Seismic Swarm VS20241028.1 Recorded West-Northwest of Beluga, Alaska

A seismic swarm designated VS20241028.1 was detected in the region 66 km west-northwest of Beluga, Alaska. The sequence began at 04:31 UTC on 27 October 2024 and concluded at 17:52 UTC on 29 October 2024, spanning 61 hours and 20 minutes. During this period, 56 earthquakes were registered.

The events exhibited low magnitudes, ranging from -0.8 to 1.0, with the majority falling below zero. Depths remained shallow, predominantly between -3 km and 7 km. The initial event measured magnitude 1.0 at 5 km depth, while subsequent activity included numerous events near or below the detection threshold. This pattern of clustered, low-energy seismicity is characteristic of swarm behavior rather than a mainshock-aftershock sequence.

The swarm occurred within the Cook Inlet basin of south-central Alaska, a tectonically active area shaped by the ongoing subduction of the Pacific Plate beneath the North American Plate along the Aleutian megathrust. This convergent margin drives regional deformation, including strike-slip faulting on structures such as the Castle Mountain Fault and distributed crustal seismicity. The basin itself contains thick sedimentary sequences overlying accreted terranes, which can influence the propagation and detection of small-magnitude events.

Historical records indicate that swarm activity in this sector is not uncommon. Since 1 January 2000, eight swarms have been documented in the broader area. Prior episodes occurred in 2021 (one swarm), 2022 (one swarm), and 2024 (six swarms). These recurrent clusters suggest episodic fluid migration or stress perturbations along pre-existing fractures within the overriding plate.

Analysis of the October 2024 swarm shows a rapid onset followed by a gradual decline in event rate. Early events on 27 October included several positive-magnitude shocks interspersed with negative-magnitude detections. Activity persisted through 28 October with continued low-magnitude releases, tapering on 29 October. The shallow focal depths align with expectations for crustal events in this forearc setting, where brittle failure occurs at limited depths due to the thermal structure of the subduction zone.

Such swarms contribute to understanding background seismicity levels and may reflect transient changes in pore pressure or aseismic slip. Continued monitoring by regional networks remains essential for distinguishing swarm activity from potential foreshock sequences preceding larger events.

References
Alaska Earthquake Center. (2024). Regional seismicity reports. University of Alaska Fairbanks.
United States Geological Survey. (2024). Earthquake catalog and tectonic summaries for Cook Inlet, Alaska. USGS Earthquake Hazards Program.
Haeussler, P. J., & Plafker, G. (2023). Neotectonics of south-central Alaska. In Geological Society of America Special Papers.