Seismic Swarm VS20240524.1: Analysis of Microearthquake Activity Near Karluk, Alaska

A seismic swarm designated VS20240524.1 was recorded 86 km NNW of Karluk on Kodiak Island, Alaska. The sequence began at 01:19 UTC on 24 May 2024 and concluded at 01:34 UTC on 25 May 2024, encompassing 32 earthquakes over 24 hours and 15 minutes. Magnitudes ranged from -0.7 to 0.9, with the majority of events falling below 0.5. Focal depths clustered predominantly between 27 km and 36 km, though a few shallower events occurred at 0–3 km.

The swarm exhibited a typical pattern of low-magnitude, closely spaced events without a dominant mainshock. Early activity on 24 May included events at depths near 31–32 km, followed by a concentration of slightly larger magnitudes (0.3–0.9) between 06:40 and 06:51 UTC. Later phases showed continued microseismicity at similar depths until the final recorded event at 01:34 UTC on 25 May. Such sequences often reflect fluid migration or stress adjustments along pre-existing faults rather than large-scale rupture.

Kodiak Island lies within the Alaska-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 seismicity, including both intraslab and interface earthquakes. The region’s crust features a complex assemblage of accreted terranes and volcanic arcs shaped by millions of years of subduction. Historical large-magnitude events, such as the 1964 Mw 9.2 Great Alaska Earthquake, originated nearby along the megathrust and caused widespread deformation across Kodiak Island.

Earthquake swarms have occurred intermittently in this sector since at least 2000. Records indicate 20 such swarms through May 2024, with notable increases in frequency during 2022 (4 swarms), 2023 (7 swarms), and the current year. Depths around 30 km align with typical intraslab activity within the subducting slab, where phase changes and dehydration reactions can promote swarm-like behavior.

Ongoing monitoring by regional seismic networks continues to track background rates and any potential escalation. While most swarm events remain imperceptible, they contribute valuable data on stress transfer within the subduction system.

References
USGS Earthquake Hazards Program – Alaska Earthquake Catalog
Alaska Earthquake Information Center – Regional Seismicity Reports
Alaska Division of Geological & Geophysical Surveys – Tectonic Framework of Kodiak Island