Seismic Swarm VS20230127.1 Near Karluk, Alaska: Detailed Analysis and Regional Context

Seismic swarm VS20230127.1 was recorded in a remote offshore area 86 km NNW of Karluk, Alaska, on Kodiak Island. The sequence began at 18:53 on 26 January 2023 and concluded at 23:51 on 15 May 2023, spanning 2620 hours and 58 minutes. During this period, 2489 earthquakes were registered, characteristic of swarm behavior where events occur in rapid succession without a dominant mainshock.

The first 100 events reveal a rapid onset followed by predominantly low-magnitude activity. The sequence initiated with a magnitude 2.7 earthquake at 17 km depth. Subsequent events were mostly below magnitude 1.0, with many registering negative magnitudes and focal depths clustered between 0 and 5 km. Magnitudes fluctuated between -0.8 and 1.1, while depths remained shallow, rarely exceeding 5 km after the initial event. This pattern indicates intense microseismicity concentrated in the upper crust, with events distributed across short time intervals and minimal depth variation.

Analysis of these early events shows clusters of near-zero and negative-magnitude quakes interspersed with occasional magnitude 1.0+ shocks. Depths stabilized around 1–4 km for the majority, suggesting a shallow source region. The progression from the opening M2.7 event to sustained low-level activity over the following days points to a diffuse release of strain rather than a classic foreshock-mainshock-aftershock sequence.

The Kodiak region lies within the 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 setting produces frequent earthquakes, including swarms, due to plate interface slip, crustal faulting, and possible fluid migration. Historical records confirm elevated swarm activity in the area, consistent with the seven documented swarms since 1 January 2000. Prior swarms occurred in 2019 (1 swarm), 2020 (2 swarms), and 2022 (4 swarms), highlighting recurring episodic seismicity in this segment of the arc.

Such swarms contribute to the broader understanding of stress transfer and seismic hazard along the Alaska-Aleutian megathrust. The shallow depths observed align with known crustal deformation patterns in the overriding plate near Kodiak Island.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification data