Seismic Swarm VS20231022.1 Near Karluk, Alaska: Geological Context and Event Analysis

The region northwest of Karluk on Kodiak Island, Alaska, sits 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 seismicity, including both large megathrust events and smaller earthquake swarms. Kodiak Island has experienced significant historical earthquakes, notably the magnitude 9.2 Great Alaska Earthquake of 1964, whose rupture extended along the subduction interface beneath the island. Ongoing deformation in this area reflects both interplate coupling and crustal adjustments within the overriding plate.

Swarm VS20231022.1 was recorded 82 km NNW of Karluk. The sequence began at 20:55 on 21 October 2023 and concluded at 05:04 on 28 October 2023, spanning 152 hours and 8 minutes. During this interval, 82 earthquakes were detected. Magnitudes ranged from −0.6 to 1.3, with the majority of events below magnitude 1.0. Focal depths were predominantly shallow, concentrated between 0 and 6 km, although a few events reached depths up to 13 km. The swarm exhibited episodic bursts of activity rather than a single dominant mainshock-aftershock pattern, consistent with fluid-driven or stress-transfer mechanisms often observed in subduction-related swarms.

Inspection of the temporal distribution shows the highest event rates during the first 48 hours, followed by a gradual decline. Early events included several near or below magnitude 0.0 at depths of 1–4 km. Later peaks on 24–25 October featured the largest events of the sequence (magnitudes 1.2–1.3). Negative magnitudes indicate microseismicity detectable only by sensitive local networks, highlighting the swarm’s low-energy character. Depth variations suggest activity migrated slightly within the upper crust without clear migration along a single fault plane.

Historical records maintained by SeismoSight document 18 swarms in the same region since 1 January 2000. The frequency has increased markedly in recent years: one swarm each in 2002 and 2016, two in 2019, four in 2020, four in 2022, and six in 2023. This trend may reflect improved detection thresholds or changes in local stress conditions, although the underlying drivers remain under study.

The occurrence of repeated swarms in this sector of the Kodiak segment underscores the dynamic nature of the subduction interface and overlying crust. Continued monitoring is essential for distinguishing background seismicity from potential precursory patterns ahead of larger events.

References

SeismoSight internal swarm catalog VS20231022.1
USGS Earthquake Hazards Program – Aleutian subduction zone tectonics
Alaska Earthquake Center – Kodiak Island seismic history