Seismic Swarm VS20190618.1 Near Adak, Alaska: Geological Context and Event Analysis

A notable earthquake swarm designated VS20190618.1 occurred approximately 38 km northeast of Adak, Alaska, in the Aleutian Islands. The swarm initiated at 15:45 on 17 June 2019 and concluded at 02:28 on 21 June 2019, spanning 82 hours and 42 minutes. During this period, 80 earthquakes were recorded, with magnitudes ranging from -1.3 to 0.0 and focal depths predominantly between 0 and 6 km. These events reflect typical microseismic activity in a tectonically active subduction zone.

Adak lies within the Aleutian volcanic arc, formed by the subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin produces frequent seismic swarms and volcanic eruptions, characteristic of the Pacific Ring of Fire. The region's geology features a complex system of faults and volcanic edifices, with Adak Island itself hosting the historically active Mount Adagdak and Moffett volcanoes.

Seismic swarms in this area often arise from fluid migration or stress adjustments along the subduction interface rather than a single large rupture. Depths in the recorded events remained shallow, consistent with crustal processes above the main Wadati-Benioff zone. Magnitudes stayed below detection thresholds for felt shaking, underscoring the swarm's low-energy nature.

Historically, the Aleutian Islands have experienced significant earthquakes, including the magnitude 8.6 event of 1957 near Adak and the magnitude 9.2 Great Alaska Earthquake of 1964, which affected the broader region. Since 2000, twelve swarms have been documented near Adak, with notable clusters in 2002 (1 swarm), 2013 (1 swarm), and 2018 (10 swarms). These recurrent episodes highlight persistent tectonic strain accumulation.

Analysis of VS20190618.1 reveals a gradual onset with events clustered in the first 24 hours, followed by diminishing frequency. The largest events reached -0.2, while most remained below -0.5, illustrating a classic swarm decay pattern without a dominant mainshock.

Ongoing monitoring by regional seismic networks supports improved hazard assessment in this remote yet strategically important area. Continued observation of swarm patterns aids in distinguishing background activity from potential precursors to larger events.

References

USGS Earthquake Hazards Program: Aleutian Arc tectonics and historical seismicity.
Alaska Volcano Observatory: Regional volcanic and seismic context for Adak Island.
SeismoSight internal classification: Swarm VS20190618.1 parameters and statistics.