New Earthquake Swarm S20260611.1 Detected West of Nikolski, Alaska

A new seismic swarm, designated S20260611.1, began at 15:13 on 10 June 2026, approximately 67 km west of Nikolski on Umnak Island in the Aleutian Islands. Over the subsequent 99 hours and 33 minutes, 72 earthquakes were recorded. Magnitudes ranged from -0.4 to 2.0, with the majority falling between 0.0 and 1.5. Depths were predominantly shallow, concentrated between 0 and 10 km, consistent with activity in the upper crust.

The swarm exhibited a typical pattern of clustered, low-to-moderate events without a single dominant mainshock. Initial activity on 10 June included events of 0.3–0.7 magnitude at depths of 0–7 km. Activity continued through 11–14 June, with notable events reaching 2.0 magnitude at 5 km depth on 13 June and multiple 1.4–1.5 magnitude shocks at 3 km depth on 12 June. Negative magnitudes indicate microseismicity detectable only by sensitive instruments.

Nikolski lies within the tectonically active Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This setting produces frequent seismicity, including both shallow crustal events and deeper Wadati-Benioff zone activity. Umnak Island forms part of the central Aleutian arc, characterized by volcanic edifices and fault systems accommodating oblique subduction. Historical records show that such swarms often occur in this region without leading to larger destructive earthquakes, though they provide valuable data on stress accumulation along the plate interface.

Since 1 January 2000, eight earthquake swarms have been identified in the immediate area according to SeismoSight internal classification. These occurred in 2010 (3 swarms), 2014 (2), 2015 (1), 2023 (1), and 2024 (1). The current swarm aligns with this episodic pattern, underscoring the persistent seismic character of the western Aleutians.

Monitoring continues through regional networks to assess any evolution in rate or magnitude. Such swarms contribute to improved understanding of precursory signals and fault mechanics in subduction environments.

References

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