Seismic Swarm VS20240220.1: Microseismicity in the Cook Inlet Forearc, Alaska

A seismic swarm designated VS20240220.1 was recorded 63 km west-northwest of Tyonek, Alaska, within the Cook Inlet forearc basin. The sequence began at 12:52 UTC on 19 February 2024 and concluded at 20:40 UTC on 20 February 2024, spanning 31 hours and 47 minutes. During this interval, 27 earthquakes were detected, with local magnitudes ranging from -0.7 to 0.3 and focal depths between 3 km and 64 km.

The Cook Inlet region lies above the eastern Aleutian subduction zone, where the Pacific Plate descends beneath the North American Plate at approximately 6–7 cm per year. This tectonic setting produces a well-developed forearc basin filled with Cenozoic sedimentary strata overlying Mesozoic basement. Active deformation occurs along a network of thrust and strike-slip faults that accommodate both plate convergence and clockwise rotation of the overriding block. The Tyonek area specifically occupies the western margin of the basin, near the projected trace of the Castle Mountain Fault system and subsidiary structures that extend offshore beneath Cook Inlet.

Seismicity in this sector is characteristically diffuse, reflecting both intraslab events within the subducting slab and shallower crustal activity. The February 2024 swarm exhibited a temporal clustering pattern typical of swarm sequences, with event rates highest during the first 12 hours followed by a gradual decline. Depths were predominantly shallow to mid-crustal (3–15 km), although two events reached 25 km and 64 km, consistent with the transition from crustal to intraslab regimes. Negative and near-zero magnitudes indicate microseismicity below the threshold routinely felt at the surface.

Historical records maintained since 1 January 2000 document only two prior swarms in the immediate vicinity: one event in 2021 and one in 2022. These earlier episodes similarly comprised low-magnitude events distributed over hours to days, suggesting that swarm-type behavior is an infrequent but recurrent mode of strain release in this portion of the forearc.

Possible physical drivers for the 2024 swarm include localized pore-pressure increases from fluid migration along pre-existing fractures or minor stress perturbations induced by nearby slow-slip processes. No associated surface deformation or volcanic unrest was reported in the vicinity of Mount Spurr or Redoubt Volcano during the same period. The absence of a clear mainshock–aftershock signature further supports classification as a swarm rather than a triggered sequence.

Continued monitoring by regional seismic networks remains essential for distinguishing background microseismicity from precursory activity that could herald larger events on regional faults. The low energy release observed in VS20240220.1 is typical of the numerous small sequences that accommodate a measurable fraction of plate-boundary strain in southern Alaska.

References

Alaska Earthquake Center, University of Alaska Fairbanks – Regional seismicity catalog (2024 update).
USGS Earthquake Hazards Program – Tectonic framework of the Cook Inlet basin.
SeismoSight internal swarm classification database (VS20240220.1 parameters).