Location:

Period:

27 Jul 2015 05:42:11 - 28 Jul 2015 22:44:00 (1 day 17 hours 1 minute)

Volcanoes in 100km radius:

Tana(97km), Herbert(99km)

Earthquakes:

6 swarms found nearby.

2007

PS20071227.1(73.4km)

26 Dec

17 hours

7 earthquakes

2008

S20080311.2(3.9km)

11 Mar

23 hours

31 earthquakes

2015

S20150727.2(3.3km)

27 Jul

8 days 16 hours

328 earthquakes

PS20150727.1(38.9km)

27 Jul

3 hours

8 earthquakes

S20150727.3(23.7km)

27 Jul

4 days 9 hours

74 earthquakes

2024

S20240502.1(3.5km)

2 May

2 days 2 hours

93 earthquakes

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Seismic Swarm S20150727.4: Analysis of Activity Near Nikolski, Alaska

Seismic swarm S20150727.4 was recorded 100 km SSW of Nikolski, Alaska, on Umnak Island in the central Aleutian Islands. The sequence began at 05:42 on 27 July 2015 and concluded at 22:44 on 28 July 2015, spanning 41 hours and 1 minute. During this period, 31 earthquakes were registered.

The events displayed typical swarm characteristics, with magnitudes ranging from 2.3 to 4.0 and focal depths between 4 km and 42 km. The largest event reached magnitude 4.0 at a depth of 5 km on 27 July at 05:47:48 UTC. Other notable events included a magnitude 3.8 quake at 35 km depth on 27 July at 13:29:59, a magnitude 3.1 at 26 km on 28 July at 03:05:18, and a magnitude 3.6 at 42 km on 28 July at 22:44:00. The majority of events clustered between magnitudes 2.4 and 2.9, with depths distributed across shallow to intermediate levels.

This swarm represents one of three sequences recorded in 2015 within the region. Historical data since 2000 indicate five swarms in total near Nikolski, with prior activity limited to single events in 2007 and 2008.

The central Aleutian Islands lie within a highly active subduction zone where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting drives frequent seismicity and volcanism along the Aleutian arc. Umnak Island hosts several volcanoes, including Mount Vsevidof and Mount Recheshnoi, consistent with the broader pattern of arc magmatism fueled by slab dehydration and partial melting in the mantle wedge.

Earthquake swarms in subduction zones often reflect fluid migration, stress transfer along faults, or minor slip events rather than mainshock-aftershock sequences. Depths observed in this swarm align with both crustal and upper-plate seismicity common in the region. The Aleutian arc experiences some of the highest rates of great earthquakes globally, with notable historical events including the 1957 Mw 8.6 Andreanof Islands earthquake and the 1965 Mw 8.7 Rat Islands earthquake.

Ongoing monitoring by regional networks continues to track background seismicity and swarm activity to assess potential links to volcanic unrest or larger tectonic strain release.

References

USGS Earthquake Hazards Program
Alaska Earthquake Center, University of Alaska Fairbanks
Global Volcanism Program, Smithsonian Institution