Seismic Swarm VS20230524.1: Analysis of Activity Near Karluk, Alaska

Seismic swarm VS20230524.1 was recorded in a region 85 km NNW of Karluk, Alaska, beginning at 04:32 on 23 May 2023 and concluding at 17:15 on 29 August 2023. Over 2364 hours and 42 minutes, the sequence produced 3637 earthquakes. This event aligns with the area's established pattern of episodic seismic clusters within the tectonically active Aleutian subduction zone.

The first 100 events exhibited predominantly low magnitudes, ranging from -0.7 to 2.0, with the majority below 1.0. Depths were generally shallow, concentrated between 0 and 5 km, though several events reached 27–35 km. Initial activity on 23 May featured microearthquakes at depths of 0–5 km, followed by a cluster on 24 May that included slightly deeper events up to 35 km and the sequence's peak magnitude of 2.0. Subsequent events through 25 May maintained similar characteristics, with isolated deeper occurrences and frequent negative-magnitude detections indicating high-resolution monitoring of subtle crustal adjustments.

This swarm occurred in a portion of southwestern Alaska influenced by the ongoing subduction of the Pacific Plate beneath the North American Plate. The regional geology features a convergent margin marked by the Aleutian Trench, where oblique convergence drives both megathrust earthquakes and distributed crustal seismicity. The Kodiak Island vicinity, including areas northwest of Karluk, lies near the eastern extent of the Aleutian arc, where volcanic and tectonic processes interact. Historical great earthquakes, such as the 1964 M9.2 event, underscore the zone's capacity for large-magnitude release, while smaller swarms reflect localized stress redistribution along faults and volcanic systems.

Seismic swarms have been documented in the region since at least 2000, with 13 episodes recorded through 2023. Prior swarms occurred in 2002 (1 event), 2019 (2), 2020 (3), 2022 (4), and 2023 (3). These clusters typically involve hundreds to thousands of events, often at shallow depths, and are interpreted as responses to fluid migration or aseismic slip within the subduction interface and overlying crust.

The 2023 swarm's duration and event count are consistent with previous activity, emphasizing the value of continuous monitoring for understanding strain accumulation in this high-hazard setting. Data from the sequence contribute to refined models of subduction-zone dynamics, aiding assessments of potential escalation to larger events.

References

SeismoSight internal swarm classification records (VS20230524.1 parameters and event catalog).
USGS Earthquake Catalog for regional historical context and Aleutian subduction zone parameters.