Analysis of Earthquake Swarm S20200223.2 Near Karluk, Alaska

Earthquake swarm S20200223.2 was recorded beginning at 05:42 on 23 February 2020 and concluding at 06:03 on 24 February 2020. The sequence occurred 93 km north of Karluk, Alaska, and comprised 26 events over a period of 24 hours and 20 minutes. Magnitudes ranged from -0.7 to 1.9, with the largest event measuring 1.9 at a depth of 3 km. Most events were shallow, occurring between 1 km and 9 km depth, and exhibited the low-magnitude characteristics typical of swarm activity rather than a mainshock-aftershock sequence.

The swarm initiated with a magnitude 1.9 event at 05:42:54 on 23 February, followed rapidly by additional small events clustered within the first several hours. Subsequent activity included multiple events near magnitude 0.0 to -0.7 at depths of 4–7 km throughout the afternoon and evening of 23 February. A secondary cluster appeared around 18:00–19:00 on the same day, featuring events of magnitude 0.8 and 0.9 at shallow depths of 1–4 km. The final recorded event occurred at 06:03:42 on 24 February at magnitude -0.5 and 2 km depth. This temporal pattern reflects episodic energy release concentrated over a brief interval.

The location lies on the northern portion of Kodiak Island within the Aleutian subduction zone. In this region the Pacific Plate converges with and subducts beneath the North American Plate at rates of approximately 6–7 cm per year along the Aleutian megathrust. The resulting crustal stress produces both large-magnitude thrust earthquakes and frequent smaller seismic swarms. Shallow focal depths recorded during the swarm are consistent with activity occurring within the overriding plate or at the uppermost portion of the subduction interface.

Historical records maintained by SeismoSight indicate that six swarms have occurred in the same general area since 1 January 2000. These include single swarms in 2002 and 2016, two swarms in 2019, and two swarms in 2020. The recurrence of such sequences suggests that localized stress conditions or fluid migration periodically trigger clusters of small events without producing a dominant mainshock.

Swarm activity of this type is common in subduction-zone margins and often indicates aseismic slip or pore-pressure changes rather than progressive tectonic loading. The 26 events documented here remained below magnitude 2.0, resulting in no reported damage or felt shaking in nearby communities. Continued monitoring of the Kodiak region remains important because the same tectonic setting has generated great earthquakes in the past, including the magnitude 9.2 event of 1964.

References
SeismoSight internal swarm classification database (S20200223.2 event catalog).
United States Geological Survey Earthquake Hazards Program, regional tectonic summaries for the Aleutian subduction zone.