Seismic Swarm VS20181127.1: Analysis of Activity Near Adak, Alaska

A notable seismic swarm designated VS20181127.1 occurred in the Aleutian Islands region of Alaska, centered 41 km east-northeast of Adak. The sequence began at 15:29 on 26 November 2018 and concluded at 23:22 on 2 December 2018, spanning 151 hours and 52 minutes. During this interval, 191 earthquakes were recorded. Analysis of the first 100 events reveals predominantly low-magnitude activity with shallow focal depths. Magnitudes ranged from -0.8 to 2.6, with the majority falling between -0.5 and 0.5. Depths clustered between -1 km and 9 km, indicating very shallow crustal sources consistent with the tectonic setting. Notable events within this subset included a magnitude 2.6 earthquake at 141 km depth on 29 November and several magnitude 1.2–1.8 events at depths of 2 km. The swarm exhibited a typical pattern of numerous micro-earthquakes interspersed with occasional larger events, without a single dominant mainshock. The Aleutian Islands lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This subduction drives intense seismicity throughout the arc, including both shallow crustal events and deeper Wadati-Benioff zone activity. The Adak region specifically occupies a segment of the arc characterized by oblique convergence and complex faulting, contributing to frequent earthquake swarms and volcanic unrest. Historical records document nine swarms in the area since 2000, occurring in 2002 (one swarm), 2013 (one swarm), and seven swarms in 2018. Such swarms are common in subduction-zone environments and often reflect fluid migration or stress adjustments along the plate interface rather than a single large rupture. The 2018 activity aligns with the broader seismic character of the central Aleutians, where background rates remain elevated due to ongoing plate motion. References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (earthquake.alaska.edu)
SeismoSight internal swarm classification (VS20181127.1)