Seismic Swarm VS20230331.1 Near Karluk, Alaska

A seismic swarm designated VS20230331.1 occurred 86 km NNW of Karluk, Alaska, beginning at 10:49 on 30 March 2023 and concluding at 22:49 on 31 March 2023. Over this 36-hour period, 25 earthquakes were recorded, with magnitudes ranging from -0.5 to 2.8 and focal depths between 0 and 6 km. The sequence featured numerous microearthquakes, including several events below magnitude 0.0, alongside a few events exceeding magnitude 2.0.

The swarm initiated with a magnitude 0.1 event at 5 km depth. Activity intensified later on 30 March, culminating in a magnitude 2.8 earthquake at 23:33. Subsequent events on 31 March included a magnitude 2.6 shock at 00:02 and a magnitude 2.3 event at 02:24, both at shallow depths. The final recorded event was a magnitude 1.0 earthquake at 2 km depth on 31 March at 22:49. Depths remained consistently shallow throughout, suggesting activity within the upper crust.

This swarm aligns with patterns observed in the region since 2000, during which 14 swarms have been documented. Prior episodes occurred in 2002 (1 swarm), 2016 (1), 2019 (2), 2020 (5), 2022 (4), and 2023 (1). Such recurrent swarms reflect ongoing tectonic stress release without a dominant mainshock-aftershock sequence.

The location lies within the tectonically active 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 megathrust setting produces frequent seismicity, including both volcanic and tectonic events. Karluk sits on the western coast of Kodiak Island, an area influenced by the Kodiak segment of the subduction interface, which has hosted significant historical earthquakes such as the 1964 magnitude 9.2 Great Alaska Earthquake. Shallow crustal swarms in this vicinity often result from fluid migration or stress transfer along subsidiary faults rather than direct megathrust rupture.

Seismic monitoring in Alaska relies on networks operated by the Alaska Earthquake Center and the U.S. Geological Survey, which provide real-time detection of events down to low magnitudes. The shallow depths and low magnitudes observed in VS20230331.1 are typical of swarm activity in this volcanic-arc environment and do not indicate immediate hazard escalation, though they contribute to cumulative strain monitoring.

Continued observation of swarm frequency supports refined models of subduction-zone dynamics in the region.

References

• Alaska Earthquake Center seismic catalog (data through 2023)
• U.S. Geological Survey Earthquake Hazards Program
• SeismoSight internal swarm classification records