Gulf of Alaska Earthquake Swarm: February 2018 Analysis

The Gulf of Alaska lies within a highly active tectonic setting at the convergent boundary between the Pacific Plate and the North American Plate. Subduction along the Aleutian Trench generates frequent seismic events, with the region forming part of the circum-Pacific seismic belt. Historical records document major earthquakes, including the magnitude 9.2 event of 1964 that originated near Prince William Sound and produced widespread deformation across south-central Alaska. Ongoing plate motion continues to drive both shallow crustal seismicity and deeper events associated with the subducting slab.

Swarm S20180208.1 began at 16:59 on 7 February 2018 and concluded at 08:56 on 9 February 2018, spanning 39 hours and 57 minutes. During this interval, 32 earthquakes were recorded in the Gulf of Alaska. The sequence exhibited typical swarm characteristics, with no single dominant mainshock followed by aftershocks; instead, events clustered in time and space without a clear foreshock-mainshock-aftershock progression. Magnitudes ranged from 2.6 to 4.8, and focal depths varied between 4 km and 29 km, indicating activity within both the upper crust and the shallow portion of the subducting slab.

The largest event reached magnitude 4.8 at 03:58 on 8 February at a depth of 18 km. Additional notable shocks included a magnitude 3.9 at 08:57 on 8 February (6 km depth) and another magnitude 3.9 at 08:56 on 9 February (10 km depth). Multiple magnitude 3.0–3.7 events occurred throughout 8 February, concentrated between 04:00 and 10:00 local time. Depths remained predominantly shallow, with many events at or above 10 km, consistent with brittle failure in the overriding plate or near the plate interface.

This swarm represents the first such sequence recorded in the Gulf of Alaska since systematic monitoring began in 2000. Three swarms have been identified in the region through the present, underscoring that clustered seismicity, while less common than isolated events, forms a recognized component of the area’s seismic behavior. The February 2018 activity did not produce reported damage or felt effects onshore, reflecting both the moderate magnitudes involved and the offshore location.

Seismic swarms in subduction zones can arise from fluid migration, aseismic slip, or stress perturbations along the plate boundary. The 2018 sequence’s temporal clustering and depth distribution align with patterns observed in other Pacific margin settings, where short-lived increases in seismicity rates occur without leading to larger triggered events. Continued monitoring by regional networks remains essential for distinguishing background seismicity from potential precursors in this tectonically dynamic environment.