Location:

Period:

3 Apr 2017 13:29:32 - 7 Apr 2017 15:43:20 (4 days 2 hours 13 minutes)

Volcanoes in 100km radius:

Moffett(79km), Kanaga(90km), Great Sitkin(91km)

Earthquakes:

296

4 swarms found nearby.

2010

S20101104.2(21.3km)

3 Nov

4 days 9 hours

177 earthquakes

2013

PS20130831.1(102.5km)

31 Aug

15 hours

5 earthquakes

2017

PS20170509.1(49.0km)

8 May

17 hours

5 earthquakes

2025

PS20250320.1(26.0km)

19 Mar

22 hours

5 earthquakes

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Seismic Swarm S20170404.1: Analysis of Activity 72 km SSE of Adak, Alaska

SeismoSight recorded swarm S20170404.1 beginning at 13:29 on 3 April 2017 and concluding at 15:43 on 7 April 2017. The sequence, located 72 km south-southeast of Adak in the Aleutian Islands, produced 296 earthquakes over 98 hours and 13 minutes. Depths clustered between 5 km and 43 km, with the majority between 10 km and 30 km, consistent with the shallow crustal and upper-plate seismicity typical of the region.

The first 100 events reveal a rapid onset followed by sustained moderate activity. The initial event measured magnitude 1.5 at 12 km depth. Peak magnitudes reached 5.1 on 4 April at 10:49 (26 km depth) and 5.0 at 12:42 (23 km depth). Additional notable events included magnitudes 4.8, 4.6, and 4.0, all within the first day. Most events remained below magnitude 3.0, with depths predominantly 15–25 km. Temporal clustering showed elevated rates between 09:50 and 15:30 on 4 April, after which activity gradually declined while maintaining similar depth ranges.

The Aleutian Islands occupy 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 generates the Aleutian volcanic arc and produces frequent earthquakes along the megathrust, within the downgoing slab, and in the overriding plate. Adak lies near the Andreanof Islands segment, a portion of the arc known for both great earthquakes and persistent background seismicity.

Historical records document two prior swarms in the study area since 2000: one event in 2010 and one in 2013. These earlier sequences were smaller in total count than S20170404.1. The 2017 swarm fits the pattern of episodic, spatially concentrated activity observed along the arc, where fluid migration or stress transfer can trigger short-lived clusters without leading to a single large mainshock.

Overall, the swarm exhibited typical characteristics of subduction-related seismicity: shallow to intermediate depths, a broad magnitude distribution dominated by smaller events, and a duration of several days. Such sequences contribute to ongoing strain release along the plate interface and provide data for refining local seismic hazard models in the central Aleutians.

References

USGS Earthquake Catalog
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification S20170404.1