Seismic Swarm VS20230903.1: Analysis of Activity Near Karluk, Alaska

A seismic swarm designated VS20230903.1 was recorded 85 km NNW of Karluk, Alaska, on Kodiak Island. The sequence began at 15:23 UTC on 2 September 2023 and concluded at 22:10 UTC on 16 September 2023, spanning 342 hours and 46 minutes. During this interval, 242 earthquakes were detected.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.6 to 2.0, with the majority below 1.0. Depths were chiefly shallow, concentrated between 0 and 5 km, although several events reached 30–35 km. The initial event measured magnitude 0.0 at 4 km depth. Subsequent activity included a magnitude 1.9 event on 3 September at 1 km depth and a magnitude 2.0 event on 4 September at the surface. Later events in the subset showed clusters at greater depths, such as multiple recordings near 30–35 km on 5–8 September.

This pattern is consistent with microseismicity often observed in subduction-zone environments. Events clustered temporally in the first 48 hours before declining in frequency, with occasional deeper events interspersed among shallow ones.

The Kodiak region lies within the Alaska-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 great megathrust events. The 1964 M9.2 Great Alaska Earthquake originated nearby, generating widespread deformation across Kodiak Island. Historical records document recurring seismic swarms in the area, with 14 such sequences identified since 2000. Prior swarms occurred in 2002 (1 swarm), 2019 (2), 2020 (3), 2022 (4), and 2023 (4).

Such swarms are commonly attributed to fluid migration along faults or minor stress adjustments within the overriding plate and accretionary prism. Depths recorded in VS20230903.1 align with both crustal and upper-plate seismicity typical of this margin. No damage or felt reports were associated with the swarm, reflecting the small event sizes.

Ongoing monitoring by regional networks continues to track background seismicity in this highly active tectonic province. Updated catalogs confirm the subduction zone remains capable of producing both swarm-like sequences and larger mainshock-aftershock events.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center, University of Alaska Fairbanks
Alaska Division of Geological & Geophysical Surveys tectonic summaries