Seismic Swarm VS20180118.1: Analysis of Earthquake Activity Near Adak, Alaska

Seismic swarm VS20180118.1 was recorded 40 km northeast of Adak, Alaska, beginning at 04:47 on 18 January 2018 and concluding at 12:13 on 20 January 2018. Over 55 hours and 25 minutes, the event sequence included 87 earthquakes. This swarm provides insight into localized seismic behavior in a tectonically active subduction setting.

The Aleutian Islands, including the Adak region within the Andreanof Islands group, lie along the convergent boundary where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–8 cm per year. This process generates frequent earthquakes and volcanic activity characteristic of the Pacific Ring of Fire. Earthquake depths in the swarm ranged primarily from 4 to 14 km, with the majority clustered between 5 and 10 km, consistent with shallow crustal or upper-plate seismicity often observed in forearc environments. Magnitudes varied from −1.0 to 1.5, with most events below zero, indicating microseismicity typical of swarm sequences rather than mainshock-aftershock patterns.

Temporal distribution showed peak activity on 18 January, with events occurring at intervals of minutes to hours, followed by a gradual decline on 19 and 20 January. Notable larger events included a magnitude 1.5 earthquake at 02:54:30 on 19 January at a reported depth of −1 km and a magnitude 1.1 event at 01:22:26 on the same day. Such shallow or negative depth readings can reflect near-surface or atmospheric influences in detection algorithms but remain within the internal classification parameters of the recording system.

Historical records indicate limited swarm occurrences in the area since 2000. Only two prior swarms have been documented: one in 2002 consisting of a single event cluster and another in 2013. This low frequency suggests that swarm VS20180118.1 represents a relatively infrequent mode of seismic release compared to the more common isolated earthquakes driven by ongoing plate convergence.

The geological setting around Adak features complex faulting associated with the Aleutian megathrust and subsidiary structures. Regional volcanism, including nearby stratovolcanoes, may contribute to fluid migration that triggers swarm activity through pore-pressure changes. Depths observed in the swarm align with typical values for tectonic earthquakes in the upper 15 km of the crust in this segment of the arc.

In summary, swarm VS20180118.1 illustrates the episodic nature of microseismic release in the central Aleutians. Continued monitoring of similar sequences can improve understanding of stress accumulation along the subduction interface.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonic framework)
Alaska Earthquake Center (Aleutian subduction zone characteristics)