Location:

Period:

27 Jul 2015 00:31:38 - 4 Aug 2015 17:03:05 (8 days 16 hours 31 minutes)

Volcanoes in 100km radius:

Tana(97km), Herbert(98km)

Earthquakes:

328

6 swarms found nearby.

2007

PS20071227.1(76.8km)

26 Dec

17 hours

7 earthquakes

2008

S20080311.2(6.3km)

11 Mar

23 hours

31 earthquakes

2015

PS20150727.1(39.5km)

27 Jul

3 hours

8 earthquakes

S20150727.3(25.1km)

27 Jul

4 days 9 hours

74 earthquakes

S20150727.4(3.3km)

27 Jul

1 day 17 hours

31 earthquakes

2024

S20240502.1(5.8km)

2 May

2 days 2 hours

93 earthquakes

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Seismic Analysis of Swarm S20150727.2 Near Nikolski, Alaska

Earthquake swarm S20150727.2 occurred approximately 98 km south-southwest of Nikolski on Umnak Island in the Aleutian Islands, Alaska. The sequence initiated at 00:31 UTC on 27 July 2015 and concluded at 17:03 UTC on 4 August 2015, spanning 208 hours and 31 minutes. During this interval, 328 earthquakes were recorded.

The Aleutian Islands form part of a classic subduction-zone setting where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic regime produces frequent seismicity, including both interplate thrust events and intraslab earthquakes, as well as associated volcanic activity along the arc. Nikolski lies within a region of elevated background seismicity, with the nearby Aleutian Trench serving as the primary locus of strain accumulation and release.

Analysis of the first 100 events reveals a rapid onset followed by sustained activity. The initial event reached magnitude 4.5 at 30 km depth. Subsequent shocks included several events of magnitude 4.0–4.7 within the first hour, with depths ranging from 15 km to 37 km. A notable escalation occurred around 04:55 UTC on 27 July with a magnitude 5.7 earthquake at 27 km depth. Peak magnitudes in the examined sequence reached 5.9 at 35 km depth and 5.7 at 39 km depth later that morning. Depths throughout the swarm clustered between 10 km and 42 km, consistent with both crustal and upper-plate seismicity in the subduction environment.

Magnitude distribution shows a predominance of events between 2.0 and 4.0, with 12 events exceeding magnitude 4.5 in the first 100 shocks. Temporal clustering indicates the highest rates of occurrence during the initial 12 hours, after which activity gradually declined while remaining elevated above background levels until early August.

Historical records maintained by SeismoSight indicate only two prior swarms in the same locale since 2000: one in 2007 comprising a single event and another in 2008 also limited to one event. The 2015 swarm therefore represents the most extensive sequence documented in the area during the observational period.

Such swarms are characteristic of subduction zones and may reflect fluid migration, stress triggering, or slow slip phenomena along the plate interface. Continued monitoring remains essential given the region’s capacity for great earthquakes, as demonstrated by historical events such as the 1957 Mw 8.6 Andreanof Islands earthquake and the 1965 Mw 8.7 Rat Islands earthquake.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
SeismoSight internal swarm classification records