Seismic Swarm VS20230923.1: Analysis of Activity Near Karluk, Alaska
A seismic swarm designated VS20230923.1 occurred northwest of Karluk, Alaska, from 09:04 on 22 September 2023 to 13:09 on 30 September 2023. The event sequence lasted 196 hours and 5 minutes, during which 121 earthquakes were recorded at a location 85 km northwest of Karluk.
Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.7 to 1.8, with the majority falling between -0.5 and 0.5. Depths were mostly shallow, concentrated between 0 and 3 km, though isolated events reached 31–34 km and one outlier registered at 91 km. The swarm initiated with microseismicity clustered near 2 km depth and showed sporadic deeper pulses around 30 km on 23–24 September. Peak magnitudes of 1.6–1.8 occurred on 26–27 September, all at shallow depths.
The 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 crustal and intraslab seismicity along the Alaska Peninsula and Kodiak Island shelf. Karluk itself sits on the western flank of Kodiak Island, placing the swarm epicenters in the offshore transition zone between the Kodiak segment and the Semidi Islands. Historical records document persistent background seismicity driven by plate-boundary locking and episodic slow-slip phenomena.
Seismic swarms have occurred in the area since at least 2000, with 15 documented episodes through 2023. Annual counts remained low until recent years: one swarm in 2002, two in 2019, three in 2020, four in 2022, and five in 2023. This progressive increase aligns with observed patterns of heightened microseismicity along the Kodiak segment, potentially linked to stress transfer following the 1964 Great Alaska Earthquake and ongoing postseismic relaxation.
The VS20230923.1 swarm exhibited classic swarm characteristics: no clear mainshock-aftershock decay, clustered shallow foci, and a rapid onset followed by gradual decline. Such sequences commonly reflect fluid migration or aseismic slip transients within the overriding plate or along the subduction interface. Depths predominantly under 5 km suggest activation of upper-crustal faults rather than deep intraslab rupture.
No damage or felt reports were associated with this low-magnitude sequence. Continued monitoring remains essential given the region’s capacity for larger events, as demonstrated by the 1938 and 1946 earthquakes along adjacent segments.
References USGS Earthquake Hazards Program Alaska Earthquake Center catalog Alaska Division of Geological & Geophysical Surveys tectonic summaries