Location:

Period:

1 Mar 2008 09:46:23 - 4 Mar 2008 14:36:15 (3 days 4 hours 49 minutes)

Volcanoes in 100km radius:

Gareloi(28km), Tanaga(74km), Takawangha(83km), Semisopochnoi(87km)

Earthquakes:

7 swarms found nearby.

2006

PS20060708.1(62.4km)

8 Jul

9 hours

10 earthquakes

PS20060826.1(47.5km)

26 Aug

47 minutes

6 earthquakes

2007

PS20071219.1(67.4km)

19 Dec

19 hours

12 earthquakes

2008

S20080416.1(28.4km)

15 Apr

8 days 6 hours

286 earthquakes

VS20080416.1(24.4km)

15 Apr

3 days 6 hours

89 earthquakes

2020

PS20200126.1(80.4km)

26 Jan

18 hours

8 earthquakes

2026

S20260617.2(20.1km)

16 Jun

1 day 5 hours

26 earthquakes

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Seismic Swarm S20080301.2 Near Adak, Alaska: Geological Context and Event Analysis

A seismic swarm designated S20080301.2 occurred approximately 186 km west of Adak, Alaska, in the Andreanof Islands region of the central Aleutian arc. The sequence began at 09:46 UTC on 1 March 2008 and concluded at 14:36 UTC on 4 March 2008, spanning 76 hours and 49 minutes. During this period, 82 earthquakes were recorded, with the majority exhibiting magnitudes between 1.9 and 3.5 and focal depths ranging from 0 to 35 km.

The Aleutian arc forms at the convergent boundary where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent earthquake swarms and volcanic activity along the 2,500 km island chain. Adak Island and surrounding areas lie within a highly active segment characterized by intermediate-depth seismicity and occasional great earthquakes exceeding magnitude 8. Historical records indicate that the region has experienced repeated seismic episodes, consistent with the three swarms documented since 2000 (two in 2006 and one in 2007).

The 2008 swarm featured two events of magnitude 5.0 or greater. The first, a magnitude 5.0 earthquake at 24 km depth, occurred early on 1 March, followed hours later by a magnitude 5.2 event at 35 km depth. Subsequent activity included multiple magnitude 3.0–4.1 shocks clustered at shallow to mid-crustal depths, reflecting typical swarm behavior driven by fluid migration or stress triggering rather than a single mainshock-aftershock sequence. Depths concentrated between 3 and 6 km for many smaller events, suggesting involvement of the overriding plate and upper portions of the subducting slab.

Swarm activity displayed temporal clustering, with peak rates on 1 and 2 March before gradually declining. Magnitudes remained predominantly below 3.5 after the initial larger shocks, and no events exceeded magnitude 5.2. This pattern aligns with known Aleutian swarm characteristics, where episodic bursts often occur without producing surface rupture or significant damage due to their offshore location and moderate energy release.

The Andreanof Islands segment has a well-documented history of large earthquakes, including the 1957 magnitude 8.6 event and the 1986 magnitude 8.0 shock, both of which released substantial strain along the megathrust. Ongoing subduction continues to load the plate interface, making swarm monitoring essential for understanding precursory signals or background seismicity rates. The 2008 sequence contributed to the regional catalog without altering established recurrence intervals for great earthquakes.

SeismoSight internal classification identifies this swarm as part of the post-2000 series, providing a basis for comparative analysis of swarm frequency and energy distribution in the central Aleutians.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
SeismoSight Swarm Database (internal classification)