Seismic Swarm VS20181204.1: Analysis of Activity in the Andreanof Islands, Aleutian Arc

The Andreanof Islands form part of the central Aleutian chain in Alaska, situated along the convergent boundary where the Pacific Plate subducts beneath the North American Plate. This tectonic setting produces one of the most seismically active regions on Earth, characterized by frequent earthquakes, volcanic arcs, and episodic seismic swarms. The subduction zone here accommodates convergence rates of approximately 6–7 cm per year, generating both shallow crustal events and deeper Wadati-Benioff zone activity.

Seismic swarm VS20181204.1 was recorded in this setting, beginning at 22:09 on 3 December 2018 and concluding at 16:20 on 11 December 2018. Over 186 hours and 11 minutes, a total of 179 earthquakes were detected. Analysis of the first 100 events reveals predominantly microseismic activity. Magnitudes ranged from −1.0 to 1.9, with the majority falling between −0.5 and 0.5. Depths were consistently shallow, spanning −1 km to 8 km, consistent with brittle failure in the upper crust above the subduction interface.

The temporal distribution shows an initial cluster of very low-magnitude events on 3–4 December, followed by sporadic larger events such as the magnitude 1.9 earthquake at 19:20 on 5 December. Subsequent activity through 7 December maintained low magnitudes and shallow depths, indicating a diffuse release of strain without progression to a mainshock-aftershock sequence. Such patterns align with fluid migration or localized stress perturbations commonly observed in subduction-related swarms.

The Aleutian arc has experienced at least ten seismic swarms since 2000, with documented episodes in 2002 and 2013, and eight additional swarms in 2018 alone. These recurrent swarms underscore the dynamic stress environment of the region, influenced by both plate-boundary locking and potential magmatic or hydrothermal processes near volcanic centers such as those on Adak and Kanaga islands.

Historical large-magnitude earthquakes further illustrate the tectonic hazard. The 1957 Andreanof Islands earthquake reached magnitude 8.6, producing significant tsunami impacts across the Pacific. More recent events, including the 1986 magnitude 8.0 earthquake near Adak, confirm ongoing strain accumulation and release along the megathrust.

This swarm provides valuable data on background seismicity rates and helps refine models of short-term seismic hazard in the central Aleutians. Continued monitoring by regional networks supports improved forecasting of potential escalation to larger events.

References

USGS Earthquake Hazards Program – Aleutian Arc Tectonic Summary
Alaska Earthquake Center – Regional Seismicity Reports
International Seismological Centre – Event Catalogues