Location:

Period:

22 Aug 2018 08:42:39 - 25 Aug 2018 16:11:48 (3 days 7 hours 29 minutes)

Volcanoes in 100km radius:

Takawangha(67km), Bobrof(71km), Tanaga(72km), Kanaga(79km), Gareloi(92km), Moffett(98km)

Earthquakes:

6 swarms found nearby.

2005

S20050805.1(25.7km)

4 Aug

1 day 17 hours

41 earthquakes

2006

PS20060708.1(103.1km)

8 Jul

9 hours

10 earthquakes

2007

PS20071219.1(104.6km)

19 Dec

19 hours

12 earthquakes

2017

PS20170509.1(65.7km)

8 May

17 hours

5 earthquakes

2018

S20180815.1(7.0km)

15 Aug

4 days 1 hours

110 earthquakes

2025

PS20250320.1(113.9km)

19 Mar

22 hours

5 earthquakes

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Seismic Swarm S20180823.1: Analysis of Earthquake Activity Southwest of Adak, Alaska

Seismic swarm S20180823.1 occurred 97 km southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 08:42 on 22 August 2018 and concluded at 16:11 on 25 August 2018, spanning 79 hours and 29 minutes. During this period, 39 earthquakes were recorded, with magnitudes ranging from 1.6 to 6.3 and focal depths between 20 km and 56 km.

The swarm featured a prominent mainshock of magnitude 6.3 at 03:35 on 23 August 2018, located at a depth of 20 km. This event was preceded and followed by numerous smaller tremors, predominantly in the 1.7–2.8 magnitude range. Depths clustered between 20 km and 35 km for most events, with several deeper occurrences exceeding 40 km later in the sequence. The temporal distribution showed peak activity on 23 August, including multiple events above magnitude 2.5 within hours of the mainshock, followed by a gradual decline through 24 and 25 August.

This activity aligns with the tectonic setting of the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. The central Aleutians experience frequent seismicity due to this oblique subduction, which generates both shallow thrust earthquakes and deeper intraslab events. Adak lies near the Andreanof Islands segment, a region characterized by historical megathrust ruptures and persistent background seismicity.

Historical records indicate five prior swarms in the area since 2000, occurring in 2005, 2006, 2007, 2017, and 2018. These episodes reflect episodic stress release along the subduction interface and associated fault structures, consistent with the high seismic productivity of the arc. The 2018 swarm fits this pattern of clustered, moderate-magnitude activity without evidence of a larger triggered mainshock beyond the recorded 6.3 event.

Such swarms provide valuable data on fault behavior and stress accumulation in subduction environments. Continued monitoring supports improved understanding of precursory patterns and regional hazard assessment in this tectonically active zone.

References

USGS Earthquake Catalog
Alaska Earthquake Center
SeismoSight internal swarm classification