Seismic Swarm VS20250507.1: Analysis of Activity Near Beluga, Alaska

Seismic swarm VS20250507.1 occurred in a tectonically active region 66 km west-northwest of Beluga, Alaska, within the Cook Inlet basin. The sequence began at 08:59 on 6 May 2025 and concluded at 23:20 on 31 May 2025, spanning 614 hours and 21 minutes. During this period, 387 earthquakes were recorded, providing valuable data on local fault behavior in a subduction zone setting.

The first 100 events displayed characteristic features of a microseismic swarm. Magnitudes ranged from -1.1 to 1.8, with the majority falling between -0.9 and 0.0. Depths were predominantly shallow, concentrated between 0 and 5 km, though occasional events reached 25–31 km. Activity initiated with a magnitude 1.8 event at 6 km depth, followed by numerous low-magnitude or negative-magnitude tremors clustered in the initial days. Temporal distribution showed bursts of activity, particularly on 7–8 May and 10–12 May, with events occurring at intervals of minutes to hours. This pattern suggests fluid migration or stress triggering along minor faults rather than a single mainshock-aftershock sequence.

The Cook Inlet region lies above the Alaska-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 framework produces frequent earthquakes, volcanic arcs, and sedimentary basins. Beluga sits near the western margin of the basin, underlain by Mesozoic and Cenozoic sedimentary rocks deformed by thrust faults and strike-slip structures. Historical seismicity includes the 1964 magnitude 9.2 Great Alaska Earthquake, whose rupture extended through adjacent segments of the megathrust.

Since 2000, 13 swarms have been documented in the broader area. These occurred in 2021 (1 swarm), 2022 (1 swarm), 2024 (10 swarms), and 2025 (1 swarm). The elevated frequency in 2024–2025 may reflect increased monitoring sensitivity or transient changes in crustal stress. Such swarms typically involve low-magnitude events at shallow depths and rarely produce felt shaking or surface rupture in this part of Alaska.

Regional geology indicates that swarm activity often aligns with mapped faults within the Cook Inlet fold-and-thrust belt. These structures accommodate part of the oblique convergence between plates. Depths recorded in VS20250507.1 are consistent with brittle failure in the upper crust above the subducting slab. No significant volcanic unrest was associated with this sequence, distinguishing it from swarms sometimes linked to magmatic processes farther south along the arc.

Continued monitoring of similar sequences enhances understanding of seismic hazard in south-central Alaska. The data from VS20250507.1 contribute to models of fault interaction and stress transfer in a region that hosts critical infrastructure and population centers near Anchorage.

References
Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity catalogs and tectonic summaries.
U.S. Geological Survey – Alaska subduction zone reports and Cook Inlet geologic maps.
SeismoSight internal swarm classification records.