Seismic Swarm S20130121.1 Near Adak, Alaska: Characteristics and Regional Context

The seismic swarm designated S20130121.1 occurred approximately 130 km west-southwest of Adak, Alaska, within the tectonically active Aleutian subduction zone. This region forms part of the Aleutian arc, where the Pacific Plate subducts beneath the North American Plate at rates of about 6–7 cm per year. The resulting compressional forces generate frequent earthquakes, including both individual events and clustered swarms, reflecting the complex interplay of slab dehydration, faulting, and magma movement in the overriding plate.

The swarm initiated at 11:32 UTC on 21 January 2013 and concluded at 22:54 UTC on 24 January 2013, spanning 83 hours and 22 minutes. During this period, 62 earthquakes were recorded. Event magnitudes ranged from 1.6 to 5.1, with the largest shock (magnitude 5.1) occurring at a depth of 15 km early in the sequence. Depths varied between 0 km and 29 km, indicating activity across the upper crust and near the plate interface. Most events clustered between 3 km and 17 km depth, consistent with shallow crustal adjustments common in subduction settings.

Temporal distribution showed peak activity on 21 January, with 21 events, followed by a gradual decline. Notable later shocks included magnitude 3.3 events on 21 and 23 January, and several magnitude 3.0 events on 22 and 23 January. The sequence lacked a clear mainshock-aftershock pattern typical of tectonic ruptures, instead exhibiting swarm-like characteristics with distributed energy release.

This swarm aligns with historical patterns in the central Aleutians. Since 2000, seven swarms have been documented near Adak, occurring in 2006 (two swarms), 2007 (one), 2008 (two), and 2012 (two). Such recurrent clustering underscores the area's persistent seismic productivity driven by ongoing subduction.

The Aleutian Islands host one of Earth's most active volcanic arcs, with Adak situated near the Andreanof Islands segment. Past large earthquakes, such as the 1957 magnitude 8.6 event and the 1986 magnitude 8.0 shock, highlight the potential for great earthquakes, though swarms like S20130121.1 typically represent lower-magnitude, non-destructive activity. Monitoring by regional networks aids in distinguishing these swarms from precursors to larger events.

References

USGS Earthquake Catalog Alaska Earthquake Center Annual Reports SeismoSight Internal Swarm Database