Seismic Swarm PS20200126.1 Near Adak, Alaska

A seismic swarm designated PS20200126.1 occurred 242 km west-southwest of Adak, Alaska, on 26 January 2020. The sequence began at 03:57 UTC and concluded at 22:33 UTC, spanning 18 hours and 36 minutes. During this interval, eight earthquakes were recorded, with magnitudes ranging from 2.7 to 6.1 and focal depths between 9 km and 39 km. The largest event reached magnitude 6.1 at a depth of 17 km.

The swarm events unfolded in a compact temporal cluster. An initial magnitude 5.6 shock at 15 km depth was followed within hours by the peak magnitude 6.1 event. Subsequent activity included five additional events above magnitude 5.0 distributed through the morning and early afternoon, concluding with a magnitude 5.7 shock at 35 km depth. Depths generally increased through the later stages of the sequence.

This region lies along the Aleutian subduction zone, where the Pacific Plate converges with and descends beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust interface and overlying volcanic arc produce frequent seismicity across a wide range of magnitudes. Adak itself sits on the central Aleutian Islands, an area characterized by active volcanism and complex faulting associated with oblique subduction and slab rollback.

Historical records since 2000 document 26 prior swarms in the vicinity. These episodes occurred in 2003 (two swarms), 2005 (two), 2006 (seven), 2007 (four), 2008 (three), and single swarms in 2010, 2012, 2013, 2014, 2015, 2017, 2019, and 2020. The 2020 swarm therefore represents the most recent in a recurring pattern of clustered activity.

A notable larger event, magnitude 7.2, struck 207 km west-southwest of Adak on 19 December 2007 at a location 36 km from the 2020 swarm center. That earthquake occurred at 36 km depth and remains one of the strongest shocks recorded in the immediate area during the past two decades.

Swarm sequences in subduction settings often reflect stress transfer along the plate interface or within the overriding crust, sometimes triggered by fluid migration or afterslip following larger regional events. The 2020 swarm’s moderate magnitudes and limited duration align with background rates observed along this portion of the Aleutian arc.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the area. Such data contribute to refined hazard assessments for the central Aleutians, where both subduction-zone thrust events and shallower crustal earthquakes pose risks to infrastructure and remote communities.

References

• SeismoSight internal swarm classification PS20200126.1
• USGS Earthquake Catalog (regional events 2000–2020)