Seismic Swarm VS20230605.1 Near Pedro Bay, Alaska: Characteristics and Regional Context

A seismic swarm designated VS20230605.1 was recorded 63 km east-northeast of Pedro Bay, Alaska. The sequence began at 02:15 on 5 June 2023 and concluded at 21:30 on 6 June 2023, spanning 43 hours and 14 minutes. During this interval, 64 earthquakes were detected.

The swarm exhibited predominantly low-magnitude events clustered at shallow depths. The majority of recorded quakes ranged between –0.9 and 0.9 in magnitude and occurred at depths of 0–5 km. Two larger events stood out: a magnitude 1.9 earthquake at 126 km depth on 5 June at 16:41 and a magnitude 1.8 event at 107 km depth on 6 June at 21:30. These deeper shocks contrast with the shallow swarm activity and may reflect separate stress release within the subducting slab.

The Pedro Bay region lies within the Cook Inlet segment of the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent crustal and intraslab seismicity as well as active volcanism. The swarm location is situated near the northeastern margin of Iliamna Lake, in proximity to the Chigmit Mountains and the volcanic centers of Iliamna and Redoubt. Crustal faults in the area accommodate both strike-slip and thrust motion associated with oblique convergence and clockwise rotation of the overriding plate.

Historical records indicate that earthquake swarms are uncommon in this specific locale. Since 1 January 2000, only three swarms have been identified in the immediate vicinity: one each in 2012, 2016, and the current 2023 sequence. The 2023 swarm therefore represents the third documented swarm in more than two decades, underscoring the generally low rate of swarm-type activity compared with more persistent aftershock sequences or volcanic-tectonic tremor observed nearer to active volcanoes.

Seismic swarms in subduction-zone forearcs can arise from fluid migration, magmatic processes, or transient slip on crustal faults. The shallow depth distribution and low magnitudes observed here are consistent with fluid-driven seismicity or minor fault slip within the upper crust, while the two deeper, higher-magnitude events align with intraslab deformation typical of the subducting Pacific Plate beneath south-central Alaska.

Ongoing monitoring by regional seismic networks continues to track background seismicity and any potential changes in volcanic unrest indicators at nearby Iliamna and Redoubt volcanoes. The 2023 swarm adds a modest increment to the long-term catalog of crustal deformation in this tectonically active corridor.

References

• Alaska Earthquake Center, University of Alaska Fairbanks (earthquake catalog data)
• U.S. Geological Survey Earthquake Hazards Program (tectonic framework and regional seismicity summaries)
• SeismoSight internal swarm classification VS20230605.1 (event parameters and timing)