Seismic Swarm VS20220902.1: Analysis of Activity Near Karluk, Alaska

A seismic swarm designated VS20220902.1 was recorded 85 km NNW of Karluk, Alaska, beginning at 05:28 on 1 September 2022 and concluding at 15:22 on 7 September 2022. Over 153 hours and 54 minutes, the swarm comprised 108 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from -0.8 to 1.9. The majority clustered below magnitude 0.5, including numerous negative magnitudes indicative of microseismicity. Depths were mainly shallow, concentrated between 0 and 6 km, though isolated deeper events reached 26–34 km on 2 September. Temporal distribution showed peak occurrence on 1–2 September, with events spaced minutes apart during initial phases. Later days exhibited sparser activity, tapering by 7 September. Notable patterns include repeated shallow events at 0 km depth and brief clusters at mid-crustal levels around 32–34 km. These characteristics align with swarm behavior rather than mainshock-aftershock sequences. Since 2000, seven swarms have occurred in the region. Prior episodes took place in 2002 (one swarm), 2019 (two swarms), 2020 (three swarms), and an earlier 2022 event (one swarm). This recurrence underscores persistent seismic restlessness in the area. The swarm location lies within the Aleutian subduction zone, where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting generates frequent earthquakes across a range of depths, from shallow crustal events to deeper intraslab activity. Kodiak Island and the adjacent Alaska Peninsula host a complex geology shaped by accretionary processes, featuring the Kodiak Formation of Cretaceous sedimentary rocks and overlying volcanic arcs. Historical seismicity includes great earthquakes, such as the 1964 magnitude 9.2 event, which ruptured segments of the megathrust nearby. Swarm activity in this zone often correlates with fluid migration or stress adjustments along faults without producing a dominant mainshock. Depths observed in VS20220902.1 are consistent with upper-plate and interface seismicity in the subduction environment. Continued monitoring remains essential given the region's proximity to populated areas and infrastructure on Kodiak Island.

References

SeismoSight internal swarm classification VS20220902.1 dataset.
USGS Earthquake Hazards Program regional tectonic summaries for Alaska subduction zone.
Alaska Earthquake Center historical seismicity records.