Seismic Swarm S20191124.1: Analysis of Earthquake Activity Southeast of Adak, Alaska

The seismic swarm designated S20191124.1 occurred approximately 92 km southeast of Adak, Alaska, in the central Aleutian Islands. It began at 00:54 UTC on 24 November 2019 and concluded at 05:46 UTC on 26 November 2019, spanning 52 hours and 52 minutes. During this interval, 84 earthquakes were recorded. The swarm initiated with a magnitude 6.3 event at 20 km depth, followed by numerous aftershocks ranging from magnitude 1.5 to 4.5, with depths predominantly between 10 km and 57 km.

This region lies within the Aleutian subduction zone, where the Pacific Plate converges with and subducts beneath the North American Plate at a rate of approximately 6–7 cm per year. The resulting compressional tectonics produce frequent seismic activity, including both shallow crustal events and deeper intraslab earthquakes. The central Aleutians host several active volcanoes and have a well-documented history of large-magnitude thrust earthquakes along the megathrust interface.

The initial magnitude 6.3 shock occurred at a focal depth of 20 km, consistent with the typical range for interface events in this segment of the subduction zone. Subsequent events showed a mix of shallow (10–20 km) and intermediate-depth (25–57 km) activity, suggesting involvement of both the plate interface and the downgoing slab. Magnitudes decreased rapidly after the mainshock, with the majority falling below magnitude 3.0, characteristic of swarm behavior rather than a classic mainshock-aftershock sequence.

Historical records indicate that swarm activity is recurrent in this area. Since 1 January 2000, six prior swarms have been identified: one in 2008, two in 2013, and one each in 2015, 2016, and 2017. These episodes demonstrate episodic clustering of seismicity without progression to a significantly larger mainshock, likely reflecting localized stress release along fault segments or fluid migration within the subduction complex.

The 2019 swarm contributed to ongoing monitoring efforts in a region that experiences some of the highest rates of seismicity in the United States. Continuous seismic networks operated by the Alaska Earthquake Center and the U.S. Geological Survey provide real-time detection, enabling detailed characterization of such sequences. Depths recorded during the swarm align with expected values for the central Aleutian arc, where the slab dips steeply and intermediate-depth seismicity is common.

In summary, swarm S20191124.1 exemplifies typical subduction-related seismicity in the Aleutians. Its moderate size, short duration, and declining magnitude trend underscore the importance of sustained monitoring to distinguish swarm activity from potential precursors to larger events.

References

USGS Earthquake Hazards Program – Aleutian Arc Tectonic Setting
Alaska Earthquake Center – Regional Seismicity Reports
Global CMT Catalog – Focal Mechanism Data for Central Aleutians