Location:

Period:

7 Apr 2018 17:14:18 - 9 Apr 2018 17:43:56 (2 days 29 minutes)

Volcanoes in 100km radius:

Recheschnoi(15km), Okmok(25km), Vsevidof(25km), Bogoslof(78km), Kagamil(95km), Uliaga(95km)

Earthquakes:

3 swarms found nearby.

2007

PS20071227.1(108.5km)

26 Dec

17 hours

7 earthquakes

2018

S20180404.1(5.7km)

3 Apr

2 days 4 hours

54 earthquakes

2022

PS20220111.1(61.3km)

11 Jan

3 hours

8 earthquakes

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Seismic Swarm S20180407.1: Analysis of Activity Near Nikolski, Alaska

SeismoSight recorded seismic swarm S20180407.1 beginning at 17:14 on 7 April 2018 and concluding at 17:43 on 9 April 2018. The sequence occurred 49 km northeast of Nikolski, Alaska, on Umnak Island in the central Aleutian arc. Over 48 hours and 29 minutes, the swarm comprised 49 earthquakes.

Magnitudes ranged from 0.2 to 2.2, with the majority below 1.5. Depths were predominantly shallow, between 0 and 19 km, indicating activity within the upper crust. Notable events included a magnitude 2.2 earthquake at 22:36 on 7 April at 8 km depth and another magnitude 2.2 at 23:59 the same day at 4 km depth. The swarm exhibited a typical pattern of clustered, low-magnitude events without a dominant mainshock.

Regional Geological Setting

Nikolski lies within 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. This tectonic interaction produces the Aleutian volcanic arc and drives frequent seismicity across the region. Umnak Island forms part of the active volcanic front, with nearby features including Mount Recheshnoi and Mount Vsevidof contributing to the area's elevated seismic and volcanic hazard profile.

The central Aleutians experience both interplate thrust earthquakes and intraslab events due to the complex geometry of the subducting slab. Historical records document recurrent moderate-to-large earthquakes, reflecting the high strain accumulation along the megathrust.

Swarm Context and Historical Statistics

SeismoSight data indicate only two swarms have occurred in the Nikolski region since 1 January 2000: one in 2007 consisting of a single event and the 2018 sequence detailed here. Such swarms are interpreted as episodes of fluid migration or stress transfer within the overriding plate rather than classic foreshock-mainshock-aftershock sequences.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Volcano Observatory, Aleutian Arc Tectonic Summary (avo.alaska.edu)
SeismoSight internal swarm classification records