Location:

Period:

10 Jul 2021 18:12:59 - 12 Jul 2021 06:32:12 (1 day 12 hours 19 minutes)

Volcanoes in 100km radius:

Kiska(31km), Segula(69km), Davidof(83km), Buldir(87km), Little Sitkin(97km)

Earthquakes:

5 swarms found nearby.

2006

PS20060614.1(23.6km)

14 Jun

1 hours

5 earthquakes

S20060614.2(9.2km)

14 Jun

18 days 19 hours

1106 earthquakes

S20060614.1(3.7km)

14 Jun

1 day 1 hours

136 earthquakes

S20060614.3(27.8km)

14 Jun

3 days 7 hours

62 earthquakes

2014

PS20140623.2(93.5km)

23 Jun

1 day 3 hours

11 earthquakes

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Seismic Swarm S20210711.2 Near Attu Station, Alaska

An earthquake swarm designated S20210711.2 occurred approximately 289 km east-southeast of Attu Station in Alaska's Aleutian Islands. The sequence began at 18:12 UTC on 10 July 2021 and concluded at 06:32 UTC on 12 July 2021, spanning 36 hours and 19 minutes. During this interval, 37 earthquakes were recorded, with magnitudes ranging from 1.6 to 5.4 and focal depths primarily between 1 km and 28 km.

The swarm exhibited typical characteristics of clustered seismic activity in a subduction-zone setting. The largest event reached magnitude 5.4 at a depth of 1 km on 11 July at 02:11 UTC, followed by several events above magnitude 3.0, including a magnitude 4.3 at 7 km depth and a magnitude 4.2 at 9 km depth. Most events clustered in the first 12 hours after initiation, with activity tapering thereafter. Depths remained predominantly shallow, consistent with tectonic release along the plate interface or overlying crust.

The Aleutian Islands form part of the circum-Pacific seismic belt, where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–8 cm per year along the Aleutian Trench. This convergent margin produces frequent earthquakes, including swarms, due to episodic slip on the megathrust and associated crustal faults. Attu Station lies near the western end of the arc, a region known for both great earthquakes and smaller clustered sequences.

Historical records since 2000 indicate five prior swarms in the broader area, occurring in 2006 (four events) and 2014 (one event). These episodes reflect the persistent strain accumulation and release characteristic of the subduction zone. The 2021 swarm aligns with this pattern, showing no immediate indication of escalation to a major mainshock.

Such swarms contribute to ongoing monitoring efforts that help refine seismic hazard assessments for remote Aleutian communities and maritime routes. Continued observation supports improved understanding of stress transfer along the plate boundary.

References:
USGS Earthquake Catalog
Alaska Earthquake Center reports
SeismoSight internal swarm classification data