Seismic Swarm VS20240426.1 Near Beluga, Alaska

A seismic swarm designated VS20240426.1 occurred 67 km west-northwest of Beluga, Alaska, in the Cook Inlet region. The sequence began at 09:12 on 25 April 2024 and concluded at 02:35 on 29 April 2024, spanning 89 hours and 22 minutes. During this period, 68 earthquakes were recorded, with magnitudes ranging from -0.9 to 1.9 and depths predominantly between 0 and 33 km. Most events clustered at shallow depths of 1–5 km, though a smaller subset reached greater depths up to 33 km. The swarm exhibited typical characteristics of clustered microseismicity, with events occurring in rapid succession early on 25–26 April before tapering. Notable larger events within the sequence included a magnitude 1.9 earthquake at 02:40 on 26 April at 1 km depth and a magnitude 1.7 event at 04:29 on 28 April at 4 km depth. Such swarms often reflect fluid migration or stress adjustments along minor faults rather than a single mainshock-aftershock pattern. The Cook Inlet region lies within the tectonically active Pacific Ring of Fire, where the Pacific Plate subducts northwestward beneath the North American Plate along the Aleutian megathrust. This convergent margin drives frequent earthquakes and volcanic activity. Beluga sits near the western margin of the Cook Inlet basin, an area underlain by Mesozoic and Cenozoic sedimentary rocks overlying accreted terranes. Historical seismicity in south-central Alaska includes both deep intraslab events and shallower crustal earthquakes associated with the subduction interface and overlying faults. Since 2000, five seismic swarms have been documented in the immediate area according to SeismoSight records: one each in 2021 and 2022, and three in 2024. These episodes underscore the region’s propensity for episodic swarm activity amid ongoing plate convergence at rates of approximately 6–7 cm per year. Analysis of event timing shows peak activity within the first 24 hours, followed by a gradual decline, consistent with swarm behavior driven by transient stress changes. Depths indicate a mix of very shallow crustal sources and slightly deeper events possibly linked to the subducting slab or volcanic processes near nearby features such as Mount Spurr. This swarm provides valuable data for monitoring tectonic strain accumulation in a high-hazard zone. Continued observation supports improved understanding of precursory patterns ahead of larger events.

References

SeismoSight internal swarm classification VS20240426.1
USGS Earthquake Hazards Program – Cook Inlet tectonics overview
Alaska Earthquake Center – Regional seismicity summary