Location:

Period:

27 Jun 2011 23:27:33 - 28 Jun 2011 16:13:32 (16 hours 45 minutes)

Volcanoes in 100km radius:

Seguam(73km), Amukta(79km), Chagulak(91km)

Earthquakes:

M 7.0+:

M 7.3; Fox Islands, Aleutian Islands, Alaska; (24 Jun 2011) (22km from the swarm center)

9 swarms found nearby.

2008

PS20081031.1(186.9km)

31 Oct

1 day 9 hours

6 earthquakes

2011

S20110628.1(11.3km)

27 Jun

2 days 22 hours

33 earthquakes

S20110903.2(17.2km)

2 Sep

3 days 5 hours

58 earthquakes

S20110902.1(20.1km)

2 Sep

2 days 7 hours

65 earthquakes

2013

PS20130904.2(198.9km)

4 Sep

1 day 7 hours

11 earthquakes

2015

PS20150727.1(165.3km)

27 Jul

3 hours

8 earthquakes

PS20150905.1(187.4km)

4 Sep

18 hours

5 earthquakes

PS20151102.1(110.1km)

2 Nov

15 minutes

5 earthquakes

2016

PS20160319.1(164.6km)

18 Mar

15 hours

5 earthquakes

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Seismic Swarm PS20110628.1: Analysis of Events Near Atka, Alaska

A seismic swarm designated PS20110628.1 occurred in the Aleutian Islands region of Alaska, approximately 180 km east of Atka. The sequence began at 23:27 UTC on 27 June 2011 and concluded at 16:13 UTC on 28 June 2011, spanning 16 hours and 45 minutes. During this period, seven earthquakes were recorded, with magnitudes ranging from 3.2 to 5.8 and focal depths between 8 km and 61 km.

The swarm initiated with a magnitude 5.8 event at 29 km depth. Subsequent activity included another magnitude 5.8 quake at 30 km depth, followed by a magnitude 5.4 event at 61 km. A smaller magnitude 3.2 shock occurred at 8 km depth, succeeded by magnitude 5.4 and 5.1 events at 25 km and 40 km depths, respectively. The sequence ended with a magnitude 5.2 earthquake at 43 km depth. These events clustered tightly in both time and space, characteristic of swarm behavior rather than a typical mainshock-aftershock sequence.

This swarm represents the second such episode in the area since 2000, with the prior occurrence documented in 2008. Notably, a magnitude 7.3 earthquake struck the Fox Islands on 24 June 2011, centered only 22 km from the swarm's focal region. The proximity suggests a possible stress transfer or triggering relationship between the large event and the subsequent swarm activity.

The Aleutian Islands lie along an active subduction zone where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–8 cm per year. This tectonic setting produces frequent seismicity and volcanism across the arc. The region experiences both shallow crustal earthquakes and deeper events associated with the subducting slab, consistent with the observed depth range in the swarm. Historical records indicate elevated seismic productivity, including great earthquakes exceeding magnitude 8, driven by the locked interface and episodic slip.

Geological studies confirm that the central Aleutians host a mix of volcanic and tectonic features, with fault systems accommodating oblique convergence. Updated assessments from regional monitoring highlight ongoing strain accumulation along the megathrust, underscoring the area's potential for significant seismic release.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center reports on Aleutian arc tectonics
SeismoSight internal swarm classification database