Seismic Swarm Near Karluk, Alaska: Insights from the September 2022 Event

The seismic swarm designated VS20220922.1 occurred approximately 84 km north-northwest of Karluk on Kodiak Island, Alaska. It began at 23:35 UTC on 21 September 2022 and concluded at 04:17 UTC on 23 September 2022, spanning 28 hours and 41 minutes during which 54 earthquakes were recorded. Magnitudes ranged from -1.0 to 1.2, with the majority of events registering below 0.5 and depths predominantly between 1 and 5 km, though a few deeper events reached 32–33 km. Activity peaked on 22 September between 13:00 and 14:00 UTC, featuring a dense cluster of low-magnitude shallow shocks.

This swarm reflects typical behavior in a subduction-zone setting. The Kodiak region sits above the Alaska-Aleutian megathrust, where the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year. Such convergence generates both great megathrust earthquakes and frequent smaller seismic sequences. Swarms—clusters of events without a dominant mainshock—are common here and often result from fluid migration or slow slip along the plate interface and overlying crustal faults.

Since 2000, ten comparable swarms have been documented in the same area, occurring in 2002, 2016, 2019 (two episodes), 2020 (four episodes), and 2022 (including the present sequence). These episodes demonstrate recurring, episodic strain release that does not necessarily precede larger earthquakes but contributes to the long-term seismic budget of the margin.

Geologically, Kodiak Island forms part of the accretionary prism built by repeated subduction of oceanic crust and sediments. The island’s bedrock records millions of years of this process, while its modern seismicity is monitored continuously by regional networks. Depths recorded in the 2022 swarm are consistent with both upper-plate crustal faulting and activity near the plate interface.

No damage or felt reports were associated with this low-magnitude sequence, underscoring the value of dense seismic monitoring in distinguishing background swarm activity from potentially hazardous events.

References

USGS Earthquake Catalog
Alaska Earthquake Center, University of Alaska Fairbanks
USGS Tectonic Summary: Alaska-Aleutian Subduction Zone