Seismic Swarm VS20180929.1: Analysis of Microearthquake Activity Near Adak, Alaska

A seismic swarm designated VS20180929.1 was recorded in the Aleutian Islands region of Alaska. The sequence began at 16:59 on 28 September 2018 and concluded at 15:56 on 30 September 2018, spanning 46 hours and 56 minutes. The epicentral area lies 41 km east-northeast of Adak, Alaska. During this interval, 33 earthquakes were detected, with magnitudes ranging from -1.7 to 0.3 and focal depths predominantly between 1 km and 5 km.

The swarm exhibited typical characteristics of clustered microseismicity, with the majority of events registering below magnitude 0.0. Earliest activity included events at 16:59:25 (-0.1, 3 km) and 16:59:30 (-0.3, 2 km). Subsequent shocks occurred at irregular intervals, peaking in frequency during the morning of 29 September. Later events on 30 September included a magnitude -0.2 shock at 15:10:41 (1 km depth) and a final magnitude 0.0 event at 15:56:03 (3 km depth). Depths remained shallow throughout, consistent with brittle failure in the upper crust.

Adak lies within the tectonically active Aleutian arc, where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin generates frequent earthquakes across a wide range of magnitudes. The island occupies a position along the Andreanof segment, a portion of the arc known for both moderate swarms and occasional great earthquakes exceeding magnitude 8. Shallow crustal swarms such as VS20180929.1 are commonly attributed to fluid migration or stress perturbations along existing fault networks rather than mainshock-aftershock sequences.

Historical records indicate that swarm activity has been documented in the Adak vicinity since at least 2000. Six distinct swarms have occurred in this period, distributed as one event in 2002, one in 2013, and four in 2018. The elevated count in 2018 suggests a possible increase in localized stress or fluid involvement during that year, although swarm recurrence remains episodic and not strictly periodic.

Geological mapping of the region reveals a complex assemblage of volcanic and sedimentary rocks overlying an active subduction interface. The shallow depths recorded in VS20180929.1 align with the upper 10 km of the crust, where hydrothermal systems and fracture networks facilitate small-magnitude slip. No surface rupture or significant ground deformation was associated with this swarm, consistent with its low energy release.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the Adak area. Such swarms provide valuable data for refining models of subduction-zone dynamics and assessing background rates of seismic activity.

References
USGS Earthquake Hazards Program – Aleutian Arc tectonics
Alaska Earthquake Center – Regional seismicity summaries
SeismoSight internal swarm classification records