Seismic Swarm Near Adak, Alaska: Analysis of Recent Activity
A seismic swarm was recorded 39 km west of Adak, Alaska, beginning at 16:12 on 3 June 2025 and concluding at 11:12 on 5 June 2025. Over this 42-hour, 59-minute period, 40 earthquakes were detected. The sequence featured a rapid onset with the two largest events occurring within minutes of initiation, followed by a gradual decline in frequency and magnitude.
The initial event registered magnitude 3.1 at 16:12 on 3 June at a depth of 18 km. Approximately four minutes later, a magnitude 4.9 earthquake struck at 16 km depth, succeeded seconds afterward by a magnitude 4.7 event at 10 km depth. Subsequent activity consisted primarily of events below magnitude 2.0, with depths ranging from 9 km to 18 km. Several microearthquakes with negative magnitudes were also recorded, indicative of very small energy releases typical in swarm sequences.
This swarm aligns with the tectonic setting of the central Aleutian Islands, where the Pacific Plate subducts beneath the North American Plate along the Aleutian Trench. The region experiences frequent seismicity due to this convergent boundary, which has produced numerous historical events exceeding magnitude 7.0. Adak lies within a zone of elevated volcanic and seismic hazard, influenced by the subduction process that generates both shallow crustal earthquakes and deeper events along the Wadati-Benioff zone.
Historical records indicate five prior swarms in the area since 2000. These occurred in 2008 (one swarm), 2017 (one swarm), 2023 (one swarm), and 2024 (two swarms). Such episodic clusters are characteristic of the Aleutian arc, where stress accumulation along the plate interface periodically triggers localized sequences without a single dominant mainshock.
The recent swarm's magnitude distribution, with a clear peak at the outset and rapid decay, is consistent with fluid migration or aseismic slip processes often observed in subduction-related swarms. Depths predominantly between 10 km and 16 km suggest activity near the plate interface or within the overriding crust.
References
- Alaska Earthquake Center, University of Alaska Fairbanks
- United States Geological Survey Earthquake Hazards Program
- SeismoSight internal swarm classification records