Earthquake Swarm S20200126.2 in the Aleutian Subduction Zone

An earthquake swarm designated S20200126.2 occurred approximately 242 km west-southwest of Adak, Alaska, in the central Aleutian Islands. The sequence began at 23:28 UTC on 25 January 2020 and concluded at 07:16 UTC on 30 January 2020, spanning 103 hours and 47 minutes. During this period, 93 earthquakes were recorded, with magnitudes ranging from 1.8 to 5.6 and focal depths primarily between 8 km and 46 km.

The swarm unfolded in distinct phases, beginning with a magnitude 4.4 event at 10 km depth. Activity intensified on 26 January, featuring a magnitude 5.6 earthquake at 15 km depth followed by numerous aftershocks clustered around 10–30 km. Subsequent days included several events exceeding magnitude 4.0, such as a 4.8 at 35 km on 26 January and a 5.1 at 10 km on 27 January. The sequence peaked again with a magnitude 5.4 event at 10 km depth on 28 January before gradually diminishing, ending with smaller events near 25 km depth on 30 January. Depths showed a bimodal distribution, with many shallow events near 10 km and a secondary cluster between 20 km and 35 km, consistent with activity along the subduction interface and within the overriding plate.

This region lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust environment produces frequent seismic swarms due to episodic slip, fluid migration, and stress transfer along the plate boundary. The central Aleutians host both interplate thrust earthquakes and intraslab events, with historical activity including great earthquakes such as the 1957 Mw 8.6 Andreanof Islands event and the 1965 Mw 8.7 Rat Islands earthquake. Modern monitoring by the Alaska Earthquake Center and USGS confirms persistent background seismicity, punctuated by swarms that reflect transient stress changes rather than mainshock-aftershock sequences.

Since 2000, five comparable swarms have been documented in the vicinity, occurring in 2005 (one swarm), 2006 (two swarms), and 2007 (two swarms). These earlier episodes similarly involved dozens of events over several days, highlighting recurring swarm behavior along this segment of the arc. The 2020 swarm aligns with this pattern, exhibiting no single dominant mainshock and a rapid decay in activity after the largest events.

Such swarms contribute to understanding subduction-zone dynamics and inform regional hazard assessments. Continued seismic monitoring remains essential given the Aleutian arc’s capacity for both moderate swarms and great earthquakes.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Alaska Earthquake Center (aeic.alaska.edu)
Global CMT Catalog (globalcmt.org)