Seismic Swarm S20180124.5 in the Gulf of Alaska

Seismic swarm S20180124.5 was recorded in the Gulf of Alaska, beginning at 09:57 on 23 January 2018 and concluding at 13:54 on 2 February 2018. Over 243 hours and 57 minutes, a total of 109 earthquakes were registered. Analysis of the first 100 events reveals a sequence dominated by moderate-magnitude tremors, with the majority occurring at depths between 10 and 30 km. Magnitudes ranged primarily from 2.8 to 4.5, including several events of 4.0 or greater, such as the 4.5 recorded on 26 January at 19 km depth and multiple 4.4 events distributed across the initial days.

The swarm exhibited a clustered temporal pattern, with heightened activity on 23–27 January featuring repeated events near magnitude 3.5–4.2 at consistent depths around 20 km. Later stages showed a gradual decline in both frequency and peak magnitudes, transitioning to isolated lower-magnitude occurrences by early February. Depths remained stable within the upper crust, suggesting a shallow tectonic source without significant migration.

The Gulf of Alaska lies within the tectonically active Pacific Ring of Fire, where the Pacific Plate subducts beneath the North American Plate along the Aleutian megathrust. This convergent boundary generates frequent seismic activity, including both individual large earthquakes and episodic swarms driven by stress accumulation and fluid migration in the subduction interface. The region has a well-documented history of major events, notably the magnitude 9.2 Great Alaska Earthquake of 1964, which originated nearby and produced widespread deformation. Since 2000, only one swarm has been classified in this area prior to the present record, underscoring the relative rarity of such clustered sequences compared to isolated mainshock-aftershock patterns.

This swarm provides valuable insight into the ongoing deformation along the subduction zone, where repeated moderate events can indicate localized strain release without escalation to a larger rupture. Depths clustering near 20 km align with the typical seismogenic zone in this segment of the margin, influenced by the geometry of the downgoing slab.

References

USGS Earthquake Catalog
Alaska Earthquake Center historical records
NOAA tectonic summaries for the Gulf of Alaska