Location:

Period:

27 Jul 2015 05:12:18 - 31 Jul 2015 14:54:15 (4 days 9 hours 41 minutes)

Volcanoes in 100km radius:

Tana(75km), Herbert(80km), Cleveland(80km), Kagamil(89km), Carlisle(91km), Uliaga(99km)

Earthquakes:

8 swarms found nearby.

2007

PS20071227.1(67.1km)

26 Dec

17 hours

7 earthquakes

2008

S20080311.2(19.8km)

11 Mar

23 hours

31 earthquakes

2015

S20150727.2(25.1km)

27 Jul

8 days 16 hours

328 earthquakes

PS20150727.1(17.4km)

27 Jul

3 hours

8 earthquakes

S20150727.4(23.7km)

27 Jul

1 day 17 hours

31 earthquakes

2017

S20170810.1(24.0km)

9 Aug

2 days 7 hours

32 earthquakes

2022

PS20220111.1(102.2km)

11 Jan

3 hours

8 earthquakes

2024

S20240502.1(20.1km)

2 May

2 days 2 hours

93 earthquakes

AI-generated article — for informational and entertainment purposes only. May contain inaccuracies. Full disclaimerFound an error?

Seismic Swarm S20150727.3: Activity Near Nikolski, Alaska

Seismic swarm S20150727.3 occurred in the central Aleutian Islands, approximately 65 km south-southwest of Nikolski on Umnak Island. The sequence began at 05:12 UTC on 27 July 2015 and concluded at 14:54 UTC on 31 July 2015, spanning 105 hours and 41 minutes. During this period, 74 earthquakes were recorded, with magnitudes ranging from 1.5 to 5.3 and focal depths between 2 km and 83 km.

The swarm featured several events above magnitude 4.0, including a peak magnitude 5.3 earthquake on 28 July at 20 km depth. Additional notable shocks reached 4.9, 4.7 (multiple occurrences), 4.6, and 4.3. Shallower events clustered near 10–25 km depth, while deeper activity extended into the 40–80 km range, consistent with intermediate-depth seismicity within the subducting slab. Activity was most intense during the first 48 hours, with a gradual decline thereafter.

This region lies along the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at roughly 6–7 cm per year. The resulting megathrust environment produces both shallow crustal earthquakes and deeper intraslab events. Umnak Island forms part of the Aleutian volcanic arc, hosting stratovolcanoes such as Mount Recheshnoi and Mount Vsevidof. Historical large-magnitude earthquakes in the central Aleutians include the 1957 Andreanof Islands event (Mw 8.6) and the 1965 Rat Islands earthquake (Mw 8.7), underscoring the zone’s capacity for great earthquakes.

Earthquake swarms in the Aleutians often reflect fluid migration or stress transfer along the plate interface rather than mainshock-aftershock sequences. Since 2000, four swarms have been documented in the broader Nikolski area, occurring in 2007, 2008, and twice in 2015. Swarm S20150727.3 represents one of the two 2015 episodes, highlighting episodic clustering in this segment of the arc.

No surface rupture or significant volcanic unrest was associated with the swarm. The events remained within expected background rates for the highly active subduction setting and did not trigger a formal tsunami warning.

References

Alaska Earthquake Center, University of Alaska Fairbanks
USGS Earthquake Catalog
Tectonic summary for the Aleutian Islands, USGS Professional Paper series