Seismic Swarm S20080311.2 Near Nikolski, Alaska

A seismic swarm designated S20080311.2 was recorded 108 km south-southwest of Nikolski, Alaska, from 14:46 UTC on 11 March 2008 to 13:58 UTC on 12 March 2008. Over 23 hours and 12 minutes, the sequence produced 31 earthquakes with magnitudes between 2.3 and 4.2. Depths ranged from 3 km to 41 km, with most events occurring between 4 km and 25 km.

The swarm began with a magnitude 2.4 event at 6 km depth. Subsequent activity included multiple events near magnitude 3.0, such as a 3.1 at 6 km and a 3.4 at 12 km. Peak magnitudes reached 4.0 at 4 km depth late on 11 March and 4.2 at 11 km early on 12 March. Later events included a 3.9 at 41 km and a 3.6 at 40 km, indicating a modest deepening trend toward the end of the sequence.

Nikolski lies on Umnak Island in the eastern Aleutian arc, where the Pacific Plate subducts beneath the North American Plate at rates of approximately 6–7 cm per year. This convergent margin generates frequent earthquakes and volcanic activity along the Aleutian subduction zone. The region experiences both shallow crustal events and deeper intraslab seismicity, consistent with the observed depth distribution of the 2008 swarm.

Historical records indicate that only one prior swarm has been identified in the same area since 1 January 2000, occurring in 2007. Such episodic clusters are characteristic of the Aleutian arc, where fluid migration or stress transfer along the plate interface can trigger short-lived sequences without a single dominant mainshock.

The 2008 swarm did not produce reported damage or felt shaking at Nikolski, reflecting the moderate magnitudes involved. Continued monitoring by regional networks remains essential given the arc’s potential for larger events.

References

• Alaska Earthquake Center, University of Alaska Fairbanks
• U.S. Geological Survey Earthquake Hazards Program
• SeismoSight internal swarm classification records