Seismic Swarm S20100815.1: Analysis of Activity Near Nikolski, Alaska

Seismic swarm S20100815.1 was recorded 53 km south-southwest of Nikolski, Alaska, in the central Aleutian Islands. The sequence began at 17:43 on 14 August 2010 and concluded at 01:40 on 16 August 2010, spanning 31 hours and 57 minutes. During this interval, 34 earthquakes were detected, with magnitudes ranging from 1.9 to 4.5 and focal depths between 1 km and 14 km.

The swarm exhibited a typical clustered pattern, with the majority of events occurring on 15 August. The largest event reached magnitude 4.5 at a depth of 10 km on 15 August at 08:38:10 UTC. Other notable shocks included a magnitude 3.0 event at 3 km depth on 15 August at 09:58:24 and several magnitude 2.7 events at shallow depths of 4–6 km. Depths remained predominantly crustal, consistent with activity along the convergent margin. Event frequency peaked in the early morning hours of 15 August before gradually declining toward the swarm's termination.

Nikolski lies within the Aleutian volcanic arc, formed by the ongoing subduction of the Pacific Plate beneath the North American Plate at rates of approximately 6–7 cm per year. This tectonic setting produces frequent seismicity, including both mainshock-aftershock sequences and earthquake swarms. The central Aleutians host numerous active volcanoes and fault systems that accommodate oblique convergence and arc-parallel extension. Historical records indicate that swarms in this region often occur without associated volcanic unrest and are attributed to fluid migration or stress transfer along the plate interface and overlying crust.

Since 1 January 2000, five swarms have been documented in the Nikolski area according to SeismoSight internal classification. These occurred in 2006 (one swarm), 2007 (one swarm), and 2010 (three swarms). Swarm S20100815.1 represents the third swarm recorded in 2010, underscoring elevated seismic clustering during that year.

The 2010 swarm's shallow depths and moderate magnitudes align with background seismicity patterns observed along the Aleutian megathrust and subsidiary faults. No damage or felt reports were associated with these events, consistent with their offshore location and modest energy release. Continued monitoring of the region remains essential given the Aleutians' capacity to generate great earthquakes.

References

• USGS Earthquake Catalog (historical Aleutian seismicity)
• Alaska Earthquake Center annual reports (tectonic setting of Nikolski region)
• SeismoSight internal swarm classification database (S20100815.1 parameters and 2000–2010 statistics)