Analysis of Seismic Swarm VS20230818.1 Near Nikolski, Alaska

Seismic swarm VS20230818.1 was recorded 74 km west of Nikolski, Alaska, on Umnak Island in the Aleutian chain. The sequence began at 07:17 on 17 August 2023 and concluded at 13:08 on 18 August 2023, spanning 29 hours and 51 minutes. A total of 26 earthquakes were detected during this interval.

The events displayed low magnitudes between -0.1 and 1.9, with focal depths ranging from -1 km to 16 km. Activity initiated with a magnitude 1.0 event at 9 km depth, followed by additional shocks clustered in the first several hours. Later events on 18 August were predominantly microearthquakes below magnitude 0.5. The swarm exhibited a typical pattern of rapid onset and gradual decay without a dominant mainshock.

Historical records indicate limited swarm activity in the immediate vicinity since 2000, with only two prior episodes documented—one in 2010 and another in 2015. This scarcity suggests that swarm-type sequences are infrequent relative to isolated tectonic events in the area.

The swarm occurred within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at approximately 6–7 cm per year. This oblique subduction drives both megathrust earthquakes and upper-plate seismicity along the volcanic arc. Umnak Island and adjacent regions host several stratovolcanoes, including Mount Okmok and Mount Recheshnoi, whose magmatic systems can interact with regional faulting to produce earthquake swarms. Crustal stresses in this segment of the arc are influenced by the subduction of the Amlia Fracture Zone to the east, contributing to heterogeneous seismicity patterns.

Seismic monitoring in the Aleutians relies on regional networks operated by the Alaska Earthquake Center and the U.S. Geological Survey, which provide real-time detection of events as small as magnitude 0.0 in well-instrumented sectors. The low-magnitude character of VS20230818.1 is consistent with background microseismicity observed throughout the central Aleutian arc.

No significant surface deformation or volcanic unrest was associated with this swarm. Continued monitoring remains essential given the region’s history of producing larger subduction-related earthquakes.

References

Alaska Earthquake Center, University of Alaska Fairbanks.
U.S. Geological Survey Earthquake Hazards Program.
SeismoSight internal swarm classification records.