Seismic Swarm S20100718.2: Analysis of Activity Near Nikolski, Alaska

Seismic swarm S20100718.2 was recorded southwest of Nikolski, Alaska, beginning at 06:14 on 18 July 2010 and concluding at 04:45 on 23 July 2010. Over 118 hours and 31 minutes, the swarm produced 165 earthquakes. The events occurred 31 km southwest of Nikolski on Umnak Island in the Aleutian chain.

The first 100 events, spanning primarily 18 and 19 July 2010, showed magnitudes ranging from 1.9 to 4.3. The largest event measured 4.3 at a depth of 4 km early on 18 July. Depths clustered between 0 and 26 km, with the majority between 3 and 10 km. Activity was most intense during the initial hours, featuring multiple events above magnitude 3.0, then gradually declined with smaller magnitudes dominating later entries. Notable clusters included several magnitude 3.4–3.6 events within the first 24 hours, followed by a steady sequence of 2.0–2.9 events.

Nikolski lies within the Aleutian subduction zone, where the Pacific Plate converges with the North American Plate at rates exceeding 6 cm per year. This tectonic setting generates frequent seismicity and volcanism across the island arc. The region experiences both shallow crustal events and deeper intraslab earthquakes associated with the subducting slab. Historical records indicate persistent seismic swarms linked to stress accumulation along the plate interface and nearby volcanic systems.

Since 2000, only two swarms have been classified in the area according to internal SeismoSight records: one in 2007 and the present 2010 sequence. Such swarms typically reflect episodic release of tectonic strain rather than isolated mainshock-aftershock sequences.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog (general regional tectonics).
Alaska Volcano Observatory regional geological summaries.