Seismic Swarm S20240303.1: Analysis of Activity Near Beluga, Alaska

A seismic swarm designated S20240303.1 occurred 62 km west-northwest of Beluga, Alaska, from 08:41 on 2 March 2024 to 12:50 on 6 March 2024. In approximately 100 hours, the swarm produced 89 earthquakes. This sequence represents the third swarm recorded in the region since 2000, following single events in 2021 and 2022.

The Beluga area lies within south-central Alaska along the northern margin of the Cook Inlet basin. Tectonic deformation here results from the ongoing subduction of the Pacific Plate beneath the North American Plate at the Alaska-Aleutian megathrust. The regional crust experiences both compressional and strike-slip stresses, producing frequent shallow seismicity. Depths recorded during the swarm ranged primarily from 5 to 10 km, with occasional events extending to 20 km, consistent with the brittle upper crust in this portion of the subduction zone.

Magnitudes remained low throughout the sequence, spanning from -1.0 to 1.6. The majority of events clustered between -0.7 and 0.5, with only a few reaching or exceeding magnitude 1.0. Notable larger events within the swarm included a magnitude 1.6 earthquake at 12:45 on 5 March and a magnitude 1.5 event at 06:05 on 6 March. Depths for these events were 8 km and 10 km, respectively. The temporal distribution showed peak activity on 2–3 March, followed by a gradual decline through 6 March.

Such swarms are characteristic of the Cook Inlet region, where small-magnitude sequences often reflect localized stress adjustments or minor fluid migration along pre-existing faults. No volcanic association is indicated for this particular swarm, though the broader Cook Inlet hosts several active volcanoes whose seismic signatures differ in character and depth distribution.

The 2024 swarm aligns with historical patterns of low-level clustered seismicity in the area. With only three documented swarms since the start of 2000, the occurrence rate remains modest relative to the high background seismicity of southern Alaska. Continued monitoring by regional networks provides essential data for tracking any evolution in activity.

References

• Alaska Earthquake Center, University of Alaska Fairbanks – regional seismic catalog and swarm classification data.
• U.S. Geological Survey Earthquake Hazards Program – tectonic framework of the Alaska-Aleutian subduction zone.