Seismic Swarm S20100903.1: Analysis of Activity Near Adak, Alaska

Seismic swarm S20100903.1 occurred in the central Aleutian Islands, approximately 69 km southeast of Adak, Alaska. The sequence began at 11:16 UTC on 3 September 2010 and concluded at 12:56 UTC on 6 September 2010, spanning 73 hours and 40 minutes. During this interval, 49 earthquakes were recorded.

The swarm initiated with a magnitude 6.5 mainshock at 23 km depth. Subsequent events ranged in magnitude from 1.7 to 3.7, with focal depths predominantly between 20 km and 30 km, though several reached 45–60 km. The largest aftershocks clustered within the first 24 hours, after which activity declined steadily, with the final recorded event measuring magnitude 2.4 at 45 km depth.

This swarm reflects typical behavior in a subduction-zone environment. Adak lies along the Aleutian arc, where the Pacific Plate subducts northwestward beneath the North American Plate at rates of approximately 6–7 cm per year. The resulting megathrust interface and overlying crustal faults generate frequent seismic sequences. Intermediate-depth events (20–60 km) commonly occur within the subducting slab or along the plate interface.

The Aleutian Islands have experienced repeated large earthquakes throughout the instrumental era. Notable historical events include the 1957 M8.6 Andreanof Islands earthquake and the 1965 M8.7 Rat Islands earthquake, both of which ruptured segments of the arc near Adak. These great earthquakes demonstrate the region’s capacity for megathrust rupture, while smaller swarms represent background stress adjustment along secondary faults.

Since 2000, only one prior swarm has been identified in the immediate area, occurring in 2008. The 2010 sequence therefore represents a relatively infrequent clustered episode superimposed on the arc’s steady background seismicity.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the central Aleutians, providing data that refine models of slab geometry and stress transfer. Such observations contribute to improved understanding of how moderate swarms relate to the potential for larger events along this highly active convergent margin.

References

USGS Earthquake Catalog
Alaska Earthquake Center annual reports
Global CMT catalog
Tectonic summaries of the Aleutian subduction zone (USGS Professional Papers)