquakes

2022

VS20220126.1(5.7km)

25 Jan

3 days 9 hours

91 earthquakes

S20220605.1(11.0km)

4 Jun

4 days 0 hours

74 earthquakes

2024

VS20240610.1(13.5km)

9 Jun

3 days 5 hours

85 earthquakes

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Seismic Swarm VS20211210.1 in the Rat Islands, Aleutian Islands

A seismic swarm designated VS20211210.1 occurred in the Rat Islands region of the Aleutian Islands, Alaska. The sequence began at 20:06 on 9 December 2021 and concluded at 08:34 on 12 December 2021, spanning 60 hours and 27 minutes. During this interval, 55 earthquakes were recorded.

The events exhibited magnitudes ranging from 0.5 to 4.5, with the majority clustered between 1.0 and 3.4. Depths varied from 1 km to 13 km, predominantly in the 3–10 km range. Notable events included a magnitude 4.5 earthquake at 19:45 on 10 December at 9 km depth, a magnitude 4.1 event at 20:15 the same day at 10 km depth, and several magnitude 3.0–3.4 shocks distributed across the first two days. Activity peaked on 10 December, with 24 events, before tapering on 11 and 12 December.

The Rat Islands form part of the western Aleutian arc, situated above the convergent boundary where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. This tectonic regime produces high seismicity through megathrust faulting, intraslab earthquakes, and upper-plate deformation. The region has hosted large historical earthquakes, including the magnitude 8.7 event of 1965, and remains one of the most active segments of the circum-Pacific seismic belt.

Historical records indicate seven swarms in the Rat Islands since 1 January 2000. These occurred in 2003 (two swarms), 2006 (one swarm), 2014 (three swarms), and 2021 (one swarm). Such episodic clusters are characteristic of subduction-zone environments, where fluid migration or stress transfer can trigger distributed seismicity without a dominant mainshock-aftershock pattern.

The December 2021 swarm fits this established behavior, with events distributed across a compact spatial and temporal window and no single earthquake exceeding magnitude 5. Depths remained shallow, consistent with activity within the overriding plate or near the plate interface. Monitoring of such swarms contributes to understanding stress accumulation along the arc and informs regional hazard assessments.

References

USGS Earthquake Hazards Program – Aleutian Islands tectonics overview
Alaska Earthquake Center – Regional seismicity summaries
SeismoSight internal swarm classification records (VS20211210.1)