Seismic Swarm S20190106.1: Analysis of Activity 131 km Southwest of Adak, Alaska

Earthquake swarm S20190106.1 occurred 131 km southwest of Adak in the Aleutian Islands, Alaska. The sequence began at 18:47 UTC on 5 January 2019 and concluded at 02:24 UTC on 7 January 2019, spanning 31 hours and 37 minutes. During this interval, 32 events were recorded, with magnitudes ranging from 1.8 to 5.9 and focal depths predominantly between 25 and 50 km.

The swarm initiated with a magnitude 5.9 earthquake at 30 km depth, followed by a rapid succession of smaller events. Notable shocks included a magnitude 5.1 event at 35 km depth on 6 January and a magnitude 4.3 event early in the sequence. Depths clustered around the 30–45 km range, consistent with intermediate-depth seismicity within the subducting slab. Activity showed a typical swarm pattern of clustered events without a single dominant mainshock-aftershock decay, with most events occurring within the first 12 hours.

The location lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at approximately 7–8 cm per year. This tectonic setting produces frequent intermediate-depth earthquakes as the slab descends beneath the overriding plate. The central Aleutians near Adak experience elevated seismicity due to the geometry of the subducting slab and the presence of the Adak fracture zone, which influences stress distribution.

Since 2000, seven prior swarms have been documented in the same region, occurring in 2005, 2006 (two episodes), 2007, 2010, 2017, and 2018. These episodes reflect episodic stress release along the subduction interface and within the downgoing slab, a characteristic behavior of the Aleutian arc.

The January 2019 swarm provides insight into short-term clustering of intermediate-depth events. Magnitudes above 4.0 accounted for five events, while the majority remained below 3.0, indicating limited energy release overall. Depths remained stable throughout, suggesting the activity originated from a confined volume within the slab rather than migrating along the plate interface.

Such swarms contribute to understanding seismic hazard in remote Aleutian locations, where monitoring relies on regional networks. Continued observation of similar sequences aids in refining models of slab dehydration and stress transfer in this highly active convergent margin.

References

USGS Earthquake Catalog
Alaska Earthquake Center regional reports
SeismoSight internal swarm classification database